OF PORTLAND CEMENT PASTE AT ELEVATED
TEMPERATURE
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op donderdag 11 juli 2013 om 10.00 uur door
Qi Zhang
Master of engineering aan de Tongji University, P.R. China geboren te Wenzhou, P.R. China
Copromotor: Dr. G. Ye
Samenstelling promotiecommissie: Rector Magnificus voorzitter
Prof.dr.ir. K. van Breugel Technische Universiteit Delft, promotor Dr. G. Ye Technische Universiteit Delft, copromotor Dr.ir. E.A.B. Koenders Technische Universiteit Delft / COPPE-UFRJ Prof.dr.ir. L. Taerwe Universiteit Gent, Belgi¨e
Prof. S. Xu Zhejiang University, China
Dr. P. Pimienta French Scientific and Technical Centre for Building, Frankrijk Prof. R.P.J. van Hees Technische Universiteit Delft
Prof.dr.ir. E. Schlangen Technische Universiteit Delft, reservelid
The work reported in this thesis is part of the STW project (code No. 07045) (Explosive spalling of concrete toward a model for fire resistant design of concrete elements)
ISBN 97890-6562-328-7
Keywords: Portland cement paste, high temperature, dehydration, microstructural change, Lattice model
© 2013 by Qi Zhang.
All rights reserved. This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognize that its copyright rests with its author and that no quotation from the thesis and no information derived from it may be published without the author’s prior consent.
throughout the course of this thesis.
Table of Contents · · · v
List of Figures · · · x
Chapter 1 Introduction · · · 1
1.1 Background . . . 1
1.1.1 Fire spalling of concrete . . . 1
1.1.2 Material deterioration of cement paste due to dehydration . . . 2
1.2 Research scope . . . 3
1.3 Problem statement . . . 4
1.4 Objectives of present research . . . 5
1.5 Research strategy and structure of this thesis . . . 5
Chapter 2 Literature study on cementitious material at high temperature 9 2.1 Behavior of cementitious material at high temperature . . . 9
2.1.1 Mechanical properties: strength and elastic modulus . . . 9
2.1.2 Creep . . . 10
2.1.3 Transport properties . . . 11
2.1.4 Thermal properties . . . 13
2.1.4.1 Thermal conductivity . . . 13
2.1.4.2 Thermal dilation . . . 13
2.1.5 Material deterioration mechanism . . . 15
2.1.6 Remarks . . . 16
2.2 Chemical transition of cementitious material . . . 16
2.2.1 Chemical composition of cement paste . . . 16
2.2.2 Phase transformation at high temperature . . . 17
2.2.2.1 Dehydration of calcium silicate hydrate . . . 21
2.2.2.2 Dehydration of calcium hydroxide . . . 21
2.2.2.3 Decarbonation . . . 21
2.2.2.4 Recrystallization . . . 22
2.2.3 Kinetics of dehydration . . . 22 v
2.3 Materials structure of cementitious material . . . 23
2.3.1 Multiscale structure at room temperature . . . 25
2.3.1.1 Microstructure of cement paste (0.1-100 µm) . . . 25
2.3.1.2 Nanostructure of CSH gel (2-100 nm) . . . 25
2.3.1.3 Atomic structure of CSH (<2 nm) . . . 26
2.3.2 Materials structure after heating cycle . . . 26
2.3.2.1 Microstructure of cement paste after heating cycle (0.1-100 µm) . . . 26
2.3.2.2 Nanostructure of CSH gel after heating cycle (2-100 nm) . . 28
2.3.2.3 Atomic structure of CSH after heating cycle (<2 nm) . . . . 29
2.3.3 Discussion . . . 31
2.4 Concluding remarks . . . 31
2.4.1 Summary . . . 31
2.4.2 Questions to be studied . . . 32
Chapter 3 Chemical transitions in heated Portland cement paste · · · 33
3.1 Introduction . . . 33
3.1.1 Phase transformation of heated cement paste . . . 35
3.1.2 Dehydration kinetics of hydration products . . . 36
3.2 Phase transformation of heated cement paste . . . 37
3.2.1 Experimental methods: X-ray powder diffraction and Rietveld refine-ment . . . 37
3.2.2 Sample preparation and experimental procedure . . . 39
3.2.2.1 Sample preparation . . . 39
3.2.2.2 Collection of XRD patterns of heated cement paste . . . 39
3.2.2.3 Rietveld refinement of the XRD patterns . . . 40
3.2.2.4 Stabilized weight loss of heated cement paste . . . 41
3.2.2.5 Phase distribution diagram of heated cement paste . . . 41
3.2.3 Experimental results of XRD/Rietveld refinement and TGA . . . 41
3.2.3.1 Changes of CH and lime in cement paste . . . 45
3.2.3.2 Changes of amorphous phase in cement paste . . . 45
3.2.3.3 Changes of alite, belite and other minor phases in cement paste 46 3.2.3.4 The relationship between dehydration and recrystallization of CSH . . . 47
3.3 Dehydration kinetics of hydration products . . . 49
3.3.1 Experimental methods: Thermogravimetric analysis . . . 49
3.3.1.1 Procedure for determining the weight loss due to dehydration of CH and CSH . . . 49
dehydration of CSH and CH . . . 54
3.3.3 Experimental results . . . 54
3.4 Conclusion . . . 57
Chapter 4 Microstructural investigation of heated Portland cement paste 59 4.1 Introduction . . . 59
4.1.1 Experimental techniques: SEM, MIP and nitrogen adsorption . . . . 60
4.2 Sample preparation and test procedures . . . 61
4.2.1 Heat treatment . . . 62
4.2.2 SEM observation . . . 62
4.2.3 MIP test . . . 64
4.2.4 Nitrogen adsorption test . . . 64
4.3 Experimental results . . . 64
4.3.1 Scanning Electron Microscope . . . 64
4.3.2 Mercury Intrusion Porosimetry . . . 69
4.3.3 Nitrogen adsorption . . . 72
4.3.3.1 Specific surface area (BET) . . . 73
4.3.3.2 Pore size distribution . . . 74
4.3.4 Comparison of pore structures determined by various methods . . . . 76
4.3.4.1 Pores >0.14 µm: SEM/MIP . . . 76
4.3.4.2 Pores in the range from 3nm to 37nm : Nitrogen adsorp-tion/MIP . . . 77
4.4 Discussion . . . 78
4.4.1 Structure of cement paste at micro scale (0.1-100 µm) . . . 79
4.4.2 Structure of CSH at nano scale (2-100 nm) . . . 79
4.4.3 Structure of CSH at atomic scale (< 2 nm) . . . 83
4.5 Summary . . . 83
Chapter 5 Modeling of microstructural changes of heated Portland cement paste · · · 85
5.1 General . . . 85
5.2 Physical model of heated cement paste . . . 87
5.2.1 General assumptions . . . 87
5.2.2 The dehydration kinetics of Portland cement paste . . . 88
5.2.3 The shrinkage of CH and CSH at high temperature . . . 89
5.2.3.1 The shrinkage of CH due to dehydration . . . 89
5.2.3.2 The shrinkage of CSH at high temperature . . . 90
5.2.4 Representative volume element (RVE) . . . 93
5.2.5 Mechanical properties of different phases in cement paste . . . 94 vii
5.3 Numerical method for simulating the microstructural changes of cement paste 96
5.3.1 Voxel-based microstructure . . . 97
5.3.2 Lattice model . . . 97
5.3.3 Build up of initial lattice structure: from voxels to beam network . . 99
5.3.4 Equivalent nodal force due to thermal dilation and dehydration (εT + εd)100 5.3.5 Nonlinear failure analysis procedure . . . 102
5.3.6 Deformed voxel microstructure: from beam network to voxels . . . . 104
5.3.7 Methods for determining porosity and pore connectivity . . . 105
5.3.7.1 Porosity . . . 105
5.3.7.2 Pore connectivity . . . 105
5.3.8 Flow chart of simulation procedure . . . 106
5.4 Validation . . . 110
5.4.1 Virtual microstructure of cement paste simulated by HYMOSTRUC3D 110 5.4.1.1 HYMOSTRUC3D . . . 110 5.4.1.2 Extensions of HYMOSTRUC3D . . . 111 5.4.2 Simulation scheme . . . 112 5.4.3 Simulation Results . . . 113 5.4.3.1 3D virtual microstructure . . . 113 5.4.3.2 Porosity . . . 113 5.4.3.3 Pore connectivity . . . 117
5.4.3.4 The effect of the voxel size . . . 118
5.5 Conclusions . . . 118
Chapter 6 Numerical and experimental investigation of mechanical prop-erties of fire-damaged Portland cement paste · · · 121
6.1 General . . . 121
6.2 Virtual tensile test of cement paste . . . 123
6.2.1 Preprocess: Beam network representing the microstructure of fire-damaged cement paste . . . 123
6.2.2 Fracture process analysis . . . 123
6.2.3 Postprocess: Stress-strain curve and cracking pattern of cement paste at micro scale . . . 126
6.3 Experimental method at meso scale . . . 126
6.4 Results . . . 128
6.4.1 Simulation results at micro scale . . . 128
6.4.1.1 Load-displacement curves of cement paste specimen in a vir-tual axial tensile tests . . . 128
6.4.1.2 Cracking pattern . . . 132 viii
6.4.2 Experimental results of bending test at meso scale . . . 133
6.5 Discussion on temperature-induced deterioration of cement-based materials . 133 6.6 Conclusion . . . 135
Chapter 7 Conclusions · · · 137
7.1 Retrospection . . . 137
7.2 Conclusions . . . 137
7.3 Contributions to Science and Engineering . . . 139
7.4 Further research . . . 140
Bibliography · · · 141
1.1 Schematic representation of fire spalling of concrete at high temperature [84] 2 1.2 The role of the material properties in fire spalling . . . 3 1.3 Interaction between the three parts of the STW project on ”Explosive spalling
of concrete: towards a model for fire resistant design of concrete elements” . 4 1.4 Outline of this thesis . . . 6 2.1 Effect of temperature on the residual (after cooling) compressive strength and
elastic modules of unsealed C70 concrete heat cycled at 2 /min under 0% and 20% load [44] . . . 10 2.2 Creep of Portland cement/porphyry aggregate concrete at various
tempera-tures [49]. Psi in the figure refers to pounds per square inch (1450 psi = 10 MPa). The points are experimental results, and the straight lines are the trend lines. . . 11 2.3 (a) Dependence of permeability on temperature and humidity (h in the
Fig-ure). a is the permeability of concrete, aa is the permeability of concrete at
room temperature and humidity of 100% after [10]. (b) flow passage in cement gel, with the neck of pore structure after [10] . . . 12 2.4 The evolution of intrinsic permeability of cement paste (after cooling) with
temperature [26] . . . 13 2.5 Thermal dilation of hydration products and clinker phases [56] . . . 14 2.6 Thermal strain and shrinkage of cement paste and concrete [91] . . . 15 2.7 Microdiffusion of chemically bound water molecules from micropores to
cap-illary pores [84]. The black dots indicate the water molecules. . . 15 xi
jennite to tobermorite: (a) A small variation of the geometry of the dissolved reactants leads to a structure that is jennite-like (J) or tobermorite-like (T), typically without the bridging tetrahedra (dotted lines); (b) the simple layer of CSH; (c) approximate dimensions of the smallest unit; (d) transformation of the tobermorite-like to jennite-like units with average calcium/silicon ratio (c/s) = 1.7 as the bridging tetrahedra are placed by scavenging the structure for isolated tetrahedra. This schematic ignores OH− that is required at places where the silicates are absent, and in Portland cement, substitution of bridging tetrahedra by aluminum and sulfate. [40] . . . 18 2.9 DTA/TGA Data of high performance cement (HPC) [47]. Ordinary Portland
cement CEM I 52.5, water/cement ratio 0.33, curing age 28 days. . . 19 2.10 X-ray diffractograms of the reference specimen (initial cement paste; curing
age 70 days), and the heated specimens at various temperatures. Key to phases: C2S (•); Portlandite (N); Calcite (F); Brownmillerite (); Ettringite
(H); Ca1.5SiO3.5· H2O (); lime () [6] . . . 20
2.11 Phase diagram of calcium hydroxide and xonotlite at high temperature and high pressure . . . 24 2.12 View of the microstructure of a 100-day old cement paste with w/c 0.30, cured
at room temperature [23] . . . 26 2.14 BSE images of the cement paste samples after fire loading (950 ) and
sub-sequently cooling [88] . . . 28 2.15 The comparison of the porosity of HPC and SCC at high temperature
mea-sured by different methods [47]. Ordinary Portland cement CEM I 52.5, wa-ter/cement ratio 0.33, curing age 28 days. . . 29 2.16 Nanoindentation modulus and nanoindentation hardness of heated cement
paste (after cooling) [22] . . . 30 xii
β-CaSiO3[77]. Large solid and open circles represent calcium ions at heights 0
and 12 in the pseudo-cell respectively. Large circles with white centers represent interlayer calcium ions ; each one shown occurs only once in every 7.3 ˚A along b. Triangles represent SiO4 tetrahcdra, with small circles for silicon atoms.
Full and open small circles indicate that the tetrahedra occurs respectively twice or once in the height (7.3 ˚A) of the true cell. Full lines indicate pseudo-cell boundaries ; broken lines indicate boundaries of two monoclinic unit pseudo-cells of β-CaSiO3. . . 30
3.1 The schematic of dehydration-induced microstructural changes of heated ce-ment paste . . . 34 3.2 The experimental procedure for determining the phase transformation of
heat-ed cement paste . . . 35 3.3 The Bragg diffraction in crystal lattice . . . 37 3.4 X-ray diffractometer . . . 40 3.5 The measured XRD patterns and calculated patterns by Rietveld refinement
of heated Portland cement paste after cooling. The peak positions of each phase are shown in the bottom. . . 43 3.5 The measured XRD patterns and calculated patterns by Rietveld refinement
of heated Portland cement paste after cooling. The peak positions are shown in the bottom. (continued) . . . 44 3.6 The stabilized weight loss of Portland cement paste at temperatures from 105
to 1000 . . . 44 3.7 Evaluation of the phase fraction of heated Portland cement paste as function
of temperature determined by XRD/Rietveld and TGA . . . 45 3.8 The relationship between recrystallization degree and dehydration degree of
CSH . . . 48 3.9 Thermogravimetic analyzer (TG-449-F3-Jupiter) . . . 49 3.10 Separation of TG curve of cement paste into TG curves of CSH and CH . . . 50 3.11 Determination of kinetic parameters from experimental TG data . . . 53
3.14 Activation energy and lnA0 of dehydration of CSH and CH determined by TGA 56
3.15 The schematic of activation energy of the dehydration of CSH at elevated temperature . . . 57 4.1 Various devices for the microstructural investigation . . . 62 4.2 The typical SEM image and grey level histrogram . . . 63 4.3 The SEM images of heated and subsequently cooled cement paste. Material:
CEM I 42.5N, w/c=0.5, curing age 28 days . . . 66 4.4 The histogram of the images of heated and subsequently cooled cement paste.
Material: CEM I 42.5N, w/c=0.5, curing age 28 days . . . 67 4.5 The phase area fraction obtained by SEM image analysis. Pore size > 0.14
µm. Material: CEM I 42.5N, w/c=0.5, curing age 28 days. Note: The total volume fraction in this figure remains 100%. This doesn’t mean that the total volume of cement paste keeps the same. As shown in Fig. 2.5(a), the total volume of cement paste decreases with increasing temperature. . . 68 4.6 The pore size distribution of heated and subsequently cooled cement paste.
Material: CEM I 42.5N, w/c=0.5, curing age 28 days . . . 70 4.6 The pore size distribution of heated cement paste. Material: CEM I 42.5N,
w/c=0.5, curing age 28 days (Continued) . . . 71 4.7 The porosity of heated cement paste determined by MIP. Material: CEM I
42.5N, w/c=0.5, curing age 28 days . . . 71 4.8 The adsorption isotherms of heated cement paste after a heating/cooling cycle.
Material: CEM I 42.5N, w/c=0.5, curing age 28 days . . . 72 4.9 The specific surface area of Portland cement paste heated as a function of
temperature. Material: CEM I 42.5N, w/c=0.5, curing age 28 days. . . 73 4.10 The pore size distribution (from 3 nm to 37 nm) of heated cement paste (CEM
I 42.5N, w/c=0.5, curing age 28 days) determined by BJH method . . . 75 xiv
mined by nitrogen adsorption. Material: CEM I 42.5N, w/c=0.5, curing age
28 days. . . 75
4.12 The comparison of the porosity determined by various methods. Material: CEM I 42.5N, w/c=0.5, curing age 28 days . . . 77
4.13 Schematic representation of the changes of the gel structure of CSH at elevat-ed temperature baselevat-ed on colloid model [40, 41] (a)→(b): volume change due to dehydration; (b)→(c): volume change due to recrystallization; (c)→(d): volume change due to recrystallization and the transformation of small crys-talline particles to large cryscrys-talline particles. . . 80
5.1 The schematic representation of the dehydration induced microstructure change 86 5.2 The schematic representation of the volume change of (a) CH due to dehy-dration and (b) CSH due to dehydehy-dration and recrystallization . . . 89
5.3 Density of CSH phases versus water content [5, 41, 81]. . . 92
5.4 2D schematic of voxel-based circle . . . 97
5.5 The displacement of 3D beam in local coordinates . . . 98
5.6 The beam generation from voxel and its cross section properties . . . 100
5.7 Linear stress/strain law with initial thermal strain . . . 100
5.8 The updated voxel-based microstructure . . . 105
5.9 A 2D schematic of burning algorithm . . . 106
5.10 Condensed flow chart of the microstructural simulation of cement paste at high temperature . . . 107
5.10 (Continued) Condensed flow chart of the microstructural simulation of cement paste at high temperature . . . 108
5.11 Schematic view of HYMOSTRUC3D concept: Growth mechanism of individ-ual particles resulting in a 3D-microstructure [85] . . . 111
5.12 The heating program in the simulation of microstructure change due to dehy-dration . . . 113
28 days. All simulation results are visualized by open source software Paraview.114 5.14 Comparison of 2D images from simulation and SEM images of Portland
ce-ment paste. In the experice-ment, the cece-ment paste was heated up to different temperatures and cooled down to room temperature. Material: CEM I 42.5N, w/c=0.5, curing age 28 days. All simulation results are visualized by open source software Paraview. . . 115 5.15 The comparison of the simulated capillary porosity and the measured capillary
porosity. Cement paste: CEM I 42.5N, w/c=0.5, curing age 28 days. . . 116 5.16 The percentage of percolated pore volume of the simulated microstructure
of cement paste heated up to different temperature. Cement paste: CEM I 42.5N, w/c=0.5, curing age 28 days. . . 117 5.17 Splitting of microstructure with voxel size of 1 µm. Cement paste: CEM I
42.5N, w/c=0.5, curing age 28 days. All simulation results are visualized by open source software Paraview. . . 118 5.18 The effect of voxel size on the porosity (> 2 µm). Cement paste: CEM I
42.5N, w/c=0.5, curing age 28 days . . . 119 6.1 The schematic of the degradation of heated Portland cement paste at different
scales . . . 122 6.2 The loading approach in virtual axial tensile test . . . 124 6.3 The schematic flowchart of analysis procedure of a virtual tensile test . . . . 125 6.4 (a) The load-displacement curve; (b) The stress-strain curve; (b) The fracture
energy . . . 127 6.5 The schematic loading method of cement paste after heating/cooling cycles . 127 6.6 The heat treatment of the samples of Portland cement paste before 3 point
bending test. . . 128 6.7 The load displacement curves of cement paste obtained in virtual tensile test
on the specimen after heating/cooling cycles (105∼ 1000) . . . 129 xvi
on the specimen after heating/cooling cycles (105∼ 1000) (continued) . . 130 6.8 (a) The beam network before virtual tensile test; (b) Load-displacement curve
of cement paste at 105; (c) Crack pattern at stage I; (d) Crack pattern at stage II; (e) Crack pattern at stage III; (f) Crack pattern at stage IV; . . . . 131 6.9 The simulated elastic modulus and tensile strength as functions of
tempera-ture. The cement type is CEM I 42.5N, water/cement ratio is 0.5, curing time is 28 days. . . 132 6.10 The simulated fracture energy. Material: CEM I 42.5N, w/c=0.5, curing age
28 days. . . 132 6.11 The measured tensile strength determined by 3 point bending test . . . 134 6.12 The normalized tensile strength of heated cement paste (after cooling) at
different scales obtained by various methods. The heat treatments of the samples in various methods were the same. . . 135
Introduction
1.1
Background
1.1.1
Fire spalling of concrete
Fire spalling of concrete structures occurs when they are exposed to high temperatures. Spalling is defined as the violent or non-violent breaking off of layers or pieces of concrete from the surface of a structural element. Fire spalling may jeopardize the structural integrity and the safety of fire loaded structures. Fires in confined spaces, like tunnels, can lead to very high temperatures and have very severe effects on the concrete. Several fire accidents and spalling phenomena in tunnels has been reported since 1940 [17]. Concerns were raised regarding fire protection of tunnel linings.
The main reasons for fire spalling are thermal stresses and pore pressure [44] (see Fig. 1.1):
Thermal stress The thermal deformation of concrete at high temperature consists of ther-mal dilation and dehydration-induced shrinkage. The reason for therther-mal stress is re-strained thermal deformation [10].
Pore pressure When concrete is exposed to fire, the heat is conducted from the heated surface to the inner zone. When the temperature exceeds 100, the capillary water in pores is changed into water vapor. Gradually, the chemically bound water in hydration products is released with gradual increase of temperature. The increased amount of water vapor in the confined pore space leads to an increase of the pore pressure. This pore pressure might lead to high stress and fire spalling.
Figure 1.1: Schematic representation of fire spalling of concrete at high temperature [84]
1.1.2
Material deterioration of cement paste due to dehydration
As said, fire spalling of concrete is caused by pore pressure and thermal stress. The magnitude of the pore pressure and the thermal stresses are influenced by the material properties of the concrete (Fig. 1.2). These material properties include mechanical properties (strength and elastic modules), thermal properties (thermal conductivity and coefficient of thermal expansion) and transport properties (gas and moisture permeability). To understand the mechanism of fire spalling, the changing properties of heated cement paste need to be known. The change of the properties of cement paste is caused by chemical transformations, micro-cracking and microstructural changes (Fig. 1.2). The most important chemical trans-formations are the dehydration of hydration products, i.e. calcium silicate hydrate (CSH) and calcium hydroxide (CH). At sufficiently high temperatures CSH and CH start to de-hydrate, causing micro-cracking and irreversible changes in the microstructure. In the past
many researchers have executed experimental studies on temperature-induced changes of material properties [10, 44]. The residual strength of heated cement paste depends on many factors [44]. Yet, the deterioration mechanism of cement paste at elevated temperature, in which dehydration and microstructural changes are involved, is not clear. In this study, in order to study the deterioration mechanism of heated cement paste, the chemical transition, microstructural changes and micro-cracking are investigated (Fig. 1.2).
Fire spalling of concrete Thermal stress Pore pressure Caused by Caused by Material properties affected by affected by Material deterioration of cement paste Micro cracking Chemical transition Microstructure change Cement paste Area of this study
Figure 1.2: The role of the material properties in fire spalling
1.2
Research scope
This study is a part of the research project on ”Explosive spalling of concrete: towards a model for fire resistant design of concrete elements” (code No. 07045) (Fig. 1.3), funded by the Dutch National Science Foundation (STW). The main objective of this research project is studing and modeling of the mechanism that is responsible for (explosive) spalling of fire-loaded concrete elements. In this project, the fire-fire-loaded concrete is examined by a so-called multi-scale approach (i.e. micro, meso and macro). Three main issues are investigated:
(1) dehydration of cement paste and its consequences for the microstructural integrity and the material properties.
(2) water vapor formation and associated development of pressure gradients acting at a pore-scale level.
(3) up-scaling the results from the microscopic investigations (1 and 2) towards a model for evaluating the response of full-scale concrete elements to a fire load.
The interaction between the individual research topics is indicated in Fig. 1.3. This study fo-cuses on the dehydration of cement paste and dehydration-induced microstructural changes, viz Ph.D 1.
13
Figure 2: Interaction between the various parts of the project
Although it is generally accepted that the processes, discussed above, play a crucial role in concrete spalling, only little is known about the boiling and dehydration process in the inner microstructure of the material. This is the reason why the focus of this research project is on these two processes in particular, and to integrate these processes and their consequences on the macro-level. The resulting macro-scale model will form the fundamental basis for the calculation of the probability of spalling of full-scale concrete elements. For the sake of understanding, the complexity of the problem addressed is illustrated in figure 1. Particular the zones situated at the concrete’s fire loaded surface are of great interest with respect to the spalling process. In this region, boiling, vapour transport, dehydration and micro-cracking are strongly interacting.
In this research proposal, it is proposed to investigate three main issues: 1) dehydration and its consequences for the microstructure’s integrity, 2) boiling and associating development of pressure gradients acting on a pore-scale level, and finally, 3) up-scaling the results from the microscopic investigations (1 and 2) towards a model for evaluating full-scale concrete elements. The interaction between the individual research topics is provided in figure 2. Next, the various research topics will be discussed in more detail.
1) Microstructural changes caused by dehydration
When producing concrete, the main constituents such as cement, aggregates and water, are mixed together to form a product with a certain consistency. A few hours after mixing, i.e. the dormant stage, the cement starts to react with water and form a microstructure, which consists of a solid phase (anhydrous cement and hydration products), a wet phase (pores filled with free and physically adsorbed capillary water) and a gaseous phase (emptied pores filled with vapour). The microstructure, formed during the hydration process, determines the properties of the final product. When
Figure 1.3: Interaction between the three parts of the STW project on ”Explosive spalling of concrete: towards a model for fire resistant design of concrete elements”
1.3
Problem statement
The dehydration of hydration products (mainly CSH and CH) is the main reason for the mi-crostructural changes of heated cement paste. Although it is generally accepted that the de-hydration of cement paste plays a crucial role in concrete spalling, only little is known about the dehydration process and its consequences for the microstructural changes. Firstly, the dehydration kinetics of cement paste at high temperature is not exactly known [2, 47]. The quantitative information on the phase transformation is not clear. Secondly, the mechanism of microstructural changes due to dehydration remains inadequately understood. Thirdly, the influence of microstructural changes on the loss of strength of heated cement paste is still insufficiently studied.
1.4
Objectives of present research
In this thesis, the dehydration-induced change of the microstructure of heated cement paste is investigated experimentally and by numerical simulation. The microstructural changes and dehydration of cement paste will be used as input for the simulation of fire spalling of concrete at meso and macro scale (see Fig. 1.3). The main objectives of this study are:
- To propose a calculation procedure for simulating the microstructural changes of ce-ment paste due to dehydration.
- To study the correlation between the microstructural changes with the decrease of tensile strength of cement paste due to dehydration.
- To obtain input for the simulation at meso/macro scale, i.e. the amount of released chemically bound water and the strength of heated cement paste.
1.5
Research strategy and structure of this thesis
This research consists of an experimental and a numerical part.
Experimental study Experimental investigations on the phase transformation of heat-ed cement paste are carriheat-ed out. The dehydration kinetics of cement paste at high temperature are studied by using thermal analysis. The microstructural changes of heated cement paste are studied by using various experimental techniques: viz Scan-ning Electron Microscope (SEM), Mercury Intrusion Porosimetry (MIP) and nitrogen adsorption.
Numerical simulation Based on these experimental data, a computer-based model for simulating the microstructural changes due to dehydration is proposed. The influence of microstructural changes on the loss of strength is simulated by a virtual tensile test. The simulation results are compared with experimental results.
The structure of this thesis is organized as follows (Fig 1.4):
In Chapter 2 the available information about the nature of cementitious materials and microstructural changes at high temperature is summarized. The chemical transfor-mation and the microstructural changes of cement paste at high temperature, which were measured by different techniques, are briefly reviewed.
Chapter 2 Literature study Chapter 4 Microstructural investigation Chapter 3 Chemical study Chapter 5 Microstructural simulation Chapter 7 Conclusions Chapter 6 Virtual tensile test Numerical simulation Experimental study
In Chapter 3 the chemical transformation of heated cement paste is investigated. The phase transformation process is quantitatively analyzed by X-ray diffraction technique and Rietveld refinement 1. The dehydration kinetics is studied by thermogravimetric
analysis.
In Chapter 4 the microstructure of heated cement paste is investigated experimen-tally. SEM, MIP and nitrogen absorption are performed, respectively, to reveal the temperature-induced microstructural changes.
In Chapter 5 a micro-level physio-chemical model is proposed to characterize the mi-crostructure damage process. By comparing the simulated mimi-crostructure with exper-imental data, this simulation model is validated.
In Chapter 6 a virtual tensile test is performed on heated cement paste. The reasons for the loss of strength of heated cement paste are discussed.
In Chapter 7 this thesis is summarized and conclusions on dehydration-induced mi-crostructural changes are drawn. The contributions of this study to science and engi-neering are discussed.
1The Rietveld refinement is a quantitative analysis method of X-ray diffraction, which will be introduced
Literature study on cementitious
material at high temperature
2.1
Behavior of cementitious material at high temperature
In this section, the experimental investigations of properties of cementitious materials at high temperature are reviewed. Models proposed in past decades to describe temperature-induced material deterioration are discussed.
2.1.1
Mechanical properties: strength and elastic modulus
The strength of cementitious material is the maximum stress that the specimen can with-stand in compression and/or in tension. Many investigations have been carried out on the compressive strength of concrete at high temperature. Factors influencing the strength of heated concrete are [44]:
- The type of concrete
- The moisture boundary condition of samples during curing - The presence of external load during heating
- Whether the measurement is taken in the ”hot” state or after cooling - Rate of heating/cooling
- Time under thermal loading - Number of thermal cycles
Figure 2.1: Effect of temperature on the residual (after cooling) compressive strength and elastic modules of unsealed C70 concrete heat cycled at 2 /min under 0% and 20% load [44]
Different factors have different (positive or negative) effects on the strength. The measure-ments reflect the sum of all these effects. Generally, the strength of concrete decreases with increasing temperature. But the strength of unsealed and unstressed specimen slightly in-creases as long as the temperature is below 200 [10, 44]. This is because very little cement paste dehydrates at temperatures below 200 . Meanwhile, the free water in the pores of the concrete is released, and the concrete becomes denser and stronger.
The temperature dependency of the elastic modules of heated concrete shows a similar tendency as the compressive strength. When the specimen is heated without load, at tem-peratures beyond 200 , the elastic modules reduces more than the compressive strength [44] (Fig. 2.1).
2.1.2
Creep
High temperature also influences the creep of concrete. Below 100 , creep of concrete at moderate stress levels mainly originates from the cement paste. That is probably due to the breaking and restructuring of bonds in the cement gel [10]. Marechal at al [49] investigated the creep of concrete under static load 1450 psi (10 MPa) at various temperatures (Fig.
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Figure 2.2: Creep of Portland cement/porphyry aggregate concrete at various temperatures [49]. Psi in the figure refers to pounds per square inch (1450 psi = 10 MPa). The points are experimental results, and the straight lines are the trend lines.
2.2). Fig. 2.2 shows that creep of concrete increases with increasing temperature. From the experimental data on the creep of concrete at high temperature, Bazant [10] suggested that the creep rate ˙ε increases with increasing temperature up to 400 , and follows a modified Arrhenius equation: ˙ ε = ˙ε0 exp Q RT0 − Q RT (2.1) where T0 is the reference temperature, ˙ε0 is creep rate at T0, and Q is the activation energy
of creep [8, 9, 49]. The dependence of creep on temperature was verified for temperatures up to 400 [49].
2.1.3
Transport properties
The permeability of heated concrete/cement paste governs migration of gas or liquid. Powers and Brownyard [59] found that the main influencing factor of permeability is the connec-tivity of capillary pores. Bazant etc. [10] investigated the influence of the humidity and temperature on permeability (Fig. 2.3). He showed that in the temperature range from 100
(a) (b)
Figure 2.3: (a) Dependence of permeability on temperature and humidity (h in the Figure). a is the permeability of concrete, aais the permeability of concrete at room temperature and
humidity of 100% after [10].
(b) flow passage in cement gel, with the neck of pore structure after [10]
to 360 the permeability increases by about two orders of magnitude. As shown in Fig. 2.3(b), Bazant [10] suggested that heating leads to smoothing of initially very rough pore surfaces, which reduces the surface energy. This would cause an increase of the average width of the necks of pores, which leads to the increase in permeability.
Farage [26] investigated the variation of the permeability of ordinary Portland cement paste (OPC) with increasing temperature. An experimental program was carried out on neat cement paste made of French commercial CEM I 52.5 cement (OPC). Thirty cylindrical samples (40 mm in diameter and 80 mm high) were prepared with a water/cement ratio of 0.4. The samples were kept at 100% humidity until demouding at 24-48 hours. Then the samples were sealed to avoid drying and kept at 20 for 7 years. Before testing, the samples were dried by heating at a rate of 0.1/min until 60 . The intrinsic permeability of cement paste decreased from 1.5 × 10−16m2 to 1.0 × 10−16m2 at temperatures from 80 to 150 ,
and then increased from 1.0 × 10−16m2 to 1.8 × 10−16m2 at temperatures from 150 to 300 (Fig. 2.4). The possible reason for the decrease of the permeability of heated cement
0.8 1 1.2 1.4 1.6 1.8 2 0 100 200 300 400 Int ri n si c p e rm e ab ili ty (10 -16 m 2) Temperature (℃)
Figure 2.4: The evolution of intrinsic permeability of cement paste (after cooling) with temperature [26]
paste at temperatures from 80 to 150 is that the hydration of clinker in cement paste is accelerated, and that the pore connectivity is reduced.
2.1.4
Thermal properties
2.1.4.1 Thermal conductivity
Harmathy [36] studied the thermal conductivity of oven-dried hardened Portland cement paste at temperatures up to 1000 . The water/cement ratios of the samples are 0.3, 0.4 and 0.5, respectively. He concluded that the thermal conductivity did not vary very much at high temperature. The experimental data from Carman and Nelson [3], however, indicated a significant reduction of the thermal conductivity of hardened cement paste between 400 and 1000 . Harada [35] and Davis [20] similarly concluded that at higher temperature the thermal conductivity decreased.
2.1.4.2 Thermal dilation
The thermal deformation of cementitious materials can be measured by thermal dilatome-try. Dilatometric investigations of both unhydrated clinker phases and hydration products were performed by Piasta [56]. He found that chemical transition of hydration products
influences the thermal dilation at high temperature (Fig. 2.5). In the temperature range from 20 to 200 , hydrates of clinker minerals and hardened cement paste exhibit a low expansion. When the temperature exceeds 200 , the thermal deformation of different hydrates was different (see Fig. 2.5(a)). Temperature-induced chemical reactions go along with the shrinkage of the solid reaction products (Fig. 2.5(a)). The clinker phase itself, however, expands at high temperature with different coefficients of thermal expansion (Fig. 2.5(b)). Thermal expansion of β-C2S is not linear in the temperature range from 600 to
700 . The volume of β-C2S increases significantly at a temperature of about 650. This
is caused by the transformation of β-C2S into α-C2S (Fig. 2.5(b)).
0.8 0 -0.8 -1.6 -2.4 -3.2 -4.0 200 400 600 800 20 1 2 3 4 5 6 7 Temperature ºC ΔL %
(a) Thermal dilation 1:C3SH; 2:β − C2SH; 3:
C3AH.; 4: C4AFH.; 5: cement paste; 6:
Ca(OH)2; 7, ettringite (index H means
hydrat-ed) 3.2 2.4 1.6 0.8 0 -0.4 ΔL % Temperature ºC 20 200 400 600 800 1 2 3 4 5
(b) Thermal dilation of unhydrated clinker 1:C3S;
2:β − C2S; 3: C3A.; 4: C4AF .; 5: cement
Figure 2.5: Thermal dilation of hydration products and clinker phases [56]
Bazant etc. investigated thermal dilation and shrinkage of cement paste and concrete at high temperatures [91]. He suggested to calculate the thermal strain εT and shrinkage
strain εd, representing the volumetric strains caused by changes of temperature T and water
content w, by the equations: εT = Z T T0 αTdT, εd= Z w w0 ksdw (2.2)
where αT is the coefficient of thermal expansion and ks is the shrinkage coefficient due to
expansion αT is about 25 × 10−6/ . The total deformation, that includes thermal dilation
and shrinkage, is shown in Fig. 2.6. The theoretical thermal dilatation of paste and concrete without simultaneous dehydration shrinkage are shown as dashed lines. We see in Fig. 2.6 that the total strain of cement paste is positive (expansion) at temperatures below 250 , and is negative (shrinkage) at temperatures beyond 250 .
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Figure 2.6: Thermal strain and shrinkage of cement paste and concrete [91]
Chemical bond
Figure 2.7: Microdiffusion of chemically bound water molecules from micropores to capillary pores [84]. The black dots indicate the water molecules.
2.1.5
Material deterioration mechanism
To explain the various experimental results concerning the temperature dependency of me-chanical properties , Ulm et al. [84] proposed underlying mechanisms that govern the ma-terial behavior of concrete at high temperature. In his model, the dehydration process is attributed to microdiffusion of chemically bound water from micropores to capillary pores. The loss of bound water, together with the chemical decomposition and dissociation of the hydration products, weakens the chemical bonds and destroys the cohesive forces in the mi-cropores (the thick black line in Fig. 2.7) . However, this model can not explain the change in transport properties and thermal properties of cement paste.
2.1.6
Remarks
From the brief literature study discussed in Section 2.1, we learned that most of the properties of cementitious material vary with increasing temperature. Compressive strength increases with increasing temperature up to 200, and subsequently decreases continuously at higher temperatures (Fig. 2.1). The gas permeability decreases with increasing temperature up to about 150, and subsequently increases continuously at higher temperatures (Fig. 2.3).
In order to interpret the evolution of various properties of cement paste with temperature, many investigations on the chemical transition and microstructure of cement paste have been carried out. Yet, there is a lack of information on the material deterioration mechanism of heated cement paste. In the following sections literature on these two aspects will be discussed.
2.2
Chemical transition of cementitious material
As a multiphase material, cementitious material at high temperature undergoes a series of chemical transitions. The chemical reactions play an important role in the deterioration of cementitious material at high temperature. This section gives a literature survey of the chemical reactions.
2.2.1
Chemical composition of cement paste
The Portland cement paste is formed by the reaction of Portland cement clinker with water. The four major minerals in the clinker are tricalcium silicate (C3S), dicalcium silicate (C2S),
tricalcium aluminate (C3A) and tetracalcium aluminate ferrite (C4AF ). The main reactions
of the individual cement constituents with water and the molecular volume of each phase are presented [12] 1:
1Notation in cement chemistry: C = CaO, S = SiO
C3S 1 + 5.3H 1.35 → C1.71.521SH4+ 1.3CH0.61 (2.3) C2S 1 + 4.3H 1.49 → C1.72.077SH4 + 0.3CH 0.191 (2.4) C3A 1 + 3C ¯SH2 0.833 + 26H 5.253 → C6 ¯ S3H32 8.249 (2.5) 2C3A 1 + C6S¯3H32 4.124 + 4H 0.404→ 3C45.269A ¯SH12 (2.6) C4AF 1 + 3C ¯SH2 1.739 + 30H 4.22 → C6A ¯5.742S3H32+ CH0.259+ F H0.5453 (2.7) 2C4AF 1 + C6A ¯S3H32 2.871 + 12H 0.844 → 3C4A ¯5.742S3H12+ 2CH0.259 + 2F H0.5453 (2.8) C3A 1 + 6H 1.21 → C31.69AH6 (2.9) C4AF 1 + 10H 1.41 → C31.17AH6+ CH0.259+ F H0.5453 (2.10)
Among these hydration products, the CSH (C1.7SH4) and calcium hydroxide (CH) are the
ones that dominate the properties of cement paste. With Eq. 2.3∼2.10, the volume fraction of each phase in hydrated cement can be determined. The CSH gel is highly amorphous. The atomic structure of CSH is still not clear. Models of the atomic structure of CSH were reviewed by Richardson [64]. It is widely accepted that CSH can be considered as a mixture of imperfect tobermorite- and jennite-like structures [79], as shown in Fig. 2.8. In contrast, calcium hydroxide (CH) is generally crystalline. CH is usually found in the empty spaces far away from the unhydrated clinker [85]. The atomic structure of CH in cement paste is almost the same as that of natural portlandite [72].
2.2.2
Phase transformation at high temperature
The hydration products contain chemically bound water. When the hydration products are heated up to temperatures beyond 105 , the chemically bound water will be released. There are many phase transformations that occur in heated cementitious material, and they are not fully reversible. Thermal techniques can be used to study the phase transforma-tions, such as thermogravimetric analysis (TGA) [47] and differential scanning calorimetry (DSC) [6] (see Fig. 2.9). By analyzing the TG and DTG 2 curves, the reactions that take
place in cement paste at high temperature can be identified. More accurate information on the crystalline phases in cement paste can be obtained by using diffraction techniques.
ba sa l s pa ci ng --- ---
-a)
b)
c)
d)
Figure 2.8: Highly schematic sequence of CSH. The circles are calcium ions, and the triangles are silicate tetrahedra. This representation shows the relationship of jennite to tobermorite: (a) A small variation of the geometry of the dissolved reactants leads to a structure that is jennite-like (J) or tobermorite-like (T), typically without the bridging tetrahedra (dotted lines); (b) the simple layer of CSH; (c) approximate dimensions of the smallest unit; (d) transformation of the tobermorite-like to jennite-like units with average calcium/silicon ratio (c/s) = 1.7 as the bridging tetrahedra are placed by scavenging the structure for isolated tetrahedra. This schematic ignores OH− that is required at places where the silicates are absent, and in Portland cement, substitution of bridging tetrahedra by aluminum and sulfate. [40]
-8 -7 -6 -5 -4 -3 -2 -1 0 0 200 400 600 800 1000 1200 Tem pe rature [oC] T e m p er at u re D if fer en ce [ oC/ m g ] -40 -30 -20 -10 0 W e ig h t lo s s [ % ] CH loss
Figure 6.4 DTA/TGA Data of HPC PPF0
The corresponding quantities are calculated and shown in Table 6.1.
Table 6.1 Weight loss of different phases Loss of capillary water
(%) Loss of chemically bound water (%) Decomposition of Ca(OH)2 (%) Decomposition of CaCO3 (%) SCC01 PPF0 1.52 9.32 1.92 11.29 HPC PPF0 1.82 11.86 2.16 0.29
As the water to powder ratio of HPC is higher than SCC01, the water loss of HPC is thus higher than SCC01. The weight loss caused by decomposition of unhydrated cement is very low, and can be disregarded. As mentioned in Chapter 3, the weight loss at the temperature range of 730 to 770 ºC is caused by decomposition of limestone.
6.2.3 Loss of Evaporable Water
Normally, water in hardened cement paste could be described as gel water, capillary water and chemically combined water in the gel particles and in calcium hydroxide [11, 61].
Due to the small size of the gel pores, gel water can also be considered as “adsorbed” or physically bound water, movement of the water molecules near the surfaces of the gel particles being restricted by van der Waal’s forces. This adsorbed gel water can be driven out and is then considered as evaporable water. The capillary water is contained in capillary pores
Figure 2.9: DTA/TGA Data of high performance cement (HPC) [47]. Ordinary Portland cement CEM I 52.5, water/cement ratio 0.33, curing age 28 days.
The heated cement paste has been studied by X-ray diffraction (XRD) by Peng [54] and Stepkowska [74]. The crystal phases in samples were identified by the position of peaks in XRD patterns. The XRD patterns indicate that CSH starts to be transformed into β-C2S
at about 600 . The dehydration of cement paste can also be measured by synchrotron X-ray or neutron diffraction. In-situ synchrotron radiation [68, 73] has been used to study dehydration/recrystallization of various natural crystalline CSH: xonotlite, tobermorite and hillebrandite. Castellote et al. [18] used in-situ neutron diffraction to monitor the change of chemical composition of cement paste when heated until 620 and cooled afterwards. From the results of in-situ synchrotron radiation, it was observed that CSH at high temperature dehydrates and is transformed into C2S and lime. Alonso etc. [6] evaluated the evolution of
cement paste at elevated temperatures and subsequent rehydration by XRD (Fig. 2.10) and 29Si MAS-NMR. He found that the CSH is transformed into C2S at 750 . Based on the
above experimental results, the following phase transformations will be discussed:
Dehydration of calcium silicate hydrate Dehydration of calcium hydroxide Decarbonation
Figure 2.10: X-ray diffractograms of the reference specimen (initial cement paste; curing age 70 days), and the heated specimens at various temperatures. Key to phases: C2S (•);
Portlandite (N); Calcite (F); Brownmillerite (); Ettringite (H); Ca1.5SiO3.5 · H2O ();
2.2.2.1 Dehydration of calcium silicate hydrate
Calcium silicate hydrate (CSH) is the main phase that dominates the properties of Portland cement paste. The chemical reaction is described as follows:
(CaO)aSiO2(H2O)b (CaO)aSiO2(H2O)amorphousb−c + c · H2Ogas (2.11)
The dehydration of CSH and the evaporation of water are the main reasons for the weight loss of cement paste in the temperature range from 105 to 1000 . During the dehydration of CSH, water vapor is released causing the raise of vapor pressure in the micro pores. The dehydration of CSH is a typical multi-step reaction, with an activation energy 3 that varies
with the extent of conversion [44]. Taylor [76] pointed out that the dehydration process of CSH is effected by both temperature and relative humidity.
Due to its amorphous morphology, the atomic structure of CSH in heated cement paste is difficult to identify by XRD. The dehydration of natural tobermorite, which has similar atomic structure as that of CSH in cement paste, has been studied by Taylor [77]. During heating, the basal spacing 4 between tobermorite layers decreases from 14˚A to 11˚A at 125
, and to 9˚A at 300 (see Fig. 2.8). Similar with CSH, tobermorite dehydrates into a mixture of wollastonite (CS) and belite (β-C2S) when the temperature exceeds 700.
2.2.2.2 Dehydration of calcium hydroxide
The dehydration of calcium hydroxide (CH) takes place at around 420 , causing a rapid weight loss of cement paste. The chemical reaction is:
Ca(OH)2 CaO + H2Ogas (2.12)
This rapid weight loss can be measured from TG curves, such as the one presented in Fig. 2.9. The dehydration of portlandite (CH) produces CaO and water vapor.
2.2.2.3 Decarbonation
During the curing of cement paste, the carbonation of cement paste takes place and produces calcite (CaCO3). The decarbonation of calcite (CaCO3) in cement paste occurs at
temper-atures above 650 . The calcite decomposes into lime (CaO) and carbon dioxide (CO2). In
3Activation energy is the minimum energy required to start a chemical reaction
formula form:
CaCO3 CaO + CO2gas (2.13)
The calcite in cement paste is produced by the carbonation of hardened cement paste in air. Because carbonation of cement paste can continue for a long time, aged cement paste contains more calcite than fresh cement paste. As shown in Fig. 2.10, the reference specimen (specimen without heating) of cement paste at the age of 70 days contains very little calcite.
2.2.2.4 Recrystallization
The amorphous CSH starts to recrystallize into β-C2S or CS (wollastonite) when the
tem-perature exceeds 700 . Recrystallization of cement paste is an exothermic reaction . In Fig. 2.10 it is observed that the C2S (•) peaks become higher with increasing
tempera-tures from 450 to 750 . The recrystallization of CSH causes shrinkage and changes the microstructure of cement paste.
2.2.3
Kinetics of dehydration
The kinetics of dehydration, i.e. the reaction rate, can be used to predict the dehydration process under any arbitrary temperature curve. The analysis of the kinetics of dehydration of cement paste can be studied by using thermal analysis techniques, such as Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC) [6, 10].
The dehydration kinetics of hydration products (CSH and CH) follow the Arrhenius equation [30, 90]:
K(T ) = A0exp (−Ea/RT ) (2.14)
where A0 is the pre-exponential factor and Ea is the activation energy. For the dehydration
of CH, the activation energy Ea is 145 [kJ/mol], the pre-exponential factor A0 is 1.7×108
[s−1] [29]. The exact values of Ea and A0 of the dehydration of CSH are not known.
2.2.4
Influence of pressure on reactions at high temperature
The dehydration process of cement paste is influenced by both temperature and pressure. Piasta investigated the influence of temperature and high pressure on the decomposition of
calcium hydroxide and natural CSH xonotlite [58]. Fig. 2.11 is the phase diagram of CH and CSH xonotlite at high temperature and high pressure. The solid line indicates the phase boundary. Fig. 2.11 shows that both temperature and high pressure can strongly influence the phase transformation. As shown in Fig. 2.11, when the pressure is lower than 10 MPa (10 MPa = 0.1 kilobar), the effect of pressure on the dehydration of CSH and CH is very small. In the experiments reported in literatures, the reported maximum pore pressure in cementitious material exposed to fire is about 4 MPa [45, 55]. This value is much lower than 10 MPa (10 MPa = 0.1 kilobar). Therefore, the influence of pressure on the dehydration of heated cement paste is negligible.
2.2.5
Evaluation
The foregoing literature study on chemical reactions of heated cement paste helps to under-stand the spalling of concrete at elevated temperature. During the material deterioration of cement paste at high temperature, two issues about chemical reactions are still insufficiently understood:
Phase transformation The phase transformation plays a critical role in the changes of the microstructure. The thermodynamic equilibrium state of heated cement paste changes with increasing temperature. At high temperature, CSH and CH are transformed into β-C2S and lime, respectively. However, the aforementioned investigations are
mostly limited to a qualitative description of the phase transformation as a function of temperature. There is a lack of knowledge of the quantitative analysis of the changes of the chemical composition of heated cement paste.
Kinetics The Arrhenius equation is widely accepted to determine the effect of temperature on the dehydration kinetics. The parameters Ea and A0 of dehydration of CH have
been determined already, but the parameters Ea and A0 for dehydration of CSH are
not clear yet.
2.3
Materials structure of cementitious material
The properties of cement paste are controlled by the microstructure of the material. In particular the pore structure plays a critical role in the transport properties and mechanical
---0.1 kilobar
Ca(OH)2
CaO
(a) Hydration and dehydration boundary of
cal-cium hydroxide. If Ca(OH)2is in the state blow
the phase boundary (solid line), the
transforma-tion of Ca(OH)2 into CaO takes place (see the
dots). If CaO is in the state above the phase
boundary (solid line), the transformation of CaO
into Ca(OH)2takes place (see the crosses).
---0.---1 --ki--l-obar---
-(b) Phase relations in the system CaSiO3-H2O in
the temperature range from 350 to 500 to 54
kilobars
Figure 2.11: Phase diagram of calcium hydroxide and xonotlite at high temperature and high pressure
properties (strength and elastic modulus). Portland cement paste is a porous material with a multi-level microstructure. At micro scale (0.1-100 µm), cement paste is a multi-phase composite consisting of pores, unhydrated clinker and hydration products (including CSH and CH) [85]. At nano scale (2-100 nm), the CSH is present as a gel-like phase that consists of many gel particles [40, 41]. At atomic scale (<2 nm), the structure of CSH can be considered as the atomic structure of imperfect tobermorite or jennite [13, 19].
2.3.1
Multiscale structure at room temperature
2.3.1.1 Microstructure of cement paste (0.1-100 µm)
At micro scale the porous CSH matrix, together with the unhydrated cement (i.e. the four clinker phases C3S, C2S, C3A, C4AF ), large Portlandite crystals (CH) and the capillary
pores form the cement paste [83]. Unhydrated remnants of Portland cement clinker are present in all or nearly all cement pastes [23]. Since the non-hydrated components in cement have much higher electron backscatter coefficients than the hydrating products, these residual unhydrated cement grains appear in backscatter scanning election microscope (SEM) images as bright entities (See Fig. 2.12). Most of these cement remnants are surrounded by, and are in close contact with grey hydration product shells of varying thickness. The main component of the hydration shell is CSH. It is generally accepted that the CSH exists in at least two different forms, a low density (LD) and a high density (HD).
2.3.1.2 Nanostructure of CSH gel (2-100 nm)
The nanostructure of CSH can not be observed directly so far. Most nanostructure models proposed for CSH are based on indirect information, such as specific surface area [40, 41]. Garci Juenger [32, 33] observed the pore size distribution and surface area of cement paste by nitrogen adsorption. Based on nitrogen adsorption data, Jennings proposed a colloid model [40, 41] to characterize the nano-scale structure of CSH. In this colloid model the CSH gel is suggested to consist of many ”globules” that are tightly packed (Fig. 2.13). Each globule consists of the layered atomic structure of CSH and water molecules, that is present in Fig. 2.13.
Residualunhydrated
cementgrain
Fully-hydrated
cementgrain
Hydration shell
Figure 2.12: View of the microstructure of a 100-day old cement paste with w/c 0.30, cured at room temperature [23]
2.3.1.3 Atomic structure of CSH (<2 nm)
CSH gel is a poorly crystalline material [79, 80]. It is widely accepted that CSH has a disordered layer structure at the atomic scale. Most of the layers are structurally imperfect jennite (Ca9Si6O32H22) and 1.4 nm tobermorite (Ca5Si6O26H18) [40] (see Fig. 2.8). Besides
jennite and 1.4 nm tobermorite, a number of models for the atomic structure of CSH is reviewed by Richardson [64]. These models are summarized and compared, and it was shown that many of them are in fact very similar to one another.
2.3.2
Materials structure after heating cycle
2.3.2.1 Microstructure of cement paste after heating cycle (0.1-100 µm)
At micro scale, the evolution of the microstructure of cement paste at high temperature is easier to measure than that at the nano scale and atomic scale. One of the most popular instruments for microstructure investigation is the Scanning Electron Microscope (SEM). Ye and Liu [47, 88] investigated the cement paste heated up to 950 by SEM technique. Because of the limitation of the image resolution, SEM can only measure capillary pores larger than the size of the pixel in the SEM image. Micro-cracks were observed in heated cement paste (see Fig. 2.14). They can be caused by different thermal deformation of clinker
Figure 2.13: A schematic representation of a globule representing subsequent stages [41]. I Single sheet of C-S-H with all evaporable water removed (left), and a saturated globule
without water adsorbed on the surface (right).
II A) Fully saturated globule with monolayer of water molecules on the surface and both interlayer.
B) Partially dried globules with much of the interlayer water and the adsorbed mono-layer removed. Volume of globule plus monomono-layer is reduced and density is in-creased.
C) All evaporable water removed. The IGP are empty leaving internal void and reducing density.
Figure 2.14: BSE images of the cement paste samples after fire loading (950) and subse-quently cooling [88]
and hydration products. Mercury intrusion porosimetry (MIP) is another useful technique to measure pore size distribution of cement paste. The pore diameters that can be measured by MIP vary from 0.001 µm to 1000 µm according to the pressure used. Ye et al. [88, 47] used MIP to determine the pore structure of heated cement paste. The porosity of cement paste was measured by SEM image analysis and MIP, respectively (Fig. 2.15). It was found that the porosity of cement paste increases with increasing temperature.
2.3.2.2 Nanostructure of CSH gel after heating cycle (2-100 nm)
The change in the nanostructure of CSH at evaluated temperature has been discussed by Matthew [22] based on the indirectly information (nanoindentation data). The cement paste was heated up to different temperatures from 105 to 700 , and cooled down to room temperature (20 ). The cooled specimens were tested by nanoindentation. The maximum indentation depth is 300 nm.
As shown in Fig. 2.16(a), the elastic modulus of both LD CSH and HD CSH decrease with increasing temperature. Fig. 2.16(b) shows that the hardness of CSH increases at temperatures up to 200 , and decrease at temperatures beyond 200 . The change of these mechanical properties (Fig. 2.16) of CSH can be explained by the degradation of the nanostructure. The possible reason for this degradation is the decrease of the packing density of CSH matrix at nano scale [22]. The packing densities of LD CSH and HD CSH are functions of temperature. With increasing temperature, the packing density gradually decreases, which leads to the decrease in hardness and elastic modulus of CSH. If we accept
(a) The porosity of HPC and SCC at high tem-perature obtained by SEM image analysis
(b) The porosity of HPC and SCC at high temperature obtained by MIP
Figure 2.15: The comparison of the porosity of HPC and SCC at high temperature measured by different methods [47]. Ordinary Portland cement CEM I 52.5, water/cement ratio 0.33, curing age 28 days.
Jennings’s model of a globular CSH structure [40, 41], the following possible consequences for the nanostructure of heated cement paste [21] are:
- A change in volume of the globules but no change in the nanoporosity. - No change in volume of the globules but a change in the nanoporosity. - A change in both the volume of the globules and the nanoporosity.
The volume changes of CSH and the breaking of chemical bonds at high temperature is the main reason for the microstructural changes of heated cement paste.
2.3.2.3 Atomic structure of CSH after heating cycle (<2 nm)
The dehydration process causes changes of the atomic structure of amorphous CSH in cement paste. What is actually occurring in amorphous CSH can not be ’seen’ directly by experi-mental techniques. The atomic structure of crystalline tobermorite at high temperature can be determined using X-ray diffraction. Since it is widely accepted that the atomic struc-ture of amorphous CSH is tobermorite-like, Taylor [77] investigated the change of atomic structure of tobermorite by X-ray oscillation photographs. During the dehydration of tober-morite, one probable atomic structure transformation from 9.35 ˚A CSH to β-CaSiO3 was
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Figure 2.16: Nanoindentation modulus and nanoindentation hardness of heated cement paste (after cooling) [22]
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Figure 2.17: Comparison of the probable structure of the 9.35 ˚A tobermorite with that of β-CaSiO3 [77]. Large solid and open circles represent calcium ions at heights 0 and 12 in
the pseudo-cell respectively. Large circles with white centers represent interlayer calcium ions ; each one shown occurs only once in every 7.3 ˚A along b. Triangles represent SiO4
tetrahcdra, with small circles for silicon atoms. Full and open small circles indicate that the tetrahedra occurs respectively twice or once in the height (7.3 ˚A) of the true cell. Full lines indicate pseudo-cell boundaries ; broken lines indicate boundaries of two monoclinic unit cells of β-CaSiO3.
characterized and shown in Fig. 2.17 [77]. This transformation destroys the interlayer Si-O-Si links (triangles in Fig. 2.17). Besides, the other atomic structure of CSH (tobermorite) after heating cycle is β-Ca2SiO4 [77].
2.3.3
Discussion
The presented experimental results on microstructural changes can explain the changes of the macro properties, such as strength and permeability. The micro cracks (Fig. 2.14) could be one of the reasons why the mechanical properties decrease at high temperature. The cracks connect the isolated capillary pores, which might have an effect on the permeability.
2.4
Concluding remarks
2.4.1
Summary
The degradation of cementitious material at high temperature is a complex phenomenon. The change of material properties at high temperature is caused by chemical transitions and microstructural changes. In this chapter three main aspects of cementitious material at high temperature were reviewed, viz: chemical reactions, microstructural changes and the resulting changes in material properties. Based on the foregoing literature review the following conclusions can be drawn:
Both the mechanical properties and the gas permeability of cementitous material change with increasing temperature. Compressive strength increases with increasing temperature up to 200 , and subsequently decreases continuously at higher tem-peratures. The gas permeability decreases with increasing temperature up to about 150 , and subsequently increases continuously at higher temperatures. The thermal conductivity and the coefficient of thermal expansion don’t vary much at high temper-ature. The change of these properties is caused by complex chemical transitions and microstructural changes.
The cement paste at high temperature undergoes a series of chemical reactions. These chemical reactions include the dehydration of CSH, the dehydration of CH and the recrystallization of CSH into β-C2S. It has been reported that the dehydration of
CSH starts at about 105, while the dehydration of CH occurs at around 420 . The amorphous CSH can be transformed into β-wollastonite or β-dicalcium silicate (β-C2S)
[76, 77]. The dehydration of CSH is the most important reaction and dominates the weight loss and the changes of mechanical properties. Still there is a lack of quantitative information on the chemical composition of heated cement paste.
The dehydration kinetics of CSH and CH is assumed to follow the Arrhenius equation. The Arrhenius parameters (Ea and A0) of dehydration of CSH are not exactly known
yet. The influence of pressure on the reactions can be neglected (see Fig. 2.11). The structure of cement paste changes with increasing temperature. The increased
temperature on cement paste causes the increase of pore volume and micro-cracking. The local mechanical properties (hardness and elastic modulus) of LD CSH and HD CSH vary with increasing temperature. This indicates that the nanostructure of CSH changes at high temperature. The changes of the nanostructure and atomic structure of CSH at high temperature have hardly been studied yet.
Most studies of the effect of temperature on the material properties are qualitative. There is a knowledge gap regarding the complex interaction between chemical transi-tions and microstructural changes. This hampers the development of numerical method for the prediction of the change of meso properties at high temperature.
2.4.2
Questions to be studied
This thesis focuses on modeling of the dehydration-induced microstructural changes of heat-ed cement paste. The dehydration and volume changes of hydration products must be quantified in view of microstructural modeling. The investigation of the chemical transitions and microstructural changes of heated cement paste will be performed in Chapter 3 and Chapter 4. The degradation mechanisms of heated cement paste will be discussed based on the experimental data in this study and literature. In Chapter 5 a comprehensive model for the dehydration-induced microstructural changes of cement paste will be proposed. In that model the chemical transitions and the microstructure evolution will be dealt with. The model will be validated by comparing the simulation results with experimental data. In Chapter 6 the mechanical properties of heated cement paste will be simulated by using a virtual tensile test.
Chemical transitions in heated
Portland cement paste
3.1
Introduction
The degradation and microstructural changes of cement paste at high temperature are mainly caused by chemical reactions. At sufficiently high temperature, CSH and CH in cement paste dehydrate. At temperatures beyond about 700 , the amorphous CSH is recrystallized into β-C2S [6]. The dehydration and recrystallization of CSH and CH cause their shrinkage [56].
Meanwhile, the unhydrated clinker particles, which are surrounded by CSH and CH, expand due to thermal dilation. This strain mismatch between shrinking hydration products and expanding clinker leads to micro-cracking and microstructural changes, as indicated in Fig. 3.1 and can also be observed in Fig. 4.3 in Chapter 4.
Since the modeling of microstructural changes of heated cement paste is the purpose of this study, the volume changes of each phase must be determined. For determining the volume change of hydration products due to chemical reactions, the dehydration and recrystallization of hydration products should be quantified.
From the literature study in Chapter 2 we learned that the dehydration of CSH and CH follows the Arrhenius equation [30, 90]:
K(T ) = A0exp (−Ea/RT ) (3.1)
where A0is the pre-exponential factor and Eais the activation energy. The activation energy
Ea and the pre-exponential factor A0 for dehydration of CH have been studied extensively
