Altered cement hydration and subsequently modified porosity, permeability and compressive strength of mortar specimens due to the influence of electrical current

Altered cement hydration and subsequently modified porosity,

permeability and compressive strength of mortar specimens due to

the influence of electrical current

Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department Materials & Environment, Stevinweg 1, 2628 CN Delft, The Netherlands

Summary

This paper reports on the influence of stray current flow on microstructural properties, i.e. pore connectivity and permeability of mortar specimens, and link these to the ob-served alterations in mechanical properties and cement hydration. Mortar specimens were partly submerged in water and calcium hydroxide (Ca(OH)2) as an external en-vironment. The electrical current density applied was 10mA/m2 as commonly used in cathodic protection (CP) applications. The results show that the stray current flowing through mortar specimens partly submerged in water tends to accelerate calcium ions leaching and results in increased porosity, permeability and decreased com-pressive strength. In contrast, a positive effect of the stray current flow was observed for mortar specimens, partly submerged in Ca(OH)2 solution, where decreased po-rosity, permeability and increase in compressive strength were recorded. These ob-servations are further discussed in terms of most plausible phenomena that link be-tween microstructural properties to mechanical and transport properties and related mechanisms.

1 Introduction

In the field of steel corrosion, science of cement-based materials and performance of reinforced concrete structures in general, the effect of electrical current on materials’ properties (both steel and concrete) is commonly considered only when steel corro-sion is of concern, e.g. stray current effects on steel corrocorro-sion initiation in close-by or remote structures that appear to be “in the way” of low resistive underground path-ways. Stray current is a current leakage e.g. from rail transit systems. Except stray currents, electrical current due to the application of impressed current Cathodic Pro-tection (CP) also flows through reinforced concrete or steel structures. Whereas stray current can be non-stable and of significantly larger scales, the current (DC only) within CP applications is generally in the range of mA/m2 steel (concrete) surface. Similarly to stray current effects, the DC current with CP applications is only moni-tored in terms of sufficiency in order to assure proper polarization (protection) from steel corrosion. However, all types of electrical current flowing through a reinforced (or not) civil structure, would pass through the concrete bulk matrix as well. The re-lated effects (positive or negative) are normally not subject to investigation. Consider-ing that a concrete bulk matrix is a porous medium with relevant humidity and ionic strength of the pore solution, logically any type of electrical current flow will exert al-terations in ion/water migration and will consequently affect the bulk matrix proper-ties.

This paper presents the effect of electrical current flow at the level of 10 mA/m2 as commonly used for CP applications on cement-based matrix properties. Mortar cubes were cast and after 24h immersed in H2O and Ca(OH)2 environment. Electrical current was applied (via metallic conductors in the medium) as a simulation of stray current. The effect of current flow was investigated in conditions where leaching was possible i.e. in H2O environment [1-5] and compared to such where leaching was almost completely avoided (Ca(OH)2).

The leaching process in cement-based materials results from concentration gradients between the pore solution of the cement-based material and the external environment, surrounding the structure. Although a relatively slow process in natural environment, calcium leaching would increase porosity, permeability and diffusivity of the concrete bulk matrix, resulting in higher risks for aggressive agents’ penetration and consequently reduced service life of concrete structures [5-11]. There are sever-al methods that can potentisever-ally accelerate csever-alcium ions leaching such as: tempera-ture, electrical field, deionised water external medium or replacement of the tap (cast-ing) water, different chemical admixtures (e.g. ammonium nitrate) [5, 12-14].

This paper reports on the microstructural properties (pore connectivity and per-meability) of mortar specimens in condition of stray current flow. The observations are linked to the recorded changes in mechanical properties and cement hydration. The results support the hypothesis for the effect of electrical current flow on micro-structural properties of cement-based materials and justify the need for further inves-tigation and/or specification of threshold levels for electrical (incl. stray) current densi-ties, expected to bring about negative alteration in (reinforced) concrete structures.

2 Experimental materials and methods 2.1 Materials

Mortar cubes of 40 mm×40 mm×40 mm (Fig.1) were cast, using OPC CEM I 42.5N with water-to-cement ratio of 0.5 and cement-to-sand ratio of 1:3. The chemical com-position (in wt. %) of CEM I42.5N (ENCI, NL) is as follows: 63.9% CaO; 20.6% SiO2; 5.01% Al2O3; 3.25% Fe2O3; 2.68% SO3; 0.65% K2O; 0.3% Na2O. After casting and prior to conditioning, the specimens were cured in a fog-room of 98% RH, 20°C for 24 hours; after de-moulding they were positioned in containers with the relevant envi-ronment (H2O or Ca(OH)2), Fig.1.

2.2 Sample designation

Two groups of specimens were investigated: 1) Control group - no DC current in-volved and 2) group “10 mA/m2”, where DC current flow in the medium and hence through the specimens was relevant at the respective current level (the set-up for DC current applications is as depicted in Fig.1). Both groups were partly submerged in water or (Ca(OH)2) solution as external environment.

2.3 Current regime

Figure 1 shows a schematic presentation of the experimental set up, including the layout for DC current application at the level of 10 mA/m2. The experimental set up is as previously used and reported [15].

Figure 1: (a) Experimental set-up [15]; (b) electrical circuit, where R is the container, where the mortar specimens are partly (1/2 of height) immersed in water/Ca(OH) solution, R =120 kΩ and R =20 kΩ

The current density was adjusted by additional resistors (R1=120 kΩ and R2= 20 kΩ) in the electrical circuit, Fig.1b). The electrical current was applied from 24h of hydra-tion age (immediately after de-moulding of the specimens) and until 112 days of age.

2.4 Methods

2.4.1 Degree of hydration

Non-evaporable water content of cement paste is commonly used to determine the degree of cement hydration. The degree of hydration can be expressed as [16]:

max             = c W c W_{n n} α (1) where: �𝑊𝑛

𝑐 � - non-evaporable water content per gram of original cement in the mixture,

�𝑊𝑛

𝑐 �_𝑚𝑎𝑥 - � 𝑊𝑛

𝑐 � for complete hydration.

The non-evaporable water content per gram of original cement in the mixture Wn/c is calculated using the following equation [1]:

1 ) 1 ( 2 1 − − = L W W c W_n (2) where: W1 is the mass of paste prior to ignition (g),

W2 is the mass of paste after ignition (g), and

L is the loss of ignition for the sample of the original dry cement powder (g/g of original cement).

In this study, the loss of ignition was measured according to NEN 5931 standard [17]. 2.4.2 Standard compressive strength

Standard compressive strength tests were performed on at least three replicates of 40×40×40 mm mortar cubes at the hydration ages of 14, 28, 84 and 112 days. The mortar specimens were taken out from the conditioning set-up, cloth-dried and tested within a 30 min time interval.

2.4.3 Mercury intrusion porosimetry (MIP)

The sample preparation for MIP tests followed generally accepted procedures [18, 19]. The MIP tests were carried out by using Micrometritics Poresizer 9320 (with a maximum pressure of 207 MPa) to determine the porosity and the pore size distribu-tion of the specimens.

2.4.4 Permeability

Permeability was calculated based on MIP data (i.e. pore size distribution and pore structure were derived) using the equation 2

c

k′ =d η ϕ, where dc is the length scale that characterizes the pore diameter, dominating fluid transport, η is dimensionless parameter that takes into account tortuosity and pore connectivity and ϕ is the poros-ity. All above parameters (dc,η andϕ ) can be associated with the threshold pore di-ameter (dc), the total porosity (η ), and the effective porosity (ϕ ) derived from MIP measurements [20-21].

3 Results and discussion 3.1 Degree of hydration

Estimation of the degree of cement hydration is important, since the related mecha-nisms are responsible for the microstructural and mechanical development of the bulk matrix. Additionally, chemical shrinkage and autogenous deformation, which in-dicate performance of cement-based materials, can also be linked to the cement hy-dration process. There are several experimental techniques used to estimate the de-gree of cement hydration, for instance: measuring the heat of hydration, the non-evaporable water content, the amount of calcium hydroxide produced in the hydration reactions [22]. In this paper, the degree of cement hydration was calculated based on the measured non-evaporable water content. Generally, the degree of hydration in-creases with cement hydration and time.

Figure 2: Degree of hydration for mortar specimens in both control and “10mA/m2” groups, partly im-mersed in H2O and Ca(OH)2 as external environment

Figure 2 shows the degree of hydration for mortar specimens, partly immersed in wa-ter and Ca(OH)2 solution in control and under stray current conditions. It was ex-pected that the stray current flow in groups “10 mA/m2” will result in altered cement hydration rate due to enhanced ion and water migration in these cases, compared to control (no current involved) conditions.

As can be seen from Fig.2, for the mortar specimens partly immersed in water, the degree of hydration for the control group is slightly higher until 28 - 84 days and more or less equal to the “10 mA/m2” group towards the end of the test - at 112 days. For the cubes in water environment, calcium leaching is relevant for both control and “under current” conditions with an additional effect of the current flow in the latter case. The calcium content influences the degree of cement hydration; next to that, following equation (2), the higher the loss of ignition, the lower the non-evaporable water content will be. Therefore, the result is a lower rate of cement hydration in the mortar cubes conditioned in water environment, which is due to calcium leaching, a process with a higher rate when the specimens are under conditions of current flow. In contrast, for the cubes conditioned in Ca(OH)2 environment, calcium leaching is not relevant (i.e. no concentration gradient between the pore water and external solu-tion applies); therefore the degree of hydrasolu-tion for both control and “10 mA/m2” groups is similar throughout the test.

3.2 Compressive strength

Compressive strength is a general indicator of concrete quality and an important con-trol test both for concrete production and cement-based microstructural properties.

and curing conditions including humidity, temperature and time [23-24]. Generally, compressive strength increases with progress of cement hydration. It is also related to microstructural development of the cement-based matrix: compressive strength depends on pore structure (i.e. porosity and pore size distribution) and the degree of cement hydration [25]. Several studies report on the link between compressive strength and microstructural properties of cement-based materials such as pore structure and pore size distribution [26-30]. The present study is focusing on the in-fluence of stray current flow on cement-based materials partly immersed in two dif-ferent environments (water and Ca(OH)2 solution). Figure 3 presents the compres-sive strength results as a function of hydration age for the mortar specimens in all studied conditions.

Figure 3: Compressive strength as a function of hydration age for mortar specimens submerged in H2O and Ca(OH)2 as external environment in control and under current (10mA/m

2

) conditions

As shown in the Fig. 3, compressive strength increases with time of cement hydra-tion, which is as expected. There is no significant influence of the external environ-ment and/or the current flow, involved for groups “10 mA/m2”. In Ca(OH)2 environ-ment, similar values of compressive strength were observed both for control and for groups “10 mA/m2”. However, a general trend of lower compressive strength values towards the end of the test is relevant for the specimens under current in water envi-ronment (group 10 mA/m2-H2O). The effect is due to the reduced rate of cement hy-dration (Fig. 2) and related microstructural changes as a consequence from en-hanced calcium leaching in this group.

3.3 Microstructural properties

Mercury intrusion porosimetry (MIP) measurement is commonly used method to characterize porosity and pore size distribution of cement-based materials. It can be used to detect the pore size diameter from several nanometers to hundreds mi-crometers [31]. Total porosity can be determined from the total volume of intruded mercury at the maximum experimental pressure divided by the bulk volume of the sample. Effective porosity is defined as the volume of mercury removed during extru-sion. It is an important parameter for the permeability in porous materials. The effec-tive porosity disregards inkbottle pores and only takes into account the effeceffec-tive flow channel in the pore structure. It can be obtained from the second intrusion curve (see Fig. 4) [32].

Figure 4: Mercury intrusion and extrusion hysteresis [32].

Figures 5 and 6 depict the MIP results for porosity and a summary of the total porosi-ty of mortar specimens partly submerged in the water and Ca(OH)2 solution obtained from MIP measurements. As seen in the Figure (5a,c), increasing porosity occurs for current regime of mortar specimens partly submerged in the water solution. Electrical current accelerates leaching of calcium ions leading to an increase total porosity of mortar specimens compared with control specimens. On the contrary, a current re-gime (Fig 5b,c) gives a lower total porosity compared with control specimens due to avoidance of calcium leaching and accelerate of cement hydration.

Figure 5: Porosity and summary total porosity obtained from MIP for mortar specimens partly sub-merged in the water and Ca(OH)2 solution

Figure 6 depicts differential pore size distribution curves for mortar speci-mens partly submerged in the water and Ca(OH)2 solution. As observed in the Fig. 6a in differential pore size curves, the pore size diameter of current re-gime increase and results in a higher total porosity compared with control specimens, as shown in the figure 5c. It can be attributed to calcium ions leaching. At 112 days, the pore size diameters are more less the same be-tween current regime and control specimens. Conversely, for mortar speci-mens partly submerged in Ca(OH)2 solutions, pore refinement occurred for current regime at 28 days. The pore diameter shifted to a lower pore diame-ters (gel pores), leading to densification of mortar bulk matrix. There is no significant difference between control specimens and current regime at 112 days.

Figure 6: Pore size distribution curves for mortar specimens partly submerged in the water (a) and Ca(OH)2 solution (b).

Fig. 7 presents ink bottle porosity, effective porosity and calculated permeability for mortar specimens partly submerged in the water and Ca(OH)2 solution. Although fluctuative values of ink bottle and effective porosity were observed for both cases, mortar specimens partly submerged in the Ca(OH)2 solution tend to have lower cal-culated permeability compared with mortar specimens partly submerged in the water solution (Fig. 7c). As expected, a current regime tends to yield lower permeability values than that of control specimens due to a higher degree of hydration (Fig. 2) leading to densification of bulk matrix. Increasing porosity (Fig. 5c) due to calcium ion leaching and a competing effect of ion and water migration for current regime in the water environment end up with higher permeability (Fig. 7c) and decreased in com-pressive strength (Fig. 3).

Figure 7: Ink bottle (a), effective porosity (b) and permeability (c) of mortar cubes in H2O and Ca(OH)2

as external environment

There are several points that can be deduced from the experimental results mentio-ned above. During the hydration process, mortar specimens partly submerged in the water solution tend to have calcium leaching due to chemical potential or concentra-tion gradient between mortar specimens and water soluconcentra-tion. Electrical current flow accelerates calcium leaching and affects cement hydration. Calcium leaching results in coarsening of the pore structure, leading to an increase of the permeability of ce-ment-based materials. Increasing porosity during electrical current flow will enhance the degradation process in terms of decreasing compressive strength of mortar spe-cimens. It can be said that there is a larger effect on microstructural and mechanical properties when leaching was involved and also accelerated by the electrical current flow. These effects are less pronounced in Ca(OH)2 environment. In that case the Ca(OH)2 solution avoided/prevented calcium ions leaching, resulting in a beneficial effect on strength development of mortar specimens both for control and current re-gime. Cement hydration was accelerated during electrical current flow leading to

in-need to be performed in order to define threshold levels electrical (including stray) current densities, expected to have negative effect on (reinforced) concrete struc-tures.

4 Conclusions

This paper deals with microstructural properties of the specimens in condition of stray current flow with time, pore connectivity and permeability and will link those to the observed alterations in mechanical properties and cement hydration. The following conclusions can be drawn:

a. Stray current flow in mortar specimens partly submerged in water tends to ac-celerate calcium ions leaching compared to control specimens, resulting in in-creased porosity, permeability and dein-creased compressive strength.

b. Positive effects of stray current flow through mortar specimens partly sub-merged in Ca(OH)2 solution were monitored. It can be attributed to avoidance of calcium ion leaching in the mortar specimens, leading to decreased porosi-ty, permeability and increased in compressive strength.

c. Comprehensive investigation and modelling work need to be performed to de-fine threshold values for negative or positive effects of stray current flow on (reinforced) concrete structures.

5 Acknowledgements

The financial support from Directorate General of Higher Education Ministry of Edu-cation Republic of Indonesia is gratefully acknowledged. The authors would like to thank technicians of Microlab, Section of Material and Environment, Delft University of Technology for supporting an experimental set up.

6 References

[1] Jain, J., N.Neithalath, Analysis of calcium leaching behavior of plain and modified ce-ment paste in pure water, Cece-ment & Concrete Composites 31 (2009) 176–185.

[2] Puertas, F., I. Garcia-Diaz, M. Palacios, M.F. Gazulla, M.P. Gomez, M. Orduna, Clin-kers and cements obtained from raw mix containing ceramic waste as a raw material. Characterization, hydration and leaching studies, Cement & Concrete Composites 32 (2010) 175–186.

[3] Marinoni, N., A.Pavese, M.Voltolini, M.Merlini, Long-term leaching test in concretes: An X-ray powder diffraction study, Cement & Concrete Composites 30 (2008) 700–705 Gaitero, J.J., I. Campillo, A. Guerrero, Cement and Concrete Research, Vol.38 (2008), p.1112.

[4] Gaitero, J.J., I. Campillo, A. Guerrero, Reduction of the calcium leaching rate of cement paste by addition of silica nanoparticles, Cement and Concrete Research 38 (2008) 1112–1118.

[5] An Cheng, Sao-Jeng Chao,Wei-Ting Lin, Jia-Liang Chang, Prediction of the Deteriora-tion Depth of Concrete by Accelerating Calcium Leaching Test, Advanced Materials

Research Vol. 365 (2012) pp 3-7.

[6] Roziere, E., A. Loukili, R. El Hachem, F. Grondin, Durability of concrete exposed to leaching and external sulphate attacks, Cement and Concrete Research 39 (2009) 1188–1198

[7] Jacques, D., L. Wang, E. Martens, D. Mallants, Modelling chemical degradation of con-crete during leaching with rain and soil water types, Cement and Concon-crete Research 40 (2010) 1306–1313.

[8] Nguyen, V.H., H. Colina, J.M. Torrenti, C. Boulay, B. Nedjar, Chemo-mechanical cou-pling behaviour of leached concrete Part I: Experimental results, Nuclear Engineering and Design 237 (2007) 2083–2089.

[9] Heukamp, F.H., Ulm, F.J., Germaine, J.T., 2001. Mechanical properties of calcium-leached cement pastes. Triaxial stress states and influence of the pore pressures. Cem. Concr. Res. 31, 767–774.

[10] Ulm, F.J., Lemarchand, E., Heukamp, F.H., 2003. Elements of chemomechanics of leaching of cement-based materials at different scales. Eng. Frac. Mech. 70, 871–889.

[11] Ulm, F.J., Torrenti, J.M., Adenot, F., 1999. Chemoporoplasticity of calcium leaching in concrete. J. Eng. Mech. (ASCE) 125 (10), 1200–1211.

[12] Saito, H., Nakane, S., 1999. Comparison between diffusion test and electrochemical acceleration test for leaching degradation of cement hydration products. ACI Mater. J. 96 (2), 208–213.

[13] Saito, H., Deguchi, A., 2000. Leaching tests on different mortar using accelerated elec-trochemical method. Cem. Concr. Res. 30, 1815–1825.

[14] Saito, H., Nakane, S., Ikari, Fujiwara, A., 1992. Preliminary experimental study on the deterioration of cementitious materials by acceleration method. Nucl. Eng. Des. 138, 151–155.

[15] Susanto A, Koleva DA, Copuroglu O, van Beek K, van Breugel K, Mechanical, electri-cal, and microstructural properties of cement-based materials in condition of stray cur-rent flow, Journal of Advanced Concrete Technology Vol. 11, 119-134, April 2013.

[16] L.E. Copeland and J.C. Hayes, The Determination of Non-evaporable Water in Hard-ened Portland Cement Pastes, PCA Bulletin 47, December 1953, also ASTM Bulletin 194, December 1953.

[17] Aggregates for concrete - Determination of loss on ignition, NEN 5931 standard, July 1990.

[18] Sumanasooriya, M.S., Neithalath,N., Stereology- and morphology-based pore structure descriptors of enhanced porosity (previous) concrete, ACI Materials Journal. 106 (5) (2009) 429-438.

[19] Hu J., Porosity of concrete, morphological study of model concrete, PhD thesis, Delft University of Technology, Delft 2004.

[20] Ma Y et al., The pore structure and permeability of alkali activated fly ash, Fuel (2012).

[21] Garboczi EJ., Permeability, diffusivity, and microstructural parameters: a critical review. Cem Concr Res 1990; 20(4):591-601.

[22] X. Feng, E.J. Garboczi, D.P. Bentz, and P.E. Stutzman, Estimation of the degree of hydration of blended cement pastes by a scanning electron microscopy point-counting procedureCement and Concrete Research3 4 (2004) 1787-1793.

[23] Nehdi and Soliman, Early-age properties of concrete: overview of fundamental con-cepts and state-of-the-art research, Construction Materials 2011, 164: 57-77.

[24] P. Kumar Mehta and Paulo J. M. Monteiro, Concrete Microstructure, Properties and Materials, October 20, 2001.

[25] P.K. Mehta, Concrete: Structure, Properties, and Materials, Prentice Hall, New York, 1993.

[26] G. Fagerlund, Proceedings of the International RILEM Symposium on Pore Structure, Praque, 1973, pp. D51–D73 (Part 2).

[27] S. Baozhen, S. Erda, in: Proceedings of the International RILEM Congress, Versailles, France, 1987, pp. 232–237.

[28] R.R. Hengst, R.E. Tressler, Fracture of Foamed Portland Cements, Cement and Con-crete Research. Vol. 13, pp. 127-134, 1983.

[29] M. Robler, I. Odler, Investigations on the Relationship between porosity structure and strength of hydrated Portland cement pastes I. Effect of porosity, Cement and Concrete Research. Vol. 15, pp. 320-330, 1985.

[30] G.C. Hoff, Porosity-strength considerations for cellular concrete, Cement and Concrete Research, Vol. 2, pp. 91-100, 1972.

[31] Aligizaki, K. K., Pore structure of cement-based materials, Taylor & Francis, (2006).

[32] Ye, G., (2003). “Experimental study and numerical simulation of the development of the microstructure and permeability of cementitious materials.” Ph.D. thesis. Delft: Delft University of Technology.