SELF-HEALING CAPACITY OF HARDENED CEMENT SUSPENSIONS
WITH HIGH LEVELS OF CEMENT SUBSTITUTION
C. Litina 1 and A. Al-Tabbaa 1
1
Engineering Department, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK – e-mail: cl519@cam.ac.uk; aa22@cam.ac.uk
Keywords: self-healing, cement suspension, compositional variation, supplementary cementitious materials, high water content
ABSTRACT
Aqueous cement suspensions without aggregates have been commonly applied as a remedial technique in structural and geotechnical applications, often comprising part of the permanent infrastructure. These suspensions derive their mechanical characteristics and properties through the same hydration processes and binding chemistry as concrete; thus they are susceptible to the same deleterious environmental factors and inherent properties. This can endanger the long-term functionality of the installation.
Therefore there is a strong incentive of compositional alteration of the basic binder by incorporating supplementary cementitious materials that have been acknowledged to improve the physical properties and enhance the durability characteristics of the hardened matrix. However the latter does not offer a robust long term solution. Therefore a compositional enhancement through biomimetic approach of damage response for the design of high performance cement suspensions is put forward. This study involves two stages, i.e. determination of the autogenous crack sealing behaviour of various optimised blended compositions and the subsequent enhancement of the intrinsic autogenous healing processes through the inclusion of microencapsulated healing agents. Herein the results of this preliminary investigation on the effect of compositional variation on the intrinsic properties of physical response to cracking are presented.
Ternary and quaternary blends of minerals -including Portland cement, slag, MgO and silica fume- are being developed and investigated. Upon cracking and water ingress, the unreacted particles are activated and yield hydration products that crystallize in the crack, sealing it off and recovering mechanical characteristics. The self-healing capacity of the samples is quantified through microscope observation, gas permeability test and three-point flexural bending. The findings on the crack healing efficiency and mechanical recovery of initially cracked specimens confirm the existence of self-healing mechanisms in supplementary cementitious materials.
1. INTRODUCTION
Aqueous cement suspensions often comprise part of the permanent infrastructure. However due to their composition they are susceptible to severe tensile cracking phenomena and environmental deterioration. This can endanger the long-term functionality of the installation. The latter is exacerbated by the remote positioning of these installations. Literature indicates that a common approach to enhance the
durability of cement pastes is to limit the percentage content of Portland cement (PC) and substitute by supplementary material [1–3]. Cementitious materials can display naturally self-healing properties as a result of intricate calcium precipitating processes [4]. However the investigation of the effect of clinker substitution on self-healing properties is limited. Hence, the work presented in this paper focuses on the investigation of the effect of compositional variation on the intrinsic properties of physical response to cracking.
2. MATERIALS
The details of the mixes tested are given in Table 1. Eight different compositions were investigated, denoted A1-A8. Type I Portland cement (52.5N); blast furnace slag (BFS), limestone powder; silica fume; and MgO were used for the preparation of ternary and quaternary blends. The water to binder (total solids) ratio adopted in this design was kept constant (w/c=0.6 by weight). This water content is similar to the upper bound water content adopted in relevant studies [5] and common for grouting applications [6].
A series of cylindrical 15mmx30mm and prismatic specimens 40mmx40mmx160mm were cast from each batch using a high shear mixer to ensure homogenisation. Specimens were demoulded after 24hours and water cured at laboratory temperature (20±1oC) until testing. 1- and 28-day-old specimens were considered in order to quantify the effect of the variant consistency and thus the effect of hydration and chemistry of the different mixes on self-healing behaviour.
Table 1: Mixture design proportion by weight
Mix Components wt (%)
PC BFS Limestone Silica fume MgO Water
A1 0.50 0.45 0 0 0.05 0.60 A2 0.45 0.45 0 0 0.10 0.60 A3 0.45 0.45 0 0.10 0 0.60 A4 0.45 0.40 0 0.15 0 0.60 A5 0.30 0.40 0.30 0 0 0.60 A6 0.20 0.50 0.30 0 0 0.60 A7 0.10 0.60 0.25 0 0.05 0.60 A8 0.05 0.65 0.20 0 0.10 0.60 3. METHODS
The experiments are divided into two categories; a preliminary phase of characterisation of basic mechanical and rheological properties of fresh and hardened compositions and a subsequent phase of establishing the self-healing potential of these blends. The principle of the latter is to perform mechanical tests aimed at pre-cracking the specimen, followed by a curing period to allow the triggering of self-recovery and then retesting in order to characterise the residual mechanical properties and clarify the self-healing capability. Therefore three-point flexural bending and permeability testing investigations were performed in order to highlight the self-healing phenomenon.
For the determination of the permeability of the hardened specimen the method and apparatus proposed were adopted [7]. Cylindrical specimens prepared for each
composition were removed from water at 1 and 28 days. Half were loaded to 80% of their respective ultimate compressive strength and then set aside for 28days in water to allow the healing process to take place. The rest of the same batch was used to create the bench mark for the healing efficacy. 10 mm-thick cylindrical disk specimens were cut from the middle of the cylinders. Subsequently, the disk specimens were vacuum-dried until testing time and then placed and sealed on the top of a cell with silica gel sealer to avoid leakage of gas vapour. The values of mass variation with time due to the vaporization of methanol liquid at a constant 40oC water bath temperature were recorded at specific time interval until a steady-state mass loss was reached. The tests were conducted in triplicates.
Mechanical characterisation by means of three-point flexural loading of prismatic specimens was performed. These were allowed to cure for 1 and 28 days-accordingly- and then artificially cracked with a 3-point flexural test fitting until ultimate flexural load was reached and a single crack at the middle of the notch was created. Then the cracked samples were placed in water for 28 days, while care was taken for the crack surfaces to remain in contact; nor force was applied neither renewal of the curing medium. A quasi-static loading speed of 0.02mm/min was selected.
4. RESULTS
The compressive and flexural strengths of the investigated mixes are presented in Figure 1. The basic mechanical behaviour indicates that as the level of cement substitution increases the compressive strength decreases (A1 to A5). Moreover the nature of the supplementary cementitious materials used in each mix seems to also play an important role on the strength gain. At the same levels of cement replacement for BFS the use of MgO yielded higher compressive strength compared to silica fume. Furthermore the use of limestone powder, as a reactivity enhancer of BFS, led to improved strength gain. On the other hand flexural strength does not follow a similar trend, but rather slightly increases with cement substitution fluctuating at 1.5MPa.
The findings on the crack healing efficiency (Healing efficiency = Efhealed/Efpristine) and mechanical recovery of initially cracked specimens confirm the existence of self-healing mechanisms in supplementary cementitious materials (Figure 2). Due to the lack of tensile support in the investigated formulations crack control at 28days did not effectively prohibit the total abruption of the fragments of these specimens. However care was taken for the crack surfaces to remain in contact even after complete abruption. A1 and A2 specimens that had been completely fractured showed partial reattachment however the bond strength was too weak to sustain the specimens in one piece. Virtual reattachment of the two segments could be attributed to the water suction/pressures between the two surfaces. The latter could also explain the dissipation of water/moisture from the crack location to the surrounding surfaces following the load increment. Among specimens produced with the same cement content, the addition of silica fume appeared beneficial for the mechanical recovery. High limestone content (A4) presented the highest self-healing efficiency exhibiting mechanical recovery (fracture stiffness recovery) up to 80% and in fact reaching 30% for samples initially completely separated. So far the presence of MgO has not hindered the manifestation of self-healing properties however a dependency on the content is inferred. Analysis of the precipitated products concentrated in the crack area will give a clearer image of the mechanisms contributing to the healing.
5. CONCLUSIONS
Preliminary results indicate recovery of mechanical properties (tensile strength, stiffness) as well as self-sealing induced permeability reduction of cracked specimen. The quaternary and ternary cementitious composites display autonomous self-healing potential even at high levels of cement substitution. The exact mechanism contributing to the exhibited phenomenon should be confirmed with further observations using SEM of the specimens’ microstructure.
Figure 1: Compressive and flexural strength of different mixes at 28days.
Figure 2: Flexural strength recovery of different mixes after 28days of water curing
ACKNOWLEDGEMENTS
Financial support from the Engineering and Physical Sciences Research Council (EPSRC) and the Hellenic States Scholarship Foundation for the PhD study of the first is gratefully acknowledged.
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