LINKING HYDRATION KINETICS TO MATERIAL PERFORMANCE

To better understand the composition-reaction-property correlations in OPC- SCM-limestone systems provisioned with PCE, an attempt was made to link the hydration kinetics to the rheological properties and structural recovery in the fresh state and compressive strength development in the hardened state. In the first part, the characteristic calorimetric parameters indicative of acceleration or retardation in hydration kinetics, i.e., the time of occurrence of the main hydration peak and the time corresponding to the end of the induction period (i.e., onset of acceleration regime), were extracted from the calorimetry profiles (Figure 4-9) to construct rheology-reactivity plots in Figure 4-14. As can be seen, the rheological properties measured at the age of 70 min (i.e., dynamic yield stress, plastic viscosity and static yield stress at rest) of the investigated mortars are well-correlated with the times at which the induction periods end, and the main hydration peaks occur. In mortars with faster early-age hydration kinetics, both the onset of acceleration regime and the main hydration peak occur earlier compared to those with slower early-age hydration kinetics. As faster hydration kinetics implies formation of larger quantities of hydration products and development of elastic properties at a given amount of time, it is expected that in mortars with higher reactivity,

the transition from a “slurry” state to “plastic” state would occur faster. This is because the nucleation of hydrates, especially C-S-H, turns the soft colloidal interaction among cementitious grains into more rigid interactions. The continuing nucleation of hydrates in such mortars increases the development of rigidity (i.e., elastic properties) of system, thus leading to a larger increase in rheological properties. In contrast, mortars with slower early-age reactivity feature slower formation of hydration products, and thus lower rheological properties.

(a) (b)

Figure 4-14 Correlations between rheology and reaction kinetics of OPC-SCM and PLC-

SCM systems provisioned with PCE: (a) time corresponding to end of the induction period vs. rheological properties and (b) time of occurrence of the main hydration peak

vs. rheological properties.

In the second part, a focus was placed to link hydration kinetics to the development of hardened properties (i.e., compressive strength). Several studies have shown that compressive strength evolution is linearly correlated to the release of heat

through hydration (Bentz et al. 2012a; Kumar et al. 2013b; a; Lothenbach et al. 2008b). To further examine the implication of this correlation for the evaluated OPC-SCM and PLC-SCM systems provisioned with PCE, the strengths at ages of 1, 3, and 7 days were plotted against the cumulative heat release in Figure 4-15. Here, the cumulative heat release was normalized by water content of the mixtures, since the initial water content serves a measure of the initial porosity of system which needs to be filled by the hydration products for the evolution of the microstructure and the development of properties. As can be seen, a generic linear relationship can be established between compressive strength evolution and cumulative heat release encompassing all systems investigated in this study. The majority of data points lies within the ± 20% bound of the linear best fit line, thus validating that the cumulative heat release of binders can be effectively utilized to estimate the compressive strength development of mortars, regardless of the binder composition or the presence/absence of dispersants (e.g., PCE). As identified in Figure 4-15, the established linear trend-line has a non-zero x-intercept, reflecting that a critical degree of hydration of the cement is needed to percolate solid- phase networks, and thus contribute to the gain in strength.

Figure 4-15 Correlation between compressive strength development at 1, 3, and 7 days

and cumulative heat release normalized by initial water content for OPC-SCM and PLC- SCM systems provisioned with PCE. The solid line represents linear best fit line with

These correlations, shown in Figure 4-14 and Figure 4-15, can be used as useful means to estimate the time-dependent material performance of binary blends in both fresh (rheological properties) and hardened state (compressive strength) by simply using the experimentally measured calorimetry profiles as the input. This approach is powerful in that it proposes a reliable means for concrete technologists to perform apriori estimation of key engineering properties of a cementitious system by simply using the experimental measurement of its heat release as the sole input.

4.5. SUMMARY

A series of PLC-SCM and OPC-SCM mortars were evaluated to compare and contrast their performance in terms of their particle packing, time-dependent rheological properties, hydration kinetics, and compressive strength evolution. All binders were provisioned with the presence of PCE to improve fluidity and enhance solid concentration of cementitious particles.

• The benefit of larger SSA in PLC systems to improve solid concentration is strongly related to the presence of PCE. PLC binders with higher SSA of particles necessitated a higher dosage of dispersant to improve fluidity and achieve maximum possible solid concentration.

• Although PLC systems necessitate higher PCE dosages due to its higher SSA as compared the OPC systems, the former systems show faster hydration rate and larger extents of heat release due to the higher solid concentration and greater SSA.

• PLC systems develop larger rheological properties (i.e., dynamic yield stress, plastic viscosity, and static yield stress at rest) compared to their OPC counterparts, especially at later ages. This is attributed to the coupled effect of higher SSA and lower inter-particle spacing in the former systems as compared to the latter mixtures.

• Aluminosilicate SCMs are more effective in terms of improving strength in PLC systems as compared to OPC systems. While SCMs are capable of invoking pozzolanic reactions (leading to formation of pozzolanic C-S-H) in either systems, the superiority of the PLC system is attributed to: (i) the higher SSA of

the PLC, which improves solid concentration and ensures faster hydration kinetics, and (ii) the carbonate-rich chemistry of PLC, which allows PLC to partake in chemical reactions with the SCM leading to the formation of space- filling carboaluminate hydrates. The combined effects of optimized particle size distribution (i.e., higher SSA), optimized particle packing, and optimized chemistry of the PLC-SCM systems lead to equivalent or even higher compressive strength than that of pure OPC or OPC-SCM systems.

• Hydration kinetics is found to correlate well with rheological properties and compressive strength development of OPC and PLC systems containing high volume of SCM replacements that are provisioned with PCE. Such correlations provide a reliable basis for apriori estimation of key engineering properties, both in the fresh and hardened state, and thus enable the optimization of binder formulation to provide more sustainable solution for concrete construction applications.

5. PHYSICO-MECHANICAL CHARACTERISTICS OF CEMENT PASTE OVER