The effect of water-to-cement ratio on the hydration kinetics of tricalcium silicate cements: Testing the two-step hydration hypothesis

Recent efforts to model tricalcium silicate based cements assume a two-step hydration mechanism where, in the first step, the empty space between cement particles rapidly fills with a low density calcium silicate hydrate (C-S-H) and in the second step, the thus formed C-S-H densifies slowly. This gives rise to models that nicely mimic the shape of experimentally observed hydration calorimetry curves and explains the transition between Stage 3 (acceleration) and Stage 4 (decreasing rate) hydration as well as post Stage 4 continued slow reaction. If this mechanism is correct, however, one would expect that the amount of heat energy released should increase with available space between particles, i.e. with water-to-cement ratio. A series of experiments, wherein the water-to-cement ratio was systematically varied showed little or no increase in total heat released when the water-to-cement ratio was doubled or tripled suggesting that the two-step mechanism may need revision.