Physical effects of limestone on the hydration of cement clinkers
Effects of limestone addition on the hydration of limestone blended cement can be classified into two aspects, viz. physical effects and chemical effects.
Numerous researchers who investigated Portland limestone cement with high level of calcite addition regard it as inert filler, e. g. by means of both thermal and BSE image analysis, it was investigated by G. Ye et al. that limestone powder does not take part in the chemical reaction at all.
The low reactivity of limestone due to its low solubility is the main cause leading to the inert property; however, even acting as filler, limestone in blended cement causes an apparent hydration acceleration of cement clinker especially at early ages.
On the one hand, the effective w/c ratio in limestone blended system is increased significantly due to the low reactivity of limestone powder, altering the hydration characteristics of cements and providing more water and space for the hydration of clinkers.
On the other hand, limestone is softer than clinker which could achieve a finer particle size when interground with Portland cement clinkers, producing an improved particle size distribution and improving particle packing, e.g., for an overall specific Blaine surface area of 420 m2/ kg, 50% of the limestone particles can be below 700nm, compared with 3µm for the clinkers;
Therefore, a large number of tiny limestone particles act as nucleation sites for the hydration of C3S (and also C2S ), which then markedly decrease the concentration of Ca and Si ions in the solution phases promoting the transformation of C3S phase to the solution phase, thus the hydration of C3S being accelerated.