The "filler effect", due to the physical presence of mineral additions in cement, is mainly known to accelerate the hydration of the clinker component. Previously, this was attributed to the surface of the filler providing nucleation sites for C-S-H as there is a clear dependence on the surface provided by the filler particles. Our results reveal that the increase in nucleation is quite low compared to the area provided. Based on the isothermal calorimetry experiments and SEM images, we demonstrate that the most important parameter is the interparticle distance. We propose that this is mainly the result of the shearing conditions rather than extra surface available for C-S-H as formerly assumed. Quantitatively slag and fly ash behave very similarly to quartz. Limestone, on the other hand, seems also to stimulate C-S-H nucleation giving it a higher efficiency in accelerating clinker hydration.

a b s t r a c t a r t i c l e i n f oIn this study, the effect of SCM in the cement paste was isolated by using ternary systems combining Portland cement, quartz and SCM. The results show clear differences in how the hydrates from the reaction of clinker, slag and fly ash fill space. The reaction of slag is more efficient than that of fly ash in modifying the porosity. Our results indicate that Portland cement (PC), slag and fly ash reactions are limited at later ages by the lack of water-filled capillary pores. The higher the space available, with increasing the water/solids ratio, the later the reaction is limited. This explains the lower degree of reaction of SCM in blended systems at high replacement levels.

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