The influence of fly ash on the hydration of OPC within the first 44 h—A quantitative in situ XRD and heat flow calorimetry study

We used in-situ XRD and heat flow calorimetry to investigate the first 44 h of hydration of Ordinary Portland Cement (OPC) blended with siliceous fly ash (FA). By recalculating the heat flow using XRD data it was possible to establish the direct influence of FA on the silicate and aluminate reaction of the OPC hydration in combination with absolute phase quantities. A retarding effect of FA on the silicate reaction was detected, probably induced by the adsorption of Ca2 + ions on the FA surface. The aluminate reaction proved to be influenced by FA. Thereby the filler effect produces an acceleration of the reaction by providing additional nucleation sites. Additionally the reaction itself is very sensitive to the chemical composition of FA. Readily available SO3 seems to be the factor responsible for retarding the aluminate reaction. The increasing of the effective w/s-ratio seems to have only a slight influence on the hydration behavior.