Investigating the role of reactive silica in the hydration mechanisms of high-calcium fly ash/cement systems

High-calcium fly ashes (ASTM Class C) are being widely used as a replacement of cement in normal and high strength concrete. In Greece such fly ashes represent the majority of the industrial by-products that possess pozzolanic properties. Even thought the contribution of factors, such as fineness and water/binder ratio, on the performance of fly ash/cement (FC) systems has been a common research topic, little work has been done on examining whether and to what extent reactive silica of fly ashes affects the mechanisms occurring during their hydration.The work presented herein describes a laboratory scale study on the influence of active silica of two high-lime fly ashes on their behavior during hydration. Volumes up to 30% of Greek high-calcium fly ashes, diversified both on their reactive silica content and silicon/calcium oxides ratio, were used to prepare mixes with Portland cement. The new blends were examined in terms of compressive strength, remaining calcium hydroxide, generation of hydration products and microstructural development. It was found that soluble silica of fly ashes holds a predominant role especially after the first month of the hardening process. At this stage, silica is increasingly dissolved in the matrix forming additional cementitious compounds with binding properties, principally a second generation C–S–H. The rate however, that fly ashes react in FC systems seems to be independent of their active silica content, indicating that additional factors such as glass content and fineness should be taken into account for predicting the contribution of fly ashes in the final performance of pozzolanic cementitious systems.