Influence of quicklime addition on the mechanical properties and hydration degree of blended cements containing different fly ashes

Even though the use of pozzolanic and latent hydraulic materials (such as fly ash, slag and natural pozzolans) in the construction sector is nowadays a common practice, certain deficits associated with these materials have initiated research that aims to reduce the negative effects that they introduce into cementitious systems. With respect to fly ash, its relatively slow pozzolanic reactivity hinders its greater utilization; hence cost efficient methods of activation are on demand. In this work, cement systems containing the two typical types of Hellenic fly ashes (of high and moderate calcium content) were chemically activated by adding industrially produced quicklime. A first step in assessing the efficiency of the above procedure was based in monitoring the compressive strength development, lime depletion values and data derived from a selective dissolution procedure (based on a mixture of picric acid–methanol). Moreover, evolution of the fly ash reaction and the morphology of the formed hydration products were examined by means of scanning electron microscopy. Evidence included in this investigation reveal that quicklime presence accelerated the reaction rate of high-calcium fly ash, whereas in the case of lower-calcium fly ash, quicklime had a positive impact only during the very early stages of hydration and thereafter acted inhibitory with respect to its reactivity development.