Influences of different types of fly ash and confinement on performances of expansive mortars and concretes

Abstract This paper evaluates the performances of expansive concrete containing fly ashes. In this study, both standard fly ashes and substandard fly ash are used as a cement replacing material in expansive mortar and concrete. A substandard fly ash which contains high CaO, free lime, and SO 3 contents is used in this study to compare it with high CaO and low CaO fly ashes which are standard fly ashes. Flow, water requirement, and setting times of the expansive mortars are studied together with unconfined and confined compressive strength, length changes, and Young’s modulus of expansive concretes. Test results indicate that both free expansion and restrained expansion of the expansive concrete are enhanced by using fly ash, especially by the substandard fly ash. Three hypotheses supporting the mechanism of fly ash on the expansion enhancement of the mixtures are described. The first hypothesis is that expansive mixtures containing fly ash produce more ettringite than that without fly ash. The second hypothesis is that a smaller stiffness leads to a higher expansion of the expansive concrete containing fly ash when compared to that of the expansive concrete without fly ash. The third hypothesis is that fly ash delays the hydration of the expansive additive in the expansive mixtures. These hypotheses are clarified by the test results of XRD-Rietveld analysis, DTG, and Young’s modulus. The use of fly ash in the expansive concrete enhances the expansion, the compressive strength, and the pore structure, especially for the expansive concrete under confinement. A mercury intrusion porosimetry (MIP) test of mortar is used to verify the enhanced compressive strength and the pore structure of the expansive concretes.