Shrinkage mitigation of alkali-activated fly ash/slag mortar by using phosphogypsum waste

Abstract

Shrinkage in alkali-activated materials (AAMs) is relatively high, and its mitigation strategies are welcome to improve the performance of AAMs for practical applications. This study investigates the replacement of fly ash with phosphogypsum (PG) waste to mitigate shrinkage in alkali-activated fly ash/slag (AFS) mortar. A range of tests were carried out to ascertain the autogenous shrinkage, drying shrinkage, setting time, and compressive strength of the AFS. SEM/EDS and XRD characterized the microstructure and hydration products of the AFS paste. It was founded that using PG shortened the setting time of the AFS mixture cured at room temperature. Incorporating 5% of PG treated at 40 °C in the AFS mortar was optimal for improving its autogenous and drying shrinkage. Using an optimal addition of PG in the AFS mortar led to the formation of ettringite with a micro-expansive effect to compensate for shrinkage. However, a high PG content used may inhibit the gel phase formation in the AFS matrix, resulting in a reduction in the compressive strength of the AFS mortar.