Properties and micro-structure assessment of building materials based on flue gas desulfurisation gypsum modified by cement and industrial waste

Abstract

Utilisation and recycling of industrial waste and by-products to prepare building materials is the green style and contributes to the resource conservation and environmental protection. In this paper, the orthogonal test and two curing methods including 7-day non-soaking curing and 28-day periodical-soaking curing are used to investigate the effect of cement, activators and waste like blast furnace slag, silica fume and fly ash on the properties of FGD gypsum-based composites. Three typical specimens from the orthogonal test are chosen, a pure gypsum specimen, a specimen with prominent combination properties and a specimen with a big fall in strength after the 28-day periodical-soaking curing, to explore their composition and structure characterisation in two curing methods by XRD, SEM-EDS and DTA-TG. It was found that the properties of the composites, especially the waterproof property, can be improved by adding an appropriate amount of OPC and waste due to the synergistic effect between them. The periodical-soaking curing accelerates the hydration of the active substances in the composites, which promotes forming more C–S–H gel, AFt crystals or tacharanite. After the 28-day periodical-soaking, the flexural and compressive softening coefficient are 79.8 % and 107.8 % higher than that of 28-day pure FGD gypsum specimens, respectively. However, improper composition can produce excessive ettringite, which can damage the mechanical properties and water resistance of the specimens due to the expansibility of ettringite.