Investigation of a durable gypsum binder for building materials

Abstract

An investigation was made to formulate a durable gypsum binder based on calcined phosphogypsum, fly-ash/granulated blast-furnace-slag and Portland cement. In this binder, phosphogypsum acts as the basic matrix and a source of sulphate in the mixture of calcium, alumina and silica derived from the fly-ash/granulated-slag. The strength development in the binder at an early age is due to the setting and hardening of calcined gypsum and Portland cement, and at a later age is due to the formation of ettringite and tobermorite. The formation of hydration products was confirmed by differential thermal analysis, X-ray diffraction and scanning electron microscopy. The gypsum binder based on fly ash exhibited lower compressive strength (22.0 N mm −2) than gypsum binder based on granulated slag (35.0 N mm −2). The effect of temperature (27–60°C) on strength development of gypsum binder in high humidity was studied. Data show that with increasing temperature, the strength of fly-ash-based gypsum binder is increased, whereas the strength of the slag-based gypsum binder is decreased. The enhancement in strength with increasing temperature is ascribed to the pozzolanic action of fly ash blended with the calcined gypsum. The fall in strength of slag-based gypsum binder can be attributed to the decomposition of ettringite with increasing temperature. The gypsum binder was found suitable for use in masonry mortars, bricks and glass-reinforced composites.