The influence of gypsum content on microstructural development, strength and expansion of cured PFA-lime mixes

The results ol the current investigation show that the presence of small amounts ol gypsum has a profound effect on the reaction of lime with PFA and on the strength development and expansion of cured PFA-lime mixes. PFA-lime-gypsum-water mixes were compressed into cylinders of dry density 1·52 g/cm³ and bulk density 1·81 g/cm³, and were cured for up to 28 dars in ]00% relative humidity at 95°C. The linear expansion during curing and the compressive strength after curing were determined, and the cured specimens were analysed using X-ray powder diffraction, thermogravimetric analysis and scanning and transmission electron microscopy with energy dispersive X-ray analysis. Gypsum additions were found to delay strength development and expansion, hut produced significant increases in the long-term strength and expansion up to an optimum gypsum addition of 4 wt%. High gypsum levels also resulted in a severe retardation in the rate of lime consumption. The analytical results show that these changes are controlled hy the nature and behaviour of the layer of surface reaction products which are observed to form on the surface of the PFA particles. The products consist of a sulphate containing fibrous and foil like C-A-S-H gel, a sulphate containing C-A-S-H globular colloidal product and crystalline ettringite and hydrogarnet. It is the development of and the relative amounts of these different products at the different gypsum concentrations which determine the physical hehaviour of the cured material.