The influence of gypsum content on the porosity and pore-size distribution of cured PFA—lime mixes

Abstract

Previous work by the Authors has shown that small gypsum additions to compacted PFA—lime mixes lead to enhanced compressive strength during high-temperature (95°C) curing but that gypsum contents in excess of an optimum level result in a fall in strength. This strength variation is related to the nature of the reaction product formed. At low gypsum concentrations this is principally a fibrous and foil-like C-A-S-H gel; at high gypsum concentrations it is predominantly a colloidal sulphate-containing C-A-S-H product. In the current work the porosity and pore-size distribution of compacted high-temperature cured PFA—lime mixes of varying gypsum content have been determined, and the water absorption and drying shrinkage have been measured. The presence of gypsum leads to a reduction in overall porosity of the cured dry material. At low gypsum contents the pore-size population consists of pores predominantly in the 4-50 nm region, whereas for high gypsum contents porosity shifts towards pore sizes in excess of 500 nm. Also, drying shrinkage is observed to increase rapidly with increasing gypsum content. The observations support the hypothesis that at high gypsum concentrations the formation of a colloidal sulphate containing C-A-S-H product leads to increased water absorption, enhanced drying shrinkage, coarsening of pores and a reduction in strength.