Influence of Process Variables on Shrinkage in Low-Calcium Fly-Ash Geopolymers

Abstract

The influence of process variables such as curing temperature and relative humidity (RH) on the shrinkage of alkali-activated fly ash (AAF) is examined in this work. The ambient conditions are varied after an initial accelerated moist curing at high-temperature. An analysis interlinking the effects of curing on AAF strength, shrinkage, reaction product content, and porosity is performed. Strength achieved and the pore structure formed for the different curing conditions depend upon the sodium alumino-silicate (N-A-S-H) gel content formed in the AAF. While water is not directly combined in the formation of N-A-S-H gel, its content is sensitive to the availability of moisture. The moisture loss due to drying during the geopolymerization reduces the N-A-S-H content formed in the AAF. Compared to the continuous moist curing at elevated temperature, there is a decrease in the N-A-S-H content on lowering the temperature or drying produced by the decrease in RH. Reducing temperature and RH following initial accelerated curing has the beneficial effect of reducing the shrinkage compared to drying at a higher temperature. Reduction in the N-A-S-H content due to decrease in temperature after the accelerated curing is more significant than the drying on lowering the RH to 50%. The autogenous shrinkage measured under sealed conditions contributes significantly to the total shrinkage in AAF. The shrinkage in the AAF is significantly lower than a comparable cement paste. While shrinkage is produced by drying, the moisture loss and shrinkage relationship is not unique. The shrinkage produced by moisture loss due to drying is primarily influenced by the pore structure formed in the AAF, which also depends on the N-A-S-H content. The influences of temperature and humidity on the strength, pore structure and shrinkage are determined by the N-A-S-H formed in the AAF.