Shrinkage in low-calcium fly ash geopolymers for precast applications: Reaction product content and pore structure under drying conditions

Shrinkage and strength gain in alkali-activated low-calcium fly ash (AAF) geopolymers are evaluated. AAF mixtures that are proportioned for strength are evaluated for typical variation of compositional variables including water and sodium contents in the mixture. Strength gain in the AAF mixture is related with the formation of the sodium aluminosilicate (NASH) gel under accelerated curing conditions. Moisture loss produced by drying results in an overall reduction in the NASH gel content formed in the AAF, while its composition is not changed. NASH content is higher in AAF with lower water content producing finer pore structure and higher strength. The excess Na does not participate in NASH formation; however, the excess Na is available like a filler, which does not contribute to the strength of the AAF while producing pore refinement. Shrinkage produced on drying depends on the water and sodium contents in the activated paste. AAF mixture made with smaller water content has a refined pore structure, which results in increased shrinkage strain on drying while the mass loss is lower. Excess sodium in AAF produces a larger shrinkage strain on drying due to the pore refinement. AAF mixtures with a low sodium content and prevention of moisture loss during strength gain is suggested for precast applications.

Automated summary

This article evaluates the shrinkage and strength gain in alkali-activated low-calcium fly ash (AAF) geopolymers. The AAF mixtures, designed for strength, were evaluated for various compositional variables, including the water and sodium contents. The strength gain in the AAF mixture is attributed to the formation of sodium aluminosilicate (NASH) gel under accelerated curing conditions. Moisture loss during drying leads to a decrease in the NASH gel content, while its composition remains unchanged. The study suggests that AAF mixtures with lower water content and prevention of moisture loss are suitable for precast applications.