Shrinkage behavior of fly ash based geopolymer pastes with and without shrinkage reducing admixture

In this study, flowability, compressive strength, drying and restrained shrinkage, cracking potential, and pore size distribution of fly ash based geopolymer (FG) pastes were investigated. The FG pastes were made with two SiO2/Na2O mole ratios (Module), two solute mass concentrations (Concentration), with and without shrinkage reducing admixture (SRA), and their properties were compared with those of Portland cement (PC) pastes. A liquid-to-binder ratio (L/B) of 0.33 was used for both PC and FG pastes. The drying condition of 25 degrees C and 50% RH was used for both free and restrained drying shrinkage tests. The results indicated that the Module and Concentration of activators affected all properties of FG pastes investigated. All FG pastes had a lower cracking tendency than the PC pastes, the latter of which cracked first although its shrinkage was not the highest. The drying shrinkage of FG pastes increased with decreasing Module and increasing Concentration. There was a critical pore size, d(s), computed from both the water loss and pore size distributions, which was strongly related with free drying shrinkage of all the pastes studied. The smaller the d(s) value, the larger the free drying shrinkage a paste will have. SRA substantially coarsened pore structure, significantly reduced free drying shrinkage, noticeably reduced restrained ring shrinkage, distinctly delayed crack initiation time, and considerably decreased crack width of all pastes studied.