The role of temperature and activator solution molarity on the viscosity and hard structure formation of geopolymer pastes

Abstract This study aimed to determine the impact of preparation temperature (10 °C to 20 °C) and different alkaline activator solution (AAS) molarities on the rheological behaviour, structure development, mechanical performance during geopolymer (GP) synthesis, and ageing of metakaolin (MK) based GP pastes. By decreasing the AAS molarity from 8 M to 5 M, the viscosity of AAS decreased by up to 1.68 times and GP pastes viscosity by up to 1.74 times. A temperature increase from 10 °C to 20 °C decreased the viscosity of AAS by up to 1.89 times and the viscosity of GP pastes by up to 1.29 times. With the AAS molarity decrease, an increase of GP paste spread diameter of up to 1.7 times was observed. At 10 °C, the spread diameter increased up to 51% and at 20 °C temperature, up to 57%. Decreasing the AAS molarity, MK/AAS ratio and preparation temperature of the samples led to a retardation in the structure formation in the GP samples during GP synthesis from 4 to 8 h. For samples prepared at 10 °C the UPV values were up to 12% lower than in the same composition samples prepared at 20 °C. Decreasing the AAS molarity and MK/AAS ratio in the samples led to a decrease in compressive strength after GP synthesis of up to 30%, however the UPV values and compressive strength in the samples prepared at 10 °C were up to 2.3% and 12.1% lower than in the samples prepared at 20 °C. The XRD results confirmed the compressive strength test results and revealed that a decrease of molarity and temperature led to a less intensive geopolimerisation reaction and hydroxide sodalite formation in the composition. After ageing, the UPV and compressive strength values in the samples mostly increased during the first 7 days, but after 56 days the UPV and compressive strength in the samples prepared at 10 °C and 20 °C practically levelled off.