Understanding the influence of compressive strength, microstructure, and mechanism for metakaolin-based geopolymer under varying rest periods in the curing process

High-temperature curing applied to varying stages of dissolution, polycondensation, and reorganization in geopolymerization would influence the reaction kinetics, microstructure, and mechanical properties of geopolymer. This work first investigated the influence and mechanism of the rest period before high-temperature curing on metakaolin-based geopolymers (MKGs) properties. MKGs cured with 0, 2, 4, 6, 8, 10, 20, and 40-hour rest periods before 60 °C curing were prepared, and those cured without high-temperature curing were the control group. The compressive strength of MKGs were measured characterizing the property macroscopically. X-ray diffraction (XRD), isothermal calorimetry, 29Si and 27Al nuclear magnetic resonance (NMR), thermogravimetry-derivative thermogravimetry (TG-DTG), mercury intrusion porosimetry (MIP), and scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) were used to characterize the microstructure of MKGs. A short rest period elevated the initial reaction rate but lowered the resulting reaction level, coarsened the average pore size, and increased the porosity, deteriorating the compressive strength MKGs. Prolonging rest period weakened the deterioration effect of the porosity structure of MKGs cured with a short rest period. Furthermore, prolonging the rest period elevated the reaction level and favored incorporating Al into N-A-S-H gel, modifying the gel structure and densifying the MKGs matrix. Compared with no high-temperature curing, although prolonging the rest period increased the porosity, the increase in reaction level dominated the improvement of mechanical properties of MKGs. The insights contribute to guiding the development of geopolymers from the theory to engineering applications. •The reaction kinetics and mechanism of MKGs under varying rest periods were analyzed.•A short rest period elevated the initial reaction rate but lowered the resulting reaction level.•Prolonging rest period favored incorporating Al into N-A-S-H gel and improved the compressive strength of MKGs.