Mechanical Performance and Microstructure of Single Component Geopolymer Mortar

In order to investigate the strength development rule and microscopic mechanism of fly ash based geopolymer mortar with different NaOH concentrations, cement-sand ratios and solution-cement ratios at various curing temperatures, the mechanical properties test were carried out. The microscopic morphology and pore size distribution were analyzed by scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP), respectively. The results show as follows: For geopolymer mortar specimens prepared using NaOH solution with a concentration of 10%, no significant strength development was observed even at very high curing temperatures. With the increase of NaOH concentration or curing temperature, the compressive strength and flexural strength of single component geopolymer mortar can obtain their best value, and the location of the value is determined by both curing temperature and NaOH concentration. The pore size distribution differential curve of the geopolymer is a unimodal distribution. Controlling the concentration of can greatly reduce the peak value of the pore diameter differential curve and significantly reduce the porosity of the geopolymer. Fly ash particles dissolve gradually under the action of NaOH and form cementitious material on its surface. When the concentration of Na⁺ is low, the geopolymer polymerization is less; controlling the concentration of NaOH can make the geopolymer become dense and improve their compressive strength. The fractal model based on thermodynamic relation is the best one to describe the structure of the geopolymer pore, followed by the pore axis model, while the spatial filling model and the sponge model can only describe the fractal dimension of the pore structure of gel pores and transition pores. The calculated values of fractal dimension based on thermodynamic relationship and sponge model are between 2.0 and 3.0, which is similar to the results of general cement-based materials. Proper adjustment of the concentration of NaOH can improve the pore structure of the geopolymer.