Optimization of Hybrid Microwave Curing Approach Based On the Performance of Metakaolin-Based Geopolymer Mortars

Geopolymer binders have been highlighted due to their low carbon emission during production and processing. While metakaolin and F-type fly ash are commonly used as raw materials for aluminosilicate-based geopolymers, the long heat-curing requirements for hardening and strength development still pose challenges. This paper investigates the possible use of a hybrid microwave curing technique to design a set-on-demand approach to reduce the duration of heat curing in metakaolin-based geopolymer. The experimental design was established for samples with three different molar ratios (MR; 1.3,1.5, and 1.7) containing metakaolin, fly ash, and silica fume. Samples were subjected to 3 different curing regimes: oven curing, microwave (MW) curing, and hybrid curing (a combination of optimized microwave and oven curing). The performance evaluation was based on compressive strength, dimensional stability, and alkali leaching (efflorescence). Implementing only MW curing resulted in a significant decrease in compressive strength compared to their counterpart oven-cured samples. The reduction of compressive strength was more pronounced at lower molar ratios. The design of a hybrid curing approach where a portion of oven curing was replaced by MW resulted in a higher strength development than those only cured with MW. Similarly, the efficiency of hybrid curing was more pronounced in samples having MR of 1.5 and 1.7. Using MW curing in the geopolymer binders did not affect the alkali leaching; however, it increased the material's drying shrinkage. Results showed that replacing a portion of oven curing with microwave curing in a hybrid approach can increase the operation speed and the hardening rate without significantly decreasing compressive strength.