Mechanical strength and the degradation mechanism of metakaolin based geopolymer mixed with ordinary Portland cement and cured at high temperature and high relative humidity

Abstract

Geopolymer is a new type of eco-friendly cementitious material, and its superior drying and high temperature resistance has been widely recognized. The service performance of geopolymer under 150 °C high-temperature hydrothermal conditions is still less discussed. In this paper, the mechanical strength of pure metakaolin system with low calcium content and metakaolin-cement system with high calcium content under hydrothermal and non-hydrothermal conditions were studied. The results show that under 150 °C hydrothermal conditions, the strength of pure metakaolin geopolymer sharply decreases by reduction rate of 81.8% compared to the sample under 150 °C drying conditions, while the strength of metakaolin-cement geopolymers is well retained with only a slight decrease of 14.4%. This is mainly because the predominantly hydration product sodium aluminosilicate (N-A-S-H) gel of pure metakaolin system undergoes the process of “dissociation–repolymerization–crystallization” under 150 °C hydrothermal conditions, resulting in the loss of cementation ability and obvious deterioration of mechanical strength. In the metakaolin-cement system, the high-calcium calcium silicate gel (C-A-S-H) gel maintains a stable structure, thereby maintaining the macroscopic strength of the material under the hydrothermal conditions.