Micro-mechanisms and compressive strength of Geopolymer-Portland cementitious system under various curing temperatures

The excellent strength of low calcium fly ash-based geopolymer paste (GP) can only be attained when high heat curing (60–90 °C) is applied. To achieve reasonable strength in ambient curing temperature (20 ± 2 °C), an inclusion of OPC to GP (GeoPC system), has been studied to accelerate (C,N)-A-S-H formation to achieve an early strength development. This paper presents the micro-mechanisms and hence development of compressive strength of GeoPC under various curing temperatures (10–70 °C) to develop a better understanding and to determine the combined effect of OPC and GP on the GeoPC systems. The results exhibited that the strength of GeoPC system tended to be similar to the GP as strength was increased when the curing temperature increased. However, with the same level of curing in a moderate temperature (20–50 °C), the strength of GeoPC system was evidently higher than that of GP due to an extra precipitation of both C-(A)-S-H and N-A-S-H gel or after internal heat from OPC simultaneously during curing process, as well as its microstructures and mechanisms. It may be concluded that the GeoPC could achieve greater strength than that of GP when the same level of moderate curing temperature was applied, leading to less heat-cured energy consumption. The optimum curing was in a mild temperature range of 30–40 °C, which could be found in hot environment area throughout summer time. The production of GeoPC, a part of self-cured geopolymer, could therefore be extended to be more convenient in practical work and be commercialised in construction industry. •Strength and micro-mechanisms of GeoPC are comparatively increased when curing temperature increased.•GeoPC consumed less heat-curing energy than GP in the same level of strength achievement.•A mixed characteristic of OPC and GP was found in GeoPC at all curing regimes.•The optimum curing temperature of GeoPC mixtures was approximately 30–40 °C in this test.•GeoPC system could possibly produce a self-cured GP in a hot climate area or through summer time.