Effect of Na/Al on formation, structures and properties of metakaolin based Na-geopolymer

Na-Geopolymer (Na-GP) has been prepared by using metakaolin as the main raw material and sodium silicate solution as alkaline activators. Sodium hydroxide (NaOH) was used to adjust the molar ratio of Na/Al and the effect of Na/Al on heat evolution of Na-GP was measured. The increase of Na/Al could increase the geopolymerization reaction rate and the cumulative heat release. A five-stage reaction process similar to that of cement hydration is proposed to describe the formation process of Na-GP according to the heat evolution results. Elevated temperature can further promote the early reaction rate. However, cumulative heat release decreased with the increase of temperature, indicating the reaction extent has been strongly limited because of rapidly setting and hardening of Na-GP at elevated reaction temperature. Hardened Na-GP pastes showed mesoporous structures with high porosity and there was an obvious decrease trend in mesopore sizes by increasing Na/Al. The effect of Na/Al on mechanical properties of Na-GP pastes and mortars was also tested. High mechanical properties of Na-GP pastes and mortars could be rapidly obtained in the early stage and would slightly increase with the prolonged curing time. Mechanical properties of the Na-GP mortar at each age showed a trend of firstly increase and then decrease with the increase of Na/Al. However, slight increase in mechanical properties has been observed for the mortar after 360 days. (C) 2019 Elsevier Ltd. All rights reserved.