Effect of vacuum dehydration on gel structure and properties of metakaolin-based geopolymers

In aluminosilicate mineral materials, water plays a significant role during the process of geopolymer reaction and is related closely to the development of the strength and structure. In this work, a geopolymer is synthesized via the reaction of metakaolin with alkaline silicate solutions, including water glass and a NaOH solution. This study investigated the effect of water on the gel structure and properties of metakaolin-based geopolymers. Geopolymer samples were found to fast release water under vacuum and allowed the development of low-defect porous materials. After 3 days of curing in suitable conditions, the samples were treated under vacuum, during which the compressive strength did not decrease. Under vacuum at 120 degrees C, the residual water of the sample produced using water glass was 8.31% and that of the sample produced using NaOH solution was only 1.08%. The results of XRD and FTIR demonstrated that the sample produced using NaOH solution consisted primarily of NaA zeolite. SEM analysis indicated that the pores in sample growed with increasing H2O/Na2O molar ratio. Additionally, vacuum dehydration can suppress the formation of micro cracks and reduce the shrinkage of the samples.