Influence of Calcium Bentonite Addition on the Compressive Strength, Efflorescence Extent and Drying Shrinkage of Fly-Ash Based Geopolymer Mortar

The influence of calcium bentonite addition on the compressive strength, efflorescence extent and drying shrinkage of fly-ash based geopolymer mortar was investigated. The results showed that appropriate amount (e.g. 15 wt%) of calcium bentonite addition favoured the enhancement of compressive strength mainly because its addition allowed the geopolymer to tolerate more water glass addition and therefore increased the Si/Al ratio of geopolymer. The maximum 28 d compressive strength of the geopolymer mortar with calcium bentonite addition reached 47.8 MPa, which was evidently larger than the maximum value of the specimen without calcium bentonite addition. Calcium bentonite addition was also beneficial for the reduction of drying shrinkage percentage of geopolymer mortar due to water adsorption and volume expansion properties of calcium bentonite. Moreover, calcium bentonite addition could reduce, even eliminate the efflorescence mainly due to Na+ adsorption by calcium bentonite through ion exchange.