Investigation of Affecting Factors on Drying Shrinkage and Compressive Strength of Slag Geopolymer Mortar Mixture

In geopolymer mixes, the amount of shrinkage is (3-6 times) higher than the OPC. Excessive shrinkage can cause cracking, which is a threat to the durability of the concrete. To that end, a solution must be found to reduce it. Studies on the type and concentration of activators, type of base material and its properties, mixing proportions, applying different curing conditions and use of different additives are among the solutions investigated to decrease the drying shrinkage of geopolymers in previous research. In this study, after a comprehensive investigation, the effects of magnesium oxide, calcium hydroxide, sodium carbonate, and Portland cement additives as well as heat treatment and moist curing conditions were evaluated in geopolymer mortar containing slag activated with sodium hydroxide and sodium silicate. Finally, the performance of polypropylene fibers was examined as well. The effects of mentioned options on compressive strength were also investigated. The results indicate the different behaviors of additives on drying shrinkage and compressive strength depending on their type, substitution content, and age. Eventually, it seems that among the available alternatives, the most effective option is to use heat treatment with complete control of the heat rate. With this approach, not only the amount of drying shrinkage reduction will reach 66% but also the early compressive strength will be increased significantly. Although the use of magnesium oxide, sodium carbonate, and calcium hydroxide improved the compressive strength of the samples, relatively different results regarding shrinkage were observed in the use of these materials. Time-dependent behavior was also evident in their influence process. Polypropylene fibers also caused a slight decrease in compressive strength (less than 7.5%) probably due to the reduced compaction capacity of fresh mortar, but it was effective in controlling shrinkage in the short term.