The Mechanical Properties of Geopolymers from Different Raw Materials and the Effect of Recycled Gypsum

Geopolymers are amorphous inorganic polymers that are mainly used in the construction industry as an environmentally friendly alternative to ordinary cement. This study compared selected mechanical properties (setting time, shrinkage, strength) of geopolymer specimens made from different main raw materials, mainly at room temperature, and investigated the effects of recycled gypsum on these. A structural analysis of the specimens was conducted with XRD and SEM. Also, the leaching of aluminium, silicon, and calcium from the specimens was investigated. According to this study, raw materials have a significant impact on the properties of geopolymers. Recycled gypsum affected the setting time of the geopolymers, but the effect was not the same for all specimens. It increased the setting time of specimens made from calcium-rich raw materials, for example, and the ground-granulated blast furnace slag specimens hardened as fast as ordinary Portland cement (about 300 min), but the addition of gypsum decreased it to 1300 min. Gypsum-containing specimens, based on Ca-deficient metakaolin or fly ash, hardened even faster than OPC, in 100–150 min. Recycled gypsum significantly reduced the plastic shrinkage of most of the 28 d specimens to lower values than those achieved for OPC (0.07%). The only exceptions were the fly-ash-based specimens. However, gypsum had no effect on the drying shrinkage, which accounted for a larger proportion of the total shrinkage in most specimens. Therefore, it had no significant effect on the total shrinkage of the geopolymer specimens. The reducing effect of gypsum on the plastic shrinkage of geopolymers was attributed to ettringite, which was observed in all gypsum-containing specimens analysed with XRD. In this study, recycled gypsum decreased the compressive strength of the specimens, which could be prevented by using a finer gypsum powder.