Influence of gibbsite and quartz in kaolin on the properties of metakaolin-based geopolymer cements

Abstract

Three different kaolins (K1, K2, K3) from sources in Cameroon were applied for producing geopolymer cements. The kaolins differ significantly in their gibbsite and quartz contents. Thermal transformation (700°C, 4h) into their metakaolins MK1, MK2, MK3 shows the total loss of crystalline kaolinite and reveal the typically rather broad bump in the X-ray pattern. Gibbsite becomes dehydrated into γ and χ-Al2O3. Geopolymer cements (GP1, GP2, GP3) were obtained using freshly prepared sodium silicate solutions (NWG) with a ratio NWG/MK=0.87. It could be observed that the initial (60/80/90min) and final (90/140/160min) setting time increases and their 28day compressive strength (49/39/30MPa) decreases in the course GP1/GP2/GP3. It is discussed that the higher content of quartz in K1 (up to 22wt.%), compared to K2 (10wt.%) and K3 (8wt.%) promotes higher strength values and decreased setting times. Gibbsite was not present in K1, but up to 11wt.% in K2 and 28 wt.% in K3, transformed in its dehydrated forms remains unreacted during geopolymerization. Therefore, the higher content of gibbsite in the kaolinite could be related to a lower strength. The reacted volumes and compositions of the geopolymer become almost the same in all three cases. A content of 30–50% of unreacted metakaolin was proved in all cases.