Influence of gibbsite and quartz in kaolin on the properties of metakaolin-based geopolymer cements: Fid-Bau Portal

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

Tchakoute, H.K. / Rüscher, C.H. / Djobo, J.N.Y. / Kenne, B.B.D. / Njopwouo, D.

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.

Highlights • Presence of gibbsite in kaolin affects the chemical structure of its metakaolin. • A large amount of gibbsite in kaolin reduces the overall rate of network formation. • Presence of quartz in raw material did not hinder the geopolymerization. • The reacted volumes and compositions of the geopolymer become almost the same.