Influence of the synthetic calcium aluminate hydrate and the mixture of calcium aluminate and silicate hydrates on the compressive strengths and the microstructure of metakaolin-based geopolymer cements

The main goal of this study is to investigate the effects of the calcium aluminate hydrate (CA04) and the mixture of calcium aluminate and silicate hydrates (CA04 + CS04) on the compressive strengths and the microstructure of metakaolin-based geopolymer cements. Bauxite, rice husk and eggshells were calcined and mixed in proper amounts to prepare CA04 and CS04. Metakaolin has been substituted by 0, 5, 10, 15 and 20 wt% of CA04, 10 and 20 wt% of (CA04 + CS04) in the formulation of geopolymer cements. The clay-like solid materials were characterized by measuring their compressive strengths, the crystalline and amorphous phases were monitored using the X-ray diffractometry analysis, their microstructures were investigated using the scanning electron microscope and mercury intrusion porosimetry and the functional groups were studied using the infrared spectroscopy. The compressive strengths of geopolymer cements containing 0, 5, 10, 15 and 20 wt% of CA04 are 49.50, 57.17, 63.59, 38.79 and 35.05 MPa, respectively. Those containing 10 and 20 wt% of (CA04 + CS04) are 40.59 and 51.19 MPa, respectively. The average pore diameters of geopolymer cements containing 0, 10 and 20 wt% of CA04 are 11.80, 10.10, 12.20 nm, respectively. Whereas the one containing 20 wt% of (CA04 + CS04) is 10.80 nm. It appears that geopolymer cement with 10 wt% of CA04 and 20 wt% of (CA04 + CS04) have higher compressive strengths and lower average pore diameters. It was found that 10 and 20 wt% of CA04 and (CA04 + CS04), respectively, could be used as additives for the preparation of geopolymer cements with higher strength development. •Calcium aluminate hydrate was prepared using calcined bauxite, calcined eggshell and rice husk ash.•Calcium aluminate and silicate hydrates were obtained by mixing CAH and CSH.•Metakaolin was substituted by different amount of CAH and CAH + CSH.•The strengths increase from 49.50 to 63.59 MPa when metakaolin were substituted by 0 and 10 wt% of CAH.•The strengths increase from 40.59 to 51.19 MPa when 10 to 20 wt% of CAH + CSH were used as additives.