Use of indurated laterites and lateritic soils as ferro-kaolinites for the synthesis of geopolymer materials: a comparative study

Indurated laterites are hardened layers within the laterite profile containing a gravel component. Lateritic soils, on the other hand, are the finer-grained material forming the uppermost part of the laterite profile, lacking the gravel component. The objective of this research work is the comparison of the compressive strength and microscopic properties of geopolymers from lateritic soils and indurated laterites. Three samples of each type of laterite were calcined at 700 °C and used as ferro-kaolinites. The chemical reagent used is a sodium waterglass with a molar ratio of SiO₂ to Na₂O of 1.6. Chemical composition demonstrates that the Fe₂O₃ content in indurated laterites is between 36.46 and 37.60 wt. %. Fe₂O₃ ranges from 17.32 to 22.48 wt. % in lateritic soils. Compressive strengths of geopolymers from indurated laterites are 29.87, 49.63 and 57.17 MPa, while those from lateritic soils are 41.75, 43.21 and 44.51 MPa. From the ⁵⁷Fe Mössbauer spectra, the relative proportions of hematite in the geopolymers using indurated laterites are 73.9, 72.1 and 64.4%. In contrast, the relative percentages of hematite for geopolymers from lateritic soils are 54.0, 33.8 and 26.9%. The mass percentages of coordinated Fe³⁺ in geopolymer materials from lateritic soils are 42.5, 29.0 and 15.5%, whereas those from indurated laterites are 6.8, 5.0 and 5.1%. It can be established that the values of compressive strength of the geopolymers from the indurated laterites are higher than those from the lateritic soils. In contrast to lateritic soils, the ⁵⁷Fe Mössbauer results support a higher hematite and a lower coordinated Fe³⁺ content in the geopolymers derived from indurated laterites.