Prospects of gibbsite-rich laterite as a source of aluminosilicates in geopolymerisation

Abstract

Laterite, an iron-rich soil widely found in the tropical and subtropical regions of the world, has shown promise for the development of eco-friendly construction materials through geopolymerisation. However, this material varies greatly in composition based on location, prevailing climate conditions, and even in depth within a given lateritic profile. The top layer of most lateritic profiles is usually low in kaolinite but rich in aluminium or iron hydroxide minerals. Despite these variations, research on the use of laterite in geopolymerisation has predominantly focused on materials rich in kaolinite. Therefore, this study explores the potential of aluminium-rich laterite as a source of aluminosilicates in geopolymerisation. In this study, the reaction kinetics, setting times, flow behaviour, strength development, phase composition, and pore structure of geopolymer derived from aluminous laterite were examined. This study also considered both calcined and uncalcined laterite as well as the influence of calcium minerals, namely calcium carbonate (CaCO3) and Portland cement, which replaced 40% of the laterite. In addition, the influence of the laterite’s properties on the performance of the derived geopolymer was also examined. The flow behaviour of the paste was found to be influenced by the viscosity of the activating solution, while the setting times and heat of reaction varied according to the type of laterite and the presence of calcium carbonate or Portland cement, which reduced the setting times and accelerated the rate of heat liberation within the first hour of the isothermal calorimetry test. The geopolymer mix based on calcined laterite displayed the highest amount of heat liberated, while its uncalcined laterite counterpart showed the lowest. All mixes within the calcined laterite series exhibited higher compressive strength than those in the uncalcined series, but only the calcined laterite mixes containing calcium minerals achieved structural strength. The uncalcined laterite mixes experienced strength regressions, with samples of the uncalcined laterite mix containing calcium carbonate developing cracks and subsequently disintegrating. The phase assemblage, porosity and pore structure were also influenced by the type of laterite and the presence of calcium carbonate or Portland cement. However, the presence of calcium carbonate also led to severe efflorescence and subflorescence, which negatively impacted the porosity and structural integrity. Also, the dissolution of gibbsite initiated the development of unstable phases in the uncalcined laterite mixes within the geopolymer and hybrid categories. Conversely, in the absence of activators, especially sodium hydroxide, as demonstrated in the binary mix containing uncalcined laterite, the dissolution of gibbsite is inhibited, resulting in the formation of stable phases.