Properties and microstructure evolution of Pisha sandstone after water/alkali leaching

Abstract

Alkali-activated aluminosilicate minerals for the production of geopolymer building materials provide low-carbon and low-energy advantages compared to OPC. Pisha sandstone (PS) is a mudstone/sandstone rich in silica and aluminium, but it cannot be used directly as an engineering material due to the expansion of soggy clay minerals. In this study, the PS was modified via water and alkali leaching pretreatments, and the modified PS was used to investigate the feasibility of eliminating or attenuating the swelling of montmorillonite, and the alkali activation behavior of the PS system, respectively. The stirring frequency, dosage and types of leaching agent, and leaching time were considered as the variables. The ion concentration of the leachate was characterized using an inductively coupled plasma mass spectrometer (ICP-MS). The modified and untreated PS were characterized using a range of analytical techniques, including particle size analysis, powder XRD, SEM/EDX, TGA, and its free swelling volume in water, as well as the mechanical properties of the specimens prepared as cylinders. The results show that water leaching reduces the particle size of PS, diminishes the span of its particle size distribution, and decreases the relative amount of montmorillonite in PS. Montmorillonite undergoes dissolution/ion exchange under alkali leaching. With increasing alkali concentration and leaching time, the phase changes from Ca-montmorillonite to Na-montmorillonite to glassy phase silica-aluminate. The amorphous N-A-S-H and C-A-S-H coexist in the system, N-A-S-H is the major hydration product. PS is still classified as a material with low pozzolanic activity, as the amount of geopolymer produced in the system is very low. The degree of water-absorption swelling and cracks on the specimen surface induced by the ASR gel deepened with increasing alkali concentration. The involvement of reactive Al and reactive Ca converted the ASR gels into (N,C)-A-S-H gels or C-(A)-S-H gels, which depended on the atomic ratios of these reactive ions, and significantly improved the strength and water resistance of the specimens.