Dissolution Characteristics and Microstructure of Waste Pisha Sandstone Minerals in Alkaline Solutions

Abstract

Waste Pisha sandstone (WPS) is the main damming material for the check dam in the Loss Plateau of northwest China. The dissolution characteristics of WPS in alkaline solutions were investigated as a basis for studying WPS modification materials and revealing the modification mechanism to further study the pozzolanic activity of WPS and the development of cementitious materials for concrete utilizing WPS. In this paper, WPS was milled and calcined at 600 °C, 700 °C, and 800 °C, respectively. After that, the activated WPS was immersed in 0.05, 0.1, 0.5, 1.0, 2.0, and 5.0 M NaOH solutions for leaching tests. The two curing temperatures of 20 °C and 80 °C were set, respectively. The ion concentration of Si, Al, and Ca in the alkaline solutions was determined using chemical titration, silicon–molybdenum blue colorimetric method, and graphite furnace atomic absorption spectrometry, respectively. After the leaching tests, the residues of WPS were characterized using XRD and SEM-EDS. The results show that the concentration of each ion in the leachate did not increase with leaching time but showed fluctuating variations with leaching time. Mechanochemical activation and thermal activation will promote the dissolution of minerals in alkaline solutions and increase the leaching efficiency of Si. However, the soluble Si in the leachate is not able to generate further gelling-like substances, limited by the total amount of available Ca in the mineral. WPS can dissolve more ions in higher concentrations of a NaOH solution, but the mineral crystallinity of its residue will be reduced. Higher curing temperatures can greatly increase the leaching efficiency of Si in a short time, which is better than thermal activation, and it can also promote the generation of newborn minerals and increase the crystallinity of minerals in WPS after leaching.