Geopolymeric mortars through acid and basic activations formulated from slate mining waste

ABSTRACT Since the mid-1970s, geopolymer mortars and concretes have been produced through the alkaline activation of mixtures containing natural raw materials or wastes rich in silica (SiO2) and alumina (Al2O3), using a two-part processing method. In general, these inorganic polymers exhibit good mechanical performance, thermal resistance, and tribochemical properties. The literature has demonstrated that mining tailings (dust) from slate dams rich in aluminosilicates can be reused to obtain geopolymer composites through alkaline activation with potassium hydroxide (KOH) or sodium hydroxide (NaOH), or even with other basic aqueous solutions such as sodium silicates. Currently, there is an attempt in the literature to improve the synthesis of geopolymer compounds by acid activation, called silicoaluminophosphates, among the various aqueous phosphate solutions and others described in the literature. Therefore, food-grade phosphoric acid (H3PO4) at a concentration of 85% was selected for this exploratory study. The scope of this research was to evaluate the potential use of slate mining residue as a raw material in the synthesis of geopolymer composites using the two-part method for civil construction applications, such as mortars for the manufacture of auxiliary lightweight paving structures, masonry enclosures, and auxiliary urban elements. The raw material was optimized by grinding in a ball mill for periods of 15 minutes, 2 hours, and 7 hours, until reaching the desired particle size for geopolymerization. The materials were subjected to laser particle size analysis and physicochemical characterization by XRF, XRD, and SEM of both the raw material and the geopolymer specimens (PCs). During the processing of the PCs, the setting time was analyzed, and then their mechanical behavior was evaluated by compression tests at 28 days. The results of the mechanical compressive strength were satisfactory, showing increases in the alkaline mixture mixes (1 and 2), with corresponding values of 100% and 47.33%, respectively. In the acidic mixture, in mixes (1 and 3), strength reached increases of 91% and 93%, indicating potential for application in non-structural masonry. It was concluded that slate residue can be applied in infrastructure, urban sanitation, paving of public roads, and auxiliary paving structures, using geopolymerization techniques.