Drying shrinkage behavior of geopolymer mortar based on kaolinitic coal gangue

Abstract

The coal industry generated tons of waste over the years of mineral extraction, and the waste disposal leads to soil and air pollution caused by acid mine drainage and spontaneous combustion. Seeking environmental sustainability, this paper investigated the valorization of coal tailings as a primary precursor in geopolymer mortar. The effect of Portland cement as a hardened admixture was evaluated on the mechanical and physical properties of geopolymer mortar, by replacing the coal tailings with a lower amount of Portland cement. Six mortars were characterized by several standard tests besides ultrasonic pulse velocity, mercury porosimetry (MIP), and drying shrinkage, the microstructure was evaluated by SEM/EDS analysis and X-Ray diffraction. The workability was slightly higher for geopolymer mortars with calcium by almost 5%, and the addition of sugarcane molasses and flue gas desulfurization increased the fluidity, reaching 220 mm. The Portland cement improved the early strength of geopolymer mortar by almost 10 MPa at 7 days, due to the acceleration of chemical reaction yielding a denser and heterogeneous matrix. The SEM/EDS analysis evidenced the co-existence of the polymeric matrix and C-A-S-H gel. The compressive strength at 28 days was higher than 20 MPa for all geopolymer mortars. A higher reaction rate provided by calcium addition leads to higher densification of the aluminosilicate matrix at early ages. The ultrasonic pulse velocity (UPV) confirmed denser matrix formation, and the geopolymer with 05 wt% of Portland cement presented UPV of 2700 m/s at 07 days, while the non-calcium geopolymer showed 2000 m/s. The denser matrix formed limited the water loss, decreased the open porosity, and improved the stiffness of geopolymer mortars, yielding a drying shrinkage almost 50% lower than for geopolymer without calcium addition. Finally, this paper established the lower replacement (5 wt%) of coal tailings for Portland cement produced efficient geopolymer mortar, with excellent physical and mechanical properties for in situ application without a thermal cure.