Specimen size effects on the mechanical behaviors and failure patterns of the mine tailings-based geopolymer under uniaxial compression

Mine tailings (MTs) with rich aluminosilicates can be successfully used in the production of geopolymer via alkaline activation; and MTs, through geopolymerization are being used as an alternative in pavement and building construction materials, including concrete and bricks. Besides the factors such as the Si:Al ratio, Si/Al: NaOH ratio, and solid/liquid ratio that influence the compressive strength of the geopolymer, the specimen size also plays a major role. However, the effects of the specimen size on the compressive strength, the damage process, and the failure mechanisms of geopolymerized MTs under unconfined compression largely are unexplored. In the study presented in this paper, the influence of the cubic specimen size on the compressive strength was investigated. Three different water/mine tailing (W/MT) ratios of 13%, 16%, and 20% were selected around the optimum moisture content of 15.4% for the MTs. The fracture and damage mechanisms of the geopolymer specimens under unconfined compression were analyzed and the stress–strain relationships under unconfined compression for different sizes were determined. The compressive strength of the geopolymer was found to decrease monotonically with increases in the specimen size for the selected W/MTs ratios. It was observed that the failure strain of the geopolymerized specimens increased with the specimen size for W/MT ratios smaller than or approximately equal to the optimum moisture content of the MTs; and this trend was reversed for W/MT ratios greater than the optimum moisture content. The fracture pattern was not found to be dependent on the specimen size; and splitting cracks and column-like cracks were found to be the major fracture patterns for all the selected specimen sizes. An analytical solution based on energy release theory was derived that was found to capture the specimen size effects. Finally, the microstructural behaviors were observed and revealed that geopolymerization and carbonization contribute to the cementation.