Climatic conditions in plateau areas can enormously affect the properties and microstructure of cement-based materials. This research investigates the strength development and microstructural changes in magnesium phosphate cement (MPC) mortars in a plateau environment. Experiments were conducted in parallel in a plateau area (Lhasa) and a plain area (Chengdu) to evaluate the effects of the water-to-binder ratio (w/b = 0.12, 0.14 and 0.16) and sand-to-binder ratio (s/b = 0.5, 0.75 and 1) on the compressive and flexural strength of MPC mortars. At the same time, hydration products were characterized via XRD, TGA, and SEM/EDX micro-analyses, and the porosity of the materials was also analyzed via MIP. The results demonstrated that curing in a plateau environment resulted in a decrease in workability and yielded higher strength at an early age (before 1 day) but degraded the long-term (180-day) strength of MPC mortars when compared with curing in a plain environment, irrespective of w/b and s/b ratios. Unlike the plain group, the plateau group revealed the deterioration of microstructures over time, including the decrease in struvite content, the morphology change in struvite crystals, and the increase in porosity, which resulted in the degradation of mechanical properties between 1 and 180 days. The strength loss can be effectively alleviated at lower w/b and s/b ratios.

There are some limitations in the application of tuff powder as a supplementary cementitious material (SCM). Exploring its feasibility in new fields will consume a large amount of silica-alumina mine solid wastes. This study has investigated the mechanical properties and mechanism in contact-hardening of tuff powder with a method of compression molding. The compressive strength of specimens was tested, and the X-ray diffraction (XRD), thermogravimetric analysis (TG), scanning electron microscopy (SEM), and Mercury intrusion porosimetry (MIP) methods were used to reveal the mechanism of contact-hardening of tuff powder from a micro-perspective. The results indicated that the compressive strength of specimens was higher when activated by sodium hydroxide compared to calcium hydroxide. Compared to calcium hydroxide, the compressive strength of TFS20 and TFF20 activated by sodium hydroxide was improved by 20% and 23%, respectively. The hydration degree of tuff powder was very low, with a water–cement ratio (w/c) of 0.15, while the hydration degree of coal gangue powder was higher. The results of TGA and SEM indicated that the sodium hydroxide had a better activating effect on slag and fly ash. Therefore, more C-S-H gels were generated in those samples activated by sodium hydroxide. Furthermore, the structure of samples was more compacted, and there was a reduction of porosity by 10% and 11% for TFS20 and TFF20, respectively, especially the proportion of harmful pores.