Investigation of mechanical properties and hydration of low-carbon magnesium and calcium-rich waste powder geopolymer paste

Magnesium and calcium-rich waste powder (MWP) has the potential to be a low-carbon geopolymer cementitious material. This study investigates the mechanical properties and hydration products of low-carbon magnesium and calcium-rich waste powder geopolymer paste (LMWP). The influences of alkali content, calcination temperature, mix proportions of raw materials and curing temperature on the compressive strength and hydration of LMWP were examined. The mechanical properties of LMWP were systematically evaluated by assessing setting time, fluidity, and compressive strength, while the pore structure was analyzed using mercury intrusion porosimetry (MIP). The hydration products and microstructures of LMWP were investigated by XRD, TG-DTG, and SEM-EDS. The results indicated that incorporating 1 % NaOH significantly enhanced the compressive strength of LMWP, whereas thermally activated MWP (800 ℃, 900 ℃) negatively affected compressive strength development. The addition of slag facilitated the reaction of MWP and improved the compressive strength of LMWP. When the slag incorporation reached 40 %, the specimen demonstrated optimal performance with a compressive strength of 27.8 MPa. The pore diameter was predominantly distributed around 10 nm, indicating well-structured porosity. Microstructural analysis revealed that the hydration products are dense calcium magnesium silicate gels (C-M-S-H), which significantly enhanced the compressive strength and optimized pore structure of LMWP. The efficiency of carbon emission reduction achieved by LMWP was evaluated. The findings indicate that, compared to traditional cement-based materials, LMWP reduces cement consumption by over 60 %, significantly decreasing CO₂ emissions. This study innovatively utilizes MWP to prepare green and low-carbon geopolymer paste materials, with the aim of replacing cement applications in the construction industry, thereby reducing carbon emissions. It explores new avenues for the low-carbon and green development of the civil engineering sector and contributes to efforts in addressing the global climate crisis.