Pore structure and sorption of MgO-hydromagnesite cement pastes – on the mechanisms of moisture-induced weakening

Magnesia-based binders are emerging as sustainable alternatives to Portland cement. A special class are blends of MgO with hydrated magnesium carbonates, such as hydromagnesite (HY), which can reach mechanical properties comparable to Portland cement. However, the moisture sensitivity of these binders raises concerns for practical use. This study investigates the water sorption behavior and microstructural evolution of MgO/HY cement pastes with varying blend ratios (70/30, 80/20, 90/10 by mass), alongside pure MgO and Portland cement references. Samples were exposed to relative humidity conditions of 0–100% RH and characterized using 1 H Nuclear Magnetic Resonance (NMR) T₂ relaxometry, supported by chemical analysis and mercury intrusion porosimetry measurements. Results reveal a very high water uptake at high RH and corresponding significant widening of water-filled pores. The water adsorption and its impact on the pore structure explain the (reversible) decrease of strength and stiffness at high RH reported in previous studies.