Reactive MgO cement with activated recycled cement powder: particle packing and carbonation

Reactive MgO cement (RMC) turns carbonation into strength for lower-carbon binders. However, simultaneously improving its fresh properties, carbonation behavior, and hardened performance remains challenging. Here, thermally activated recycled cement powder (TARCP), calcined at 800 °C, is introduced into RMC mixtures. TARCP improves flowability by reducing yield stress, although excessive dosage increases apparent viscosity and limits further flow improvement through reduced water film thickness. TARCP also refines the pore structure and promotes densely packed hydromagnesite-like carbonation products, leading to a 28-day compressive strength of approximately 55 MPa. The RMC mixture containing 7.2 vol% TARCP shows approximately 20% lower embodied CO2 per unit strength after 28 days. However, the overall carbonation degree plateaus at higher TARCP contents, limiting further strength gain. These effects are governed by the physicochemical characteristics of TARCP and its influence on the granular skeleton, particularly through particle packing, water film thickness, Ca-Mg carbonation reactions, and product-layer formation. This study clarifies the coupled rheology-carbonation mechanisms of TARCP-RMC systems and provides guidance for designing lower-carbon MgO-based binders.