Inferring the effect of chemical additives on early-stage hydration and phase assemblage of magnesia-clay-based cement

Early-stage hydration of magnesia-clay-based (MCB) cements is largely governed by pore-solution chemistry. Despite early-stage reactions within the first 48 h accounting for over 90% of heat release, the influence of chemical additives on this window remains unreported. In this study, isothermal calorimetry, time-resolved pH, ionic conductivity, dissolved Mg, Si, and Al concentrations, XRD, and 27 Al/ 29 Si MAS NMR are used to identify how citric acid (CA), sodium citrate (SoCi), and sodium carbonate (SC) modify MCB cement suspensions' hydration kinetics and phase assemblage. Results show that CA amplifies heat release via dissolved Mg species, driven by acid-induced surface protonation of MgO, but limits metakaolin dissolution at low pH, yielding only brucite at 48 h. SoCi sustains high alkalinity and suppresses brucite, with Mg(citrate) − as the dominant computed Mg species. It promotes hydrotalcite by 24 h and trace Al incorporation into the precursor M-A-S-H network. SC produces the most pronounced heat-flow maximum and the most extensive Al(IV) incorporation into M-A-S-H. This coincides with MgCO₃(aq) as a dominant computed Mg species, and yields the highest hydrotalcite content, together with the strongest NMR- and XRD-inferred M-A-S-H signals (increased Al(IV) incorporation and Q 1 , Q 2 , Q 3 sites for Si) by 48 h. Overall, this evidence suggests that of the three additives, sodium citrate (2 wt%) best promotes MgO-metakaolin reaction while suppressing excess brucite formation within the first 48 h, whereas sodium carbonate (2 wt%) most effectively drives Al(IV) incorporation into the M-A-S-H structure. These findings inform a durable design pathway for this promising MCB cement.