Early-age formation of MgO-SiO$_2$ binders and the effect of hydromagnesite and dehydrated hydromagnesite: An in situ synchrotron X-ray diffraction and pair distribution function study

MgO-SiO$_2$ binders are promising low-CO$_2$ alternatives to Portland cement that can permanently store CO$_2$ by incorporating magnesium carbonate phases. Adding a magnesium carbonate such as nesquehonite or hydromagnesite is known to change the initial hydration product from crystalline brucite (Mg(OH)$_2$) to disordered hydrous carbonate-containing brucite (HCB), yet how these binders evolve during the initial hours of reaction remains largely unknown. Here, in situ synchrotron X-ray diffraction (XRD) and pair distribution function (PDF) analysis follows an MgO-SiO$_2$ paste with and without hydromagnesite (HY, Mg$_5$(CO$_3$)$_4$(OH)$_2$$\cdot$4H$_2$O) or dehydrated hydromagnesite (DHY, HY heated at 325 °C to remove water) over the first 24 h, with ex situ measurements to 42 days. Real space phase quantification reveals that magnesium-silicate-hydrate (M-S-H) is detected in the first scan (~45 min) of every paste, before formation of any brucite or HCB. The PDF results also show that in the neat MgO-SiO$_2$ paste, half of the M-S-H present at 42 days has formed by 24 h, while no M-S-H is detected by XRD at any age due to the intrinsic disorder present in this phase. Both carbonate sources (HY and DHY) are found to increase the extent of MgO dissolution, where no unreacted MgO is detected at 24 h in contrast with >7 days for the neat paste. However, DHY is seen to slow the initial stages of MgO dissolution (prior to ~6 h) compared with the neat paste, a period during which significant DHY rehydration is seen to occur without any HCB formation. Hence, this investigation shows that the onset of HCB precipitation in MgO-SiO$_2$-(D)HY paste is influenced by the hydration state of the DHY phase, where precipitation is delayed while DHY is rehydrating.