Effects of MgO-based cementitious binder on smectites

Abstract

Earth materials are subsoils used in construction due to their natural cementing properties. These properties originates from its clay fraction, which becomes cohesive during drying. Unlike common cement, earth materials are recyclable and have no CO₂ emission apart from manufacturing. Unfortunately, earth materials containing the abundant clay mineral smectite exhibit large swelling and shrinkage strains. Such earth materials are considered unsuitable for construction unless a stabiliser is added, which is commonly Portland cement or quicklime. This study explored the use of MgO-based cementitious Binder (MB) as alternative with a focus on the mineralogical effect of MB on smectite during hydration. A series of MB/smectite blends and MB components/smectite was cured up to six months to investigate the mineralogical changes and the formation of magnesium (alumino) silicate hydrate. The results showed that MB transformed smectite into Mg-hydroxy-interlayered smectite (Mg-HIS) within hours. The reason is the dissolution of MgO, a main constituent of MB. The dissolved Mg precipitates as Mg-hydroxy in the interlayer and transforms the smectite to Mg-HIS in this process. This is causing a pH increase and may prevent a complete HIS formation as the MgO dissolution mechanism will change once the pH is above the point-of-zero charge of MgO. An advantage of the smectite to Mg-HIS transformation is the strongly reduced the swelling/shrinkage properties of the clay. This suggests that adding MB to smectite could be a superior binding approach compared to quicklime, which primarily causes clay particle flocculation.