Improving the Sustainability of Magnesium Oxalate Cements Using Clay and Calcium Sulfoaluminate Clinker

Abstract

Magnesium oxalate cement (MgOx) is an alternative binder that forms through the acid–base reaction between dead-burned magnesium oxide (MgO1500) and an acid salt containing oxalate ions. Previously, industrial wastes were used to replace MgO1500, and fly ash was used to produce oxalate acid salt. In an attempt to further reduce the carbon footprint of MgOx and use more abundant starting materials, this study used a calcium sulfoaluminate cement (CSA) clinker in place of MgO1500 and a natural, clayey soil to produce oxalate acid salt. The mineralogy and microstructure of pastes were studied, as well as the strength development of mortars. Although full replacement of MgO1500 with CSA yielded samples with 28-d compressive strength ∼30 MPa, it reduced ultimate pH from ∼9 to ∼5, shortened setting times, and negatively impacted strength retention after immersion in water. An equal-part mixture of MgO1500 and CSA gave optimal results. Glushinskite was detected as the main reaction product. No ettringite formed from ye’elimite in CSA due to the low pH of the system. This research contributes to the ongoing progress of an innovative construction binder, magnesium oxalate cement (MgOx), which was recently proposed as an environmentally friendly alternative to traditional portland cement. Portland cement is known for its significant carbon footprint, and MgOx aims to be low carbon or carbon neutral by incorporating CO2 indirectly into the mixture in the form of oxalates (C2O42−). The study evaluated the replacement of the two main components in conventional MgOx. Dead-burned magnesia, the source of magnesium ions, is replaced with a lower–carbon-footprint powder, whereas the oxalate ion source is prepared using an abundant natural material instead of a limited industrial waste. Most alternative binders that directly incorporate CO2 into the mixture do so by curing in a controlled temperature and humidity chamber, which limits the product to a precast shape. MgOx can be cast into any shape, on site, greatly increasing its versatility. It is also strong and resistant to water, similar to portland cement mixtures. By making MgOx cement more affordable and reducing its environmental impact, this research supports the construction industry’s shift toward greener building practices.