Design and fabrication of photocatalytic magnesium oxychloride cement with improved moisture stability: A step towards sustainable construction

The photocatalytic magnesium oxychloride cement (MOC) represents a promising alternative building material due to its advantageous properties, such as high compressive strength, and reduced environmental impact. In addition, photocatalytic cements promote self-cleaning, air purification, and antimicrobial properties compared to traditional cements, giving them an additional advantage. The MOC cement requires few raw materials to be formed (MgO, MgCl2, TiO2, and H2O) at room temperature; however, its moisture stability remains a challenge. As alternative, this work proposes the fabrication of photocatalytic self-cleaning and antimicrobial MOC samples with optimized moisture stability through the modifications of the MgO/H2O molar ratios (M/H) and using low-cost additives, e.g., citric acid (CA) and fly ash (FA) by a multivariable factorial design. The characterization of the MOC samples showed needle-like morphology, characteristic of the phase 5 [5Mg(OH)(2)(.)MgCl(2)(.)8 H2O] of this cement, that favored the formation of high surface areas (up to 328 m(2)g(-1)). The MOC samples were exposed to humid conditions to demonstrate its moisture stability, which results revealed that their mechanical performance was influenced by the M/H molar ratio, resulting in morphology changes. The addition of citric acid improved the moisture resistance of the MOC samples. This is supported by the increased moisture stability coefficient (MSC) values, which ranged from 0.59 to 0.69. The optimal conditions proposed were 5/15 M/H, 1%CA, 3%FA with 3%TiO2, which allow the design of a MOC with an initial strength of 39 MPa, high moisture stability and high photocatalytic activity (up to 100%) to remove four model pollutants: reactive black 5, methylene blue, rhodamine B, and methyl yellow under solar irradiation. Meanwhile, antimicrobial activity was demonstrated against the Escherichia coli, Staphylococcus aureus, Salmonella sp., and Listeria monocytogenes bacteria, showing high zones of inhibition > 22 mm under visible light. Overall, this study demonstrates the potential of MOC as a sustainable material with enhanced photocatalytic activity and resistance to high humidity environments, making it suitable for outdoor applications.