High-Bond-Strength, Water-Resistant Magnesium Oxychloride Cement-Based Inorganic Adhesive Inspired by Adsorbed Heavy Metal Ions

Magnesium oxychloride cement (MOC) has good compressive strength, cures in air, adheres to the surface of many materials including wood, and is being developed as a wood adhesive; however, MOC has poor water stability and easily loses mechanical strength and bonding ability in aqueous environments, and the penetration of wood is nonhomogeneous. Inspired by the adsorption of heavy metal ions, diethylene triamine pentaacetic acid (DTPA) was introduced to improve the water stability and homogeneous penetration of MOC composites. The −COOH of DTPA has a chelating effect on Mg2+, and the 5-phase crystals formed during hydration of MOC composites grow in situ on DTPA, which protects the 5-phase crystals from hydrolysis in aqueous environments. The physical anchoring effect and the interaction with the hydrogen bonding provided better interfacial bonding of the MOC composites when used as wood adhesives. The compressive strength of MOC with 5% DTPA reached 31.23 MPa after 7 days of water soaking, 774.79% higher than that of pure MOC. The MOC adhesive with 1% DTPA achieved a wet lap strength of 2.36 MPa, 59% higher than that of the pure MOC adhesive. Overall, DTPA can significantly improve the water stability and homogeneous penetration of MOC composites and can be applied as a wood adhesive, which provides theoretical support for developing and utilizing formaldehyde-free adhesives for wood construction.