In depth characterization of the addition of an alumina by-productinto the magnesium phosphate cementitious matrix formulatedwith magnesium oxide by-product

Abstract

This study investigates the incorporation of an alumina-rich by-product, PAVAL®, into a magnesiumphosphate cement matrix formulated with low-grade magnesium oxide (LG-MgO), aimingto valorize industrial residues while improving the sustainability of chemically bonded cements.The research focuses on characterising the potential chemical interaction between aluminiumspecies in PAVAL® and the K-struvite matrix, and evaluating whether aluminium interacts withmatrix providing a better bonding.Micromortars containing up to 35 wt% PAVAL® were prepared and characterised using XRD,FTIR-ATR, BSEM-EDS, 27Al and 31P MAS-NMR, and TG/DTG. The addition of PAVAL® significantlyreduced the setting time from 84.5 min (0 wt% PAVAL®) to 16.5 min (35 wt% PAVAL®),which is a beneficial outcome for rapid applications, and enhanced compressive strength from25.3 MPa (0 wt% PAVAL®) to a maximum of 40.5 MPa at 17.5 wt% PAVAL®. Although XRD andNMR did not confirm the formation of crystalline aluminophosphates, FTIR-ATR spectra showedphosphate band shifts, and BSEM-EDS mapping revealed Al-rich particles embedded within the Kstruvitematrix, with signal overlap at particle boundaries.31P MAS-NMR and TGA techniques confirmed the formation of amorphous Mg2KH(PO4)2⋅15H2O, increasing from 0.0 wt% (0 wt% PAVAL®) to 4 wt% (35 wt% PAVAL®).Concurrently, the K-struvite content decreased by 7.1 % compared with the theoretically expectedamount in 35 wt% PAVAL mortars, while the Al(OH)3 content also decreased by 3.5 % under thesame comparison, suggesting partial solubilization and subsequent retention of aluminium as Al(OH)4- through interaction with phosphate species. These findings support Al may influence thematrix through physical embedding, surface adsorption or diffusion in matrix, although theformation of defined aluminophosphates remains inconclusive under the studied conditions.