In depth characterization of the addition of an alumina by-product into the magnesium phosphate cementitious matrix formulated with magnesium oxide by-product

This study investigates the incorporation of an alumina-rich by-product, PAVAL®, into a magnesium phosphate cement matrix formulated with low-grade magnesium oxide (LG-MgO), aiming to valorize industrial residues while improving the sustainability of chemically bonded cements. The research focuses on characterising the potential chemical interaction between aluminium species in PAVAL® and the K-struvite matrix, and evaluating whether aluminium interacts with matrix providing a better bonding. Micromortars containing up to 35 wt% PAVAL® were prepared and characterised using XRD, FTIR-ATR, BSEM-EDS, 27 Al and 31 P MAS-NMR, and TG/DTG. The addition of PAVAL® significantly reduced 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 from 25.3 MPa (0 wt% PAVAL®) to a maximum of 40.5 MPa at 17.5 wt% PAVAL®. Although XRD and NMR did not confirm the formation of crystalline aluminophosphates, FTIR-ATR spectra showed phosphate band shifts, and BSEM-EDS mapping revealed Al-rich particles embedded within the K-struvite matrix, with signal overlap at particle boundaries. 31 P MAS-NMR and TGA techniques confirmed the formation of amorphous Mg 2 KH(PO 4 ) 2 ·15H 2 O, 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 expected amount in 35 wt% PAVAL mortars, while the Al(OH) 3 content also decreased by 3.5 % under the same comparison, suggesting partial solubilization and subsequent retention of aluminium as Al(OH) 4 - through interaction with phosphate species. These findings support Al may influence the matrix through physical embedding, surface adsorption or diffusion in matrix, although the formation of defined aluminophosphates remains inconclusive under the studied conditions. • Alumina by-product addition accelerates magnesium phosphate cement setting. • XRD and FTIR-ATR do not confirm clear interaction between alumina and K-struvite. • More alumina by-product increases amorphous phosphate formation in the matrix. • NMR confirms K-struvite and disordered phosphates in alumina-rich samples. • Al(OH) 3 solubilization reduces K-struvite in high alumina by-product mortars.