Influence of phosphates on phase formation in alkali-activated MgO-Al2O3-SiO2-P2O5 cements

The impact of phosphates on phase formation in low Ca alkali-activated materials (AAMs) is investigated using a polymer-assisted sol-gel process to fabricate MgO-Al2O3-SiO2-P2O5 cement precursors covering a broad range of compositions activated with sodium hydroxide. X-ray diffraction and magic angle spinning-nuclear magnetic resonance are used to examine the crystalline and amorphous phases that form over 470 days of curing (35 degrees C, 90% relative humidity). The results confirm that Al is preferentially incorporated into hydrotalcite-like layered double hydroxides (LDH) over zeolites. Zeolites form when more Al is present than can be incorporated into the LDH. Little evidence of phosphate incorporation into aluminosilicate networks (such as zeolites or disordered aluminosilicate hydrate) was observed. The phosphates present in the precursor favor reaction with sodium to form water-soluble sodium phosphate phases. In most cases, the remainder of the phosphates become adsorbed to the surface of other phases and are not intercalated into the LDH, though at high phosphate concentrations (26.6 wt. % P2O5) and extended curing times (470 days), phosphates are observed to intercalate into LDH phases. These results provide preliminary evidence that phosphates are compatible with low Ca AAMs, which is consequential as there is a growing interest in both the use of AAM and phosphate-based corrosion inhibiter in steel-reinforced concrete.