Insights on the synergistic dispersion of sodium hexametaphosphate and sodium silicate in poured earth concrete

Poured earth is increasingly attracting interest in the construction sector due to its intrinsic lower carbon footprint and easy processing stemming from the liquidity achieved by the usage of clay dispersants. In this paper, the reactions between phosphate and silicate species, and the associated dispersion mechanisms in liquid mixed formulations, were investigated using ATR-FTIR, NMR, and zeta potential. The results show that the co-presence of hexametaphosphate (HMP) and silicate (Sil) produces a synergistic dispersion effect, increasing the dispersing effect compared to either HMP or Sil used alone at comparable dosage. The synergistic effect arises from the inherent structures of the phosphate and silicate groups: phosphate preferentially adsorbs at the edge of the clay mineral, thereby exerting electrostatic repulsion, while silicate provides steric repulsion. By combining, the newly formulated HMP-Sil performs either electrostatically or electro-sterically. NMR measurements reveal that the polyphosphate groups undergo an initial deprotonation followed by gradual dissociation into smaller phosphate species over time, whereas silicate chains remain largely unchanged. Despite this reaction path, the formulated HMP–Sil dispersants retain their dispersing efficiency for at least six months. Finally, the applicability of HMP-Sil was further evaluated using representative illitic and smectitic clays, confirming its high dispersing efficiency for a broader range of clay-rich earth materials.