Influence of Phosphates on Pore Solution pH in Alkali‐Activated MgO‐Al ₂ O ₃ ‐SiO ₂ ‐P ₂ O ₅ Cement

ABSTRACT Nuclear waste storage applications require a new type of cementitious material having lower pH than Portland cement and compatibility with embedded steel. Since phosphates can be used as both pH buffers and corrosion inhibitors in carbon steel, a series of phosphate‐containing alkali‐activated materials were prepared from MgO‐Al 2 O 3 ‐SiO 2 ‐P 2 O 5 precursor materials with various MgO, Al 2 O 3 , SiO 2 , and P 2 O 5 concentrations. These samples were activated with sodium hydroxide and cured for 28 days at 35°C. The impact of precursor phosphate content on pore solution composition and pH was examined using cold‐water extraction (CWE) and Gibbs energy minimization (GEM) modeling. For samples activated with the same amount of sodium, higher phosphate content in the precursor material leads to lower pH in the pore solution. When the pore solution is saturated with sodium phosphate and the soluble Na/P ratio is between 2 and 3, the pH decreases from 15.1 ± 0.1 in the absence of phosphates to 11.0 ± 0.1 with phosphates. The resulting pore solution and a precipitated mixture of di‐ and tri‐sodium phosphate have a buffering effect on the pore solution's pH and Na + , keeping the pH and Na + concentration near 11.0 and 1.0 mol/kg, respectively. This pH is within the range desired for certain nuclear waste storage applications.