Performance of magnesium hydroxide gel at different alkali concentrations and its effect on properties of magnesium oxysulfate cement

This paper investigated the influence of the alkali concentration on the nanostructure of synthetic magnesium hydroxide and examined the effect of magnesium hydroxide on the performance of magnesium oxysulfate (MOS) cement at different magnesium-to-sulfur molar ratios (Mg/S). Magnesium hydroxide precipitated at different pH values, namely 9.4, 9.6, 9.8, 10.0, 10.2, and 10.4, was obtained by adding different amounts of sodium hydroxide solution to magnesium sulfate solution. The results showed that microcrystalline or nanocrystalline structures formed in all the samples were the magnesium hydroxide gel and MgO–MgSO4–H2O gel. Increasing the pH reduced the content of the magnesium hydroxide gel but increased the content of the MgO–MgSO4–H2O gel, but the specific surface area of the magnesium hydroxide gel contributed the most, a higher specific surface area leads to a lower thermal stability. According to FT-IR and TG-DTG results, MgO-MgSO4-H2O gel exhibited a high water content. In addition, magnesium hydroxide preferred orientation in the case of small crystal size and tended to grow toward crystal plane (0 0 1). It was also found that the proper amounts of the magnesium hydroxide gel and MgO–MgSO4–H2O gel were beneficial to the mechanical properties of the MOS cement. The cement samples with a reactive-magnesium-to-sulfur ratio of 5.0 and 8.0 had a ratio of the crystal phase to the amorphous phase of 2.91 and 1.29 respectively and compressive strength of 65.4 and 89.6 MPa respectively. In addition, the higher alkali concentration environment will also lead to lattice distortion of 517 phase and lower thermal stability of 517 phase.