Magnesium silicate hydrate (MSH) gel was formed by reacting magnesium oxide (MgO) with silica fume (SF) in distilled water. The MSH was prepared using a MgO/SF molar ratio of 1.0 (40:60 weight ratio). Samples were analyzed during hydration process up to 300 days at room temperature. The MSH characterization has been carried out using a range of analytical techniques. Quantitative analysis was achieved using thermogravimetric analysis (TG/DTG) with a de-convolution technology. The structure of MSH gel was characterized using solid state nuclear magnetic resonance (²⁹Si NMR) and Fourier transform infrared (FT-IR) spectroscopy. Transmission electron microscopy (TEM) was used to investigate MSH microstructure. Compared with natural magnesium silicate hydrate minerals, the structure of MSH gel is highly disordered and generates on the surface of particles, producing a shell structure with cavity. The molecular structure of MSH phase is layered. The results also show that the extent of polymerization of MSH gel is related to the solution pH during hydration.

The radionuclide cesium (Cs) was solidified using magnesium silicate hydrate (M–S–H) cement. The influence of Cs⁺ on the reaction of the M–S–H gel system was evaluated by measuring the compressive strength and microscopic properties of the solidified body. By testing the impact resistance, leaching resistance and freeze–thaw resistance of the solidified body, the immobilizing ability of Cs⁺ by the M–S–H cement was analyzed. Results indicate that Cs⁺ only slightly affects the reaction process of the M–S–H gel system, and only slows down the transformation rate of Mg(OH)₂ into the M–S–H gel to a certain extent. The M–S–H cement exhibits superior performance in solidifying Cs⁺. Both the leaching rate and cumulative leach fraction at 42 d were considerably lower than the national requirements and better than the ordinary Portland cement-solidified body. The curing effect of the M–S–H cement on Cs⁺ is mainly physical encapsulation and chemisorption of hydration products.

Magnesium silicate hydrate (M–S–H) cement, formed by reacting MgO, SiO₂, and H₂O, was used to encapsulate strontium (Sr) radionuclide. Samples were prepared using light-burned magnesium oxide and silica fume, with sodium hexametaphosphate added to the mix water as a dispersant. The performance of the materials formed was evaluated by leach testing and the microstructure of the samples was also characterized. The stabilizing/solidifying effect on Sr radionuclide in the MgO–SiO₂–H₂O system with low alkalinity is demonstrated in the study. The leaching rate in a standard 42-day test was 2.53 × 10⁻⁴ cm/d, and the cumulative 42-day leaching fraction was 0.06 cm. This meets the relevant national standard performance for leaching requirements. Sr²⁺ was effectively incorporated into the M–S–H hydration products and new phase formation resulted in low Sr leaching being observed.

The effects of carbonation on the strength and microstructural evolution of MgO-activated reactive SiO₂ (M-A-R-S) mortars under different conditions have been investigated. M-A-R-S mortars with a binder-to-silica sand ratio of 50% were prepared by light-calcined MgO and silica fume (SF) under accelerated carbonation at 20% CO₂ condition for different periods. Carbonation significantly increases the compressive strength of M-A-R-S mortars. The specimens cured for 28 days in air and placed to accelerated carbonation for ∼14 days achieved the highest strength of ∼83 MPa and the lowest porosity of 13.5%. The increased strength and density resulted from the formation of the hydrated magnesium carbonate (i.e. nesquehonite (MgCO₃·3H₂O) with a needle/column-like morphology. However, it was also found that continuous carbonation for longer periods (e.g. 28 days), associated with excessive carbonated phase(s) formed, caused micro-cracking and a reduction in compressive strength.

In this study, we investigated the impact of aluminium ion (Al³⁺) incorporation on the microstructure and the phase transformation of the magnesium silicate hydrate system. The magnesium silicate hydrate system with aluminium was prepared by mixing magnesium oxide and silica fume with different aluminium ion contents (the Al/Si molar ratios of 0.01, 0.02, 0.05, 0.1, 0.2) at room temperature. The high degree of polymerization of the magnesium silicate hydrate phases resulted in the limited incorporation of aluminium in the structure of magnesium silicate hydrate. The silicon-oxygen tetrahedra sites of magnesium silicate hydrate layers, however, were unable to substitute for silicon sites through inverted silicon-oxygen linkages. The increase in aluminium ion content raised the degree of polymerization of the magnesium silicate hydrate phases from 0.84 to 0.92. A solid solution was formed from residual aluminum-amorphous phases such as hydroxyl-aluminum and magnesium silicate hydrate phases. X-ray diffraction (XRD), field emission scanning electron microscope (F-SEM), and ²⁹Si and ²⁷Al MAS NMR data showed that the addition of Al³⁺ promotes the hydration process of MgO and has an obvious effect on the appearance of M-S-H gel. The gel with low aluminum content is fluffy, while the gel with high aluminum content has irregular flakes. The amount of Al³⁺ that enters the M-S-H gel increased with the increase of Al³⁺ content, but there was a threshold: the highest Al/Si molar ratio of M-S-H gel can be maintained at about 0.006.

The current investigation focuses on the stability of the magnesium oxide-based cementitious system under the action of sulfate attack and the dry-wet cycle. The phase change in the magnesium oxide-based cementitious system was quantitatively analyzed by X-ray diffraction, combined with thermogravimetry/derivative thermogravimetry and scanning electron microscope, to explore its erosion behavior under an erosion environment. The results revealed that, in the fully reactive magnesium oxide-based cementitious system under the environment of high concentration sulfate erosion, there was only magnesium silicate hydrate gel formation and no other phase; however, the reaction process of the incomplete magnesium oxide-based cementitious system was delayed, but not inhibited, by the environment of high-concentration sulfate, and it tended to turn completely into a magnesium silicate hydrate gel. The magnesium silicate hydrate sample outperformed the cement sample, in terms of stability in a high-concentration sulfate erosion environment, but it tended to degrade considerably more rapidly, and to a greater extent, than Portland cement, in both dry and wet sulfate cycle environments.