Magnesium silicate hydrate (M-S-H) cement is a low-carbon cementitious material. This paper discusses the mechanism of dipotassium phosphate (DKP) accelerating the hydration of hydrated magnesium silicate cement, and confirms that the introduction of DKP accelerated the formation of M-S-H gel due to the alkali induction effect and K-struvite seed crystal induction effect. This improves the compressive strength and subsequent strength of hydrated magnesium silicate cement, which can increase 3.25, 13.78, 22.23, 34.58 MPa and 37.96 MPa at the age of 1, 3, 7, 14 d and 28 d, respectively. Meanwhile, different additives can induce the formation of different M-S-H crystal phases. The acceleration of the hydration process by these additives is confirmed through X-ray diffraction (XRD), ²⁹Si nuclear magnetic resonance spectroscopy (NMR), scanning electron microscopy (SEM) and density functional theory calculations. M-S-H is the main source of strength for hydrated magnesium silicate cement, and this paper also emphasizes that suitable additives can induce a faster formation of M-S-H structures with stronger cementitious properties, thereby improving the hydration rate and early compressive strength of magnesium silicate hydrate cements, and further expanding the application range of magnesium silicate hydrate cements.

MgO: SF mass ratio for preparing MSHC samples was 40 ∶ 60 (i.e., the amount of substance was 1∶1), the water/cement (w/c) ratio was 0.45, the SHMP content was 1% (mass fraction) of the total solid mass, and the samples were all cured at 25 ℃ and 95% RH for 28 d. When the samples were cured at different ages (i.e., 1, 3, 7, 14 d and 28 d), they were broken into small pieces with the particle sizes of 1-3 cm and soaked in isopropyl alcohol to stop their further hydration. The broken samples were dried in a vacuum drying oven at 40 ℃ until there was no more mass loss.

The crystal phases of the samples were determined by a model Empyrean Rietveld X-ray diffractometer (XRD, Malvern Panalytical Co., The Netherlands). The morphology of the samples was analyzed by a model JSM-IT300 scanning electron microscope (SEM, JEOL Ltd., Japan). The nuclear magnetic resonance map of the samples was analyzed by a model Advance III 400 nuclear magnetic resonance spectroscope (NMR, Bruker Co., Germany).

This work was also analyzed based on density functional theory calculations (DFT). All the models were geometrically optimized and subsequent calculations were performed by a software named Vienna Ab-initio Simulation Package (VASP). The projection enhanced plane wave method (PAW) was used in the calculation, and the cutoff energy was set to 500 eV, which was obtained after the convergence test. The exchange-correlation potential was approximated by Perdew-Burke-Ernzerhof (PBE) under generalized gradient approximation (GGA). A 15×15×9 K-point grid was used to sample the first Brillouin region for the stress-strain calculation. Young's modulus and Poisson's ratio of each M-S-H crystal phase were calculated via vaspkit.

The DKP increases the compressive strength of MSHC by 3.25, 13.78, 22.23, 34.58 MPa and 37.96 MPa at 1, 3, 7, 14 d and 28 d, respectively. This mechanism is since the DKP induces the formation of more suitable M-S-H crystal phase with a higher mechanical strength, and the DKP accelerates the hydration rate of M-S-H, especially the early hydration rate. The SEM images confirm the acceleration effect of the DKP on the hydration. The results of Rietveld XRD and DFT calculation indicate that the DKP can induce the formation of M-S-H crystal structure with a greater gellability. The M-S-H crystal phase-D induced by SHMP is of the lowest Young's modulus and the highest Poisson's ratio. Also, its M-S-H crystal phase generation is rather small, compared with other groups, resulting in its lowest theoretical compressive strength. The M-S-H principal crystal phase induced by DKP is a superhydrous phase B, which is of the highest Young's modulus and the lower Poisson's ratio, leading to the highest theoretical compressive strength. This indicates that the DKP induces the formation of M-S-H crystalline phase with a greater compressive strength. The main crystal phase of M-S-H induced by KOH and K-struvite are talc and anthophyllite, respectively, which are of higher Young's modulus and lower Poisson's ratio, thus improving the compressive strength.

The admixture DKP could improve the compressive strength of MSHC via inducing the MSHC system to generate more suitable and higher mechanical strength M-S-H crystal phase, accelerating the hydration process of MSHC, and increasing the compressive strength of MSHC samples (i.e., 3.25, 13.78, 22.23, 34.58 MPa and 37.96 MPa) at different ages (i.e., 1, 3, 7, 14 d and 28 d). The results by Rietveld X-ray diffraction refinement showed that different admixtures induced the hydration of MSHC to form different M-S-H crystal phases, which could affect the cement strength. The DKP induced M-S-H to form a superhydrous phase B, which was a M-S-H crystal phase with a high Young's modulus and a low Poisson's ratio. This was one of the mechanisms due to the DKP enhancing MSHC. This wor'k indicated that suitable admixtures could induce the formation of M-S-H crystal structure with a greater cementitious property, thereby improving the hydration rate and early strength of MSHC.