Magnesium silicate hydrate (M-S-H) cementitious material is regarded as a potential alternative to conventional Portland cement due to its excellent sulfate erosion resistance, heavy metal ion solidification performance, and low pH value during hydration. However, the industrialization process of M-S-H cementitious materials is currently restricted due to the high cost and unstable supply of traditional silicon sources that rely on industrial by-products (such as silica fume, fly ash, and slag). Natural layered silicates such as montmorillonite and kaolin have a similar tetrahedral-octahedral stacking unit structure to M-S-H, and can exhibit a good pozzolanic activity after mechanical or thermal activation. Exploring the use of calcined montmorillonite and kaolin as alternative silicon sources to partially or completely replace silica fume broadens the source of raw materials for M-S-H cementitious materials and reduces production costs. This work was to investigate the effects of partial or complete replacement of silica fume by calcined montmorillonite (MT) and calcined kaolin (KL) on the structure and strength of M-S-H cementitious materials, clarifying the action mechanism of aluminum in the two clay minerals on the M-S-H hydration process and layered structure.

Light-burned magnesia (LM) was used as a magnesium source and silica fume (SF) as a reference silicon source. Montmorillonite and kaolin were calcined in a muffle furnace at 750 ℃ for 1 h at a heating rate of 10 ℃/min to obtain calcined montmorillonite and calcined kaolin. The reference sample was prepared via mixing MgO and silica fume at a mass ratio of 4 : 6. On this basis, different mass fractions (i.e., 10%, 30%, 50%, 70%, and 100%) of calcined montmorillonite and kaolin were used to replace silica fume, and the samples were named as MT-10~MT-100 and KL-10~KL-100, respectively according to the replacement ratio. The mortar strength specimens were prepared with the samples above, superplasticizer, tap water, and standard sand, and the compressive strength was tested according to the standard GB/T 17671—1999 "Methods of testing cement- Determination of strength". The paste was prepared at a water-to-solid ratio of 0.5 and 0.7% superplasticizer based on the total mass of solids, cured in sealed glass tubes with liquid paraffin, and terminated hydration with isopropanol at a certain age, then dried in a vacuum oven at 40 ℃ for 48 h. The mineral phase, thermal behavior, functional groups, microstructure, and element distribution of the samples were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and solid-state ²⁹Si and ²⁷Al magic-angle spinning nuclear magnetic resonance (MAS NMR).

Calcined kaolin exhibits a great early strength enhancement effect, and its compressive strengths at 3 d and 7 d have a positive correlation with the dosage (i.e., correlation coefficients are 0.996 and 0.905, respectively). At the dosage of calcined kaolin of 70%, the strength at 3 d reaches 39.37 MPa and the strength at 7 d reaches 49.58 MPa, which are 464.0% and 400.3% higher than those of the reference sample, respectively. In contrast, the strength enhancement effect of calcined montmorillonite is weak. The strengths at 3 d and 7 d increase slowly with the increase of montmorillonite dosage. At the montmorillonite dosage of 70%, the strength at 3 d is only 8.94 MPa and the strength at 7 d is 14.28 MPa, which are 36.5% and 53.7% higher than those of the reference sample, respectively. The strength increases slowly in the montmorillonite dosage range of 10%-50%.

The microscopic analysis shows that calcination at 750 ℃ causes more thorough damage to the crystal structure of kaolin than that of montmorillonite. The XRD diffraction peaks of kaolin disappear completely after calcination, and the layered structure collapses completely, resulting in a higher pozzolanic activity. After calcination, the interlayer spacing of montmorillonite decreases, and some characteristic diffraction peaks are still retained, indicating that its crystal order is not completely destroyed and the pozzolanic activity is lower than that of calcined kaolin. The FTIR and NMR analysis further confirms that the structural hydroxyl groups of calcined kaolin are completely removed, and a large number of active Al and Si sites are exposed. Although montmorillonite also undergoes dehydroxylation and structural damage after calcination, the degree of disorder is lower than that of kaolin.

Aluminum in both calcined montmorillonite and kaolin can enter the M-S-H phase in the early stage of hydration, and form magnesium aluminosilicate hydrate (M-S-A-H) gel via substituting Si⁴⁺ in the silicate tetrahedron or Mg²⁺ in the magnesium-oxygen octahedron, which significantly improves the polymerization degree of the product. The formed M-S-A-H is external hydration products, mainly formed through element diffusion between particles. The analysis by the XRD, FTIR, and DTG shows that Mg-Al layered double hydroxide (Mg-Al LDH) is generated within the first 28 d of hydration in both raw material systems. The enhancement of cementitious material strength by calcined kaolin and montmorillonite is related to the formation of M-S-A-H. The strength improvement effect of calcined kaolin is higher than that of calcined montmorillonite due to the more thorough destruction of the crystal structure, higher degree of network disorder, more active Al and Si sites, and higher content of Al(V).

Calcined kaolin and montmorillonite could be used as alternative silicon sources for M-S-H cementitious materials. Calcined kaolin had a great early strength enhancement effect, and its compressive strength had a positive correlation with the dosage. At the dosage of 70%, the compressive strength at 7 d was increased by more than 400%, compared with the sample prepared with pure SF. The strength enhancement effect of calcined montmorillonite could be weak, and the strength was only increased by 53.7% at the same dosage and age. The difference in strength enhancement effect between calcined kaolin and montmorillonite could be due to the fact that the calcination at 750 ℃ caused more thorough damage to the crystal structure of kaolin than that of montmorillonite, resulting in a higher pozzolanic activity. Aluminum in both calcined montmorillonite and kaolin could enter the M-S-H phase in the early stage to form M-S-A-H gel, significantly improving the polymerization degree of the product. The formed M-S-A-H were external hydration products, and Mg-Al LDH was generated within the first 28 d of hydration.