Effect of Aluminum Incorporation on the Reaction Process and Reaction Products of Hydrated Magnesium Silicate

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.


INTRODUCTION
As we all know, the hydration product of CaO-SiO 2 -H 2 O system is calcium silicate hydrate (C-S-H) gel, like C-S-H, at normal temperatures and pressures, and the main hydration product of MgO-SiO 2 -H 2 O system is magnesium silicate hydrate (M-S-H)gel (Li et al., 2014;Jia et al., 2017a). The molecular structures of magnesium silicate hydrate (M-S-H) and calcium silicate hydrate (C-S-H) phases are significantly different. The former is an amorphous precursor of talc or sepiolite with layered silica structure (Lothenbach et al., 2015;Bernard et al., 2019), while the latter is a silicadeficient solid solution and is a precursor of two crystalline phases, tobernorite and jennite (Maruyama et al., 2014;Li et al., 2020a). Various studies show that the system has many excellent properties, such as excellent adsorptive properties, especially for heavy metal ions like Cu, Ni (Jia et al., 2016). The M-S-H has a similar structure with sepiolite: it has a large specific surface area and there are unique nano-scale pores in molecular structure, and it has the ability to adsorb heavy metals (Ji et al., 2014;Jia et al., 2017b;Jia et al., 2019;Liu et al., 2021). Magnesium silicate hydrate (M-S-H) phases can be considered as a potential cementitious material for nuclear waste immobilization (Walling et al., 2015) owing to its moderate pH value (varying from ∼9.5 to ∼10.5) and the Radionuclide (Cs, Sr) sorption potential (Li et al., 2014;Zhang et al., 2020).
The interface region between cement-based material and clay can be observed in the generation of M-S-H, the main components of clay are SiO 2 and Al 2 O 3 (Bonen and Cohen, 1992;Santhanam et al., 2020). This study aims to understand the influence of Al 3+ content on the phase transformation and the structural change of M-S-H. However, the impact of Al 3+ on the reaction processes during the hydration of M-S-H is poorly investigated. On the contrary, in the presence of Al 3+ , the molecular structure of C-S-H will change (Song et al., 2021). According to Richardson et al.(Richardson, 1999) the incorporation of aluminium ions (Al 3+ ) could change the molecular structure of the C-S-H phase. For example, tetrahedrally coordinated Al 3+ [Al (Bernard et al., 2019), Al for tetra-coordination] can substitute for silicon sites in the structure of tobermorite, occupying the bridging tetrahedra sites of C-S-H chains (Bernard et al., 2020;Li et al., 2020b). Magnesium silicate hydrate is formed by reacting MgO with silica fume (SF) and MgO dissolved in water, with one part disassociating with Mg 2+ and the other part hydrated with water to form Mg(OH) 2 , and at the same time the SiO 2 dissociates in water to form H 2 SiO 4 2− , and finally the Mg(OH) 2 , SiO 2 , and H 2 SiO 4 2− react to form M-S-H (Li et al., 2014;Tang and Chen, 2020). The MgO dissolves slowly in water and forms a poorly soluble weak electrolyte, and the Mg(OH) 2 is incomplete, which leads to a decrease in the hydration rate of M-S-H (Bernard et al., 2017). The hydration rate of M-S-H can be increased by adding Al 3+ and thereby can increase the early strength of M-S-H . The M-S-H hydration product was prepared by mixing the reactive MgO, silica fume (SF) and Al(NO 3 ) 3 , and the synthesized solids were characterized after 300 days. Advanced testing methods were used to characterize the M-S-H phases prepared under different Al 3+ contents.

Materials
Light burned technical grade MgO (Martin Marietta Magnesia Specialties, United States) and SF (Elkem, China) were used to synthesize M-S-H phase. Aluminium nitrate (Al(NO 3 ) 3 , (Chempur, China) was used as the aluminium salt. The chemical composition data of the raw materials as reported by the manufacturers are presented in Table 1. The median particle sizes of the MgO and silica fume were 3.5 and 0.5 μm respectively.

Experiment Method and Analytical Techniques
The aluminium magnesium silicate hydrate (AMSH) specimens were prepared by mixing the aqueous solutions of MgO, SF, and Al(NO 3 ) 3 at Mg/Si molar ratio of 1:1 and Al/Si molar ratios of 0.01, 0.02, 0.05, 0.1, and 0.2 were presented in Table 2. Complete hydration was obtained by adopting a water/solids (W/S) ratio of 10, where S was the total mass of MgO and SF.
In order to speed up the reaction process and make sure the samples were homogenized, the prepared samples were placed in a horizontal oscillator to oscillate for 7 days, and then the solutions were stored at room temperature (25 ± 1 C) in 250 ml sealed polyethylene bottles for up to 300 days. After 300 days the granular residue was separated by filtration and the collected solids were soaked in absolute ethyl alcohol for 24 h to inhibit further hydration, and then dried at 40°C for 48 h. The specimens were characterized using multiple techniques, such as X-ray diffraction analysis (XRD D/Max 2400 V diffractometer with Cu Kα radiation at a scan rate of 0.5 2θ min −1 ), thermogravimetric and derivative thermo-gravimetric analysis (TGA/ DTG, alumina crucibles were used and sample were heated in a nitrogen atmosphere between 50°C and 1,000°C at 10°C/min), and field emission scanning electron microscopy (F-SEM: NOVA Nano-SEM 450) on gold coated samples that had been sputter coated for 2 min using 15 mA and 30 Pa pressure and 29 Si/ 27 Al nuclear magnetic resonance spectroscopy (NMR). Figure 1 shows the phases transformation in the MgO-SiO 2 -H 2 O system with different Al 3+ content curing for 1 day and 300 days. Curing for 1 day it is obvious that with the increase of the dosage of Al (NO 3 ) 3 , the content of residual MgO in the system decreases greatly, while the content of Mg (OH) 2 increases gradually, which indicates that the addition of Al 3+ promotes the hydration of MgO and the formation of Mg (OH) 2 ( Figure 1A). At the same time, a dispersion peak appears around 2θ about ∼10.6 o and it is gradually obvious with the increase of Al 3+ content, it indicates that there was the formation of a new phase and amorphous type of micro-crystalline hydroxy aluminium. By the later stage of hydration (300 days), the crystalline phases were completely transformed to the amorphous phases due to the reaction of MgO and Mg (OH) 2 with silica fume (SF) ( Figure 1B). The cementitious system is all amorphous phase; and the characteristic peak of M-S-H gel at the broad diffraction at 22-30°becomes weaker with the increase of the dosage of Al 3+ , which may be the result of the change of molecular structure. The broad diffraction at 20-28°, 33-40°and 58-62°r anges are attributable to the M-S-H phases Jia et al., 2016) (Figure 1B) while the diffraction peak between 8°a nd 12°represents the Al-amorphous phases ( Figures 1A,B).

RESULTS AND DISCUSSION
Three weight loss stages occurred during the curing of M-S-H for 300 days (Figure 2). The first weight loss occurred in the temperature range from 50 to 200°C and is attributed to the removal of free/bound water. The second and third weight losses in the 200-320°C and 320-700°C ranges are attributed to the removal of constitutional water in aluminum hydroxide (Alamorphous phases) and M-S-H phase respectively (Jia et al., 2016;Nied et al., 2016) (Figure 2A). As the dosage of Al 3+ increases, the weight loss of the phase in the cementitious system increases gradually in the weight loss range of 200°C-400°C, and the weight loss phase in this range may be amorphous or microcrystalline hydroxyl aluminum. According to the weight   loss data, the contents of free water and bound water in different samples are basically consistent, indicating that the addition of Al 3+ did not affect the generation of M-S-H gel when the curing age is long enough and the initial ratio of Al/Si is less than 0.20. As the mass of free and bonding water depends on the porous channel in the MgO-SiO 2 -H 2 O system but not on the aluminium incorporated, the original structure of the M-S-H phase was unaffected by the incorporated aluminium ( Figure 2B). The mass percents of various components in AMSH samples after curing for 300 days were shown in Table 3. The molecular structure of the solid solution which included aluminum-amorphous phases (hydroxyl-aluminum) and M-S-H phases was characterized using the NMR. The coordination of the spectrum of 29 Si NMR is usually expressed by Q n , where n represents the bridge oxygen number between each siliconoxygen tetrahedral unit and other Si atoms. The chemical migration of 29 Si in silicate minerals ranges from -60 to -120 ppm and according to different coordination, it can be divided into the following four intervals: -60 to -70 ppm represents a single free island silicate (Q 0 ); -70 to -83 ppm represents the endpoint of p-silicate or chain silicate (Q 1 ); -83 to -90 ppm represents the silicon-oxygen tetrahedron in the chain silicate (Q 2 ); -90 to -100 ppm represents the silicon-oxygen tetrahedron in the chain silicate (Q 3 ); -100 to -120 ppm represents the silicon-oxygen tetrahedron in the threedimensional reticular silicate (Q 4 ) (Wei et al., 2006;Wei et al., 2011). The transformation of the microstructure of the M-S-H phase is indicated by the peaks at -85.5 ppm (Q 3 -a) and -92.9 to -95.1 ppm (Q 3 -b) ( Figure 3A). In Figure 3A, Q 3 -a indicates the Si unit via inverted Si-O-Si linkages, while Q 3 -b reflects the Si unit in the Si-O tetrahedral layer (Tonelli et al., 2016). Figure 3B shows that the coordination of a few Al 3+ that entered the structure of the M-S-H phase and changed to the tetrahedral coordination [Al (Bernard et al., 2019)]. The chemical shift from -92.9 to -95.1 ppm suggests that Al 3+ only occupied the Q 3 -b tetrahedra sites of the M-S-H layers and did not substitute for the Si 4+ via inverted Si-O-Si linkages (Q 3 -a). Although the initial structure of the M-S-H phases was remained unchanged the average condensation degree was increased from 0.84 to 0.92 after the incorporation of Al 3+ ( Figure 3C).
Using the deconvolution technique to fit the data of the NMR Test curve, we can figure out the percentage of Si and Al with different coordinations, and the specific data are shown in Table 4. After curing for 300 days, the polymerization average degree of M-S-H and Al/Si molar ratio (M(Al: Si)) in M-S-H can be computed by formula 1 and formula 2.
Where n is the initial Al/Si molar ratio in M-S-H, I(Al IV ) is the percentage of moles of tetrahedral-Al. Si NMR chemical shifts (ppm) and relative intensities (%) from de-convolution of the 29 Si/ 27 Al NMR spectra for the AMSH-2 ∼ 5 samples after curing for 300 days. Key: p-peak, sh-shoulder, Q 1 -Q 1 (3OH), Q 2 -Q 2 (2OH), Q 3 -a-Q 3 (OH) as continuous layer silicates, Q 3 -b-Q 3 (OH) as inverted silicates, Q 3 -SF-Q 3 (OH) in SF. According to Figure 3C, with the increase of Al 3+ incorporation, the polymerization average degree of M-S-H increase from 0.85 to 0.93, but obviously the adding amount of Al 3+ is still low. When the initial Al/Si molar ratio exceeds 0.1, both the adding amount of Al 3+ and the polymerization average degree of M-S-H remain at a constant value. The adding of Al 3+ plays a filling role, as it connects the silicon oxygen tetrahedron chains together and increases the degree of polymerization of the stratified structure. The molecular structure of M-S-H was already highly aggregated, which limits the adding amount of Al 3+ . It also shows that the number of Si sites in the silicon oxygen tetrahedral chain replaced by Al 3+ is not large, and that the main function is to connect.
The F-SEM data show that the M-S-H phase has honeycomb morphology and grown on the surface of the SF particles extending to the gaps ( Figure 4A). The Mg/Si molar ratio of pure M-S-H phases was 0.95. After the incorporation of Al 3+ , the Mg/Si molar ratio remained unchanged in the M-S-H phases grown on the surface of SF particles, however, increased to 1.24 in M-S-H phases grown in the gaps ( Figures 4A,B). After curing for 300 days, samples AMSH-1 and AMSH-5 both A and B had only amorphous specimens. The amorphous substance in sample AMSH-1 was still M-S-H gel with villous shape, the villi size was about 10-20 nm and there were a lot of nano-scale pores between the villi. Aluminium incorporation changed the morphology of M-S-H phases from honeycomb to petaline shape ( Figure 4B).

CONCLUSION
The hydration process of MgO in the magnesium silicate hydrate system was accelerated by the addition of Al 3+ , and the rate of conversion from MgO to Mg(OH) 2 increases with the increase of Al 3+ content. However, it does not influence the formation of hydration products (M-S-H phases) and it slows down the M-S-H generation. According to TGA/DTG we know that Mg(OH) 2 crystals with amorphous hydroxy aluminum are mixed together, which led to the position of weight loss peak being offset, and the increase of the weight loss range from 200°C to 300°C proved that there is a new phase formation in the M-S-H.
Irrespective of the amount of Al 3+ doped, a limited amount of Al 3+ entered the structure of the M-S-H phase, consequently, the initial molecular structure of the M-S-H remained unchanged. Aluminium ions occupied the Q 3 -b tetrahedra sites of the M-S-H layers and did not substitute for Si 4+ via inverted Si-O-Si linkages (Q 3 -a). The average condensation degree and the Mg:Si molar ratio of the M-S-H phase raised with the increase of Al 3+ content. Hydroxyl-aluminum, a residual aluminium amorphous phase, and the M-S-H phases formed a solid solution. The addition of Al 3+ morphology of the M-S-H phase from honeycomb-like to petal-like.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

AUTHOR CONTRIBUTIONS
YJ: Conception and design of study. YZ and XZ: Drafting the manuscript. YJ and FL: analysis and/or interpretation of data. WM, HY and RH: revising the manuscript critically for important intellectual content.