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Article

Powders Synthesized from Water Solutions of Sodium Silicate and Calcium and/or Magnesium Chlorides

by
Tatiana V. Safronova
1,2,*,
Alexandra S. Sultanovskaya
2,
Sergei A. Savelev
2,
Tatiana B. Shatalova
1,2,
Yaroslav Y. Filippov
2,3,
Olga V. Boytsova
1,2,
Vadim B. Platonov
1,
Tatiana V. Filippova
1,
Albina M. Murashko
2,
Xinyan Feng
2 and
Muslim R. Akhmedov
4
1
Department of Chemistry, Lomonosov Moscow State University, Building, 3, Leninskie Gory, 1, 119991 Moscow, Russia
2
Department of Materials Science, Lomonosov Moscow State University, Building, 73, Leninskie Gory, 1, 119991 Moscow, Russia
3
Research Institute of Mechanics, Lomonosov Moscow State University, Building, 1, Michurinsky Pr., 119192 Moscow, Russia
4
Department of Space Research, Lomonosov Moscow State University, Building, 3, Leninskie Gory, 1, 119991 Moscow, Russia
*
Author to whom correspondence should be addressed.
Compounds 2025, 5(2), 22; https://doi.org/10.3390/compounds5020022
Submission received: 25 April 2025 / Revised: 18 May 2025 / Accepted: 12 June 2025 / Published: 16 June 2025
Browse Figures
Versions Notes

Abstract

:
Powders with phase composition including quasi-amorphous phases and calcium carbonate CaCO3 in the form of calcite or aragonite and sodium halite NaCl as a reaction by-product were synthesized from 0.5M aqua solutions of sodium silicate and 0.5M aqua solutions of calcium and/or magnesium chlorides. Starting solutions were taken in quantities which could provide precipitation of hydrated calcium and/or magnesium silicates with molar ratios Ca/Si = 1 (CaSi), Mg/Si = 1 (MgSi) or (Ca+Mg)/Si = 1 (CaMgSi). Hydrated calcium and/or magnesium silicates, hydrated silica, magnesium carbonate, hydrated magnesium carbonate or hydrated magnesium silicate containing carbonate ions are suspected as components of quasi-amorphous phases presented in synthesized powders. Heat treatment of synthesized powders at 400, 600, 800 °C and pressed preceramic samples at 900, 1000, 1100 and 1200 °C were used for investigation of thermal evolution of the phase composition and microstructure of powders and ceramic samples. Mass loss of powder samples under investigation during heat treatment was provided due to evacuation of H2O (m/z = 18), CO2 (m/z = 44) and NaCl at temperatures above its melting point. After sintering at 1100 °C, the phase composition of ceramic samples included wollastonite CaSiO3 (CaSi_1100); enstatite MgSiO3, clinoenstatite MgSiO3 and forsterite Mg2SiO4 (MgSi_1100); and diopside CaMgSi2O6 (CaMgSi_1100). After sintering at 1200 °C, the phase composition of ceramics CaSi_1200 included pseudo-wollastonite CaSiO3. After heat treatment at 1300 °C, the phase composition of MgSi_1300 powder included preferably protoenstatite MgSiO3. The phase composition of all samples after heat treatment belongs to the oxide system CaO–MgO–SiO2. Ceramic materials in this system are of interest for use in different areas, including refractories, construction materials and biomaterials. Powders prepared in the present investigation, both via precipitation and via heat treatment, can be used for the creation of materials with specific properties and in model experiments as lunar regolith simulants.

1. Introduction

The CaO−MgO−SiO2 system one is of great importance in geochemistry [1,2,3] and in different research areas of material science, including construction materials [4,5], refractories and biomaterials [6], with amorphous, quasi-amorphous and polycrystalline structures [7]. Taking into account that the earth’s crust and lunar regolith is predominantly composed of silicon dioxide (SiO2), with a notable content of magnesium oxide (MgO), calcium oxide (CaO) in the form of crystalline structures such as olivine and pyroxene [8], the synthesis and investigation of calcium and magnesium silicates seems to be a very important task, also for model experiments as simulants of lunar regolith. The following silicates are in this system: MgSiO3, Mg2SiO4, CaMgSi2O6, CaMgSiO4, Ca2MgSi2O7, Ca3Mg(SiO4)2 [9], CaSiO3, Ca2SiO4, Ca3SiO5, Ca3Si2O7 [10,11,12,13]. Calcium and/or magnesium silicates with molar ratios Ca/Si = 1, Mg/Si = 1 and (Ca+Mg)/Si = 1, i.e., wollastonite CaSiO3, enstatite/clinoenstatite MgSiO3, and diopside CaMgSi2O6, are of particular interest due to the possibility of using them in the creation of materials with specific properties. Wollastonite CaSiO3, enstatite/clinoenstatite MgSiO3 and diopside CaMgSi2O6 can be prepared at high temperatures as a result of solid state or hetero phase reactions. Powdered or compacted mixtures of starting components containing precursors of calcium, magnesium and silicon oxides have to be used [14,15,16,17]. Calcium and/or magnesium silicates can be prepared via crystallization from the glass melts [18,19].
Fine powders of wollastonite CaSiO3, enstatite/clinoenstatite MgSiO3 and diopside CaMgSi2O6 can be prepared via heat treatment of precipitated hydrated calcium and/or magnesium silicates with preset molar ratios Ca/Si = 1, Mg/Si = 1 and (Ca+Mg)/Si = 1 from different soluble calcium salts and silicates. Amorphous magnesium silicates with different MgO/SiO2 and hydrated calcium silicates with different CaO/SiO2 mass or molar ratios can be synthesized from water solutions of sodium silicate nonahydrate Na2SiO3·9H2O or sodium silicate pentahydrate Na2SiO3·5H2O and calcium nitrate tetrahydrate Ca(NO3)2·4H2O, calcium acetate Ca(CH3COO)2 [20,21], or magnesium nitrate tetrahydrate Mg(NO3)2·4H2O, magnesium sulfate MgSO4 [22] and magnesium chloride MgCl2 [23], also in the presence of surfactants [24,25]. Colloidal SiO2 or Si(OC2H5)4 (TEOS) as precursors of SiO32−-ion and water solutions of calcium or magnesium nitrates were used for enstatite MgSiO3 and wollastonite CaSiO3 preparation [26,27,28]. Powders of hydrated calcium silicate consisting of fine particles were synthesized by precipitation from aqua solutions of sodium silicate Na2SiO3 and different calcium salts, including nitrate Ca(NO3)2, chloride CaCl2 and acetate Ca(CH3COO)2 [29,30], also in the presence of surfactants [31]. Mechanochemical synthesis was used for synthesis of amorphous magnesium silicates or hydrated calcium silicates [32,33]. The combustion method also was used for synthesis of wollastonite from calcium nitrate tetrahydrate Ca(NO3)2·4H2O, colloidal SiO2, ammonium nitrate NH4NO3 and citric acid C6H8O7·H2O [34].
The possibility of preservation of reaction by-products in the powders synthesized via precipitation gives an opportunity to take them as components forming the phase composition during later heating via hetero phase or solid-state interaction [35] or influencing densification in the case of absence of hetero phase interaction with the formation of new phases. So, using a water solution of sodium silicate Na2SiO3 can provide the presence of sodium salts in the synthesized powder as a reaction by-product, no matter what calcium salt or magnesium salt would be used in the exchange reactions.
The aim of the present work consisted in preparing and investigating powders synthesized from water solutions of sodium silicate Na2SiO3 and calcium CaCl2 and/or magnesium MgCl2 chlorides via precipitation at molar ratios Ca/Si = 1 (CaSi), Mg/Si = 1 (MgSi) or (Ca+Mg)/Si = 1 (CaMgSi). The possible influence of sodium chloride NaCl as a reaction by-product preserved in synthesized powders on the microstructure and phase composition formation of powders after heat treatment at different temperatures and ceramics after firing was also under investigation. The present investigation will provide the new insights into the combination of several methodological approaches in the preparation of both hydrated powders in the system CaO−MgO−SiO2−H2O and calcium or/and magnesium silicates in the system CaO−MgO−SiO2. Among them are (1) synthesis from solutions via precipitation, (2) using mixed-cationic solution, (3) the positive role of reaction by-products preserved in synthesized powders. The advantages of synthesis from solutions via precipitation consisted in the possibility of active powder preparation both from the point of view of chemical interactivity and from the point of view of activity in the sintering process. Using a mixed-cationic solution for the synthesis of one (CaMgSi) of three powders can help to reach a more uniform distribution of cations in the prepared powder. A reaction by-product which has the ability to disappear from the synthesized product during heat treatment can play its unique role both in the formation of the phase composition and the morphology/microstructure of heat-treated powders/ceramics.

2. Materials and Methods

2.1. Materials

Calcium chloride CaCl2 (CAS no. 10043-52-4, RusKhim, Moscow, Russia), magnesium chloride hexahydrate MgCl2·6H2O (CAS no. 7791-18-6, RusKhim, Russia) and sodium metasilicate pentahydrate Na2SiO3·5H2O (CAS no. 10213-79-3, RusKhim, Russia) were used for the synthesis of powders.

2.2. Synthesis of Powders

The following aquations ((1), (2), (3)) were used for calculation of quantities of starting salts (CaCl2, MgCl2·6H2O, Na2SiO3·5H2O) for solutions preparation:
Na2SiO3 + CaCl2 + xH2O = CaSiO3·xH2O + 2NaCl
2Na2SiO3 + CaCl2 + MgCl2 + xH2O = CaMgSi2O6·xH2O + 4NaCl
Na2SiO3 + MgCl2 + xH2O = MgSiO3·xH2O + 2NaCl
Sodium metasilicate Na2SiO3·5H2O, calcium chloride CaCl2 and magnesium chloride hexahydrate MgCl2·6H2O were dissolved in distilled water to obtain 0.5 M solutions. Also, 500 mL of mixed-cationic aqua solution containing calcium and magnesium chlorides with a molar ratio of Ca/Mg = 1 according Equation (2) was prepared. The labeling of prepared powders, volumes and concentrations of aqua solution used are presented in Table 1.
Then, solutions of calcium and/or magnesium chloride were added to the sodium metasilicate solutions with constant stirring on a magnetic stirrer. The resulting suspension was stirred at room temperature for 30 min. Next, the precipitates were separated from the mother liquors by vacuum filtration and evenly distributed over a large surface area and left to dry for 1 week. The dried precipitate was crushed in an agate mortar and sieved through a polyester sieve with 200 μm mesh. The scheme of powders and ceramic sample preparation is presented at Figure 1.
Transparent mother liquors were collected and products dissolved in them were extracted from solutions via crystallization due to water evaporation at 60 °C. The substances extracted from mother liquors which were separated from precipitates during syntheses of powders CaSi, CaMgSi and MgSi were labeled as by-CaSi, by-CaMgSi and by-MgSi respectively.

2.3. Heat Treatment of Synthesized Powders

A small batch of synthesized powders were heat treated at 400, 600, 800 °C to investigate the evolution of their phase composition. Heating in the furnace was carried out at a rate of 5 °C/min. The holding time at these temperatures was 2 h. Powder samples were labeled as CaSi_400, CaMgSi_400 and MgSi_400; CaSi_600, CaMgSi_600 and MgSi_600; CaSi_800, CaMgSi_800 and MgSi_800, showing the composition and temperature of heat treatment. Additionally, powder MgSi_1300 was prepared to confirm the molar ratio Mg/Si = 1 preset in the synthesis.

2.4. Preparation of Ceramic Samples

To prepare preceramic samples crushed and sieved through a 200 μm mesh sieve powders were pressed in the form of simple disks by uniaxial one-sided pressing on a manual press (Carver Laboratory Press model C, Fred S. Carver, Inc., Wabash, IN, USA) using a steel die with diameter 12 mm. Pressing was carried out at a pressure of 100 MPa. Preceramic powder samples pressed into disks were fired at 900, 1000, 1100 and 1200 °C. Heating in the furnace was carried out at a rate of 5 °C/min. The holding time at these temperatures was 2 h. Ceramic samples were labeled in the same way as powders, showing the composition and temperature of heat treatment.

2.5. Methods of Analysis

The linear shrinkage of the ceramic samples was estimated as a comparison of the diameter of ceramic and preceramic samples using Equation (4).
D/D0 = Dheat treatment/Dpress×100, %,
where
  • D/D0—relative diameter of the sample after heat treatment, %;
  • Dheat treatment—diameter of the sample after heat treatment, cm;
  • Dpress—diameter of the preceramic sample after pressing, cm.
The apparent density of the preceramic samples and ceramic samples after heat treatment were calculated using Equation (5).
ρ = m/(h × πD2/4), g/cm3,
where
  • ρ—density of the sample, g/cm3;
  • m—weight of the sample, g;
  • h—thickness of the sample, cm
  • D—diameter of the sample, cm.
The mass and the linear dimensions of the samples were measured with accuracy ± 0.001 g and ± 0.01 mm, respectively, before and after heat treatment.
Thermal analysis, including thermogravimetry (TG) and differential scanning calorimetry (DSC), was performed using an STA 409 PC Luxx thermal analyzer (NETZSCH, Selb, Germany) during heating in air (10 °C/min, 40–1300 °C), the specimen mass being at least 10 mg. The gas-phase composition was monitored by a Netzsch QMS 403C Aëolos quadrupole mass spectrometer (NETZSCH, Selb, Germany) coupled with a Netzsch STA 409 PC Luxx thermal analyzer. The mass spectra were registered for the following m/z values: 18 (H2O); and 44 (CO2).
The phase composition of powders obtained after synthesis was determined by X-ray powder diffraction (XRD) analysis using a Rigaku D/Max-2500 diffractometer (Rigaku Corporation, Tokyo, Japan) with a rotating anode (Cu–Ka radiation), angle interval 2Ѳ: from 2° to 70° (step 2Ѳ–0.02°). Phase analysis was performed using the ICDD PDF2 database and “Match!” software (version 3.15, https://www.crystalimpact.com/, accessed on 18 April 2025).
Scanning electron microscopy (SEM) images of synthesized powders, powders after heat treatment and SEM images of ceramic samples were studied using a LEO SUPRA 50 VP scanning electron microscope (Carl Zeiss, Oberkochen, Germany); the imaging was performed in a low vacuum mode at an accelerating voltage of 3–21 kV (SE2 detector).

3. Results and Discussion

According to the XRD data of synthesized powders shown in Figure 2, the phase composition of all powders synthesized from aqua solutions of sodium silicate Na2SiO3 and calcium CaCl2 and/or magnesium MgCl2 chlorides included the reactions’ by-product halite NaCl (PDF card # 5-628; #96-900-8679). And this is a true confirmation of the fact that the exchange reactions ((1), (2), (3)) took place in spite of the absence any reflexes of hydrated silicates of calcium and/or magnesium. It is possible to assume that the main quasi-amorphous components of white precipitates/powders with labels CaSi, CaMgSi and MgSi could be described with the formulas CaSiO3·xH2O, CaMgSiO3·xH2O or MgSiO3·xH2O, respectively. Precipitation of quasi-amorphous hydrated silicates with a molar ratio (Ca,Mg)/Si < 1 (for example, Ca1.5SiO3.5·xH2O [PDF card 33-306]) and hydrated silica SiO2·xH2O is also possible [36]. The phase composition of CaSi synthesized powder also included calcium carbonate CaCO3 in the form of two modifications: calcite (PDF card # 5-586; #96-900-0966) and aragonite CaCO3 (PDF card # 41-1475; #96-210-0188). The phase composition of CaMgSi synthesized powder included calcium carbonate CaCO3 in form of calcite (PDF card # 5-586; #96-900-0966). The possibility of CaCO3 formation can be connected with high pH level of aqua solution of sodium silicate having possibility to catch CO2 from the atmosphere. This phenomenon was earlier reflected in the previous investigations devoted to synthesis of calcium silicates via precipitation from aqua solutions of sodium silicate and calcium nitrate [37]. Precipitated MgSi could also adsorb CO2 from the atmosphere due to the basic pH level of aqua solution of sodium silicate, or even contain CO32−-ion in structures of quasi-amorphous phases, for example, as amorphous magnesium carbonates [38], amorphous hydrated magnesium carbonates [39] or hydrated magnesium silicate containing CO32− carbonate ions [40].
The XRD data presented at Figure 3 confirm that phase composition of reaction by-products extracted from mother liquors collected after vacuum separation of precipitates consisted of only one mineral—halite NaCl (PDF card # 5-628; #96-900-8679). According to conditions of precipitation and Equations (1–3), the maximum quantity of halite NaCl forming in these reactions is 0.5M or 29.2 g. The mass of by-CaSi extracted from mother liquor was 15 g, mass of by-CaMgSi—22 g and mass of by-MgSi—21 g (Table 2). The possibility of reaction by-products to remain in the precipitates and then in the synthesized powders after drying could be due to both the protocol/procedure of filtration used and due to such properties of synthesized particles as the morphology and dimension that define the specific surface of powder. Taking into account differences in the distribution of NaCl between the synthesized powders and the mother liquors, it is possible to make the conclusion that the dimensions of the particles in the CaSi powder were smaller than particles in the CaMgSi and MgSi powders.
SEM-images of synthesized powders are presented in Figure 4. One can see smaller aggregates of particles in CaSi powder, and bigger aggregates of particles in CaMgSi and MgSi powders.
Thermal analysis data of the synthesized powders are shown in Figure 5 and Figure 6. The total mass loss of CaSi, CaMgSi and MgSi powders were 41.5%, 38.1% and 39.3% at 1300 °C, respectively. The mass loss in the interval 40–700 °C with maximum at 130 °C for all powders and for MgSi in 600–770 °C according MS data (Figure 6) was due to evacuation of H2O adsorbed and H2O from hydrated silicates or hydrated silica. CO2 evacuation was in the interval 200–900 °C with maxima at 590 °C for CaSi powder and at 630 °C for CaMgSi powder, and in the intervals 500–530 °C and 560–900 °C with maxima at 120 °C and 675 °C for MgSi powder. Evacuation of CO2 could be due to degassing at relatively low temperatures and due to decomposition of calcium carbonate, detected in the phase composition of the synthesized powders CaSi and CaMgSi. The ion current corresponding to both H2O (600–770 °C) and CO2 (560–900 °C) is observed in about the same region for MgSi powder. Decomposition of quasi-amorphous hydrated magnesium carbonate or probably more complicated quasi-amorphous hydrated magnesium silicate containing carbonate ions CO32− could be a reason. Total mass loss at 800 °C for CaSi, CaMgSi and MgSi powders were 18.1%, 23.6% and 27.1%, respectively.
An endothermic effect is observed on the DSC curve in the interval 40–200 °C with a peak at 125 °C. A sharp endothermal peak without loss of mass was detected for all powders in the interval 780–820 °C with a minimum at 808 °C. Taking into account the presence of NaCl in all synthesized powders, this endothermal peak can be attributed to melting of this salt. According to phase diagrams, the melting point of NaCl is 801 °C. In conditions of the heating rate used (10 °C/min), a little bit higher meaning of the NaCl melting point could be detected. According to TG data, the end of mass loss was at 1090 °C for CaSi powder, 1150 °C for MgSi powder and 1220 °C for CaMgSi powder. The mass losses due to NaCl evacuation were 23.4% CaSi powder, 12.2% for MgSi powder and 14.5% for CaMgSi powder. These data are in partial agreement with the estimation of the quantity of NaCl captured by synthesized powders via weighing of the by-product extracted from separated mother liquors. The bigger quantity of NaCl left in the mother liquors, the smaller the mass loss of powders due to NaCl evacuation during heating. The greater the certain mass fraction of NaCl in the sample in the synthesized powder (see Table 2), the more intense the endothermic effect.
Figure 7 shows XRD data of all samples CaSi both synthesized and after heat treatment.
The phase composition of CaSi powders before and after heat treatment and the ceramic sample after firing determined using “Match!” software are summarized in Table 3.
The synthesized CaSi powder and CaSi_400 powder annealed at 400 °C contained a by-product NaCl and calcium carbonate in the form of calcite and aragonite. No silicate phases could be detected at an annealing temperature of up to 600 °C, and therefore it can be concluded that during synthesis, the main products, hydrated calcium silicates, are deposited in an X–ray amorphous form. Only at a temperature of 800 °C does the formation of the crystalline phase of calcium silicate (wollastonite) begin; however, at this temperature, crystallization does not complete during annealing, as evidenced by a significantly lower mass fraction of the by-product (NaCl) at a higher temperature. Further, traces of NaCl are removed between 1000 °C and 1100 °C, and between 1100 °C and 1200 °C a complete phase transition of wollastonite to pseudo-wollastonite occurs [41].
Figure 8 shows XRD data of all samples MgSi both synthesized and after heat treatment.
The phase composition of MgSi-powders before and after heat treatment and the ceramic sample after firing determined using “Match!” software are summarized in Table 4.
The phase composition of synthesized MgSi powder, MgSi_400 and MgSi_600 powders annealed at 400 °C and 600 °C, respectively, contained a by-product, NaCl. No silicate phases could be detected in powder samples after annealing at temperatures of up to 600 °C, and therefore it can be concluded that the target product in these powders was present in quasi-amorphous form. After heat treatment at 800 °C, the phase composition of MgSi_800 powder consisted of halite NaCl and enstatite MgSiO3. And this result confirmed that powder was contained the product with the preset molar ratio Mg/Si = 1. After heat treatment of ceramic samples at temperatures in the interval 900–1200 °C, no NaCl was detected. After heat treatment at 900 and 1000 °C, in the ceramic samples MgSi_900 and MgSi_1000, enstatite MgSiO3 and forsterite Mg2SiO4 were detected. After heat treatment at 1100 °C, the phase composition of the ceramic samples MgSi_1100 included enstatite MgSiO3, forsterite Mg2SiO4 and clinoenstatite MgSiO3. After heat treatment at 1200 °C, the phase composition of the ceramic samples of MgSi_1200 included protoenstatite MgSiO3, forsterite Mg2SiO4 and clinoenstatite MgSiO3. Formation of some quantity of forsterite having the molar ratio Mg/Si = 2 can be explained by the possibility of silica transaction to the amorphous phase, which can form due to the presence of NaCl melt in samples during firing at temperatures above its melting point, i.e., at 900–1200 °C. Phase transactions of magnesium metasilicate MgSiO3 was described earlier [42]. And the presence of clinoenstatite and protoenstatite in phase composition of ceramic samples after heat treatment at 1100 and 1200 °C is in agreement with information about phase transactions revealed earlier. The presence of mineralizers can also influence the temperature of phase transactions [43].
Figure 9 shows XRD data of all CaMgSi samples, both synthesized and after heat treatment.
The phase composition of CaMgSi-powders before and after heat treatment and the ceramic sample after firing determined using “Match!” software are summarized in Table 5.
The phase composition of synthesized CaMgSi powder, and CaMgSi_400 powder annealed at 400 °C contained a by-product NaCl and calcite CaCO3. After heat treatment at 600 °C, only one phase of NaCl was detected in the powder. No silicate phases could be detected in powder samples after annealing at temperatures of up to 600 °C, and therefore it can be concluded that during synthesis, the main products, hydrated calcium and magnesium silicates, are deposited in an X–ray amorphous form. After heat treatment at 800 °C, the phase composition of the CaMgSi_800 powder included NaCl and diopside CaMgSi2O6. After heat treatment at 900 °C, the phase composition of the CaMgSi_900 ceramic sample preferably consisted of diopside CaMgSi2O6 and a very little quantity of NaCl. After firing at temperatures 1000, 1100 and 1200 °C, the phase composition of ceramic samples was represented by diopside CaMgSi2O6. After heat treatment of ceramic samples at temperatures in the interval 1000-1200 °C, no NaCl was detected. The main feature of NaCl as a reaction by-product preserved in synthesized powders consisted in its relatively high melting point and chemical indifference in respect to calcium and magnesium silicates, as XRD analysis confirmed. It should be noted that after heat treatment at temperatures higher then melting point, NaCl is evacuating from the ceramic samples. And after heat treatment at 1100 °C (for CaSi, the biggest content of NaCl in synthesized powder (Table 2)), at 900 °C (for MgSi, the lowest content of NaCl in synthesized powder), at 1000 °C (for CaMgSi, the middle content of NaCl in synthesized powder), NaCl was not detected in the phase composition of ceramic samples.
Microstructures of CaSi powders after heat treatment are shown in Figure 10. Microstructures of MgSi powders after heat treatment are shown in Figure 11. Microstructures of CaMgSi powders after heat treatment are shown in Figure 12. When the heat treatment temperature of the powders increases, the particle sizes of CaSi, MgSi and CaMgSi powders increase.
The particle sizes of CaSi powders (Figure 10) after heat treatment at 400 and 600 °C are significantly less than 100 nm. After heat treatment at a temperature of 800 °C, the particle size of CaSi powders can be estimated as belonging to the range of 50–200 nm.
MgSi powder (Figure 11), after heat treatment at 400–600 °C, consisted of quite big particles with dimension 2-14 μm. Magnification at 20 kx gives no opportunity to make a clear conclusion about the structure of these big particles, which probably could be strong aggregates of very small particles. The dimension of particles which can be seen in micro photos of powder MgSi after 800 °C give us the opportunity to estimate the dimensions of particles at 100–200 μm.
CaMgSi powder (Figure 12), after heat treatment at 400–600 °C, consisted of both small particles and quite big particles with dimension 2–10 μm. These big particles look like aggregates of very small particles, of a dimension which is very difficult to estimate. At micrographs of powders after 400 and 600 °C, particles of NaCl of cubic form can be found. Above 800 °C, particles of elongated morphology with dimension 2–4 μm and with prismatic form with dimension 0.2–1 μm can be found in the micro photos. According to XRD, NaCl and diopside CaMgSi2O6 are the crystalline phases presented at this temperature. Probably, it was the particles of these phases that entered the lens aperture.
The apparent density of preceramic samples prepared from CaSi powder was 1.15–1.20 g/sm3; the apparent density of preceramic samples prepared from MgSi powder was 1.30–1.35 g/sm3; the apparent density of preceramic samples prepared from CaMgSi powder was 1.35–1.40 g/sm3 (Figure 13). Taking into account the theoretical density of wollastonite (2.910 g/cm3, PDF card #27-88), enstatite/clinoenstatite/forsnterite (3.210 g/cm3, PDF card #71-786/3.206 g/cm3, PDF card 35-610/3.194 g/cm3, PDF card #71-1081) and diopside (3.272 g/cm3, PDF card 72-1497), the relative densities of pressed powder compacts were about 41–43%. The highest densities of ceramic samples after firing were 1.60 g/sm3 (CaSi_1100, relative density—55%), 1.70 g/sm3 (MgSi_1200, relative density—53%) and 1.85 g/sm3 (CaMgSi_1200, relative density—56%). The decrease in CaSi_1200 sample density (1.33 g/cm3, relative density—46%) can be explained by the phase transition of wollastonite to pseudo-wollastonite (2.886 g/cm3, PDF card # 74-874).
The liner shrinkage of ceramic samples was 28, 23 and 21% for samples of Ca_1100, Mg_1100 and CaMg_1100 after firing at 1100 °C (Figure 14). In spite of relatively obvious changes in dimensions, no high densities or dense microstructures of ceramics were reached. The high content of volatile components which were evacuated practically up to 1100 °C can be taken as a reason of the low density of the ceramic samples under investigation.
So, during heat treatment at the temperatures below melt formation, NaCl can be treated as a component preventing sintering due to its presence in the powder compact. After melt forming, we could treat it as an environment which provided conditions for processes of particle re-arrangement and densification due to the capillary effect and for processes of melting/crystallization. At the same time, just after melting (according TA data, interval 780–820 °C with minimum at 808 °C), NaCl starting its evacuation from the ceramic samples. And this evacuation of NaCl can prevent densification of the ceramic samples. In fact, the content of NaCl in synthesized powders under investigation was different (Table 2). Shrinkage of all samples was notable, but the densities of samples were not high. The low density of preceramic samples and the presence of volatile components (H2O, NaCl) can be taken as obvious reasons of the low density of ceramic samples. The observed behavior of NaCl during ceramic preparation gives the opportunity to take NaCl as an inorganic pore-forming agent which can be evacuated during heating in the contrast with other inorganic porogens that can be removed by washing [44].
Microstructures of ceramic samples after firing at 1100 °C are shown in Figure 15. Cross-sections of ceramic samples presented at Figure 15 demonstrate the porous microstructure of ceramics prepared from synthesized powders. The ceramic sample looks like under sintered. Pores with dimensions 4–10 μm can be seen in the microstructure of CaSi_1100. Pores with dimensions 2–4 μm can be seen in the microstructure of MgSi_1100. The low apparent density of this sample (Figure 13) gives the opportunity to make a conclusion about closed porosity, which cannot be seen in the micro photo of MgSi_1100 ceramic sample. Pores with dimensions 10–20 μm can be seen in the microstructure of CaMgSi_1100.

4. Conclusions

Aqua solutions of sodium silicate Na2SiO3 and calcium CaCl2 and/or magnesium MgCl2 chlorides were used with the preset molar ratios Ca/Si = 1 (CaSi), Mg/Si = 1 (MgSi), and (Ca+Mg)/Si = 1 (CaMgSi) for synthesis of powders with phase composition including quasi-amorphous phases and calcium carbonate CaCO3 in the form of calcite or aragonite and sodium halite NaCl as a reaction by-product. Hydrated calcium and/or magnesium silicates, hydrated silica, magnesium carbonate, hydrated magnesium carbonate or hydrated magnesium silicate containing carbonate ions are suspected as components of the quasi-amorphous phase presented in synthesized powders. Mass loss of powder samples under investigation during heat treatment was due to the evacuation of H2O (m/z = 18), CO2 (m/z = 44) and NaCl at temperatures above its melting point. After sintering at 1100 °C, the phase composition of ceramic samples included wollastonite CaSiO3 (CaSi_1100); enstatite MgSiO3, clinoenstatite MgSiO3, forsterite Mg2SiO4 (MgSi_1100), and diopside CaMgSi2O6 (CaMgSi_1100). After sintering at 1200 °C, the phase composition of ceramics CaSi_1200 included pseudo-wollastonite CaSiO3. After heat treatment at 1300 °C, the phase composition of MgSi_1300 powder preferably included protoenstatite MgSiO3. The remarkable content of halide NaCl in the synthesized powders did not change the preset molar ratios of Ca/Si = 1, and (Ca+Mg)/Si = 1, as it was confirmed with XRD data of samples after firing at 800–1200 °C. It is possible to assume that during heating of MgSi powder and ceramics with preset molar ratio Mg/Si = 1, a rearrangement of SiO2 between high temperature amorphous and crystalline phases takes place, with the possibility of forsterite Mg2SiO4 formation. The phase composition of all samples after heat treatment belongs to the oxide system CaO–MgO–SiO2. Ceramic materials in this system are of interest for use in different areas, including construction materials, refractories and biomaterials. Powders prepared in the present investigation, both via precipitation and heat treatment, can also be used for the creation of materials with specific properties and in model experiments as lunar regolith simulants.

Author Contributions

Conceptualization, T.V.S.; methodology, T.V.S.; investigation, T.V.S., A.S.S., S.A.S., T.B.S., Y.Y.F., O.V.B., V.B.P., T.V.F., A.M.M., X.F. and M.R.A.; resources, T.B.S., O.V.B. and V.B.P.; writing—original draft preparation, A.S.S., S.A.S. and T.V.S.; writing—review and editing, T.V.S.; visualization, T.V.S., A.S.S., S.A.S., T.B.S., Y.Y.F., O.V.B., V.B.P., A.M.M., and M.R.A.; supervision, T.V.S.; project administration, T.V.S.; funding acquisition, M.R.A. All authors have read and agreed to the published version of the manuscript.

Funding

This work was carried out with the support of the MSU Program of Development, Project No 23-SCH01-16.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

This research was carried out using the equipment of the MSU Shared Research Equipment Center “Technologies for obtaining new nanostructured materials and their complex study”, which had been purchased by MSU within the framework of the Equipment Renovation Program (National Project “Science”), and within the framework of the MSU Program of Development.

Conflicts of Interest

The authors declare no conflicts of interest.

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Compounds 05 00022 g001
Figure 1. Scheme of powders and ceramic sample preparation.
Figure 1. Scheme of powders and ceramic sample preparation.
Compounds 05 00022 g001
Compounds 05 00022 g002
Figure 2. XRD of synthesized powders: o—halite NaCl (PDF card # 5-628; #96-900-8679); c—calcite CaCO3 (PDF card # 5-586; #96-900-0966); a—aragonite CaCO3 (PDF card # 41-1475; #96-210-0188).
Figure 2. XRD of synthesized powders: o—halite NaCl (PDF card # 5-628; #96-900-8679); c—calcite CaCO3 (PDF card # 5-586; #96-900-0966); a—aragonite CaCO3 (PDF card # 41-1475; #96-210-0188).
Compounds 05 00022 g002
Compounds 05 00022 g003
Figure 3. XRD of reaction by-products extracted from mother liquors collected after vacuum separation of precipitates: o—halite NaCl (PDF card # 5-628; #96-900-8679).
Figure 3. XRD of reaction by-products extracted from mother liquors collected after vacuum separation of precipitates: o—halite NaCl (PDF card # 5-628; #96-900-8679).
Compounds 05 00022 g003
Compounds 05 00022 g004aCompounds 05 00022 g004b
Figure 4. SEM images of the powders synthesized from aqua solutions of sodium silicate and calcium chloride—CaSi (a); sodium silicate and calcium and magnesium chlorides—CaMgSi (b); and sodium silicate and magnesium chlorides—MgSi (c).
Figure 4. SEM images of the powders synthesized from aqua solutions of sodium silicate and calcium chloride—CaSi (a); sodium silicate and calcium and magnesium chlorides—CaMgSi (b); and sodium silicate and magnesium chlorides—MgSi (c).
Compounds 05 00022 g004aCompounds 05 00022 g004b
Compounds 05 00022 g005
Figure 5. Thermal analysis data of the synthesized powders: TG (a) and DSC (b).
Figure 5. Thermal analysis data of the synthesized powders: TG (a) and DSC (b).
Compounds 05 00022 g005
Compounds 05 00022 g006
Figure 6. Mass spectra of the synthesized powders for m/z = 18 (a) and m/z = 44 (b).
Figure 6. Mass spectra of the synthesized powders for m/z = 18 (a) and m/z = 44 (b).
Compounds 05 00022 g006
Compounds 05 00022 g007
Figure 7. Thermal evolution of phase composition for samples based on CaSi powder: o—halite (NaCl, # 5-628; #96-900-8679); c—calcite CaCO3 (PDF card # 5-586; #96-900-0966); a—aragonite CaCO3 (PDF card # 41-1475; # 96-210-0188); w—wollastonite CaSiO3 (PDF card #27-88, #96-900-5779); p—pseudo-wollastonite CaSiO3 (PDF card # 74-874, #96-900-2251).
Figure 7. Thermal evolution of phase composition for samples based on CaSi powder: o—halite (NaCl, # 5-628; #96-900-8679); c—calcite CaCO3 (PDF card # 5-586; #96-900-0966); a—aragonite CaCO3 (PDF card # 41-1475; # 96-210-0188); w—wollastonite CaSiO3 (PDF card #27-88, #96-900-5779); p—pseudo-wollastonite CaSiO3 (PDF card # 74-874, #96-900-2251).
Compounds 05 00022 g007
Compounds 05 00022 g008
Figure 8. Thermal evolution of phase composition for samples based on MgSi powder: o—halite NaCl (PDF card # 5-628; #96-900-8679); e—enstatite MgSiO3, (PDF card #71-786, #96-900-1594); f—forsterite Mg2SiO4, (PDF card #71-1081, #96-900-7378); I—clinoenstatite MgSiO3 (PDF card 35-610, #96-900-8078); !—protoenstatite MgSiO3 (PDF card # 11-273, #96-154-8550).
Figure 8. Thermal evolution of phase composition for samples based on MgSi powder: o—halite NaCl (PDF card # 5-628; #96-900-8679); e—enstatite MgSiO3, (PDF card #71-786, #96-900-1594); f—forsterite Mg2SiO4, (PDF card #71-1081, #96-900-7378); I—clinoenstatite MgSiO3 (PDF card 35-610, #96-900-8078); !—protoenstatite MgSiO3 (PDF card # 11-273, #96-154-8550).
Compounds 05 00022 g008
Compounds 05 00022 g009
Figure 9. Thermal evolution of phase composition for samples based on CaMgSi powder: o—halite NaCl (PDF card # 5-628; #96-900-8679); c—calcite CaCO3, (PDF card # 5-586; #96-900-0966); d—diopside CaMgSiO6 (PDF card 72-1497, #96-900-4554).
Figure 9. Thermal evolution of phase composition for samples based on CaMgSi powder: o—halite NaCl (PDF card # 5-628; #96-900-8679); c—calcite CaCO3, (PDF card # 5-586; #96-900-0966); d—diopside CaMgSiO6 (PDF card 72-1497, #96-900-4554).
Compounds 05 00022 g009
Compounds 05 00022 g010
Figure 10. SEM images of CaSi powders after heat treatment at different temperatures.
Figure 10. SEM images of CaSi powders after heat treatment at different temperatures.
Compounds 05 00022 g010
Compounds 05 00022 g011
Figure 11. SEM images of MgSi powders after heat treatment at different temperatures.
Figure 11. SEM images of MgSi powders after heat treatment at different temperatures.
Compounds 05 00022 g011
Compounds 05 00022 g012
Figure 12. SEM images of CaMgSi powders after heat treatment at different temperatures.
Figure 12. SEM images of CaMgSi powders after heat treatment at different temperatures.
Compounds 05 00022 g012
Compounds 05 00022 g013
Figure 13. Densities preceramic compacts, and ceramic samples after firing at 900, 1000, 1100 and 1200 °C.
Figure 13. Densities preceramic compacts, and ceramic samples after firing at 900, 1000, 1100 and 1200 °C.
Compounds 05 00022 g013
Compounds 05 00022 g014
Figure 14. Linear dimensions of ceramic samples after firing at different temperatures.
Figure 14. Linear dimensions of ceramic samples after firing at different temperatures.
Compounds 05 00022 g014
Compounds 05 00022 g015aCompounds 05 00022 g015b
Figure 15. SEM images of ceramic samples after firing at 1100 °C.
Figure 15. SEM images of ceramic samples after firing at 1100 °C.
Compounds 05 00022 g015aCompounds 05 00022 g015b
Table 1. Labeling of prepared powders, volumes and concentrations of aqua solutions of Na2SiO3·5H2O, CaCl2 and MgCl2·6H2O used for the powder syntheses.
Table 1. Labeling of prepared powders, volumes and concentrations of aqua solutions of Na2SiO3·5H2O, CaCl2 and MgCl2·6H2O used for the powder syntheses.
№LabelingConcentration × Volume
Na2SiO3·5H2OCaCl2MgCl2·6H2O
1CaSi0.5 M × 0.5 л0.5 M × 0.5 л-
2CaMgSi0.5 M × 0.5 л0.5 M × 0.25 л0.5 M × 0.25 л
3MgSi0.5 M × 0.5 л-0.5 M × 0.5 л
Table 2. Distribution of NaCl between the synthesized powders and the mother liquors.
Table 2. Distribution of NaCl between the synthesized powders and the mother liquors.
LabelingExpected Mass of NaCl, molExpected Mass of NaCl, gMass of NaCl Extracted from Mother Liquor, Mass of NaCl in Powder (Calculated), %
g%
by-CaSi0.2529.2155248
by-CaMgSi0.2529.2227723
by-MgSi0.2529.2217327
Table 3. Phase composition (weight %)1 of synthesized powder CaSi powder, CaSi powder after heat treatment and ceramics after firing.
Table 3. Phase composition (weight %)1 of synthesized powder CaSi powder, CaSi powder after heat treatment and ceramics after firing.
SamplePhases Detected (Crystalline Phases Only)
Halite
#96-900-8679
NaCl		Calcite
#96-900-0966
CaCO3		Aragonite
#96-210-0188
CaCO3		Wollastonite
#96-900-5779
CaSiO3		Pseudo-Wollastonite
#96-900-2251
CaSiO3
CaSi (synthesized powder)47%23%30%--
CaSi_400 (powder)55%15%30%--
CaSi_600 (powder)100%----
CaSi_800 (powder)24%--76%-
CaSi_900 (ceramics)8%--92%-
CaSi_1000 (ceramics)9%--91%-
CaSi_1100 (ceramics)---100%-
CaSi_1200 (ceramics)----100%
1 According to data from “Match!” software.
Table 4. Phase composition (weight %)1 of synthesized MgSi powder, MgSi powder after heat treatment and ceramics after firing.
Table 4. Phase composition (weight %)1 of synthesized MgSi powder, MgSi powder after heat treatment and ceramics after firing.
SamplePhases Detected (Crystalline Phases Only)
Halite
#96-900-8679		Enstatite
MgSiO3
#96-900-1594		Forsterite
Mg2SiO4
#96-900-7378		Clinoenstatite
MgSiO3
#96-900-8078		Protoenstatite
MgSiO3
#96-154-8550
MgSi (synthesized powder)100%- --
MgSi_400 (powder)100%- --
MgSi_600 (powder)100%- --
MgSi_800 (powder)16%84% --
MgSi_900 (ceramics)-63%37%--
MgSi_1000 (ceramics)-73%27%--
MgSi_1100 (ceramics)-26%43%30%-
MgSi_1200 (ceramics)--25%28%47%
MgSi_1300 (powder)--6%-94%
1 According to data from “Match!” software.
Table 5. Phase composition (weight %) 1 of synthesized CaMgSi powder, CaMgSi powder after heat treatment and ceramic CaMgSi after firing.
Table 5. Phase composition (weight %) 1 of synthesized CaMgSi powder, CaMgSi powder after heat treatment and ceramic CaMgSi after firing.
SamplePhases Detected (Crystalline Phases Only)
Halite
#96-900-8679
NaCl		Calcite
#96-900-0966
CaCO3		Diopside
#96-900-4554
CaMgSi2O6
CaMgSi (synthesized powder)55%45%-
CaMgSi_400 (powder)60%40%-
CaMgSi_600 (powder)100%--
CaMgSi_800 (powder)54%-46%
CaMgSi_900 (ceramics)2-98%
CaMgSi_1000 (ceramics)--100%
CaMgSi_1100 (ceramics)--100%
CaMgSi_1200 (ceramics)--100%
1 According to data from “Match!” software.
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Safronova, T.V.; Sultanovskaya, A.S.; Savelev, S.A.; Shatalova, T.B.; Filippov, Y.Y.; Boytsova, O.V.; Platonov, V.B.; Filippova, T.V.; Murashko, A.M.; Feng, X.; et al. Powders Synthesized from Water Solutions of Sodium Silicate and Calcium and/or Magnesium Chlorides. Compounds 2025, 5, 22. https://doi.org/10.3390/compounds5020022

AMA Style

Safronova TV, Sultanovskaya AS, Savelev SA, Shatalova TB, Filippov YY, Boytsova OV, Platonov VB, Filippova TV, Murashko AM, Feng X, et al. Powders Synthesized from Water Solutions of Sodium Silicate and Calcium and/or Magnesium Chlorides. Compounds. 2025; 5(2):22. https://doi.org/10.3390/compounds5020022

Chicago/Turabian Style

Safronova, Tatiana V., Alexandra S. Sultanovskaya, Sergei A. Savelev, Tatiana B. Shatalova, Yaroslav Y. Filippov, Olga V. Boytsova, Vadim B. Platonov, Tatiana V. Filippova, Albina M. Murashko, Xinyan Feng, and et al. 2025. "Powders Synthesized from Water Solutions of Sodium Silicate and Calcium and/or Magnesium Chlorides" Compounds 5, no. 2: 22. https://doi.org/10.3390/compounds5020022

APA Style

Safronova, T. V., Sultanovskaya, A. S., Savelev, S. A., Shatalova, T. B., Filippov, Y. Y., Boytsova, O. V., Platonov, V. B., Filippova, T. V., Murashko, A. M., Feng, X., & Akhmedov, M. R. (2025). Powders Synthesized from Water Solutions of Sodium Silicate and Calcium and/or Magnesium Chlorides. Compounds, 5(2), 22. https://doi.org/10.3390/compounds5020022

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