Composite Powders Synthesized from the Water Solutions of Sodium Silicate and Different Calcium Salts (Nitrate, Chloride, and Acetate)

: Composite powders were synthesized from the water solutions of sodium silicate and different calcium salts (nitrate, chloride, and acetate) at a Ca/Si molar ratio of 1.0. According to the XRD data, all the synthesized powders included hydrated calcium silicate Ca 1,5 SiO 3,5 · xH 2 O (Ca/Si molar ratio = 1.5) and calcium carbonate CaCO 3 (Ca/Si molar ratio = ∞ ). The presence of H 2 SiO 3 or SiO 2 · xH 2 O in the synthesized powders was assumed to be due to the difference between the Ca/Si molar ratio of 1.0 speciﬁed by the synthesis protocol and the molar ratio of the detected products. Reaction by-products (sodium nitrate NaNO 3 , sodium chloride NaCl, and sodium acetate NaCH 3 COO) were also found in the synthesized powders after ﬁltration and drying. According to the XRD data phase composition of all powders after washing four times consisted of the quasi-amorphous phase and calcium carbonate in the form of calcite. Calcium carbonate in the form of aragonite was detected in powders synthesized from calcium chloride CaCl 2 and calcium nitrate Ca(NO 3 ) 2 before and after washing. Synthesized powders containing reaction by-products and washed powders were used for the preparation of ceramics at 900, 1000, and 1100 ◦ C. The phase composition of the ceramic samples prepared from the washed powders and powder containing NaCl after ﬁring at 900 and 1000 ◦ C consisted of β -wollastonite β -CaSiO 3 , and, after ﬁring at 1100 ◦ C, consisted of both β -wollastonite β -CaSiO 3 and pseudo-wollastonite α -CaSiO 3 . The phase composition of the ceramic samples prepared from powders containing sodium nitrate NaNO 3 and sodium acetate NaCH 3 COO after ﬁring at 900, 1000, and 1100 ◦ C consisted of calcium sodium silicates, i.e., Na 2 Ca 2 Si 3 O 9 (combeite) and Na 2 Ca 3 S i2 O 8 . Synthesized and washed composite powders can be used for the preparation of biocompatible materials, in the technology of construction materials, and as components of lunar soil simulants.


Introduction
Silicon, calcium, and sodium belong to the category of rock-forming elements [1].These elements are present in the Earth's upper continental crust in the form of different minerals, primarily as silicates and aluminosilicates, including hydrated ones and others.These natural minerals are used as starting components for producing materials for different uses, primarily for producing construction materials [2].Silicon and calcium oxides dominate not only in the composition of Earth's upper continental crust, but in lunar soil also.The list of minerals presenting in lunar soil includes plagioclase, clinopyroxene, orthopyroxene, olivine, apatite and others [3].It is well known due to the investigation of probes of lunar soil that silicon and calcium oxides are among the dominating ones, and sodium oxide presents in noticeable quantities in the composition of lunar regolith [4,5].The CaSiO 3 , Ca 2 SiO 4 , Ca 3 SiO 5 , Ca 3 Si 2 O 7 , Na 2 Ca 2 Si 3 O 9 , Na 2 Ca 2 Si 2 O 7 , Na 2 Ca 3 S i2 O 8 , Na 2 CaSiO 4 , and Na 2 SiO 3 that exist in the Na 2 O-CaO-SiO 2 system [6][7][8] are of particular interest.These minerals are of great importance as bonding components (Ca 2 SiO 4 and Ca 3 SiO 5 ) in the production of cement materials [9]; as filler (CaSiO 3 ) increasing the strength of cement or polymer materials [10][11][12]; as raw materials (CaSiO 3 and Na 2 Ca 3 S i2 O 8 ) for the production of ceramics or powders with electrical insulating or photoluminescent properties [13,14]; as components of biocompatible and bioresorbable materials (CaSiO 3 , Ca 2 SiO 4 , Ca 3 SiO 5 , and Na 2 Ca 2 Si 3 O 9 ) [15,16]; as a component of binders in the production of different construction materials with flame-retardant properties (Na 2 SiO 3 ) [17]; and as potential components for the preparation of lunar soil simulants [18].
Most of these minerals can be prepared from powdered or compacted mixtures of components containing precursors of calcium, silicon, and sodium oxides as a result of the solid-state or hetero-phase reactions taking place at high temperatures [19][20][21][22][23]. Sodium calcium silicates and calcium silicates can be prepared via crystallization from glass melts [24][25][26].Wires of CaSiO 3 with an acicular structure grown from the melts may have fine dimensions [27].The particles of powders prepared using a solid-state or hetero-phase reactions under heating ordinarily have larger dimensions in comparison with those prepared via bottom-up synthesis methods.
The task of creating synthetic analogues of natural minerals arises in case of the necessity of preparing active nanosized powders and fine ceramic materials with special functions or properties.To prepare powders or ceramic materials in the Na 2 O-CaO-SiO 2 system, starting powders should be made using bottom-up methods of synthesis.For this reason, the synthetic precursors of calcium silicates such as powdered hydrated calcium silicate should be considered.The formation of hydrated calcium silicates is a basic chemical process in the creation of Portland cement stone [28,29].There are many scientific articles devoted to the investigation of this process.
The syntheses of hydrated calcium silicates in the form of powder via precipitation from solutions in some of these articles were carried out to understand the special features of the processes taking place during cement stone formation [29][30][31].
Hydrated calcium silicates with different CaO/SiO 2 mass ratios of 1.0, 1.3, 1.5 [30], and 1.4 (CaO/SiO 2 molar ratio = 1.5) were synthesized from water solutions prepared from sodium silicate nonahydrate Na 2 SiO 3 •9H 2 O and calcium nitrate tetrahydrate Ca(NO 3 ) 2 • 4H 2 O [29,31].A product containing nanowires of tobermorite Ca 5 (Si 6 O 16 )(OH) 2 •4H 2 O and calcium oxide CaO was prepared from the water solutions of sodium silicate Na 2 SiO 3 and calcium nitrate Ca(NO 3 ) 2 in the presence of surfactants, with n-hexane as the oil phase, and an ammonia solution via hydrothermal synthesis [32].Then, nanowires of CaSiO 3 were synthesized via the heat treatment of this product.
The availability of water-soluble sodium silicate in the form of pentahydrate Na 2 SiO 3 •5H 2 O or nonahydrate Na 2 SiO 3 •9H 2 O as reagents was probably the reason for using these salts as a source of SiO 3  2− ions in the syntheses of hydrated calcium silicates, including cases of later heat treatment for the production of ceramics [33,34].
There are some articles devoted to the syntheses of hydrated calcium silicates or wollastonite using colloidal SiO 2 or Si(OC 2 H 5 ) 4 (TEOS) as precursors of SiO 3 2− ions.For example, the synthesis of hydrated calcium silicates with different CaO/SiO 2 mass ratios of 0.75, 1.0, and 1.33 was conducted using the mechanochemical conditions from silica (Aerosil 200) and freshly obtained CaO from CaCO 3, mixing them in deionized water using a roller mill for 2 days [35].Mechanochemical synthesis was used for the synthesis of calcium silicate from CaCO 3 and silica at room temperature [36].Calcium nitrate tetrahydrate Ca(NO 3 ) 2 •4H 2 O, colloidal SiO 2 (25 wt% of silica with an average diameter of 15 nm), ammonium nitrate NH 4 NO 3 , and citric acid C 6 H 8 O 7 •H 2 O were used as precursors in the combustion method of wollastonite synthesis [37].Wollastonite α-CaSiO 3 ceramics were made at 1400 • C from β-CaSiO 3 powder, which was prepared from product synthesized from Ca(NO 3 ) 2 •4H 2 O and Si(OC 2 H 5 ) 4 (TEOS) gel and NH 4 OH as the precipitant.The former reagent (0.0455 mol) was dissolved in 100 mL of ethanol containing a small amount of distilled water (2 mL), while the latter reagent (0.05 mol) was dissolved in 100 mL of absolute ethanol [38].Samples of amorphous calcium silicates were prepared from Ca(NO 3 ) 2 •4H 2 O, Si(OC 2 H 5 ) 4 (TEOS) taken at different ratios of Ca/Si, and HNO 3 as the catalysator of the TEOS hydrolysis and later heat treatment at 700 • C [39].
The powders of hydrated calcium silicate consisting of fine particles were synthesized via precipitation from aqua solutions of sodium silicate Na 2 SiO 3 and different calcium salts including nitrate Ca(NO 3 ) 2 , chloride CaCl 2 , and acetate Ca(CH 3 COO) 2 [40].Synthesis from solutions of sodium silicate Na 2 SiO 3 and acetate Ca(CH 3 COO) 2 was also made in the presence of amino acids [40].From the listed examples, one can see that CaO, CaCO 3 , Ca(NO 3 ) 2 , CaCl 2 , and Ca(CH 3 COO) 2 can be used as a precursors of Ca 2+ in the syntheses of hydrated calcium silicates.
The preservation of the 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 interactions [41].So, using a water solution of sodium silicate Na 2 SiO 3 can provide the presence of sodium salts in the synthesized powder as a reaction by-product when any calcium salt would be used as a precursor of Ca 2+ in the exchange reactions.
The aim of the present work consisted in preparing and investigating two series of powders synthesized from the water solutions of sodium silicate Na 2 SiO 3 and different soluble calcium salts (nitrate Ca(NO 3 ) 2 , chloride CaCl 2 , and acetate Ca(CH 3 COO) 2 ) via precipitation at the molar ratio Ca/Si = 1.0.The preservation in synthesized powders of sodium salts, i.e., NaNO 3 , NaCl, and NaCH 3 COO as reaction by-products, was planned for the first series as a possibility of creating a powder mixtures for the preparation of composite powders or ceramic materials in the Na 2 O-CaO-SiO 2 system.The powders of the second series were prepared from the synthesized powders after filtration and drying, as well as washing four times to remove the reaction by-products, for the comparison and clarification of the role of sodium salts as reaction by-products in the formation of microstructure and the phase composition of ceramics or heat-treated powders.

Materials
Sodium silicate pentahydrate Na 2 SiO  In total, 500 mL of the 0.5 M solutions of sodium silicate pentahydrate Na 2 SiO 3 •5H 2 O were added to 500 mL of the 0.5 M solutions of the calcium salts.Prepared slurries of white synthesized products were kept in the mother solutions for 15 min under stirring.Then, the synthesized products labeled as Ca_N, Ca_Cl, and Ca_Ac (see Table 1) were separated from the mother liquors using vacuum filtering and dried for a week at room temperature.The collected mother liquors were kept at 40 • C for weeks to extract the solved reaction by-products.In total, 20 g of each of synthesized powder (Ca_N, Ca_Cl, and Ca_Ac) was placed in 200 mL of distilled water for the preparation of suspension and to remove the reaction by-products captured by the synthesized powders.After vacuum filtration, the washing liquid was collected and the solved products were extracted using drying at 40 • C. The procedure of washing and the collection of washing liquid was repeated four times.Then, the synthesized and washed powders labeled as Ca_NW, Ca_ClW, and Ca_AcW (see Table 1) were dried for a week at room temperature.After drying, the synthesized powders containing the reaction by-products and powders went through the washing stage four times were ground in an agate mortar and passed through a sieve with a cell size of 200 µm.

Preparation of Ceramic Samples
Pre-ceramic samples with diameter 12 mm and height 2-3 mm were prepared via uniaxial one-sided pressing at 100 MPa using a steel die and a manual press (Carver Laboratory Press model C, Fred S. Carver, Inc., Wabash, IN, USA).Then, pre-ceramic powder compacts were fired in the air in a furnace at 900 • C, 1000 • C, and 1100 • C, with exposure at the specified temperatures for 2 h (the heating rate of the furnace was 5 • C/min).

Methods of Analysis
The relative changing of the linear dimension of the samples after the heat treatment and the density of the samples before and after the heat treatment were calculated using Equations ( 4) and (5), respectively, D/D 0 -relative diameter of the sample after the heat treatment, %; D heat treatment -diameter of the sample after the heat treatment, cm; D press -diameter of the sample after pressing, cm; where: ρ-density of the sample, g/cm 3 ; 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 before and after the heat treatment were measured with an accuracy of ±0.001 g and ±0.01 mm, respectively.
The phase composition of the powders obtained and the ceramic samples after firing was determined with X-ray powder diffraction (XRD) analysis using a Rigaku D/Max-2500 diffractometer (Rigaku Corporation, Tokyo, Japan) with a rotating anode (CuKα radiation) and angle interval 2Θ: from 2 • to 70 • (step 2Θ − 0.02 • ).The phase analysis was performed using the ICDD PDF2 database and Match software (version https://www.crystalimpact.com/, 30 June 2023).
Scanning electron microscopy (SEM) images of the synthesized powders and powders after washing were characterized using an NVision 40 microscope (Carl Zeiss, Jena, Germany) using an in-lens detector.SEM images of powders were also taken using a VPSE detector in high vacuum conditions, and SEM images of ceramic samples were taken using a secondary electron imaging mode (SE2) detector with the electron microscope LEO SUPRA 50VP (Carl Zeiss, Jena, Germany; auto-emission source) with an accelerating voltage of 20 kV.The samples of powders and ceramic samples were pre-coated with a layer of chromium or gold.A chromium (up to 20 nm in thickness)/gold layer (up to 10 nm in thickness) on the surface of the powders and ceramic samples was applied to the samples (Quorum Technologies spraying plant, Q150T ES, Great Britain, London, UK).

Results and Discussion
According to the XRD data (Figure 1, Table 2), the phase composition of the powders synthesized from the 0.5 M water solutions of sodium silicate, calcium nitrate, chloride, and acetate included an artificial hydro silicate with the composition Ca 1,5 SiO 3,5 •xH 2 O. Peaks with the interplanar spacing of 3.02 A (29.54 • ) and 1.81 A (50.45 • ) possibly belong to calcium silicate hydrate, which has a variable Ca/Si ratio from 0.8 to 1.5 [42,43].The main peaks of calcium silicate hydrate are 29.54,33.25, and 50.45 • .Peaks with an interplanar spacing of 3.04 A (29.34 • ), 0.278 A (32 • ), and 1.82 A (50.45 • ) in work [31] were attributed to calcium silicate hydrate, synthesized from solutions at a Ca/Si molar ratio of 1.5 (CaO/SiO 2 mass ratio = 1.4).It should be noted that CaCO 3 presented in the form of calcite in the synthesized powders Ca_N, Ca_Cl, and Ca_Ac, and in the form of aragonite in the powders Ca_N and Ca_Cl also.According to the XRD data, sodium nitrate NaNO3 and sodium chloride NaCl as reaction by-products were found in the powders Ca_N and Ca_Cl, respectively.Sodium acetate NaCH3COO was not detected in the powder Ca_Ac using the XRD analysis in spite of the fact that the reaction of calcium silicate synthesis took place.
To explain the presence of CaCO3 in the phase composition of the synthesized powders, the hydrolysis of sodium silicate in the water solution should be taken into account.The first stage of sodium silicate hydrolysis can be reflected by Equation ( 9) and the second stage of sodium silicate hydrolysis can be reflected by Equation (10) [44]: NaHSiO3 + HOH ⇄ H2SiO3 + NaOH (10) These reactions clearly explain the highly basic pH level in sodium silicate aqueous solutions.The presence of NaOH in the reaction zone could provide the opportunity for the following Reactions (11)- (13)  According to the XRD data, sodium nitrate NaNO 3 and sodium chloride NaCl as reaction by-products were found in the powders Ca_N and Ca_Cl, respectively.Sodium acetate NaCH 3 COO was not detected in the powder Ca_Ac using the XRD analysis in spite of the fact that the reaction of calcium silicate synthesis took place.
Reactions ( 6)- (8) give us the opportunity to assume the formation of silicic acid H 2 SiO 3 or hydrated silica SiO 2 •xH 2 O and the presence of these components in the synthesized powders in the form of amorphous or quasi-amorphous components.According to the synthesis protocol, the Ca/Si molar ratio was established as equal 1.0.So, the formation of hydrated silicon oxide or silicic acid can be assumed as it appears in Reactions ( 6)- (8).
To explain the presence of CaCO 3 in the phase composition of the synthesized powders, the hydrolysis of sodium silicate in the water solution should be taken into account.The first stage of sodium silicate hydrolysis can be reflected by Equation (9) and the second stage of sodium silicate hydrolysis can be reflected by Equation (10) [44]: NaHSiO 3 + HOH H 2 SiO 3 + NaOH (10) These reactions clearly explain the highly basic pH level in sodium silicate aqueous solutions.The presence of NaOH in the reaction zone could provide the opportunity for the following Reactions ( 11)-( 13) in all syntheses performed: 2NaOH + Ca(NO 3 ) 2 = Ca(OH) 2 + 2NaNO 3 (11) 2NaOH During process of drying, fine Ca(OH) 2 can react with the CO 2 presenting in the air according to Reaction (14).
According to the XRD data (Figure 2, Table 2), reaction by-products such as sodium nitrate NaNO 3 or sodium chloride NaCl were not detected in the washed powders Ca_NW or Ca_ClW, respectively.Calcium carbonate CaCO 3 in the form of calcite definitely presented in all powders after washing.Calcium carbonate CaCO 3 in the form of aragonite was detected in the powders Ca_NW and Ca_ClW.It should be noted that, in the case of preventing an interaction with the CO 2 , presenting in the air, graphs of XRD confirms the absence of calcium carbonate [34].Reflexes and places of possible reflexes of the hydrated calcium silicate are marked in Figure 2, but the intensities of these reflexes are very low.The appearance of the XRD graphs give us the opportunity to draw a conclusion about the presence of the amorphous or quasi-amorphous phase in all powders after washing.During process of drying, fine Ca(OH)2 can react with the CO2 presenting in the air according to Reaction (14).
According to the XRD data (Figure 2, Table 2), reaction by-products such as sodium nitrate NaNO3 or sodium chloride NaCl were not detected in the washed powders Ca_NW or Ca_ClW, respectively.Calcium carbonate CaCO3 in the form of calcite definitely presented in all powders after washing.Calcium carbonate CaCO3 in the form of aragonite was detected in the powders Ca_NW and Ca_ClW.It should be noted that, in the case of preventing an interaction with the CO2, presenting in the air, graphs of XRD confirms the absence of calcium carbonate [34].Reflexes and places of possible reflexes of the hydrated calcium silicate are marked in Figure 2, but the intensities of these reflexes are very low.The appearance of the XRD graphs give us the opportunity to draw a conclusion about the presence of the amorphous or quasi-amorphous phase in all powders after washing.

Powder Phase Composition According XRD
The weights of the reaction by-products extracted from the mother liquors after the separation of precipitates are presented in Figure 3 and in Table 3.According to the estimation of the weight of by-products, the maximum relative part (32.7%) of the by-product The weights of the reaction by-products extracted from the mother liquors after the separation of precipitates are presented in Figure 3 and in Table 3.According to the estimation of the weight of by-products, the maximum relative part (32.7%) of the byproduct was kept in the synthesized Ca_N powder, and the minimum relative part (16.6%) of the by-product was kept in the Ca_Ac powder.According to the SEM images presented in Figure 4, all synthesized powders consisted of nanosized particles with morphology close to isometric form and with dimensions of 10-50 nm in all synthesized powders Ca_N, Ca_Cl, and Ca_Ac.These data on the microstructure of powders and particle dimensions differ slightly from the data on powders, in particular, those synthesized from calcium nitrate presented in the article [40].As it was noticed before, the quantity of the same by-product captured by the synthesized powder can give some base for the estimation of the specified surface area and the dimension of particles [33].The larger the weight of the captured by-product, the larger the specific surface area is and the smaller the particle dimension is.In this investigation, we can assume that the difference in nature of adsorbed and occluded by-products can make a difference in the captured mass.Moreover, we can guess that aggregates of particles in the powder Ca_Ac were denser than aggregates in the other synthesized powders, probably due to the action of some kind of "glue" formed as a result of the acetate-anion having an organic nature and hydrated silica interaction.The quantity of captured by-product in the Ca_Ac powder was relatively lower for this reason.
differ slightly from the data on powders, in particular, those synthesized from calcium nitrate presented in the article [40].As it was noticed before, the quantity of the same byproduct captured by the synthesized powder can give some base for the estimation of the specified surface area and the dimension of particles [33].The larger the weight of the captured by-product, the larger the specific surface area is and the smaller the particle dimension is.In this investigation, we can assume that the difference in nature of adsorbed and occluded by-products can make a difference in the captured mass.Moreover, we can guess that aggregates of particles in the powder Ca_Ac were denser than aggregates in the other synthesized powders, probably due to the action of some kind of "glue" formed as a result of the acetate-anion having an organic nature and hydrated silica interaction.The quantity of captured by-product in the Ca_Ac powder was relatively lower for this reason.differ slightly from the data on powders, in particular, those synthesized from calcium nitrate presented in the article [40].As it was noticed before, the quantity of the same byproduct captured by the synthesized powder can give some base for the estimation of the specified surface area and the dimension of particles [33].The larger the weight of the captured by-product, the larger the specific surface area is and the smaller the particle dimension is.In this investigation, we can assume that the difference in nature of adsorbed and occluded by-products can make a difference in the captured mass.Moreover, we can guess that aggregates of particles in the powder Ca_Ac were denser than aggregates in the other synthesized powders, probably due to the action of some kind of "glue" formed as a result of the acetate-anion having an organic nature and hydrated silica interaction.The quantity of captured by-product in the Ca_Ac powder was relatively lower for this reason.The weight of the reaction by-products extracted from the washing liquids as a result of 1st, 2nd, 3rd, and 4th washing is presented in Figure 5.According to the diagram, the quantity of the collected products diminished substantially from the 1st to the 4th washing.All washing liquids (solutions) collected just after washing were translucent.A camera photo of the glasses containing washing liquids after some time being held in a drying box at 40 °C (Figure 6) shows that the washing liquids (solutions) after the 2nd, 3rd, and 4th washing of the Ca_Ac powder became opalescent.This observation may indicate the formation of colloidal SiO2 particles during the drying of the collected washing liquids (solutions) and confirms the formation and existence of hydrated silica SiO2•xH2O or silicic acid H2SiO3 in the synthesized powders.The same transformations were observed in the similar series of glasses of solutions collected after washing of Ca_N and Ca_Cl powders after some time being held in a drying box at 40 °C.The weight of the reaction by-products extracted from the washing liquids as a result of 1st, 2nd, 3rd, and 4th washing is presented in Figure 5.According to the diagram, the quantity of the collected products diminished substantially from the 1st to the 4th washing.All washing liquids (solutions) collected just after washing were translucent.A camera photo of the glasses containing washing liquids after some time being held in a drying box at 40 • C (Figure 6) shows that the washing liquids (solutions) after the 2nd, 3rd, and 4th washing of the Ca_Ac powder became opalescent.This observation may indicate the formation of colloidal SiO 2 particles during the drying of the collected washing liquids (solutions) and confirms the formation and existence of hydrated silica SiO 2 •xH 2 O or silicic acid H 2 SiO 3 in the synthesized powders.The same transformations were observed in the similar series of glasses of solutions collected after washing of Ca_N and Ca_Cl powders after some time being held in a drying box at 40   The weight of the reaction by-products extracted from the washing liquids as a result of 1st, 2nd, 3rd, and 4th washing is presented in Figure 5.According to the diagram, the quantity of the collected products diminished substantially from the 1st to the 4th washing.All washing liquids (solutions) collected just after washing were translucent.A camera photo of the glasses containing washing liquids after some time being held in a drying box at 40 °C (Figure 6) shows that the washing liquids (solutions) after the 2nd, 3rd, and 4th washing of the Ca_Ac powder became opalescent.This observation may indicate the formation of colloidal SiO2 particles during the drying of the collected washing liquids (solutions) and confirms the formation and existence of hydrated silica SiO2•xH2O or silicic acid H2SiO3 in the synthesized powders.The same transformations were observed in the similar series of glasses of solutions collected after washing of Ca_N and Ca_Cl powders after some time being held in a drying box at 40 °C.According to the XRD data (Figure 7a-c), the phase composition of the collected byproducts and products after the 1st washing was presented by sodium nitrate NaNO3 (powder Ca_N), sodium chloride NaCl (powder Ca_Cl), and sodium acetate NaCH3COO (powder Ca_Ac).According to the XRD data (Figure 7a-c), the phase composition of the collected byproducts and products after the 1st washing was presented by sodium nitrate NaNO3 (powder Ca_N), sodium chloride NaCl (powder Ca_Cl), and sodium acetate NaCH3COO (powder Ca_Ac).The phase composition of products extracted from the 3rd and 4th washing solutions has not been determined with the help of the ICDD PDF2 database or Match software.But, the XRD graphs for these products look very similar regardless of the starting pair of precursors.The main XRD reflex of the products extracted from the 3rd and 4th washing (Figure 7d) solutions of the Ca_N and Ca_Ac powders are in 2Θ interval 8.77-8.98• , and Ca_Cl in 2Θ interval 8.64-9.26• .

Ca_Ac Ca_Ac Ca_Ac
The SEM images of the powders after washing four times (Ca_NW, Ca_ClW, and Ca_AcW) are presented in Figure 8. Similar to the synthesized powders, those after washing also consisted of nanosized particles combined into aggregates.The morphology of the particles remained close to isometric form and the dimensions were about the same 10-50 nm in the powders Ca_N and Ca_Cl, and 20-50 nm in the powder Ca_Ac.
The phase composition of products extracted from the 3rd and 4th washing solutions has not been determined with the help of the ICDD PDF2 database or Match software.But, the XRD graphs for these products look very similar regardless of the starting pair of precursors.The main XRD reflex of the products extracted from the 3rd and 4th washing (Figure 7d The thermogravimetry (TG), differential thermogravimetry (DTG), and differential scanning calorimetry (DSC) curves for the synthesized powders are shown in Figure 9.The thermogravimetry (TG), differential thermogravimetry (DTG), and differential scanning calorimetry (DSC) curves for the synthesized powders are shown in Figure 9.The total mass loss of the Ca_N powder when heated to 1000 °C was 32.6% (Figure 9a).The four steps of mass loss can be found in the DTG curve of the Ca_N powder.The first step 40-168 °C according to the mass spectrometry data was due to the physically bonded and hydrated water removal.The two next slight steps (180-290 and 290-410 °C) The total mass loss of the Ca_N powder when heated to 1000 • C was 32.6% (Figure 9a).The four steps of mass loss can be found in the DTG curve of the Ca_N powder.The first step 40-168 • C according to the mass spectrometry data was due to the physically bonded and hydrated water removal.The two next slight steps (180-290 and 290-410 • C) were due to CO 2 evacuation.The final step of mass loss (410-730 • C) according to the mass spectrometry data was connected with nitrogen oxides evacuation and confirmed by NO (m/z = 30) detected in the gas flow.The following reactions can be suggested to reflect the chemical transformation in the Ca_N powder during heating: Ca 1,5 SiO Reactions ( 15) and ( 16) describe the dehydration of the hydrated calcium silicate and hydrated silica.In the presence of extra sources of calcium such as calcium carbonate and compounds containing silicon, the formation of dicalcium silicate is possible (reactions ( 17) and ( 18)) [45].Taking into account the phase diagram CaO-SiO 2 , we can imagine that the way from CaCO 3 as a precursor of CaO and increasing quantity of SiO 2 can be the following: CaO → Ca 3 SiO 5 → Ca 2 SiO 4 → Ca 3 Si 2 O 7 → CaSiO 3 .But, this way can possibly be realized in the absence of sodium salts.Reactions ( 19) and ( 20) can explain the mass loss due to the evolution of nitrogen oxides, which were detected with the help of mass spectrometry for one of them with m/z = 30 (NO) [46].These two reactions ( 19) and ( 20) can also reflect the variants of sodium calcium silicates formation during heating from the mixture of silica, calcium carbonate, and sodium nitrate.Sodium calcium silicates with formulars Na 2 Ca 2 Si 3 O 9 (combeite) [47] and Na 2 Ca 3 Si 2 O 8 in the Na 2 O-CaO-SiO 2 system [8] were detected by means of XRD analysis in the phase composition of ceramics made from Ca_N powder after firing at 900, 1000, and 1100 • C (Figure 10a).
were due CO2 evacuation.The final step of mass loss (410-730 °C) according to the mass spectrometry data was connected with nitrogen oxides evacuation and confirmed by NO (m/z = 30) detected in the gas flow.The following reactions can be suggested to reflect the chemical transformation in the Ca_N powder during heating: Ca1,5SiO3,5•xH2O = Ca1,5SiO3,5 + xH2O (15) H2SiO3 + 2CaCO3 = Ca2SiO4 + H2O + 2CO2 (18) 3SiO2 Reactions ( 15) and ( 16) describe the dehydration of the hydrated calcium silicate and hydrated silica.In the presence of extra sources of calcium such as calcium carbonate and compounds containing silicon, the formation of dicalcium silicate is possible (reactions ( 17) and ( 18)) [45].Taking into account the phase diagram CaO-SiO2, we can imagine that the way from CaCO3 as a precursor of CaO and increasing quantity of SiO2 can be the following: CaO → Ca3SiO5 → Ca2SiO4 → Ca3Si2O7 → CaSiO3.But, this way can possibly be realized in the absence of sodium salts.Reactions ( 19) and ( 20) can explain the mass loss due to the evolution of nitrogen oxides, which were detected with the help of mass spectrometry for one of them with m/z = 30 (NO) [46].These two reactions ( 19) and ( 20) can also reflect the variants of sodium calcium silicates formation during heating from the mixture of silica, calcium carbonate, and sodium nitrate.Sodium calcium silicates with formulars Na2Ca2Si3O9 (combeite) [47] and Na2Ca3Si2O8 in the Na2O-CaO-SiO2 system [8] were detected by means of XRD analysis in the phase composition of ceramics made from Ca_N powder after firing at 900, 1000, and 1100 °C (Figure 10a).The total mass loss of the Ca_Cl powder when heated to 1000 °C was 36.5% (Figure 9b).The curves of TG and DTG give us the opportunity to make a guess as to the three steps of mass loss.The first step 40-350 °C according to the mass spectrometry data was due to physically bonded and hydrated water removal.The evacuation of CO2 started from 340 °C and the second step of mass loss took place due to this process up to 690 °C.One can see the sharp endothermic effect at 795 °C, and then no signals at the mass spectrometry curves for m/z = 18 or for m/z = 44 were detected.According to the scientific literature, the melting point for NaCl presenting in this Ca_Cl powder was determined as 810 °C [48], 801 °C [49], 1070K (797 °C) [50], or 1064 ± 14 K (791 ± 14 °C) [51].It was shown earlier that NaCl can transfer to the gaseous phase from the powder during heating above the melting point [52].So, the mass loss at the third step from the melting of sodium chloride up to 985 °C was due to NaCl transfer to the gaseous phase.According to the XRD data (Figure 10b), the phase composition of ceramics based on powder Ca_Cl was presented by wollastonite CaSiO3 after firing at 900 and 1000 °C and by wollastonite CaSiO3 and pseudo-wollastonite CaSiO3 after firing at 1100 °C.According to the literature, the The total mass loss of the Ca_Cl powder when heated to 1000 • C was 36.5% (Figure 9b).The curves of TG and DTG give us the opportunity to make a guess as to the three steps of mass loss.The first step 40-350 • C according to the mass spectrometry data was due to physically bonded and hydrated water removal.The evacuation of CO 2 started from 340 • C and the second step of mass loss took place due to this process up to 690 • C. One can see the sharp endothermic effect at 795 • C, and then no signals at the mass spectrometry curves for m/z = 18 or for m/z = 44 were detected.According to the scientific literature, the melting point for NaCl presenting in this Ca_Cl powder was determined as 810 • C [48], 801 • C [49], 1070K (797 • C) [50], or 1064 ± 14 K (791 ± 14 • C) [51].It was shown earlier that NaCl can transfer to the gaseous phase from the powder during heating above the melting point [52].So, the mass loss at the third step from the melting of sodium chloride up to 985 • C was due to NaCl transfer to the gaseous phase.According to the XRD data (Figure 10b), the phase composition of ceramics based on powder Ca_Cl was presented by wollastonite CaSiO 3 after firing at 900 and 1000 • C and by wollastonite CaSiO 3 and pseudowollastonite CaSiO 3 after firing at 1100 • C. According to the literature, the transformation of wollastonite to pseudo-wollastonite takes place at 1398 K (1125 • C) [53].So, the presence of both phases is possible due to the expected phase transformation and due to the kinetics of the transformation process.No phases containing sodium were detected.This fact gives an additional confirmation of NaCl escaping from the Ca_Cl samples (powder or ceramics) during heating at temperatures above the melting point.
The total mass loss of the Ca_Ac powder when heated to 1000 • C was 32.1% (Figure 9c).The curves of TG and DTG give us the opportunity to make a guess as to the three steps of mass loss.The first step 40-270 • C according to the mass spectrometry data was due to physically bonded and hydrated water removal.Reactions ( 15) and ( 16) describe the dehydration processes.The second step of mass loss was in the interval of 270-550 • C. Reaction (21) describes the thermal decomposition of sodium acetate presented in powder Ca_Ac with the evolution of CO 2 and H 2 O in the atmosphere containing oxygen.
The third step of mass loss was in the interval of 550-750 • C. According to the mass spectrometry data, the evaluation of CO 2 (m/z = 44) was the reason of mass loss.According to the XRD data (Figure 10c), the phase composition of ceramics based on powder Ca_Ac after firing at 900, 1000, and 1100 • C included sodium calcium silicates with formulars Na 2 Ca 2 Si 3 O 9 and Na 2 Ca 3 Si 2 O 8 .An evaluation of CO 2 can be taken as a result of reactions ( 22) and ( 23), reflecting the possible way of formation of the phase composition of ceramics prepared from powder Ca_Ac containing reaction by-product-sodium acetate NaCH 3 COO.
TG, DTG, and DSC curves for the Ca_NW, Ca_ClW, and Ca_AcW powders (washed four times using distilled water and not containing reaction by-products) are shown in Figure 11.It should be noted that TG, DTG, and DSC curves for powders Ca_NW, Ca_ClW, and Ca_AcW not containing reaction by-products look very similar.
The total mass loss of the Ca_NW powder when heated to 1000 • C was 24.4% (Figure 11a).The total mass loss of the Ca_ClW powder when heated to 1000 • C was 28.7% (Figure 11b).The total mass loss of the Ca_AcW powder when heated to 1000 • C was 25.7% (Figure 11c).Two steps of mass loss can be found for all washed powders, taking into account the TG and DTG curves.The first step of mass loss for the Ca_NW, Ca_ClW, and Ca_AcW powders in the interval of 40-400 • C according to the mass spectrometry data was mainly due to physically bonded and hydrated water removal.Reactions ( 15) and ( 16) describe the dehydration processes.
The second stage of mass loss for the Ca_NW and Ca_AcW powders was in the interval of 400-800 • C, and for the Ca_ClW powder it was in the interval of 400-825 • C. According to the mass spectrometry data, the mass loss in the second stage was only due to CO 2 evolution.
One can see exothermic peaks at 839 • C in the interval of 811-865 • C at the DSC curve for the Ca_NW powder, at 827 • C in the interval of 792-863 • C for the Ca_ClW powder, and at 837 • C in the interval of 813-867 • C for the Ca_AcW powder.These peaks reflect the phase transformation of dicalcium silicate from γ-Ca 2 SiO 4 (calcioolivine) [54] to α'-Ca 2 SiO 4 taking place at 1120 K (848 • C) according to the literature [53]).This transformation can be taken as evidence of dicalcium silicate formation according to reactions (24) and (25) from anhydrous calcium silicate and anhydrous silica in the presence of a local excess of calcium carbonate.2Ca 1,5 SiO 3,5 + CaCO 3 = 2Ca 2 SiO 4 + CO 2 (24) According to the XRD data, the phase composition of the ceramic samples prepared from the Ca_NW, Ca_ClW, and Ca_AcW powders only included the phase of wollastonite CaSiO 3 after firing at 900 and 1000 • C and the phases of wollastonite CaSiO 3 and pseudo wollastonite CaSiO 3 after firing at 1100 • C (Figure 12).So, the following hetero-phase reactions ( 26) and ( 27) then took place: 2Ca According to the XRD data, the phase composition of the ceramic samples prepared from the Ca_NW, Ca_ClW, and Ca_AcW powders only included the phase of wollastonite CaSiO3 after firing at 900 and 1000 °C and the phases of wollastonite CaSiO3 and pseudo wollastonite CaSiO3 after firing at 1100 °C (Figure 12).So, the following hetero-phase reactions ( 26) and ( 27) then took place: 2Ca1,5SiO3,5 + SiO2 = 3CaSiO3 (26)  The bulk densities of powders, the preceramic powder compact densities, and the densities of samples after firing are presented in Table 4.It should be noted that the bulk densities of washed powders (0.33-0.37 g/cm 3 ) were lower than those of synthesized powders (0.44-0.54 g/cm 3 ).Also, the densities of preceramic powder compacts made from washed powders (0.85-0.87 g/cm 3 ) were lower than those made from synthesized powders (1.08-1.41g/cm 3 ).The presence or absence of reaction by-products could make an impact in this property.Ceramic samples made from Ca_Cl and Ca_N powders had low densities after firing at any temperature in the interval of 900-1100 °C despite the demonstrated ordinary dependence of linear shrinkage from temperature (Figure 13).The largest densities in the used firing conditions were demonstrated by the ceramic samples made from Ca_Ac powders (1.33-1.35g/cm 3 ).This slight firing success was probably due to the short-term presence of Na2CO3 with a melting point of 854 °C.
Ceramic samples made from Ca_Ac powder had the largest linear shrinkage (18-20%) among the samples prepared from powders containing reaction by-products (Figure 13).The linear shrinkage of ceramic samples made from powders (Ca_NW, Ca_ClW, and Ca_AcW) not containing reaction by-products did not demonstrate any dependence on firing temperature in the diapason used and was 15-17% for Ca_ClW ceramic samples and 18-21% for the Ca_NW and Ca_AcW ceramic samples.One of the reasons for the demonstrated low tendency towards densification consisted in mutual hetero-phase reactions taking place in the ceramic samples which prevented densification.Another reason, at least for the ceramic samples based on washed powders and based on Ca_Cl powder, The bulk densities of powders, the preceramic powder compact densities, and the densities of samples after firing are presented in Table 4.It should be noted that the bulk densities of washed powders (0.33-0.37 g/cm 3 ) were lower than those of synthesized powders (0.44-0.54 g/cm 3 ).Also, the densities of preceramic powder compacts made from washed powders (0.85-0.87 g/cm 3 ) were lower than those made from synthesized powders (1.08-1.41g/cm 3 ).The presence or absence of reaction by-products could make an impact in this property.Ceramic samples made from Ca_Cl and Ca_N powders had low densities after firing at any temperature in the interval of 900-1100 • C despite the demonstrated ordinary dependence of linear shrinkage from temperature (Figure 13).The largest densities in the used firing conditions were demonstrated by the ceramic samples made from Ca_Ac powders (1.33-1.35g/cm 3 ).This slight firing success was probably due to the short-term presence of Na 2 CO 3 with a melting point of 854 • C.
Ceramic samples made from Ca_Ac powder had the largest linear shrinkage (18-20%) among the samples prepared from powders containing reaction by-products (Figure 13).The linear shrinkage of ceramic samples made from powders (Ca_NW, Ca_ClW, and Ca_AcW) not containing reaction by-products did not demonstrate any dependence on firing temperature in the diapason used and was 15-17% for Ca_ClW ceramic samples and 18-21% for the Ca_NW and Ca_AcW ceramic samples.One of the reasons for the demonstrated low tendency towards densification consisted in mutual hetero-phase reactions taking place in the ceramic samples which prevented densification.Another reason, at least for the ceramic samples based on washed powders and based on Ca_Cl powder, consisted in the phase transformation of wollastonite to pseudo-wollastonite.The relatively low firing temperatures used in comparison with the melting point of CaSiO 3 (1544 • C [7]) and Na 2 Ca 2 SiO 9 (higher than 1289 • C [55]) probably could not activate diffusion processes to provide the sufficient transport of matter.consisted in the phase transformation of wollastonite to pseudo-wollastonite.The relatively low firing temperatures used in comparison with the melting point of CaSiO3 (1544 °C [7]) and Na2Ca2SiO9 (higher than 1289 °C [55]) probably could not activate diffusion processes to provide the sufficient transport of matter.All ceramic samples had low density and according to the SEM micrographs were under-sintered (Figures 14 and 15).The dimensions of grains increased with the increasing firing temperature.The grains of calcium sodium silicates in the microstructure of Ca_N ceramic samples after firing at 900 °C were 0.3-0.5 µm, after firing at 1000 °C-0.5-1.0 µm, and after firing at 1100 °C-1.0-2.0 µm (Figure 14, first row).The grains of calcium silicate CaSiO3 in the microstructure of Ca_Cl ceramic samples after firing at 900 °C were 100-200 nm, after firing at 1000 °C-100-300 nm, and after firing at 1100 °C-300-500 nm, (Figure 14, second row).The apparent absence of Ca_Cl ceramic samples integrity after firing at any used temperature and noticeable differences in the grains' dimensions from those in Ca_N and Ca_Ac ceramic samples allow us to draw a conclusion about the crucial role of sodium chloride NaCl presenting in this powder and having a relatively low melting point and ability to transfer to the gas phase when heated above the melting temperature.The submicron dimensions of grains in Ca_Cl ceramic samples made from powder containing sodium chloride as a reaction by-product turns our mind to the recollection about the high-temperature surface activity of this melt [56].The grains of calcium sodium silicates in the microstructure of Ca_Ac ceramic samples after firing at 900 °C were 0.1-0.3µm, after firing at 1000 °C-0.5-1.0 µm, and after firing at 1100 °C-1.0-2.0 µm (Figure 14, third row).The microstructures of Ca_Ac ceramic samples after firing at 900 °C and 1000 °C demonstrate signs of melt action during sintering.One can even see the bi-modal porosity both after firing at 900 °C (pores 1.0-2.0µm /10.0-20.0µm) and after firing at 1000 °C (pores 0.1-0.5 µm/5-10µm).All these features escaped in Ca_Ac ceramics after firing at 1100 °C.And, we can admit the identity of the phase composition and microstructure of ceramic samples made from the Ca_N and Ca_Ac powders after firing at 1100 °C.
The grains of calcium silicate CaSiO3 in the microstructure of the Ca_NW ceramic samples after firing at 900 °C were 0.1-0.3µm, after firing at 1000 °C-0.1-0.5 µm, and after firing at 1100 °C-2.0-5.0 µm (Figure 15, first row).One can see in the microstructure of the Ca_NW ceramics the presence of a lot of pores with dimensions of 1.0-5.0µm after firing at 1000 °C and pores with dimensions of 1.0-10.0µm after firing at 1100 °C.The grains of calcium silicate CaSiO3 in the microstructure of the Ca_ClW ceramic samples after firing at 900 °C were 100-200 nm, and after firing at 1000 °C-100-300 nm.As it All ceramic samples had low density and according to the SEM micrographs were under-sintered (Figures 14 and 15).The dimensions of grains increased with the increasing firing temperature.The grains of calcium sodium silicates in the microstructure of Ca_N ceramic samples after firing at 900 • C were 0.3-0.5 µm, after firing at 1000 • C-0.5-1.0 µm, and after firing at 1100 • C-1.0-2.0 µm (Figure 14, first row).The grains of calcium silicate CaSiO 3 in the microstructure of Ca_Cl ceramic samples after firing at 900 • C were 100-200 nm, after firing at 1000 • C-100-300 nm, and after firing at 1100 • C-300-500 nm, (Figure 14, second row).The apparent absence of Ca_Cl ceramic samples integrity after firing at any used temperature and noticeable differences in the grains' dimensions from those in Ca_N and Ca_Ac ceramic samples allow us to draw a conclusion about the crucial role of sodium chloride NaCl presenting in this powder and having a relatively low melting point and ability to transfer to the gas phase when heated above the melting temperature.The submicron dimensions of grains in Ca_Cl ceramic samples made from powder containing sodium chloride as a reaction by-product turns our mind to the recollection about the high-temperature surface activity of this melt [56].The grains of calcium sodium silicates in the microstructure of Ca_Ac ceramic samples after firing at 900 • C were 0.1-0.3µm, after firing at 1000 • C-0.5-1.0 µm, and after firing at 1100 • C-1.0-2.0 µm (Figure 14, third row).The microstructures of Ca_Ac ceramic samples after firing at 900 • C and 1000 • C demonstrate signs of melt action during sintering.One can even see the bi-modal porosity both after firing at 900 • C (pores 1.0-2.0µm /10.0-20.0µm) and after firing at 1000 • C (pores 0.1-0.5 µm/5-10µm).All these features escaped in Ca_Ac ceramics after firing at 1100 • C. And, we can admit the identity of the phase composition and microstructure of ceramic samples made from the Ca_N and Ca_Ac powders after firing at 1100 • C.
The grains of calcium silicate CaSiO 3 in the microstructure of the Ca_NW ceramic samples after firing at 900 • C were 0.1-0.3µm, after firing at 1000 • C-0.1-0.5 µm, and after firing at 1100 • C-2.0-5.0 µm (Figure 15, first row).One can see in the microstructure of the Ca_NW ceramics the presence of a lot of pores with dimensions of 1.0-5.0µm after firing at 1000 • C and pores with dimensions of 1.0-10.0µm after firing at 1100 • C. The grains of calcium silicate CaSiO 3 in the microstructure of the Ca_ClW ceramic samples after firing at 900 • C were 100-200 nm, and after firing at 1000 • C-100-300 nm.As it clearly can be seen in the SEM photos, Ca_ClW ceramic samples after firing at 900 • C and 1000 • C had no satisfying integrity.The microstructure of the Ca_ClW ceramics after firing at 1100 • C was extremely non-uniform and consisted of 1.0-2.0µm grains and tightly sintered segments with dimensions of 2-8 µm consisting of 0.2 nm grains (Figure 15, second row).Th grains of calcium silicate CaSiO 3 in the microstructure of the Ca_AcW ceramic samples after firing at 900 • C were 0.1-0.2µm, and after firing at 1000 • C-0.1-0.3 µm.After firing at 1100 • C, the microstructure of the Ca_AcW ceramics consisted of grains with dimensions 1.0-5.0µm and monoliths elongated up to 10.0 µm (Figure 15, third row).One can see porosity after firing at 1100 • C with a wide pore size distribution (1.0-10.0µm).
clearly can be seen in the SEM photos, Ca_ClW ceramic samples after firing at 900 °C and 1000 °C had no satisfying integrity.The microstructure of the Ca_ClW ceramics after firing at 1100 °C was extremely non-uniform and consisted of 1.0-2.0µm grains and tightly sintered segments with dimensions of 2-8 µm consisting of 0.2 nm grains (Figure 15, second row).Th grains of calcium silicate CaSiO3 in the microstructure of the Ca_AcW ceramic samples after firing at 900 °C were 0.1-0.2µm, and after firing at 1000 °C-0.1-0.3 µm.After firing at 1100 °C, the microstructure of the Ca_AcW ceramics consisted of grains with dimensions 1.0-5.0µm and monoliths elongated up to 10.0 µm (Figure 15, third row).One can see porosity after firing at 1100 °C with a wide pore size distribution (1.0-10.0µm).

Conclusions
Composite powders consisting of hydrated calcium silicate Ca1,5SiO3,5•xH2O (molar ratio Ca/Si = 1.5), calcium carbonate CaCO3 in the form of calcite or aragonite (molar ratio Ca/Si = ꝏ), and silicic acid H2SiO3 or hydrated silicon oxide SiO2•xH2O were synthesized from the water solutions of sodium silicate and different calcium salts (nitrate, chloride, and acetate) at the molar ratio Ca/Si = 1.0.After synthesis, filtration, and drying, these powders included relevant reaction by-products (sodium nitrate NaNO3, sodium chloride NaCl, or sodium acetate NaCH3COO).Powders not containing reaction by-products were prepared by washing them four times.The presence of calcium carbonate CaCO3 in the form of calcite or aragonite in the washed powders became more obvious, suppressing the XRD visibility of the other components of the powders.
The phase composition of ceramic samples prepared from the washed powders and powder containing NaCl after firing at 900 and 1000 °C consisted of wollastonite β-CaSiO3 and after firing at 1100 °C consisted of wollastonite β-CaSiO3 and pseudo-wollastonite α-CaSiO3.The phase composition of ceramic samples prepared from powders containing  The prepared ceramic samples did not demonstrate the appropriate integrity after firing at 900 and 1000 • C. So, heat treatment at these temperatures can be recommended for the preparation of submicron powders with phase composition including wollastonite and/or calcium sodium silicates.
Target-phase composition including both wollastonite and sodium calcium silicate with specified ratios can be reached by managing the protocol of removing reaction byproducts, i.e., sodium salts (NaNO 3 , and NaCH 3 COO).
All composite powders prepared in the present work, except the powder containing sodium nitrate, consist of biocompatible components and can be recommend as a filler for the creation of biocompatible and bioresorbable composites with both inorganic and polymer matrixes.Growing interest in the understanding of rock formation both in the earth and in space (for example, on the moon) demands the creation of model powder systems for investigation of the processes in systems including rock-forming oxide such as SiO 2 , CaO, and Na 2 O. So, prepared composite powders can be used as components for the preparation of lunar soil simulants with the possibility of managing particle size distribution via variation in thermal treatment regimes.The powders of wollastonite and sodium calcium silicates prepared in this investigation can be recommend as special additives in the technology of construction materials.The suggested way of the syntheses of wollastonite and sodium calcium silicates treated as matrixes can be used for the preparation of inorganic materials with special (for example, luminescent) properties.

Figure 3 .Figure 3 .
Figure 3. Weights of the reaction by-products extracted from the mother liquors.

Figure 3 .
Figure 3. Weights of the reaction by-products extracted from the mother liquors.Dimension of bar is 10 µm Dimension of bar is 1 µm Dimension of bar is 200 nm

Figure 4 .
Figure 4. SEM images of the synthesized powders containing reaction by-products.

Figure 5 .
Figure 5. Weight of the reaction by-products extracted from washing liquids.

Figure 4 .
Figure 4. SEM images of the synthesized powders containing reaction by-products.

Figure 4 .
Figure 4. SEM images of the synthesized powders containing reaction by-products.

Figure 5 .
Figure 5. Weight of the reaction by-products extracted from washing liquids.Figure 5. Weight of the reaction by-products extracted from washing liquids.

Figure 5 .
Figure 5. Weight of the reaction by-products extracted from washing liquids.Figure 5. Weight of the reaction by-products extracted from washing liquids.

Figure 6 .
Figure 6.Camera photo of glasses with solutions collected after the washing of the Ca_Ac powder after some time being held in a drying box at 40 °C.

Figure 6 . 24 Figure 6 .
Figure 6.Camera photo of glasses with solutions collected after the washing of the Ca_Ac powder after some time being held in a drying box at 40 • C.According to the XRD data (Figure7a-c), the phase composition of the collected by-products and products after the 1st washing was presented by sodium nitrate NaNO 3 (powder Ca_N), sodium chloride NaCl (powder Ca_Cl), and sodium acetate NaCH 3 COO (powder Ca_Ac).

Figure 8 .
Figure 8. SEM images of synthesized powders washed four times using distilled water (powders not containing reaction by-products).

Figure 8 .
Figure 8. SEM images of synthesized powders washed four times using distilled water (powders not containing reaction by-products).

Figure 13 .
Figure 13.Linear dimensions of the ceramic samples after firing at different temperatures.

Figure 13 .
Figure 13.Linear dimensions of the ceramic samples after firing at different temperatures.

Figure 14 .
Figure 14.SEM images of cross-sections of ceramic samples prepared from synthesized powders containing reaction by-products.Dimension of bar is 10 µm for ceramic samples Ca_N and Ca_Ac, and 1 µm for ceramic samples Ca_Cl.

Figure 14 .Figure 15 .
Figure 14.SEM images of cross-sections of ceramic samples prepared from synthesized powders containing reaction by-products.Dimension of bar is 10 µm for ceramic samples Ca_N and Ca_Ac, and 1 µm for ceramic samples Ca_Cl.

Figure 15 .
Figure 15.SEM images of cross-sections of ceramic samples prepared from synthesized powders washed four times using distilled water (powders not containing reaction by-products).Dimension of bar is 10 µm.

Table 2 .
Phase composition of prepared powders.

Table 2 .
Phase composition of prepared powders.

Table 3 .
Reaction by-products expected, extracted, and remaining in powder.

Table 3 .
Reaction by-products expected, extracted, and remaining in powder.

Table 3 .
Reaction by-products expected, extracted, and remaining in powder.