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Powders Synthesized from Calcium Carbonate and Water Solutions of Potassium Hydrosulfate of Various Concentrations

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19 August 2024

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22 August 2024

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Abstract
Powders with phase composition including syngenite K2Ca(SO4)2·H2O and/or calcium sulfate di-hydrate (gypsum) CaSO4·2H2O were synthesized from powder of calcium carbonate CaCO3 and water solutions of potassium hydrosulfate KHSO4 of various (0.5M, 1M and 2M) concentrations. Molar ratio of starting salt KHSO4/CaCO3=2 were used to provide formation of syngenite K2Ca(SO4)2·H2O. But when using 0.5M water solution of potassium hydrosulfate KHSO4 the phase composition of synthesized powder was presented by calcium sulfate dihydrate (gypsum) CaSO4·2H2O. When using 1M and 2M water solution of potassium hydrosulfate KHSO4 the syn-genite K2Ca(SO4)2·H2O was found as predominant phase in synthesized powders. According es-timations made from thermal analysis data when 1.0M and 2.0M water solutions of potassium hydrosulfate KHSO4 were used the content of calcium sulfate dihydrate (gypsum) CaSO4·2H2O in these powders were not higher then as 7.9 and 1.9 % respectively. Phase composition of products isolated from mother liquors via water evaporation consisted of syngenite K2Ca(SO4)2·H2O and potassium sulfate (arcanite) K2SO4. Synthesized powders can be used in preparation of biocom-patible bioresorbable materials with phase composition in the K2O‑CaO‑SO3‑H2O system; as matrix of thermo- or photo-luminescent materials; as components reducing the setting time and increasing strength of sulfate cements; in the fertilizing industry and also as a components of Martian regolith simulant.
Keywords: 
Subject: Chemistry and Materials Science  -   Materials Science and Technology

1. Introduction

The following mineral salts belong to the K2O-CaO-SO3-H2O system: potassium sulfate K2SO4, potassium pyrosulfate K2S2O7, potassium hydrosulfates KHSO4, K3H(SO4)2, K9H7(SO4)8·H2O, calcium sulfate hemihydrate CaSO4·0.5H2O, calcium sulfate dihydrate CaSO4·2H2O, syngenite K2Ca(SO4)2·H2O, and gorgeyite K2SO4·5CaSO4·H2O [1,2]. These minerals existing in different subsystems of K2O-CaO-SO3-H2O system [3,4,5,6,7], especially syngenite K2Ca(SO4)2·H2O, are important for several applications, among which are fertilizer industry [8,9], production of construction materials [10,11,12], inorganic materials with specific properties [13], creation of bioresorbable and biocompatible materials [14,15], creation of Martian regolith simulants [16,17,18].
Syngenite K2Ca(SO4)2·H2O can be synthesized using several different techniques: by crystallization at room temperature (20 ◦C and ∼50% relative humidity) during evaporation of a solution containing calcium sulfate CaSO4 and potassium sulfate K2SO4 in a molar ratio of 1:50 [19]; via interaction of hot solutions (80 oC, 100 oC) of calcium nitrate Ca(NO3)2 and potassium sulfate K2SO4 [20]; via interaction of calcium sulfate CaSO4 and potassium chloride KCl in water medium [21]; or under mechanical activation in planetary mill from powder mixture of potassium sulfate K2SO4 and calcium sulfate dihydrate CaSO4·2H2O [17,22].
Syngenite K2Ca(SO4)2·H2O as a solid cement stone was synthesized by adding of water to the powder mixture consisting of calciolangbeinite K2Ca2(SO4)3 and potassium sulfate K2SO4 [15] or to the powder mixture of calcium sulfate CaSO4 and potassium sulfate K2SO4 [23].
The aim of this investigation consisted in the development of robust method of synthesis of syngenite K2Ca(SO4)2·H2O powder not containing reaction by-products which need to be removed via washing of precipitate. Chemical interaction of powder of calcium carbonate CaCO3 and potassium hydrosulfate KHSO4 water solution will provide forming both target precipitate and unstable reaction by-product H2CO3 which can decompose during synthesis forming H2O and CO2. The release of gaseous CO2 from the reaction zone will be an expected sign of a chemical reaction.

2. Materials and Methods

2.1. Materials

Powders of CaCO3 (GOST 4530-76, Rushim, Moscow, Russia) and KHSO4 (GOST 4223-75, Rushim, Moscow, Russia) were used for syntheses of powders.

2.2. Synthesis of Powders

Quantities of reagents were calculated according to Reaction (1).
2KHSO4 + CaCO3 = K2Ca(SO4)2·H2O + CO2,
Molar ratio of starting salts KHSO4/CaCO3 in each synthesis was equal 2 and taken according to reaction (1) to produce syngenite K2Ca(SO4)2·H2O. Table 1 contains labeling and synthesis conditions of powders under investigation. 400 ml of 0.5M, 1.0M, 2.0M KHSO4 water solutions were prepared and used in the experiment. Calculated quantities of CaCO3 power were added to each solution. Suspensions were kept on magnetic stirrer during 3 hours until the gas release stopped. Conditions of powders’ synthesis and labeling of samples under investigation are presented in Table 1.
Precipitates were separate from the mother liquor using vacuum filtration, placed in the plastic trays and left to dry for a week. Then powders were collected, weighted, crushed in the agate mortar and passed through the sieve with the 200 μm cells. Transparent mother liquors were collected and products solved in were extracted from solutions via evaporation when keeping them at 40 oC for water evacuation and products crystallization. Substances extracted from mother liquors separated from synthesized powders SP0.5M SP1.0M and SP2.0M were labeled as Ex-SP0.5M, Ex-SP1.0M, Ex-SP2.0M respectively.

2.3. Methods of Analysis

The phase composition of the powders obtained after the synthesis and drying was determined by X-ray powder diffraction (XRD) analysis using Rigaku Miniflex 600 diffractometer (CuKα radiation, Kβ filter, and D/teX Ultra detector) in Bragg–Brentano geometry (Rigaku Corporation, Tokyo, Japan) with an angle interval 2Ѳ from 5° to 70° (step 2Ѳ − 0.02°). Phase analysis was performed using the ICDD PDF2 database [24] and Match software (version https://www.crystalimpact.com/, 15 August 2024).
Bulk densities of samples were calculated as mass of 1,0 ml of synthesized powders after drying, crashing in the agate mortar and passing through the sieve with 200 μm sells.
Scanning electron microscopy (SEM) images of the synthesized powder were characterized by SEM on Tescan Vega II (Tescan, Brno, Czech Republic) at accelerating voltages from 1 to 20 kV in secondary electron imaging mode (SE2 detector). Gold layers (≤10 nm in thickness) on the surface of the powder samples were applied (Quorum Technologies spraying plant, Q150T ES, Great Britain, London, UK).
Thermal analysis (TA) including thermogravimetry (TG) and differential thermal analysis (DTA) was performed using an NETZSCH STA 449 F3 Jupiter thermal analyzer (NETZSCH, Selb, Germany) during heating in air (10 °C/min, 40–1000 °C), the specimen mass being at least 10 mg. The gas-phase composition was monitored by a Netzsch QMS 403 Quadro quadrupole mass spectrometer (NETZSCH, Selb, Germany) coupled with a NETZSCH STA 449 F3 Jupiter thermal analyzer (NETZSCH, Selb, Germany). The mass spectra were registered for the following m/Z values: 18 (H2O); 64 (SO2).

3. Results and Discussion

According to the XRD data of synthesized powders shown in Figure 1, the phase composition of SP0.5M powder included CaSO4·2H2O (PDF card 33-313). The phase composition of SP1.0M and SP2.0M powders was represented preferably by syngenite K2Ca(SO4)2·H2O (PDF card 28-739) and CaSO4·2H2O in small extent. Reflexes of CaSO4·2H2O in XRD graph of SP2.0M powder hardly can be seen in the Figure 1 but in case of greater magnification very small reflexes could be seen. No traces of starting salts presented in the synthesized powders.
Phase composition determined using Match software and mass of synthesized powders are summarized in Table 2.
The calcium sulfate dihydrate CaSO4·2H2O present in the synthesized powder could be obtained according in accordance with the reaction (2).
2KHSO4 + CaCO3 + H2O = CaSO4·2H2O + CO2 +K2SO4,
Using Match software gypsum was determined in quantity of 7.1 and 1.3 mass % in powders SP1.0M and SP2.0M respectively. One can see that the syngenite K2Ca(SO4)2·H2O content in the synthesized powder depended on the conditions of the synthesis and increased with increasing of concentrations of the initial reagents (Table 2).
Camera photos of products extracted via evaporation from the transparent mother liquors are shown in Figure 2. There is no noticeable difference in the appearance of products collected from mother liquors except obviously bigger quantity of Ex-SP0.5M. All extracted products consisted of white transparent elongated crystals.
According XRD the phase composition of products extracted via evaporation from the mother liquors (Figure 3) consisted of potassium sulfate (arcanite) K2SO4 (card PDF 5-613) and syngenite K2Ca(SO4)2·H2O.
The presence of K2SO4 in the extracted products was provided due to reaction (2), and K2Ca(SO4)2·H2O (Ksp (K2Ca(SO4)2·H2O) = 1.88 × 10−4) formed due to presence of Ca2+, K+ and SO42- in the mother liquors during water evaporation. The phase composition determined using Match software and mass the of extracted products are shown in Table 3. The higher the concentration of water solutions of potassium hydrosulfate KHSO4 the lower the mass of products extracted from mother liquors and lower the contents of potassium sulfate K2SO4 in the extracted products are.
SEM-images of synthesized powders are presented in Figure 4.
Powder SP0.5M with phase composition presented by gypsum CaSO4·2H2O consisted of particles having elongated prismatic morphology, which was typical for this mineral, with 5-20 μm long and 2-4 μm wide. Powders SP1.0M and SP2.0M with phase composition presented preferably by syngenite K2Ca(SO4)2·H2O consisted of particles having plate morphology which is typical for this mineral [15] with dimension 4-20 μm and 0.5-2 μm thick. It should be noted that syngenite K2Ca(SO4)2·H2O earlier was found to form elongated crystals [25] or even can also form “felt-like structure consisting of long, rail-like crystals” with a length of 10–20 μm in the paste based on the СаSO4·2H2O and К2SO4 [26].
Plate and elongated morphology of particles and their small dimensions can be taken as a reason of low balk density of synthesized powders which was 0.85, 0.80 and 0.86 g/cm3 for powders SP0.5M, SP1.0M and SP2.0M respectively (Figure 5). Taking into account calculated density of gypsum CaSO4·2H2O (2.310 g/cm³, # 96-901-7314, Match) and syngenite K2Ca(SO4)2·H2O (2.575 g/cm³, # 96-900-8129, Match) relative densities of powders SP0.5M, SP1.0M and SP2.0M were 37, 34 and 33 % respectively.
Thermal analysis data for the synthesized powders are shown at Figure 6.
Total mass loss of powder SP0.5M at 1000 oC was 20.7% and this was in the good agreement with the possible mass loss of gypsum CaSO4·2H2O (20,9%) according to equation (2).
CaSO4·2H2O = CaSO4 + 2H2O
Total mass loss of powders SP1.0M and SP2.0M were 6.7% and 5.7% at 1000 oC respectively. No traces of SO2 (m/Z=64) were registered in the released gas phase during heating. Mass spectra for H2O (m/Z=18) confirmed that mass loss of all synthesized powders during heating were due to H2O evacuation (Figure 7). H2O left sample SP0.5M in interval 90-200 with maximum at 144 oC. Mass loss of SP1.0M and SP2.0M went through two stage. First stage for powders SP1.0M were in the interval 106-155 oC with maximum at 134 oC and SP2.0M in the interval 99-153 oC with maximum 128 oC. This mass loss for powders SP1.0M and SP2.0M also could be due to decomposition of the smallest quantity of gypsum CaSO4·2H2O which were found by means of XRD (Figure 1, Table 2). Mass loss at the first stage of TA data give as opportunity to estimate quantity of gypsum CaSO4·2H2O as 7.9 and 1.9 % in powders SP1.0M and SP2.0M respectively. It worth to note that the estimation made taking into account TA data is close to the estimation of quantity of gypsum CaSO4·2H2O made by using Match software. The second stage of mass loss of SP1.0M and SP2.0M were in interval 230-310 oC with maximum 276 oC. This interval corresponds to the interval 250-300 oC determined previously [27]. Temperature interval 230-280 oC was used earlier for investigation of syngenite K2Ca(SO4)2·H2O decomposition in the isothermal conditions [28]. Taking into account XRD data reaction (3) could reflect the process of thermal decomposition of syngenite K2Ca(SO4)2·H2O in this interval.
K2Ca(SO4)2·H2O = K2Ca(SO4)2 + 2H2O
Endothermal effect for SP05M at interval 100-230 oC with minimum at 147 oC which was due to thermal dehydration of gypsum CaSO4·2H2O (Figure 8).
There are 5 endothermal effects at the DSC curves of powders SP1.0M and SP2.0M. Endothermal effect in the intervals 90-180 oC with minimum at 130 oC (SP1.0M) and 90-160 oC with minimum at 126 oC (SP2.0M) correspond to thermal decomposition of gypsum CaSO4·2H2O presenting in these powders (reaction (2)). Endothermal effect in the interval 240-310 oC with minimum at 280 oC for powders SP1.0M and SP2.0M was due to dehydration of sygenite K2Ca(SO4)2·H2O (reaction (3)). The following three endothermal effects for SP1.0M and SP2.0M powders are at 557, 877 and 950 oC. According to the binary system K2SO4-CaSO4 the following phases potassium sulfate K2SO4, calcium sulfate CaSO4 and calciolangbeinite K2Ca2(SO4)3 exist in this system before 500 oC [29]. Reactions (4) and (5) can reflect formation of all these minerals [30,31].
3K2Ca(SO4)2 → K2Ca2(SO4)3 + 2K2SO4 + CaSO4
2 K2Ca(SO4)2 → K2Ca2(SO4)3 + K2SO4
There is transformation of β-K2SO4 to α-K2SO4 at 550 oC, formation of eutectic melt at 875 oC and transformation of K2Ca2(SO4)3 to K2Ca2(SO4)3 at 940 oC in the binary system K2SO4-CaSO4. Endothermal effects at the experimental curves for SP1.0M and SP2.0M powders are in the obvious agreement with the events possible according with existing information about binary system K2SO4-CaSO4.

4. Conclusions

A new method for the powder synthesis with phase composition represented preferably by syngenite K2Ca(SO4)2·H2O from 1M and 2M water solutions of potassium hydrosulfate KHSO4 and powder of calcium carbonate CaCO3 as starting reagents has been proposed. By using of TA and XRD data it was found that content of calcium sulfate dihydrate (gypsum) CaSO4·2H2O in powders synthesized from 1M and 2M water solutions of potassium hydrosulfate KHSO4 were not higher then as 7.9 and 1.9 % respectively. According SEM images these powders consisted of particles with plate morphology. When using 0.5M water solution of potassium hydrosulfate KHSO4, powder of calcium sulfate dihydrate (gypsum) CaSO4·2H2O consisting of particles with elongated prismatic morphology was obtained. The phase composition of extracted products isolated from the mother liquors collected after all syntheses was represented by potassium sulfate K2SO4 and syngenite K2Ca(SO4)2·H2O. Synthesized powders including precipitated and separated from mother liquors can be used in preparation of biocompatible bioresorbable materials with phase composition in the K2O-CaO-SO3-H2O system, as matrixes of luminescent materials, as components reducing the setting time and increasing strength of sulfate cements, in the fertilizing industry and also as a component of Martian regolith simulant.

Author Contributions

Conceptualization, T.V.S.; methodology, T.V.S.; investigation, T.V.S., P.D.L., A.I.Z., X.L., T.B.S., O.V.B., D.R.K., M.M.A., Z.X. and M.R.A.; resources, D.R.K., T.B.S., O.V.B.; writing—original draft preparation, P.D.L., A.I.Z., T.V.S.; writing—review and editing, T.V.S.; visualization, T.V.S., P.D.L., A.I.Z., D.R.K., T.B.S., O.V.B., M.M.A. 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.

Acknowledgments

In this section, you can acknowledge any support given which is not covered by the author contribution or funding sections. This may include administrative and technical support, or donations in kind (e.g., materials used for experiments).

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 1. XRD of starting salts and synthesized powders: + - KHSO4 (card PDF 11-649); * - K2Ca(SO4)2·H2O (card PDF 28-739); o - CaSO4·2H2O (card PDF 33-311); c – CaCO3 (PDF card # 5-586).
Figure 1. XRD of starting salts and synthesized powders: + - KHSO4 (card PDF 11-649); * - K2Ca(SO4)2·H2O (card PDF 28-739); o - CaSO4·2H2O (card PDF 33-311); c – CaCO3 (PDF card # 5-586).
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Figure 2. Camera photos of products extracted from mother liquors separated from precipitates via filtration and evaporation: Ex-SP0.5M (a), Ex-SP1.0M (b), Ex-SP2.0M (c).
Figure 2. Camera photos of products extracted from mother liquors separated from precipitates via filtration and evaporation: Ex-SP0.5M (a), Ex-SP1.0M (b), Ex-SP2.0M (c).
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Figure 3. XRD of products extracted from mother solutions separated from precipitates after water evaporation and KHSO4 given for comparison: * - K2Ca(SO4)2·H2O (card PDF 28-739); # - K2SO4 (card PDF 5-613); + - KHSO4 (card PDF 11-649).
Figure 3. XRD of products extracted from mother solutions separated from precipitates after water evaporation and KHSO4 given for comparison: * - K2Ca(SO4)2·H2O (card PDF 28-739); # - K2SO4 (card PDF 5-613); + - KHSO4 (card PDF 11-649).
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Figure 4. SEM images of the powders synthesized from powder of CaCO3 and water solutions of KHSO4 with concentration 0.5M (a,b); 1.0M (c,d); and 2.0M (e,f).
Figure 4. SEM images of the powders synthesized from powder of CaCO3 and water solutions of KHSO4 with concentration 0.5M (a,b); 1.0M (c,d); and 2.0M (e,f).
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Figure 5. Bulk densities of synthesized powders.
Figure 5. Bulk densities of synthesized powders.
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Figure 6. Thermal analysis data of the synthesized powders.
Figure 6. Thermal analysis data of the synthesized powders.
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Figure 7. Mass spectra of the synthesized powders for m/Z=18.
Figure 7. Mass spectra of the synthesized powders for m/Z=18.
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Figure 8. DSC of the synthesized powders.
Figure 8. DSC of the synthesized powders.
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Table 1. Conditions of powders’ synthesis. Labeling used for synthesized powders under investigation.
Table 1. Conditions of powders’ synthesis. Labeling used for synthesized powders under investigation.
Labeling1 Concentration of the KHSO4 solution, mol/l Volume of the solution, ml The amount of substances by reaction, mol Mass of reagents, g Expected mass of K2Ca(SO4)2·H2O, g
KHSO4 CaCO3 KHSO4 CaCO3
SP0.5M 0.5 400 0.2 0.1 27.2 10.0 32.8
SP1.0M 1 400 0.4 0.2 54.4 20.0 65.6
SP2.0M 2 400 0.8 0.4 108.8 40.0 131.2
1 SP – synthesized powder.
Table 2. Phase composition and mass of synthesized powders.
Table 2. Phase composition and mass of synthesized powders.
Labeling Expected mass of K2Ca(SO4)2·H2O, g Mass of synthesized powder, g Phase composition of synthesized powder1, mass %
K2Ca(SO4)2·H2O
(#96-900-8129)1 		CaSO4·2H2O
(#96-901-7314)1
SP0.5M 32.8 15.5 0 100
SP1.0M 65.6 58.2 92.9 7.1
SP2.0M 131.2 114.3 98.7 1.3
1 According to data obtained using Match software.
Table 3. Phase composition and mass of products extracted from mother liquors.
Table 3. Phase composition and mass of products extracted from mother liquors.
Labeling Mass of extracted product, g Phase composition of extracted products1, mass %
K2Ca(SO4)2·H2O
(#96-900-8129)1 		K2SO4
(#96-900-7570)1
Ex-SP0.5M 17.21 62.2 37.8
Ex-SP1.0M 5.8 49.1 50.9
Ex-SP2.0M 5.6 83.3 16.7
1 According to data from Match software.
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