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Article

Materials in the CaO-K2O-SO3-H2O System Based on Powder Mixtures including Calciolangbeinite K2Ca2(SO4)3 and Calcium Sulfate Anhydrite CaSO4

by
Alexander I. Kuznetsov
1,
Tatiana V. Safronova
1,2,*,
Tatiana B. Shatalova
1,2,
Yaroslav Y. Filippov
2,3,
Leonid A. Vaymugin
4,
Vyacheslav S. Vlasenko
5 and
Maxim S. Likhanov
1
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, Michurinsky Pr. 1, 119192 Moscow, Russia
4
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii Pr. 31, 119071 Moscow, Russia
5
Department of Physics, Lomonosov Moscow State University, Building 2, Leninskie Gory 1, 119991 Moscow, Russia
*
Author to whom correspondence should be addressed.
Ceramics 2023, 6(3), 1434-1448; https://doi.org/10.3390/ceramics6030088
Submission received: 1 June 2023 / Revised: 23 June 2023 / Accepted: 28 June 2023 / Published: 5 July 2023
Browse Figures
Versions Notes

Abstract

:
Materials (cement stone samples) in the CaO-K2O-SO3-H2O system with the target phase compositions, including syngenite K2Ca(SO4)2·H2O and calcium sulfate dihydrate CaSO4·2H2O, were prepared from powder mixtures of calcium sulfate anhydrite CaSO4, and/or calciolangbeinite K2Ca2(SO4)3, and potassium sulfate K2SO4 via hydration reactions at a water/powder ratio within an interval of 0.5–0.9. It was revealed that samples with contents of 25, 50, 75 and 100 mol% of syngenite K2Ca(SO4)2·H2O demonstrated a nonlinear dependence of their respective microstructures on their phase compositions. The microstructures of samples with phase compositions of 25 and 75 mol% of syngenite K2Ca(SO4)2·H2O consisted of pillar crystals. The microstructures of samples with phase compositions of 50 and 100 mol% of syngenite K2Ca(SO4)2·H2O consisted of plate crystals. An explanation of microstructure formation was set forth, taking into account equilibria of the dissolution–crystallization processes during cement stone formation. Materials obtained in the CaO-K2O-SO3-H2O system consisting of biocompatible and resorbable (soluble in water) phases can be recommended for testing as potential substances for bone defect treatments.

1. Introduction

Bioresorbable materials are in demand in modern medicine for creation matrices that promote and support bone regeneration, thus eliminating the “gold standard” of using bone material taken from the human body [1,2]. Materials based on calcium phosphates with a molar ratio of 0.5 < Ca/P < 1.67, calcium carbonates [3] and calcium sulfates [4] have the ability to resorb when used in regenerative methods of bone defect treatment [5,6,7,8,9]. Different composite materials containing calcium sulfates and other components such as calcium phosphates [10,11,12,13,14] or calcium citrate tetrahydrate [15] in the phase composition are now under investigation.
Minerals in the CaO-K2O-SO3-H2O system, such as calcium sulfate hemihydrate CaSO4·0.5H2O, calcium sulfate dihydrate CaSO4·2H2O, syngenite K2Ca(SO4)2·H2O [16], and gorgeyite K2SO4·5CaSO4·H2O [17] are well known in the area of cements production for construction and in the fertilizer industry. Hydrated calcium potassium sulfates formed during storage of Portland cement clinker are generally regarded as undesirable constituents [18]. Hydrated calcium potassium sulfates such as syngenite K2Ca(SO4)2·H2O and gorgeyite K2SO4·5CaSO4·H2O are used as additives, which can increase the strength and hardening rate of sulfate cements [19,20]. These double salts are used both as complex activators for the hardening of sulfate binders and, on the other hand, as collimating agents [21]. Hydration of gypsum in K2SO4 solutions takes place only partially [22]. Double sulfates such as syngenite K2Ca(SO4)2·H2O and gorgeyite K2SO4·5CaSO4·H2O can be used for the preparation of luminophores based on calciolangbeinite K2Ca2(SO4)3. Materials with photoluminescent properties, based on both pure calciolangbeinite K2Ca2(SO4)3 and its mixtures with rare-earth elements, namely, europium Eu and terbium Tb, can be prepared [23,24].
Minerals presented in the CaO-K2O-SO3-H2O system mentioned above can be resorbed during implantation when interacting with physiological fluids. Some of these minerals are used as components [4,25,26,27] of biocompatible materials. Some of them can be used due to the possibility of providing a neutral pH level by the obvious mechanism of dissolution of salts formed by a strong acid and a strong base [28].
There are three crystal modifications of calcium sulfate: calcium sulfate dihydrate (gypsum) CaSO4·2H2O, calcium sulfate semihydrate CaSO4·0.5H2O and calcium sulfate anhydrite CaSO4 [29,30]. It has been demonstrated that mechanochemically synthesized, syngenite crystals with submicron size can act as seeding material for flue gas desulphurization (FGD) anhydrite and promote gypsum crystallization more effectively than an equimolar mass composed of only potassium sulfate. Mechanochemically synthesized syngenite has been used as a seeding material in a fully finished dry self-leveling floor screed based on FGD anhydrite [16].
There are several approaches to syngenite synthesis, preferably in the form of powder. Syngenite K2Ca(SO4)2·H2O can be prepared by reacting calcium sulfate anhydrite CaSO4 with potassium sulfate K2SO4 in an equimolar ratio in the presence of water [31]. Syngenite K2Ca(SO4)2·H2O was prepared from the homogenised and calcined in a dry nitrogen atmosphere at 300 °C mixture of CaSO4 and K2SO4 by keeping it in air. This mixture left in the air quickly reacts with moisture. A total of 80% of the theoretical amount of syngenite K2Ca(SO4)2·H2O was formed within the first 10 h, and 95% was formed after a week. The addition of distilled water to the powder mixture of CaSO4 and K2SO4 provokes the reaction of syngenite K2Ca(SO4)2·H2O phase formation (Reaction (1)) [32]. Syngenite K2Ca(SO4)2·H2O is formed after 25–30 min in the case of using calcium sulfate anhydrite CaSO4 and potassium sulfate K2SO4.
CaSO4 solid+ K2SO4 solid+ H2Ovapor/liquid → K2Ca(SO4)2·H2Osolid
Syngenite K2Ca(SO4)2·H2O can also be formed by the interaction of calcium potassium carbonate K2Ca(CO2)2 with sulfuric acid H2SO4 (Reaction (2)) [33].
K2Ca(CO3)2 + 2H2SO4 → K2Ca(SO4)2∙H2O + H2O + 2CO2
Additionally, syngenite K2Ca(SO4)2·H2O can be synthesized at a temperature of 22–25 °C from a powder mixture including calcium sulfate dihydrate CaSO4·2H2O and potassium sulfate K2SO4 within a duration of 15–20 min (Reaction 3).
CaSO4·2H2Osolid + K2SO4 solid+ H2Oliquid → K2Ca(SO4)2·H2Osolid + H2Oliquid
Low-soluble syngenite K2Ca(SO4)2·H2O can be obtained from a solution of potassium sulfate K2SO4 and calcium sulfate CaSO4 [34].
The formation of hydrated potassium calcium sulfates is strongly temperature dependent; no gorgeyite K2SO4·5CaSO4·H2O crystallisation is observed at temperatures below 40 °C [30,35]. Syngenite K2Ca(SO4)2·H2O was by chance synthesized due to the interaction of hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ and potassium hydrosulfate KHSO4 in an acetone medium in conditions of mechanical activation (Reaction (4)) [36].
Ca₁₀(PO₄)₆(OH)₂ + 8KHSO4 + 2H2O → 4K2Ca(SO4)2·H2O +6CaHPO₄
According to our knowledge, novel inorganic composites in the CaO-K2O-SO3-H2O system consisting of syngenite K2Ca(SO4)2·H2O and calcium sulfate dihydrate CaSO4·2H2O in different proportions have not been under investigation up to now. Notwithstanding this, the physico-chemical properties of syngenite K2Ca(SO4)2·H2O and the well-known biocompatibility of calcium sulfate dihydrate CaSO4·2H2O give us a reason to pay attention to the development of these composites.
The aim of the present work was to create materials (samples of cement stones) in the CaO-K2O-SO3-H2O system consisting of syngenite K2Ca(SO4)2·H2O and calcium sulfate dihydrate CaSO4·2H2O based on powder mixtures, including calciolangbeinite K2Ca2(SO4)3 calcium sulfate anhydrite CaSO4 and/or potassium sulfate K2SO4. Special attention was paid to phase and microstructure formation during the hydration process of cement stone specimens in the CaO-K2O-SO3-H2O system.

2. Materials and Methods

2.1. Samples Preparation

2.1.1. Preparation of Powder of Calcium Sulfate Anhydrite CaSO4

The synthesis of CaSO4·2H2O was performed by the method described in article [37] from aqueous solutions of Ca(NO3)2·4H2O (GOST 4142-77, Ruskhim, Moscow, Russia) and (NH4)2SO4 (GOST 10873-73, Ruskhim, Moscow, Russia) at a molar ratio of Ca/S = 1 in initial solutions of salts according to Reaction (5):
Ca(NO3)2 + (NH4)2SO4 + 2H2O → CaSO4·2H2O + 2NH4NO3
A quantum of 500 mL of 1 M aqueous solution of (NH4)2SO4 was added to 500 mL of 1 M aqueous solution of Ca(NO3)2. After adding an aqueous solution of (NH4)2SO4, the suspension was kept on a magnetic stirrer at room temperature and a setting of constant stirring for 15 min. Then, the precipitate was separated from the mother liquid using a Buechner funnel. The resulting synthesized product was placed in 500 mL distilled water, and then the precipitate was separated again over a Buechner funnel. This action was repeated 4 times to remove the reaction by-product (ammonium nitrate NH4NO3). After washing was completed, the precipitate was placed in plastic containers and air dried for one week. The CaSO4·2H2O powder was disaggregated in a planetary mill (Fritch Pulverisette, Idar-Oberstein, Germany) in an acetone medium for 15 min at a rotation speed of 600 rpm. Containers and grinding media made from zirconia were used. A total of 10 g of powder and 50 g of grinding media were placed in each container. After acetone evaporation, dry powders were passed through a sieve with 200 μm mesh.
Powder of calcium sulfate anhydrite CaSO4 was prepared from calcium sulfate dihydrate CaSO4·2H2O at 900 °C in porcelain boats at a heating rate of 5 °C/min by holding at a given temperature for 120 min according to Reaction (6):
CaSO4·2H2Osolid → CaSO4 + 2H2O

2.1.2. Preparation of Powder of Calcium Langbenite K2Ca2(SO4)3

A powder of calcium langbenite K2Ca2(SO4)3 was prepared from a homogenized powder mixture of potassium sulfate K2SO4 and calcium sulfate anhydrite CaSO4 at 900 °C in porcelain boats at a heating rate of 5 °C/min by holding at a given temperature for 120 min, according to the equation expressed in Reaction (7):
K2SO4 solid + 2CaSO4 solid → K2Ca2(SO4)3 solid

2.1.3. Preparation of Starting Powder Cement Mixtures

Powder mixtures for cement stone preparation, including calcium sulfate anhydrite CaSO4, and/or calciolangbenite K2Ca2(SO4)3, and potassium sulfate K2SO4, were further homogenized in a planetary mill (Fritch Pulverisette, Idar-Oberstein, Germany) in acetone medium for 15 min at a rotation speed of 600 rpm.
Table 1 shows the composition of the starting powder cement mixtures consisting of calcium sulfate anhydrite, and/or calciolangbeinite and potassium sulfate. The target phase composition of cement stones samples containing 25, 50, 75 and 100% of syngenite content are presented in Table 1.
The calculation of the composition of starting powder cement mixtures was carried out using hydration-based Reactions (8) and (9) for samples Syn-25; Reaction (9) for sample Syn-50; and Reactions (9) and (10) for sample Syn-75 and Reaction (10) for sample Syn-100.
CaSO4 + 2H2O → CaSO4·2H2Osolid
K2Ca2(SO4)3 solid + 3H2O → K2Ca(SO4)2·H2Osolid + CaSO4·2H2Osolid
K2Ca2(SO4)3 solid + K2SO4 solid + 2H2O → 2K2Ca(SO4)2·H2Osolid
Starting powder cement mixtures were then disaggregated and homogenized in acetone for 15 min at a rotation speed of 600 rpm (see conditions above). Drying was carried out in the air for one hour. Sieving was carried out through a sieve with a mesh size of 200 μm.

2.1.4. Preparation of Cement Stone Samples from Powder Cement Mixtures

The samples were formed by filling a latex mold (30 × 10 × 10 mm) with 3 g of dry powder cement mixtures, followed by hand compaction. Distilled water was used as a mixing liquid at a water/powder (W/P) mass ratio of 1 ÷ 0.5. The liquid/solid (L/S) ratio was calculated under the assumption that potassium sulfate K2SO4 is a soluble salt and therefore not included in the mass of the solid phase. The W/P and L/S ratios for each powder mixture are presented in Table 1. The obtained cement stones were left for a week for the completion of hydration reactions.

2.2. Characterization

2.2.1. Linear Shrinkage and Geometric Density

The linear shrinkage and geometric density of the samples were determined taking in account demensions of latex mold and by measuring samples’ mass and dimensions after hydration, with an average of over 5 samples for each composition using Equations (11) and (12), respectively.
ΔLrel = (Lmold − Lcement sample)/Lmold × 100, %,
where:
  • ΔLrel—linear shrinkage of the cement stone sample after hardening, %;
  • Lcement sample—length of cement stone sample after hardening, cm;
  • Lmold—length of the mold, cm.
ρ = m/(L × h × w), g/cm3,
where:
  • ρ—density of the sample, g/cm3;
  • m—weight of the sample, g;
  • L—length of the sample, cm.
  • h—thickness of the sample, cm
  • w—width of the sample, cm
The mass and the linear dimensions of the samples were measured with accuracy of ±0.001 g and ±0.05 mm, respectively.

2.2.2. Characterization of Phase Composition and Microstructure of Samples

The diffractometer Rigaku D/Max-2500 with a rotating anode (Tokyo, Japan), a 2Θ angle range of 2–70° with a step of 0.02°, a rate of spectrum registration of 5°/min, CuKα radiation (λ = 1.5406 Å) was used for investigation of the samples by X-ray powder diffraction (XRD). The phases were determined using the ICDD PDF-2 database [38].
The electron microscope LEO SUPRA 50VP (Carl Zeiss, Jena, Germany; auto-emission source), with an accelerating voltage of 20 kV (SE2 detector), was used for the investigation of samples by scanning electron microscopy (SEM). The samples were pre-coated with a layer of chromium (up to 20 nm).

2.2.3. Thermogravimetric and Mass Spectrometric Analysis

Thermal analysis (TA) was performed to determine the thermogravimetric weight loss (TG) of the cement stone samples at heating up from 40 °C to 1000 °C in the air using Netzsch STA 409 PC Luxx thermal analyzer (NETZSCH, Selb, Germany). The sample weight was at least 10 mg. The gas phase composition was monitored by the quadrupole mass spectrometer QMS 403 Quadro (NETZSCH, Selb, Germany) combined with a thermal analyzer Netzsch STA 409 PC Luxx. The mass spectra (MS) were registered for the m/Z = 18 (H2O); the heating rate was 10 °C/min.

3. Results

XRD data of initial powders used for preparation of powdered cement mixtures (Table 1) and further cementing in the CaO-K2O-SO3-H2O system are presented in Figure 1 and Figure 2.
The phase composition of the powders represented by calcium sulfate anhydrite and calciolangbeinite agrees with the results obtained in paper [22].
Micrographs of the starting powders used for the preparation of cement powder mixtures and further cement stone production are shown in Figure 3 and Figure 4.
Gypsum particles had columnar morphology and were 4–10 µm long and 1–4 µm thick, while calciolangbeinite particles were close to isometric (distorted dodecahedron), and within a size range of 20–40 µm.
The W/P (Table 1) ratio varies greatly for samples because it does not take into account the ability of some components of the mixture (potassium sulfate) to dissolve. The L/S ratio (Table 1) varies from sample to sample because the original powders had different bulk densities (Table 1); the ratio is approximately 0.9 for samples Syn-25, Syn-75 and Syn-100. The amount of liquid is also related to the particle size, the effect being most pronounced for sample Syn-50. The original powder of sample Syn-50 consisted only of calciolangbeinite, the particles of which are much bigger (Figure 4) than those of calcium sulfate anhydrite (Figure 3), and therefore less water is needed than would be needed for, e.g., Syn-25 containing calciolangbeinite and calcium sulfate anhydrite.
When water was added, the formation of cement stone phases took place in accordance with Reactions (8) and (9) for the 1st formulation (Syn-25), Reaction (9) for the 2nd formulation (Syn-50), Reactions (9) and (10) for the 3rd formulation (Syn-75) and Reaction (10) for the 4th formulation (Syn-100) (Table 1).
The phase composition of obtained cement stone samples, as expected, contains only two minerals: calcium sulfate dihydrate CaSO4·2H2O and syngenite K2Ca(SO4)2·H2O (Figure 5 and Figure S1). The intensity of syngenite peaks increases with a decrease in the content of calcium sulfate anhydrite in the starting powder mixture (Table 1). Sample Syn-100 was a single-phase material including only syngenite. This result is agreement with the data presented in the paper [24]. No traces of gorgeyite were found in the cement stone samples prepared in the present work, which is in agreement with the results observed earlier [20].
Thus, the phase compositions of the obtained samples of cement stones coincide with that of the target.
Figure 6 shows microphotographs of the obtained cement stone samples. The structure of the cement samples does exhibit an obvious non-linear dependence from composition. Samples Syn-25 and Syn-75 have a columnar structure, in contrast to samples Syn-50 and Syn-100, which have a lamellar structure. This dependence may be explained by the equilibria of dissolution/crystallization processes existing between the water solutions and the minerals formed during cement stone formation.
Let us discuss the equilibria in the dissolution/crystallisation processes during cement stone formation in the samples under investigation.
The respective solubility levels of calcium sulfate dihydrate CaSO4·2H2O and syngenite K2Ca(SO4)2·H2O are approximately equal [39,40]. However, since syngenite dissociates into a larger number of particles, the solubility of syngenite is higher than that of gypsum. Potassium sulfate K2SO4 is a soluble salt. Solubility constants Ksp of calcium sulfate dihydrate CaSO4·2H2O and syngenite K2Ca(SO4)2·H2O are presented below:
Ksp (CaSO4·2H2O) = 2.25 × 10−4
Ksp (K2Ca(SO4)2·H2O) = 1.88 × 10−4
Reactions (13) and (14) show dissociation equations for syngenite K2Ca(SO4)2·H2O and calcium sulfate dihydrate CaSO4·2H2O accordingly:
K2Ca(SO4)2·H2Osolid ↔ 2K+ + Ca2+ + 2SO42 + H2O
CaSO4·2H2Osolid ↔ Ca2+ + SO42 + 2H2O
The analysis here considers the equilibria of dissolution/crystallisation processes during cement stone formation and the composition of solutions. The Syn-25 sample contains gypsum and syngenite, with an excess of ions Ca2+ and SO42− (underlined) originating from the gypsum (Reactions (15) and (16)). Calcium and sulfate ions share common products for dissolution reactions, so they are the ones that affect the shifting of equilibria. Due to the suppression of syngenite dissociation (gypsum content is higher in comparison of syngenite), gypsum is more involved in the dissolution–crystallisation process. The microphotograph (Figure 6a) shows that the Syn-25 sample consisted of crystals with a columnar habitus. Column length averaged 10–20 µm, and the thickness 0.5–2 µm.
K2Ca(SO4)2·H2Osolid ↔ 2K+ + Ca2+ + 2SO42 + H2O
CaSO4·2H2Osolid (excess) ↔ Ca2+ + SO42 + 2H2O
Conclusion: In the sample Syn-25 during cement stone formation dissolution of syngenite is suppressed, and gypsum is significantly involved in the dissolution–crystallisation processes.
There is an excess of better-dissociating syngenite in the Syn-50 sample, which suppresses the dissociation of gypsum, as expressed in Reactions (17) and (18). Calcium Ca2+ and sulfate SO42− ions (underlined) are common products for dissolution reactions, so they are the ones that affect the shifting of equilibria. Due to the suppression of gypsum dissociation (gypsum is less soluble than is syngenite), syngenite is more involved in the dissolution–crystallisation process. The micro-photograph (Figure 6b) shows that crystals in the sample Syn-50 are characterised by a plate habitus. The thickness of the plates was 0.1–0.2 μm, width 0.5–2 μm, length 0.5–2 μm.
K2Ca(SO4)2·H2Osolid (excess) ↔ 2K+ + Ca2+ + 2SO42 + H2O
CaSO4·2H2Osolid ↔ Ca2+ + SO42 + 2H2O
Conclusion: In the sample Syn-50 during cement stone formation syngenite is significantly involved in the dissolution–crystallisation processes.
Soluble potassium sulfate K2SO4 (Reaction (19)) in the Syn-75 sample suppresses the dissociation of syngenite more than it takes place due to the dissociation of gypsum, as described in Reactions (20)–(21). Calcium, potassium and sulfate ions (underlined) are products for dissolution reactions, so they are the ones that affect the shifting of equilibria. Due to the suppression of syngenite dissociation (potassium ions only inhibit the dissolution of syngenite), gypsum is more involved in the dissolution–crystallisation process. Microphotograph (Figure 6c) shows that crystals in the Syn-75 sample are characterised by a columnar habitus. Column length averaged 10–20 µm and the thickness 0.5–2 µm.
K2SO4 ↔ 2K+ + SO42
K2Ca(SO4)2·H2Osolid ↔ 2K+ + Ca2+ + 2SO42 + H2O
CaSO4·2H2Osolid ↔ Ca2+ + SO42 + 2H2O
Conclusion: In the sample Syn-75 during cement stone formation gypsum is significantly involved in the dissolution–crystallisation processes.
In the Syn-100 sample, only syngenite as low-soluble substance is present within the Reactions (22) and (23). Due to the absence of a gypsum phase in sample Syn-100, only syngenite is involved in the dissolution–crystallisation process. The micro-photograph (Figure 6d) shows that crystals in the sample Syn-100 are characterised by a plate habitus. The thickness of the plates was 0.1–0.2 μm, the width 0.5–2 μm and the length 0.5–2 μm.
K2Ca(SO4)2·H2Osolid ↔ 2K+ + Ca2+ + 2SO42 + H2O
K2SO4excess ↔ 2K+ + SO42
Conclusion: In the sample Syn-100 during cement stone formation syngenite is the only mineral involved in the dissolution–crystallisation processes.
The effect of the dissolution–crystallisation processes of the minerals in the cement stone samples during hydration and hardening on their microstructure is presented in Table 2.
Thermal analysis data, namely TG and MS for m/Z = 18, of cement stone samples, consisting of syngenite K2Ca(SO4)2·H2O and calcium sulfate dihydrate CaSO4·2H2O, are presented in Figure 7. The main mass loss in the Syn-25, Syn-50 and Syn-75 samples (Figure 7a) occurs in two steps: decomposition of gypsum CaSO4·2H2O and decomposition of syngenite K2Ca(SO4)2·H2O. When heating the Syn-100 sample, only one step of mass change is observed, and only syngenite K2Ca(SO4)2·H2O is decomposed. Decomposition of calcium sulfate dihydrate CaSO4·2H2O occurs at approximately 150 °C (Figure 7b), with the material changing into calcium sulfate anhydrite CaSO4 in Reaction (24). At 280–300 °C (Figure 7b), syngenite decomposes, presumably forming a mixture of calcium sulfate anhydrite CaSO4, potassium sulfate K2SO4 and calciolangbeinite K2Ca2(SO4)3 (Reaction (25)).
CaSO4·2H2O → CaSO4 + 2H2O↑
3K2Ca(SO4)2·H2O → 2K2SO4 + CaSO4 + K2Ca2(SO4)3 + 3H2O↑
Residual mass of every sample under investigation according TA as a result of thermal decomposition is in a good correlation with theoretical residual mass calculated according to the target composition and Reactions (22) and (23) (Table 3). This correlation can be treated as indirect confirmation of the fact that composites with given quantitative ratio of target phases of syngenite K2Ca(SO4)2·H2O and calcium sulfate dihydrate CaSO4·2H2O in the system CaO-K2O-SO3-H2O were created.
These Reactions (24) and (25) of thermal decomposition are complete, as the theoretical mass change within the measurement error is the same as the practical one. For most samples, the amount of real mass lost is slightly higher than the theoretical one (tenths of a percent), which can be explained by the presence of unbound water in the material. On heating of samples Syn-25, Syn-50 and Syn-75, Reactions (24) and (25) take place; for sample Syn-100, only Reaction (25) takes place.
Figure 8 and Figure 9 show plots of linear shrinkage as well as density of cement stone samples after molding, hydration and drying depending on target phase composition of samples of cement stones.
The average density of the samples does not change linearly, because the samples have different compositions, structures and linear shrinkage. In general, the density of the cement mix increases with the increase of the molar content of syngenite in the samples, but due to the different size of the mixed particles, there is some non-linearity in the change of properties.

4. Conclusions

Cement stone samples with phase compositions including minerals in the system CaO-K2O-SO3-H2O, namely, calcium sulfate dihydrate CaSO4·2H2O and syngenite K2Ca(SO4)2·H2O, were obtained. Starting powder mixtures consisted of calcium sulfate anhydrite CaSO4 and/or calciolangbeinite K2Ca2(SO4)3 and potassium sulfate K2SO4. The phase compositions of the samples of cement stone in the system CaO-K2O-SO3-H2O were formed due to reactions of hydration. The microstructures of the prepared cement stone samples demonstrated an obvious dependence from the equilibria of dissolution–crystallization processes during cement stone formation. The dependence of the microstructure on the composition of the cement stone through the equilibria that occur during the hydration reaction has been explained. It was revealed that samples with contents of 25, 50, 75 and 100 mol% of syngenite K2Ca(SO4)2·H2O demonstrated a nonlinear dependence of microstructure on their phase composition. The microstructures of samples with phase compositions of 25 and 75 mol% of syngenite K2Ca(SO4)2·H2O consisted of pillar crystals. Finally, the microstructures of samples with phase compositions of 50 and 100 mol% of syngenite K2Ca(SO4)2·H2O consisted of plate crystals. The cement samples prepared can be classified as quick-curing, so there is a potential use of these powder cement mixtures in inkjet printing technology. The obtained materials can be recommended for in vitro tests to assess the biocompatibility of the materials obtained.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ceramics6030088/s1, Figure S1. Graphs of XRD reflexes of K2Ca(SO4)2·H2O (card PDF 74-2423) (a) and CaSO4·2H2O (card PDF 33-311) (c), and XRD data with symbols for peak indexing (b) of cement stone samples obtained in the CaO-K2O-SO3-H2O system: •—CaSO4·2H2O (card PDF 33-311); ♦—K2Ca(SO4)2·H2O (card PDF 74-2423).

Author Contributions

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

Funding

This work was carried out with financial support from the Russian Science Foundation (grant no. 22-19-00219).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

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”, and 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 conflict of interest.

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Figure 1. XRD data of the calcium sulfate anhydrite CaSO4 powder synthesized from CaSO4·2H2O and used for powder cement mixture preparation v—CaSO4 (card PDF 20-867).
Figure 1. XRD data of the calcium sulfate anhydrite CaSO4 powder synthesized from CaSO4·2H2O and used for powder cement mixture preparation v—CaSO4 (card PDF 20-867).
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Figure 2. XRD data of the calciolangbeinite K2Ca2(SO4)3 powder synthesized from a mixture of potassium and calcium sulfates and used for powder cement mixture preparation *—K2Ca2(SO4)3 (card PDF 33-311).
Figure 2. XRD data of the calciolangbeinite K2Ca2(SO4)3 powder synthesized from a mixture of potassium and calcium sulfates and used for powder cement mixture preparation *—K2Ca2(SO4)3 (card PDF 33-311).
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Figure 3. SEM image of calcium sulfate anhydrite CaSO4 powder.
Figure 3. SEM image of calcium sulfate anhydrite CaSO4 powder.
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Figure 4. SEM image of calciolangbeinite K2Ca2(SO4)3 powder.
Figure 4. SEM image of calciolangbeinite K2Ca2(SO4)3 powder.
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Figure 5. XRD data of cement stone samples obtained in the CaO-K2O-SO3-H2O system: •—CaSO4·2H2O (card PDF 33-311); and ♦—K2Ca(SO4)2·H2O (card PDF 74-2423).
Figure 5. XRD data of cement stone samples obtained in the CaO-K2O-SO3-H2O system: •—CaSO4·2H2O (card PDF 33-311); and ♦—K2Ca(SO4)2·H2O (card PDF 74-2423).
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Figure 6. SEM images of obtained samples of cement stones in the CaO-K2O-SO3-H2O system: (a) Syn-25; (b) Syn-50; and (c) Syn-75; (d) Syn-100.
Figure 6. SEM images of obtained samples of cement stones in the CaO-K2O-SO3-H2O system: (a) Syn-25; (b) Syn-50; and (c) Syn-75; (d) Syn-100.
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Figure 7. TG (a) and MS (m/Z=18) (b) curves of cement stone samples consisting of syngenite K2Ca(SO4)2·H2O and calcium sulfate dihydrate CaSO4·2H2O.
Figure 7. TG (a) and MS (m/Z=18) (b) curves of cement stone samples consisting of syngenite K2Ca(SO4)2·H2O and calcium sulfate dihydrate CaSO4·2H2O.
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Figure 8. Plot of linear shrinkage versus cement stone composition after moulding and drying.
Figure 8. Plot of linear shrinkage versus cement stone composition after moulding and drying.
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Figure 9. Diagram of the geometric density of cement stone as a function of cement stone phase composition.
Figure 9. Diagram of the geometric density of cement stone as a function of cement stone phase composition.
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Table
Table 1. Target phase composition of cement stone samples and composition of starting powder cement mixtures.
Table 1. Target phase composition of cement stone samples and composition of starting powder cement mixtures.
LabelingTarget Phase Composition of
Samples of Cement Stones, mol%		Composition of Starting Powder Cement Mixtures, mol%Bulk Density, g/cm3W/P RatioL/S RatioReactions for Calculation
K2Ca(SO4)2·H2OCaSO4·2H2OCaSO4K2Ca2(SO4)3K2SO4
Syn-25 25755050-0.6970.90.9(8), (9)
Syn-50 5050-100-0.7980.50.5(9)
Syn-757525-66330.7460.80.9(9), (10)
Syn-1001000-50500.8350.60.8(10)
Table
Table 2. Formation of the microstructure of cement stone specimens, depending on the composition and the possibility of phase dissolution and crystallisation processes.
Table 2. Formation of the microstructure of cement stone specimens, depending on the composition and the possibility of phase dissolution and crystallisation processes.
Cement SamplesDissolution–Crystallization Is
Prefarable for the Phase		Shape of the Particles
Syn-25Calcium sulfate dihydrate CaSO4·2H2O Pillar
Syn-50Syngenite K2Ca(SO4)2·H2OPlate
Syn-75Calcium sulfate dihydrate CaSO4·2H2O Pillar
Syn-100Syngenite K2Ca(SO4)2·H2OPlate
Table
Table 3. Calculated and real weight-loss on hardening of cement stone specimens.
Table 3. Calculated and real weight-loss on hardening of cement stone specimens.
SampleTheoretical Residual Mass as a Result of Thermal Decomposition, wt%Residual Mass According TA as a Result of Thermal Decomposition, wt%
Syn-2585.185.5
Syn-5089.289.0
Syn-7592.291.9
Syn-10094.594.2
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Kuznetsov, A.I.; Safronova, T.V.; Shatalova, T.B.; Filippov, Y.Y.; Vaymugin, L.A.; Vlasenko, V.S.; Likhanov, M.S. Materials in the CaO-K2O-SO3-H2O System Based on Powder Mixtures including Calciolangbeinite K2Ca2(SO4)3 and Calcium Sulfate Anhydrite CaSO4. Ceramics 2023, 6, 1434-1448. https://doi.org/10.3390/ceramics6030088

AMA Style

Kuznetsov AI, Safronova TV, Shatalova TB, Filippov YY, Vaymugin LA, Vlasenko VS, Likhanov MS. Materials in the CaO-K2O-SO3-H2O System Based on Powder Mixtures including Calciolangbeinite K2Ca2(SO4)3 and Calcium Sulfate Anhydrite CaSO4. Ceramics. 2023; 6(3):1434-1448. https://doi.org/10.3390/ceramics6030088

Chicago/Turabian Style

Kuznetsov, Alexander I., Tatiana V. Safronova, Tatiana B. Shatalova, Yaroslav Y. Filippov, Leonid A. Vaymugin, Vyacheslav S. Vlasenko, and Maxim S. Likhanov. 2023. "Materials in the CaO-K2O-SO3-H2O System Based on Powder Mixtures including Calciolangbeinite K2Ca2(SO4)3 and Calcium Sulfate Anhydrite CaSO4" Ceramics 6, no. 3: 1434-1448. https://doi.org/10.3390/ceramics6030088

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