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Powders with a phase composition including syngenite (K2Ca(SO4)2·H2O) and/or calcium sulfate dihydrate (gypsum, CaSO4·2H2O) were synthesized from the powder of calcium carbonate (CaCO3) and water solutions of potassium hydrosulfate (KHSO4) of various concentrations (0.5 M, 1 M, and 2 M). A molar ratio of starting salts, KHSO4/CaCO3 = 2, was used to provide the formation of syngenite (K2Ca(SO4)2·H2O). But when using a 0.5 M water solution of potassium hydrosulfate (KHSO4), the phase composition of the synthesized powder was presented by calcium sulfate dihydrate (gypsum, CaSO4·2H2O). When using 1 M and 2 M water solutions of potassium hydrosulfate (KHSO4), the syngenite (K2Ca(SO4)2·H2O) was found as the predominant phase in synthesized powders. According to estimations made from thermal analysis data, powders synthesized using 1.0 M and 2.0 M water solutions of potassium hydrosulfate (KHSO4) contained no more than 7.9 and 1.9 mass % of calcium sulfate dihydrate (gypsum, CaSO4·2H2O), respectively. The 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 biocompatible bioresorbable materials with phase compositions in the K2O-CaO-SO3-H2O system; as matrix of thermo- or photo-luminescent materials; as components reducing the setting time and increasing the strength of sulfate cements; in the fertilizing industry; and also as components of Martian regolith simulants. [ABSTRACT FROM AUTHOR]

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 Ca1,5SiO3,5·xH2O (Ca/Si molar ratio = 1.5) and calcium carbonate CaCO3 (Ca/Si molar ratio = ∞). The presence of H2SiO3 or SiO2·xH2O 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 NaNO3, sodium chloride NaCl, and sodium acetate NaCH3COO) 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 CaCl2 and calcium nitrate Ca(NO3)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-CaSiO3, and, after firing at 1100 °C, consisted of both β-wollastonite β-CaSiO3 and pseudo-wollastonite α-CaSiO3. The phase composition of the ceramic samples prepared from powders containing sodium nitrate NaNO3 and sodium acetate NaCH3COO after firing at 900, 1000, and 1100 °C consisted of calcium sodium silicates, i.e., Na2Ca2Si3O9 (combeite) and Na2Ca3Si2O8. 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. [ABSTRACT FROM AUTHOR]