Article Highlights• Phosphoanhydrite was obtained from neutralized phosphogypsum at temperatures 800 and 900 °C • The properties of phosphoanhydrite were improved upon grinding it to surface area 600 m 2 /kg • The properties of phosphoanhydrite were improved using setting activators K 2 SO 4 and Na 2 SO 4 • Syngenite and glauberite were formed in the samples with additives K 2 SO 4 and Na 2 SO 4 • The compressive strength of dry specimens was 58.5 MPa Abstract Phosphoanhydrite was obtained by processing dihydrate phosphogypsum generated during phosphoric acid production at Joint Stock Company (JSC), Lifosa (Lithuania). The influence on the properties of phosphoanhydrite of the treatment process parameters was determined, including: the conditions of phosphogypsum neutralisation in the lime suspension, temperature and duration of calcination of neutralized phosphogypsum to phosphoanhydrite, fineness of ground anhydrite, selection of additives activating the hydration of anhydrite and their amounts. Upon adding activators of anhydrite hydration, K 2 SO 4 or Na 2 SO 4 , XRD and IR spectral analysis data show that the specimens, after 1 day of phosphoanhydrite hydration with additives, contained complex salts -syngenite upon using K 2 SO 4 additive and glauberite in the case of Na 2 SO 4 additive. These additives act as crystallization centers and accelerate hydration. It was determined that the specimens of phosphoanhydrite calcined at a temperature of 900 °C with 2% K 2 SO 4 additive have the best physical and mechanical properties. Already after 1 day of hardening the specimens acquired a compressive strength of 21.18 MPa, and after 28 days of hardening they had a compressive strength of 58.5 MPa (dry specimens).

In this work, properties of sulphate technogenic raw material – flue gas desulphurization (FGD) gypsum have been investigated in order to obtain α-hemihydrate gypsum as gypsum binding material. FGD gypsum is produced as a by-product during the process of desulphurization of exhaust gas emitted in power stations heated by fuel containing sulphur. It has been determined that α-hemihydrate gypsum can be obtained from FGD gypsum under hydrothermal conditions at 125 °C and 130 °C. The compressive strength of gypsum samples is 13 MPa – 16 MPa after 2 h of hardening and it depends on temperature and time of isothermic curing. The compressive strength of the same samples in dry state is 49 MPa – 67 MPa. SEM analysis data showed that the properties of α-hemihydrate gypsum significantly depend on the shape and size of its crystals: when crystals have near form to regular hexagonal – shaped prism with length and width ratio approx. of 4 : 1, the gypsum exhibits the high quality mechanical and physical properties.

DOI: http://dx.doi.org/10.5755/j01.ms.19.2.4438

Tirta galimybė, naudojant mikrobangas (buitinę mikrobangų krosnelę), atlikti dihidračio gipso (CaSO 4 2H 2 O) dehidrataciją iki -pushidračio gipso (-CaSO 4 0,5H 2 O). Dėl mikrobangų poveikio termiškai apdorojant reagentinį gipsą ir fosfogipsą, stebėta, ar įmanoma sudaryti sąlygas, kad iš dihidračio gipso atskiltų pusantros molekulės vandens ir gautume pushidratį gipsą, turintį rišamųjų savybių.Dihidračio gipso dehidratacija vykdyta 100, 300, 450 ir 600 W galios režimais buitinėje mikrobangų krosnelėje SAMSUNG. Dehidratacijos kinetika tirta tam tikrais laiko tarpais iš trijų skirtingų krosnelės vietų paimant po 3 g dehidratuojamos medžiagos ir nustatant kaitinimo nuostolius 400 °C temperatūroje.Tyrimais nustatyta, kad, parinkus atitinkamus mikrobangų krosnelės parametrus (galingumą) ir terminio apdorojimo parametrus (temperatūrą ir trukmę), galima CaSO 4 2H 2 O dehidratuoti iki -CaSO 4 0,5H 2 O. Gautos gipsinės rišamosios medžiagos savybės priklauso nuo gipso žaliavos. Iš reagentinio gipso gautos rišamosios medžiagos rišimosi pradžia 18 min, rišimosi pabaiga 28 min, o gniuždymo stipris po 2 h siekia 5,5 MPa, o iš fosfogipsoatitinkamai 25 min ir 37 min bei 1,6 MPa.

While dehydrating gypsum with additives at the temperatures of 800°C and 900°C the influence of alkali additives on both the crystalline structure of anhydrite and properties of anhydrite binder was investigated. The industrial and household wastes including other lowcost materials were used as additives. Having heated them with gypsum the anhydrite with alkali activation properties was obtained. The properties of such substances were evaluated using the methods of chemical, diffractive X-ray scanning and scanning electron microscopy (SEM) analyses. Some additives, e.g. 5 % ground glass waste, were found to increase crystal agglomerate formation of anhydrite binder, accelerate the hydration process of anhydrite and double the compressive strength of hydrated samples compared to samples without additives

Influence of Ce³⁺ on the formation of α-semi-hydrate gypsum was investigated in this work. α-semi-hydrate gypsum is normally obtained by heat conversion of the dihydrate gypsum. The model system: CaSO₄·2H₂O–Ce₂(SO₄)₃×8H₂O was designed. The amount of Ce₂(SO₄)₃×8H₂O corresponding to 0.1; 0.5 and 1 % of Ce₂O₃ from the mass of dry materials (gypsum) was added to solution. Maleic anhydride (C₄H₂O₃) was used as crystal modifier and 0.1 % of C₄H₂O₃ from the mass of dry materials (gypsum) was added to solution. Water/gypsum of the suspension was equal to 2. The system was processed under hydrothermal conditions at 128 °C ±2 °C and 138 °C ±2 °C temperature; the duration of isothermal curing was 3 hours. The product obtained was treated at 400 °C temperature. The X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR) scanning electron microscopy (SEM) was used to characterize the product.

DOI: http://dx.doi.org/10.5755/j01.ms.18.4.3102