STUDY OF THE IMPACT OF PHOSPHOGYPSUM ADDITIVE AND FERROUS SULPHATE SOLUTION ON THE MECHANISM OF THE MAGNESIA-BISHOFIT COMPOSITION

Phosphogypsum is a very pure raw material [16] with a negative, from a technical point of view, elongated shape of calcium dihydrate crystals (due to the formation conditions), which remains the same after both dehydration in dry air and grinding, this makes technical difficulties relating to further processing of the product into a binder, consequently, water resistance and plasticity of magnesium solutions can be improved by adding primary and secondary phosphates without strength reduction. Famous experts on phosphogypsum Yu. P. Meshcheryakov and N. A. Kolev [8] found, that when adding phosphogypsum instead of natural gypsum during grinding of Portland cement, which is intended to control the setting time of the mixture, there occurred reduction in the initial setting time from 1 h 55 min up to 1 h 36 min (when adding 4 % agent), and the final setting time was decreased from 6 h 15 min up to 6 min. After grinding of phosphogypsum, there appeared a fresh surface, on which the electron emission phenomenon occurs, however, not the entire surface emits, but only the active centres, where field strength reaches 108 V/cm. The positively charged active centres have low CaSO 4 2H 2 O concentration on their surface. Purpose. Specifying the optimal amount of phosphogypsum and ferrous sulphate solution added, studying their impact on the curing mechanism of the magnesia-bishofit composition. Conclusion. To increase the sulphate compound, required for the crystallinity [10] reduction and magnesia stone sealing [4], the ferrous sulphate solution have been added to the composition formulation, ferrous sulphate anions immediately polarize free calcium ions with the formation of CaSO 4 ?2H 2 O. P. P. Budkov's experiments [8] prove that the larger the magnesia cement-to-sulphate stone ratio, the lower the setting time of the composition, and the higher the tensile strength of the stone. Moreover, V. V. Shchelyaghin [15] recommended adding ferrous sulphate, formed in tanks for black metal etching with sulphuric acid, to the magnesia-bishofit composition. The scholar replaced up to 50 % of the magnesium chloride solution MgCl 2 for the ferrous sulphate solution FeSО 4 ?7H 2 О, thus improving water resistance of the stone. He also established that ferrous sulphate accelerates setting time of magnesia mixtures and reduces the chance of efflorescence formation on the product. Therefore, based on the results of the research conducted, 5 % of phosphogypsum is the optimal amount to be added to the magnesia-bishofit composition. The additive reduces the setting time of the mixture and increases the strength of the stone, and together with the 5 % ferrous sulphate additive it enables formation of a sealing newgrowth CaSO 4 2H 2 O. Clogging of pores with dehydrite particles and bonding flakes of Fe(OH) 3 improves deformation phenomena during curing of the specimens, reduces their water absorption capacity and increases strength.