ChemInform Abstract: 3Mg(OH)₂·MgSO₄·8H₂O: A Metastable Phase in the System Mg(OH)₂—MgSO₄—H₂O.

Abstract

Mg(OH)2-MgSO4-H2O. -3Mg(OH)2·MgSO4·8H2O (3-1-8 phase) and 5Mg(OH)2·MgSO4·2H2O (5-1-2 phase) are obtained from OHsupersaturated MgSO4 solutions which are obtained by suspending highly reactive MgO in differently concentrated MgSO4 solutions followed by filtration and storage at 25 (3-1-8 phase) or 40°C (5-1-2 phase). The 3-1-8 phase is metastable, the 5-1-2 phase stable at room temperature. The former is characterized by thermal analysis, Raman spectroscopy, and high--resolution synchrotron powder XRD (space group C2/c, Z = 4). Its crystal structure consists of parallel double chains of edge-linked distorted Mg (OH2)2(OH)4 octahedra along the [-110] and [110] directions forming a pattern of crossed rods. Isolated SO4 tetrahedra and interstitial H 2 O molecules separate the stacks of parallel double chains. -(DINNEBIER*, R. E.; PANNACH, M.; FREYER, D.; Z. Anorg. Allg. Chem. 639 (2013) 10, 1827-1833, http://dx.doi.org/10.1002/zaac.201300128 ; MPI Festkoerperforsch., D-70569 Stuttgart, Germany; Eng.) -J. Schramke 43-004

Research on technology of performance improvement of basic magnesium sulfate cement—BMS

“…20,21 MgO s 4). 8 The molecular arrangement of phase of 5•1•7 is closer than that of phase of 3•1•8. In addition, the main hydration products of magnesium oxysulfate cement, the 3•1•8 phase, begin to decompose by heating at 40 C. In contrast, the 5•1•7 phase only decomposes at 81 C. 12 Therefore, the thermal stability of 5•1•7 phase is superior to 3•1•8 phase.…”

“…In addition, the main hydration products of magnesium oxysulfate cement, the 3•1•8 phase, begin to decompose by heating at 40 C. In contrast, the 5•1•7 phase only decomposes at 81 C. 12 Therefore, the thermal stability of 5•1•7 phase is superior to 3•1•8 phase. 8 This makes the better resistant to hydrothermal performance of BMS than magnesium oxysulfate cement. 22 But the hydrothermal resistance of BMS is lower than that of Portland cement.…”

“…12 There are plenty of pores in the hydration products of MOS with low strength. 12 After the hydration of BMS, the whiskers of 5Á1Á7 phase interlace, which are 11 F I G U R E 2 An octahedron chain arrangement in the 5Á1Á7 phase 10 F I G U R E 3 Crystal packing diagram of 3Á1Á8 phase 8 uniformly distributed and compact. 10,12 When BMS is under load, these dense whiskers can withstand greater pressure than the lamellar crystals of Mg(OH) 2 .…”

“…Figure3shows the crystal packing diagram of 3•1•8 phase. Each octahedron shares four edges and five vertices with four surrounding octahedrons (see Figure4) 8. The molecular arrangement of phase of 5•1•7 is closer than that of phase of 3•1•8.…”

“…6,7 However, it was pointed out by Robert that the 3•1•8 phase is not a stable phase at room temperature. 8 It was pointed out by Urwongse that if the MOS was prepared with active MgO and MgSO 4 or the solution of MgSO 4 at room temperature, the content of the 3•1•8 phase in the hydration product would not exceed 50%. This leads to the lower intensity of MOS than that of MOC, 9 which is also the main reason for poor water resistance of MOS.…”