Periclase

Total MgO content in clinker is normally maintained at less than 2.0% to avoid expansion in the autoclave test. Belite containing periclase (at an early stage of crystallization) indicates coarse grains of MgO-bearing silicates, such as diopside, pyroxene, and hornblende in the raw feed. Dendritic periclase occurs in slowly cooled clinkers from large kilns (200 t/hr) and is hardly observed in clinkers from small kilns (20 t/ hr), according to Ono (1995). 

Microscopical details of dolomitic raw materials and the resulting clinkers were described by Dreizler (1988) in relation to national MgO limits in cements, soundness, and the availability of suitable limestone deposits. Soundness of periclase-rich cement was said to notably improve with decrease in free lime (0.10 percent in the case study presented). 

Periclase, free lime, or coarsely crystalline CgA (slowly cooled), or any combination thereof, have been shown to be responsible for excessive autoclave expansion (Gonnerman, Lerch, and Whiteside, 1953). 

Gebauer (1988) has convincingly shown the lack of expansive effects of periclase in 4 year old, outdoor-stored concretes, concluding that the ASTM test for 

5 Crystal field spectra of transition metal ions 

Oxide (mineral) Cation V (cm ) CFSE (cm-1) CFSE (kJ/g.ion) Sources of data 

The optical spectra of natural and synthetic transition metal-doped corundums have been extensively studied both theoretically and experimentally. Data reviewed elsewhere (Bums and Bums, 1984a Bums, 1985a) are summarized in table 5.2. The Ae and CFSE values are only approximate because the ground-state splittings, 6, of the tlg orbital group (cf. fig. 2.9) are unknown but could amount to 1,000 cm-1 for some cations in corundum. 

---

Two types of magnesia, caustic-calcined and periclase (a refractory material), are derived from dolomitic lime. Lime is required in refining food-grade salt, citric acid, propjiene and ethylene oxides, and ethylene glycol, precipitated calcium carbonate, and organic salts, such as calcium stearate, lactate, caseinate. 

Other Processes. Dead Sea Periclase (DSP, Mishor Rotem, Israel) converts magnesium chloride into MgO by spray-roasting, then hydrates the MgO to Mg(OH)2. The Mg(OH)2 is washed and dmm filtered. DSP purchases the brine from Dead Sea Works, which collects and stores enriched brine from the southern margins of the Dead Sea (77). 

Flame-Retardant Filler. Demand has increased for Mg(OH)2 as a nonhalogenated, flame-retardant filler for thermoplastics used in the aerospace, microelectronics, and cable and wire manufacturing industries (90). Producers of nonhalogenated, flame retardant fillers include Kyowa, Aluisuisse-Lonza (Magnifin product line), Morton, and a Dead Sea Periclase/Dead Sea Bromine joint venture (91). 

Dead Seas Periclase Ltd., on the Dead Sea in Israel, uses yet another process to produce magnesium oxide. A concentrated magnesium chloride brine processed from the Dead Sea is sprayed into a reactor at about 1700°C (127,128). The brine is thermally decomposed into magnesium oxide and hydrochloric acid. To further process the magnesia, the product is slaked to form magnesium hydroxide which is then washed, filtered, and calcined under controlled conditions to produce a variety of MgO reactivity grades. A summary of MgO purities, for the various processes is given in Table 20. 

Other Phases in Portland and Special Cements. In cements free lime, CaO, and periclase, MgO, hydrate to the hydroxides. The in situ reactions of larger particles of these phases can be rather slow and may not occur until the cement has hardened. These reactions then can cause deleterious expansions and even dismption of the concrete and the quantities of free CaO and MgO have to be limited. The soundness of the cement can be tested by the autoclave expansion test of Portiand cement ASTM C151 (24). 

Magnesium oxide. The natural minerals, i.e., magnesite (MgCO ), brucite [Mg(OH)9], etc., after being crushed to predetermined size, are calcined at temperatures varying from 1055 to 2000 K, depending upon whether a caustic or a dead-burned produc t (periclase) is being... 

Figure 4.28. Measured and calculated shock temperatures versus pressure for for-sterite for low-pressure (olivine), mixed phase, and high-pressure phase regime (possibly MgO periclase) -I- MgSi03 (perovskite)). Shock temperatures in the mixed phase regime (Ahrens et al., 1969).

The number of oxide type minerals is quite large. Rostov (1956) has identified 160 specific minerals, grouped them into classes (chrysoberyl, spinel, corundum, periclase, etc.), and proposed a classification system. Only a few examples will be discussed here. 

Chesney A process for extracting magnesium from seawater. Developed by British Periclase in 1937. 

Figure 4.3 The Mg0-Al203 phase diagram. Both MgO (periclase) and MgAl204 (spinel) have broad composition ranges at high temperatures.

Dead Sea periclase (DSP) magnesia manufacture, 15 402, 412 Dead Sea Works magnesium... 

Many reactions of geological interest have crystalline (c) solids as reactants and products. Even when such reactions have large values of AH, the enthalpy of reaction cannot be measured directly because the reactions are very slow. An important reaction of this kind is that of MgO (periclase) and SiOa (quartz) to form forsterite (Mg2Si04). 

---