MgO periclase

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). 

Ca- and Mg-oxides (lime, periclase [MgO]) may occur as small crystals embedded within the glass or they may be located on its surface depending on temperature and furnace conditions where the minerals formed. Low-temperature minerals such as anhydrite (CaS04) may form on the surface of the fly ash grains after they have left the high-temperature zones in the furnace (Linton et al. 1977 Soroczak et al 1987 Ainsworth et al. 1993 Fishman et al. 1999). 

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100 1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH- layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

Crystal field spectral measurements of transition metal ions doped in a variety of silicate glass compositions (e.g., Fox et al., 1982 Nelson et al., 1983 Nelson and White, 1986 Calas and Petiau, 1983 Keppler, 1992) have produced estimates of the crystal field splitting and stabilization energy parameters for several of the transition metal ions, examples of which are summarized in table 8.1. Comparisons with CFSE data for each transition metal ion in octahedral sites in periclase, MgO (divalent cations) and corundum, A1203 (trivalent cations) and hydrated complexes show that CFSE differences between crystal and glass (e.g., basaltic melt) structures,... 

Difference doped periclase, MgO (table 5.1), and reference [3] glasses (basalt B and granite G)... 

Fig. 2.22. Electronic (optical) absorption spectrum of octahedrally coordinated Fe-+ ions in the periclase (MgO) structure (after Goto et al., 1980 reproduced with the publisher s permission).

Periclase (MgO) has the rocksalt structure, which is face-centered cubic with each Mg-+ cation surrounded by six 0 anions in a regular octahedral arrangement, and each O " similarly coordinated to six cations at the corners of a regular octahedron. By far the most stable surface for oxides having the rocksalt structure is the (100), illustrated in Fig. 8.13... 

The following data are for a single crystal of periclase, MgO relative permittivity, 9.65 refractive index, n, 1.736 cubic unit cell, oq, 0.4207 nm, Z = 4 formula units of MgO. 

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