Free MgO

MgO-spinel clinker is obtained by the same procedure, but with a substantial proportion of free MgO (75-85%). MgO-spinel is produced by sintering at temperatures of 1600-1800°C and a dense product is obtained. MgO-spinel refractory has better high temperature mechanical properties in arc furnace roof application compared to magnesia-chrome refractories, but it has lower strength than a high-alumina refractory. 

A number of papers have examined the adsorption of metal atoms other than Au on MgO. Fernandez et al. systematically studied all first row transition metals (K-Zn) on defect-free MgO(lOO), at coverages corresponding to epitaxial overlayers and at coverages representative of isolated atoms on the surface. The trends in adsorption energy for these species were dominated by the interaction between the adsorbed atoms and O atoms in the surface, although interactions with adjacent surface Mg atoms also contribute for the larger adsorbed species e.g. K and V). It is not clear from these results whether these trends would transfer in a simple way to defects on the oxide surface. 

Zhang C, Yoon B, Landman U (2007) Predicted oxidation of CO catalyzed by Au nanoclusters on a thin defect-free MgO film supported on a Mo(lOO) surface. J Am Chem Soc 129 2228... 

Au clusters which do not bind CO as strongly were not poisoned by reversing the order of gas exposure. As mentioned earlier, Au clusters were in fact inactive on defect-free MgO films and must be activated by defects. Clusters composed of eight Pd atoms (Pd ) are already active on defect-free films producing two CO molecules per cluster in TPR experiments [7] Pd octamers on defective films did show an increase in activity to approximately three CO molecules per cluster. 

To examine the role of corner sites in small Au clusters, Molina and Hammer performed RPBE calculations of CO, 02, and CO 02 adsorption on an AU34 cluster on defect-free MgO(lOO).175 CO and CO 02 adsorption was found to... 

Accurate Chinese production figures are difficult to obtain however, it is estimated that the total production capacity of magnesia in China is about 2.7 Mt per annum. This comprises of 1.8 Mt of dead-burned magnesia and 900,000 tonnes of caustic-calcined magnesia. The purity of the magnesite ranges from 91 to 98% loss-free MgO content. 

We systematically studied the bonding of transition metal atoms to O ions of a regular, defect-free MgO(OOl) surface and we obtained results from nonrela-tivistic d-, 4c/-atoms) and scalar relativistic Ad-, 5c -atoms) GGA DF calculations [167]. The investigation comprised nine atoms of three triads of the periodic system Cr, Mo, W Ni, Pd, Pt Cu, Ag and Au. The substrate was represented by stoichiometric clusters embedded only in rigid PC arrays (see Section 2.2). Two conceivable adsorption sites were considered on-top adsorption on O anion and bridge positions over two adjacent O anions. 

Hawkins, P., and Hayden, K.L., "The Effect of Free MgO in Clinker on Volume Stability," Cement Chemists Seminar, Portland Cement Association, Skokie, Illinois, 1976,21 pp. 

Free MgO, also called periclase, is present in Portland clinkers only at high MgO contents in the raw meal (usually in the form of MgC03). It may be tolerated if present in amormts of only a few per cent, but may cause concrete damage if present in higher amormts (see also section 2.9). 

In ordinary Portland clinker (corresponding to ASTM type I Portland cement) tricalcimn silicate is the most abundant phase, present in amormts between about 50% and 70%. Dicalcium silicate usually constitutes 15-30% of the clinker. Typical amounts of tricalcium aluminate are 5-10%, and of the ferrite phase 5-15%. In special Portland clinkers the individual clinker pliases may be present in increased or reduced amounts, or may be absent entirely. Free lime, free MgO and alkali sulfates are minor constituents of Portland clinker that are present only in small amounts or may be absent entirely. Calcium sulfate is only rarely a constituent of Portland clinker, but is intergroimd in limited amounts with it, to obtain Portland cement. In Table 2.1 different types of Portland cement are compared. 

In the hydration of cement, Mg ions present in the crystalline lattices of clinker minerals are incorporated into the stmcture of the formed hydrate phases. Free MgO, present as periclase, also hydrates, yielding hexagonal magnesium hydroxide [Mg(OH)2], called brucite ... 

As an alternative solution it has been suggested that a clinker should be produced directly that contains free MgO, by burning a pertinent raw meal at 1380°C (Wang and Lou, 1992). In laboratory experiments a cement with 5% MgO hydrating at 50°C achieved complete conversion from MgO to Mg(OH)2 within a year and produced a linear expansion of 10 mm/m. 

Unlike MgO calcined at low temperatures or present in clinker in limited amounts, free magnesimn oxide (periclase) that is present in Portland cements made from raw meals with high MgCOj contents must be considered detrimental if present in amounts above 4-5 wt% MgO, as it tends to l drate at ambient temperature for years, and ultimately may cause cracking in the concrete stmcture. For additional data on Portland cement that contains elevated amounts of free MgO see section 2.9. 

Some important data relating to the clinker phases are given in Table3. Fig. 9 shows the strength development of these phases. As already stated, free CaO and free MgO (periclase) may also occur in the clinker. 

Eschka-Mixiure—Thoroughly mix 2 parts by weight of sulphur-free MgO with 1 part by weight of sulphur-free anhydrous Na2C03 and grind to pass through an ordinary flour sieve. 

The enhancement of catalytic activity of supported gold clusters such as Aug by moisture was also studied by Bongiorno and Landman [98] with first-principles calculations (Figure 11.20). The results indicated that the introduction of H2O in the feed gas would greatly enhance the adsorption of O2 on Aug. Furthermore, Oj would react with H2O to form hydroperoxyl-hke complexes, even if Aug clusters were supported on a defect-free MgO(lOO) film. The formed hydroperoxyl-like complex was responsible for CO oxidation to CO2 with a small activation barrier of 0.5 eV. 

---