MgO cements

Excessive expansion and crack formation of high-MgO cement may also be prevented... 

Chatteiji, S. (1995) Mechanism of expansion of concrete due to the presence of dead burnt CaO and MgO. Cement and Concrete Research 25,51-56. 

Ben Haha, M., G. Le Saout, F. Winnefeld and B. Lothenbach (2011). Influence of slag chemistry on the hydration of alkali activated blast-furnace slag - Part I Effect of MgO . Cement and Concrete Research 41(9) 955-963. 

An aqueous solution of mono ammonium phosphate [10361-65-6] reacts with MgO to form ammonium magnesium phosphate hexahydrate [15490-91-2], NH MgPO 6H20. Several other species of hydrated phosphates are created during this reaction which takes place quickly and produces compounds that have desirable properties as cementing agents. The hexahydrate is the most prevalent. Properties are given in Table 22. 

Portland Cement. Portland cement is obtained by calcining a mixture of substances to produce an appropriate ratio of the oxides CaO, MgO, AI2O2, Fe202, and Si02 (9) (see Cement). 

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). 

Calcination or dead burning is used extensively to dehydrate cements (qv) and hygroscopic materials such as MgO, and to produce a less water sensitive product. Calcination is also used to decompose metal salts to base oxides and to produce multicomponent or mixed oxide powders for... 

Certain oxides of divalent metals, those of ZnO, CuO, SnO, HgO, and PbO, form cements that are hydrolytically stable in addition MgO, CaO, BaO and SrO form cements that are softened when exposed to water. Compressive strengths of these materials range from 26 to 83 MPa, the strongest being the copper(II) and zinc polyacrylate cements (Table 5.1). Crisp, Prosser Wilson (1976) found that for divalent oxides the rate of reaction increased in the order... 

Most practical cements contain Mg " which is less strongly bound to the polyacrylate than Zn (Gregor, Luttinger Loebl, 1955a). Magnesium oxide forms a paste with PAA which sets to a plastic mass this is not hydrolytically stable, for when placed in water it swells and softens (Hornsby, 1977 Smith, 1982a). Moreover, if ZnO powder contains more than 10% MgO, the resultant cement deteriorates under oral conditions. 

Lastly, there was the cementitous reaction which Kingery (1959b) reported with BeO, Be(OH)2, CuO, CujO, CdO, SnO and PbjO. In addition, calcined ZnO and MgO formed cements. 

Although an acid phosphate matrix cannot be excluded it is not essential for cement formation. In fact, it must be remembered that when these cements are prepared the oxide or silicate powder is normally in excess of that required for the reaction. Under these conditions most oxides (MgO... 

Cement formation between MgO and various acid phosphates involves both acid-base and hydration reactions. The reaction products can be either crystalline or amorphous some crystalline species are shown in Table 6.5. The presence of ammonium or aluminium ions exerts a decisive influence on the course of the cement-forming reaction. 

The reaction between MgO and ammonium dihydrogen phosphate (ADP) in aqueous solution yields struvite, MgNH4P04. bHjO, and schertelite, Mg(NH4)2(HP04)2.4H2O, as the main reaction products. Both may be regarded as cementing species. Only minor amounts of MgO are consumed during these reactions as it is present in excess. The reaction is exothermic. Sodium tripolyphosphate (STPP) or borax may be added to retard the reaction. The main course of the reactions may be represented thus ... 

Finch Sharp (1989) found the mole ratio of MgO to A1(H2P04)3 to be an important parameter that affected both the reaction rate and the nature of the reaction products. The critical mole ratio was 2 1. When the ratio was less than 2 1 cements were not formed at all, and when it was exactly 2 1 the paste set slowly and always remained tacky. Further increases in the ratio caused cements to set faster with greater evolution of heat. Finch Sharp (1989) also found that this ratio affected the proportion of crystalline phase to amorphous phase in the cement matrix. The proportion of newberyite in the matrix reached a maximum when the MgO/A1(H2P04)3 ratio was 4 1 and decreased to a low level when the ratio was 8 1. 

Figure 6.12 Microstructure of MgO-aluminium hydrogenphosphate cement (Finch Sharp, 1989).

Matkovic, B., Popvic, S., Rogic, V., Zunic, T. Young, J. F. (1977). Reaction products in magnesium oxychloride cement pastes. System MgO-MgClj-HjO. Journal of the American Ceramic Society, 60, 504-7. 

There have been a number of studies aimed at understanding the chemistry of the curing and setting of magnesium oxychloride cements and at identifying the phases that are present in the final material. Investigations in the first half of the twentieth century revealed that cement formation in the MgO-MgCla-HaO system involves gel formation and crystallization of... 

MgO, produced by different but well-defined routes and having different reactivities towards aqueous MgCl in this way, it was possible to study the cementation reactions in some detail and to ensure a reasonably close approach to equilibrium. 

Sorrell Armstrong formulated cements in proportions corresponding to the 5 1 8 and 3 1 8 compositions. The initial mixtures were thick slurries with no observable tendency to separate provided a sufficiently reactive oxide was used. They tended to set within about 90 minutes, at which time samples were prepared for X-ray determination. Initially, although the preliminary hardening process was apparently complete, the only crystalline phase that could be found was MgO moreover, this material was found in amounts that approximated to the quantity in the initial mixture. 

After some two hours, the X-ray diffraction pattern corresponded to either the 5 1 8 or the 3 1 8 phase warming of the sample had also occurred. Growth of the crystalline oxychloride phases continued rapidly up to about 15 hours, and more slowly thereafter, until after four days there was no trace of MgO in the diffraction pattern of the cement. 

The fact that the initial setting process for magnesium oxychloride cements takes place without observable formation of either the 5 1 8 or the 3 1 8 phase is important. It indicates that formation of an amorphous gel structure occurs as the first step, and that crystallization is a secondary event which takes place from what is effectively a supersaturated solution (Urwongse Sorrell, 1980a). This implies that crystallization is likely to be extremely dependent upon the precise conditions of cementition, including temperature, MgO reactivity, heat build-up during reaction and purity of the components in the original cement mixture. 

This work was of value in constructing that part of the phase diagram involving solutions in water. Of greater value in understanding cement formulations were the results obtained in the earlier study (Sorrell Armstrong, 1976) for the MgO-rich portion of the phase diagram. 

Table 8.4. Properties of MgO, CuO, Cu O, Bi O, La O phosphonic acid cements Ellis, 1989 Ellis Wilson, 1990)...

The low permittivity of these liquids compared with water inhibits dissociation of the acids so that cement formation demands much more reactive basic oxides. Oxides and hydroxides that are capable of cement formation are ZnO, CuO, MgO, CaO, Ca(OH)2, BaO, CdO, HgO, PbO and BiaOj (Brauer, White Moshonas, 1958 Nielsen, 1963). In practice these are confined to two calcium hydroxide and special reactive forms of zinc oxide. 

Eugenol, 4 allyl-2-methoxy phenol, is capable of forming cements with ZnO, CuO, MgO, CaO, CdO, PbO and HgO (Brauer, White Moshonas, 1958 Nielsen, 1963). Other 2-methoxy phenols are also capable of forming cements with metal oxides, provided the allyl group is not in a 3- or 6-position where it sterically hinders the reaction (Brauer, Argentar Durany, 1964). These include guaiacol, 2-methoxyphenol, and the allyl and propylene 2-methoxy phenols. 

Moshonas (1958) investigated the reactions between zinc oxide and a large number of chelating agents. Of these, EBA proved to be the most promising. They then examined cement formation between EBA and various metal oxides. Cement formation was found with MgO, CaO, BaO, ZnO, CdO, HgO and PbO. 

---