Page 455 - Handbook of Thermal Analysis of Construction Materials

430 Chapter 10 - Non-Portland Rapid Setting Cements

4.0 MAGNESIUM OXYCHLORIDE AND
MAGNESIUM OXYSULFATE CEMENT
SYSTEMS

Magnesium oxychloride cement, also known as Sorel cement, is
formed by mixing finely divided magnesium oxide with an aqueous
solution of magnesium chloride. [17] It possesses many properties superior
to those of portland cement. These include fire resistance, low thermal
conductivity, higher abrasion resistance, and compressive and flexural
strengths. Its excellent binding capability permits the use of many organic
and inorganic aggregates which would be unsuitable for making portland
cement concrete.
Four types of oxychloride complexes [18] are known to form in the
MgO–MgCl -H O system. They are 5Mg(OH) •MgCl •8H O (5-form),
2 2 2 2 2
3Mg(OH) •MgCl •8H O (3-form), 2Mg(OH) •MgCl •5H O (2-form), and
2
2
2
2
2
2
9Mg(OH) •MgCl •6H O (9-form). The 2 and 9-forms are formed in solu-
2 2 2
tions at temperatures above 100%. The formation of magnesium
chlorocarbonate, Mg(OH) •MgCl •2MgCO •6H O, is detected upon con-
2 2 3 2
tinuous exposure of the oxychloride to air.
The stabilization of hardened oxychloride cement products against
attack by water has been studied, as it is known that these products lose
strength on prolonged exposure to water.
Magnesium oxysulfate cements can be produced by adding magne-
sium chloride solutions to calcium sulfates or calcium phosphate-sulfate
mixtures. [19] The cements formed can be regarded as variants of Sorel
cements. The following phases have been identified depending upon the
temperature and pressure conditions: 3Mg(OH) •MgSO •8H O;
4
2
2
3Mg(OH) •MgSO •3H O; Mg(OH) •MgSO •5H O; Mg(OH) •2MgSO
2 4 2 2 4 2 2 4
•3H O; 2Mg(OH) •3MgSO •5H O; MgSO •H SO •3H O; 3Mg(OH) 2
2
4
2
4
2
4
2
2
•MgSO •4H O. DTA and DTG curves of the 3, 5, 2, and 9-forms are
4 2
presented in Fig. 22. The numbers on the curves refer to the number of water
molecules per mole remaining. The peaks and their corresponding tempera-
tures are given in Table 1.
There is a close correspondence between the DTA and the DTG
curves. The former occurs at a temperature of about 45°C higher
than the latter. The corresponding heating rates were 10°C/minute and
0.4°C/minute.