Page 458 - Handbook of Thermal Analysis of Construction Materials

Section 4.0 - Magnesium Oxychloride and Oxysulfate 433

Compacts of hydrated magnesium oxychloride paste (designated
System III) were also studied. [17] The effect of immersion in water at 85%
for 5 hours on the compacts of paste (designated System III) hydrated
magnesium oxychloride cement (chloride solution – solid ratio = 0.59) are
illustrated in Fig. 23. [17] The endothermal dip in the DSC curves for System
III at 425°C was due only to Mg(OH) . It is apparent the oxychloride
2
complex became unstable in hot water.
Magnesite is used as a source material for making magnesium
oxychloride cement. [20] Variations in crystallinity and composition of
magnesite can affect the quality of oxychloride-based products including
their mechanical strength. Significant strengths are obtained with crypto-
crystalline magnesite with low iron and calcium content. The presence of
forsterite (Mg SiO ) is not desirable and was not detected in the two
2
4
samples that gave the best results. Formation of dicalcium and tricalcium
silicate can occur if the CaO/SiO ratio is greater than 1.87. This would
2
result in good strength as all the MgO is available to form oxychloride and
additional hydraulic reactions of the calcium silicates can occur. There was
no evidence for the presence of these silicates in the work cited.
The two superior magnesites are designated 1 and 3 in Fig. 24.
Figure 25 contains thermograms of the magnesium oxychlorides produced
with these magnesites. Figure 24 indicates that the content of the magne-
sium carbonate (peak at 700°C) is similar for all samples (verified by
chemical analyses). Samples 2, 4, 5, and 6 show higher endothermal effects
due to FeCO and CaCO at 500°C and 800–925°C respectively, and the
3
3
presence of these compounds is not conducive to the development of
strengths. Further, it was demonstrated that an oxychloride cement giving
a higher endothermal peak area at 400°C gave higher flexural and compres-
sive strengths. [21] This peak represents the primary strength-contributing
component, 3Mg(OH) •MgCl •8H O in the set cement. It was also shown
2
2
2
that larger dehydration losses between 50–250°C would mean greater
uncombined magnesium chloride in the set cement and lower strength.
The thermograms in Fig. 25 (obtained after 28 days reaction) show
that the oxides of samples 1 and 3 have definitely resulted in the formation
of larger amounts of 3Mg(OH) •MgCl •8H O than those of the others as is
2
2
2
evident by the large endothermal peaks at 400°C. The oxychloride cements
prepared from samples 2, 4, 5, and 6 show more uncombined MgCl than
2
that prepared from samples 1 and 3 as exhibited by the endothermal peak
between 50–250°C.