Page 383 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Magnesium Oxysulfate Cement 361

Four magnesium oxysulfate complexes are formed in the MgO-
MgSO -H O system in the temperature range, 30–120°C. These are:
4
2
5Mg(OH) •MgSO •2–3H O
2
4
2
3Mg(OH) •MgSO •8H O
2 4 2
Mg(OH) •MgSO •5H O
4
2
2
Mg(OH) •2MgSO •3H O
2
2
4
The complex 3Mg(OH) •MgSO •8H O is stable below 35°C. The complex
2
4
2
5Mg(OH) •MgSO •3H O has also been detected below 35°C.
2 4 2
Magnesium oxysulfate cement systems have many similar charac-
teristics to those of magnesium oxychloride cement systems. They are,
however, less sensitive to elevated temperatures, hence more suitable to
some industrial processes incompatible with the use of magnesium oxy-
chloride cement.
3.2 Strength Development
Magnesium oxysulfate cement pastes are generally considered to
be weaker than oxychloride pastes. [17] Beaudoin and Ramachandran exam-
ined the strength (related to microhardness)–porosity characteristics of
paste-hydrated and compacted magnesium oxysulfate cement pastes. [18]
The curves in Fig. 3 show the effect of particle size on the performance of
compacted materials in addition to the behavior of the paste-hydrated
materials. The pastes were prepared at different solution-solid ratios. A
series of pastes made using a saturated solution of MgSO •7H O was
4 2
included in the investigation. There is a linear semi-logarithmic relation for
all the paste-hydrated materials. The compacted specimens show signifi-
cantly greater strength than the paste-hydrated specimens at porosity values
in the range 10–35%. The compaction process evidently increases the
number of interparticle bonds possibly involving comminution processes.
Larger values of microhardness for compacts made with finer starting
material lend support to this argument. Increased strength at lower porosi-
ties (compacted samples) is attributed to an increase in the volume concen-
tration of finer pores in the specimens and an increase in the number of
contacts between particles.