Page 380 - Handbook of Thermal Analysis of Construction Materials

Section 2.0 Magnesium Oxychloride Cement 357

Two compounds, 9Mg(OH) •MgCl •5H O and 2Mg(OH) •MgCl 2
2
2
2
2
•4H O (9 and 2 form) are known to be present at temperatures higher than
2
100°C. Carbonation of MOC can occur on prolonged exposure to ambient.
This generally results in the formation of magnesium chlorocarbonate,
Mg(OH) •MgCl •2MgCO •6H O.
2
3
2
2
Small quantities of hydraulic aluminate minerals can cause the
[7]
conversion of the reaction products in MOC paste. Pastes with a molar
ratio of MgO/MgCl greater than 5 generally contain the 5 form as the main
2
reaction product. The same paste containing CA or C AF additions results
4
in formation of the 3 form. The conversion is attributed to the hydration
products of the hydrated aluminate minerals. Small additions of the latter
do not appear to influence strength of the hardened MOC paste. It is argued
that the polycrystalline composite containing either the 5 form or the 3 form
in combination with MgO or Mg(OH) behaves similarly in response to
2
mechanical loads. The addition of the inert aluminate minerals (e.g., C AS)
2
does not activate the phase conversion effect.
2.3 Microstructure Development
[8]
The evolution of microstructure is described as follows. A needle
morphology is observed at about 2 hours after mixing the MOC paste. Short,
stubby prismatic crystals with substantial intergrowth form a fine grained
structure within 4 hours. Crystal morphology in the form of longer prisms
with larger aspect ratio is more predominant in regions of high porosity.
Monolithic structures prevail in regions of low porosity.

2.4 Strength Development

[8]
A study by Matkovic and Young suggests that the low porosity
regions in MOC paste are analogous to contact zones in C S pastes critical
3
to strength development. The fracture process is consistent with that of
crystalline material. Mechanical interlocking is not considered to be a
major strength contributing factor.
A strength development curve is shown in Fig. 1. Strength (related
to pulse velocity) changes little after one day. The first appearance of
needle-like crystals corresponds to the onset of rigidity concommitant with
rapid microstructural development during the next few hours. Strength
development is a result of dense structures forming in limited space.
Formation of needle-like crystals is not a requirement.