Page 382 - Handbook of Thermal Analysis of Construction Materials

360 Chapter 9 - Non-Portland Cement Binders and Concrete

Strength development due to mechanical bonding of strong
whisker crystals of hydrates on MgO crystals was a mechanism consid-
ered by Tooper and Cartz. [10] The bonding sites were augmented by
interpenetration of crystals and solid-solid contact with other scroll-type
crystal morphologies.

2.5 Resistance To Water

MOC paste undergoes significant strength retrogression (up to
50%) when continuously immersed in water. [11]–[13] This is a dissolution
process and requires continuous contact with water for leaching of magne-
sium chloride to be significant. The water resistance of MOC can be
improved by addition of ferrous phosphate and aluminum phosphate with
magnesium oxide powder. [11] Coatings with bitumen emulsion have also
been effective in minimizing strength retrogression. Sulphur impregnation
of MOC paste improves water resistance significantly. [14] Loss in strength
is reduced from 48.5% to 12.6% after complete immersion in water for
seven days. Re-impregnation after exposure to water for 28 days increases
strength and stabilizes the materials on further exposure to water. The more
readily soluble material is removed through exposure of the initially
impregnated material to water. Subsequent impregnation results in a body
with more sulphur distributed in a matrix more resistant to water.
Additives which have been shown to be effective in lessening the
weakening effects of water include the following: [15] fly ash, slag, baked
clay, diatomite, silicate cement, coal gangue, baked diatomite, H PO ,
4
3
Al(H PO ) , H BO , Na SO , Na PO , and K PO . The addition of copper
2 4 3 3 3 2 4 3 4 3 4
powder in the amount of 10–12% has also been reported to increase strength
and water resistance of MOC paste. This is attributed to formation of
3CuO•CuCl •3H O.
2
2

3.0 MAGNESIUM OXYSULFATE CEMENT

3.1 Hydration

Magnesium oxysulfate cement pastes can be produced through
reaction of MgO and an aqueous solution of MgSO •7H O. [16] Alterna-
2
4
tively magnesium chloride solutions can be added to calcium sulfates or
calcium phosphate–sulfate mixtures. The presence of phosphates enhances
the water resistance of this cement system.