TITLE

Factors Influencing the Phase Composition and Properties of Magnesium Oxychloride Cements

AUTHOR(S)
Tawfik, A.; Serry, M. A.
PUB. DATE
March 2013
SOURCE
Interceram: International Ceramic Review;Mar2013, Vol. 62 Issue 1, p25
SOURCE TYPE
Trade Publication
DOC. TYPE
Article
ABSTRACT
Precursor MgO/MgCl2 and H2O/MgCl2 molar ratios were varied (from 3 to 6 and 12 to 24, respectively) to determine their effect on the phase composition and properties of pure magnesium oxychloride (MOC) cement pastes after air curing. XRD and DTA were applied to reveal the phase composition of hydrated pastes. The water required for workability and initial and final setting times were determined to assess the cement's hydraulic properties. Physico-mechanical properties of hardened samples were also evaluated by means of bulk density (BD) and compressive strength (CS) measurements. Selected samples, doped with 0-1.33 mass-% of sodium hexametaphosphate (NaPO3)6 were investigated for water resistance after 7-14 d immersion in water. This was done by comparing BD values and computing a compressive strength retention ratio (CSRR), as well as examining changes in component functional groups (by FTIR spectroscopy) after water curing. he results indicate that increasing the molar MgO/MgCl2 ratio from 3 to 6 leads to a gradual increase in the amount of water needed for workability and consequently, H2O/MgCl2 molar ratio, with little change in the range of initial and final setting times (25-35 and 120-180 min, respectively) for all samples. CS increased to its maximum value at a molar ratio of 3.5 MgO/MgCl2 and then gradually decreased to a minimum as the ratio increased to 6. Meanwhile, BD lowered gradually from ~1.6 to 1.0 g/cm³. The XRD and DTA investigation revealed that with molar MgO/MgCl2 ratio 3-3.5, phase-3 [3Mg(OH) 2·MgCl2·8H2O] is the main product component with lesser amounts of phase-5 [5Mg(OH) 2·MgCl2·8H2O] and chlorartinite [Mg2 (OH)ClCO3·3H2O] also detected. The latter phase predominated as the molar ratio increased up to 6.0 at the expense of phase-5 and phase-3, mainly due to the relatively higher stability and resistance of phase-3 against atmospheric CO2 gas than phase-5. Doping of selected MOC samples, having MgO/MgCl2 molar ratio 3.5, with up to 1.33 % SHMP led to a gradual decrease in compressive strength and CSRR of the air-cured samples and a decrease of bulk density. Water resistance of these samples improved with increased SHMP up to 0.66-1.33 %, (~1.0 %), which gave the lowest BD (<1.0 g/cm³) and the highest CSRR (1.0-1.4), after water-curing up to 14 d. This is mainly attributed to phosphate anions stabilizing the major MOC P-3 phase at the expense of -OH groups and discouraging dissolution in water, as confirmed by the FTIR spectra.
ACCESSION #
86442206

 

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