The influence of composition and temperature on hydrated phase assemblages in magnesium oxychloride cements

Abstract

Due to their nonhydraulic nature, magnesium oxychloride cements (MOCs) are susceptible to degradation following contact with water. Improving the water resistance of these materials requires better understanding of hydrated phase relations and the sensitivity of hydrated phases to water. Toward this end, a series of targeted experiments and complementary thermodynamic calculations were carried out to assess hydrated phase assemblages in the system MgO-MgCl 2 -H 2 O across a range of compositions. Focus is placed on appraising the effects of composition and reaction temperature. In broad agreement with literature data, under ambient conditions, hydrated MOCs are noted to contain Phase 3 (P3), Phase 5 (P5), and brucite, but the mass partitioning of these phases is highly dependent on H 2 O/ MgCl 2 and MgO/MgCl 2 molar ratio as well as curing temperature and age. At room temperature, P3 is favored at lower water contents, however, P5 and/or brucite are favored to form as water availability increases. Thermodynamic calculations indicate that P3 is "more stable" than P5 at lower temperatures-an outcome which impacts the engineering properties, for example, strength and volume stability. The impacts of the accuracy and self-consistency of currently available thermodynamic data and their implications on predicted phase assemblages are discussed. K E Y W O R D S hydration, magnesium oxychloride cement, mechanical properties, temperature, thermodynamics, volume change Angeles, CA.

Super-hydrophobic magnesium oxychloride cement (MOC): from structural control to self-cleaning property evaluation

“…It should be noted that for all the MOC specimens, the mineralogical phases consist of major Phase 5, minor Mg(OH) 2 and unreacted MgO and MgCO 3 . This is in agreement with the previous studies [2,4,6]. As the Phase 5 is the most favourable crystalline phase [2,6], the molar ratio of MgO/MgCl 2 is desired to be higher than 5 and the molar ratio of H 2 O/MgCl 2 is in the range of 6-15.…”

“…Magnesium oxychloride cement (MOC) has attracted extensive attention, as it is generally associated with ambient temperature curing and excellent materials properties such as fast setting, high mechanical strength, good resistance to abrasion and fire [1][2][3][4][5]. The behavior of MOC is mainly regulated by the composition and microstructure of the hydration products, produced through neutralization-hydrolysis-crystallization process in the ternary system of MgO-MgCl 2 -H 2 O [1,6].…”

Effect of alcohol leachable chloride on strength of magnesium oxychloride cement

“…It has early been concluded that the aqueous reactions between MgO and MgCl 2 in the system MgO‐MgCl 2 ‐H 2 O can yield Mg 3 (OH) 5 Cl·4H 2 O (5‐phase), Mg 2 (OH) 3 Cl·4H 2 O (3‐phase), Mg(OH) 2 , the gel and mixtures thereof, which are highly dependent on the precursor molar ratios, temperature, and MgO reactivity (ie, the reaction rate of MgO in MgCl 2 solution) 17‐23 . Besides, the equilibrium phases in the system also involve unreacted MgO and residual chlorides.…”

Phosphoric acid‐modified magnesium oxychloride: Study of water stability, kinetics, and pair distribution function

“…For structural applications, MOC has the stoichiometric formula Mg 3 (OH) 5 Cl·4H 2 O and it is commonly referred to as 5‐phase MOC . MOC has an alternative stoichiometric formula (Mg 2 (OH) 3 Cl·4H 2 O, 3‐phase MOC) that is thermodynamically more stable at low water content curing conditions, but 5‐phase is usually the preferred phase for construction applications.…”

“…Magnesium oxychloride is known to slowly degrade in water and recrystallize into magnesium hydroxide or other magnesium species, which can be accelerated with increased temperature. This results in issues regarding the retention of structural properties of MOC composites when exposed to weathering conditions, such as rain and high humidity.…”