Synthesis, Structure, and Thermal Stability of Magnesium Oxychloride 5Mg(OH)2∙MgCl2∙8H2O

Abstract

Today, low-energy and low-carbon footprint alternatives to Portland cement are searched because of huge CO2 emissions coming from Portland clinker calcination. Because of some superior properties of magnesium oxychloride cement (MOC) and the lower carbon footprint of its production, MOC became an intensively studied material with high application potential for the design and development of construction products. In this contribution, magnesium oxychloride with stoichiometry 5Mg(OH)2∙MgCl2∙8H2O (Phase 5) was prepared and characterized. The kinetics of formation and the phase composition of the material were determined using X-ray diffraction and consequent Rietveld analysis. The morphology was studied by scanning electron microscopy, and the chemical composition was determined by both energy-dispersive spectroscopy and X-ray fluorescence. Moreover, the simultaneous thermal analysis in combination with mass spectroscopy and Fourier-transform infrared spectroscopy was employed to study the thermal stability. Using mass spectroscopy, we were able to clarify the mechanism of water and hydrochloric acid release, which was not previously reported. The observed structural and chemical changes induced by exposure of studied samples to elevated temperatures were linked with the measured residual macro and micro parameters, such as bulk density, specific density, porosity, water absorption, compressive strength, and pore size distribution. The Phase 5 revealed a needle-like crystalline morphology which formed rapidly and was almost completed after 96 h, resulting in relatively high material strength. The four-day compressive strength of magnesium oxychloride cement was similar to the 28-day compressive strength of Portland cement. The thermal stability of Phase 5 was low as the observed disruptive thermal processes were completed at temperatures lower than 470 °C.

Carbon Dioxide Uptake by MOC-Based Materials

“…This process MOC phases and MOC-based composites have been intensively studied in recent years. Synthesis, structure, and the thermal stability of magnesium oxychlorides in particular were studied in detail [6,7]. MOC is a non-hydraulic versatile binder with specific properties, which make it superior to PC.…”

Water-to-Cement Ratio of Magnesium Oxychloride Cement Foam Concrete with Caustic Dolomite Powder

“…The results show that a 5•1•8 phase was present in crosslinked needle-and rod-shaped crystals in the MOC, which endowed it with good mechanical properties [23,24]. When the molar ratio of MgO/MgCl 2 was less than 6, the MgO content in the hydration products increased with an increase in the MgO/MgCl 2 molar ratio, which made the original solution more alkaline and ensured the stability of the MOC phase [7].…”

“…The strength of MOC can be reduced with the introduction of waste [14]. The MgCl 2 used in the laboratory or small amounts of MgCl 2 used to study MOC material are obtained as high-purity raw materials [23,24], and some errors may be involved in practical industrial applications when an impure raw material is used [25]. Due to the number of raw materials, their relationships with concrete performance have a great impact [26,27].…”

Foam Glass Lightened Sorel’s Cement Composites Doped with Coal Fly Ash

“…Previous studies describing the development of phase 3 and phase 5 in the reaction process show the progress of their formation. The stoichiometric phase 3 is fully formed (>99 wt.% of the sample) after only 36 h. The study of formation of the phase 5 described the formation process similarly, determining the formation time of the pure phase 5 (>99 wt.% of the sample) to 96 h [ 28 , 29 ]. Concerning the strength development, preceding research has shown the significant increase in the compressive strength in the first seven days of curing.…”