(90f) Characterisation and Enhancement of Water Stability and Mechanical Properties of Magnesium Oxychloride Cement

Abstract

In recent decades, the research community has expressed significant interest in the development of sustainable building materials, particularly low-carbon cement, with the aim of minimising carbon dioxide emissions. Among these materials is magnesium oxychloride (MOC), which is produced through a chemical reaction between magnesium oxide (MgO) and a magnesium chloride (MgCl2) solution. Additionally, reactive magnesium oxide (MgO) is created by calcining magnesite (MgCO3) at temperatures below 750°C, considerably lower than the 1450°C required for sintering Portland cement clinker. This process conserves energy by reducing fuel consumption and contributes to the overall reduction of greenhouse gas emissions. This cement is often referred to as "eco cement" due to its lower CO2 emissions (20-40%) and reduced energy consumption during production, compared to ordinary Portland cement.

Magnesium oxychloride cement, also known as Sorel cement, is formed by mixing powdered magnesium oxide (MgO) with magnesium chloride (MgCl₂). The optimal basic formula for this cement is determined by a molar ratio of MgO/MgCl₂ (M) of 9 and H₂O/MgCl₂ (H) of 13. This specific ratio has been found to yield the best performance in terms of the cement's properties. Magnesium oxychloride (MOC) is a crystalline ceramic material that holds significant promise as a structural and fire-resistant construction material. It is widely regarded as one of the most robust cement types, offering distinct advantages over Portland cement. MOC is known for its exceptional strength, superior bonding capabilities, and rapid setting characteristics, achieving impressive early strength without the need for humid curing. Its durable, stone-like composition makes it fireproof, as extensively documented in the literature. The needle-like structure of MOC allows the crystals to interlock, resulting in a very tough and robust material. This interlocking crystal structure contributes to its high strength and durability, making it highly resistant to mechanical stress and enhancing its overall performance as a construction material.

The interlocking needle-like crystal structure of MOC makes it suitable for both lightweight and heavy-duty flooring applications. This cement exhibits remarkable load-bearing capacity, able to withstand vibrations from heavy cast iron wheel movements without displaying any signs of cracks or fissures. Its exceptional durability and strength make it ideal for demanding environments where both structural integrity and resistance to mechanical stress are required. Additionally, MOC is relatively lightweight, with a specific gravity of 2.4. It features a low coefficient of thermal expansion and demonstrates negligible volume change during setting, contributing to its dimensional stability and making it suitable for applications where minimal thermal stress and expansion are critical. These properties further enhance its suitability for use in a variety of structural and flooring applications. Its versatile applications extend across a wide range of industries, including industrial flooring, ship decks, railway passenger coach floors, hospital floors, ammunition factory floors, missile silos, underground armament factories, and bunkers. The exceptional strength, durability, and fire resistance of MOC make it an ideal choice for environments that demand high-performance materials capable of withstanding heavy traffic, vibrations, and extreme conditions. While MOC offers numerous advantages for construction purposes, it can exhibit limited water stability. This can lead to issues such as degradation or weakened structural integrity when exposed to prolonged moisture or water. As a result, enhancing its water resistance is an important consideration in expanding its application and ensuring long-term durability in various environments.

Unfortunately, the limited water stability of MOC is due to the inherent poor water resistance caused by the decomposition of its hydrate products, specifically 3 Mg(OH)₂·MgCl₂·8H₂O (P3) and 5 Mg(OH)₂·MgCl₂·8H₂O (P5), into Mg(OH)₂. This decomposition leads to a significant reduction in compressive strength, compromising the material's structural integrity when exposed to moisture or water over time. It is important to note that the mechanical strength of magnesium oxychloride cement (MOC) primarily arises from the formation of 5-phase crystals during hydration. By optimizing the molar ratio of MgO, H₂O, and MgCl₂, the formation of these five-phase crystals can be enhanced, leading to an increase in the overall strength of the MOC. This optimization is crucial for improving the material's performance and ensuring its suitability for various structural applications. The reduction of chloride is crucial, as free chloride ions in cement can lead to the formation of hydrochloric acid. This acid accelerates the oxidation of iron, which compromises the tensile strength of reinforced steel bars and undermines the overall structural integrity of the material. Controlling chloride levels is therefore essential to maintaining the durability and strength of reinforced concrete structures.

This research presents methodologies designed to investigate and enhance water stability, reduce free chloride content, and optimize the 5-phase composition of materials. These objectives are achieved through the use of X-Ray Diffractometry (XRD), X-Ray Fluorescence (XRF), and titration techniques, which provide detailed particle characterisation, material’s structural and chemical properties.

The composition and crystal size of a sample were analysed using XRD, with the crystal size determined through the Scherrer equation. Rietveld refinement was employed to accurately quantify the sample's composition. Achieving optimal peak fits required adjustments to crystallite peak broadening, microstrain peak broadening, and site occupancy. The site occupancy of chloride determined from XRD data was further validated through titration analysis to quantify both the free chloride and the chloride leached from the crystal structure during water stability testing. The crystal size of the 5-Phase was also characterized in relation to the mechanical properties of the cement and its water stability, in order to evaluate the potential influence of crystal size on parameters such as compressive strength and overall water resistance.