Characterisation of magnesium oxysulphate (MOS) cement pastes produced with MgO extracted from magnesium silicate rocks

• Ano de defesa: 2024
• Autor(a) principal: Freitas, Taís Oliveira Gonçalves
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: Alternative cements; Cimento de oxisulfato de magnésio (MOS); Cimentos alternativos; Hidróxido de magnésio; Magnesium hydroxide; Magnesium oxysulphate cement (MOS); Magnesium silicate; Microestrutura; Microstructure; Silicato de magnésio
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/74/74133/tde-01042025-085828/

The construction industry faces the challenge of reducing its environmental footprint while providing essential infrastructure in developing countries. Portland cement is responsible for around 8 per cent of global anthropogenic CO2 emissions. Although it is not possible to completely replace Portland cement, great efforts are being made globally to mitigate its environmental impact. One promising alternative is the development of new binders, such as magnesium cements, whose main raw material is magnesium oxide (MgO). MgO is mainly produced by calcining magnesite (MgCO3) or extracted from hydrated magnesium silicate rocks using alkaline dissolution or acid leaching, which allow the production of Mg(OH)2 without releasing CO2, offering a sustainable alternative. Magnesium oxysulphate cement (MOS) is known for its good properties, such as fire resistance, low density and low thermal conductivity, and is produced from a mixture of MgO with an aqueous solution of magnesium sulphate (MgSO4) and weak acids. Most studies on MOS cements use MgO obtained by calcining magnesite, but the influence of using MgO obtained from Mg(OH)2 extracted from magnesium silicate rocks has not yet been studied. This study aims to fill this gap by assessing the impact of MgO obtained by calcining Mg(OH)2 on the hydration reactions of MOS cements. In the first stage, three formulations of MOS cement produced with MgO obtained from analytically pure Mg(OH)2 were studied to determine the best MgO molar ratio. The pastes showed a complex process of phase formation, with those with a lower molar ratio showing better mechanical behaviour due to better proportions of stable phases at 7 and 28 days, while higher molar ratios of MgO/MgSO4 influenced the formation of Mg(OH)2, reducing the strength of the paste. In the second stage, the best performing formulation was adopted to develop MOS cement pastes with MgO obtained from hydrated magnesium silicate rocks. The pastes were evaluated after 6 hours, 1, 3, 7 and 28 days to determine their physical, mechanical and microstructural properties. Although there were changes in the hydration kinetics and the appearance of new hydration products in the microstructure due to impurities in the raw material extraction process, the greatest variation in the physical and mechanical properties was observed only in the first 6 hours. The cement paste made with MgO extracted from magnesium silicate rocks hydrated faster due to its greater specific surface area, reaching 50% hydration in less time and providing greater compressive strength in the first 6 hours. Both pastes reached values greater than 22 MPa in 1 day of analysis, showing potential for applications requiring rapid hydration. After 28 days, the pastes showed good mechanical strength (around 56 MPa), which, due to their low alkalinity, low density and high porosity, makes them suitable for different types of applications such as construction materials, and may be suitable for composites reinforced with vegetable fibers, for example.