Improving moisture resistance of MDF cement composites with coatings and additives

Abstract

Macro-Defect Free (MDF) cement composite, developed and patented by scientists from Imperial Chemical Industry (ICI) in the early 1980s, are cement-polymer composites designed to enhance strength by minimizing large voids. These composites are made by adding small amounts of polymer and water to the cement and processing the mixture using a method similar to rubber production. The material is passed between two roller mills at different speeds, where high shear forces eliminate macro voids, a crucial step in production. The composite is then cured under moderate temperature and pressure, which further reduces voids, with the polymer believed to fill them during this stage. A key feature of MDF cement composites is their unusually high flexural strength, reaching 150-300 MPa, which is comparable to the strength of steel and far exceeds the 5-10 MPa of conventional cement paste. While initial claims attributed this strength to void elimination, later research suggested that crosslinking reactions between cement ions and polymer chains play a more significant role. Different cement and polymer combinations can be used to produce MDF cement composite, with calcium aluminate cement (CAC) and poly(vinyl alcohol-co-vinyl acetate) (PVAc) copolymers achieving the highest flexural strength, followed by ordinary Portland cement (OPC) with poly(acrylamide). However, these composites have poor durability when exposed to water, experiencing swelling and rapid strength loss. Therefore, this thesis explores strategies to improve the strength and durability of Macro Defect-Free (MDF) cement composites by employing various coatings and additives. The primary focus is on assessing the effects of nano-coatings, silane and siloxane coatings, and carbon nanotube additives on the physical and chemical properties of MDF cement composites. It investigates the microstructural interactions between these treatments and the cement matrix, with a specific emphasis on mitigating water sensitivity and enhancing mechanical strength. The research employs two approaches: applying hydrophobic coatings and incorporating additives to reduce water sensitivity in MDF. The coatings examined include nano-coatings, silane and siloxane, silica-based, and polyurethane, while the additives tested include lignosulphonates, silica-based material, and carbon nanotubes at varying concentrations. Biaxial flexural strength tests were conducted on dry and wet specimens to evaluate the impact of water immersion on strength. Additional tests, such as water absorption and water contact angle measurements, were performed to assess permeability and hydrophobicity before and after water immersion. Microstructural analysis using Scanning Electron Microscopy (SEM) provided insights into the quality of adhesion and changes within the cement matrix, while X-ray Diffraction (XRD) analysis quantified the crystalline phases in the treated samples, shedding light on the effects of different treatments. The experimental results revealed that silane and siloxane coatings significantly reduced the strength loss due to water immersion and performed exceptionally well in water absorption tests, demonstrating low permeability. Water contact angle measurements showed that these coatings maintained high hydrophobicity, even after exposure to water. SEM images confirmed a uniform coating layer for silane and siloxane treatments, unlike other coatings, which displayed incomplete coverage. While XRD analysis showed minimal differences for most treatments compared to the control, carbon nanotube additives exhibited patterns that correlated with strength retention, as evidenced by SEM analysis, indicating their potential to enhance the cement matrix. The findings highlight the superior performance of silane and siloxane coating in improving the durability and water resistance of MDF cement composite. Although nano-coatings and carbon nanotubes provided some mechanical and microstructural advantages, the consistent performance of silane and siloxane across all tests makes them particularly suitable for practical applications. Future research should aim to further optimize these treatments and investigate how additives like carbon nanotubes can reinforce the cement matrix. In summary, this thesis offers a detailed examination of how different coatings and additives influence the strength, durability, and water sensitivity of MDF cement composite. The results suggest that silane and siloxane coatings are promising solutions for reducing water sensitivity in cementitious materials, contributing valuable insights to the field of construction materials and the improvement of MDF cement composite performance in various environmental conditions.