Cement-based materials with hydrophobic micro-textured surface

Abstract

The use of hydrophobic additives is a relatively new development within the field of durability design of concrete structures. This approach relies mainly on chemical modification of the cement paste matrix. Another promising, but less well-studied approach is by adopting the Cassie-Baxter mechanism. This can be achieved by creating a suitable micro texture on the surface of the cementitious material, which traps air when in contact with water leading to the separation between water and solid. The focus of this study is to develop concretes with hydrophobic micro textured surfaces and to study the properties and durability of such concretes. Hydrophobic Precipitated Calcium Carbonate (HPCC) made by dry ball milling calcium carbonate and stearic acid is used to enhance hydrophobicity of the textured surfaces. The effects of HPCC on the wettability and mass transport properties of cementitious materials were investigated. By adopting soft lithography, micro textures on hydrophobic lotus and taro leaves were transferred onto the surfaces of cementitious samples. Furthermore, research on developing hydrophobic surfaces using a fine microgrid to mimic the micro texture of hydrophobic plant leaves was undertaken. The hydrophobicity of micro textured surfaces was enhanced by adding HPCC into cementitious mixture or by coating the surface with Polydimethylsiloxane (PDMS). Water contact angle measurements were carried out to determine the effect of surface micro textures on wettability and imbibition experiments were carried out to determine the effect they have on water transport properties. Optical microscopy was used to observe the formation and evolution of air trapped between the micro texture. Furthermore, their durability and hydrophobicity when exposed to a range of conditions were investigated. Changes in morphology due to continuous hydration, atmospheric carbonation and abrasion, and wettability of deteriorated textured surfaces were investigated. The research was successful in developing cementitious materials with micro textured surfaces that displayed hydrophobicity, high water contact angle and strong adhesion between droplets and substrate. The addition of HPCC to both micro textured and non-textured samples showed decreasing wettability with increasing HPCC content. For example, taro textured cementitious samples containing 16% HPCC had a water contact angle of 143˚, compared to 123˚ for taro textured samples without HPCC. Furthermore, micro textured samples containing 16% HPCC showed around 40% reduction in water sorptivity while the textured samples with PDMS coating showed a reduction of 70% compared to non-textured samples. The decrease in water absorption is mainly due to formation of trapped air cushions when the sample is submerged underwater. However, samples containing less than 8% HPCC do not form such air cushion, and therefore, are less effective in decreasing water absorption. This indicates that the performance of micro textures is related to HPCC content. Durability studies showed that abrasion and efflorescence could affect the morphology of textured surfaces and increase their wettability. In contrast, the influence of continuous hydration and carbonation was negligible. Replacing 10% wt. of cement with silica fume can prevent efflorescence and increase the abrasion resistance of surface textures, which yielded a more durable textured surface. PDMS coating on micro textured surfaces was resistant to cyclic wetting and elevated temperature of 50˚C, indicating that the coating was stable and capable of providing long-term protection to cementitious materials. Overall, this research has shown that the combination of hydrophobic treatments (HPCC / PDMS coating) and microtextured surfaces is a promising approach to deliver innovative hydrophobic concrete for durable structures.