Ismael Flores-Vivian

Self-Assembling Particle-Siloxane Coatings for Superhydrophobic Concrete

We report here, for the first time in the literature, a method to synthesize hydrophobic and superhydrophobic concrete. Concrete is normally a hydrophilic material, which significantly reduces the durability of concrete structures and pavements. To synthesize water-repellent concrete, hydrophobic emulsions were fabricated and applied on portland cement mortar tiles. The emulsion was enriched with the polymethyl-hydrogen siloxane oil hydrophobic agent as well as metakaolin (MK) or silica fume (SF) to induce the microroughness and polyvinyl alcohol (PVA) fibers to create hierarchical surfaces. Various emulsion types were investigated by using different mixing procedures, and single- and double-layer hydrophobic coatings were applied. The emulsions and coatings were characterized with optical microscope and scanning electron microscope (SEM), and their wetting properties, including the water contact angle (CA) and roll-off angle, were measured. A theoretical model for coated and non-coated concrete, which can be generalized for other types of materials, was developed to predict the effect of surface roughness and composition on the CA. An optimized distance between the aggregates was found where the CA has the highest value. The maximal CA measured was 156° for the specimen with PVA fibers treated with MK based emulsion. Since water penetration is the main factor leading to concrete deterioration, hydrophobic water-repellent concretes have much longer durability then regular concretes and can have a broad range of applications in civil and materials engineering.

The Development of Hydrophobic and Superhydrophobic Cementitious Composites

Freezing and thawing of water in saturated concrete induces stresses, cracks, spalling, and eventually allows chlorides, sulphates and other ions to penetrate through the porous space of concrete causing the corrosion and ultimately structural failure. These detrimental factors are limited if concrete has the ability to repel water. Water repellant concrete was achieved through internal and external surface applications. Hydrophobicity, overand super-hydrophobicity are demonstrated by the ability of a surface to repel water and are characterized by contact angles. In cementitious composites internal hydrophobization can be achieved through the addition of superhydrophobic admixtures to fresh concrete resulting in an air void system with small, well-dispersed air bubbles to provide superior resistance to freezing and thawing. Hydrophobic, overand super-hydrophobic surface coatings were achieved by the design of hierarchical surfaces tailoring the fiber content, mixture proportion, and superhydrophobic emulsions. Contact angle tests were used to characterize the developed coatings. The use of internal hydrophobization improves freezing and thawing resistance of fiber-reinforced composites as demonstrated by a durability factor of 100 through as many as 700 accelerated (-50°C to 20°C) cycles in 5% NaCl solution.

The Effect of Nano-SiO2 on Cement Hydration

The nanoparticles of SiO2 with a size range of 5–20 nm were synthesized in acidic and basic medium from tetraethoxysilane (TEOS) using sol–gel method. The effect of agglomerated nanoparticles on the hydration of portland cement systems was analyzed by X-Ray Diffraction technique. Experimental results demonstrate that developed nano-SiO2 produced in acidic medium accelerate the hydration by creating nuclei sites for the formation of C-S-H, whereas the nano-SiO2 produced in basic medium reacted like a pozolanic material. Most of the portlandite was consumed and transformed into C-S-H due to a pozolanic reaction producing a dense microstructure characteristic for composites with both types of nanoparticles.

Nano-engineered Superhydrophobic and Overhydrophobic Concrete

The use of superhydrophobicity and overhydrophobicity in cementitious materials can lead to improved durability performance. Ingress of water with dissolved chemicals can have detrimental effects on the performance of cementitious materials. Additionally, reducing the amount of water and ice on concrete surfaces can improve the reduction of scaling, thereby preserving the pore system and integrity of concrete. The hydrophobic, overhydrophobic, and superhydrophobic surfaces can be realized by using particle deposition and siloxane coatings. Thin layers of such water repellant material can create a strong barrier to water. Another method to improve the durability of concrete is based on the addition of superhydrophobic admixtures of polymethyl hydrosiloxane emulsions and nano particles. These admixtures can produce a 3-dimensional volume hydrophobization by releasing hydrogen in alkali environments enabling the formation of air voids throughout a cementitious matrix. The walls of these voids are then coated with varying size particles producing the surface roughness required for superhydrophobicity. This paper compares different approaches and benefits of superhydrophobic concrete and discusses the improved durability properties and the future applications.