Moisture phenomena in hydrophobic porous building material Part 1: Measurements and physical interpretations

This paper adresses the hygric behaviour of ceramic and calcium silicate brick treated with a water repellent oligomeric alkyl-alkoxysilane. Although the hydrophobic treatment succesfully blocks capillary effects, diffusion-adsorption processes of water molecules into the hydrophobic treated material still can take place. This process determines the particular hygric behaviour of hydrophobic treated materials. First, the contact angle of a drop deposited on an hydrophobic surface changes continuously over time due to absorption of water from the droplet into the hydrophobic material. This absorption process is found to be drop-size dependent. As a result large droplets, that remain on the hydrophobic surface for a longer time, are preferentially used to accurately measure the contact angle. Second, the water vapour permeability can decrease substantially due to clogging of connective pores caused by the hydrophobic treatment. Porous building materials showing an important midsize pore system within the length of the polymer chains are very sensitive to clogging of connective pores and therefore susceptible to an important increase in water vapour transmission resistance. Due to micro-droplet and water island formation on the hydrophobic network, the water vapour permeability increases with relative humidity. Finally, liquid water break-through in hydrophobic treated cracks is explained in view of microdroplet and water forming process in the crack. When liquid water islands become connected in the crack, capillary forces arise leading to the break-through