Water uptake of porous building materials with extremely small air entrapment effects

When a porous material is near capillary saturation, entrapped air is expected to affect water transfer through the pores. Based on the results of water absorption tests at reduced air pressure, Janssen et al. (Energy Procedia, 2015) demonstrated that air entrapment prevents water absorption above capillary saturation. In this study, to understand the air entrapment effects on water transfer in the high-water-saturation region, we further examined the water transfer characteristics corresponding to extremely small air entrapment effects. First, using three common porous building materials, water uptake experiments were conducted at low air pressure near vacuum (several kilopascals), and the time evolution of the water absorption was measured. The results show that low air pressure accelerates water uptake by brick and aerated concrete specimens, whereas the water uptake by calcium silicate board specimens is not significantly affected. These differences among the materials are discussed from the viewpoint of the pore structure. Furthermore, the results of simultaneous water and air transfer calculations confirm that air entrapment and pressure development in the pores can significantly reduce the rate of water uptake and the water content that a material can reach after capillary absorption.