Abstract:The effect of hydrophobic agent on the efflorescence of Portland cement based decorative mortar (PCDM) cured for 1180d and the mechanism of the effect for PCDM cured for 1d were investigated. The results show that the early efflorescence of PCDM cured for less than 28d is significantly reduced by adding hydrophobic agent, but the efflorescence of PCDM cured for 28180d is only little affected. For PCDM cured for 1d, the addition of hydrophobic agent increases its larger capillaries with 50nm1μm. However, the addition of hydrophobic agent decreases the concentrations of K+, Na+ions in the pore solution, and prohibits their mobility, so that the total leachability of alkalis ions on the surface of PCDM is decreased, and the leachabilty of surface salts and efflorescence of PCDM is reduced.