Experimental study on mortar with the addition of hydrophobic silicone oil for water absorption, strength, and shrinkage

•Mortar was bulk-treated with silicone oil by directly mixing the oil into mixture or spraying it to fine aggregates.•Water absorption of pre-cracked mortar indicated that the treatments were both effective to achieve bulk water repellency.•Drying shrinkage and autogenous shrinkage after both the treatments were found to be smaller than that of untreated mortar. In this study, a hydrophobic agent of silicone oil was used to treat mortar by directly mixing into the mixture and spraying it on the sand for pre-treatment, respectively, aiming to offer the mortar bulk water repellency that can be sustained even on the occurrence of cracking. Contact angle was measured for surface wettability, and water absorption and chloride penetration were tested on sound specimens that were pre-dried under two different conditions. Water absorption was further tested on the pre-cracked specimens to characterize water repellency with cracks. The mortar was also tested for compressive strength, Young’s modulus, drying and autogenous shrinkage as well as setting times. The results indicated significant reduction of water absorption and chloride penetration with increasing addition and spraying amount of the agent than those of untreated mortar. Furthermore, for the pre-cracked specimens, water absorption was remarkably lower than that of untreated mortar, particularly under wetting/drying cycles, indicating that bulk water repellency can be preserved at a certain degree even if cracks occur. On the other side, the silicone oil reduced compressive strength and Young’s modulus for both treatments. Autogenous shrinkage and drying shrinkage tended to decrease compared to untreated mortar. For direct mixing, the reduced drying shrinkage was partially attributed to its less water loss than that of untreated mortar, whereas for sand spraying, the weakened ITZ of the mortar was inferred to play a major role in reducing drying and autogenous shrinkage.