Effect of internal hydrophobation, silica fume and w/c on compressive strength of hardened cement pastes

Internal hydrophobation by adding hydrophobic agents during the mixing process is a method for reducing water permeability of cement based materials. It can be used as an alternative to other methods such as reducing water cement ratio (w/c) or using silica fume (SF). However, it may affect other properties of cement based materials such as compressive strength. In this paper the results of an experimental study on compressive strength of different hcps with main variables w/c, SF and hydrophobic agents are presented. Rapeseed oil and alkyl alkoxysilane were selected as hydrophobic agents. Although, a low dosage of hydrophobic agents can be more effective than lowering w/c or adding SF in reducing water permeability, an obvious reduction was observed in compressive strength by this way of internal hydrophobation compared to the other above mentioned methods. Different reasons such as lower hydration degree, chemical reactions of hydrophobic agents and non-uniform distribution of hydrophobic materials in the hcp could have resulted in lower compressive strength of hydrophobed samples. Using other types of hydrophobic agents or impregnation after the curing process can be other alternatives which would have less effect on compressive strength.