Effect of water-repellent admixtures on the behaviour of aerial lime-based mortars

Two different anionic surfactants, sodium oleate and calcium stearate, commercialized as water repellents for cement-based mortars, were added to lime-based mortars in order to check whether they were improved by these admixtures. Different properties of lime-based mortars were evaluated: fresh state behaviour through water retention, air content and setting time, hardened state propel-ties such as density, water absorption through capillarity, water vapour permeability, long-term compressive strengths, pore structure through mercury intrusion porosimetry, and durability assessed by means of freezing-thawing cycles. A clear improvement in lime-based mortars was achieved when sodium oleate was added: strong capillarity reduction and excellent durability in the face of freezing-thawing processes, without any compressive strength drop. The mechanism for this improvement was related to air void formation due to the air-entraining ability of these surfactants. Insolubility of calcium stearate turned out to be responsible for fewer air bubbles - as SEM examination revealed - and showed lower effectiveness. (C) 2009 Elsevier Ltd. All rights reserved.