Durability of air entrained cement mortars under combined sulphate and freeze-thaw attack

摘要

The paper deals with experimental results of combined sulphate expansion and freeze-thaw attack on air entrained (AE) and non-air entrained (nAE) ordinary portland cement mortars. After prior freezing and thawing of the 4x4x16 cm mortar bars the part of them was subjected to sulphate attack of the 5% Na2SO4 solution. Another part of mortar bars was immersed in the sulphate solution for 110 days and then these bars were frozen and thawed. The resistance of nAE and AE mortars to regular sulphate and freeze-thaw attacks alone was tested as well. In order to compare damage degree of mortars in all environments, there were investigated long term linear strains (expansion), and compressive strength of mortars. Also the SEM and XRD analyses of mortars were carried out. Compared to the individual sulphate attack alone, prior freeze-thaw and following it sulphate attack affected significant acceleration of sulphate expansion of nAE and AE mortars as well, despite much better resistance of AE mortars to freezing and thawing. However, the prior sulphate attack significantly reduced AE mortars resistance to freezing and thawing. The SEM analysis showed that air voids in AE mortars were partially filled with ettringite. Its precipitation in air voids of AE mortars caused that the volume of air was diminished and freeze-thaw resistance of AE mortars was significantly decreased and it was as low as resistance of nAE mortars was. The complete destruction of AE mortars subjected to sulphate attack alone occurred earlier than that of nAE mortars did. Based on SEM analysis results it is suggested that lower sulphate resistance of AE mortars was due to higher porosity of the air void-paste interface and its higher initial w/c ratio compared to 'bulk paste' farther away from.