Influence Of Salt Admixture On The Air-void System Ofair-entrained Concrete

Nowadays, it is important for improving anti-frozen properties of concrete inNorth China, especially in Northeast region. To create the desired air void system,air is entrained in concrete by using air entraining admixture (AEA) that formamount of uniform and stable air bubbles. A mass of hardening acceleratingadmixtures and anti-freezing admixtures are added to a concrete mixture in winterconstructing, in order to enhance the early strength of the concrete and lower itsfreezing point. The influence and mechanism of salt admixture on the air-voidsystem in air-entraining concrete are studied furtherly, for boosting the durability ofconcrete structures to provide theoretical support and technical support in Northarea.Five different conventional hardening accelerating admixtures andanti-freezing admixtures were used which contained calcium chloride, calciumnitrate, sodium sulfate, sodium nitrite and glycol. Two different AEAs were used:SJ-2(nonionic Surfactant) and DH-9(anionic surfactant). This article primary wasstudied influence of above-mentioned salt admixtures and glycol on foam stabilityof AEAs and the air-void system of air-entrained concrete, combined with themorphology, distribution of air-voids and parameters of the air-void system thatcontains average radius, air content, specific surface, spacing factor etc.The results indicated that calcium chloride and calcium nitrate could reducethe foam stability of SJ-2. However, calcium chloride and calcium nitrate couldreduce the foaming power of DH-9, and had no effect on the foam stability. On theother hand, blending in sodium sulfate and sodium nitrite could increase more thefoaming power, but with the increase of added content, the foaming power andfoam stability were decreasing gradually. In addition, glycol could enhance thefoam stability of SJ-2, but had no effect on the foam stability of DH-9.Results from this research suggested that salt admixtures and glycol have greateffect on air content of fresh concrete and air-void system of hardened concrete. ForSJ-2, calcium chloride and calcium nitrate could reduce air content, but sodiumsulfate, sodium nitrite and glycol could increase the air content; with the increase ofadded content, calcium chloride, calcium nitrate, sodium sulfate and sodium nitritecould enhance spacing factor, and had negative influence on air-void system;moreover, glycol could decrease spacing factor, and improve air-void system.However, sodium sulfate, sodium nitrite and glycol reduced the compressive strength of air-entraining concrete.For DH-9, calcium chloride, calcium nitrate, sodium sulfate and sodiumnitrite, with the growth of the added content, would raise air content in freshconcrete. Expect for sodium sulfate, the other salts could reduce spacing factor, andimprove the air-void system in hardened concrete, so that it could influence theanti-frozen properties of concrete; however, when sodium sulfate was used in freshconcrete, the spacing factor was larger than air-entraining concrete that there wasno salts. In addition, sodium sulfate had an adverse effect on the air-void system;glycol with the increasing of added content could reduce spacing factor, andimprove the air-void system in hardened concrete.Additionally, the paper made a study of the effect of salts admixture and glycolon the morphology of entrained air voids in hardened cement paste. It was observedthat the walls of air voids in cement paste without using AEAs were compact. It wasa distinct shell that formed around most of the entrained air voids. This shellconnects with capillary pores. When salts admixture were used in cement paste, itcaused the morphology of the walls to change certainly, and the needlelike ettringiteexisted in the walls of air voids. After blending in glycol in cement pastes, themorphology of air voids was similar to that of air-entraining cement pastes.