Effect Of Inorganic Salts On Pore Structure And Frost Resistance Of Concrete

As high-early strength agent, anti-freeze agent or corrosion inhibitor, inorganic salts are widely used in concrete structure. Especially in some regions with cold climate, inorganic salts added into concrete can increase the early strength of concrete or make concrete ice-free before hardeding, which accelerates construction progress. Deterioration due to freeze-thaw action is also important durability problem of concrete in those places. In this work, pore structure of cement paste, air voids strcture, saturation degree, frost resistance and deicer-scaling resistance of concrete with an inorganic salt, such as CaCl2,NaCl,Na2SO4,NaNO2,Ca(NO2)2 and Ca(NO3)2, were investigated to find out effect and mechanism of different inorganic salt on frost resistance of conceret, which can provide theoretical basis for the proper usage of inorganic salt in the concrete structure.The main research work and compliments of this paper are:The influence of inorganic salts on pore structure, hydration products and their morphology, and hydration degree of cement paste with water-cement ratio of 0.5, 0.4 and 0.3 after curing 3d and 28d were investigated using Mercuty Intrusion Porosimetry , SEM and XRD. The results showed that the hydration action of cement containing 1% inorganic salt was unaltered essentially. However, the hydration rate and degree were influenced by inorganic salt obviouly, which affected forming and developing of pore structure in cement paste with inorganic salt. At the dosage of 1%, the order of refinement action of different inorganic salts on cement paste pore structure at early age were: chlorides>Na2SO4> nitrites>Ca(NO3)2. For the cement pastes cured for 28d, the order were: Ca(NO3)2>chlorides>nitrites>Na2SO4. Chlorides can refine pore structure of cement paste at early age as well as after a long curing time, while the refinement action of Na2SO4 was reduced with increasing of curing time. The refinement action of Ca(NO2)2 and NaNO2 were more obvious at 28d. Different from the other salt, Ca(NO3)2 had not improvement on the hydration degree of cement paste and made the pores more coarse at early age, but it had refinement effect on the pore structure of cement paste cured longer time. The air voids strcture of concrete with inorganic salt were investigated by Linear-Traverse Method using microscope. The results showed that inorganic salt was disadvantage for the air viod structure of concete. Fluidity and viscidity of fresh cement paste and hydration products formation rate of cement were influenced by inorganic salt added, which resulted in change of air viod structure of non-air entraining concrete. In the six salts, NaNO2 decreased air content and average air void size and air void spacing appreciably; Na2SO4 increased air content, air voids with size more than 1000μm, average air void size and air void spacing of non-air entraining concrete; Ca(NO3)2 increased average air void size and air void spacing obviously; clorides and Ca(NO3)2 had no remarkable influence. Inorganic salt was harmful for the air viod structure of air-entraining concete since surface tension of air-entraining agent solution was raised and stability of air viod was reduced by salt. Ca(NO2)2 and NaNO2 increased surface tention of air-entraining agent solution remarkable, and they increased average air void size and air void spacing of air-entraining concrete with the same dosage air-entraining agent.Effect of inorganic salt on surface tension of salt solution, saturation calcium hydroxide solution, and rise height of liquid in capillary were investigated. Except for the influence on paste pore structure, the medium properties which permeating through paste pores was also affected by inorganic salt: the surface tension, density and viscidity of salt solution and saturation calcium hydroxide solution with inorganic salt were increased. Therefore, the different pore structure of cement paste with inorganic salt and medium properties led to lower permeability and higher incremental saturation degree for the surface layer of concrete with salt.Frsot resistance and deicer-sacling resistance of concrete containing inorganic salt were investigated particularly. The results showed that inorganic salt was harmful for the frsot resistance, especially for the deicer-sacling resistance of concrete, which was general results of higher saturation degree for the surface layer and change of permeability, air voids strcture and compressive strength of concrete with salt. Frsot resistance of concrete reduced with increasing of calcium salt dosage, so the dosage of CaCl2,Ca(NO2)2 or Ca(NO3)2 should be no more than 2%. Na2SO4 redused frost resistance of concrete obviously. Frost resistance decreased appreciably when NaNO2 dosage less than 1%, and freeze-thaw cycles can be more than that of concrete without salt with increasing of NaNO2 dosage. NaCl added into concrete also can cause severe scaling although there was no deicer around concrete. In the freeze-thaw process, the saturation degree increasing rate of concrete with inorganic salt was greater than that of concrete without salt, and the saturation degree increasing rate of concrete freeze-thaw in 3.5% NaCl solution was greater than that in water. This was the reason for the low frost resistance and deicer-sacling resistance of concrete containing inorganic salt. Entraining air voids into concrete was effective for improving frsot resistance and deicer-sacling resistance of concrete containing inorganic salt, but air entraining agent used must be consistent with inorganic salt in order to reach enough air content and air voids strcture demanded.