Elucidation of Mechnisms of Deterioration of Concrete Caused by Chemical Attack of De-icing Salts and its Preventive Measures

对于除冰盐化学侵蚀引起的混凝土劣化，发现高浓度的CaCl_2对硅酸盐水泥混凝土具有很强的侵蚀性，且在低温下的侵蚀性比在高温下的侵蚀性更强，而在NaCl溶液中，不存在与温度和溶液浓度无关的劣化现象。CaCl_2溶液的进入引起的变质可能是由于氢氧化钙的溶解和络合盐的形成。在掺加粉煤灰、高炉矿渣、硅灰等矿物掺合料的砂浆中，氢氧化钙含量的降低和氯离子渗透率的降低对矿物掺合料水化过程的共同作用，有效地提高了其抗氯化钙侵蚀性能。在高浓度CaCl_2溶液中，掺入空气可提高混凝土抗氯化钙侵蚀性能，但降低水灰比对后期混凝土抗氯化钙侵蚀性能影响不大。有趣的是，在CaCl_2溶液中反复干湿会导致进一步的劣化并伴随开裂。对于碱-硅反应引起的混凝土劣化问题，在碱含量较低的含活性骨料砂浆中，孔隙溶液中OH^-离子浓度的升高促进了其在1N NaCl溶液中的膨胀。孔隙溶液中OH^-离子浓度的增加是NaCl侵入活性骨料砂浆的主要原因。

With regard to the deterioration of concrete due to the chemical attack caused by de-icing salts, it was found that a high-concentration CaCl_2 was very aggressive towards Portland cement concrete, and its aggressivity became more severe at a low temperature than at a high temperature, while no sign of deterioration was observed in NaCl solution idependent of both the temperature and the concentration of solution. The deterioration caused by the ingress of CaCl_2 solution seemed to be attributed to the dissolution of calcium hydroxide and the formation of complex salts. In the mortars incorp orating mineral admixtures such as fly ash, blast-furnace slag and silica fume, the combined effect of a decrease in the content of calcium hydroxide and a reduced chloride ion permeability on the process of hydration of mineral admixtures effectively improved the resistance to calcium chloride attack. Furthermore, it was confirmed that the entrained air in concrete improved the resistance to the calcium chloride attack, but the reduced water : cement ratio had little influence at later stages of exposure time in a high-concentration CaCl_2 solution. Interestingly, a further deterioration accompanied by cracking was found to be caused by the repetitive drying and wetting in CaCl_2 solution.With the regard to the deterioration of concrete due to alkali-silica reaction, in a reactive aggregate-bearing mortars with relatively low alkali contents, the promotion of their expansions in 1N NaCl solution was caused by a rise of OH^- ion concentration in pore solution. The increase in OH^- ion concentration in the pore solution was responsible for the intrusion of NaCl into the mortars containing a reactive aggregate.