chapter  4
Effects of sodium chloride on the hydration products in the interfacial zone between cement paste and alkali-reactive aggregate
Pages 10

Abstract X-ray diffraction and quantitative thermogravimetry analysis were carried out to evaluate the amount of Ca(0H) 2, ettringite and Friedel's salt, and the degree of orientation of Ca(0H) 2 crystals in the cement paste-reactive aggregate interfacial zone. The analysis showed that the use of reactive aggregate significantly affected the amount and the degree of orientation of Ca(0H) 2 crystals in the interfacial zone. The result that the intrusion of 1N NaCl solution promoted the precipita­ tion of Ca(0H )2 crystals in the zone was also obtained. In most composite specimens, ettringite was found to precipitate in the interfacial zone. The specimens soaking 1N NaCl solution had greater amounts of ettringite in their interfacial zone than those soaking water. Friedel's salt was also formed in the interfacial zone of the composite specimens containing NaCl at the stage of mixing and those without added NaCl soaking 1N NaCl solution for 8 weeks. Keywords: Alkali-silica reaction, Calcium hydroxide, Ettringite, Friedel's salt, Sodium chloride, X-ray diffraction, Thermogravimetry

1 In t roduc t ion

Chemical and microstructural characteristics of the interfacial zone between paste and aggregate have been considered to influence the durability as well as the mechanical properties of concrete. Diamond (1989a) suggested that Ca(0H) 2 in the interfacial zone took part in the alkali-silica reaction in concrete. It is also well known that NaCl supplied from the surrounding media promotes the expansion of concretes and mortars containing reactive aggregate (Hobbs, 1988). Recently, Kawamura and Diamond (1991) found that great amounts of chloridebearing ettringite and Friedel's salt existed within mortar specimens containing a Japanese opal stored in 1N NaCl solution at 38 °C for longer than one year. However, at present, it is ambiguous whether the formation of the chloride-bearing ettringite is related to the expansion of the mortars in the NaCl solution. On the other hand, Monteiro and Mehta (1985) reported that greater amounts of ettringite were formed in the interfacial zone than in bulk cement paste. The chloride-bearing ettringite may be formed in the interfacial zone around reactive aggregate grains in mortars in NaCl solution, although no sulfate ions are supplied.