Adding Slag and Limestone Powder to Concrete to Increase the Durability of Concrete in Sulfate Environment

Durability of concrete in the environments containing sulfate ion has been accompanied with special concern for many reinforced concrete structures such as bridge decks and piers. Such structures as well as many other reinforced concrete structures have been suffering from corrosion due to existence of sulfate ions in soil or water adjacent to structure. Conventional methods like use of sulfate resistance cement in concrete has been utilized for long time; however, due to its limited effect and its unsuitability against some other harsh environments, alternative solutions like substitute of pozzolanic materials with a part of cement has been attracting for many researchers and practical engineers.In the current study, to introduce a concrete with relatively high durability against sulfate ions, concretes containing slag and limestone powder were examined in sulfate environment. Since the concentration of sulfate ion in site is low and the corrosion of concrete adjacent to soil or water containing sulfate ion occurs during a long time, for evaluation of the effect of sulfate ion on concrete within a shorter period of time in the laboratory, it is necessary to perform accelerated experiments. The accelerator tests can be done by increasing the concentration of the sulfate ion and/or performing wetting and drying cycles on the concrete specimens. Accordingly, the solutions of 5% magnesium sulfate, 5% sodium sulfate and pure water were used as laboratory environments to perform the tests. To design the concrete mixes, ACI 363R was used to determine the ingredients; however according to ACI 211.4R-93, 30% reduction in water amount was utilized based on using super plasticizer in the mix. Liquid super plasticizer with Melamine base was used and the slump was stabilized about 80-100 mm. Besides the aforementioned codes, some modifications were made in the amount of solid ingredients based on the recommendations Mostofinejad and Nozhati. In order to evaluate the concrete deterioration process in sulfate ion, the reduction in compressive strength plus the weight changes in all specimens were measured; hence, the effect of the type of sulfate ion in different water-to-cement ratios and different ratios of limestone powder and slag on the decrease of weight and compressive strength of concrete was investigated. To do so, 27 mix designs including 15% and 30% substitute limestone powder and 10% and 20% substitute slag with the water-to-cement ratios of 0.3, 0.4 and 0.5 were made and cast in 243 cubes. The test results on the specimens after 70 and 140 days showed that a combination of 10% slag and 15% limestone powder substitute to cement not only provides an economical mix design, but also fairly increases the durability of concrete. Such a combination of cementitious materials could be recommended for concrete in sulfate environments; e.g. for construction of bridge piers.