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Granite rubble is one of the most frequently used aggregates in concrete manufacturing. Granite rubble is considered to be a non-reactive aggregate, however, depending on the quarry, granite may have various impurities including harmful minerals containing active SiO2. The alkalisilica reaction is among the chemical reactions that have a significant effect on the durability of concrete. During the alkalisilica reaction certain forms of silica present in concrete aggregates react with high alkali content leading to formation of hygroscopic gel that expands in humid environment and slowly, although strongly enough degrades concrete structures. Concrete rubble of 3 different fractions was used for the test: 2/8 (Mix D); 11/16 (Mix E); 5/11 (Mix G). The tests revealed that granite rubble used for the tests contained few reactive rocks containing amorphous silica because after 14 days the expansion did not exceed 0.1 % ((D Mix expanded by 0.059 %, E Mix expanded by 0.066 %, G Mix expanded by 0.079 %) according to RILEM AAR-2 test method. After 56 days of testing none of the test specimens demonstrated significant micro-cracking and scaling specific to alkali corrosion; only gel deposits on the surface were observed.

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Concrete mix made of cement CEM I 42.5 R, silica fume, fine aggregate sand of 0/4 fraction and superplasticizer was tested. Five batches of specimens were made with different silica fume content, where up to 10 % of the binding material was replaced with silica fume. Compressive strength, water absorption, density and resistance to alkali silica reaction were tested in concretes modified with different amounts of silica fume addition. The effect of silica fume addition on the expansion of modified concrete conditioned in 1M NaOH solution for 56 days was tested. Concrete modified with silica fume at 10 % by weight of cement was found to be the most appropriate for structures exposed to alkaline environment. The control specimen without mineral addition had the expansion of 0.113 %, which exceeds the limit value by 0.1 %, whereas the expansion of specimens modified with 2.5 % of silica fume was equal to the limit value, i.e. 0.1 %. The expansion values reduced in specimens modified with 5 % and 7.5 % of mineral addition. After 56 days of testing, the expansion values of these specimens were 0.093 % and 0.082 % respectively. The lowest expansion value was obtained in specimens with the highest content of silica fume, i.e. 10 %. The expansion of these specimens was 0.07 % after 56 days of conditioning in 1M NaOH solution of 80 °C temperature. The tests revealed that concrete modified with silica fume replacing 10 % of cement had better resistance to alkali silica reaction and better durability characteristics and thus can be used as structural concrete.

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