Test Methods For Evaluating The Oxidization Potential Of Sulphide –Bearing Aggregates And Its Effects On Concrete Durability

This research project focuses on the development and validation of test methods to evaluate the potential oxidation of sulphide-bearing aggregates, which can cause severe damage when used in concrete. The mechanism of damage is believed to consist of two parts: (a) the oxidation of sulphide minerals, which results in the formation of ferric hydroxide, and (b) the formation of sulphuric acid, which reacts with calcium hydroxide in concrete leading to an internal sulphate attack. Both parts produce a volume increase, damaging the concrete. A simple, quick and economical test method was developed and used to test thirty-one aggregates with different sulphur content. This test involves soaking the aggregate in an oxidizing agent at room temperature, washing the aggregate on a specific sieve, and drying it at 80°C. The soaking and drying cycle is repeated and the disintegration of the aggregates is measured as % mass loss. The composition of the oxidizing solution was evaluated, and the assessment of the aggregate was related to the presence of iron and sulphur ions in the solution after the test. The aggregate oxidation test developed here is anticipated to be adopted as a screening test method by North American standards due to its simplicity and applicability to a wide range of aggregates. The expansion of recently developed mortar bar samples containing a limited number of aggregates proves that the test can show expansion in aggregates with sulphide as well as high silica content; however, the high-silica aggregate did not show significant expansion in the second stage of the test, unlike the sulphide-bearing aggregates. The test was examined for its ability to evaluate the effects of supplementary cementing materials (SCM`s) on mitigating the damage in mortars containing sulphide-bearing aggregates. The results revealed that extended exposure to the oxidizing agent caused damage in the bar due to reasons other than the oxidation of sulphide phases when SCM with high reactive alumina is used. In addition, the results revealed that silica fume and low-calcium fly ash were effective in mitigating the damage, however, the efficacy of SCM`s is mainly linked to their ability to reduce the penetration of oxidizing agents into the mortar bars. These results need to be validated using field investigations. Concrete samples were tested under different conditions in an attempt to replicate the damage mechanisms in concrete samples under lab conditions. Some of the testing regimes showed promising results and are recommended for future studies.

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