Effect of burnt oil shale on ASR expansions: A petrographic study of concretes based on reactive aggregates

•Effect of burnt oil shale (BOS) on concretes based on reactive aggregate is studied.•The expansion decreases from 17wt% BOS with a flint aggregate.•Expansions decrease with the BOS addition rate for siliceous limestone aggregates.•Without BOS, ASR gel, microcracks and altered silica with alkalis are observed.•The addition of BOS reduces the aggregate degradation and modifies gel products. Some aggregates are potentially reactive and can be used because of a pessimum effect that allows reducing expansion. However some potentially reactive aggregates do not have this property and therefore cannot be employed. Thus, to valorise these potentially reactive aggregates, Supplementary Cementitious Materials (SCMs) can be added such as burnt oil shale which has been demonstrated to improve mechanical properties and inhibit ASR. This study proposed to employ burnt oil shale (BOS) from Germany and reports data on concrete based on a potentially reactive flint with a pessimum effect and two potentially reactive siliceous limestones. The action of burnt oil shale was observed in concrete expansions and aggregate degradation by SEM. At a microstructural scale, without burnt oil shale, aggregates are altered by ASR, i.e. the microcrystalline quartz for flint aggregate and finely dispersed micro-quartz for siliceous limestone aggregates present alkalis thanks to EDS analyses, which result in concrete expansion. The addition of burnt oil shale allows the reduction of aggregate degradation and modifies gel products with a lower C/S and a mix of CSH and ettringite for the flint concrete. Therefore, concrete expansion decreases approximately 90% from 17wt% BOS with flint aggregate because of exploitation of the pessimum effect and with 30wt% BOS for siliceous limestones. The burnt oil shale addition generates a competition between ASR and the pozzolanic reaction. The presence of micro-quartz in burnt oil shale would play a significant role in the pozzolanic reaction by fixing alkalis and thus inhibiting ASR.