ALKALI LIMIT IN CEMENT WITH SUPPLEMENTARY CEMENTING MATERIALS – A REVIEW
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The alkali silica reaction (ASR) may cause deleterious cracking in concretes as a result of the reactions of reactive aggregates in concrete systems that contain elevated alkali contents. Current strategies applied in the mitigation of ASR are based on limiting the alkali content (Na2Oe) of the cement and concrete and through the screening of aggregates with additional surety provided by the use of supplementary cementitious materials (SCMs) in the partial replacement of cement. These strategies pose significant issues for the construction materials industry through increased manufacturing costs and reduction in volumes of viable raw materials that meet the imposed criteria. The effective mitigation of deleterious ASR using SCMs should change the focus of regulators and standards authorities to risk management through the assessment of the risk profile of a concrete mix in a particular application. Using a risk profile to assess alkali limits has the potential to relax alkali limits in cements. To achieve this aim a deep understanding of ASR in cement-SCM-aggregate concrete mixes is required through laboratory testing correlated with long-term field performance. This paper reviews ASR, reactivity assessment of aggregates and the role of SCMs in ASR mitigation and proposes a change in the focus to a balanced alkali limit based on assessed risk for the occurrence of deleterious ASR.
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