Combined Influence of Lithium Nitrate and Metakaolin on the Reaction of Aggregate with Alkalis

The best known and effective methods for the reduction of the negative effects of an alkali–silica reaction in concrete include the application of mineral additives with an increased aluminium content and reduced share of calcium, as well as chemical admixtures in the form of lithium compounds. Because both aluminium and lithium ions increase the stability of reactive silica in the system with alkalis, it is possible to presume that the application of both corrosion inhibitors together will provide a synergistic effect in the ASR limitation. The paper presents the results of studies on the influence of combined application of metakaolin and lithium nitrate on the course of corrosion caused by the reaction of opal aggregate with alkalis. The potential synergistic effect was studied for the recommended amount of lithium nitrate, i.e., the Li/(Na + K) = 0.74 molar ratio and 5%, 10%, 15%, and 20% of cement mass replacements with metakaolin. The effectiveness of the applied solution was studied by measurements of mortars expansion in an accelerated test, by microstructure observations, and by determination of the ASR gels composition by means of SEM-EDS. The influence of metakaolin and the chemical admixture on the compressive and flexural strengths of mortars after 28 and 90 days of hardening were also analysed. The results of the studies revealed a synergistic effect for mixtures containing metakaolin at 15% and 20% cement replacement and lithium nitrate admixture in alkali–silica reaction expansion tests. It was found that corrosion processes in mortars with 5 and 10% levels of metakaolin became more severe after adding a lithium admixture to mortars with metakaolin only. The obtained results were confirmed by observations of the mortars’ microstructures. There was no synergistic impact of lithium nitrate and metakaolin on compressive strength characteristics. The compressive strength of mortars containing a combination of metakaolin and lithium nitrate decreased both after 28 and after 90 days, compared to mortars with metakaolin alone.