Project summary

The alkali-silica reaction (ASR) is one of the main chemical deterioration mechanisms in concrete. Prevention strategies for a damaging ASR include the use of supplementary cementitious materials (SCMs, e.g. calcined clays) in concrete. 

During the first funding period, we investigated the effect of the chemical environment, in particular the effect of aluminum (Al) on the structure, composition and properties of ASR gels. The results show that Al is structurally incorporated into ASR gel, which increases the degree of crosslinking in the gel. The increased degree of crosslinking leads to a reduced swelling capacity, since the absorbed water is more strongly bound into the gel structure and thus free swelling pressure is reduced. The concrete test results show a correlation between the reactivity of Al-rich SCMs and their ASR mitigation potential in concrete. This correlation could be explained by the incorporation of Al into the ASR gels, as a similar amount of gel but differences in the expansion of the concrete with metakaolin and the less reactive metaclay could be observed. The results obtained provide new knowledge on the effect of Al-rich SCMs on ASR and enhance the understanding of the beneficial effect of SCMs with regard to mitigating a damaging ASR. 

In the second funding period, further changes in the concrete properties due to the addition of Al-rich SCMs (calcined clays) will be investigated. The focus will be set on changes in the hydration phase assemblage and porosity of the hydrated binder in concretes as well as the release of Al and alkalis (Na, K). The distribution of alkalis between the pore solution and solid phases, such as C-(A)-S-H and ASR gels will be analyzed. Furthermore, the Al incorporation and the change in composition, morphology and distribution of ASR gels in laboratory concretes (early phase vs. increasing maturation) will be investigated as a function of time. The question of how the storage temperature, composition, and morphology of the ASR gels affect the expansion in concrete will be addressed. For this, a variety of analytical methods, including µ-XRF, µ-indentation and SEM-AQM/EDX, will be used. The knowledge gained will provide a comprehensive understanding of the damaging ASR progression and serve as a basis for future ASR prevention strategies as well as for assessing the development of damage in ASR affected structures.