Effect Of The Ratio Of Al₂O₃ To SiO₂ In Mineral Admixtures On Alkali-Silica Reaction

Alkali-silica reaction （ASR） is one of the important factors affecting durability of concrete constructions. Inhibiting ASR with mineral admixtures had been studied by many experts almost from the view of the functions of SiO₂ and Al₂O₃. In this paper, the effects and mechanisms of Al₂O₃ to SiO₂ ratio （A/S） on ASR were presented from the view of A/S of admixtures.The effects of calcination condition on reactivity of calcined bauxite, reactive bauxite prepared by calcination, were analyzed by means of XRD and complexometry. The results revealed that the maximum reactivity of calcined bauxite occurred at 500oC×20min in calcination condition of rapid calcining and quick cooling.The Accelerated Mortar Bar Test was conducted to study long-term effectivities of five species of aluminous substances including Al₂O₃, Al（OH）3, flyash, bauxite and calcined bauxite, two species of siliceous materials such as silica fume and micro powder of quartz glass, and various composite admixtures in different A/S on ASR inhibition. The morphology and composition of the hydrated products at interface between cement paste and glass were analyzed by means of SEM and EDS. The results showed that ASR could be inhibited effectively by aluminous substances not only in short term within 14 days but also in long term; while ASR could be inhibited effectively by siliceous materials in short term, but could not in long term. The synergistic effect of Al₂O₃ and SiO₂ was showed for inhibiting ASR, and there was a proper A/S in the composite admixtures in which ASR was inhibited most effectively.C-S-H gel was prepared with the mixture of C₃S and C₂S extracted from Portland cement clinker and respectively with silica fume, Al₂O₃ and their mixture to analyze the effects of aluminous substances, siliceous materials and the A/S on compositions and structures of the gel by means of XRD and IR. The results indicated that aluminous substances could provide substituting Al³⁺ for Si⁴⁺ and participate in formation of zeolite minerals, while siliceous materials could lengthen silicate chains and increase silanol groups in C-S-H structures, and that A/S affected the degree of substituting Al³⁺ for Si⁴⁺.C-S-A-H gel was synthesized with chemical method, and the effect of A/S on alkali content in the gel was investigated. Through measuring theζpotential of the gel and the coordination number of Al³⁺ in the structure of the gel, and studying the composition and morphology of the gel, the functional mechanisms of A/S on alkali content in the gel and on ASR were analyzed. The results showed that the alkali content in the gel increased and came up to a maximum, then decreased with the A/S increasing. The effecting mechanisms can be concluded that Si⁴⁺ in the structure of gel can be substituted by Al³⁺, which makes the gel possess negative charges, and the alkali ions with positive charge are bound in the structure of gel. When the amount of Al³⁺ is small, Al³⁺ is in the form of [AlO₄] tetrahedron, and occupys the site of bridging [SiO4] tetrahedron of silicate chains. With A/S increasing, more Si⁴⁺ are substituted by Al³⁺, and more negative charges appear in the structure of gel, so the alkali content in the gel increases. However, the amount of Si⁴⁺ which can be substituted by Al³⁺ is limited, so the alkali content in the gel come up to a maximum. With Al³⁺ continuing to increase, redundant Al³⁺ are in the form of [AlO₆] octahedron, and exist in the interlayer of C-S-H structure, which reduces the negative charges of gel, so the amount of alkalis bound in the gel also reduces. Therefore, the content of harmful alkali in concrete will be changed following the variation of alkali content in the gel, and the effectiveness of admixtures on ASR will be affected.