Study of alkali-silica reaction gel in concrete by high-resolution nuclear magnetic resonance / Estudo da reação álcali-sílica em concretos através de ressonância magnética nuclear de alta resolução

AUTOR(ES)
DATA DE PUBLICAÇÃO

2009

RESUMO

The alkali-silica reaction (ASR) occurs between structurally distorted or disordered silica and aqueous solutions of alkaline hydroxides, as KOH or NaOH. The product of the ASR is an alkali-silicate gel, which may expand upon water absorption. When the ASR occurs in mineral aggregate used in concrete, the process of reaction and expansion cause the decrease in the mechanical resistance of the structure. In this work, a structural study was carried out to analyze the effect on the gel of two processes proposed respectively to mitigate the expansion and prevent the ASR: the treatment with LiNO3 and the application of methyltrimethoxysilane (MTSM, Si(OCH3)3CH3). Also, the reactivity of three minerals (basalt, granite and quartzite) commonly used as aggregates was analyzed, simulating the ASR in laboratory conditions. Nuclear Magnetic Resonance (NMR) techniques of 29Si, 7Li, 23Na, 13C e 1H in the solid state were applied to analyze the structure of the gel, the minerals and the products resulting from treatments and the attack with alkaline solutions (KOH). The results show that the silicate network in the gel is a lamellar structure, composed mainly by Q3 silicates, which is transformed into a linear structure upon the treatment with LiNO3 solutions. This modification, eliminating the inter-lamellar spaces capable to host water and cause the collective expansion of the structure, explain the reduction in the expansion observed in mortar or concrete structures treated with Li. On the other hand, after exposing gel samples to MTMS aqueous solutions, no interaction of the silicate groups in the gel with SiCH3 groups were detected, indicating auto-condensation of silane in these samples. With respect to the attack of the reactive minerals, some changes were detected in the distribution of Qn silicate species in the three rocks. In attacked basalt and granite there is an increase in the fraction of Q1 and Q4with respect to the original minerals. In attacked quartzite the differences are mores strong and depend on the reaction parameters: temperature and time of attack. The increment in temperature dissolve the amorphous silicates of low and medium connectivity (Q0, Q1 and Q2) resulting in a highly connected silicate with Q3 and Q4. Conversely, no appreciable transformation in the amorphous silicates were observed during longer attack times at room temperature, but the dissolution of bigger silica particles with well defined Q4 species was observed.

ASSUNTO(S)

29si gel gel 29si alkali-silica reaction estrutura ressonância magnética nuclear reação álcali-sílica nuclear magnetic resonance quartzito

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