Influence of Storage Conditions and Concrete Composition on the Effectiveness of Different Silica Fumes Against ASR

Abstract

Cement paste samples incorporating various amounts of silica fume were made and stored in humid air at 23 degrees C and 38 degrees C. Their pore solution was expressed under high pressure at different times, then analyzed for its alkali content. The storage temperature did not significantly affect the alkal concentration in the short term (0-28 days), where most reaction products (hydrates) are formed. However, all control and blended pastes stored at 38 degrees C recycled very significant amounts of alkali in the pore solution between 28 days and 1.5 years, while not those stored at 23 degrees C. Consequently, the long-term effectiveness of silica fume against alkali-silica reaction (ASR) should be better than expected from a number of recent experimental studies all involving concrete expansion tests conducted at 38 degrees C, for field concretes exposed at an average lower temperature, e.g. in many regions of the world. This would explain at the same time why satisfactory field performance is reported in Iceland, for instance, while the only one case reported until now relating to poor field performance of silica fume against ASR took place under the hot South African climate. Results from concrete prism expansion tests also indicated that ASR expansion can develop in high performance concrete, even with moderately reactive aggregates. Other expansion results confirmed that the higher the degree of reactivity of the particular reactive aggregate to counteract, the alkali content in the silica fume, the alkali content in the portland cement used, and the cement dosage, in other words the higher the total concrete alkali content, the higher is the amount of silica fume required to counteract ASR expansion. The study also indicated that pelletizing the silica fume before mixing with the clinker at the grinding stage did not reduce its effectiveness against ASR, provided the grinding process is effective in dispersing the silica fume.