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Title: Combined Use of Calcium Nitrate and Fly Ash to Build ASR-Free Concrete Industrial Floors

Author(s): Mario Collepardi, Jean Jacob Ogoumah Olagot and Roberto Troli

Publication: Symposium Paper

Volume: 354


Appears on pages(s): 247-254

Keywords: alkali silica reaction; calcium nitrate; dry shake-hardener; fly ash; set accelerator

DOI: 10.14359/51736078

Date: 7/1/2022

The present paper preliminarily illustrates the mechanism of damages caused by the alkali-silica reaction (ASR) between the high alkali content of the dry shake-hardener due to the high cement content on the top of the concrete industrial floors and the alkali-reactive coarse aggregate in the concrete substrate. To mitigate or prevent these damages a special dry shake-hardener, based on the partial replacement of the Portland cement by siliceous fly ash, is used. The beneficial influence of the fly ash, as well as that of other fine pozzolanic materials, is due to the distribution of a very large number of amorphous silica-based fine particles which can potentially react with the alkali in the same way as the amorphous or badly crystallized silica of the alkali-reactive coarse aggregates. The introduction of a very high number of pozzolanic particles significantly reduces the alkali availability for the reaction with the few alkali-reactive coarse aggregates. In other words, the alkalis instead of concentrating their aggression on a few grains of the alkali-reactive coarse aggregates, usually 5 to 15 mm (2 to 6 in.) in size, spread their action on a large number of very fine pozzolanic particles so that their expansive and destructive power is lost. However, another problem can arise when the Portland cement is partially replaced by fly ash due to the longer setting time, particularly in cold weather, of the dry shake-hardener, so that the workers must wait a very long time before the mechanical troweling and the opening of the finished surface to the pedestrian traffic. To avoid this drawback a combined use of the siliceous fly ash and a setting accelerator, based on tetra-hydrate calcium nitrate in powder form [4H2O∙Ca(NO3)2 > 4H2O∙CaO∙N2O5 > H4CN2] has been studied at three different temperatures: 35°C (95°F), 20°C (68°F) and 5°C (41°F). In warm weather, at temperatures as high as 35°C (95°F), there is no need for H4CN2 since the Portland cement hydration occurs at a very great rate and only the dry shake-hardener containing fly ash without H4CN2 can be applied within few hours and incorporated into the concrete substrate. At 20°C (68°F) the delay in the setting times caused by the partial replacement of Portland cement by fly ash can be compensated by the use of H4CN2 at 1% by weight of the cementitious materials. In cold weather, such as that caused by a temperature as low as 5°C (41°F), a much higher percentage of H4CN2, up to 5% by weight of the cementitious materials, must be used to reduce the setting times at approximately the same values as those recorded at 20°C (68°F) when the dry shake-hardener without fly ash is used.