Avoidance of Alkali Reactions by Fly Ash - Examinations of concretes during long-term storage
Project Number 374
In the previous projects (VGB Projects 344/300/245) it was shown that the alkali silica reaction is affected by the different chemical properties of the pore solution where the dominating factor is the alkali concentration.
The use of fly ash results in the dilution of alkali concentration thus reducing the susceptibility of concrete to ASR damage. The pozzolanic reaction of fly ash consumes OH- and forms aluminium containing C-AS-H phases which are able to bind alkalis; thus reducing the susceptibility of concrete to ASR.
Fly ash increases the aluminium concentration of the pore solution significantly. Instead of swelling silicate gel, non-swelling alumino silicates may be formed which do not damage concrete. Thus even at relatively high total alkali contents, concretes made with sufficient amount of fly ash exhibit no ASR damage. Up to now, concrete prisms and cubes made with 25 to 35% fly ash have shown no signs of ASR damage.
At present, it is not possible to estimate the applicability of these laboratory results to equivalent concrete specimens currently being exposed to natural climatic conditions. Nevertheless, when considering that the beginning of damage is 14 months for concretes made with fly ash as opposed to 6 months for reference concretes, it is clear that fly ash plays an active part in avoiding ASR damage.
Till now, after a storage time of 7 to 8 years in natural climatic conditions, no damage was detected for the concrete mixtures containing 30 % fly ash or more. Some of the other concretes with less or no fly ash show small cracks after the same storage time. It is likely that the same positive effect of concretes containing 30 % fly ash, which was observed under laboratory conditions, will also be seen in outdoor conditions. To proof this effect it is necessary to continue the investigations and observe the concrete cubes in the future.
With these results a modification of the German alkali-guideline with the aim of an enlarged application area for fly ash concrete could be possible.
The research project is supervised by the VGB-Technical Committee Power Plant By-products