Fly-ash (pfa)
209
there is so much reactive silica that all alkali is used up during an initial reaction, leaving none to cause problems later, however reactive the coarse aggregate. (c)
Surface chemistry effects
It appears that fly-ash can act as a catalyst or a starting point for crystal growth in the cement paste. Such effects are beyond the scope of this book but it should be realised that there is more to the story than has been told earlier. This may provide some explanation for a smaller early age strength reduction than chemical effects alone would predict when equal mass substitutions are made. Dr Malcolm Dunstan (in the UK) and Mohan Malhotra in Canada (Malhotra and Ramezanianpour, 1994) have done interesting work on roller compacted and other concrete with 50–60% of fly-ash substitution. A very revealing point is that good results are obtained with high fly-ash in either earth dry concrete (roller compacted) or concrete with a normal slump attained through using a superplasticiser. However poor results are obtained with high fly-ash at normal water contents. It could be said that the w/c v strength relationship is even more marked in the case of fly-ash than in the case of cement. Dangers to avoid with fly-ash 1
2
3
Since fly-ash is lighter (and cheaper) than cement it might be thought that it would be especially useful in low strength concrete. In fact it does produce much better looking, more segregation resistance and less bleeding prone concrete for a given (relatively high) water to cementitious ratio. However this is sometimes its undoing. Uninformed or thoughtless people tend to over water it to a greater extent than plain concrete, yet in fact its strength is more affected by a given amount of excess water. Thus fly-ash should be used with care and conservatism for low strength requirements. Properly used it is valuable for such uses but is less resistant to over-watering abuse. Because strengths take longer to develop, more efficient and prolonged curing is necessary for fly-ash concrete. It is true that fly-ash concrete is substantially less permeable than plain concrete of similar strength, and therefore may be to some extent ‘self-curing’ in larger masses (and especially for below ground or on ground foundations). However, this does not help the outside 20 mm of exposed concrete, which has to protect reinforcement. Fly-ash concrete reacts extremely well to steam curing. The same calcium hydroxide that has the disadvantages of being soft, weak and easily dissolved by water or chemicals, is the source of the alkalinity which protects steel from corrosion. Therefore, by combining with it, fly-ash reduces the chemical protection available for the reinforcing steel. The question is whether or not this is compensated for by the reduced impermeability of the fly-ash concrete. The answer lies in the curing, yes if well cured, no if not well cured.