On the 2nd street project we are doing a lot of underpinning and laying a concrete slab in the cellar. This requires a lot of concrete. Cement uses up a lot of energy to produce and creates a lot of CO2 Substituting up to 50% of the cement with fly ash is considered a more ecological idea.
Fly ash is an industrial byproduct from burning coal so to use it is to recycle and keep it from the landfill. You are substituting something that is costly to create (cement) with something that is there in abundance anyway. Very green.
Fly ash does have a lot of chemicals in it. But when mixed with cement they are held in the concrete and do not leach. In fact concrete with fly ash is much more waterproof than normal concrete mixed with only cement. i.e. the concrete does not mix with water well.
Concrete mixed with fly ash takes longer to cure. Because of this it is important to use a lot less water. But once cured it is considerably stronger than normal concrete.
The only problem is that I have not been able to buy fly ash in the NY area. Cement companies buy it in bulk and won’t sell a mere 1000 pounds of it. And because the underpinning has to be done in little parcels at a time it isn’t possible to bring out a truck to pour concrete. We have to mix the concrete by hand in little batches.
So alas the concrete slab and underpinning of 22 2nd street will not have fly ash in the concrete….