I`m looking for an explanation that what is the reason behind the decrease in water absorption values when we replace the cement with flyash in pervious concrete.
Dear Tarunbir,
fly ash is very fine and almost completely amorphous (different grade of amorphous content depends on calcining temperature and type of fly ash that you are using) industrial waste so ,when you partially substitute Portland cement with this ,the water absorption decreases because of its pozzolanic effect. Fly ash reacts with hydrolisys lime from OPC hydration and water creating microcrystals of CSH (calcium silicate hydrates) or CASH (calcium alumino silicate hydrates) that reduce water absorption and your pervious concrete becomes less porous. But at the same time you stabilize shrinkage and increases mechanical properties and Frost defrost resistance. I think you can solve adding an air entering additive ,balanced if necessary to obtain only microporosity , with little amount of defoamer.
All best
Alex Reggiani
When water reacts with C3S resulted in C-S-H gel (nano-sized material) formation along with Ca(OH)2. When we add fly ash (which is pozzolanic in nature) to this system, Ca(OH)2 further reacts with silicates present in fly ash and forms C-S-H gel. This extra C-S-H formed in the system makes it more denser and leads to the reduced porosity. So lesser the porosity lesser will the water absorption. Fly ash also acts as a filler in cementitious materials because its particle size is lesser than that of cement particles. Hope this is sufficient for basic understanding.
Dodatek popiołu lotnego powodujesz uszczelnienie stosu okruchowego i ułatwia zagęszczanie mieszanki oraz przyspieszenie hydratacji cementu we wczesnym etapie (ziarna popiołu jako zarodki krystalizacji) oraz zwiększenie ilości wody związanej w fazie C-S-H (zmniejszenie porowatości kapilarnej betonu).
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