Q.1 If we add 50% fly ash with 50 % PPC cement, will that composition leads to concrete become more durable and strengthy and if yes, how many days and how much water curing should be done in that type of concrete.
PPC cement has 35% Fly Ash already added into it and further to this if you add 50% more fly ash, if the concrete does not attain the required strength it wont give the durability.
You will have total 85% Fly ash in your mix, where concrete itself may not have good bond cement and its ingredient.
Q.2. And what if we put the same ratio of fly ash in wall plaster?
You can try for this as doing this can only lead into understanding of either the material is not sticky enough to bond with wall surface or it is getting removed as powder after drying. you may loose some money but this experiment will help in understanding the properties of cement mortar.
Q.3. And if we paint the iron TMT bars before installation, will they become more resistant to corrosion or will they not hold the cement / concrete firmly ?
the layer will help in reducing the oxidation (corrosion) of reinforcement steel bars and also may not have the impact on the bond strength. but coating of the bar is preferred when the concreting of element is to be done in far future after placement of reinforcement bars (like after 6 months, year or 2 years etc.), though in such cases the coated slurry is removed from all dowel bars and further work is started, this is done for all temporary protection of reinforcement bars. If you are worried about the corrosion of the rebars after construction of the structure, you can buy the pre-coated steel bars with zinc chromate or any better method.
This I am talking for constructing a home of G+4 structure.
If you want to make saving and also a durable home, do not go for Fly Ash because it is not an active ingredient but dependent of cement content this is why replacement is not done for more than 35%.
You can go for a GGBS which takes part in active process of developing the strength, gives the superior durability and can replace cement up to 70%. but its good to have replacement with GGBS below 50% and when you blend the cement with GGBS, you need to ensure de-shuttering of the slab is done delayed and not as per the regular practice.
Whenever you do blend cement or replacement with cementitious material, always use OPC 53 grade cement only.
If you are going to construct the house in normal construction cycle, there is not need to coat the bars, but when you left the dowel for future works, consider coating those portion of the bar with Anti-rust coat by removing the rust from it using rust remover sprays.
