What is the maximum content of poly carboxylic super plasticizer to use when increasing the flew ability of HVFA concrete?
The thing is, adding more superplasticizer will have you decrease the water amount if you work at constant workability. And this decrease in water will help increase at least 28 day-strength.
In your case with high volume fly ash concrete early (1-3 days) strength is usually very low. I would say do not go beyond 1% by weight of total binder.
In order to be more precise, at which age of concrete do you consider strength ?
I meant early age strength and how I can overcome the early age strength reduction?
[1]High Volume Fly Ash(25-50% cement replacement) is potentially a ‘Green’ solution because of reduced cost of water demand as the increased flowability reduces the amount of admixtures required to achieve the SAME RESULT with the straight mix.
[II]With 0.15–0.3% by cement weight, the super plasticizers allow a water reduction up to 40%. By applying a simple analogy, a HVFA sample containing 25% cement replacement should ideally require 0.10- 0.2% of the super plasticizer by cement weight present in HVFA.
[III] Setting time of a geopolymer depends upon the composition of the alkaline solution and the ratio of the alkaline liquid to the fly ash by mass. Of course, most important being the curing temperature whose rise decreases the setting time[Chanh,NV;Trung, BD and Tuan,DV(2008)] because the polymerization process becomes more rapid and the concrete can gain 70% of its strength with in 3-4 hrs of curing[Kong KLY and Sanjayan,JG (2008)].
To improve the wokability, superplasplastisizer or extra water can be added but the former adversely affects the strength of geopolymer. So extra water should be preferred.
Now coming to the compressive strength- for all types of concretes, it depends upon the curing time and curing temperature and is expected to increase with both the factors.
When the curing time is in range of 60-90C, with in first 1-3 days, the compressive strength of the concrete rises to 400-500 kg/cm^2. The highest compressive strength was obtained by using an activator like sodium silicate solution. The compressive strength of geopolymer is highly dependent on fly ash particles with best results obtained when its size is in the range of 43 μM .
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