Fly ash is a fine powder obtained as a by-product when pulverized coal undergoes combustion in an electric generation power plants. The fly ash particles so obtained come out of the boiler along with the flue gases and subsequently captured using electrostatic precipitators or bag filters. Fly ash acts as a pozzolan i.e. it reacts with calcium hydroxide (released when cement reacts with water) to form additional cementitious products which increases the properties of concrete. One of the parameter that determines the pozzolanic potential of a fly ash is reactive silica. Reactive silica in fly ash is that fraction of the total silica which is soluble in both acids and alkalis. The IS method [IS 3812 (Part-1) 2013 Annexure-B] of determining reactive silica is incomplete as it proposes treatment with HCl alone. So we would like to propose a method in which reactive silica is determined gravimetrically by subtracting the silica content of the residue obtained by subsequent acid and alkali treatment from the total silica content.

Experimental and Results:

Four fly ash samples were taken and the respective reactive silica was determined gravimetrically using the following method.

  • At first total silica was determined by fusing 0.1g of oven dried sample with fusion mixture and sodium peroxide. The fused mass was extracted using dilute HCl and the silica subsequently dehydrated by fuming with perchloric acid. The silica obtained was filtered and the filter paper along with silica was ignited in a platinum dish. The silica obtained was volatilised as SiFby treating with HF and the percentage of total silica was calculated (w1) from the weight difference. Fuming with perchloric acid dehydrates and precipitates the entire silica present in the sample and hence none of the silica enters into the solution as soluble silica. This is a modification of a proposed method.1
  • 0.1g sample was taken in a 250 ml beaker and hard baked twice after treatment with dilute HCl. After dissolution of the baked mass with dilute HCl, it was filtered and the precipitate was subsequently treated with 25% KOH. The solution was refluxed for 4 hours and filtered. It was washed with 1:9 HCl and then with hot water until free of chloride ions. The filter paper along with the precipitate was then ignited in a platinum dish. The precipitate was then treated with HF when the silica was volatilised as SiFand the procedure continued till constant weight. The percentage silica (w2) was calculated from the difference in weight. The proposed method in the literature report standing of the KOH solution for 16 hours and then refluxing for 4 hours. However, it has no effect on the final result and refluxing for 4 hours only is sufficient.

% Reactive silica= w1-w2



Reactive silica is an important parameter for fly ash as per requirements of IS method [IS 3812 (Part-1) 2013]. The proposed method is based on the classical analysis 2,3 and the method is modified compared to the existing methods.1,4 The method is very simple, does not require drastic conditions and gives good repeatability and can be completed within 2 days. The method is an in-house development and in future may be adopted for the determination of reactive silica of fly ash.


  1. Akbar H., G. Krishan, S. D. Prajapati and R. Saini, Rasayan J. Chem, 9, 27-30.
  2. Method of Test for Pozzalonic Materials, Indian Specification, IS-1727-1967 (Reaffirmed 2004).
  3. Pulverized Fuel Ash, Indian Specification, IS-3812 (part1) 2013.
  4. Rapid estimation of reactive silica in fly ash, N. K. Katyal, A. K Dhawan, S. C. Sharma and M. M. Ali.

Contributed by: Dr. Satirtha Sengupta