The Effect of using Fly Ash as Supplementary Cementitious Material on Conventional Concrete

Abstract

Fly Ash Class C, which is rich in calcium and has strong pozzolanic properties, is commonly used as an additive in concrete to improve its workability and early strength. Typically, fly ash can replace up to 35% of the cement in concrete, depending on the desired performance. This makes it an attractive option, reducing the amount of cement needed while also preventing fly ash waste. In this study, we explored the use of Fly Ash Class C in concrete mixtures, substituting cement at rates of 0%, 20%, 25%, 30%, and 35%, based on the guidelines from SNI-03-2834-2000. The fresh concrete was tested to measure the slump value. The samples were tested at the Construction Material Laboratory of Universitas Islam Indonesia (UII) to measure compressive strength, tensile strength, and elasticity. The concrete was formed into 150 x 300 mm cylindrical specimens for these tests. The results showed that the optimal substitution rate was 20%, which led to the best balance of workability, compressive strength, tensile strength, and elasticity. While adding 20% fly ash enhanced these properties, increasing the fly ash content beyond this point actually weakened the concrete in terms of strength and elasticity.