Optimization of concrete mixture with hybrid blends of metakaolin and fly ash using response surface method

Abstract

This paper presents the experimental results of a research carried out on the strength and permeability related properties of high performance concretes made with binary and ternary cementitious blends of fly ash (FA) and metakaolin (MK). The replacement ratios for FA were 10% and 20% by weight of Portland cement and those for MK were 5% and 10%. Compressive strength, chloride permeability, water sorptivity, and water absorption properties of concretes were obtained in this study for different testing ages up to 90days. The influences of fly ash, metakaolin, and testing age on the properties of concretes have been identified using the analysis of variance. The statistical based regression models and the response surface method with the backward stepwise techniques were employed in the multi-objective optimization analysis. That is carried out by maximizing compressive strength while minimizing chloride permeability, water sorptivity, and water absorption. It was observed that fly ash and especially metakaolin were very effective on the aforementioned properties of the concretes, depending mainly on replacement levels and duration of curing. The results indicated that the ternary use of fly ash and metakaolin with the approximate cement replacement values of 13.3% and 10% respectively has provided the best results for the testing age of 90days, when the optimized strength and permeability based durability properties of the concretes are concerned.