Effect of fly ash and grading agent on the properties of mortar using response surface methodology

•Milling agent and clinker play a significant role in compressive strength development at different curing age•Pastes compressive strength increased with the increase of the milling agent debit but with the decrease of fly ash•Milling agent facilitates clinker grinding by improving cement fineness

In this study, the concept of Response Surface Methodology (RSM) was presented to optimize and compare the effect of fly ash and grinding agent on the cement compressive strength at 7, 28 and 90 days.This method showed that the optimum paste mix design with the fly ash (FA) to binder at 0%, clinker (KK) to binder at 66.4% and debit grinding agent (GA) to binder at 306 kg/t produced a spread compressive strengths of the hardened paste at 29.0, 38.0 and 50.4 MPa after 7, 28 and 90 days of curing, respectively. The fly ash began to play a significant role for the compressive strength after 28 days of curing, whereas grinding agent was reactive after 7 days of curing, indicative of time dependent contribution of fly ash and grinding agent to the development of compressive strength. These were further supported by the SEM microstructure analysis. Such a delayed involvement of fly ash and grinding agent in the cement chemistry should be taken into consideration with care when translating laboratory research results typically based on 28 days strength to field practice where a shorter curing is typically provided for cost reasons.