Integrated Effects of Metakaolin and Nano-silica in Superplasticizer-Free Mortar: An Analysis of Mortar Compressive Strength with Relative Strength, K-Factor and Clinker Savings

Abstract

The study examined the effect of substituting cement with metakaolin and nano-silica in binary and ternary blended cement mortars, with replacement levels of 10, 15, and 20% for metakaolin and 1.5, 3, and 4.5% for nano-silica, on their compressive strength. No superplasticizers were added to eliminate any influence on the mechanisms being studied. The relative strength of different combinations of metakaolin and nano-silica was compared. Each composition's pozzolanic efficiency, k-factor, was evaluated based on compressive strength. The possible saving of the clinker was evaluated as a function of the k-factor. The findings indicate that the ternary mortar blends containing 10% metakaolin and 1.5%, 3%, and 4.5% nano-silica attained the highest strength, reaching 47.1 MPa, 50.3 MPa, and 51.2 MPa at 28 days, and 48.5 MPa, 51.4 MPa, and 51.8 MPa at 56 days, respectively. TGA, XRD and SEM analyses were conducted for microstructural and morphological studies. The study highlights that incorporating metakaolin and nano-silica into mortar mixtures led to enhanced compressive strength due to the improved pozzolanic action of metakaolin in the presence of high surface area nano-silica particles. The ternary blended mortar exhibited higher mass losses in thermal analysis than the reference mortar, indicating increased decomposition of CSH and CAH. Conversely, the reference mortar showed more significant CH decomposition at 400-500 ºC and 600-900 ºC, suggesting a higher CH concentration. XRD patterns aligned with TG analysis, revealing the presence of important crystalline minerals and affirming the impact of pozzolanic activity from metakaolin, nano-silica and their combination in reducing CH content in the mortars. The study also revealed that integrating ternary compositions with 10% metakaolin and nano-silica up to 4.5% in mortar substantially decreased clinker usage, promoting environmental sustainability. The findings from this investigation in the mortar will be extrapolated to forthcoming studies in concrete, focusing on enhancing both eco-efficiency and economic efficiency of the concrete. © 2024 by authors, all rights reserved.