Synergistic Effects of Fly Ash and Metakaolin on the Performance of Sustainable Concrete

Abstract

This study examines the development of environmentally sustainable concrete incorporating fly ash (FA) and metakaolin (MK). Concrete mixtures of grade M60 were produced with a fixed replacement of 20% FA to promote long-term matrix densification, alongside varying MK contents of 4%, 8%, 12%, and 16%. Mechanical properties, including compressive, split tensile, and flexural strengths, as well as durability characteristics such as sorptivity, water absorption, chloride permeability, and homogeneity, were assessed at curing ages of 7, 28, and 90 days. The results demonstrate that the mixture containing 20% FA and 12% MK delivered the most favourable performance among the SCM combinations, leading to improvements of approximately 24–28% in compressive strength, 21–25% in split tensile strength, and 22–24% in flexural strength relative to the control concrete. Life cycle assessment indicated notable reductions in global warming potential, ozone depletion potential and acidification offering substantial environmental benefits.

Introduction

The construction industry contributes significantly to global environmental problems, accounting for about 38% of CO? emissions and 35% of total energy consumption. To reduce these impacts, the use of green concrete has been promoted by partially replacing ordinary Portland cement with supplementary cementitious materials (SCMs) such as fly ash and metakaolin. These materials improve concrete performance through pozzolanic reactions, pore refinement, and micro-filling effects, which enhance strength, durability, and sustainability while reducing cement consumption and environmental impact.

In this study, M60 grade concrete was developed using combinations of fly ash (FA) and metakaolin (MK). Several mixes were prepared with 20% fly ash and varying metakaolin contents (4–16%), along with a control mix. Mechanical properties (compressive, split tensile, and flexural strength) and durability properties (capillary absorption and chloride permeability) were tested.

Results showed that FA–MK blended concretes performed better than the conventional mix. Strength improvements ranged from 15–27% in compressive strength, 12–31% in split tensile strength, and 18–28% in flexural strength. The mix containing 20% FA and 12% MK produced the best results due to the synergistic interaction between the two materials, which created a denser microstructure. Durability tests also indicated lower water absorption and reduced chloride permeability, confirming improved resistance to moisture and chemical attack.

Additionally, life cycle analysis showed that replacing cement with SCMs significantly reduced environmental impacts, including global warming potential, ozone depletion potential, and acidification. Overall, the study demonstrates that the combined use of fly ash and metakaolin can produce stronger, more durable, and environmentally sustainable concrete suitable for modern construction.

Conclusion

Based on the experimental observations, the following conclusions are given below 1) Incorporation of FA and MK significantly improved compressive, tensile, and flexural strengths, with 20% FA + 12% MK identified as the optimum blend. 2) The synergistic pozzolanic action of MK and FA produced a denser microstructure, enhancing long-term performance. 3) Sorptivity decreased with curing age, and the 20% FA + 12% MK mix showed the lowest water absorption. 4) Chloride permeability was substantially reduced in blended mixes, indicating improved durability. 5) Cement replacement with SCMs significantly lowered GWP, ODP, and acidification impacts. 6) Overall, optimized FA–MK blends provide a sustainable and high-performance alternative to conventional concrete.

References

[1] Global, A. B. C. \"Global status report for buildings and construction.\" Global Alliance for Buildings and Construction 10 (2020). [2] Mehta, P.K., 2010, June. Sustainable cements and concrete for the climate change era–a review. In Proceedings of the second international conference on sustainable construction materials and technologies, Aneona, Italy (pp. 28-30). [3] Jiang, J., Lu, Z., Niu, Y., Li, J. and Zhang, Y., 2016. Investigation of the properties of high-porosity cement foams based on ternary Portland cement–metakaolin–silica fume blends. Construction and Building Materials, 107, pp.181-190. [4] Aswathy, M., 2017. Experimental study on light weight foamed concrete. International Journal of Civil Engineering and Technology, 8(8). [5] Krishnan, G. and Anand, K.B., 2018, February. Industrial waste utilization for foam concrete. In IOP Conference Series: Materials Science and Engineering (Vol. 310, p. 012062). IOP Publishing. [6] Falliano, D., De Domenico, D., Sciarrone, A., Ricciardi, G., Restuccia, L., Ferro, G., Tulliani, J.M. and Gugliandolo, E., 2020. Influence of biochar additions on the fracture behavior of foamed concrete. Fracture and Structural Integrity, 14(51), pp.189-198. [7] Sarazin, J., Davy, C.A., Bourbigot, S., Tricot, G., Hosdez, J., Lambertin, D. and Fontaine, G., 2021. Flame resistance of geopolymer foam coatings for the fire protection of steel. Composites Part B: Engineering, 222, p.109045. [8] Yuan, H., Ge, Z., Sun, R., Xu, X., Lu, Y., Ling, Y. and Zhang, H., 2022. Drying shrinkage, durability and microstructure of foamed concrete containing high volume lime mud-fly ash. Construction and Building Materials, 327, p.126990. [9] Gökçe, H.S., Hatungimana, D. and Ramyar, K., 2019. Effect of fly ash and silica fume on hardened properties of foam concrete. Construction and building materials, 194, pp.1-11. [10] [Vishavkarma, A. and Venkatanarayanan, H.K., 2024. Assessment of pore structure of foam concrete containing slag for improved durability performance in reinforced concrete applications. Journal of Building Engineering, 86, p.108939. [11] Ahmaruzzaman, M., 2010. A review on the utilization of fly ash. Progress in energy and combustion science, 36(3), pp.327-363. [12] [Li, G., Zhou, C., Ahmad, W., Usanova, K.I., Karelina, M., Mohamed, A.M. and Khallaf, R., 2022. Fly ash application as supplementary cementitious material: a review. Materials, 15(7), p.2664. [13] Uliasz-Boche?czyk, A. and Mokrzycki, E., 2020. The potential of FBC fly ashes to reduce CO2 emissions. Scientific Reports, 10(1), p.9469. [14] Paino, J., Perera, S., Alashwal, A. and Rodrigo, M.N., 2019. Impact of fly-ash on carbon emissions in different concrete grades. In World Construction Symposium (pp. 368-377). University of Moratuwa. [15] Dindi, A., Quang, D.V., Vega, L.F., Nashef, E. and Abu-Zahra, M.R., 2019. Applications of fly ash for CO2 capture, utilization, and storage. Journal of CO2 Utilization, 29, pp.82-102. [16] Rashad, A.M., 2013. Metakaolin as cementitious material: History, scours, production and composition–A comprehensive overview. Construction and building materials, 41, pp.303-318. [17] [Sabir, B.B., Wild, S. and Bai, J., 2001. Metakaolin and calcined clays as pozzolans for concrete: a review. Cement and concrete composites, 23(6), pp.441-454. [18] Muduli, R. and Mukharjee, B.B., 2020. Performance assessment of concrete incorporating recycled coarse aggregates and metakaolin: A systematic approach. Construction and Building materials, 233, p.117223. [19] Bohá?, M., Palou, M., Novotný, R., Másilko, J., Všianský, D. and Stan?k, T., 2014. Investigation on early hydration of ternary Portland cement-blast-furnace slag–metakaolin blends. Construction and Building Materials, 64, pp.333-341. [20] Siddique, R. and Klaus, J., 2009. Influence of metakaolin on the properties of mortar and concrete: A review. Applied Clay Science, 43(3-4), pp.392-400. [21] Duan, P., Shui, Z., Chen, W. and Shen, C., 2013. Effects of metakaolin, silica fume and slag on pore structure, interfacial transition zone and compressive strength of concrete. Construction and Building Materials, 44, pp.1-6. [22] Akkaya, Y., Ouyang, C. and Shah, S.P., 2007. Effect of supplementary cementitious materials on shrinkage and crack development in concrete. Cement and Concrete Composites, 29(2), pp.117-123. [23] Darquennes, A., Staquet, S., Delplancke-Ogletree, M.P. and Espion, B., 2011. Effect of autogenous deformation on the cracking risk of slag cement concretes. Cement and Concrete Composites, 33(3), pp.368-379.

Copyright

Copyright © 2026 Kakitapalli Sai Tirumala, Gillula Anuradha, Amujuru Tharun, Varre Harika, Gollangi Vamsi, Surla Lovaraju. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.