Revolutionizing Ultra-High Performance Concrete: Unleashing Metakaolin and Diatomaceous Earth as Sustainable Fly Ash Alternatives

Document Type

Article

Publication Date

1-24-2025

Publication Title

Construction and Building Materials

Abstract

Ultra-high-performance concrete (UHPC) incorporating supplementary cementitious materials (SCMs) presents a novel and promising material with remarkable mechanical and durability properties. However, the utilization of silica fume in such SCMs is hindered by its high cost while that of class F fly ash by its availability. Towards exploring alternative economical and eco-friendly SCMs, as well as reduce the quantity of silica fume and cement in UHPC, this study reports on the benefits of using metakaolin (MK) and diatomaceous earth (DE) as replacements for fly ash in non-proprietary UHPC blends. MK and DE were added to replace fly ash (FA), up to 100 % in 25 % increments, without changing the silica fume (SF) content. In separate mixtures, MK and DE were added 100 % each to replace both FA and SF. The resulting mixtures were extensively characterized for their flow, rheological properties, compressive strength, flexural strength, split tensile strength, and modulus of elasticity, under two curing conditions. Scanning electron microscopy, BET surface area, and particle size analysis were performed to investigate the size and shape, surface area, and morphology of the cementitious powders. Results indicate that the addition of MK and DE resulted in increased apparent viscosity and shear stress with rising shear rate, while yield stress values decreased (p < 0.05). The flow curves of all the modified UHPC mixtures fitted well (R-2 > 0.99) with the linear Bingham Plastic, modified Bingham Plastic, and Herschel-Bulkley models. UHPC mixtures with 100 % replacement of FA with MK and DE achieved compressive strength values of 139 MPa and 128 MPa, respectively. At 28 days, MK, DE, and MK-DE mixtures showed 3.5 % - 10 % higher compressive strengths than the control mixture (p < 0.05). MK-DE mixture showed compressive strength improvements of 9 % and 17.5 % over the control and DE-modified mixtures, respectively (p < 0.05). The greatest modulus of rupture (MOR) value of 10.9 MPa at 28 days was observed in the control mixture, while DE modified UHPC exhibited the lowest MOR values among all the cases. At net deflection of L/150, the MK-DE modified UHPC mixture exhibited the greatest toughness value of 52.5 Joules at 28 days, which is 5 % greater than that in control UHPC mixture. Both MK and DE modified UHPC mixtures showed toughness values similar to the control UHPC mixtures at 28 days. In conclusion, the MK-DE modified UHPC mixtures that were prepared, without SF and FA, exhibited characteristics comparable to the control mixture, showcasing potential alternatives for non-proprietary UHPC mixtures.

Comments

The SEM facility used in this work was partially supported by the National Science Foundation (NSF) under Grant No. 1126126.

DOI

10.1016/j.conbuildmat.2024.139822

Volume

460

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