Improving UHPC Durability and Shrinkage with Metakaolin and Diatomaceous Earth

Abstract

This study investigates the durability and shrinkage characteristics of ultrahigh-performance concrete (UHPC) mixtures incorporating metakaolin (MK), diatomaceous earth (DE), and their combination as supplementary cementitious materials. Four UHPC mixtures were evaluated through comprehensive experimental testing, focusing on freeze–thaw resistance, chloride ion penetrability, sulfate resistance, and drying shrinkage for different curing regimes. All mixtures demonstrated excellent freeze–thaw durability, maintaining 100% relative dynamic modulus and no mass loss after 300 cycles. Modulus of rupture tests revealed minimal strength loss in mixtures with MK or the MK–DE combination. Chloride resistance improved significantly, with one mixture showing more than a 50% reduction in permeability and over a fourfold increase in surface resistivity compared to the control. Sulfate resistance was confirmed through minimal length changes and favorable microstructural analysis. Mixtures containing MK consistently exhibited the lowest drying shrinkage and cracking potential in both air-dry and moist curing conditions. Mixtures with DE showed improved shrinkage behavior for continuous moist curing. The combination of both materials resulted in balanced performance across all durability and shrinkage parameters. These findings demonstrate that MK and DE are effective and sustainable alternatives to silica fume and fly ash in UHPC, contributing to improved durability, reduced shrinkage, and environmentally responsible infrastructure solutions. Although the results are promising, it is important to note that the findings are based on accelerated laboratory testing with fixed replacement levels; further validation in long-term field conditions and for broader dosage optimization are necessary to fully generalize the results.