Cementitious Paste-Aggregate Interface in High-Performance Concrete Based on Silica Fume: Insights from a Microstructure of the Interfacial Transition Zone (ITZ)

Abstract

This study investigates the chemical and mechanical properties of the paste–aggregate interface in High-Performance Concrete (HPC) incorporating Silica Fume (SF). Limestone and dolomite aggregates were used to assess the influence of aggregate type on interfacial behavior. Mechanical tests, including compressive, flexural, and splitting tensile tests, showed that the compressive and tensile strengths increased with increasing SF content in the cementitious matrix. This improvement is attributed primarily to the high pozzolanic reactivity of SF, which consumes portlandite (CH), a hydration product of the silicate phases C₃S and β-C₂S, and promotes the formation of additional C-S-H phases. Mechanical testing, X-ray diffraction (XRD), and Scanning Electron Microscopy (SEM) demonstrated that incorporating 10% SF into the cementitious matrix significantly improved the mechanical performance of the concrete. Compared with the reference concretes, the mixtures containing SF exhibited a 19% increase in compressive strength and a 4%–11% increase in splitting tensile strength. In addition to these macroscopic improvements, the results indicated that SF improved the paste–aggregate Interfacial Transition Zone (ITZ), resulting in enhanced interfacial bond strength for both limestone and dolomite aggregates. To address the difficulty of characterizing the interface within concrete containing irregularly shaped aggregates, a geometry-controlled direct tensile interface test was developed to evaluate the mechanical behavior of the paste–aggregate interface. This test, combined with XRD and SEM analyses, provided complementary evidence of the changes in interfacial bonding and microstructure associated with SF incorporation. The integrated experimental approach provides insights into the role of SF in modifying the concrete ITZ and offers a methodology for evaluating paste–aggregate interfacial behavior.