Influence of Composite Mineral Admixtures and Expansive Agent on the Mechanical Properties, Durability and Microstructure of Cementitious Materials

To improve volume stability and durability of cementitious materials, a composite mineral admixture including ground granulated blast furnace slag, fly ash and silica fume was incorporated at a fixed 12% replacement ratio together with varying dosages of an expansive agent (EA). Results show that the composite admixture delays setting while increasing EA shortens setting time. Mechanical properties first increase and then decrease with EA content. The mixture with 6% EA (A4) achieves the best performance, with a 28-day compressive strength of 51.6 MPa. Expansion behavior continuously increases with EA content, but the expansion coefficient peaks near unity at an optimal EA level and then decreases, which indicates effective shrinkage compensation at moderate EA dosages. Durability properties including sulfate resistance and chloride migration resistance follow a similar non-monotonic trend, enhanced at moderate EA levels but degraded at excessive dosages. The A4 mixture exhibits the highest sulfate resistance coefficient and the lowest chloride diffusion coefficient. Microstructural analyses via XRD, EDS, SEM and MIP reveal differences in hydration products and pore structure characteristics among the mixtures. The composite mineral admixtures contribute to pore refinement through hydration and filler effects, while an appropriate EA dosage provides shrinkage compensation and improves microstructural stability. These combined effects reduce harmful pore content and pore connectivity, thereby improving resistance to sulfate and chloride transport. Excessive EA leads to over-expansion, inducing microstructural defects and disrupting pore structure integrity, which ultimately compromises durability. Therefore, durability enhancement of the composite system is governed by a balance between expansion compensation and pore structure refinement, rather than simply the increase in hydration products.