Performance and Mechanistic Insights into an Early-Strength Eco-Friendly UHPC Grouting Material Modified with Nano-Calcium Carbonate

Ultra-high-performance concrete (UHPC) has potential as a grouting material for emergency landslide remediation due to its superior mechanical properties and dense matrix without coarse aggregates. However, its field application is restricted by the requirement for high-temperature curing, low early-age strength, and poor constructability under standard curing conditions. To enhance early-strength development under conventional standard curing conditions, nine eco-friendly UHPC mixtures incorporating steel slag micro-powder were prepared using an orthogonal experimental design. Four factors, including lithium carbonate as an early-strength admixture, nano-calcium carbonate as a reinforcing filler, steel fibers, and polyvinyl alcohol (PVA) fibers, were investigated. The effects of these factors were evaluated through compressive strength, flexural strength, and flowability tests at different curing ages. Field-emission scanning electron microscopy (SEM) and X-ray diffraction (XRD) were further employed to analyze microstructural characteristics and hydration-phase evolution. The results indicate that increasing the dosages of the four additives decreases fresh paste flowability. An appropriate dosage of lithium carbonate promotes ettringite formation and enhances early-age compressive strength, while nano-calcium carbonate improves the matrix compactness through pinning and pore-filling effects. The optimized eco-friendly steel-slag-based UHPC achieved compressive/flexural strengths of 23.4/6.4 MPa at 12 h and 45.5/13.8 MPa at 1 d under standard curing conditions. The developed UHPC exhibits potential for rapid emergency grouting applications in landslide remediation.