Novel lime-silica fume-modified limestone calcined clay cement (LC3) binder system for sustainable pavement construction

Abstract

Limestone calcined clay cement (LC3) has emerged as a promising sustainable alternative to ordinary Portland cement (OPC), offering comparable mechanical properties while significantly reducing carbon emissions. Conventional LC3 cement typically consists of approximately 50% ordinary Portland cement (OPC), 30% calcined clay, 15% limestone, and 5% gypsum. In this study, we refer to the binary blend of limestone and calcined clay as LC2 for convenience. This study explores two strategies to enhance LC3’s sustainability: (i) increasing the LC2-to-binder ratio and (ii) replacing OPC entirely with a calcium oxide (lime)–silica fume blend, where silica fume acts as a pozzolan to enhance hydration and strength development (all mixes contain 5% gypsum as well). A detailed investigation of 22 mix designs is conducted to evaluate compressive strength and feasibility for pavement applications, with comparison to conventional OPC-based LC3 and OPC-fly ash mortars. Microstructural analyses, including X-ray diffraction (XRD), scanning electron microscopy (SEM)-energy dispersive X-ray spectrometry (EDS), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA), further corroborate the findings. Results indicate that a balanced CaO/silica fume ratio is crucial for achieving strength comparable to 70:30 OPC-based LC3; excessive CaO leads to detrimental expansion and mix instability. A maximum compressive strength of 27.6 MPa was observed for LCS-70 mixes (LCS-70-4.0), and a maximum of 29.2 MPa was observed for the LCS-50 mixes (LCS-50-1.8). Among the investigated mixes, LCS-70-1.0—where LCS refers to the LC2-CaO–silica fume system, 70 represents the LC2-to-binder ratio (70:30), and 1.0 denotes the CaO-to-silica fume ratio—emerges as the most optimal, offering a sustainable balance of strength, cost, and environmental impact. A total score of 4.46 is observed for this mix, which is higher than any other. The study concludes that lime-silica fume-modified LC3 is a viable alternative to both OPC and conventional LC3, making it suitable for low-volume pavement applications while significantly reducing embodied carbon and energy consumption.