The accumulation of construction and demolition waste requires resource-efficient and scalable solutions based on the principles of the circular economy and the use of low-carbon materials. This study investigated the effectiveness of the integrated application of recycled concrete fines (RCF) in the stabilisation of two soil types differing in their particle size distribution and mineralogical composition. RCF was utilised as a granulometric correction material, a microfiller, and a component of a low-carbon slag binder. The positive effect of co-grinding ground granulated blast furnace slag (GGBFS) with 20 wt.% RCF on the microstructure development and mechanical properties of the low-carbon slag binder for soil stabilisation was demonstrated. The compaction characteristics determined using the Modified Proctor test, mechanical properties, water resistance, freeze–thaw resistance, and phase formation of soils stabilised with a reference mixture containing 10 wt.% Portland cement (PC) and the developed low-carbon slag binder were investigated. A differentiated approach to the application of RCF depending on the soil particle size distribution was substantiated. RCF in the fine sand fraction proved to be the most effective for highly plastic soils, whereas ground RCF with a specific surface area of 3000 cm²/g was the most effective microfiller for soils with a higher sand content and a lower clay fraction. The developed low-carbon slag binder was shown to enable the effective partial replacement of PC in soil stabilisation. Furthermore, the mineralogical composition of the soil was found to govern the phase formation processes in stabilised soils. The obtained results confirm the potential of the integrated use of RCF and the developed low-carbon slag binder for resource-efficient soil stabilisation and the recycling of construction and demolition waste in geotechnical engineering.