Comparative assessment of activation pathways for recycling CaCO₃-rich paper waste in cementitious systems

This study provides a systematic evaluation of three processing routes, dried lime mud (DLM), calcination (CLM), and alkaline activation (ALM), for recycling CaCO₃-rich paper waste as a partial replacement for cement. Standardized R³ reactivity testing, isothermal calorimetry, compressive strength testing, FTIR, XRD, TGA, SEM, MIP, and cradle-to-gate life-cycle assessment were used to evaluate the chemical reactivity, mechanical performance, microstructural development, and environmental impact of each route. The R³ results showed that DLM and ALM exhibited negligible heat release, while CLM showed the highest response among the LM-derived materials due to CaO hydration; however, the cumulative heat release of CLM remained below the 100 J/g SCM threshold commonly used to classify reactive supplementary cementitious materials. At 25% replacement, DLM showed only a 7.6% reduction in 28-day compressive strength. In comparison, CLM showed a 14.3% reduction and the densest pore structure among the LM-modified mixtures. In contrast, 50% replacement led to strength losses exceeding 50%, and 75% replacement resulted in severe strength reductions of approximately 80–90%, accompanied by pore coarsening. A cradle-to-gate life-cycle assessment showed that minimally processed DLM provided the largest environmental benefit, with emissions more than 90% lower than those of cement. In comparison, CLM achieved approximately 70–85% emission reductions but required higher processing energy due to calcination. Overall, the results demonstrate that LM should be classified as a low-carbon filler rather than a reactive supplementary cementitious material, and that approximately 25% replacement represents a practical upper limit for maintaining mechanical performance while reducing cement-related environmental impacts.