From waste to value: Enhancing strength and CO2 sequestration of cement composites with lime mud under carbonation curing

Lime mud (LM), a highly alkaline by-product from the paper industry, is usually disposed of by landfilling, causing environmental risks and resource waste. Previous attempts to use LM in cementitious materials often relied on calcination, which increases energy use and CO 2 emissions, or direct blending, which leads to reduced strength and poor durability. To address these limitations, this study investigates the direct incorporation of uncalcined LM, alone or with slag, into cement-based materials (CBMs) subjected to carbonation curing. The hydration–carbonation behavior, pore structure, mechanical properties, and high-temperature resistance were comprehensively evaluated. The results show that LM provided nucleation sites and high alkalinity, accelerating CO 2 uptake and enhancing early strength under carbonation curing. At 14 days, LM20 under carbonation curing reached a compressive strength of 52 MPa, 54 % higher than its normally cured counterpart. When combined with slag, the LM20A10 mixture achieved the best balance of strength and CO 2 sequestration, showing a greater carbonation degree and a CO 2 uptake exceeding 11 wt%. Multi-scale analyses (XRD, TG-IR, FTIR, SEM/TEM, MIP) confirmed the formation of N,C-A-S-H gels and CaCO 3 –SiO 2 composites, which densified the matrix and linked microstructure with macroscopic performance. Furthermore, carbonation-cured specimens retained over 60 % of their compressive strength at 600 °C, whereas normally cured specimens showed severe degradation, indicating improved thermal resistance. This study demonstrates a sustainable strategy for LM valorization, integrating solid waste recycling, CO 2 sequestration, and performance enhancement, thereby contributing to the development of low-carbon and durable construction materials. • LM inhibits hydration normally but boosts early strength and CO 2 uptake under carbonation curing. • During carbonation, LM densifies the microstructure via the formation of CaCO 3 and SiO 2 gel. • LM-cement-based materials exhibit better high-temperature resistance after carbonation curing. • LM enhances composite performance by activating the pozzolanic reactivity of slag.