Mechanical Performance and Sustainability Assessment of Mortar Incorporating Recycled Chicken Bone Powder

This study investigated the feasibility of using hydroxyapatite (HAp) derived from pyrolyzed waste chicken bones as a sustainable cement replacement material for cement mortar. Commercial tricalcium phosphate (TCP), which belongs to the same calcium phosphate family but possesses distinct crystalline characteristics, was used as a comparative material. HAp and TCP were incorporated as partial cement replacements at 2, 5, 10, and 20% by weight, and the workability, compressive strength, flexural strength, microstructure, and CO2 emission characteristics of the resulting mortars were evaluated. The results showed that low replacement ratios improved early-age strength owing to the micro-filler effect of fine calcium phosphate particles. In particular, the HAp mixtures exhibited superior long-term performance compared with the TCP mixtures, with the 2% HAp mixture achieving the highest compressive strength of 54.5 MPa at 56 days. Flexural strength results showed a similar trend, with HAp effectively suppressing microcrack propagation through improved matrix densification and interfacial bonding. However, replacement ratios exceeding 10% reduced mechanical performance due to cement dilution, increased porosity, and particle agglomeration. SEM observations confirmed that HAp replacement levels of 2–5% densified the mortar matrix, whereas excessive replacement caused localized agglomeration and microstructural defects. The carbon emission assessment indicated that pyrolysis reduced direct CO2 emissions compared with incineration by immobilizing part of the carbon in solid char; however, laboratory-scale pyrolysis increased total emissions because of high electricity consumption. Nevertheless, process integration with cement clinker production could enable waste valorization and carbon reduction by utilizing existing high-temperature kiln systems. Overall, chicken bone-derived HAp–carbon composite demonstrated strong potential as an eco-friendly cement replacement material, with an optimal replacement ratio of 5% or less.