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Highly crosslinked thermoset plastic waste is generally regarded as unrecycled materials and dumped into landfills. To divert the waste from landfills, recycling thermoset waste as fine particles in cementitious composites is a potential approach. However, weak interface bonding between thermosets and cement hydrates causes significant strength degradation in cementitious composites, which has constituted a long-term stumbling block. Herein, we innovate a green recycling solution for recycling thermoset waste as fillers in order to engineer sustainable cement mortar with the help of a green surface adhesion promoter – a silane coupling agent. The feasibility of the proposed approach was experimentally validated in terms of workability performance and compressive strength in order to meet requirements in practice. The results show that thermoset waste can substitute for at least 15% of sand in cement mortar, following workability requirements. In addition, the optimal replacement level of thermoset waste for sand in cement mortars with the highest compressive strength is 5%, beyond which the compressive strength exhibits a continuing drop. The strength alteration mechanisms are elucidated by scanning electron microscope (SEM) analyses. We demonstrate that the developed thermoset cement mortars with 5% thermoset waste have the great potential to achieve a highly dense microstructure, robust interfacial transition zone, and better hydrate crystal growth, which are the microscopic origins determining the inherent strength enhancement. The findings of this study enable the design of eco-friendly cement mortar formulations, which can both benefit the construction industry and relieve the severe environmental burden caused by landfilling thermoset waste after its end-of-life use. • A novel strategy for recycling thermoset waste into cement mortars is devised. • The interfacial bonding between thermosets and hydrates is enhanced by silanization. • The workability and compressive strength of the cement mortars are evaluated. • The mortars with 10% thermoset show performances on par with control groups. • Microstructural origins behind variations in the compressive strength are revealed. [ABSTRACT FROM AUTHOR]