Exploring coffee extract as a renewable admixture to prepare mortars with better performance

Intending to decarbonize the chemical admixtures and promote the sustainable development of the construction industry, this paper investigated the feasibility of coffee extract as a renewable natural admixture to adjust the performance of cementitious materials. The influences of coffee extract with various concentrations on the compressive strength, setting time, hydration properties and microstructure development of cementitious materials were studied. Results showed that both hydration promoting and hydration retarding effects were brought about when using the coffee extract as the mixing water in preparing cementitious materials. The hydration promotion was attributed to the polyphenols in the coffee extract, which functioned as a Ca2+ captor in the pore solution of pastes through a complexation mechanism. The complex of polyphenols-Ca may form the initial seeds for the promoted hydration of cementitious materials. A low concentration of the coffee extract (≤0.05%) exerted a prominent hydration promoting effect on the hydration of cement, while the hydration retarding effect gradually became apparent for a coffee extract with high concentrations. Overall, the compressive strength of cementitious mortars with 0.05% coffee extract was increased by 18.6% at 28d owing to the formation of more hydration products, a 25.1% reduction in porosity, and the thick Ca(OH)2 crystals. As a result, a 5% optimum concentration of coffee extract achieved the best economic and environmental benefits for cementitious mortars. This study contributes a new solution for sustainable development in the construction industry, as the coffee extract is low carbon, natural, non-toxic, renewable and can be a superior alternative to chemical admixtures.