Coffee by-products in cement mortar and concrete: a comprehensive bibliometric and systematic review of its potential for the South African construction industry

This article systematically reviews the use of coffee waste, mainly Spent Coffee Grounds (SCG) and their derivatives, in cement and concrete, focusing on studies of mechanical performance, durability, and microstructural behaviour worldwide. A systematic search of four databases yielded 1,797 records, which, after duplicate removal and screening, left 200 reports for full-text assessment; following exclusions for lack of quantitative data, irrelevant content, or lack of South African relevance, 63 studies were finally included in the review. It synthesizes experimental, analytical, and microstructural research conducted regionally and globally to assess their applicability within the South African sustainability context. In this review, findings are categorized into four main areas: mechanical properties, durability, microstructural evolution, and environmental impact. Results show that the mechanical performance of concrete containing coffee waste depends heavily on the treatment and dosage of the waste. Optimal improvements were observed with 1.1% raw SCG by cement weight, 5% SCG ash as a cement replacement, and 15% SCG biochar as a fine-aggregate substitute, resulting in up to a 30% increase in compressive strength. However, untreated SCG reduced workability and increased shrinkage due to its high water absorption and organic content. To evaluate durability, including the Rapid Chloride Permeability Test (RCPT), water absorption, and drying shrinkage tests, it was found that raw SCG increases permeability and shrinkage. Conversely, pyrolyzed biochar significantly reduces chloride ion penetration and enhances the matrix density. Microstructural analyses reveal that raw SCG disrupts the Interfacial Transition Zone (ITZ), increasing porosity, whereas biochar promotes additional C–S–H gel formation and leads to a more refined pore structure. In the South African context, SCG biochar shows strong potential as a supplementary cementitious and insulating material, complementing fly ash and calcined clays to promote low carbon construction. Overall, the review concludes that converting SCG into biochar or ash supports sustainable waste valorization while delivering measurable improvements in mechanical performance (up to + 30%) and durability, along with a 25% reduction in chloride permeability.