Abstract Anaerobic digestion (AD) is a well-known technology for organic waste treatment with recognised environmental benefits including the generation of renewable energy (methane) and other added-value products. However, when applied to very easily biodegradable substrates, there is a need to mitigate the sudden pH drop, in order to improve the stability and feasibility of the process. In this work, and for the first time, waste-based geopolymer spheres were used as pH regulators in AD. The influence of the binder composition and content on the ability to control pH and on the methane yield was evaluated. The pH buffer capacity of the geopolymers can be controlled by the fly ash content in the composition, with higher contents leading to higher alkalis leaching and narrower pH fluctuation over time. The spheres promoted an excellent control of pH in batch operated anaerobic reactors (up to 70 days), which increases the stability and efficiency of the systems. The pH range in the reactor without spheres remained mostly between 3.84 and 4.38, preventing methane production, whereas in reactors with spheres pH was mostly kept between 6.45 and 7.94. The fast pH stabilization during the entire experiments, which occurred with fly ash-based geopolymers, stimulated the early methane production, demonstrating the huge potential of these innovative spheres as a pH buffering material. Using this innovative waste-based material, instead of commercial alkaline materials, enhances process simplicity (prevent the need for continuous pH adjustment) and sustainability, which may overcome the limitations associated with the existing applications such as the use of substrates with a high acidogenic potential and, therefore, contribute to the spread of AD technology.