One of the five key elements of the cement industry’s roadmap to achieving climate neutrality by 2050 is the 5C cement challenge to develop future commercial products with a lower clinker-to-cement ratio (≤0.60) than those currently manufactured. According to the data available as of 2021, clinker content in the Spanish cement industry stands at around 0.8 (clinker/cement ratio), a high factor exacerbated by the economic crisis in the sector, COVID-19, lower demand for cement, and lower availability of traditional additives [1,2]. This combination makes it imperative to continue the search for new supplementary cementitious materials (SCMs) that offer an eco-friendly alternative to those traditionally used (fly ash, silica fume, and natural/calcined clay) [3]. In recent years, the scientific community has conducted ongoing research into the different industrial waste streams that, due to their chemical, mineralogical, and pozzolanic characteristics, offer scientifically, technically, and environmentally viable substitutes [4,5,6,7,8,9,10]. Within these emerging streams, ash from biomass combustion is attracting particular attention because of the vast volumes generated globally (170 Mt/yr) [11], mainly driven by the new international scenarios promoting the use of clean energy from renewable sources [12]. There is a wide variety of biomass ash rich in reactive silica and alumina (rice husk, bagasse cane, bamboo leaf, and paper sludge, among others) capable of providing cement with the improved properties required by the transition to a low-carbon economy [13,14,15,16,17]. However, biomass ash has several drawbacks, such as its heterogeneity (time of year generated, energy process, burning temperature, etc.) and its contribution of potentially negative elements/oxides to the cement matrix. The latter is the case with high-potash (K₂O) ash, which requires pre-washing to reduce its content and prevent reactivity and durability issues (alkali-aggregate reaction) in the new eco-cements [18,19,20,21,22].