Carbonate-rich coffee husk ash as an alternative potassium source for metakaolin-based geopolymers

Abstract This study investigates the use of carbonate-rich coffee husk ash (CHA) as an alternative potassium source for synthesizing metakaolin-based geopolymers, a prominent class of chemically bonded ceramics. A fundamental constraint of raw CHA is its high carbonate content, which restricts the alkalinity of the reaction medium and hinders the dissolution of the aluminosilicate network. To overcome this bottleneck, Ca(OH) 2 was introduced as a targeted chemical modifier to trigger in situ causticization. Five systems were evaluated at a constant K 2 O/metakaolin mass ratio of 0.40: KOH, K 2 CO 3 , CHA, K 2 CO 3 –Ca(OH) 2 , and CHA–Ca(OH) 2 . Phase evolution and microstructural characteristics were systematically investigated using XRD, ATR–FTIR enhanced by second-derivative analysis, and SEM. The results demonstrate that Ca(OH) 2 addition successfully converts soluble carbonates into insoluble crystalline phases (butschliite and calcite), unlocking the alkalinity required to sustain geopolymerization. This chemical interaction promoted a compact matrix with reduced porosity (29%) and a competitive compressive strength of ~ 10 MPa compared to the unmodified carbonate systems.