Use of agro-industrial waste in alkali-activated cement: A study with metakaolin and garlic peel ash

Portland cement remains the most widely used binder worldwide, although its production is highly energy-intensive and represents a major source of CO2 emissions due to clinker calcination. Alkali-activated cements (AACs) have emerged as low-carbon alternatives capable of eliminating clinker, incorporating solid wastes, and achieving competitive mechanical and durability performance. However, the use of unconventional agro-industrial wastes as precursors remains insufficiently explored. This study investigates an alkali-activated cement produced from metakaolin combined with garlic peel ash, an abundant but underutilized agricultural waste. Metakaolin was partially replaced by 10%, 20% and 30% of ash, using a 1:1:0.60 binder-to-sand-to-water ratio and 5 M NaOH as the alkaline activator. The garlic peel ash was characterized chemically, mineralogically and granulometrically, and the AAC were tested in both fresh and hardened states. The incorporation of 10% ash increased the flow diameter due to its lower density, whereas higher replacement levels reduced workability because of the ash's high fineness and corresponding increase in water demand. A slight reduction in density was observed with increasing ash content, associated with both the lower specific mass of the waste and volumetric changes during reaction product formation. Compressive strength improved with higher ash contents, particularly under NaCl curing, a behavior linked to enhanced particle packing, the high alkali content of the ash, and mineralizing effects of SO3 and Cl−. NaCl curing promoted greater ionic diffusion, intensifying the alkaline activation reactions. XRD, SEM and TGA analyses confirmed higher formation of sodalite and faujasite in the 30% ash mixture cured in NaCl, which also exhibited greater mass loss between 300 and 700°C, indicating higher amounts of crystallized N-A-S-H gel. This composition achieved the highest compressive strength, demonstrating the potential of garlic peel ash as an effective alternative precursor for AACs.