Increasing the yield and quality of natural indigo cake using low-cost calcium hydroxide

This research aimed to improve the yield and quality of natural indigo cake by using low-cost calcium hydroxide produced from household waste, specifically golden apple snail shells and eggshells, as alternatives to commercial lime. Quicklime (CaO) was prepared by calcination in a custom-designed 50-liter vertical kiln at ≈ 900 °C for 4 hours, followed by slaking to obtain hydrated lime (Ca(OH)₂). XRD, SEM, and FT-IR analyses confirmed that both limes consisted primarily of Ca(OH)₂, with snail shell lime showing higher purity and rougher particle surfaces, while eggshell lime contained minor impurities. Indigo cakes produced from snail shell lime had slightly lower yield (29.71% of commercial lime) but higher color intensity (+28.84%) with a less greenish-blue hue, and exhibited superior storage stability for up to 12 weeks compared to commercial indigo. Eggshell-derived indigo cakes had lower yield (38.21%) and color intensity (43.18% of commercial lime) but produced brighter, distinctly blue tones. The preparation of indigo dyeing solution from these indigo cakes required only 10 minutes before being ready for use, whereas commercial lime-based indigo required longer preparation. Cotton fabrics dyed with snail shell indigo exhibited superior colorfastness to sunlight (ΔE* = 19.12%) and washing (ΔE* = 4.38%) compared to fabrics dyed with commercial lime, while eggshell indigo produced fabrics with high color intensity (K/S = 67.41%) but lower sunlight fastness (ΔE* = 6.62%). All dyed fabrics showed blue-green hues (a* < 0, b* < 0). These results demonstrate that lime produced from household waste, especially golden apple snail shells and eggshells, can effectively replace commercial lime, reduce dye preparation time, and promote sustainable use of local resources in community-based indigo production. GRAPHICAL ABSTRACT HIGHLIGHTS A 50-liter vertical kiln using waste vegetable oil as fuel was developed to calcine calcium-rich waste materials at 800–900 °C for 4 hours, suitable for community-scale quicklime production. Golden apple snail shell-derived quicklime showed high purity, fine texture, and better suitability for indigo cake production compared to eggshell-derived and commercial lime. Indigo cake made from snail shell lime enabled faster preparation of leuco-indigo solution and resulted in dyed fabrics with superior colorfastness to sunlight and washing. The approach offers a low-cost, sustainable alternative to commercial lime and promotes efficient use of household waste in local natural dyeing communities.