<![CDATA[Rice husk-derived lignin (RHL) is a promising bio-admixture capable of supporting sustainable mortar production while improving agricultural waste use. Therefore, this study aimed to investigate the influence of non-sulfonated RHL on mortar workability, setting behavior, compressive strength, crack pattern, and microstructural characteristics at different dosage levels. RHL was extracted from rice husk through alkaline-acid precipitation without combustion treatment and incorporated into mortar mixtures at dosages of 0–0.5% by cement weight. Setting time, mortar flow, compressive strength, and crack behavior were evaluated at curing ages of 7, 14, and 28 days, supported by SEM–EDX, XRD, and FTIR analyses. The results showed that low to moderate RHL dosages improved mortar flow, accelerated setting behavior, and increased compressive strength. The optimum dosage of approximately 0.3% produced the highest 28-day compressive strength of 48.64 MPa, corresponding to an increase of about 15.8% compared with normal mortar. Crack pattern observations showed less severe surface fragmentation and more localized cracking in lignin-modified mortar. SEM–EDX, XRD, and FTIR analyses also suggested improved matrix compactness and more homogeneous hydration product development at the optimum dosage. However, excessive RHL incorporation reduced compressive strength despite continued improvements in flowability. These results showed that RHL could function effectively as a non-sulfonated bio-based mortar admixture without requiring combustion-based processing.]]>
