In this work, the effects of three additives (sewage sludge, marble sludge, and clay sludge) on the sintering behaviors of two types of biomass ash (wheat straw and wood waste ash) were investigated. The ability of the additives to abate sintering was evaluated by performing standard ash fusion characterization and laboratory-scale sintering tests on mixtures of biomass ash and additives. The possible mechanisms underlying the anti-sintering effects of the additives were examined using a combination of X-ray diffraction (XRD) and scanning electron microscopy energy-dispersive X-ray spectrometry (SEM-EDX) analyses of residues from the sintering tests. The best anti-sintering effect was achieved when marble sludge was used. The diluting effect of the marble sludge on the biomass ashes is considered to be the main reason for the decreased degree of ash sintering. In addition, Ca from the marble sludge may promote the formation of high-temperature melting silicates and phosphates with low K/Ca ratios: These: chemical reactions and consequent products are favorable for reducing ash Melt formation and sintering tendency. Sewage sludge served as a suitable additive to mitigate the sintering of the studied biomass ashes. Upon addition of the sewage sludge, compositions of the biomass ash changed from low-temperature melting silicates to high-temperature melting silicates, phosphates, and oxides. The shift of ash chemistry had a considerable positive effect on biomass melting and sintering temperatures. Clay sludge exhibited a poor ability to reduce the sintering tendency of wheat straw ash. Moreover, the addition of clay sludge decreased melting temperatures and caused severe sintering behavior of wood waste ash. SEM-EDX and XRD analyses revealed that, as clay sludge was added, more Si-rich melts were formed in wood waste ash. This occurred because clay sludge provides thermodynamically reactive Si-containing species and promotes the formation of low-melting-temperature alkali silicates.