The objectives of the present work were to investigate the effects of sewage sludge and marble sludge addition on slagging tendency and obtain better understandings for slag formation processes during the combustion of problematic wood waste pellets. Wood waste pellets produced with and without additives were combusted in a boiler (1.2 MWth) with continuous measurements of the combustion temperature and flue gas composition. The chemical composition, mineral phase, and microchemistry of the collected bottom ash and slag were examined by X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM/EDX). Reference wood waste pellets showed a high slagging tendency with 34 wt % of the ingoing fuel ash formed as slag on the grate. XRF and SEM/EDX analyses revealed high contents of Si, K, and Na in the slag and a clear correlation among the three elements. It implies melted fractions of the slag consist of alkali silicates indirectly observed as glass phases by XRD analysis. The severe slagging of wood waste pellets was reduced distinctly upon addition of marble sludge. For two marble sludge added combustion tests, less than 3 wt % of the ingoing fuel ash formed as low sintering degrees slag. The mineral compositions of the resultant slag were changed from low melting point silicates to high temperature melting calcium based silicates, oxides, and hydroxides. SEM/EDX revealed enhancement of Ca and Mg in the melted slag, which were possibly originated from marble sludge and dissolved in ash melts. This may lead to release of alkali metals from the ash melts thereby reducing the formed melt amount. In addition, marble sludge addition restrained sintering and accumulation of melted ash into a continuous phase and resulted in fragile slag particles. Addition of 4 wt % sewage sludge has a minor effect on the slag formation of wood waste pellets combustion. The sizes and sintering degrees of the formed slag were considerably decreased and the mineral compositions in the slag were dominated by high temperature melting corundum and calcium silicates. The enrichment of kalsilite observed in the slag was probably caused by the reactions of aluminum silicates in the sewage sludge with potassium from the fuel. The corundum, calcium silicates, and kalsilite have higher melting temperatures and thus gave a lower ash melt fraction in the slag. However, as a result of the addition of sewage sludge, both the ash content and the Si level of ingoing fuel pellets were enhanced. Thereby the formed slag amount increased slightly for the combustion of the 8 wt % sewage sludge blended wood waste pellets.