This paper deals with the valorization of wastepaper sludge ash (WSA) as the main component of Hydraulic Road Binder (HRB). The physical, chemical and mineralogical characteristics of this WSA were studied and this characterization was completed by a study of the reactivity of WSA in presence of lime and gypsum. This study led to the optimization of HRB rich in WSA whose characteristics were tested on mortars and on samples of treated soil. The characterization of WSA and the study of its reactivity showed that WSA had hydraulic properties because of the presence of hydraulic minerals: lime (CaO), mayenite (Ca12Al14O33) and alpha'-Ca2SiO4. The compressive strength of WSA-containing mortars was strongly penalized by the high water demand of the ash but this was not the case for the samples of treated soils. The results obtained on clayey soil treated by a binder optimized by the addition of 20 wt% of gypsum were better than those obtained with conventional commercial binders: their behavior was comparable in the long term but the optimized binder was more efficient in the short term. All the results obtained in this study permit us to conclude that WSA is suitable for use as the main component of HRB. This study is a good example of waste valorization: firstly, the calorific value of sludge produced by the paper recycling industry is recovered at the industrial plant and, secondly, the ash formed during this incineration process is reused as the main component of HRB with interesting added value.