The blast furnace slags are a by-product of the production process of cast iron, during which large amounts of liquid slags are formed. The composition of the blast furnace slags depends on the actual quality and proportion of the minerals and fluxes present in the blast furnace charge. Every year the steel industry in Europe produces 2900 tons of slags that, if left untreated, represent an industrial waste to be sent to landfills with serious environmental impact. In the RESLAG (Turning waste from steel industry into valuable low cost feedstock for energy intensive industry) project, the use of waste products deriving from iron and steel plants as new feedstock in different fields is considered: recovery of precious metals, thermal energy storage systems for steel-making and CSP industry, production of innovative refractory ceramic compounds. Within this framework, ENEA is investigating the possibility of using pebbles made with slags produced by the steel industry as a filler in high-temperature packed-bed thermocline TES systems, using a binary mixture of molten salts (60% NaNO3 40% KNO3) as HTF. More specifically, the pebbles are obtained by processing and sintering Electric Arc Furnace (EAF) slags produced during the manufacture of crude steel. Here, the lessons learned and the first experimental results collected during the commissioning phases of the PBTTES pilot plant developed within the RESLAG project are reported. In particular, the paper firstly reports about a preliminary set of tests carried out to check the chemical compatibility of the slags with the molten salts. Subsequently, the pilot plant is described and the results of the first commissioning tests, which were aimed at flooding the packed-bed with molten salts and checking the installed instrumentation, are reported.