Among the technological routes for recycling mining and beneficiation slate wastes, thermal expansion seems particularly attractive, yielding products adequate for use in civil construction, especially as pozzolanic material for cement manufacture. The present investigation aims at analysing the properties of slate before and after the thermal expansion, searching for the reasons of expansion and the variables that affect the characteristics and properties of the products. Results of standard test procedures recommended by the construction industry are presented and discussed. (C) 2005 Elsevier Ltd. All rights reserved.