Effect of NaOH molarity on the formation of hybrid cements from sulfate-bearing clay and Pietra Serena sludge

•Hybrid alkaline cements are prepared from sulfate-bearing clay and Pietra Serena.•Different alkalinities of NaOH solution were used (4,6 and 8 M).•CaO from sludge takes part in alkali reaction to form hybrid cement.•SO42- from clay can be trapped by ettringite, U-phase and SO42- bearing zeolites.