Reuse of waste sandstone sludge via alkali activation in matrices of fly ash and metakaolin

This work aims at investigating the possibility of reusing sludge from sandstone sawing in alkaline cements. In particular, the behaviour towards alkali activation process of Pietra Serena sludge (sPS), either alone or in binary mixtures with class F fly ashes and metakaolin, was evaluated. The binders were synthesised with 8 M and 12 M NaOH solutions and cured at 85 °C for 5 or 20 h. Mechanical strength was determined after one day and the reaction products were characterized by XPRD, FTIR and SEM/EDX. Also colorimetry tests were performed. The alkali activation of sPS gives a material with limited amount of amorphous gel, in which unreacted particles can be still recognized, and showing a compressive strength of 3.6(2) MPa. The use of fly ashes and metakaolin in different proportions enhances mechanical strength up to 36(2) MPa. In these blends, the sludge acts basically like a filler. However, calcium carbonates present on the sludge partially dissolve and react in the alkaline mixture favouring the formation of some local high-calcium, high Si/Al ratio (C-A-S-H-type gel) areas co-existing with N-A-S-H-type gel.