The recycling of subway tunnel muck: a low alkalinity cementitious material for ecological concrete preparation

Abstract

Ecological concrete (EC) has received special attention recently, but its application has been largely limited by the high alkalinity caused by cement. In this study, a novel low alkalinity cementitious material (cementitious material developed from subway tunnel muck, S-CM) was developed from subway tunnel muck (STM). STM was activated by ball milling and the optimal time was 2 h. It was found that the solubilities of Si and Al of STM were enhanced by 1.74 and 2.36 times after mechanical activation. The leaching kinetics of Si and Al could be well described by Avrami model. The activated STM was a reactant in the cementitious system, and the higher Si and Al solubilities led to better gel generation. S-CM-2h exhibited a denser structure with lower porosity (34.88%), higher density (1.67 g/mL) and fewer macropores. The compressive strength of S-CM saw a notable increase from 4.06 to 10.14 MPa as a result of the denser microstructure and enhanced gel generation. Further study indicated that S-CM had much better environmental compatibility than sulphoaluminate cement in EC preparation. EC developed from S-CM had a 13.8% lower alkalinity, better root growth and a 47.5% higher dry weight of plants than EC developed from cement. This study provides a new approach for high value-added recycling of STM.

Graphical Abstract