Sustainable utilization of shield muck in geopolymer composites: insights into activation mechanisms and carbon emission reduction strategies
Kai Wang, Kai Cao, Shaoqi Zhang, Zhiguo Yan, Xi Jiang, Hehua Zhu
Abstract
Shield muck is a typical waste generated during the shield tunnel construction, posing a serious threat to local ecological safety and people's lives. However, due to its complex composition and poor engineering properties, its comprehensive utilization rate is still very low. In this paper, a composite activation approach was proposed to strengthen the cementitious reactivity of shield muck. An L 16 (4 5 ) orthogonal test was conducted to determine the optimum parameter configuration. Furthermore, a series of advanced characterization techniques were used to investigate the phase compositions, the phase transitions, and microstructure evolutions of composite activated shield muck-based geopolymer (CASMG). Results indicate that the reactivity of shield muck could be significantly enhanced by the proposed composite activation strategy, and the maximum compressive strength of CASMG pastes could reach 31.2 MPa. Appropriate mechanical and thermal activation can increase the chemical reaction area and the amorphous phase content of raw shield muck, thereby improving the efficiency of subsequent chemical activation. During chemical activation, the reaction products with a high degree of crosslinking, primarily N-A-S-H gels, can be quickly generated around shield muck particles and occupy the nearby pores. As the increase of the alkali content, the pore structure is refined, while the proportion of relatively stable Si-rich N-A-S-H rises significantly. The carbon emission and embodied energy of the CASMG are 51 % and 46 % lower than the OPC-32.5 respectively, which exhibits significant sustainability and cost-effectiveness. These findings provide valuable guidance for the efficient utilization of shield muck in the future.