Coupled thermo-hydro-mechanical–chemical behaviour of cemented paste backfill in column experiments: Part II: Mechanical, chemical and microstructural processes and characteristics

• Paste backfill is subjected to strong coupled thermo-hydro-mechanical–chemical processes.
• High column experiments have been carried out to understand these THMC processes.
• The mechanical, chemical and microstructural processes significantly influence the behavior of CPB
• The findings can contribute to a better understanding of the behavior and the design of CPB.

The evolution of coupled thermal (T), hydraulic (H), mechanical (M) and chemical (C) (THMC) properties of underground cemented paste backfill (CPB) has been studied by means of experiments with insulated–undrained high columns. This paper which compliments Part I (Ghirian and Fall, 2013), presents the mechanical, chemical and microstructural processes and characteristics. Two columns have been built and filled with a specific CPB mix and equipped with different sensors. The vertical deformation and drying shrinkage for a period of 150 days are measured. Also, four other columns are cured for 7, 28, 90 and 150 days and then small samples are taken out from the columns to investigate the evolution of unconfined compressive strength (UCS), shear strength parameters, microstructural properties, and pore fluid chemistry. The combined results of the two part experiment show that strongly coupled THMC processes control CPB behavior. This study has demonstrated that mechanical properties are coupled to chemical reactions due to cement hydration and temperature changes inside the columns. Also, suction development due to self-desiccation can significantly increase the uniaxial compressive strength values with time. Chemical analysis including ion concentration measurements with time has revealed that change in pore fluid concentration affects the refinement of pore voids, and thereby results in improvement in hydro-mechanical performance as well as microstructural evolution of the CPB. External environmental loading, such as surface evaporation, can affect the durability performance of CPB structures. The results show that there is degradation of strength following surface shrinkage as well as an increase in saturated hydraulic conductivity due to existence of micro-cracks. The obtained results support that coupled THMC effects should be taken into account in field conditions to understand CPB behavior where stronger interplay reactions take place.