"Effects of High Accelerator Dosages on the Physical, Chemical, and Morphological Properties of a Hydrating Portland Cement Paste"

Abstract

This paper describes how alkali-free flash setting accelerators are widely used in the area of underground construction as they allow a very rapid hardening of cementitious systems. These chemicals are commonly intermixed with an air flow into a fluid concrete which is rapidly projected on the rock wall to form a consolidating shield of variable thickness. The typical accelerator dosages range between 6 and 8 % by cement mass. Nevertheless, sometimes, due to concrete fluidity loss, low air flow pressure, or poor homogenisation of the concrete/accelerator system, a very high local accelerator concentration could result in the sprayed mixture. Practical experiences point out that these conditions could cause significant strength loss of the sprayed layer, thus compromising the safety of the tunneling operations. In order to elucidate the influence of high accelerator concentrations on hydrating cement, setting time and mechanical strength development of cement paste samples, admixed with several dosages of different alkali-free accelerators, were measured. Furthermore morphological (ESEM-FEG) and crystal-chemical (TG/DSC, XRD) analyses were also performed to clarify the reasons responsible for these reduced mechanical strengths.