Effects of setting regulators on the efficiency of an inorganic acid based alkali-free accelerator reacting with a Portland cement

Abstract

Today, in the field of underground constructions, alkali-free accelerators are commonly employed, during tunnel excavation, to allow flash concrete setting. In this way, the cementitious sprayed material can firmly bond to the tunnel walls, controlling the convergence (the tendency of the section to squeeze). Their efficiency may be related to many parameters like: cement type, setting regulator, concrete composition, working temperature. Nevertheless, the influence of such factors on the accelerator performance has not been clarified yet. The accelerator efficacy is evaluated by real spraying test in job site or, when only laboratory equipment are available, by measuring the final setting times of cement systems admixed with the accelerator. Several alkali-free flash setting admixtures are available on the market. They can be divided into two main categories both containing aluminium sulphate complexes stabilized either by inorganic acids or by organic acids. In this paper, the influence of different setting regulators on the performances of an inorganic acid based alkali-free accelerator was analysed. Portland cement samples were obtained by mixing clinker with gypsum, α-hemihydrate, β-hemihydrate or anhydrite. The setting regulator instantaneous dissolution rates were evaluated through conductivity measurements. The setting time of cement pastes with and without the accelerator was measured. It was found that the shorter the final setting time (therefore the more efficient is the accelerator) the lower the setting regulator instantaneous dissolution rate. In order to understand this phenomenon, a comparison was performed between accelerated cement paste samples containing the setting regulator with the highest (β-hemihydrate) and the lowest instantaneous dissolution rate (anhydrite). The analytical work included morphological (Environmental Scanning Electron Microscopy–Field Emission Gun — ESEM–FEG), crystal–chemical (X-Ray Powder Diffraction — XRD), physical–chemical (hydration temperature profile) and chemical (Induced Coupled Plasma–Atomic Emission Spectroscopy — ICP/AES) evaluations. The results revealed significant morphological differences among the investigated samples.