Characterisation of slag blends and correlation with their sulphate resistance in static and semi-dynamic conditions

摘要

Sustainability requirements increasingly impose the use of Supplementary Cementitious Materials (SCMs) to partly substitute Portland Cement (PC) in concrete formulations. For an optimised use, a better understanding of the mechanisms contributing to concrete performance is necessary, especially in terms of durability. Blast furnace slags constitute one of the most important and efficient types of SCMs but are particularly rich in Al2O3 and its role in sulphate attack processes deserves special attention. In this work, slag was blended with PC at levels of replacement of 40% and 70%. 3-month hydrated mixes were characterised by Quantitative X-ray Diffraction (QXRD) and Backscattered Electron Image Analyses (BSE-IA) to obtain their phase assemblages. The sulphate resistance of their corresponding mortars was tested for up to 24 months following two different modes of operation: without renewing the sulphate solution (static mode), and by frequently renewing it (semidynamic mode). The concentration of all sulphate solutions was fixed at 3.0 g L-1 of Na2SO4 with the aim of being close to realistic field conditions. To accelerate the penetration of sulphates into the microstructure, the surfaces of the prisms were cut off prior to sulphate exposure. A major difference between the modes of testing was the pH to which samples were subject, which appeared to have an important influence on their behaviour. In the static mode, where pH was kept steady above 11.5 for most of the time, the tests were significantly shortened with respect to those conducted in semi-dynamic mode, where pH periodically ranged between 8.0 and 9.5. A fundamental finding in both modes and for both levels of replacement was the detection of secondary monosulphoaluminate forming from reaction of the slag with external sulphate. The identification of this intermediate hydrate contributes to understanding the global mechanism of sulphate attack in slag blended cements and, particularly, can justify the correlation between the availability of Al2O3 after curing and the final sulphate resistance. The anhydrous slag at the time of beginning the test plays an important role in the longterm behaviour in sulphate solutions.