Hydration of a silica fume blended low-alkali shotcrete cement

• Published in: Physics and chemistry of the earth. Parts A/B/C 2014, Vol.70-71, p.3-16
• Main Authors: Lothenbach, Barbara, Rentsch, Daniel, Wieland, Erich
• Format: Article
• Language: English
• Subjects: Low pH cement; Pore solution; Set accelerator; Supplementary cementitious material; Thermodynamic modelling
• ISSN: 1474-7065; 1873-5193
• DOI: 10.1016/j.pce.2013.09.007

•Rate of silica fume reaction in Portland cements decreases with time due to the drop in pH.•40% Silica fume destabilises portlandite and monocarbonate, decalcifies C–S–H and lowers the pH to 11–11.5.•Aluminium sulphate based accelerator leads to massive ettringite formation and a moderate decrease of pH.•C–S–H and thaumasite expected to be the stable hydrates in the long-term. Ettringite and C–S–H are the main hydrates formed during the hydration of the low-alkali cement “ESDRED” consisting of 60% CEM I, 40% microsilica and 4.8% set accelerator. Small quantities of portlandite and hemicarbonate present as intermediate phases destabilise within a few weeks. The use of a set accelerator leads to massive ettringite precipitation, a moderate decalcification of C–S–H and reduction of pH due to presence of dissolved formate. The slow reaction of the silica fume during hydration decalcifies the C–S–H and decreases the alkali concentration to 30mM and the pH value of the pore solution to 11.5 after 1year and longer. The further reaction of the silica fume is expected to be slow and to result in a decrease of pH to 11. Further, the destabilisation of ettringite to thaumasite is expected. The long-term stability of C–S–H and the pH of approximately 11 make ESDRED a good candidate for usage in contact with the clay-based barriers of a repository for radioactive waste.