Long-term mechanical properties of lime-pozzolan mortars: The role of amorphous Al₂O₃

¹ Brno University of Technology, Faculty of Civil Engineering, Veveří 331/95, 602 00 Brno, Czech Republic
² Kielce University of Technology, Faculty of Civil Engineering and Architecture, al. Tysiąclecia Państwa Polskiego 7, 25-314 Kielce, Poland

^* Corresponding author: martin.vysvaril@vut.cz

Abstract

Today, lime-pozzolan mortars are being used in the repair of historic buildings, replacing the lime-cement mortars that were previously used in this practice, but were incompatible with historic mortars. It has been observed in laboratory research and also in practice that some lime-pozzolan mortars can lose strength over time. The cause of this behaviour is not fully understood, but the main reason seems to be the instability and breakdown of the aluminate phases. In this paper, the role of amorphous Al₂O₃ content in pozzolans on the long-term mechanical properties of lime-pozzolan mortars is studied. Lime-pozzolan mortars with 40% replacement of lime with pozzolans with different contents of amorphous Al₂O₃ (metakaolin, pumice, pozzolanic earth, trass, zeolite) were prepared and the changes in strength, porosity and mineralogical composition of the mortars (determined by XRD and TG/DTA) over time were monitored. Mortars containing pozzolans with a high content of amorphous Al₂O₃ (over 30% by weight) showed a dramatic decrease in strength, changes in porosity and a decrease in the content of monocarboaluminate (C₄AcH₁₁) with a simultaneous increase in the content of vaterite and aragonite. The negative effects of carbonation of the aluminate phases are particularly evident in pozzolans with a high content of amorphous Al₂O₃.