Assessment of the thermal and microstructural properties of metakaolin-air lime based materials at an early age

Abstract

Hydraulic lime is an ancient building repair material, and its good early performance is necessary to ensure a successful application. In this study, hydraulic lime (metakaolin (MK)-air lime (AL), MK-AL) were prepared by the amount of MK was used for different mixtures in AL, which provided the materials with both hydraulicity and the ability to air harden. The early-age (before 28 days) properties of natural hydraulic lime (NHL5), AL and MK-AL (MK20-AL and MK40-AL) pastes were studied through an examination of their physical and mechanical properties, hydration heat, thermal properties and microstructure. The strength growth trend of the MK-AL paste was similar to that of the NHL5 paste, and the compressive strength of the MK40-AL paste reached 21.06 MPa at 28 days. There was only a carbonation reaction in the AL paste, C-S-H formed mainly by the hydration reaction of C2S in the NHL5 paste, the pozzolanic reaction and carbonation reaction occurred in the MK-AL pastes, and the carbonation reaction in the MK-AL pastes was at a low level before 28 days. The MK-AL pastes had a longer hydration stage than the NHL5 paste, and there were two exothermic hydration peaks during the pozzolanic reaction for the MK-AL pastes. Laminar Ca(OH)2 and amorphous C-S-H gel comprised the skeleton in the NHL5 paste, and the large laminar Ca(OH)2 structures that formed the skeleton were converted into an interweaved structure of the pozzolanic reaction products and carbonated products in the MK-AL pastes. There were many large pores (>200 nm) in the AL, NHL and MK20-AL pastes, while the pore size of the MK40-AL paste was relatively small (<100 nm). The porosity of all lime pastes had the following trend: NHL (38.96%) < MK40-AL (42.89%) < MK20-AL (45.47%) < AL (51.22%).