Depth dependence of hardness and reaction in metakaolin‐based geopolymers cured at low humidity

Abstract Compared to metakaolin‐based sodium and potassium geopolymers cured at 20°C/100% relative humidity, geopolymers, which were cured at 20°C/40% relative humidity, were found to have considerable changes in the nanoscale structure and physical properties near the surface due to dehydration tha...

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Bibliographic Details
Published in:Journal of the American Ceramic Society 2024-01, Vol.107 (1), p.543-560
Main Authors: Samuel, Devon, Kriven, Waltraud M.
Format: Article
Language:English
Subjects:
Balancing
Cations
Dehydration
Exposure
Geopolymers
Humidity
Metakaolin
Microhardness
Moisture content
NMR spectroscopy
Physical properties
Potassium
Relative humidity
Sodium
Three dimensional printing
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Summary:Abstract Compared to metakaolin‐based sodium and potassium geopolymers cured at 20°C/100% relative humidity, geopolymers, which were cured at 20°C/40% relative humidity, were found to have considerable changes in the nanoscale structure and physical properties near the surface due to dehydration that occurred before completely hardening. For low‐humidity samples, the density in a thin near‐surface region was higher than the bulk, while porosity and BET specific surface area were greatly diminished. The microhardness at the exposed surface was low but increased sharply within a small depth before decreasing with depth to a constant value, which depended on the initial water content and charge‐balancing cation. 29 Si NMR spectroscopy showed that the extent of reaction also varied with distance from the exposed surface. Both the elevated microhardness and incomplete reaction were detected at distances of up to 1.5 cm from the exposed surface, indicating that premature dehydration can cause microstructural changes over length scales that may be significant in large, ambiently cured 3D‐printed geopolymers. These effects are reduced by the use of sodium rather than potassium as the charge‐balancing cation and by raising the initial water content of the geopolymer, although higher water content caused greater surface deformation during drying.
ISSN:0002-7820
1551-2916
DOI:10.1111/jace.19450