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Autogenous self-healing of post-fire damaged concrete enables structure performance auto-
recovery leading to reduced repair costs, less generated waste, and lower CO2 emissions. In
this paper, to improve the efficiency of this process and understand the underlying mechanism,
the self-healing of 0.1 wt% MWCNT-modified and pure cement paste subjected to novel envi-
ronmental stimulators was tested. High-temperature damage was induced at 200 ◦C and 400 ◦C,
followed by a self-healing cyclic treatment with water, a mixture of water with phosphate-based
retarding admixture, and limewater. The self-healing efficiency of the proposed solutions were
compared based on crack closure, strength regains, porosity, and chemical composition changes.
The surface crack closure after 200 ◦C varied between 33% and 60%, whereas for 400 ◦C, only
retarding admixture exposure obtained over 50% crack closure and the most considerable
decrease in average crack width of 33% for MWCNT-modified paste. The highest values of
compressive strength recovery, equal to 18% and 14%, exceeding the intact specimen’s
compressive strength, were observed for the MWCNT-modified paste healed in water and lime-
water. Water exposure with an extended wetting phase enhanced the compressive strength re-
covery of the MWCNT-modified materials. Strong (r = 0.87) and moderate (r = 0.52) positive
correlations were observed between temperature loading and compressive and flexural strength
recovery parameters, respectively. Higher porosity and interconnected crack network, caused by
high temperature, facilitated the self-healing process. Porosity changes before and after healing
were pronounced in contrast to the amount of unhydrated cement, which did not exhibit
noticeable changes. The healing mechanism included three processes: calcite formation, further
hydration inside the cracks, and rehydration of the bulk cement paste.
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