Abstract
In-situ carbonation has been developed as a novel cement mineralisation and carbon dioxide (CO2) capture technique, which adopts porous materials to adsorb, desorb, and therewith permanently sequester CO2 in hardened Portland cement composites. However, there are limited systematic studies on validating its viability, illuminating its reaction mechanism, scrutinising its effect on durability, and defining its environmental acceptability.This thesis explores the utilisation of an in-situ carbonation technique based on CO2-impregnated faujasite, LTA 5A zeolite, and LTA 4A zeolite, with the addition of plain zeolites as references. Results show that incorporating 3─12 wt% plain zeolites significantly increases 1-day compressive strength due to increased hydration products originating from the hydration acceleration effect and rapid secondary hydration of the zeolites. The addition of 3─12 wt% plain zeolites fails to significantly raise 7- and 28-day strength, mainly attributed to reduced workability along with introduced weak spots and porosity. Compared to using the plain zeolites, in-situ carbonation can significantly enhance 1-, 7-, and 28-day compressive strength. Results indicate that in-situ carbonation can reduce porosity, facilitate hydration product formation, and improve the mean chain length of C-(A)-S-H gel, with calcium carbonate content increasing by 1.9, 5.3, and 4.8 wt% for the mixtures containing 12 wt% faujasite, LTA 5A zeolite, and LTA 4A zeolite, respectively. To wrap up, LTA 5A zeolite is the most suitable CO2 carrier among the zeolites, considering strength enhancements and CO2 sequestration volumes.
This thesis subsequently adopts LTA 5A zeolite as a CO2 carrier to investigate the effect of in-situ carbonation on the resistance against external sodium sulphate attack. Results indicate that the addition of 10 wt% plain LTA 5A zeolite jeopardises sulphate resistance. This is attributed to an increase in moisture accessibility, exacerbating the cracking of mortar. In contrast, mortar with 10 wt% CO2-impregnated LTA 5A zeolite exhibit enhanced sulphate resistance (i.e., lower strength loss) due to a lowered pH value and increased initial contents of alumina-ferric oxide-tri (AFt) and mono-carboaluminate (Mc).
To depict the big picture on environmental sustainability, this thesis employs the ReCiPe midpoint and endpoint approaches to quantitatively determine the environmental impact potentials of in-situ carbonation. Results show that adding synthetic zeolites significantly increases all midpoint impacts, while CO2 impregnation further marginally increases the impacts. In sensitivity analysis, even if 30% of material and energy consumption is reduced during zeolite production, the endpoint impact values of mortar with zeolites remain much higher than those of the control mix. This indicates that zeolite-production-induced environmental burden is a critical constraint towards practically applying in-situ carbonation.
To conclude, this thesis pinpoints three synthetic zeolites to verify the viability of using synthetic zeolite as an in-situ carbonation adsorbent and elucidate mechanisms behind changes in compressive strength and sulphate resistance. Although this technique shows deficiencies as to environmental impact, the findings of the thesis still count as a foundation in the carbonation field and enlighten researchers to seek for new suitable adsorbents.
| Date of Award | 15 Jul 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Cheng Heng Pang (Supervisor), Kien Woh Kow (Supervisor), Bo Li (Supervisor) & Edward Lester (Supervisor) |