A study of carbonation in stabilised peat
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In addition to the traditional drivers of cost and timely delivery, embodied energy (EE) and embodied carbon (EC) have emerged as major considerations in all aspects of large construction projects. This is due to stricter environmental regulations introduced in Ireland and the European Union, which have resulted in geotechnical engineers beginning to compare the EC/EE associated with various piling and ground improvement options as part of an overall appraisal of scheme feasibility for road construction projects. Where construction involves the modification or removal of peat, these calculations become more challenging as allowances should be made for the impact on the carbon stored within the peat and the gases potentially released. To incorporate these allowances in the EE and EC summations in a life cycle assessment (LCA) for a construction project requires the inclusion of peat-related factors such as peat drainage, drainage systems, slope stability, restoration and clearance of vegetation/forest. As there is a dearth of methodologies to quantify EC and EE of ground improvement techniques for road construction on peat, new LCA methodologies need to be developed to more accurately quantify emissions and energy consumed in order to produce more sustainable solutions. Traditionally, excavate-and-replace is the most commonly used method in Ireland, but other techniques such as piling and dry soil-mixing can also be considered. In projects employing an excavate-and-replace solution, peat is often removed to disposal areas and the subsequent drying causes a significant release of CO2 into the atmosphere. To date, however, in dry soil-mixing the impact of stabilising peat on on-site CO2 emissions has not been considered and is, therefore, the primary focus of this thesis. A preliminary study of nine columns containing peat and peat mixed with cement was set up in a temperature control room to investigate CO2 emission rates. Gas samples were collected using the closed chamber method and analysed using a gas chromatograph. The result of the experiment showed that the stabilised peat absorbed CO2 both from the air and CO2 released by the oxidised peat due to the carbonation process. Subsequent to this substantial finding, two extensive column studies were undertaken to examine the carbonation potential of stabilised peat whose binders consisted of cement and cement and ground granulated blast slag (GGBS) combinations. As in the first experiment, the CO2 intake rate decreased with time as the depth of the carbonation front grew. Variables which significantly influenced the CO2 intake rate included time, initial CO2 concentration, cement content, GGBS content, water table and surcharge. Besides calculating CO2 intake rates, the depth of the carbonation front was analysed to produce k-rate factors, representing the rate at which the carbonation front increases. As this was the first time the carbonation front in stabilised peat was examined, several techniques were used to establish the most suitable procedure to explore this carbonation front: phenolphthalein indicator, pH of stabilised peat slurries, X-ray powder diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), loss on ignition method (LOI), and water evaporation method. From the results, the carbonation front obtained by XRD was similar to those from LOI and FTIR. In contrast, the phenolphthalein method underestimated the carbonation front, while the pH of stabilised peat slurries technique showed a carbonation front greater than the other methods. The laboratory results provided crucial information on the environmental impact of dry soil-mixing and its on-site impact. Using these results and a novel LCA methodology, an LCA to examine EE and EC was performed on a case study of a section of motorway built on peat. To ascertain the optimum solution for EE and EC, several ground improvement scenarios were studied, including excavate-and-replace, dry soil-mixing and a combination of excavate-and-replace and piling. From laboratory results, it was taken for the dry soil-mixing scenario that emissions from the stabilised peat were zero for the lifetime of the road. The LCA also investigated emissions from peat under various management practices and restoration techniques, assessing their strength in terms of hydrology and carbon storage potential. The results showed that dry soil-mixing had a minimal on-site impact and that the EC of the binder could be lowered markedly if the binder type was changed. On the other hand, even though the EC and EE of materials used in the excavate-and-replace solution were low, the on-site impact was immense due to extensive peat disturbance. In the case study which forms part of the subject of this research, a combination of excavate-and-replace and piling was found to be most favourable solution, but the preferred ground improvement technique in general was found to change according to the project and site location. In this research, novel laboratory studies were undertaken and a new LCA methodology was developed. Various techniques to understand the carbonation process in stabilised peat were utilised, with advantages and disadvantages found for all of them. The body of work has international implications as it points to dry soil-mixing having a lower environmental impact than previously thought. Additionally, it provides a platform for further studies in this area. From this research, the environmental impact of building on peat can be quantified more accurately and the understanding of stabilised peat enhanced in terms of emissions and carbonation depth and potential.
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