Effect of Binder Content on the Carbon Sequestration Capacity of Stabilized Soft Clays, Master´s Thesis Beyza Asar

Deep mixing method (DMM) is one of the most used methods to stabilize soft clay in Finland. DMM is a process that involves combining soil at its inherent moisture content with stabilizing agents such as cement and lime. Nevertheless, it can be noted that both lime and cement have a high carbon footprint. This has a negative effect on the environment and ecosystem. To deal with this problem, Aalto University has been studying the effect of carbonation on the reaction of alternative binders.

The main objective of this research is to assess how the amount of binder and carbonation duration affects the ability of stabilized soft clays to carbon sequestration capacity. Four different binders were used in laboratory for stabilizing soft clay from Malmi area of Helsinki, Finland. These binders are GTC (a mixture of gypsum, slaked lime, and cement type I) provided from Nordkalk Oy, CEMI (cement type I), CEMIII (cement type III) sourced from Finnsementti Oy, and UPM (fly ash) obtained from UPM-Kymmene Oy + CEMII (cement type II) sourced from also Finnsementti Oy.

Unconfined compression strength (UCS) tests and thermogravimetric analysis (TGA) were performed. The aim was to observe the effect of carbonation on the strength of the soft clay based on UCS tests. In addition, TGA was used to determine temperature range that weight loss of the soft clay occurs. The amount of carbon dioxide that was sequestered can be established by measuring the weight reduction that occurs during TGA.

CEMI binder was the most effective binder on the strength of the soft clays with 18% and 10% increasing strength due to carbonation for 150 kg/m3 and 175 kg/m3, respectively. In addition, CEMI binder had the second highest amount of carbon dioxide (CO2) absorption. However, the highest amount of net CO2 was found for CEMI due to CO2 emissions from manufacturing. GTC binder absorbed the most amount of CO2 with between 12 and 17 kg CO2 eq. e. /m3 compared to other binders, and the amount of CO2 emissions from manufacturing of it was the least. Nevertheless, GTC binder did not show any positive effect on strength of the soft clay due to carbonation. The amount of weight loss of the samples increased by carbonation for all binder types, and it was seen that the amount of weight loss of the samples increased by rising binder content of the samples, except UPM+CEMII binder. Based on the UCS tests results, it was observed that higher amounts of binder, particularly at concentrations of 150 kg/m3 and 175 kg/m3, positively impacted the strength of the stabilized soft clays, except GTC binder.