When oil and gas wells reach the end of their production life, they need to be permanently plugged and abandoned. Due to the maturity of the fields in the North Sea, in the next decades many of the wells will have to be permanently plugged and abandoned therefore, Plug and Abandonment (P&A) has become a major focus in the petroleum industry. P&A is the last phase of the life cycle of a well and hence no return of capital from it is expected. Furthermore, the responsibility of the operators for the abandoned well lasts long after the wellbore has been plugged, therefore an efficient plugging procedure will be the main goal for any permanent abandonment. The requirements for a permanent barrier state that it must cover the entire cross-section of the wellbore, including all annuli, sealing permanently that region avoiding any fluid leakages through the barrier itself. This thesis evaluates fly-ash based geopolymers as an alternative sealant material for establishing a cross-sectional barrier instead of the commonly used Portland cement which presents some weaknesses when exposed to well conditions, mainly high temperatures and corrosive environments. In order to investigate the reliability of fly-ash based geopolymers, a series of experiments were conducted using fly ash-based geopolymers which were produced by the alkali-activation of fly ash. Several different recipes were prepared using different molarities and activators. Uniaxial Compressive Strength tests were performed to compare the results for each recipe and those results showed that higher concentrations of alkali solutions developed higher compressive strength. In addition, durability tests were performed to evaluate the resistance of geopolymers in corrosive environments. In this case, a solution of 15% HCl was used and geopolymers revealed a better resistance when in contact with this corrosive substance. The promising results showed that there is a great potential in geopolymers usage. Moreover, if geopolymers make less of an impact to the environment, geopolymers might be regarded as a good alternative for the industry.