This research studies the green geopolymer incorporated with phase change material (PCM) for energy saving in buildings. First class F fly ash (FA) based-geopolymer binder was studied. In order to improve the mechanical properties, low calcium slag (SG) was added to the FA to produce geopolymer. The effect of different factors including SG content (at different relative amounts FA/SG = 0/100, 25/75, 50/50, 75/25 and 100/0), NaOH solution at different concentrations (7.5, 10 and 15 M), various curing times (1, 2, 4, 7, 14 and 28 days) and curing temperatures (25 (ambient), 45, 60, 75 and 90°C) was investigated. The unit weight and uniaxial compressive strength (UCS) of the geopolymer specimens were measured. Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffraction (XRD) were also performed to characterize the microstructure and phase composition of the geopolymer specimens. The results show that the incorporation of SG not only improves the strength of the geopolymer specimens but also decreases the initial water content and thus the NaOH consumption at the same NaOH concentration required for geopolymer production. In addition, the inclusion of SG increases the unit weight of the geopolymer specimens, simply because SG has a much greater specific gravity than FA. The results also show that the strength of the FA/SG-based geopolymer develops rapidly within only 2 days and no obvious gain of the strength after 7 days. The optimum curing temperature (the curing temperature at which the maximum UCS is obtained) at a FA/SG ratio of 50/50 is around 75°C. Second, FA-based geopolymer concrete was synthesized and the effect of different factors including sodium silicate to sodium hydroxide (SS/SH) ratio, aggregate shape, water to fly ash (W/FA) ratio, curing time, water exposure and PCM inclusion on the compressive strength of the geopolymer concrete specimens cured at different ambient temperatures was studied. The results show that the UCS of the specimens increases with higher SS/SH and W/FA ratios up to a certain level and then starts to decrease at higher ratios. The results also indicate that a major portion of the strength of the specimens cured at ambient temperatures develops within the first four weeks. In addition the strength of the FA-based geopolymer concrete is slightly decreased with water exposure and PCM incorporation. Third, the mechanical and thermal properties of geopolymer mortar synthesized with FA and different amount of PCM were studied and the effect of incorporated PCM on the unit weight and UCS of geopolymer mortar was evaluated. SEM imaging was performed to identify the change of micro structure of the geopolymer mortar after incorporation of PCM. The thermal properties of the geopolymer mortar containing different amount of PCM were also characterized using differential scanning calorimetry (DSC) analysis. In addition model tests were performed using small cubicles built with geopolymer mortar slabs containing different amount of PCM to evaluate the effectiveness of geopolymer mortar wall with incorporated PCM in controlling the heat flow and internal temperature. The results indicate that both the unit weight and UCS of the geopolymer mortar decrease slightly after PCM is incorporated, mainly due to the small unit weight and low strength and stiffness of the PCM, respectively. However, the compressive strength of geopolymer mortar containing up to 20% PCM is still sufficiently high for applications in buildings. The results also show that the incorporation of PCM leads to substantial increase of heat capacity and decrease of thermal conductivity of the geopolymer mortar and is very effective in decreasing the temperature inside the cubicles. Finally, a numerical study on the thermal performance of geopolymer with incorporated PCM was carried out. In order to simulate the heat transfer through geopolymer containing PCM, a simplified method was first presented. The influence of phase transition was linked to the energy balance equation through variable specific heat capacity of the PCM-geopolymer. The thermal properties of the geopolymer containing PCM for the numerical analysis were determined using DSC and guarded heat flow (GHF) tests. The simplified method was validated based on the good agreement between the numerical and experimental results. With the validated model, the effect of various factors including the specific heat capacity, thermal conductivity and wall thickness on the thermal performance of PCM-geopolymer walls was studied. Then a modified numerical method was proposed for simulating the whole thermal transfer processes and the simulation results were used to conduct the economic evaluation of PCM-geopolymer walls for energy savings in buildings.