The present study investigated the effects of utilizing the treated flue gas desulfurization (FGD) gypsum on the improvement of properties of high volume fly ash (FA) cement pastes. The FGD gypsum was subjected to heat treatment at 150 °C in a kiln to change its compositions and convert into β-hemihydrate gypsum. The ordinary Portland cement (OPC) was partially used in the FA-cement paste at two different levels: 50 wt% for Set I mixture and 30 wt% for Set II mixture. The β-hemihydrate was added to partially replace the FA with 3, 5, and 10 wt%, respectively, for both Sets. Experimental works, including workability, setting time, compressive strength, dynamic moduli, and microstructural analysis by scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests were conducted. The workability of fresh blended pastes was found to be decreased with the increase of β-hemihydrate amount, while the compressive strength, dynamic moduli and thermal conductivity were improved. Lower amount of β-hemihydrate was found to be more effective in improving the early-age strength, while higher amount of β-hemihydrate had more effect on enhancing the later-age strength. Based on the microstructural analysis, the addition of β-hemihydrate increased the sulfate compound in the binder and in turn increased the formation of ettringite.