In coal-fired power plants, selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are frequently used to reduce NOx emissions. Liquid ammonia (NH3) is injected into the system in the SCR and SNCR processes, where it mixes with NOx in the flue gas to produce diatomic nitrogen (N2) and water (H2O). Ammonia slip, or unreacted ammonia in the flue gas left over from the NOx reduction process contaminated the fly ash and produced fly ash tainted with ammonia. The two most common forms of ammonia contamination are ammonium sulfate (NH4)2SO4 and ammonium carbonate (NH4)2CO3. Properties of fly ash are also affected by changes in coal properties. At the Mae Moh power plant, the CaO content in fly ash is increasing and the SiO2 content is decreasing. The decrease in SiO2 may affect the pozzolanic behavior of the fly ash. This work aims to examine how pastes and mortars’ characteristics are affected when very high CaO fly ash (CaO = 29.82%) from Mae Moh Power Plant is contaminated with ammonia. The tested properties in this study are normal consistency, autoclave expansion, mortar flow, compressive strength, autogenous shrinkage, and total shrinkage. The results showed that (NH4)2SO4 and (NH4)2CO3 decreased the normal consistency and produced fresh pastes using less water. The mortars’ compressive strength decreased with (NH4)2SO4. However, (NH4)2CO3 increased the mortars’ compressive strength. The addition of (NH4)2SO4 or (NH4)2CO3 increased the autoclave expansion of pastes. The paste and mortar specimens were thought to expand due to the production of gypsum and ettringite. Autogenous shrinkage and total shrinkage of mortars were both reduced by (NH4)2SO4 and (NH4)2CO3.