The development of building materials for civil construction with good thermal performance is essential, mainly in tropical climate countries such as Brazil. A material with growing interest is the alkali-activated binder (AAB), which has been seen as an alternative to Portland cement with lower carbon footprint. However, thermal performance of AAB-based products under natural exposure to natural sunlight is still unknown. This paper evaluates the thermal performance of an alkali-activated mortar (AAM) exposed to the environment, comparing its results with a conventional Portland cement mortar (PCM). The AAB was comprised of rice husk ash and metakaolin as precursors, and NaOH as the activator. Mortar boards were produced and exposed to sun under the same environmental conditions. Thermocouples and infrared thermal equipment registered the temperature of the materials. Thereafter, the boards were white painted to evaluate the color influence. The results showed that mortar board thickness influences the thermal performance – the higher the thickness, the lower the temperature registered. Unpainted board results showed that AAM heated more than PCM ones, what is related to the dark color of material. However, painted board results showed that AAM had a similar or better performance than PCM. The use AAM as an alternative material to PCM can provide material with similar thermal performance when the board is previously white painted.