Hello. This is a normal symptom of oversaturation. The mortar contains a high proportion of Ca(OH)2, the pore solutions are strongly alkaline, and with increasing partial pressure of CO2 (increasing CO32-), the supersaturation of Ca carbonate solutions increases. Ostwald's laws say that something with the lowest activation energy in the system will be precipitated and that something will further recrystallize (most likely ACC -> vaterite -> aragonite/calcite).