Efflorescence occurred on the surface of mortar blocks in which sand produced in Sanuki City, Kagawa was employed. To investigate the efflorescence behavior caused by aggregates, bulk and surface compositions were analyzed for the effloresced sand and non-effloresced sand as a reference. Leaching behavior of relevant components related to the efflorescence was observed after immersing the sands in water, in Ca(OH)(2) slurry, which simulates the conditions in mortar, and in alkaline KOH solution. Soluble Na(2)SO(4) was leached from the effloresced sand in water; and in addition to Na(2)SO(4), Na salts originating from sodium silicate in silica were released in Ca(OH)(2) slurry and KOH solution. It seems likely that these Na salts react with Ca(OH)(2) in mortar to produce strongly alkaline NaOH, which facilitates absorption of CO(2) in the atmosphere to yield carbonates such as Na(2)CO(3), NaHCO(3), and CaCO(3), promoting the efflorescence on the mortar blocks. Gypsum (CaSO(4)), which was not detected in the efflorescence material on the mortar blocks, was leached from the non-effloresced sand immersed in water and the alkaline solutions.