Environmental monitoring and rock-property investigations are performed to explain the mechanism of deterioration observed at two buildings constructed in the medieval age and the modern age at Orval Abbey (Belgium). The medieval building is composed of two limestones, while the modern building is composed of reconstituted stone agglutinated using cement containing crushed natural limestones. Deterioration due to salt efflorescence is observed only at the ground floor wall of the modern building. Four measuring sites are set up at these buildings to monitor air temperature and relative humidity. Moisture content and Equotip hardness are measured on a wall at each site. Salt at each site, soils around buildings, underground river water, and three types of stone are also sampled for further laboratory investigation. From the XRD analysis, only calcite (CaCO₃) is detected from salts at sites of the medieval building, whereas calcite, thenardite (Na₂SO₄), mirabilite (Na₂SO₄·10H₂O), and gypsum (CaSO₄·2H₂O) are detected at sites of the modern building. The source of Na and S in these salts is underground river water, not the reconstituted stone. Therefore, crystallization of sodium sulfates from constituents of the river water is considered to be the main cause of the deterioration of the modern building wall.