In the West, Portland cement replaced magnesia for masonry uses in the 20th century when energy was cheap (see energy efficiency) and mold infection was poorly understood.
However, some projects continued to use magnesia. New York City’s Brooklyn Bridge base is made from locally mined cement, a mixture of calcium oxide and magnesia cement commonly called Rosendale Natural Cement, the only natural non-fired cement made in the US.
Magnesia cement boards were approved for construction use in the US around 2003.