When I had the same question regarding sidewalks for my WWI era mining town set in the western U.S., a quick trip to the library answered my questions. As I learned, concrete sidewalks were pretty standard even in young mining towns by 1900, although they did not have the same construction methods as today. It looks like the sidewalks were made to different dimensions and of diffferent materials for individual buildings. In some photos i found, it looks like some buildings still had wooden sidewalks in 1900, while the adjacent building's sidewalk was concrete.
kevin