Page:Popular Science Monthly Volume 60.djvu/165

Rh is performed quickly and is quickly arrested when the process is completed.

Experience has proved that the burning of cement is an important factor in its manufacture as determining its qualities. A cement that is properly proportioned, thoroughly mixed and well burned and finely ground will set in about two hours after mixing with water, will harden well after setting and will continue to harden through long periods, even to several years. Ground gypsum is frequently added to cement to control the setting.

Cement should be finely ground. Ninety-two to ninety-three per cent, should pass a sieve of 100 meshes to the linear inch.

Until within the last decade, the Portland cements manufactured in the United States were generally inferior to the best English, German, Belgian and French brands. While these foreign cements have been maintained with the highest degree of excellence the American brands have been greatly improved until at the present time there are no better cements made anywhere in the world than in the United States.