This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Concrete may be made with selenitic cement mortar as the matrix. Portland cement is sometimes added in small quantities to the selenitic cement. From a series of experiments it appears that a mixture of 1 part Portland, 4 of selenitic cement, and 25 of sand, was if anything superior to the same Portland used with 4 of sand. The directions for preparing the concrete are as follows: 4 full-sized pails of water, 2 bush, prepared selenitic lime, 2 bush. clean sand. These ingredients are mixed in the edge-runner or tub, and then turned over 2 or 3 times on the gauging-floor, to ensure thorough mixing with 12 or 14 bush. ballast. When the tub is used, the sand will be first mixed dry with the ballast, and the lime poured into it from the tub and thoroughly mixed on the gauging-floor. An addition of 1/6 best Portland cement will be found to improve the setting.