This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The white deposit found on the surface of concrete, brick, and stone masonry is called efflorescence. It is caused by the leaching of certain lime compounds, which are deposited on the surface by the evaporation of the water. This is believed to be due primarily to the variation in the amount of water used in mixing the mortar. An excess of water will cause a segregation of the coarse and fine materials, resulting in a difference of color. In a very wet mixture, more lime will be set free from the cement and brought to the surface. When great care is used as to the amount of water, and care is taken to prevent the separation of the stone from the mortar when deposited, the concrete will present a fairly uniform color when the forms are removed. There is greater danger of the efflorescence at joints than at any other point, unless special care is taken. If the work is to be continued within 24 hours, and care is taken to scrape and remove the laitance, and then, before the next layer is deposited, the scraped surface is coated with a thin cement mortar, the joint should be impervious to moisture, and no trouble with efflorescence should be experienced.
A very successful method of removing efflorescence from a concrete surface, consists in applying a wash of diluted hydrochloric acid. The wash consists of 1 part acid to 5 parts water, and is applied with scrubbing brushes. Water is kept constantly played on the work, by means of a hose, to prevent the penetration of the acid. The cleaning is very satisfactory, and for plain surfaces costs about 20 cents per square yard.