The carbon black, when so used, not only effects a marked saving in the time and power required in grinding, but also improves the properties of the cement finally obtained. In the final cement, it serves as a separating or dispersing element or agent, making the cement particles more available for hydration and enables the erties. production of concrete mixes having a finer texture and better working properties and containing less water than comparable mixes made with ordinary cement, such mixes yielding concrete which is less permeable and has fewer voids and a denser structure. Further, the carbon black is inert and insoluble, and has no deleterious action on either the cement or concrete produced therefrom, nor does it tend to leach out of the concrete leaving voids or otherwise impairing it.

By the addition of such a small amount of dry carbon black, the time, and hence the power, required to grind the clinker, or reduce it by attrition to the finely divided state, is radically reduced, reductions as high as 20 to 30% attained.

Advantage may be taken of this reduction in the time required for grinding by the addition of carbon black in dry form in various ways. For example, for the production of a Portland cement of a given particle size, i. e., a given specific surface, the time, and the power, required for the grinding operation may be radically reduced; or conversely, the clinker may be subjected to the same amount of grinding as usual, with the production of a cement having a greatly increased specific surface, that is,a more finely divided cement, the addition of the dry carbon black serving to prevent the marked drop in grinding efficiency which ordinarily occurs when the cement has been reduced to a certain more or less critical particle size. Furthermore, the cement so produced will have improved properties due to yg the presence of the extremely finely divided car9,186,792 bon black free from soluble dispersing agents and the like. Of course, a balance between these factors may be effected, as by carrying out the grinding operation in such a way as to decrease the time of grinding somewhat less than referred to 6 above, while producing a cement of finer particle size than could be produced without the addition of the carbon black.