CoUoidal Carbon as a Grinding Aid in Portland Cement Manufacture.--Colloidal carbon, commonly known as carbon black, is an effective grinding aid in the manufacture of portland cement, according to the results of research carried out by the Columbian Carbon Company's Industrial Fellowship at Mellon Instutute, Pittsburgh. The report of this investigation discloses that a carbon dosage as low as 0.32 per cent. on the clinker increases the fineness of the cement by 30 per cent. when the time of grinding is constant. and that the same carbon dosage decreases the grinding time by 28 per cent. when the grinding is run to constant fineness. With a I per cent. dosage these improvements become 5 ยฐ per cent. and 34 per cent., respectively. In terms of power saving and increased output these results are of practical significance. The cements prepared with carbon present as a grinding aid compared to the controls showed improved strength properties in tensile and compression tests on mortars. These benefits are attributed in the main to the increased fineness of the cement, although there is evidence that the carbon per se is contributory. The use of carbon in dosages up to I per cent. does not alter appreciably such standard properties of portland cement as consistency, setting time, and soundness. color excepted, and has no noticeable effect on the reistance of cement mortars to freezing and thawing treatment. The effectiveness of colloidal carbon as a grinding aid, together with the existence of a reinforcing effect, is thought to be established by these experiments. A new application for colloidal carbon, which has already reached important dimensions, is thus indicated for the cement and related industries. The colloidal carbon used throughout this study was the grade termed "Cem Beads," a pellet form of a standard channel carbon manufactured from natural gas, with physical and chemical properties essentially similar to the colloidal carbon pigment used in the rubber industry. The diameter of the ultimate carbon particles in the pellet averages about 3o millimicrons (o.oooooi inch), thus establishing the colloidal nature of the pigment. Copies of a publication descriptive of the research may be obtained by writing to Mellon Institute. R. H. O.