Studies on Setting Free of Lime from Cement, Clinker and Its Hydraulic Compounds by Heating

Abstract

The setting free of lime from cement by reheating is a very interesting problem with regrrds to the cement chemistry. There are many reports in these years with regards to this problem, e. g., S. L. Meyers, Rock Products, 1930, 33, No. 8, 78; E. T. Carlson, Rock Products, 1931, 34, No. 25, 52; G. E. Bessy, Cement and Cement Manufacture, 1932, 5, 233; D. Steiner and B. Bartos, Tonind. =Ztg., 1932, 56, 1040; G. Ashkenasi, Tonind. =Ztg., 1932, 56, 1065. But it is very sorry that the earliest study about this problem by Prof. Haruhiko Cho (The Journal of the Society of Chemical Industry, Japan, 1929, 32, 57C; Technical News of the Kyushu Imperial University, Japan) was not introduced and paied attention by the European and American cement chemists, owing to the want of foreign abstract to this paper.In the present paper, it is principally reported the results of studies on the source of setting free of lime by reheating cement, clinker, and various hydeaulic compounds, i.e., calcium silicates and calcium aluminates. The brief summary from the original Japanese paper is as follows:(1) The setting free of lime by reheating cement does not be made clear only by the decomposition of tricalcium silicate to dicalcium silicate and lime, i.e., 3CaOSiO2→2CaO SiO2+CaO.(2) This decomposition of 3CaOSiO2 to 2CaO2 SiO and CaO is clearly accelerated by mixing gypsum, dicalcium ferrite (2CaO·Fe2O3), calcium fluoride (CaF2), etc., but not by the presence of calcium phosphate [Ca3(PO4)2], dicalcium silicate (2CaOSiO2), tricalcium aluminate (3CaOAl2O3), magnesia, etc.(3) Tricalcium aluminate, dicalcium silicate and dicalcium ferrite do not set free lime by heating But tricalcium aluminate sets free lime only by the presence of gypsum.(4) From these results, it will be recognized that the main part of free lime produced by heating clinker is due to the decomposition of tricalcium silicate by the presence of dicalcium ferrite and the amount of free lime by this decomposition is not so much, owing to the weaker effect of dicalcium ferrite than that of gypsum.But the decomposition of free lime by heating cement is very considerable. This is mainly due to the presence of gypsum, which has the strong effect to the decomposition of tricalcium silicate to dicalcium silicate and lime by heating about at 1050-1150°C. Sometimes the amount of free lime becomes greater than that of one molecule of lime from tricalcium silicate, i.e., 3CaOSiO2→2CaOSiO2+CaO. This excess of lime clearly comes from the decomposition of tricalcium aluminate by the prerence of gypsum, which is easily explained by the result above cited (3). So that, the amount of free lime produced by heating cement (the ground product from the clinker with gypsum) is the total sum of the lime from (1) the decomposition of tricalcium silicate by the effects of gypsum and dicalcium ferrite, and (2) the decomposition of tricalcium aluminate by the effect of gypsum.Further studies on this problem will be continued and reported hereafter by the present authors.