The cement industry has come a long way in the last half century in the selection and control of raw materials, and in developing methods of manufacturing a high-quality product. But cement producers still work with basic natural ingredients which will vary widely so it is only natural that variations may occur in the finished product although instances in which the cement is not suitable for general use are rare. Variations can occur in setting time. Whenever cement has been subjected to atmospheric moisture and carbon dioxide, the setting time will often vary in an erratic fashion. This occurs because, although uptake of water by the cement compounds generally causes retardation, exposure of the cement to carbon dioxide can cause either acceleration or retardation of setting, depending on prevailing circumstances and the extent of the reaction. Conversely, other factors, such as ambient conditions, a low/water ratio, hot weather, and use of heated material, may shorten setting time. A number of factors affect the amount and rate of bleeding experienced with a concrete mix, among these are the characteristic of the cement being used. The bleeding tendency of portland cement is related directly to the specific surface of the cement. This relationship is an inverse one; that is, the lower the specific surface, the greater will be the amount of bleeding experienced. Generally speaking, cements with a specific surface of less than approximately 2800 (Blaine) tend to result in concrete with an undesirably high amount of bleeding. Although cement accounts for only about 10 percent of concrete by volume, it typically constitutes 25 to 30 percent of the cost of the concrete mixture. It is recognized that the strength producing properties of brands of cement vary a great deal. For example, in New York City area the cube-test strength of cements is reported to have ranged from 3,500 psi to 6,500 psi during one recent test. On just one job which made use of five brands of cement, cube-strengths varied by more than 2,300 psi due to cement alone. This resulted in a 2,000 psi variation for concrete designed for 3,500 to 4,000 psi 28 day strength.