Portland cement is manufactured from carefully selected materials under closely controlled processes. The process starts by mixing limestone of marl with such other ingredients as clay, shale or blast furnace slag in a rotary kiln at a temperature of approximately 2700 degrees F. to form a clinker. The clinker is cooled and then pulverized, with a small amount of gypsum added to regulate the setting time. The pulverized product is the finished portland cement. It is ground so fine that nearly all of it will pass through a sieve with 40,000 openings to the square inch. When portland cement is mixed with water, a paste is formed which first sets and then hardens for an indefinite period. The setting and hardening are brought about by chemical reactions between the cement and water, called hydration. The selection of aggregates is of particular importance in the making of concrete. Both the cost and the quality of the concrete are affected by the kind of aggregates selected. Mixing water should be clean and free from oil, alkali or acid. An admixture is defined in ASTM Designation C125 as "A material other than water, aggregates, and portland cement that is used as an ingredient of concrete and is added to the batch immediately before or during its mixing." They can be divided into 10 groups: accelerators, retarders, air-entraining agents, gas-forming agents, cementitious materials, pozzolans, alkali-aggregate expansion inhibitors, dampproofing and permeability-reducing agents, workability agents and grouting agents. The three used most by cement masons are accelerators, retarders and air-entraining agents. Accelerators increase the rate of early strength development in concrete to: reduce the waiting time for finishing operations to be started, permit earlier removal of forms and screeds, reduce the required period for curing in certain types of work, advance the time when a structure can be placed in service, partially compensate for the slow gain in strength of the concrete even with proper protection during cold weather, and reduce the period of protection required for initial and final set in emergency repair and other work. Retarders are used to overcome the accelerating effect that temperature has on setting during hot weather concreting operations and to delay early stiffening action of concrete placed under difficult conditions. Air-entraining admixtures used in concrete will improve the workability and durability, and in the case of exposed flatwork will produce a concrete resistant to severe frost action and to the effects of applications of salt for snow and ice removal.