【正文】 or gravel are mixed. The sand and stone or gravel together make up the aggregate of a concrete mixture. The aggregate serves no structural function. It is merely ,a filler that adds lowcost bulk to the cement paste。 1 CONCRETE AND MORTAR 1. Early History of Cement and Concrete Shelter from the very beginning of man/ existence, has demanded the application Of the best , available technology of the contemporary era. In the earliest ages, structures consisted of rammed earth, or stone blocks laid one on another without benefit of any bonding or cementing medium. Stability of the stone structures depended on the regular setting of the heavy stones , The earliest masonry probably consisted of sundried clay bricks, set in regular courses in thin layers of moist mud. When the moist mud dried, a solid clay wall resulted. Construction: of this kind was mon in the dry desert areas of the world. Burnt gypsum as a cementing material was developed early in the Egyptian period and was apparently used in construction of some of the pyramids. Later the Greeks and Romans discovered methods of burning limestone to produce quicklime which was subsequently slaked for use in making mortar. Both the Greeks and the Romans learned that certain fine soil or earth, when mixed with the lime and the sand, produced a superior cementing material. The Greek material, a volcanic tuff from the island of Santorin , is still used in that part Of the world. The best of the materials used by the Romans was a tuff or ash from the vicinity of Pozzuoli near Mt. Vesuvius, hence the name “ pozzolan” used to identify a certain type of mineral admixture used in concrete today. The cement produced by the Romans was a hydraulic cement, that is, it had the capability of hardening under water. Many of the Roman structures were constructed of a form of concrete, using these materials, and stone masonry was bonded with a mortar similarly posed. During the Middle Ages of history, the art of making good mortar was nearly lost, the low point having been reached in about the llth century, when much inferior material was used. Quality of the lime started to improve at this time and in the 14th century or later the use of pozzolans was again practised. One of the most famous projects of the paratively recent period was the construction of the new Eddystone Lighthouse off the coast of England in 175759. John Smeaton, the engineer and designer of the structure, investigated many materials and methods of bonding the stones for the building. Engineering and scientific development was beginning to move rapidly at this time, and many researchers in several countries were investigating cementing agents made from gypsum, limestone 2 and other natural materials. One discovery was a method of making a cement by burning a naturally occurring mixture of lime and clay. properties of the natural cement were very erratic because of variations in the proportions in the natural material, although use of this natural cement continued for many years. In 1824 Joseph Aspdin , a brickmason of Leeds, England, took out a patent on a material he called Portland cement, so called because concrete made with it was supposed to resemble the limestone quarried near Portland, England. Aspdin is generally credited with inventing a method of proportioning limestone and clay, burning the mixture at high temperature to produce clinkers, then grinding the clinkers to produce a hydraulic cement. His small kiln, producing about 16 tons of clinker at a time, required several days/or each burn. Expansion and development of cement manufacturing was slow for a number of years. About 1850, however,