【正文】 s 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, the industry had bee well established not only in England, but also in Germany and Belgium. Shipments to the United States were started in 1868 and reached a peak about 1895, at which time production was well under way in the United States. Meanwhile the United States production of natural cement had been started early in the 19th century as a result of the demand for cement for construction of the Erie Canal and related works. Subsequent development of the rotary kiln led to large scale production of cement throughout the world. The use of concrete was expanded by the construction of railroads, bridges ,buildings and street pavements. Research in reinforcing concrete with steel rods had been started in France, and the year 1875 saw first use of reinforced concrete in the United States. Much39。 it usually makes up about 75 percent of a given mass of concrete, by volume, although a poor aggregate can reduce the strength of a batch of concrete considerably, good aggregate adds only slightly to the strength of the cement. The two principal advantages of concrete as a construction material are its relative cheapness and the ease with which it can be handled and placed while it is in the plastic stat