Expansive cements first appeared on the American market in the 1960's, following work done in the previous decade by Alexander Klein and his associates at the University of California. There are now three types of expansive cements, with the major differences relating to variations in the starting materials that are blended to make the cements, though there are minor differences in properties. Thinner pavements that are more free of cracking after numerous load applications, more resistant to propagation of cracks that do occur, and more free of loosened rubble between cracks appear to be possible by using steel-fiber reinforced concrete. These considerations may bring such concrete into significant use in airport pavements. Regulated cements are characterized by times of set that can be controlled for any period varying from less than 1 minute after mixing to as long as 70 minutes, although 45 minutes is usually a practical maximum; control may be effected either by the formulation as specified or by admixtures added by the user. The other characteristically different property of reg-set cement is that very early strengths are unusually high. A compressive strength of 1,000 psi is obtained in 1 hour with an ASTM C 109 mortar, and even higher strengths are obtained by some formulations because strength increases in two somewhat independent stages. The strength at age 1 day may be only slightly greater than that achieved by Type III cement, but at age 2 and 3 days it is higher than Type III again. Synthetic aggregates are aggregates that don't occur in nature or in any that have required some processing addition to or other than crushing, drying, or screening. Table one in the article gives a reasonably complete list. Although compressive strength of polymer-impregnated concrete my be increase about 4 times over that of regular concrete, concretes have been made with strengths up to 28,000 to 30,000 psi. The strength is very dependent upon the extent of polymer loading, or perhaps more accurately the relative completeness of filling of available space, since it does not appear to make much difference what the former concrete strength is. The quality of the aggregate perhaps makes a difference.