CEMENT FOR A MODERN STREET. 145 



CEMENT FOE A MODERN STREET. 



By Dr. S. F. PECKHAM. 



AMODERIS' street consists of a concrete foundation which extends 

 from curb to curb, upon which is laid a wearing surface of 

 asphalt, brick or other material. 



The use of these concretes has an instructive history, which might 

 be profitably preceded by a discussion of the uses of mortars and 

 coinents in antiquity, did space permit. So far as I have been able to 

 learn, all the different varieties of cementing materials, including ordi- 

 nary lime mortars, have been experimented with for the construction 

 of concrete foundations. It is therefore proper that these different 

 materials should be briefly described. 



Mortar, in the ordinary sense of the term, designates a mixture of 

 lime and sand. The lime is prepared by heating limestone in kilns, until 

 tlie carbonic acid of the limestone is expelled and oxide of calcium 

 remains, which readily absorbs water and slacks, as it is termed, and 

 in time reabsorbs the carbonic acid that was driven off. The lime is 

 mixed with the water when it is slacked and the carbonic acid is 

 absorbed from the atmosphere. When mortar is made, the lime is first 

 made into a thin paste with water, and sand is added imtil the mass 

 ceases to be sticky. Such mortar acquires strength slowly. The excess 

 of water first dries out and then the lime by slow absorption of carbonic 

 acid forms thin particles of limestone between the grains of sand, 

 until the mortar becomes a coherent mass. That this process goes on 

 very slowly is shown by the fact that the mortar between the bricks of 

 chimneys centuries old is found to contain a considerable percentage 

 of unchanged lime. This mortar, when first laid, will not bear wetting, 

 and will set only in dry air. 



The Romans had learned before the Christian Era that the addi- 

 tion to lime mortar of volcanic ash or pozzuolana would make the 

 mortar set under water and with additional strength. The so-called 

 Roman cement was noted in antiquity for its superior strength when 

 compared with ordinary lime mortar. Where they could not obtain the 

 pozzuolana they used pulverized brick and pottery. 



During the middle ages, for more than a thousand years, the art of 

 making hydraulic cement ivas lost, and, with every other art, the art of 

 making good mortar declined until the beginning of the eighteenth 

 century, when attempts were made to revive the art of making Roman 

 cement, but with only slight success. 



VOL. LX. — 10. 



