52 



BULLETIN 120, U. S. DEPARTMENT OF AGRICULTURE. 



cipal reason for it arises from the reaction between the various alkali 

 salts and the calcium hydroxid of the cement. The new compounds 

 formed have a greater volume than the replaced hydroxid and their 

 formation weakens or destroys the concrete by forcing apart the 

 particles of cement. In order for this action to occur it is necessary 

 for the water containing alkali to percolate into or through the con- 

 crete. Under field conditions : "these reactions referred to are much 



Fig. 1. — Briquets showing repellent action of oil-cement concrete on alkali water. (From tests by 

 the Office of Public Roads, United States Department of Agriculture.) Top row contained 10 per 

 cent of semiasphaltic oil; middle row contained 5 per cent of semiasphaltic oil; bottom row con- 

 tained no oil. Briquets were immersed one year in a 10 per cent solution of sodium sulphate. 



retarded if not entirely suspended in most cases, due probably to 

 the carbonization of the lime of the cement near the surface or the 

 formation of an impervious skin or protective coating by saline 

 deposits." 



It is also doubtless true that 2 "wetting and drying or freezing and 

 thawing will hasten the destruction of the cement by extending the 

 cracks already started." 



i U. S. Dept. Com., Bur. Standards Technol. Paper 12 (1912). 

 2 Montana Sta. Bui. SI (1910). 



