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crack and disintegrate. Some cement companies take proper precautions 

 in the treatment of raw materials and storage of the finished product, 

 such that very rarely does an unsound cement leave their mill. Other 

 companies, in the rush of business, do not properly mix and grind their 

 raw material and finished cement, and do not store the cement long 

 enough for the hydration of the free lime present. These are conditions 

 that tend to place more or less unsound cement on the market. The future 

 of the concrete block manifestly depends to a great extent upon the use 

 of a sound cement. 



For maximum strength in concrete, the cement must be finely ground, 

 but fine grinding is expensive and consequently this part of the manu- 

 facture is often slighted. The cement should also be slow setting, as a 

 cement that reaches its initial set in two or three hours will be stronger 

 at the end of seven days or a mouth than a quicker setting cement that 

 reaches its initial set in forty or fifty minutes. 



The cement to be used in concrete blocks should in all cases pass 

 the specifications of the American Society for Testing Materials. 



Too frequently the reason for poor concrete is ascribed to poor cement, 

 and no thought is given the other materials entering in, namely ; sand, 

 gravel or broken stone. The selection and proportioning of the aggregate 

 for the best concrete is very important in the building block industry. It 

 is well known that the proportions of cement, sand and stone which will 

 give the densest mixture of concrete will also give the highest strength. 

 It is also recognized that a rich, dense mixture of concrete is the most 

 nearly waterproof that concrete alone can be made. So that for a strong, 

 water-proof block, it is important that the cement and aggregate be prop- 

 erly proportioned. This may be done by actual trial mixtures to deter- 

 mine the densest concrete. An aggregate containing coarse stones and 

 sand has greater density than sand alone and consequently is better for 

 use in concrete blocks. 



According to Wm. B. Fuller, an eminent authority on concrete, the 

 most nearly perfect gradation of sizes of particles in an aggregate may 

 best be known by the process of mechanical analysis and subsequent re- 

 proportioning. In ease the business warrants it, samples of the gravel 

 should be submitted to a reliable testing laboratory for mechanical analy- 

 sis to determine the proper proportions, 



