io8 PRACTICAL TREE REPAIR 



with cement is apt to lead to ineffective results 

 when the work is subjected to conditions at all try- 

 ing. A person who wants to do good work with 

 cement, work suited to the materials used and 

 adapted to the end aimed at, must make himself 

 familiar with the properties of cement and of con- 

 crete and with the principles governing the selec- 

 tion of materials entering into the concrete, the 

 mixing of those materials, and the placing and 

 hardening of the mixture. There is a number of 

 cheap and excellent handbooks dealing with these 

 matters, and one of them should be familiar to 

 every user of cement. In this book, however, 

 only the elements can be presented, and such de- 

 tails as bear directly on the use of concrete as a 

 filling for trees. 



It may be well to begin with a few definitions. 

 Concrete is an artificial stone made up of small 

 pieces of natural stone held together by a matrix 

 produced by the crystallization of hydraulic ce- 

 ment and water. The materials held together, 

 such as sand, gravel, or broken stone, are called 

 the aggregates. Portland cement, the hydraulic 

 cement commonly used for making concrete, con- 

 tains lime, silica, alumina, and other elements, and 

 is made by melting a limy substance with a clayey 

 substance and grinding the resultant clinker to a 

 very fine powder. It is this material alone which 

 is correctly called cement, the mixture of it with 



