Volume Changes in the Hardened Paste

It was stated earlier that the volume of the hydration products is smaller than the combined volumes of the reacting cement and water by approximately 25% of the water volume. Under normal conditions this reduction in the volume of the cement-water system increases the porosity of the paste and is not reflected in the bulk dimensions. A change in bulk dimensions may be caused by the presence of excessive free lime or magnesia in the cement or, as will be seen later (Chapter 6), due to chemical attack of aggressive solutions, etc. These types of volume change take place only under special conditions and involve chemical changes in the cement paste. The following discussion, however, is limited mostly to volume changes caused by physical factors such as external loading and changes in moisture content and temperature, and involve no chemical changes. An exception is carbonation shrinkage which is included in this discussion.