Cement additives have been classified into six categories. In fact, this classification is crude. The World Oil Cementing Supplements regularly review commercially available cementing additives in certain World Oil issues. Here comprehensive listings of cementing products and additives available from major suppliers can be found. The products are grouped into functional categories as shown in the following: basic cements, accelerators and salts, extenders and density-reducing additives, silica to reduce or prevent high temperature strength retrogression, dispersants, bond improving and expanding agents, retarders, fluid loss additives, antigas migration agents, antifoam and defoaming agents, additives and mixtures to reduce or prevent lost circulation, density-increasing or weighting agents, free water control and solid suspending agents, spacers and chemical washes or preflushes. Products commercially available also can be found readily on the World Wide Web. Individual compounds may emerge in more than one of the categories above. For example, rubber particles reduce the density of the slurry and are also suitable as lost circulation additives. Also, there is a difference between fluid loss and lost circulation. Fluid loss refers to filtration of certain components of the fluid into the formation. Lost circulation is the total material lost into high-permeability thief zones.