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* Increase slurry yield
[[Glossary:Bentonite |Bentonite]] is typically used at concentrations of 1 to 16% [[Glossary:BWOC|BWOC]]. It may be dry-blended with the cement or [[Glossary:Prehydrated|prehydrated ]] in the mixing water. In prehydrating, the effect of 1% BWOC prehydrated is approximately equal to 3.5% BWOC dry-blended, but the [[Glossary:Yield point|yield point ]] is much higher. For best results, the prehydrated bentonite/water mixture should be used for mixing the cement slurry shortly after prehydration has been completed. Laboratory testing is advised to determine the proper gel concentration and mixing procedure for prehydrated bentonite. Tech grade or “mud gel” should not be substituted for cement-grade bentonite. Lignosulfonate is commonly used as a [[Glossary:Dispersant|dispersant ]] and [[Glossary:Retarder|retarder ]] in high-gel cements to reduce the slurry viscosity.
===Attapulgite (salt gel)===
This is a more effective extender than bentonite in seawater or high-salt slurries, but it is not regulated or does not have a specification. [[Glossary:Attapulgiteclay|Attapulgite]], (Mg,Al)<sub>2</sub> (OH/Si<sub>4</sub>O<sub>10</sub>)•12H<sub>2</sub>O, is composed of clusters of fibrous needles that require high shear to be dispersed in water. It produces many of the same effects as bentonite, except that it does not reduce fluid loss.
A disadvantage of attapulgite is that, because of the similarity of the fibers to those of asbestos, its use has been prohibited in some countries. Granular forms are available that may be permitted as a replacement.
This is an asphaltic material, or solid hydrocarbon, found only in Utah and Colorado. It is one of the purest naturally occurring bitumens. [[Glossary:Gilsonite |Gilsonite]] can be used with slurry densities as low as 11 lbm/gal at a normal concentration of 5 to 25 lbm/sack (sk) of cement, and it will plug float equipment and bridge tight annuli. The low densities obtainable with gilsonite result from its low density (1.07 g/cm<sup>3</sup>). Because gilsonite is an organic material, it is highly buoyant and will float out of the slurry unless inhibited. Bentonite is commonly added at a concentration of 2 to 6% to prevent bridging in the wellbore.
===Crushed coal===
A number of pozzolanic materials are available for use in producing lightweight cement slurries. These can be either natural or artificial and include:
* [[Glossary:Fly ash|Fly ash]]* [[Glossary:Diatomaceous earth|Diatomaceous Earth (DE)]]
* Microsilica
* Metakaolin
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