Calcium Sulfate Hemihydrate
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Calcium Sulfate Hemihydrate can be blended with Portland cement to reduce thickening and setting times in oilwell cementing. When combined with diatomaceous earth, these additives can create a cement slurry with the desired properties for well cementing. This is because gypsum is a highly effective retarder, which slows down the cement’s setting. By adding gypsum to the cement slurry, the thickening and setting times can be controlled to ensure that the cement sets at the right time and achieves the desired strength.
However, it should be noted that gypsum requires significantly more water addition at maximal application temperatures of 70–80°C. This is because, at high temperatures, gypsum can lose its water content and turn into anhydrite, which can cause the cement slurry to lose its workability and set too quickly.
To overcome this issue, sodium silicate and gypsum can be added to the cement slurry. Sodium silicate is a type of accelerator that can counteract the retarding effect of the gypsum and help to speed up the setting time of the cement. This allows the cement to set at the desired time and achieve the desired strength, even at high temperatures.
Additionally, when Calcium Sulfate Hemihydrate is combined with Sodium silicate, it can be used for cement slurries with diatomaceous earth at up to 7%. Diatomaceous earth is a sedimentary rock commonly used in oilwell cementing as a lightweight additive. Combining gypsum, sodium silicate, and diatomaceous earth can create a cement slurry with the desired properties for well cementing, including reduced thickening and setting times and the ability to withstand high temperatures and pressures.