Admixture generally refers to the mixing of drilling fluid, or "mud." It may refer to the action of mixing ingredients such as bentonite or barite into an oil or water base (e.g., mixing up an admixture of additives and a base to form drilling fluid). It may also refer to the additives in the oil or water base that form the mud. Finally, it may also refer to the drilling fluid itself.
Trenchlesspedia Explains Admixture
The admixture of bentonite and barite with an oil base forms an admixture called drilling fluid which is suitable for use in horizontal directional drilling. The admixture process is performed at a mixing plant by the plants technically skilled operators who, when given the mud specifications by the mud engineer, and an admixture of the appropriate ingredients – most often they write or bentonite – to the appropriate base, whether the base is one of water oil.