See primer on Mixture DOE published by Stat-Ease at link, and also attached at the bottom of this page.
NIST - What is a mixture design? [5.5.4.]
Mixture screening designs [5.5.4.1.]
Simplex-lattice designs [5.5.4.2.]
Simplex-centroid designs [5.5.4.3.]
Constrained mixture designs [5.5.4.4.]
Other references include: