These are covered by BS 8204, Screeds, bases and in-situ floorings, Part 3: Polymer modified cementitious screeds and wearing screeds – code of Practice (2004).

They are a mixture of cement, agregate and water, modified with a polymer dispersion where the proportion of polymer solids based on mass of dry cement is at least 4%.

The main advantages of Polymer modified cementitious screeds are:
• Enhanced adhesion to the concrete base
• Thinner applications
• Resistance to cracking
• Improved toughness, durability and resilience
• Improved resistance to impact, abrasion and dusting
• Low permeability and reduced absorption of liquids and greases.

These are usually propriatary materials, and should be formulated in accordance with BS EN 13813

Further reading: PYE, PW and HARRISON, HW. Floors and Flooring, BRE, 2003.