Most construction chemical companies make latex or polymer based admixes for screed which are advertised as increasing strength.
The problem with these products is that in order to be able to achieve the strengths that they advertise, the ratio of admix to water/cement and sand is very high.
This makes the products very expensive and also makes the screed very sticky to work with and with a strong curling effect when drying that increases the risk of the screed delaminating.
In our experience, most installers don't strictly follow the dosage recommendations and dose significantly less admix than specified. This saves money and leaves the screed easier to work with and have less risk of going drummy, but the low dosage rate means that the strength won't be achieved.
It effectively has a placebo effect. They dose a little bit of admix so they can say, "we dosed XYZ admix" but it has little effect because of the low dose rate and they know that if they use the full dose rate, the screed becomes sticky and unworkable (as well as very expensive) so they don't do that.
In general, we discourage the use of latex based admixes for this reason. They are not the best way to achieve high strength screeds and result in sticky screeds that dry too fast and curl resulting in delamination and a low dose which doesn't have these problems, doesn't make a substantial difference to the ultimate screed strength.
At ScreedPro we have more sophisticated ways of achieving high strength screed. Our preferred approach is to mix an engineered screed rather than simply adding latex or polymers to a sand cement screed. An engineered screed achieves high strength and other attributes such as fast drying or retarded curing without the need for high ratios of latex dosing.