I have been laying a 65 mm floor screed. I mixed it very dry as people advice. I also added fibre reinforcing. But in places the surface is loose after 2-3 days of laying. You can erode it with the finger until 3-4mm deep before reaching a hard surface. This happens especially in day joints and where I flattened it with a straight edge that had a sharp edge, before realising that rounding off the edge was better. The sharp edge was dragging the fibres and tiny pebbles in the sand when passing it over the surface.
I have checked the core and it is quite firm.
The question is what do I do about it now? I am going to lay tiles on top of it and it has underfloor heating below.
Another question. I am using polythene sheets to control the drying. For how long should I leave the the sheets in the present weather in the Midlands?