I would suppose a lot depends on the grind - how fine it has been produced to. Any info on that as this would be one controlling factor? In India (and I think UK), they have OPC33, OPC43 and OPC53 - with the later being much more finely ground.
The book is called Reinforced Concrete Designer's Handbook Seventh Edition (Revised) by Chas. E. Reynolds BSc. (Eng), C.Eng, F.I.C.E. published by Cement and Concrete Assocation (first edition 1932, Seventh Edition 1972).
They indicate angle of repose is 40 to 44 degrees. I don't understand what they mean by angle of surcharge which is approximately 1/2 of this - perhaps saturated angle of repose?