I had a quick look at the addmixtures. Maybe an accelerant would be beneficial. The strength of concrete runs like a parabolic curve, it quickly gets hard then spends a long time getting harder then spends a long time getting softer. If you can make the peak strength happen sooner...
Also you have no reo or metals in the cube so you can use higher rates of accelerant.
http://www.nrmca.org/aboutconcrete/cips/15p.pdf