[quote=DanG]Surface cracking sounds like a moisture problem to me.

Possibilities:
The forms are dry (absorb water) or allow water to escape rather
than hydrate and cure the concrete. Are you using some type of
form release? You might try wetting the form work to aid in
concrete placement and to prevent form material absorbing
moisture.

You mention applying fiber reinforcement. It would be normal here
to blend the fiber throughout the concrete mixture.

You mention keeping the mixture quite dry and plasticizing with a
waterproofing agent. I assume this is to avoid shrinkage cracking
and to maximize ultimate strength. You have no aggregate to speak
of and are using dust rather than sand. Many powder type products
demand mixing time and a resting time to absorb water and a
retempering.
Are you bulking by volume or by weight? It still sounds like loss
of moisture, insufficient moisture, or absorption. I would not
depend on a waterproofing additive to provide curing. I assume
your mixture is a hydraulic concrete that will set under water.
Can you provide wet cure? - flood, submerge, steam, tented moist
cure.

Dear Dan, You are obviously streets ahead of me. Please may I very rudely refer you to the answer that I gave Richard.

What can I tell you other? The climate is very humid here so water loss from the concrete is much slower than you might envisage. I think we get a pretty complete hydration because when 'set' it rings like a bell and is extremely hard. If there is any sun exposure I have the casting covered with damp sacks which are kept moist for days. The shuttering most certainly does not absorb an iota of moisture.

I could not see the necessity for any larger aggregate as the final moulded layer is no more than 100mm thick and averages 50mm. It is simply an applied fascia on a very stable base. I simply cannot see why sometime it develops a few hair line cracks after continuous vibration and at other time emerges perfect from the crysallis.

I will try mixing in short fiber threads on the next ones.

Thanks for your help. Giles