Janosh, I can speak only from my own experience. Portland so far is the only thing that really works for me to make a hard, tough, and durable block. When we started using metakaolin we were elated that set time was reduced to hours opposed to days, and shrinkage reduced considerably. What we didn't know at the time was that the metakaolin, at least in the proportion we used, reduced strength and hardness. At least most of the time, which brings me to the next point: The thing that I'm currently really curious about is that the quality of the blocks we made using the same mix design on the same day (same weather, etc.) varies so greatly. I've been wondering if it isn't water content at the time of pouring that makes the difference...I put it this way (time of pouring) because typically the mix poured first off the top of the barrel is wetter than that poured last from the bottom (despite all our efforts to make it homogenous). ...But then again maybe it is the reverse--the wetter blocks cured harder. We just don't know, and are busy building with the blocks we have, not making more just to experiment. I've wondered if adding lime to the mix wouldn't aid the pozzolanic reaction of the metakaolin, based on our positive experience with the mortar mix.
I definately (as in really, really, really!) want to hear from you what the results of your experiments are. You might stumble across the answer to some of my questions about our blocks. Also, I'm curious about the effect of climate on papercrete set and cure characteristics. We observed the blocks poured on cold days (with colder nights!) took longer.
I know this post doesn't address many of your questions but I hope it adds some useful data.
Eli
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