I'm a bit concerned about the way you are curing your pieces. Phrases like "is still damp inside" and "pieces which dried in the sun" suggest a misunderstanding. Concrete, and hypertufa too, does not set by drying. It sets by a process of crystallisation called hydration, and this only occurs when they piece remains wet. Allowing a piece to dry out, and particularly in the sun, will cause it to develop a hard but very brittle crust. Drying it through completely will produce something that crumbles to dust.

Curing is done by keeping a piece wet for at least a week, ideally for a month. The cement which should have thoroughly coated all the aggregates when it was mixed will very slowly crystallise and hold everything together. These crystals require water to form. When the cement is allowed to dry out, the crystals stop forming. Even if you wet it again there are breaks in the crystals that weaken the result. Crystals form quickly at higher temperatures but may develop as small separate crystals that weaken the structure. The strongest results form at lower temperatures where the cement can set slowly without flaws. A month at 50F is a very practical way to achieve a strong result.

Shrubs hit it on the nose! Cement doesn't dry, it cures. Slow and steady does it every time.
When I have portland "go bad" it turns into a solid lump! Keep the bags dry and off the ground if possible. An open bag needs to be tightly closed and used ASAP. If I open a bag that may not be used in one project or at one time I put the bag in a plastic garbage bag so that the open bag of cement is double sealed.

I'm wondering about the "bottle of Quickrete buff color" that you added. I don't know the product, but it seems like an awful lot of ANYTHING to add to a mix.

Further signs of bad portland:It stays out late at night and drinks too much of your booze!!
Cheers..from here

16" Solid ? Wow ! . Any previous success with this size mass or is this your first attempt ?

I don't have any experience with 'Tufa, but I have gone through enough cement that I can tell you for certain... there is indeed a LOT of bad Portland out there. The Big Box stores are possibly the worst source for good Portland on the planet. Cement has to fresh to perform predictably and you rarely get it that way through the distribution systems at the Depot or Lowe's. Don't get me wrong. I love those stores, it's just that their inventory and stocking systems don't move it through quickly enough. If you can find a commercial supply house to the concrete trade, you will get a much fresher product usually at a little less than even the big discounters. If you open the bag and there is anything but fine, consistently textured powder, then either take it back or just use it for post-holes, 'cause that's about all it's good for. If you have to "bust lumps" of any size...you are dealing with a highly degraded product and there is no way to tell how it will ultimately perform.

Thanks All,

I had put this bag of cement (from the Big Box store) in a plastic storage container in the garage about a month ago; it had been opened. There were small flakes at the surface when I opened it to use it this time. I know that the cement we use here at work is always fresh and easily fluffed. I have had a couple of projects not set up as well as others but it seems to always be a large tufa project that deals me grief. It may be my lack of patience but most of my tufa seems to get quite firm in 24 hours.

Shrubs_n_bulbs: I understand your concern with my comments about how I treated the broken pieces. I was not attempting to save them nor cure them. However, I was not sure how tufa would act if it was allowed to simply dry out in the sun. Are you saying that it will usually make a weak, easily crumbled structure?

Bamatufa: I have had pleasant success with cement spheres but I really want the aged look of tufa for a couple of large ones at my front steps. I made some that were basketball sized and I am very pleased with the finish - buff color, black sand, white pearlite, nice pitted surface. I'll try again; I just don't want another $40 bucks laying crumbled in the driveway.