for what it's worth I'll make a contribution of my experience in this area.
About 10 years ago, I repaired a Stalagmite and a Stalactite (only about 0.5m) though.
The Stalagmite was easy, as someone said above weight and therefore compression are on your side. Make sure you have a clean break and then check you can position it accurately. Clean off both surfaces with a water spray and brush, then dab them with a towel to remove any excess water, then coat the base with superglue. Position and hold in place for a couple of minutes. Job done. (Superglue likes moisture which is why it sticks your skin together so easily.)
The stalactite was more tricky as you can imagine. As the bottom of it was about a metre off the floor I would have had to construct a wooden platform and maybe a jack to support it if using resin. I opted to use a pin, so I positioned the two parts best I could, made a small mark on the side of each half, then drilled a hole into each. This is the tricky bit of this method, getting the position of the holes in the right place. Anyway, I did it and again used Superglue. The trouble with that is, it doesn't allow for re-positioning and once the glue was applied to the pin it worked fine, but I'd cocked up and hadn't drilled the
holes deep enough, so the two parts didn't quite join up!
Damn. There was about a 3mm gap. If I'd have some appropriate resin I could have pulled the pin to drill the upper hole a bit deeper and tried again, but the Superglue had already taken hold and I didn't want to prise the parts apart for fear of causing damage.
It's still up there after all this time, just a bit wobbly! Something to inject into the gap would be great. Many lessons learnt...
Regards Mel. DCA Conservation Officer.