What kind of mortar and grout? For inside or outdoor projects?

Are you sure you don't have the two mixed up? Using grout as mortar? I have done this and it does not stay stuck! Oops?

Well, it's premixed mortar that is supposed to be good for outside and for vertical holding. Expensive, won't be buying it again.
Some pieces are stuck stuck stuck. Then a few do this.
No one has had this happen?

Ah HA!!! Flag used to use that pre-mixed mortar "for outside", said she, and I kept telling her it won't work. Janice is finding out why now. She continually has pieces falling off Flags things. It won't work, ERNIE, for outside. You have to use cement-based mortar for outside projects. Like I told y'all a couple yrs. ago, when the rooster I mosaiced for my Bro was under five ft. of water for two months during the flood in his area, when I cleaned it up/scrubbing hard, there was not one tile that fell. You can't beat the cement-based mortar.

I have a window box shaped planter I made from a styrofoam fish box a few years ago. WEll, I got the fish box and dh stuccoed it for me.
I never liked the color of the stucco so I put a think coat of mortor on it and also some stones for decoration. Most of the stones fell off over the summer.
It is quick crete mortor and I used 50:50 admix:water.

The only thing I can think of is that the stones which were from the dollar store may have a coating that I should have scrubbed off. Or moisture from the soil traveled through the stucco and the mortor? Would that do it?

Kathy

I've not had good luck w/using mortar as an adhesive, but I didn't put in the admixture. I only use Mapei Ultra Flex II, which has the polymer added. I've used all kinds of stones - even the slick ones - w/it, and it is really sticky stuff - it holds vertically beautifully.

Slow is totally correct=all premixed thinset is mastic based and will eventually fail. If it stays good in a bucket it cannot be cement based. Once cement based thinset is mixed the chemical reaction begins and its unstoppable. things like temperature and moisture can slow it or speed it up, but it will eventually set, even under water. I find that working with thinset in cooler conditions slows the cure down considerably but makes it much stronger.

I use thin set for my rocks and have never yet (knock on wood) had any fall off...

I'm with Slow and WW on this, mastic does rehydrate

Makes sense, if it will keep in a bucket, it will keep on the tile!
So that is the problem. Guess I should take it back.
A home improvement show last night was saying that mortar is porous and will absorb water. They were fixing leaks in a brick house.
So if it does this, why do they build houses and fences with it? ??
I found it at Lowe's. Just to be sure, I DO NOT have to buy the extra bucket of liquid to mix it with? It will mix with water?
Wonder if this is why Home Depot doesn't carry any Mapei products. They like to sell that extra bucket of material.
Harrumph.
Thanks all for your detective work.

Im not sure by your comment you got my meaning so I will rephrase. If it comes premixed-it is mastic based. and mastic will fail. for thinset-think cement kind of. yes it will absorb water. and over time-much much time it will eventually fail like everything else. But think about roman mosaics-thousands of years. Im sure they didnt use mastic.

Lowe's is the only place I've ever seen the Mapei. You don't HAVE to use Mapei. Just ask for thinset, and get the admixture to add to it. It comes in a powder form. I did that in the beginning, adding 50/50 water/admixture to regular thinset. This was b/f I discovered the qualities of Mapei. I called their 800 number, and they were very informative about their product.