Bricks, Masonry, Asphalt and Concrete - Mortar Gone Bad?

I bought some bags of mortar two years ago to reset some bricks. All went fine and is still fine. However, I recently decided to replace some flagstone mortar joints and used an opened bag of mortar (from the brick work) and a second bag.

The mortar did not set and I do not understand why not. Does something in the mortar mix deteriorate over time? I'm more curious than concerned - I can wash out the mortar and redo it with a NEW bag but I would like to know what happened before I do the work again.

What happened?

Cementious materials abosorb humidity and "set". They can then be broken up and reused, but the actual chemical reactions required for good mortar have already taken place. An open bag of premixed mortar is good for days, not years, and even un-opened bags have a shelf life of weeks to a couple of months, depending upon the condition in which they are stored.

Interesting - I did not know that. The mortar looked and handled like a new bag. Thanks - I will certainly remember that.