Cement (mortar mix) shelf life

Yes it does. I was told when you buy it choose the warm one, as that is freshest. I do exactly the same thing (use old cement) I am glad to hear others do and would also love to hear an official (-sell by?) and unofficial (-use by) date for non-professionals from those in the know.

Usually the problem is getting the stuff to bond well since the old crack is already hard and (often) dirty. Some people use a little muriatic acid to clean it up, but a safer approach is to just brush it well and use a latex patching mix.

As far as I'm aware there is no shelf life for a bag of cement. The problem is moisture, as it will start the hydration process. As long as it's air tight it should be fine for a life time.

Maybe there was a same amount of moisture that got into the "old" bag. Or you used a different water/ cement ratio. A 0.35 ratio is best.

For example. Pour a litre (1/4 gallon) of water into a bucket. Get a handful of cement (use gloves, face mask and glasses as it can give you chemical burns ). It may be best if you use a low-heat cement instead of the regular Portland General Purpose cement. Low heat also will take longer to set giving you more time to play.

Mix the cement into the bucket of water.

Add another handful of cement. Keep adding until you get a consistency of mayonnaise. Now use half a handful of cement and mix it in. The object is to get it just thick enough that when you grab a handful of mix and turn your hand upside down the mix will stick to your hand with a glue like consistency.

Once you lay the cement, keep it moist until it sets. Should be solid within a day no matter what temperature it is. Maximum strength is reached in 28 days. But I don't thick you need cement that has a Unconfined Compressive Strength of about 50 MPa.