When the temperature is over 100 degrees, is it better to use mortar that is more wet or wet the cinder blocks first?
I'm a little confused. I was told that if you were laying a mortar bed for a tile job, vs. using cement boards, that you want to use a drier mix because it will be stronger. But then if you were pouring a slab, you need to mist it so it doesn't dry too fast.
Anyway, does the high temp just mean the mortar will just dry faster and everything will be fine? Or does this mean the mortar will dry too fast and will not bond well to the blocks?
Yes, it is confusing. When mixing a batch of mortar or concrete, less water = more strength. The problem is with hydration or the rate of "drying".... too fast and that will render the material weak and prone to faults, thus the need to keep it damp. You must however have enough water to make the material usable. A very dry mortar mix will not adhere well to the block when you try to butter it and put it in place... very frustrating when the mortar drops off the the side of the block. Too wet a mixture and your block will squeeze most of the mortar out of the joint. A concrete mix needs to have enough fluidity in order to settle the aggregate and allow time to trowel a good finish. Too much water and the mix becomes soupy, takes a long time to set, and will be weaker.
Dry cinder blocks will suck the moisture out of the mortar and weaken it. In the temperatures that you describe it would be best to dampen the blocks vs overwatering the mortar. I would recommend spraying down the blocks the night prior, cover with a tarp (optional), and they will be in good condition for the next day. I don't think spraying down mortar joints is an accepted practice but definitely keep a slab moist after it has initially set.
After enough practice you will get a feel for when the mix is right based on it's slump, the ambient temperature, and the relative humidity.
