I'm going to do some brickwork on my own. Last time it was quite the job to mix up mortar and get it laid before it started to go off (I know, working in pairs would be helpful!). I know I can extend the working life of mortar a bit by making a looser slurry (at the expense of some strength), can I also keep my mortar workable for longer by continually agitating it in a cement mixer? If so, how much longer is it likely to remain workable - just a couple of minutes, or indefinitely?
A mixer might help you mix smaller batches more quickly, so that you can easily adjust the size of a batch and keep up. But most mixers will make you think you should be making an even larger batch, and that's not a great plan.
It won't perform magic, and trying to extend the working time of cement products is heading down a road that is better left untravelled. Far better to sort whatever you need to sort to make small batches accurately and make more of them.
Without knowing what method you were using, you may be better served by a proper mortar hoe that has two big holes in the blade to promote mixing than a mixer, since your problem was having too much mortar mixed up to get through. A beneficial side effect of smaller batches is that they are easier to hand-mix.
I recall being instructed to make sure the mixing pan and tools were cleaned between each batch, to prevent the partially set earlier batch from speeding up the set of the next batch.
If you just want to extend the working life, then chilling everything (water, sand, cement) will slow the reaction somewhat. Chilling only one part (water being easiest) will slow it somewhat less, as the rest of the materials will bring the temperature back up, and the reaction itself is exothermic. But mixing only the right-sized batch for your rate of work is easier. I use a scale that reads in grams when I mix tile mortar and grout, working by myself on fussy small tiles. I can't possibly get through a whole bag at a time within the working time, and I don't even try. –
Making mortar too "loose" will simply make a big mess over the brick since it has less reason to stay in place. Mixing it to the proper consistency will allow for easy clean up off the joints with out it smearing everywhere. When I was coming up in the trade, my boss would tell me to "temper" the mix, by adding a bit of water to it to make it more pliable again. This was only don once per batch since it does weaken the mix.
As mentioned in the comments, once it is mixed, there is nothing that stops the setting up reaction. If you use a mixer to try to extend the workability, you will end up cleaning out a difficult mess.
Stirring it does slow the setting time, this is why concrete trucks have those revolving barrels on the back.
However over stirring it also weakens the end product, and also makes for a thicker coat of cememnt on the stirrer that will eventually need to be chipped off.
so you can, but you should not.
Adding more water will not make your mortar weaker. It will take longer to dry but once fully cured it will have similar strength. To keep your mix workable for longer you can either add a plasticizer or cover your mix with a thick plastic. This will slow down the drying process so your mix will remain workable for longer. Make sure the plastic is as sealed as possible to avoid air/wind getting under. This is also recommended for mortars & renders once laid. Slowing down the drying process reduces the chance of shrinkage cracking and allowing the mortar to cure slowly hence achieving maximum strength.