At the moment I am redecorating my house and today I needed to fix a nasty crack that had opened up. So, using the hammer drill as a router, I opened it up and cleaned it out to about 1 inch wide. Vacuumed it and then sprayed it with water to pre-moisten the existing concrete.
Now, we all know what usually happens. You fill it with sand and cement mix, it dries too fast because the surrounding material absorbs the water, and as it dries it cracks.
I wondered ... is there anything in the lab I might use to improve the cement mix? So I went for a mooch.
I decided to add a small amount of Xanthan gum to the dry mix, and mix it in really well. Then I added some washing-up liquid. Proceeded as usual, the more I mixed it the more plastic-like it became. It resisted the water-loss problem just fine, was usable much longer, and the cracking problem was almost entirely eliminated. Hooray for gums!