The best product for a “cold joint” between old and new concrete, mortar, or any cementitious material is a liquid acrylic bonding agent. They sell it at HD and Lowes. About $10 or $12 per gallon. It’s in the section over by the bags of concrete. It’s looks like milk. You brush it on. Just be sure to follow the directions. In addition to brushing it on the old concrete or mortar surface, I always pour a little in whatever cementitious product I’m mixing. Replace about 20% of the water with the liquid acrylic. It makes the mortar / concrete, etc. super strong and helps it stick better. It also helps it with freeze cracking, because it kind of seals out moisture once it’s cured.