Rock salt.. good idea!! BTW, pond makers use pineapple to help cure their concrete ponds faster.... same principal as vinegar...
BTW, making concrete is almost an art... water to cement ratio is one of the factors, more water more viscous but less strength.. the other is type, size, amount of aggregate (pebbles, sand, gravel, etc) this gives other properties like texture, additional strength, etc etc..
Best way is to trial and error.. or get tried and proven recipies from abroad...