[dmun]

The whole pompeii oven project is designed for people who never mixed mortar before. Stacking blocks and filling is something anyone can do - If you don't like how it looks, or something isn't level, you can go back and restack them before you fill the cores. Mortaring cement blocks is a difficult thing to do. You are basically setting up a thin rim of mortar on the edges of the hollow block, unlike a brick, where you are covering the whole flat surface. I built my whole internal chimney with mortared four inch blocks, as well as my internal oven base. It's not much to look at, and now I am covering it with tile. I'm frankly in awe at a mason who can lay up a wall of blocks with smooth even mortar joints. It's a commonplace thing, but it's not easy to do.