If you have plaster interior walls and want them to have a specific look like concrete, you can employ a faux technique to make the plaster appear to be cement. There are two methods to accomplish this transformation: painting and applying mortar. The first requires a lot of skill and prior experience, having substantial trial and error under your belt to get the right balance and look. The other method is more simple and direct.
Fill a bucket with water and add a couple of drops of dish soap. Wash the plaster with a sponge to get any dust or dirt off so you have a clean surface for mortar to adhere. Dry with clean cloths.
Rinse the bucket thoroughly and combine one part thin-set mortar with three parts masonry sand. Stir with a wooden trowel. Add water incrementally until the mortar turns to a pancake batter consistency.
Brush a concrete bonding agent onto the plaster wall. This will allow the thin-set mortar to stick to the plaster. Without it, the mortar will not adhere.
Stir the mortar mix and spread onto the plaster wall with a wooden trowel. Swirl and twist the trowel's spade to create your desired look. Sprinkle masonry sand onto the mortar to create a more textured look, if you want.
Allow the thin-set mortar to dry overnight. Once the mortar dries and sets into the bonding agent, your plaster walls will have a concrete look.