Clean out the crack to remove any loose debris. Scrub the hole with water and a stiff brush.
Pour hydraulic cement into one side of a wheelbarrow. Pour water into the other end. Mix the batch of hydraulic cement together with the water in the wheelbarrow until it has a thick consistency. Use the mixing instructions on the side of the bag for the correct water amounts.
Put on plastic gloves. Pack the hydraulic cement into the wall. Add rock to the crack if it is large to keep the hydraulic cement from coming out of the crack.
Buff the hydraulic cement with a cement grinder to even the cement with the wall. Paint if necessary.