Clean the surface of your cement block chimney using a scrub brush and water. Remove any debris and allow the surface to dry thoroughly.
Fill a plastic bucket one-fourth full with sand mix. Sand mix, which is sold under a number of different brands, is a mixture of Portland cement and sand.
Add water to the bucket, mixing the sand mix and the water with a trowel, until the mixture is textured, but easy to work with. Do not add too much water or the mixture will become soupy and drip off the wall.
Wet the area of the cement block chimney that you are working on with a hose.
Spread the mixture onto the dampened cement blocks of the ceiling using a trowel.
Keep wetting the chimney and applying the mix until you have finished the batch. You should only make small batches of the sand mix at once because it dries within 30 minutes, which makes it impossible to work with.
Repeat the process until the entire cement block chimney is covered. You will need a ladder to reach the upper portions of the chimney.