Waterproofing Cinder Block Walls and CMU's

What Makes Cinder Blocks Different?Cinder block wall sealer

Concrete blocks and cinder blocks, as well as split face blocks and lightweight blocks are all referred to as CMUs (concrete masonry units). CMUs are used for the construction of load-bearing foundation walls, basement walls, partition walls, exterior walls, retaining walls, and non-load bearing partition walls. Employing reinforcement bars and filling the hollow cores with grout results in strong structural walls. CMU's may look similar but there are big differences among them.

Standard (high-density) concrete blocks are made from cast concrete – Portland cement, sand, and gravel. Compared to typical structural concrete, they are made with a higher percentage of sand and a lower percentage of gravel and water for a stiffer mixture which holds its shape when removed from the block mold. The typical high-density concrete block (8 by 8 by 16 inches with two cores) weighs 36 to 42 lbs. per block.

Lower density blocks may use industrial wastes, such as fly ash or bottom ash, as an aggregate instead of the sand or fine gravel used to make true concrete.

"Cinder block" is an archaic term from the time when power plants and coke ovens in steel mills produced huge amounts of "cinders" from burning coal. They were patented back in 1917. Modern cinder blocks are usually made with volcanic pumice instead of cinder unless there is a nearby coal-fired power station or cement kiln to supply pulverized cinders or fly ash.

There are builders that prefer cinder blocks because they are much easier to work with. A typical cinder block weighs only 26-33 lb. However, cinder blocks are more subject to moisture problems because of their high porosity and are most often used in interior non-moisture applications or outdoors for barrier walls and fences.

Did You Know – Some cinder blocks release mercury and other heavy metal contaminants which is another reason for sealing them.

Can you tell the difference between cinder blocks and regular concrete blocks? They look similar but cinder blocks are typically a darker gray color and their surface texture is more porous.

In lightweight CMUs, gravel is replaced with shale or clay expanded at extreme temperatures which makes the blocks much lighter at 22 to 28 lbs. each. Split-faced block have a rough, stone-like texture on one face of the block instead of a smooth face. Haydite block is another example of an expanded shale products. Lightweight blocks cannot be used in retaining walls or exterior building walls because they readily seep water.

Why Cinder Block Walls Leak

Cinder blocks are more porous than standard concrete blocks or poured concrete. Only a thin wall (1-1/4 inch) separates the hollow core of concrete blocks from wind-driven rain or water-soaked ground and, after the core fills up with water, from the indoor space. Cinder blocks are classified as permeable to water and vapor (>10 perms). Standard concrete blocks are semi-permeable – about 5 perms or, when the cores are filled with grout, about 2 perms. In comparison, a good quality newly poured concrete wall 8-10 inches thick is impermeable (0.4-0.3 perms).

If your foundation walls are cinder block, they are much more likely to leak than poured concrete walls after the exterior waterproofing coating deteriorates. It may take 5 or 10 years but the cinder blocks will be on their own to fight against water pressure. It's a losing battle – they are permeable!

Exterior above-grade cinder block walls are not subject to hydrostatic water pressure but wind-driven rain pushes water right into the blocks and eventually, through the wall.

Sealing cinder blocks will extend their life by protecting them against water, the main enemy of concrete.

Prevention is much easier and cheaper than cure! If you do not seal the cinder blocks with RadonSeal now, they may become unsealable in the future. Water seepage will enlarge the pores and efflorescence will leach out the concrete, making it harder to seal.

Extensive water seepage may carry soil through the concrete and fill the pores the blocks. The telltale sign would be brown or "reddish" color on the interior surface of the wall. The plugged pores would prevent RadonSeal from penetrating and reacting with the concrete. Thus, blocks unable to be sealed by penetrating sealers, waterproofing paints or parge coating. There is no cure!

How To Waterproof Your Cinder Block FoundationRadonSeal Concrete Sealer

To waterproof, strengthen, and preserve the overall appearance of your cinder block walls, seal the blocks permanently against moisture and efflorescence with RadonSeal Plus Penetrating Concrete Sealer, the strongest concrete sealer available. RadonSeal® is easy to apply by homeowners or waterproofing contractors. Follow these steps:

1. Remove Paint and Efflorescence – The surface of the blocks must be bare and porous. Remove any paint either mechanically (wire brushing, grinding, sand-blasting) or with our industrial strength Lightning Strip paint remover. Brush off crumbling concrete, dust, and dirt. Light efflorescence can be removed with a wire brush or wire-wheel attachment. Or spray on our Efflorescence Cleaner.

2. Allow the Cinder Block Walls To Dry Out – RadonSeal cannot react if the pores are filled with water. If the wall appears to be damp (wet spots) allow them to dry out. Ventilate your basement or garage by opening windows and doors. Install a fan to improve airflow. If the blocks are continuously wet, drill weeping holes using a masonry bit at the bottom of each core.

3. Apply RadonSeal® Plus – Apply four back-to-back applications of RadonSeal using a common hand-pump "garden" sprayer.

4. Optional: Ion-Bond Armor – Let RadonSeal cure for at least 10 days, remove any purged efflorescence, and spray on Ion-Bond Armor Subsurface Elastomeric Concrete Sealer. This is also a penetrating concrete sealer which disappears into the concrete, where it bonds and reacts to seal the concrete even tighter. For more porous concrete like cinder blocks, we strongly recommend using Ion-Bond Armor as the second sealer.

You can also read the article Waterproofing Concrete Block Foundations

How to Seal Hard-to-Seal CMUs!

Ion-Bond Concrete SealerCombing RadonSeal along with Ion-Bond Armor for use on cinder blocks has proved foolproof for many a homeowner. However, older cinder blocks and sandstone foundations, exposed to decades of water seepage, require special steps to properly fill and seal the enlarged pores.

There are many different types of concrete blocks that vary widely among regions and manufacturers. Since RadonSeal seals concrete by reacting with cement, it simply can not be guaranteed to seal blocks with little to no Portland cement. Sealers in general, will have difficulty sealing blocks with unusually large pores. For example;

  • Lightweight Blocks – Split face blocks, Haydite blocks, and dry-pressed blocks (all being much lighter than the 38 lbs. of standard concrete blocks). They can be usually sealed and waterproofed with our LastiSeal Brick & Concrete Sealer.
  • "Popcorn Blocks" – made with a very course aggregate with large toothpick-size holes.

The Thinset Mortar Method – Proved successful in many apparently hopeless situations:

1. Purchase thinset mortar – Good-quality thinset mortar (without vinyl additives) is available in the tile section of home improvement centers. We recommend Mapei "Keraset Professional Grade Dry-Set Mortar" because it is inexpensive (about $20 for a 50-lb. bag), spreads easily, adheres very well, and does not shrink.

2. Wet the surface and apply the mortar on the blocks –  First, apply RadonSeal Plus on the blocks. When still wet, push the mortar very hard into the surface using the smooth edge of a 3/16" notched trowel holding it at a 45° angle. The goal is to push the thinset into the pores rather than building up a layer. There will be lots of droppings on the ground. You can finish it off neatly by going over the mortar joints with a round tucking tool and make it look like a brand new block wall. Or, if you prefer the look of a poured concrete wall, leave a smooth layer (say 1/8") on the surface.

3. Seal the mortar using Ion-Bond Armor– Thinset is also a porous cementitious material and would soon leak. RadonSeal cannot react with the thinset because of additives. Allow four days for the thinset to cure and then, seal the wall with our Ion-Bond Armor in two successive back-to-back applications about 20 minutes apart. Ion-Bond penetrates through the thinset, bonds with RadonSeal inside the blocks, and permanently seals both the cinder blocks and the mortar.

More on RadonSeal Uses, Limitations, and Application

Spot Sealing with ElastiPoxy – A stronger alternative to thinset is troweling on a mix of our ElastiPoxy Crack Filler with sand, particularly if the blocks are in a very bad shape or already crumbling. But first, use RadonSeal Concrete Sealer to strengthen and waterproof the inside of the concrete blocks. You can use the same epoxy-sand mix to also fill cracks, holes, and other defects. The epoxy-sand mix delivers a repair that is stronger than the concrete itself. It will last forever! Check out the Picture Guide on Repairing Cracks in Concrete Block Walls.