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What are Cinder Blocks?

Michael Pollick
Updated May 17, 2024
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Some construction projects call for a larger masonry block than a standard brick, but solid concrete blocks can be very expensive and very heavy. One common compromise are largely hollow masonry blocks known as cinder blocks. These are also sometimes referred to as concrete blocks, breeze blocks, or concrete masonry units (CMUs), though these terms have nuanced differences among them. Cinder blocks are generally lighter than solid concrete blocks, which makes them easier for brick masons to place in position. The hollow spaces in the blocks also provide some natural insulation or allow grout to be poured inside the rows of masonry.

Cinder blocks differ from concrete blocks in other ways besides their hollow design. Concrete blocks are made from a slurry of Portland cement and small aggregate, such as small stones or gravel. This makes them heavier and smoother than cinder blocks, which are made from a combination of Portland cement and cinders, the dusty remnants of burned coal.

When bricklayers work with cinder blocks, they generally use techniques similar to standard brick laying. The alternative rows of blocks are carefully offset so that the second layer stabilizes the first. A line of mortar is put down between each layer, so the actual dimensions of a standard cinder block may be adjusted slightly to accommodate the mortar. Corners may be finished out with half blocks, or interlaced to create a four-cornered structure.

Because cinder blocks do not have a significant amount of tensile strength, concrete is often poured vertically into the hollow chambers to provide more stability and strength. An metal rod called rebar is often placed vertically in the hollow chambers as well to reinforce the poured grout and the wall in general. It is not unusual to see rows of blocks with lengths of exposed rebar on construction sites.

The problem with using grout with standard cinder blocks is block placement. The blocks must be carefully threaded over the rebar before they can be put into place. This may not be a problem for shorter projects such as home foundations, but it would be difficult and time consuming to thread individual blocks over 20 feet sections of rebar. There is a solution to this problem, however. Some cinder blocks, called speed blocks in the construction industry, are open-ended, generally shaped like the letter H. These speed blocks can be maneuvered around existing rebar and tilted into place by a skilled bricklayer.

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Michael Pollick
By Michael Pollick
As a frequent contributor to About Mechanics, Michael Pollick uses his passion for research and writing to cover a wide range of topics. His curiosity drives him to study subjects in-depth, resulting in informative and engaging articles. Prior to becoming a professional writer, Michael honed his skills as an English tutor, poet, voice-over artist, and DJ.
Discussion Comments
By anon988979 — On Feb 15, 2015

Cinder blocks and concrete blocks are not the same thing. Many younger folks say "cinder block" when speaking of concrete (cement) blocks.

A cinder block was (is?) made using cinders from burning coal or other fuels as the aggregate required to hold the block together. Mostly cinders from coal burning power generation plants were used.

To my knowledge the last cinder blocks in the U.S. were made during World War II. Concrete was being used for war time construction. Thus, the cinders were used for the aggregate.

These blocks were not as strong nor waterproof as were concrete blocks. Which is why they are not used much , if at all, in the US.

They are still being used elsewhere. They dark gray in color.

By anon268242 — On May 13, 2012

I want to install a hinged television bracket. What are the risks and what are the precautions I can take?

The bracket is quite heavy.

By anon256886 — On Mar 23, 2012

This is an answer for anon148112. For a 4 feet by 40 feet, the first run is 42 blocks, counting for corners. Multiply that by however many blocks high you're going to go. You will end up with extra blocks, but this is a good thing, because it's much better than getting halfway into your project and finding faulty cinder blocks that cannot be used. Therefore, you will have enough to finish your project.

Most places like Home Depot and Lowes will allow for a return on the blocks that were not used for a refund. This is a measurement for cinder/concrete blocks, a.k.a "breeze" blocks, and make sure at least eight of those blocks are corner blocks. If you end up with more corner blocks than planned, no worries. These can be used in straight runs with a slight increase of mortar application, and make sure to keep your mortar mix consistent to allow for "settling" of the blocks.

By anon148112 — On Jan 31, 2011

How many cinder blocks are needed to build a wall 4' x 40'?

By anon136905 — On Dec 24, 2010

I was told that cinder blocks float in water and do not sink. Is this true?

By anon117413 — On Oct 10, 2010

How old is cinder block technology?

By anon110150 — On Sep 10, 2010

The white stuff is called efflorescence. It can be cleaned off with a weak acid solution. Read up on the process and be careful as most folks use hydrocloric acid.

If your cabinets are hanging on a cinder wall you shouldn't have a problem so long as you use proper wall anchors (not nails). An expanding anchor should do the trick.

By shore — On Aug 12, 2010

My basement cinder blocks are bleeding a white color. what is it?

By anon91697 — On Jun 23, 2010

i like cinder stone a lot because i have my kitchen constructed on cinder stone.

By anon73030 — On Mar 25, 2010

our cinder blocks are bleeding a white color, almost like a stain, could someone tell me what it is. thanks

By anon71864 — On Mar 20, 2010

quality wood construction is actually lighter than melamine or particle man made substitutes. Newer refrigerators are way lighter than the old ones from the 70'2 and 80's. Don't you have a refrigerator in your kitchen now? Most of today's products are lighter in weight, because it is all cheap stuff compared to the old products they replaced.

By anon62350 — On Jan 26, 2010

Our kitchen is constructed on cinder blocks and I want to add to the cupboards but am cautious about the extra weight on the cinder blocks. The new cupboards will be wood, and the existing cupboards are melamine, plus a fridge. Anyone's opinion would be appreciated.

Michael Pollick
Michael Pollick
As a frequent contributor to About Mechanics, Michael Pollick uses his passion for research and writing to cover a wide...
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