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Jim_In_Redondo 04-19-2011 12:09 PM

Applying mortar to cinderblocks
I am interested in updating an exterior cinderblock wall by giving it a smooth surface. Something like you might see in mediterranean, or some adobe, construction. Although kind of old (built in 1955, or so), the wall is in very good shape structurally. Water seepage and cleanliness are not current problems. The main problem is that the block is PINK and very, therefore, dated; also, it has never been painted. I expect to paint the finished project.

The current surface looks fairly porous, so I'm thinking that some sort of mortar would work. Any specific suggestions from experienced persons would be greatly appreciated.

I should add that I have construction experience in multiple phases of constuction including concrete flat-work and tile. It's just that this type of remodeling project is new to me, but I'm willing to tackle most things.

Thanks very much,

CSmith 04-20-2011 03:42 PM

Type S Portland Lime Cement(PLC). A typical type S mortar will work but will not give you the same bond strength because of the additives and fillers in a blended bag, there is simply less cementitious material.

mudmixer 04-20-2011 05:48 PM

For that application, a Type N mortar may be preferable.

Bond strength is not the same as adhesion, which you are after. The ASTM specs for mortar suggest using the mortar with the lowest strength possible because of the benefits of workability. - That all applies to use as mortar between masonry units and there are no tests to measure "bond".

You may want to consider using Thoroseal (cement based mixture) that is sold in the dry form and mixed just before use (follow instructions exactly). It will have a consistency of pancake batter and is a chore to put on and the wall must be wetted prior to the application. You can apply a second coat at the time listed on the instructions and it can be textured within reason. Using a latex additive (Acryl 60 or similar) in the mixing water.

Thoroseal has been used for decades in concrete repair and restoration and as base for other coating on architectural precast. - It is difficult to find in a big box store, but a good masonry or concrete supply store should have it, (white or gray).

CSmith 04-21-2011 03:45 AM

Bond strength can't be calculated but if you have ever had to tear down or clean a brick wall laid with PLC compared to a blended bag of the same type,there is no comparison. Anyone with any first hand experience with PLC know exactly what I am talking about.

Those are two ways I have tested adhesion and bond strength in the field, besides the obvious ingredient difference.

If the op desires to go with mortar S or N, a PLC mix the best way to go for his application. PLC adheres considerably better than a blended bag (because of the ingredients) with the same hardness destination. You want a stucco mix your not laying brick.

siriuschaos 04-21-2011 01:47 PM

Most of the exterior stucco systems will work on your project especially in your climate. A product I've worked with is 'Drivit' but many others exist. Just about any colour you desire can be had. If you can plaster you can do this easily.

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