I could get all scientific, as I spent many hours researching these two types of aggregate and how they differ…and suggest you do the same…however, for this article I am going to focus on the aspects that stood out to me the most and have more impact in the concrete overlay and decorative concrete market.
First you need to know that there are quite a few ingredients in concrete overlay or decorative concrete mixes that make them unique to the concrete industry. Decorative concrete, concrete overlay, Stamped concrete overlay, and customizable concrete veneer mixes are different from the concrete or mortar mixes you buy at your local hardware store or that come out of those big cement mixing trucks you see driving down the road.
Most decorative concrete overlay mixes have a blend of special ingredients, including polymer or acrylic, that are added to the portland cement and sand in a precisely measured powdered form at the factory or in a liquid form at the job site. The polymer and additives help give the concrete mix a unique workability and stickiness that creates a chemical and physical bond to hold the mix to the wall or floor and add a bit of strength and flexibility or “give” to the cured concrete…or flexural strength.
The real defining factor of why decorative concrete and concrete overlay ingredients are in the mix…is the extended open time they provide. The longer drying time (open time or working time) is the key and what allows the contractor time to not just quickly lay the concrete…but take a bit more time and work the mix, shape it, stamp it, carve it, etc. Standard concrete, ready mix, or truck pour concrete dries fast and gives an average 15 to 30 minutes or less of workable time, so you have to start stamping immediately and use heavy tampers to help drive the stamps into the concretes surface before it gets away from you and becomes too hard to stamp. Decorative concrete and concrete overlays, with all their ingredients, have an average 30 minutes to 6 hours of open time and the really good ones do not require any tamping.
Concretes strength comes from the combined ingredients…but especially the amount of portland cement, polymer and the “type” of sand used. Sand type is a critical component in concrete, decorative concrete and concrete overlays because some sand types break down quickly and will weaken the concrete over time. Some sands are so weak that they will cause the other ingredients to fail as the sand breaks down and creates voids, turns to powder, and causes a catastrophic breaking of the physical bond between all the other ingredients.
Yes…and not only have many contractors and homeowners experienced the difference as their overlays have failed, broken down and disintegrated in a short amount of time…science backs up what I am saying. When you buy, apply and sell concrete and concrete overlays, the word CONCRETE, for almost everyone including a customer, is synonymous for strength, hardness and longevity. But if you use the wrong sand in your mix…you can lose all those traits that make concrete…concrete. Let me be clear; All concrete, like natural stone, breaks down over time…but…if you use a weak sand, your concrete overlay will break down faster, lose its “look” and color quicker, last only a couple years and fail if exposed to the elements and any type of foot or automobile traffic.
Quartz and Limestone sand are the two main sands used in decorative concrete and concrete overlays…with Quartz sand being the sand used worldwide in almost every concrete mix where strength, durability and longevity are a factor.
Limestone sand is a softer sand…yes, sand can be soft…but I’m not talking soft in the way of a stuffed animal soft. Limestone sand is soft enough that the angular edges of the sand grains can be easily crushed, broken, chipped off and turned to powder. You can even do it in your bare hands by rubbing a handful of limestone sand together and watch as it starts creating powder. Limestone sand-based concrete overlays have trouble supporting foot traffic, chairs, tables, and even automobiles because it will scuff, scratch, chip, wear too fast, and even crumble and fail. Limestone sand in repair mortar has worked great for centuries when fixing columns and sculptures…but as a functional concrete overlay for patios, driveways and pool decks…you are taking a chance.
To the left is the MOHS hardness scale that has been designed to help people around the world choose the best mineral (sand) for their specific use. See MOHS Hardness Scale Below.
So, according to MOHS, you can clearly see why quartz is used when making concrete, decorative concrete and concrete overlays….it will simply stand up to more abuse and last longer.
I mentioned angular sand grain edges above…which may seem like they would be a strength factor as angular or sharp edges can help many things fit together more tightly than semi-angular or non- angular or rounded edges. However, in the case of crushed limestone…angular edges, although seemingly a strength…are actually part of their weakness. Because limestone is a softer mineral, It’s the angular edges, the sharp points and ridges that easily chip, break, and disintegrate with the elements and especially with any type of foot or automobile traffic.
Sand…by percentage (%)…is the main ingredient in concrete and adds much of the strength. When your main strength ingredient starts losing its edges, breaks down, turns to powder, and creates voids as the angular edges break or wear away…it can, and in most cases will, cause the other ingredients to lose their physical bond with each other and ultimately…fail or look worn and shabby. The most common Quartz sand is not crushed (crushed sand all has angular edges) and has “semi-angular” edges that lock together. Because quartz is so hard…the semi-angular edges lock together and even with foot or automobile traffic, they do not break off, wear easily or weaken the concrete.
Again, just because someone says their mix has Crushed angular sands, if it’s Limestone sand and you will be walking, driving, or exposing it to the elements…that is not a benefit. There is a reason Quartz sand is used worldwide for concrete mixes, quartz sand adds strength, durability, longevity, and the same lasting beauty as when it was applied.
Having seen all types of finished overlay projects made with all types of products. It is a combination of all the ingredients that add the realism with sand “size” and not the sand “type” being one of the larger factors in adding a more realistic look. Quartz and Limestone sand can look similar and come in many sizes, so if one mix looks more realistic than another, it could be the size of the sand used and not the type of sand used.
You really do not see the sand itself…the sand is immersed, coated and covered with all the cement and ingredients that when combined create the concrete mix, and just because someone says their sand has a unique look, you typically do not see the sand itself once you’ve mixed your mix and applied it.
To get what I mean about the look of the sand…see the picture below. There are 4 quartz and 4 limestone sand samples in the picture…can you tell for sure which are Limestone, and Which are Quartz? And remember…Most Concrete, Concrete Overlay, and Decorative Concrete mixes come in Gray or White…which means that even when dry in the bag of mix or already mixed and applied…you do not see the sand color because the sand grains are completely immersed and coated with the combined gray or white mix.
Another factor in a realistic look is the type of polymer or acrylic used. Many acrylics are denser than polymers and can create a plastic look as they coat the sands and ingredients instead of letting the sand grains help create a realistic look.
Also, sealers, coloring and skill level of the contractor are a big determining factor in how real the final look is, because a skilled contractor can make almost anything look real…while an unskilled contractor can ruin the look of even the best mix.
Closing the pores of the concrete in the mortar joints, like some stamps can do, can make it look fake. All factors to consider…and none of the ones I just listed are sand related.
In my research, I found Quartz sand, because of its unique properties, has other benefits over Limestone and is used throughout the concrete, paint and adhesive industry. Quartz sand helps paint and other products be more resistant to chemicals. Quartz is naturally acid and wear resistance because of its hardness and ability to take scrubbing, and it improves durability and flow ability of paint.
Additionally, quartz sand finds extensive applications in both silicone and industrial rubber where they are incorporated for their reinforcement qualities (the same reason most concrete overlays use quartz sand).
Quartz sand is even used in tire linings as it offers superior adhesion, tear resistance and heat aging properties, and when added to adhesives for tiles, improves considerably its tensile strength and impact resistance…again…all reasons it is also used over most other sands in concrete overlays and decorative concrete.
So…to answer the big questions of which is better for strength, realistic look, workability, resistance to wear and tear, chemical (stain) resistance, and more…in my research…Quartz Sand is clearly the better ingredient in a decorative concrete or concrete overlay mix.
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