As portland cement hydrates or hardens it produces calcium hydroxide (Ca(OH)2), as a by-product — becoming 15- to 25 percent of the mass of cement paste. This means an average mix design develops quite a lot of calcium hydroxide, as much as 100 to 140 pounds per cubic yard for an average mix of concrete. It readily dissolves in water and can move in solution through the capillary pore structure to the surface of a slab. It is alkaline and combines with carbon dioxide from the atmosphere to make calcium carbonate, that white cloudy stuff that makes plain concrete have a whitish cast and colored concrete to have a lighter shade — often referred to as efflorescence.
But calcium hydroxide also reacts with silicate compounds to produce calcium silicate hydrate (C-S-H), the primary chemistry that gives concrete its strength and hardness. These compounds are typically applied on the surface of hardened slabs where they partially fill pores; increasing the wear-ability, gloss, and interesting colorations in the case of acid etch stains. But what happens if a floor is hard-troweled, producing a very dense surface layer that liquid can’t penetrate? Would there be any significant result? What would happen if a unique silica solution was mixed into the surface of fresh concrete when finishing procedures are initiated? That’s what Multiquip Inc., Carson, Calif. had in mind when they developed the SlabArmor solution for floors.
Harnessing the power of calcium hydroxide
Ben Wiese, Product Manager/Application Specialist for Multiquip Inc. says their goal is to use available calcium hydroxide in the surface region of fresh concrete to chemically react with their product to create additional C-S-H. This creates high performance floors by creating more cementitious material throughout the densified, finished surface. By reducing the calcium hydroxide in the surface region of a floor the pH also goes down.
Steps for constructing dense finishes
Wiese stresses their patented and proprietary process is both mechanical and chemical. Good finishing processes must be observed to achieve the desired results.
After fresh concrete is struck off, workers spray an application of SlabArmor on the freshly struck concrete. This can be achieved by using hand held sprayers right behind the strike-off board or if an area is too large to reach with a sprayer, the first application can also be sprayed immediately in front of the first pass with pan floats on troweling machines. Some contractors doing this work also attach spray nozzles to their laser screeds, applying the material as the concrete is screeded. So the surface layer of bull-floated and pan-floated concrete is thoroughly mixed with SlabArmor.
Anthony DeCarlo Jr, Vice President of TWC Concrete Services, Cincinnati, Ohio, states that his company has used SlabArmor on several projects and says the Step 1 of this two part product prevents moisture loss, leading to a more consistent setting of the concrete. It also retards initial set slightly, depending on temperature, so finishers must wait a little longer before they continue finishing operations. This moisture retention quality is extremely beneficial in warmer temperatures when concrete set time is accelerating and when surface moisture loss occurs rapidly.
Wiese says they require three applications (1,200 sq. ft/gal/application) of their product during the panning process, easily achieved by placing it in the spray tanks of a finishing machine and spraying the slab on each pass.
Aside from the application of SlabArmor and a small time extension in the finishing process there are no differences in finishing procedures. During the hard-troweling process DeCarlo says the product can help finishers close the surface and combat typical surface finishing problems like trowel marks and surface tearing from the trowel machines due to high surface moisture loss.
Though the product has been available since 2009, SlabArmor continues to be tested to verify what Multiquip Inc. thinks the benefits are. Tests include surface wear-ability and abrasion resistance, strength, stain resistance and possible use as a curing material. Here’s a list of benefits:
- When slabs are finished to provide dense surfaces, water (water of convenience) is trapped in the thickness of the concrete providing there is a good vapor barrier under the slab. The result is better cement hydration and reduced curling.
- Though the densified layer of finish on floors is largely a matter of the mechanical finishing process, the formation of more C-S-H from the SlabArmor and calcium hydroxide reaction increases the density and the wear-ability of the finish.
- The finishing process tends to go more smoothly despite irregularities between concrete loads and more extreme ambient conditions.
- The shine on the floor after final troweling is greater because surfaces are harder and denser. Floors can approach the look of a diamond polished appearance.
- There’s a cost benefit if a floor is going to be densified anyway — i.e. No need to mobilize and pay a crew to come in and clean and densify a slab if you treat it while finishing. Also there are construction schedule savings of a few days that would otherwise be spent cleaning and densifying.
- When floors are diamond polished, the resulting finish is noticeably more shiny and reflective.
DeCarlo states the cost of using Step 1 of this product can be mitigated by using it to replace standard evaporation retardant and curing compounds. The SlabArmor Step 2 “Closer” application improves gloss and further densifies the surface, adding to owner satisfaction.
Though it doesn’t represent a direct cost saving, colored floors can become very expensive after construction is complete if efflorescence covers the colored concrete with a whitish layer that changes the color that people see. SlabArmor interacts with nearly all the calcium hydroxide near the surface, virtually eliminating the possibility for efflorescence.
As part of the diamond polishing process today installers typically apply lithium silicate hardeners when the metal-bond diamond polishing sequence is complete, just before resin-bond final polishing is started. SlabArmor floors do not require this step and can produce denser surfaces resulting in much higher gloss readings.
The introduction of SlabArmor represents new technology. To date there is significant experience to suggest the value of using the product. As time passes the testing currently under way should provide the hard evidence needed. In the meantime contractors see the value of using it to get the results they want.