First of all, the dust is not typically a problem for most concrete products. Over time, as the concrete is exposed to the environment, the mild acids from rain and carbon monoxide will reduce or eliminate this dust. The dust, or powder, is calcium oxide (lime). The primary problem with the dust is that paints and coating will not stick to the dusty surface. Also, since the lime reacts quickly with acids to neutralize it, the acidic degradation will etch the concrete and lead to more problems later in the life cycle. In some cases, as with Microbial Induced Corrosion, the reaction will form calcium sulfate (gypsum) which further reacts to form an expansive gel within the concrete.
In the hydration process, water is used to hydrate cement to form a gel called C-S-H, or Calcium Silicate. For every unit of gel created, about two units of a by-product called calcium hydroxide are released. Some, but not all of the calcium hydroxide is used to form other compounds. Any calcium hydroxide that is left will eventually dehydrate, leaving free lime on the surface.
One of the most frequent causes of surface dusting, especially in precast concrete, is poor curing. Concrete products are often stripped from forms while the hydration process is still occurring. The form provides an impermeable skin that hold the moisture in the concrete. When stripped, the moisture evaporates quickly unless the concrete is placed in a moist curing environment.
Another cause of dusting is a mix design with too much water. This is especially true in flat work such as floors, sidewalks, and parking lots. The excessive water evaporates from the surface too quickly leaving behind a weak and porous surface. This is made worse as foot and vehicle traffic "grind" the surface into a fine powder.
There are several methods which can be used to reduce or eliminate the dusting effect in your concrete products. In this blog, I will explain three methods that I advise precasters to follow. The first method involves mix design, the second method requires better curing practices, and the third method is a product that can be applied after stripping the casting.
The best way to produce high quality products with durable, dust free surfaces, is to use an appropriate mix design. Have an engineer or mix design specialist create a volumetric mix that follows the requirements outlined in ACI 211.1, the publication for proportioning normal weight concrete. A low water to cement ratio is important for durability and low porosity. In addition, select a material to use as a supplement to the ordinary Portland cement. Some examples are: slag, fly ash, silica fume, colloidal silica, etc. The first three are readily available in the United States. Colloidal silica is the latest technology, and is gaining support in the concrete community. Each of these are secondary cementitious materials (SCM's), and they react with the free calcium hydroxide by product in the hydration process to form calcium silicate.
In addition to a good, high quality mix design, proper curing is essential. Concrete hydration occurs in stages. The first three phases occur within the initial 10 to 20 hours, and involve a releasing large amount of heat during the hydration. Stage four will primarily occur over the first few days, but continues for weeks. Actually, as long as concrete has access to moisture, the curing process of stage four will continue. Concrete that is exposed to air at an earlier stage will have a lower strength and an increased porosity. The chart below shows how significant this effect is.
The last method I will mention is the application of a penetrating reactive sealer. There are several types of concrete sealers on the market. For the purpose of this blog, I am going to focus on the reactive silicate type. A reactive sealer is one with very small molecules which penetrate deeply into concrete. Some claim to penetrate several inches, although this is more of a property of the capillaries in the concrete than the physical properties of the chemical. Most of these are now available in water based formulas which do not have serious environmental concerns. One brand in particular is made by Concrete Sealants, Inc. in New Carlisle, OH. It is called ConBlock SH.
Products like ConBlock SH penetrate the concrete and react immediately with the lime (calcium oxide) to form calcium silicate (C-S-H gel). The reaction begins initially, making the concrete dust free within a few minutes. The complete reaction takes a few weeks. Some of the benefits to the concrete include a more durable, hard, abrasion resistant surface. It also reduces the porosity of the concrete. The concrete has better resistance to many acidic products like acid rain, carbonation, vinegar, and pickle juice. The surface is also more resistant to freeze thaw damage when exposed to a chloride solution.
The photo above shows two blocks made from the same, non air-entrained concrete. Each were cycled 100 times through freezing and thawing with a chloride solution on the surface. The block treated with ConBlock SH outperformed the untreated block. Another benefit of ConBlock SH is that is is a great primer for other paint, flooring, or sealant application. To learn more about ConBlock SH, click on the hyperlink, or call 800-332-7325. If you are not a precast concrete company, then contact your local precaster and ask them how you can purchase ConBlock SH.
Concrete dusting is a naturally occurring process. It can be reduced or eliminated using good production practices. Commercially available products can be applied to concrete surfaces soon after they are removed from the mold, or many years later, to improve the surface qualities of most concrete surfaces.
I had no idea that the first three phases occur within the initial 10 to 20 hours. I thought it would have taken much longer. It's good to know that the first three phases go faster. Thanks for sharing! http://www.cpcaustralia.com/products
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