Why Densifiers and Hardeners Do Not Always Fix Dusting Issues w/ Concrete?

Posted by The Concrete Guy on Aug 11th 2020

The primary use of densifiers and hardeners is to strengthen the top part of a surface to the depth of penetration to increase the strength, abrasion and wear resistance, and durability of a surface. Common types of densifiers and hardeners are sodium silicates, potassium silicates, lithium silicates, and, now more recently, colloidal silicas. They are routinely used on high traffic/ heavy use smooth troweled concrete surfaces typically found in garages, workshops, pole barns and sheds, warehouses, distribution centers, manufacturing facilities, etc. where the long term durability and wearability of a floor is important. Another use of densifiers and hardeners is as a polishing aid to harden the top part of a surface before it is subjected to the polishing process for burnished/ polished concrete surfaces such as those commonly found in Big Box stores.

Densifiers and hardeners work exceptionally well on concrete that is of a good mix design and was poured, placed, finished, and cured properly. They are not intended to remediate defects in concrete surfaces and, as such, may not always fully address a defect such as chronic dusting or chalking. Simply put, applying a densifier and hardener to bad concrete does not all of a sudden convert it into good concrete. With structurally sound concrete that has no integrity issues or existence of any current dusting or chalking issues, a densifier and hardener significantly adds to the strength (up to 25% improvement) of the top 1/8" or so of a smooth troweled or polished concrete surface, thereby, prolonging the durability and wearability of the top part of a surface. The type of dusting that a densifier and hardener is typically effective at controlling is any future dusting issue that is the result of wear or weathering of the top part of a surface that occurs over time.

However, many people, professionals and DIYers alike, mistakenly assume that densifiers and hardeners will correct all forms of dusting problems including ones related to defective concrete where a surface was compromised at installation and a dusting or chalking issue is already present on the top part of a surface. While a densifier and hardener may provide some benefit in these situations by mitigating or lessening the issue, especially in cases where the defect is small and dusting or chalking is limited, a densifier and hardener may not always address dusting issues, in whole or part, where the defect is more pronounced and the dusting issue is more severe. In cases where a densifier and hardener are deemed to be the appropriate fix (and confirmed by testing) on a compromised surface for strengthening the top part of a surface and at least reducing a dusting or chalking issue, it is best practice to put down 2-3 full applications of a densifier and hardener following the manufacturer's instructions and allowing the product to fully dry between applications.

So, what renders a slab defective and causes a dusting or chalking problem to begin with? Dusting or chalking is the result of a thin, weak layer, called laitance, composed of water, cement and fine particles on the top part of a surface. There are many possible causes but almost all relate to the cement to water ratio being altered at the top part of a surface during placement and finishing or excessive evaporation of moisture during the curing period. Either scenario does not allow concrete to set up properly in terms of its requisite strength, rigidity, and hardness. Instead, it results in a soft, brittle, dry and "punky" surface that is prone to dusting and chalking. An affected surface also easily scratches with a nail, sluffs off without much abrasion, and powders under light foot or vehicle traffic or by mere sweeping of the surface. Wiping your hand over such a surface can also result in dust or chalk accumulating on your hand.

Common culprits that can lead to defective concrete and dusting and chalking of a concrete surface are:

  • Prematurely troweling concrete with bleed water still on it
  • Water inappropriately applied to a surface during the finishing process
  • Exposure to rain, snow or other moisture during the finishing process
  • Troweling condensation moisture from warm humid air back into a concrete surface
  • Poor mix design with low cement content or too wet of mix
  • Direct placement over a moisture barrier (ex. polyethylene) which reduces normal absorption from beneath and increases bleed water on top of a surface
  • Spreading dry cement over a surface to remove bleed water and accelerate finishing
  • Inadequate curing (rapid surface drying due to heat, low humidity, or wind) or freezing of a surface
  • Carbon dioxide from heaters can cause carbonation on top of a surface which greatly reduces surface strength
  • Insufficient compacting or poor finishing techniques can also result in a weak top layer

Bottom line, during the placement and finishing of concrete, the objective is to keep excess water and moisture out. During the curing period, the objective is to keep as much moisture in as possible.

Of course, age of a surface can also lead to a surface dusting or chalking but that is a different type of dusting than that caused by defective concrete. An old or aged surface, especially one subjected to significant traffic, use, wear, or weathering, may also dust or chalk due to the top part of the surface beginning to weaken or deteriorate. While concrete is a very durable building material, it has a finite life and at some point it begins to deteriorate and erode. Surface dusting or chalking is one of the first signs of deterioration on old surfaces along with cracking, pitting, scaling, spalling, delaminating, salt/ deicing chemical damage, corrosion of reinforcement steel, and eventual crumbling.

In situations where a densifier and hardener is determined to not be the right solution to resolve a dusting or chalking issue, other more effective solutions may be:

  • Grinding off the top part of a surface and then honing it back up (only if a smooth surface to begin with)
  • Applying a topical acrylic sealer to a surface
  • Using rugs or mats over the top of a surface
  • Covering a surface with carpeting or tile
  • Installing a topical coating such as an epoxy, polyurea, or polyaspartic over a surface
  • Using a resurfacing compound or overlay material to fix the top layer of a surface
  • Removing and replacing a concrete slab altogether