What is Efflorescence in Concrete? An Explanation
Concrete is a fundamental building material used in countless structures around the world. From sidewalks and driveways to the pillars supporting skyscrapers, concrete provides strength, durability, and versatility. However, this ubiquitous material can sometimes display a mysterious white, powdery substance on its surface known as efflorescence.
Efflorescence on concrete may look like a minor cosmetic issue, but it can indicate underlying problems that impact the concrete's structural integrity and lifespan. By understanding what causes efflorescence and how to prevent it, contractors, crews, and even DIY homeowners can take proactive steps to avoid bigger headaches down the road.
What Is Efflorescence?
Efflorescence refers to white crystalline deposits that form on concrete and masonry surfaces. It typically occurs when water molecules dissolve soluble salts within the concrete, carrying them to the surface. As the water evaporates, it leaves the salt crystals behind, creating a white, chalky substance on the concrete.
These soluble salts can originate from the concrete materials themselves or from external influences in the environment. Common examples include calcium hydroxide, sodium carbonate, potassium sulfate, and magnesium sulfate. When exposed to water, these minerals dissociate into their ionic forms, become mobilized, and later recrystallize when the water evaporates.
Primary vs. Secondary Efflorescence
While the mechanism is essentially the same, there are two main types of concrete efflorescence:
- Primary efflorescence happens during the initial concrete curing process when excess water evaporates from the slab. This leaves any inherent soluble salts from the concrete mix right at the surface.
- Secondary efflorescence occurs later, after the concrete has cured. External moisture penetrates the slab from underneath or above, dissolves salts within, and transports them to deposit at the surface as efflorescence.
Causes of Efflorescence
Pinpointing what specifically causes efflorescence involves investigating factors during concrete's mixing, transporting, placing, finishing, and curing processes. Efflorescence tends to result from excess moisture exposure combined with the presence of soluble salts.
High Alkaline Chemical Reaction
As cement hydrates during mixing and curing, highly alkaline calcium hydroxide forms within the concrete. When this compound contacts moisture, it produces a solution with a pH around 12 or 13. If this caustic solution penetrates pores within the slab, it can react with silica compounds in aggregates or admixtures to form more soluble salts that later manifest as efflorescence.
Water Penetration
External water penetrating a concrete slab also plays a major role. Sources of moisture can include rain saturation, excess landscape irrigation, high water tables, improperly drained bases beneath slabs, hydrostatic pressure, and groundwater contamination. As this water seeps into the concrete, it dissolves salts and transports them towards the surface through capillary pores in a wicking action.
Excess Moisture
Finally, excess water during concrete placement, finishing, or curing can increase bleed water, leading to a higher concentration of salts at the surface. Using excessive amounts of water for cleaning or preparing slabs can also introduce more moisture. Additionally, variations in temperature and humidity during curing may exacerbate efflorescence by increasing evaporation rates before salts have a chance to crystallize within the slab.
Identifying Efflorescence
Detecting efflorescence simply involves looking for some telltale signs:
White, Powdery Deposits
The most obvious feature is white or greyish deposits that form on concrete surfaces. These powdery substances occur when water evaporates after transporting soluble salts to the surface. The texture resembles chalk and can easily smear if touched.
Located on Surface of Concrete
While staining may penetrate deeper into the concrete matrix, efflorescence always deposits right on the concrete's surface. It typically appears on horizontal slabs but may also occur on vertical surfaces when ample moisture enables wicking action through tiny pores.
Effects of Efflorescence
Understanding efflorescence's potential effects makes it easier to prioritize treatment.
Cosmetic Issues
The most apparent impact of efflorescence lies in aesthetics. The white deposits present an unsightly film that compromises the look of decorative concrete. If left untreated, the continuing cycle of moisture exposure and salt deposit formation causes efflorescence to recur, exacerbating visual flaws.
Potential Structural Damage
Although rare in conventional slabs, severe efflorescence may indicate more serious issues. As moisture repeatedly infiltrates, moves through, and evaporates from concrete, it can gradually weaken surfaces through subflorescence. This is where crystals grow in air pores underneath the slab, causing internal structural damage.
Preventing Efflorescence
Taking proactive measures during concrete placement provides the best defense against efflorescence:
Proper Concrete Mix Design
Optimizing concrete materials minimizes excess moisture and soluble salts. Using aggregates with fewer soluble compounds lowers the concentration of constituents that may contribute to efflorescence later on. Monitoring slump also helps moderate free water. Specifying air entrainment introduces deliberate voids that give salts a place to crystallize internally rather than being pushed to the surface.
Curing Concrete Properly
Following ideal curing protocols limits excessive moisture evaporation that can draw salts upwards. Controlling temperature changes prevents accelerated bleed water evaporation. Using damp burlap or plastic sheeting retains moisture. Allowing adequate time for complete hydration reduces permeability from excess pores while enabling salts to fully integrate into concrete's matrix.
Applying Sealants
Sealing finishes like curing compounds, densifiers, epoxies, and acrylics helps prevent external water sources from penetrating slabs after placement. Impregnating sealers chemically react within the concrete itself to reduce capillary forces that wick dissolved salts. Meanwhile, film-forming sealants create protective barriers against moisture intrusion from above the surface down into the slab.
Removing Efflorescence
Despite the best efforts, efflorescence sometimes still occurs. Addressing it quickly reduces potential recurrence. Tactics like pressure washing, chemical cleaners, brushing, or grinding physically remove deposits. However, unless underlying factors get addressed, efflorescence will likely return. Correcting deficiencies in drainage, irrigation, sealants, or mix designs provides the only way to prevent repeat occurrences.
Dry Brushing
For minor powdery deposits without underlying moisture issues, stiff bristle brushes may mechanically clean surfaces. This works best on fresh efflorescence before hard crystallization.
Chemical Cleaners
On older, stubborn accumulations, using acidic washes helps dissolve crystals so they can be rinsed away. Products often contain hydrochloric, phosphoric, acetic, or sulfamic acids. However, chemical cleaners should only be used with proper handling precautions due to health hazards associated with acidic exposure.
Pressure Washing
Adjustable pressure washers provide another physical cleaning method to remove efflorescence from concrete. The pressurized water jets away buildup without damaging surfaces. This works well on horizontal slab and vertical surfaces. Vacuums attached to the pressure washers can also extract dissolved efflorescence as it gets liberated.
Efflorescence vs Other Concrete Issues
In addition to efflorescence, concrete may experience several other common issues that share some similar traits. However, understanding the distinctions helps troubleshoot the appropriate remedies.
Scaling
Like efflorescence, scaling also causes surface deterioration and deposit buildup on concrete. However, scaling stems from freeze/thaw cycles rather than a moisture and evaporation transport mechanism. Additionally, scaling penetrates deeper than surface-level efflorescence. Appropriate air entrainment, limiting deicer exposure, and protective sealants help prevent scaling damage.