Chalky Concrete? How Efflorescence Forms and How to Stop It

We've all seen it - that white, powdery deposit that mysteriously appears on concrete surfaces. Known as efflorescence, this chalky substance can form on fresh concrete slabs or existing buildings, leaving behind an unsightly stain. But what causes these annoying white stains? And more importantly, how do we get rid of efflorescence?

Efflorescence refers to white salts appearing on concrete surfaces as a result of a moisture process. It forms when water dissolves certain soluble salts within the concrete, carrying them towards the surface. As the water evaporates, it leaves the salts behind on the concrete, creating those telltale white stains.

Appearance and Composition

Visually, efflorescence resembles a white powder or chalky crust on concrete surfaces. The deposits are typically composed of soluble salts like calcium hydroxide, sodium carbonate, potassium sulfate, and calcium carbonate. These minerals dissolve in water and migrate upwards through the concrete. While the salts themselves are harmless, their staining effect can be an eyesore.

Not a Structural or Health Issue

The good news is efflorescence itself doesn't pose any structural or health issues. It's mainly an aesthetic problem that doesn't affect the strength or integrity of concrete. However, severe or recurring efflorescence can indicate an underlying moisture issue that may require further investigation.

What Causes Efflorescence to Form?

For efflorescence to occur, three key factors must be present:

• Soluble salts within the concrete
• Moisture to dissolve the salts and transport them
• Surface pathways allowing salt solutions to migrate

Soluble Salts in Concrete

All Portland cement contains alkali salts, either naturally occurring in the raw materials or added during manufacturing. Hydrated cement also produces calcium hydroxide. These soluble compounds provide a ready supply of efflorescing salts.

Moisture Migration

Water is necessary to dissolve concrete salts and carry them through capillaries towards the surface through a wicking action. This moisture may already exist within fresh concrete, or can come later from external sources like the soil, landscaping, cleaning processes etc.

Concrete Permeability

While cement paste is relatively impermeable to moisture, concrete contains pores and microcracks that allow water and salt solutions to migrate. Without these pathways, soluble salts would have no route to reach the surface and effloresce.

Types of Efflorescence

Broadly speaking, efflorescence is categorized as either primary or secondary efflorescence based on when it occurs.

Primary Efflorescence

Primary efflorescence takes place in newly poured concrete, appearing days or weeks after initial placement while the concrete is still green. It results from the water used to mix and place concrete evaporating upwards through the slab. This moisture carries dissolved salts which deposit on the surface.

Secondary Efflorescence

Secondary efflorescence occurs after concrete has hardened and cured. Also called recurrent efflorescence, it happens when external moisture penetrates the slab from underneath, the side edges, or the surface. Common culprits include moisture rising from the soil, improper slab drainage, landscape watering, or excessive cleaning water.

Preventing Efflorescence

While a minor amount of primary efflorescence is common, severe or recurring secondary efflorescence indicates a moisture problem. Here are some ways to avoid efflorescence:

Concrete Mix Design

Specifying low-alkali cement and avoiding excess mixing water minimizes the amount of soluble salts available to effloresce. Using less water also reduces bleed channels.

Proper Curing

Allowing the appropriate concrete cure times controls moisture evaporation from the slab surface. Pre-sealing the slab also prevents bleed water channels.

Surface Treatments

Sealers, coatings, and surface hardeners protect concrete from external moisture sources. However, trapping salts below an impermeable sealer can worsen efflorescence.

Site Planning

Installing proper site drainage, vapor barriers under slabs, and slope/grading to prevent pooling eliminates moisture sources leading to secondary efflorescence.

Removing Existing Efflorescence

Light Efflorescence

Minor surface dusting often diminishes over time. Alternatively, light deposits can be removed by dry brushing followed by water washing. Commercial efflorescence removers also available.

Severe Efflorescence

Heavier chalky buildup may require pressure washing or acidic removers to dissolve. Abrasive sandblasting can also mechanically remove stubborn deposits.

When Professional Help is Needed

While surface efflorescence is mainly an aesthetic nuisance, severe or recurring deposits can indicate substantial moisture issues leading to structural defects over time. Consult a specialist if you observe:

• Extensive cracking or concrete deterioration
• White staining reappearing rapidly after removing
• Efflorescence covering large areas
• Interior floors showing deposits

Professionals can diagnose underlying causes by testing slab alkali levels, ground moisture content, drainage adequacy etc.

To prevent future efflorescence, adopt these long-term moisture protection strategies:

• Install proper below-grade waterproofing and drainage
• Apply protective surface treatments to repel water
• Maintain sealers, repair cracks
• Adjust landscape watering to avoid wetting slab edges

efflorescence can be controlled with vigilance and preventative steps.