Causes and solutions for efflorescence in autoclaved aerated concrete blocks.

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Alkali efflorescence (or salt efflorescence) in aerated concrete blocks is a phenomenon where soluble salts and alkaline substances within the block migrate to the surface with moisture and crystallize. The core causes and systematic solutions are as follows.

 The Root Causes of Efflorescence

Efflorescence is a complex process involving both physical migration and chemical reactions.

1. Internal Factors: The presence of soluble substances. This primarily includes the large amount of calcium hydroxide produced during cement hydration, but may also include potassium and sodium sulfates from the raw materials.

2. External Factors: Water acts as a carrier. Whether it's residual moisture within the block that hasn't dried completely, or later rainwater and moisture penetration, water dissolves these soluble substances.

3. Path and Result: Migration and crystallization. Under the pressure of water evaporation, the alkaline solution migrates to the surface through the capillary pores within the block. After the water evaporates, the calcium hydroxide remains on the surface and reacts with carbon dioxide in the air to form insoluble white calcium carbonate (the common white efflorescence); other salts crystallize directly.

The porous and loose structure of aerated concrete blocks provides more channels for moisture migration, making them more susceptible to efflorescence when raw materials or construction practices are inadequate.

Systematic Prevention and Solutions

Solving efflorescence requires a comprehensive approach, focusing on "blocking alkali, isolating water, and ensuring a dense structure," with control throughout the entire process.

I. Prevention is Key: Control throughout the entire process from production to construction

Optimizing raw materials and mix proportions: Use low-alkali cement; add active admixtures (such as fly ash, slag powder, and silica fume) to the mix. These react with calcium hydroxide, converting it into a stable gel, fundamentally reducing soluble alkali. Simultaneously, strictly control the water-cement ratio, minimizing water usage while meeting process requirements.

Improving production processes: Ensure that the blocks undergo sufficient steam curing to ensure more complete hydration and a more stable structure. Innovative processes, such as curing with carbon dioxide-containing flue gas, can neutralize and solidify internal alkali in advance, with significant results.

Standardizing construction and maintenance:

Ensuring dry blocks: Before installation, ensure that the blocks have sufficient curing time and meet the moisture content standards.

Using specialized materials:  It is essential to use special mortar and plaster for aerated concrete, whose water retention and bonding properties can prevent internal moisture and alkali from migrating outwards.

Ensuring waterproofing and drainage: Install a damp-proof layer at the bottom of the wall (such as a concrete guide wall); the exterior wall finish must be waterproof and breathable, and drainage should be provided at window sills, eaves, and other critical points to prevent rainwater penetration.

Avoiding early moisture exposure: During and immediately after construction, protect the walls from rain.

II. Treatment Methods for Existing Efflorescence

For existing white deposits, treatment depends on the severity:

1. Dry brushing: For mild, powdery efflorescence, it can be removed directly with a stiff brush or sandpaper.

2. Acid cleaning: For thicker, solid crusts (mainly calcium carbonate), a 1:10 dilute hydrochloric acid solution can be used for brushing and cleaning. Always wet the wall surface first, and immediately rinse thoroughly with plenty of water after acid cleaning to avoid internal corrosion.

3. Sandblasting: For large areas of severe efflorescence, light sandblasting can be used for physical removal. 4. Radical Treatment: Remove all efflorescence and damp decorative layers (down to the masonry base), and after thorough drying, apply a special anti-alkali sealing primer to the wall. Then, re-plaster and apply the finishing layers according to specifications. This is the most thorough method.

Key Points Summary:

In summary, for efflorescence in aerated concrete blocks, "water" is the direct trigger, "alkali" is the material basis, and the "porous structure" is the pathway. Prevention is better than cure. The core is to interrupt the moisture migration path and reduce soluble substances by using active admixtures, controlling moisture content, using special mortar, and ensuring proper waterproofing at joints. If efflorescence occurs, reliable moisture-proof sealing measures must be taken after removing the surface manifestations to prevent recurrence.