Views: 0 Author: Site Editor Publish Time: 2026-07-17 Origin: Site
In the manufacturing of Autoclaved Aerated Concrete (AAC), achieving a flawless, structurally sound block is a complex balancing act of chemistry, thermodynamics, and rheology. When production lines begin yielding blocks with horizontal fissures, vertical fractures, or micro-cracking, plant managers often look first to mechanical faults: a misaligned cutting wire, excessive vibration during transport, or an improper temperature gradient in the autoclave.
However, one of the most critical, yet frequently misunderstood, variables in diagnosing AAC cracking lies at the very beginning of the chemical process: the aluminum paste.
As the primary gas-generating agent, the quality, dispersion, and reaction kinetics of the aluminum paste directly dictate the internal pore architecture of the "green cake" (the pre-cured AAC block). If the gas evolution profile of the paste is misaligned with the rheological thickening of the concrete slurry, structural defects are inevitable. This article explores the scientific correlation between aluminum paste performance and AAC cracking, providing diagnostic insights for production engineers.
To understand why aluminum paste causes cracking, we must first examine the foundational chemistry of AAC. When aluminum paste is introduced to the alkaline environment of the slurry (comprising quicklime, cement, sand/fly ash, and water), it reacts to produce hydrogen gas. The generalized chemical reaction can be expressed as:
2Al+3Ca(OH)2+6H2O⟶3CaO⋅Al2O3⋅6H2O+3H2↑
This hydrogen gas creates millions of micro-bubbles, causing the slurry to expand in volume. For a flawless block to form, the gas evolution rate must perfectly synchronize with the setting rate (thickening) of the slurry. If these two curves do not match, the structural integrity of the matrix is compromised, leading directly to various types of cracking.
The Symptom: "Boiling" at the surface, internal horizontal fissures, and collapsed, oversized pores. The Cause: If the aluminum paste reacts too vigorously and releases hydrogen gas before the slurry has developed sufficient viscosity, the mixture cannot trap the expanding gas. The bubbles migrate upward, coalesce into larger voids, and escape the surface in a phenomenon known as "boiling." The Resulting Crack: Because the gas escapes rather than forming independent, uniform pores, the final block suffers from a lack of expansion height and severe horizontal stratification. When the block is transferred to the autoclave, the weak, irregular internal layers cannot withstand the high-pressure thermal shock, resulting in extensive horizontal cracking and catastrophic structural failure.
The Symptom: Settling cracks, surface tearing, and internal micro-cracking during the pre-curing stage. The Cause: Conversely, if the aluminum paste features poor reactivity or overly thick protective coatings, gas generation is delayed. In this scenario, the cement and lime begin their exothermic hydration, and the slurry begins to harden (lose plasticity) while the aluminum is still generating hydrogen gas. The Resulting Crack: The delayed gas expansion exerts immense internal pressure against an increasingly rigid matrix. Because the slurry can no longer stretch to accommodate the gas, it fractures. This typically manifests as vertical cracks, surface tears, or internal stress fractures within the green cake that widen drastically once subjected to the cutting machine or the autoclave's extreme heat.
Beyond reaction timing, the physical dispersion of the aluminum paste within the water-based slurry is a paramount factor in crack prevention. Modern factories use aluminum paste over dry powder to eliminate explosion hazards and improve mixing, but not all pastes are formulated equally.
Aluminum is naturally hydrophobic (water-repelling). Manufacturers must treat the paste with specific solvents or water-based surfactants to ensure it mixes seamlessly into the slurry. If a factory utilizes a low-quality paste with poor hydrophilic properties, the aluminum particles will agglomerate, forming concentrated clusters.
The Consequence: Areas with concentrated aluminum will over-expand, becoming ultra-porous and weak. Conversely, areas lacking aluminum will remain dense and heavy.
The Cracking Mechanism: During the high-pressure steam curing inside the autoclave (190℃ at 12 bars), the dense areas and the porous areas expand and contract at completely different rates. This severe differential thermal stress rips the block apart from the inside out, leading to unpredictable, jagged cracking patterns upon exiting the autoclave.
It is essential to recognize that aluminum paste performance is not static; it is highly sensitive to ambient temperature. A paste that performs flawlessly in the summer may cause severe cracking in the winter.
Cold Weather: Lower slurry temperatures slow down the chemical reaction of the aluminum. If the factory does not switch to a faster-reacting paste or adjust slurry temperatures, the result is delayed gas evolution and subsequent internal pressure cracks.
Hot Weather: High temperatures accelerate gas release, leading to premature foaming and boiling before the slurry can set. High-quality suppliers often formulate specific "Summer" and "Winter" grades of aluminum paste, adjusting the particle size and chemical coatings to maintain a consistent gas evolution curve regardless of the season.
If your plant is experiencing high rejection rates due to cracking, conducting an audit of your gas-generating agent is a highly recommended first step. Here is a troubleshooting protocol:
Gas Evolution Curve Testing: Before scaling to production, laboratory technicians should conduct gas volume tests on the aluminum paste at the exact temperature of the factory's current slurry. Plot the gas release over a 30-minute window to ensure the peak gas release matches the slurry's initial setting time.
Evaluate Active Aluminum Content: Verify the Certificate of Analysis from your supplier. Low active aluminum content indicates high levels of impurities (such as alumina or residual milling agents), which create unpredictable reaction kinetics and localized weak points.
Assess Suspension and Dispersion: Perform a simple water dispersion test. The paste should easily disperse into a uniform, milky-gray suspension without aggressive mechanical agitation and should not leave a heavy film of unmixed clumps at the surface.
While AAC block cracking can undoubtedly stem from mechanical mishandling or improper autoclaving cycles, the internal structural integrity of the block is forged entirely during the chemical expansion phase. Aluminum paste is not a commodity material where factories can simply prioritize the lowest price. It is a highly engineered chemical catalyst.
By sourcing high-purity, well-dispersed, and properly timed aluminum paste, AAC manufacturers can eliminate the internal stresses that cause pre-curing and post-autoclaving cracks. Investing in the correct gas-generating agent is ultimately an investment in product yield, minimizing waste, and guaranteeing the high structural performance that the modern construction industry demands.
