The mechanism of aluminium in AAC production

During mixing, aluminium reacts with water in the alkaline environment of the slurry. The reaction is promoted by the presence of calcium hydroxide. This chemical reaction produces hydrogen gas, which subsequently expands to form a network of air pores throughout the green cake. This process yields a lightweight block with a significantly reduced density.

The overall chemical reaction can be represented as follows:

2Al + 3Ca(OH)₂ + 6H₂O → 3CaO·Al₂O₃·6H₂O + 3H₂

As a standard industry guideline, coarser aluminium powders are used to produce higher-density blocks, whereas finer grades are typically used for lighter-density products. The particle size directly influences both the volume and rate of gas evolution, which in turn establishes the density of the AAC blocks.

Required data from aluminium suppliers

To maintain consistent production results, AAC manufacturers should obtain the following critical data points from their aluminium suppliers:

• D50 particle size: This parameter provides an indication of how the powder will react during production. Suppliers must provide laser diffraction particle size distribution reports, which allow manufacturers to evaluate the material and modify their formulations using the provided D10, D50, and D90 metrics.

• Gas evolution curves: This information helps identify the timing of peak gas evolution from the aluminium paste or powder, enabling manufacturers to adjust their formulations accordingly.

• Blaine surface area: By assessing the surface area, AAC producers can calibrate their formulations to maximize material efficiency and reduce the overall consumption of aluminium powder or paste.

• Additive content and percentage: Complete transparency regarding additives is necessary to prevent unforeseen chemical interactions within the mix.

Arasan Aluminium Industries: product specifications

Arasan Aluminium Industries offers aluminium powder grades tailored to different density requirements. The currently available grades are summarized in Table 1.

Table 1: Available powder grades from Arasan Aluminium Industries.

Note: All listed grades can be supplied as a paste featuring controlled gas evolution, custom particle size distributions, and specified metal content.

Advantages of using multiple aluminium grades

Even when raw-material temperatures are carefully controlled, ambient conditions can vary significantly. Differences between morning and evening temperatures, warmer afternoons, and seasonal transitions (such as dry versus humid, or winter versus summer) directly impact reaction kinetics. Relying on a single grade of aluminium powder or paste under these fluctuating conditions frequently results in cracking, structural collapse, or an inconsistent rise.

Typically, finer grades are required in colder climates to ensure adequate gas generation. Conversely, in warmer or more humid environments, blending finer and coarser grades generally yields superior stability.

To address this need for greater flexibility, Dutch company Aircrete Europe has developed a Dual Aluminium Dosing System capable of handling two different aluminium grades simultaneously (see article in AAC Worldwide, pages 52-54, issue 4, 2023).

Case study: single vs. mixed aluminium grades

The Arasan Aluminium Industries partnered with a customer to conduct comparative trials using fly ash-based mixes. The trials evaluated the performance of a single grade of aluminium powder against a mixed-grade formulation. The trials were conducted at different times of day to capture variations in temperature and assess their effects on the final product.

Table 2: Comparative trial results (variable climatic conditions – fly ash based mix)

Conclusion

Relying exclusively on a single grade of aluminium paste or powder can create manufacturing challenges when temperatures fluctuate. Most notably, this can cause irregular pore structures, surface cracks, and mould collapse during the hotter periods of the day.

By having access to both coarser and finer grades, and by adjusting the blend to accommodate prevailing environmental conditions, manufacturers can significantly reduce rejection rates while achieving higher compressive strength and superior pore structure. Ultimately, consistent product quality depends on selecting the appropriate aluminium paste or powder and maintaining the flexibility to adapt to changing production conditions.