Fly ash + phosphogypsum: building acid-resistant and corrosion-resistant baking-free brick

The service life of traditional acid-resistant brick is gradually difficult to meet the engineering requirements under the action of acid medium erosion in chemical industry park and long-term rainwater acidification in acid rain area. Aiming at this challenge, based on the concept of industrial solid waste resource utilization, the synergistic combination of fly ash and phosphogypsum provides an innovative path for the preparation of high-performance acid-resistant bricks. Through microstructure control and macro-performance optimization, the composite system shows remarkable technical advantages in corrosion resistance mechanism, process parameters and engineering application, and its performance improvement is supported by multi-dimensional data system.

1.  Anti-corrosion mechanism

Phosphogypsum (CaSO₄・2H₂O content ≥85%), a multi-level protection system constructed by two components, is used as a key component to release Ca²+ in an alkaline excitation environment. On the one hand, it activates the pozzolanic reaction between active Al₂O₃ and SiO₂ in fly ash to generate C-S-H gel and ettringite (AFt) crystals; On the other hand, the needle-like ettringite crystals grow crosswise in the pores, forming a dense protective layer with an average thickness of 0.8-1.2μm, which reduces the porosity of the brick body by 60%. The Al₂O₃ (content 15%-20%) in fly ash undergoes protonation reaction in acidic environment, resulting in harmful pores filled with gelled Al (OH) ₃ with a pore diameter of more than ＞50nm. By mercury intrusion test, the permeability coefficient of acidic medium in brick body is reduced by 75% compared with that of single gelled system. Electrochemical impedance spectroscopy (EIS) shows that the charge transfer resistance (Rct) of the composite system is 12.5 KΩ cm, which is nearly four times higher than that of the traditional acid-resistant brick (3.2 KΩ cm), which proves that its acid corrosion resistance is significantly enhanced.

2.  The production process: accurate control to achieve structural performance optimization

(I)  Technical parameters of raw material pretreatment

1)  Phosphogypsum aging: natural aging for 30 days, the content of free acid (calculated by H₂SO₄) decreased from the initial 1.2% to less than 0.3%, to avoid later frosting and volume instability.

2)  Refining of fly ash: grinding to a specific surface area of ≥ 450 m/kg (equivalent particle size ≤400 mesh), and controlling the loss on ignition < 5% to ensure the effective release of active ingredients (Al₂O₃ + SiO₂ ≥ 70%).

(II) Synergistic effect of optimized proportion The optimal composition is determined by orthogonal test:

Cementing body: fly ash 55% (active carrier) + phosphogypsum 20% (calcium source supply)

Reinforcing component: slag 15% (micro-aggregate filling)

Excitation system: Na₂CO₃ 5% (alkaline excitation) + triethanolamine 1% (early strength and densification). Under this ratio, the fluidity of the slurry reaches 180 ± 5mm and the initial setting time is 45min, which provides ideal working performance for the molding process.

(III) Densification control of molding and curing

1) Vibration molding: 30Hz high-frequency vibration for 30s, particle packing density increased from 1.2 g/cm³ to 1.6 g/cm³.

2) Autoclaved curing: curing at 1.2MPa for 8 hours, which promoted the directional growth of ettringite crystals, and finally the porosity of brick decreased to 16%-18%, which was 35% lower than that of natural curing

3. Performance testing: quantitative data to verify technical advantages

The mass loss rate is 0.8% (traditional acid-resistant brick ≥3%)

The retention rate of compressive strength is 98% (85% for traditional brick). Scanning electron microscope (SEM) shows that there is only shallow surface corrosion (depth < 50μ m) on the surface of the brick after erosion, and the internal structure remains intact.

Compressive strength of 20.5MPa (standard requirement ≥15MPa)

The flexural strength is 4.3MPa (standard requirement ≥4.0MPa).

The water absorption rate is 8.2% (standard requirement ≤9%), and all indexes are better than the requirements of GB/T 8488 "Acid-resistant Brick".

4. Engineering application: performance verification of the actual scene

(I) A phosphorus chemical base in Sichuan was applied to the foundation masonry of 3 000 m sulfuric acid storage tank in 2019, and after five years of service:

Apparent inspection: no cracks, peeling and pulverization.

Maintenance cost: 62% lower than traditional acid-resistant brick (the average annual maintenance cost is reduced from 120 yuan/㎡ to 45 yuan/㎡).

Structural monitoring: the settlement is less than 3 mm, which is better than the design allowable value (5mm).

(II) After the external walls of buildings in acid rain areas in Guangdong have been used for 3 years in an environment with an average annual rainfall of pH 4.2:

Color difference detection: Δ e < 3 (Δ e > 8 for ordinary bricks), indicating that the surface erosion degree is significantly reduced.

Impermeability test: The rainwater penetration depth is 12mm (35mm for ordinary bricks), and the erosion resistance is improved by 2.9 times.

5. Technological innovation: Expanding the application boundary of surface coating technology

Aiming at chloride ion erosion environment (such as coastal chemical industry zone), the developed phosphogypsum surface coating technology can realize:

The coating thickness is 5-10μm, and the chloride ion diffusion coefficient is reduced from 1.2× 10^-12m/s to 4.5× 10^-12m/s (ASTM C1202 standard test).

The electrochemical noise test showed that the pitting time was prolonged from 72h to over 280h.

6. Conclusion

Through the high added value utilization of industrial solid waste, the fly ash-phosphogypsum acid-resistant brick has constructed a three-stage corrosion-resistant system of "active excitation-dense molding-surface strengthening". From the stoichiometric control of raw material ratio, to the crystal growth control of curing process, to the long-term performance monitoring of engineering application, a complete data support chain has been formed. With the integrated application of new technologies such as surface coating, this material is expected to achieve wider engineering applications in complex corrosive environments, providing a typical example for the coordinated development of solid waste recycling and high-performance building materials.

Source：https://mp.weixin.qq.com/s/UJ3GUSh-UKFPoXofKfCXDA