Enhancing Fired Clay Bricks: Boosting Engineering Properties and Reducing Environmental Impact with Industrial Sludge Waste – A Study in Scientific Reports

The makeup of clay and sludge from both the BS and PS processes largely depends on the raw materials and methods used during production. Analysis shows that silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃) are major components of clay, with SiO₂ ranging from 55.77% to 61.83%, and Al₂O₃ between 15.50% and 24.40%. Moreover, the mosaic sludge also revealed similar trends, indicating a predominant presence of these two elements. The organic content in both BS and PS sludge was low, between 7.01% and 7.92%, marking them as inorganic.

When examining heavy metal content, both clay soil and sludge displayed significant concentrations of elements such as Zr, which peaked at 2738 ppm in PS sludge and 2507 ppm in BS sludge. Other metals like Ba, Fe, Cu, Zn, and Mn were also higher in PS sludge compared to BS sludge, except for Cr, which was notably present in higher amounts in clay soil. The results underscore the varying compositions across sludge and clay samples, which highlighted important variations in heavy metal concentrations.

The optimal moisture content (OMC) for making bricks was determined through compaction tests. OMC is crucial because it influences the density and strength of the final product. The study found OMC values ranging from 12% to 24%, with bricks requiring more moisture due to the finer particles in the sludge. Excessive moisture, however, can lead to shrinkage and cracks, especially during the drying process.

Atterberg limit tests classified the soil in this study as silty clay or clayey silt. The plastic index was determined to be 13.4%, while specific gravity values were similar across the clay and mosaic sludge, indicating that sludge could potentially replace a percentage of clay in the mix.

The firing shrinkage of bricks varied with different sludge content, with the lowest shrinkage at 0.34% for 5% BS bricks. Higher percentages resulted in increased shrinkage rates, with the 30% BS brick showing the maximum at 0.69%. The PS bricks demonstrated a similar trend, with results peaking with a 30% sludge content.

Densities of the bricks also varied based on sludge content. For BS bricks, the 30% sludge sample had a density of 1665.24 kg/m³, while densities generally increased as sludge proportions decreased. In terms of compressive strength, the strongest BS brick was the one with 30% sludge, measuring 26.58 N/mm², outdoing the PS brick with a similar sludge concentration.

Tests measuring the initial rate of suction assessed how quickly bricks absorbed moisture. The results indicated that BS bricks with lower sludge content showed higher suction rates. For instance, the peak was noted at 12.65 g/mm² for the 1% BS brick, while lower rates were observed as the sludge content increased.

Microstructure analysis further revealed that bricks containing higher concentrations of sludge displayed a finer surface structure and smaller pore sizes. Blueprints from the scanning electron microscope (SEM) indicated a more compact formation in bricks with 20% and 30% loads of sludge.

The sizes of pores in the bricks were also characterized, revealing that those made with mosaic sludge were significantly smaller compared to traditional clay bricks. This made the sludge-rich bricks lighter, leading to improvements in structural integrity.

Results from toxicity assessments showed that heavy metal concentrations in BS and PS bricks fell within safe limits outlined by environmental guidelines, indicating they are safe for land disposal. Various leaching tests also affirmed that bricks with incorporated sludge performed well, with lead levels remaining notably below the permitted guidelines.

When testing for total volatile organic compounds (TVOCs) and carbon emissions, results indicated that the bricks met safety thresholds for indoor use. Control bricks emitted higher levels of CO₂ compared to those made with sludge, suggesting the incorporation of sludge may lead to lower carbon emissions.

The emission rates of other gases like ozone and formaldehyde were examined as well, pointing out that the bricks produced were safe and did not exceed recommended standards. Particulate matter levels were monitored and remained within acceptable limits for all types of bricks tested.

In summary, using mosaic sludge in brick production not only meets environmental safety standards but also improves the quality and performance of the bricks. The findings advocate for utilizing these materials in construction, ensuring both durability and sustainability.