Geoenvironmental use of CaCo₃ sludge in subgrade soil for a greener sugar industry

The sugar refinery industry in Thailand produced million tons of sugar annually, generating an even larger amount of calcium carbonate sludge as a byproduct. This study examined the compaction enhancement and efficiency of subgrade material made of CaCO₃ sludge (10 to 30%), ordinary Portland cement, OPC (1 to 3%) and unqualified crushed rocks. A total of 12 specimens were tested by CBR ad electric resistivity measurements in addition to a reference specimen (100% unqualified crushed rock). The CBR test results showed that using OPC (with a limited dose) and CaCO3 sludge (up to 30%) in the mix designs could give an optimal balance in compaction enhancement (higher CBR value, lower dry density or smaller blow number) and reduction in cost and CaCO3 emission of the mixed material. For example, the samples (SC_1,2,3-10) with 10% CaCO3 sludge and 1%, 2-3% of OPC have the CBR value increased from 29 to 83, 113, and 146, respectively, or by comparing between the SC₀-0 unmixed sample with SC₀-10 (having 10% of CaCO3 sludge) it is observed a reduction in dry density from 2,189 to 2,109 gm/cc, while the CBR value remained the same at 29%. The SC₁-30 sample (1% OPC, 30% CaCO₃ sludge) gave the best balance of cost saving (15% in cost reduction), decarbonization (15% reduction in CO₂ emmision) and compaction enhancement (22% increase in CBR value) compared to the SC₀-0 sample of 100% unqualified crushed rock. The study highlighted the successful application of electrical resistivity measurements in testing and monitoring the quality of pavement mixed with CaCO₃ sludge. A strong correlation between CBR value and electrical resistivity was found, allowing a good prediction of the former from the latter. The findings of this research are expected to serve as a valuable reference for improving soil compaction efficiency, promoting zero-waste practices, and developing high-quality subgrades made from recycled materials like CaCO₃ sludge that can become a viable and cost-effective alternative of subgrade material in road construction practice.