CALCINATION AND SYNERGISTIC RATIO OPTIMISATION IN THE PRODUCTION OF HYBRID AGRICULTURAL POZZOLANS FROM AGRICULTURAL WASTES FOR THE CONSTRUCTION INDUSTRY

Abstract

Cement is an important material in the construction industry. However, the environmental problems associated with the mining and calcination of limestone for cement production necessitates the search for supplementary cementitious materials with minimal threat to the environment. Periwinkle and clam shells, processed into ash are potential substitutes for cement in concrete. This study was designed to investigate the suitability of Periwinkle Shell Ash (PSA) and Clam Shell Ash (CSA) and a hybrid of both as partial replacements for cement in concrete. Shells were sourced from Amassoma, Bayelsa State, Nigeria, washed, sun dried and calcined at varying temperatures (200, 400, 600 and 800 oC). These were pulverised to pass through 75 µm sieve to produce PSA, CSA, and hybrids of both, mixed at 70:30, 60:40, 50:50, 40:60 and 30:70. Grade M20 concrete specimens were produced at varying Portland limestone cement (PLC) replacement levels (0, 20, 30, 40 and 50%) with 0% as control, and cured for 28 days prior to testing. Specific gravity, flexural strength, compressive strength, Water Absorption Index (WAI), Chloride Induced Strength Loss Index (CISLI), and Sulphate Induced Strength Loss Index (SISLI), were conducted in accordance with British Standards. Combined mixture methodology was used to develop regression models from laboratory data, analysed and optimised at 95% confidence interval to ascertain Optimum Pozzolanic Reactivity (OPR) based on Strength Activity Index (SAI). Emissions of CO2 associated with calcination of samples were analysed and compared to that of PLC using data from energy demand and carbon footprint. Specific gravities were 3.12±0.24, 2.96±0.01, 3.00±0.10, 3.14±0.03, while flexural and compressive strengths were 5.29±0.14, 2.83±0.10, 2.83±0.08, 3.36±0.21 and 27.74±1.41, 18.27±1.10, 19.21±0.25, 21.67±2.29 N/mm2 for PLC, PSA, CSA and 60% PSA:40% CSA, respectively, at 40% cement replacement level, produced at 600 oC. Compressive strengths of 60% PSA:40% CSA, were 78.1% of PLC, 118.6% of PSA and 112.8% of CSA; implying that the hybrid was stronger than PSA or CSA. Also, WAI, CISLI, and SISLI, were 1.98±0.19, 2.84±0.08, 3.25±0.17, 2.75±0.13; 24.56±2.15, 19.34±0.37, 0.44±0.08, 6.10±0.84; and 16.47±1.43, -10.97±1.32, - 16.88±0.68, 4.09±0.62 for PLC, PSA, CSA and 60% PSA:40% CSA. The PLC had the least WAI but diminished more in strength due to chemical attacks and absence of pozzolans compared to PSA and CSA. The OPR based on SAI was 55.5% PSA:44.5% CSA, and cement replacement levels were 19.7, 23.2, and 44.0%, for PSA, CSA and 55.5% PSA:44.5% CSA, respectively, produced at 425, 527, and 607 oC. A uniform compressive strength of 20.8 N/mm2 was obtained for PSA, CSA and 55.5% PSA:44.5% CSA, produced at 425, 527, and 607 oC, respectively, satisfying the SAI criteria. Optimised WAI, CISLI, and SISLI of PSA, CSA and 55.5% PSA:44.5% CSA were 2.46, 2.52, 2.51; 2.10, 3.06, 12.72; and -33.16, -31.48, -0.08%, when produced at 425, 527, and 607 oC. A kilogram of pozzolan emitted 350g of CO2 at calcination, while that of PLC was 490g; providing a 28.60% savings in CO2 emissions. Periwinkle and clam shells enhanced cement replaceability and satisfied the strength activity index pozzolanic criteria, when synergised and ashed optimally. Consequently, environmental pollution associated with excessive limestone harvest and processing was mitigated.