Enhancing the reactivity of clay Pozzolona through mechanical activation

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Inadequate provision of shelter in Ghana due to the over-dependence on imported clinker has rendered about 12.5% of the population homeless. This has led to the springing up of slums, increase in communicable diseases and generally a decrease in productivity. Alternative and cheaper locally manufactured materials to Portland cement clinker such as clay pozzolana can be employed to reduce the cost of construction. The study focused on improving the reactivity of four clay deposits through mechanical activation. These deposits are at Mankranso, Mfensi, Tanoso and Mankessim. Pozzolana was produced from the clays by calcining them at 800°C. They were subjected to various degrees of milling using a hammer mill, ball mill and a Raymond type ring roll mill. After chemical analysis, the activated pozzolanas were used to replace 30-50% of ordinary Portland cement (OPC) and the physical properties such as standard consistence, setting times and compressive strength at various ages of curing of mortars determined. All the pozzolanas were chemically suitable with total SiO2+Fe2O3+Al2O3 oxide contents greater than 70%, the minimum required by ASTM C 618 of natural or calcined pozzolanas. The 28-day compressive strengths of blended cements containing various amounts of the pozzolanas showed that after 36 h ball milling, most of the activated pozzolanas could be used to replace 40-50% OPC. Tanoso pozzolana attained the highest strength of 35.8 MPa followed by Mfensi (30.5 MPa); Mankranso (27.7 MPa) and Mankessim (25.6 MPa) pozzolanas at 40% cement replacement. Although lower compressive strengths were obtained at 50% cement replacement relative to strengths attained at 30-40% replacement, values were mostly above the 24.1 MPa minimum as stipulated in ASTM C 595. The lowest value at 50% replacement of 23.1 MPa was attained by Mankessim pozzolana. Roll milling gave higher 28-day compressive strengths than ball milling with Tanoso pozzolana gaining the highest of 36.9 MPa compressive strength; followed by Mfensi (32.1 MPa); Mankessim (30.3 MPa) and Mankranso (29.6 MPa) at 40% replacement. The highest 28-day compressive strength at 50% replacement was attained by Tanoso pozzolana at 34.2 MPa whereas the lowest was gained by Mankessim pozzolana at 23.9 MPa. The compressive strengths attained by 36 h ball milled and roll milled pozzolanas after 90 days curing satisfied both the ASTM C 595 and EN 197-1 requirements for all the pozzolana cement mortars evaluated. Hence the activated pozzolanas can be used to replace more than the currently recommended 30% for non-activated pozzolanas. They are suitable for up to 50% of OPC replacement for most ordinary housing construction purposes.
M. Phil. Thesis Presented to the Department of Materials Engineering Faculty of Chemical and Materials Engineering College of Engineering Kwame Nkrumah University of Science and Technology, Kumasi In Partial Fulfillment of the Requirement for the Degree of Master of Philosophy in Materials Engineering