Comparative Study of Chemically and Mechanically Activated Clay Pozzolana

Abstract

Burnt clay pozzolana produced from a clay deposit at Mankranso in Ghana has been activated by mechanical means through roll milling and ball milling as well as chemically by the addition of 1% - 4% m/m Na 2 SO 4 . The pozzolana sample was chemically suitable with total SiO 2 + Al 2 O 3 + Fe 2 O 3 content ≥70% as stipulated by the ASTM C 618 standard. The particle sizes, surface characteristics and specific surface areas obtained by the types/degrees of milling were analyzed and their effect on the strength development of Portland pozzolana cement mortar cubes prepared from the pozzolana samples was evaluated. Compressive strengths obtained showed that the activated pozzolana could be used to replace up to 40% ordinary Portland cement (OPC) and satisfy the EN 197-1 and ASTM C 595 standard requirements. Comparatively, the chemically activated pozzolana cement mortars attained higher compressive strengths than the mechanically activated pozzolana cement mortars at equal ages of tests and the same pozzolana content levels. The chemically activated pozzolana cement mortars attained higher 2 day strengths than OPC at sulphate concentrations of 3% and 4% for all pozzolana content levels between 30% - 40%. SEM image and insoluble residue in HCl of a 2 day old chemically activated pozzolana cement paste confirmed dissolution of fine pozzolana particles in the alkaline media which influenced higher early age strengths. The highest 28 day compressive strength of 54.2 MPa was obtained at 4% sulphate concentration and 30% pozzolana content for the chemically activated pozzolana. The highest 28 days compres sive strength for the mechanically activated pozzolana was 35.6 MPa—obtained for the roll milled product at 30% pozzolana content. Standard consistence of the pozzolana cement pastes increased with increasing pozzolana fineness and pozzolana content. Increasing Na 2 SO 4 concentration however had no effect on standard consistence. Setting times decreased with increase in both fineness and sulphate concentration. The EN 197-1 standard for initial setting time was satisfied by the chemically activated pozzolana cement mortars at all pozzolana content levels. Pozzolana samples activated by roll milling and 36 h ball milling had faster initial setting times than the EN 196-1 standard at all pozzolana content levels beyond 30%. The ASTM C 595 requirement for initial set was however satisfied at all pozzolana content levels.