Influence of metakaolin surface area on properties of cement-based materials

Two metakaolins, with similar mineralogical composition but which vary in their surface area (11.1 versus 25.4 m(2)/g), were evaluated for use as supplementary cementitious materials through measurements of workability, setting time, strength, elastic modulus, heat evolution, calcium hydroxide (CH) content, and surface area. Compressive and flexural strength of concrete were greater and increased at a faster rate when the finer metakaolin was used, as expected. The addition of metakaolin increased early age (i.e., 1-3 days) flexural strength by as much as 60%. The effect of metakaolin surface area on compressive strength was particularly evident at the lower water-to-cementitious materials ratios (w/cms) examined and generally at later ages (i.e., 7 days or later). However, although greater in the metakaolin-cement concretes than the ordinary concretes (particularly at the lowest w/cm examined, 0.40) elastic modulus measured at 28 days, was not affected by the metakaolin surface area. The greater surface area metakaolin caused a greater and more rapid heat evolution, indicating a higher reactivity and a greater rate of hydration product formation. Both metakaolins decreased CH content compared to controls, with the consumption of CH extending beyond 14 days. Surface area measurements indicated a more refined pore structure relative to controls by 28 days. These analyses illustrate the effect of metakaolin fineness on pozzolanic reactivity, associated CH consumption, and pore structure refinement, and suggest links to the observed increased mechanical properties of metakaolin-concretes.