Effects of metakaolin on mechanical properties, pore structure and hydration heat of mortars at 0.17 w/b ratio

In the last two decades, metakaolin (MK) has received considerable attention as a supplementary cementitious material. However, few studies have evaluated the performance of cementitious composites employing MK under a very low water to binder (w/b) ratio. This paper presents our findings on the pozzolanic reactivity of MK and the effects of MK on mechanical properties, pore structure and hydration heat of mortars by replacing the cement with MK at 0%, 6%, 10%, and 14% by weight at a constant 0.17 w/b ratio. The experimental results show that the pozzolanic reactivity of MK is higher than that of silica fume (SF), especially at 28 d. For mortars without steel fibers, the flexural strength decreases with the inclusion of MK, whereas 10% MK blended mortars show the highest compressive strength (96.1 MPa at 90 d) compared to the others. For reinforced mortars with 2% steel fibers by volume, both flexural and compressive strength are remarkably enhanced. In addition, three-point bending tests demonstrate that the flexural toughness is improved with the addition of MK as well, although MK decreases the flexural strength of plain mortars. Mercury intrusion porosimetry (MIP) results indicate that increase tendency in porosity is detected with the elevating MK contents. Heat of hydration tests results show that 6% MK accelerates the cement hydration, with an advanced accelerating period starting time, and maximize the heat flow which indicates maximum temperature rise. Although 14% MK blended mortar minimizes the total heat evolved, the optimal MK content is 10% with the consideration of mechanical properties.