Compressive Strength Improvements of Cement-Based Composites Achieved with Additional Milling of Metakaolin

Kaolin from a Serbian deposit, having a high content of mica and quartz, disordered kaolinite and a high specific surface area, was used to prepare metakaolin (MK). The calcination at temperatures of 700 °C or 750 °C for 30 to 180 min resulted in MK having high pozzolanic activity, but also significant aglomeration of particles. In order to disperse aglomerates, MK was milled, which resulted in increased pozzolanic activity and reduced particle size. The effects of MK and additionally milled metakaolin (MKmill) on the composite strengths and microstructure of the pastes were compared. We prepared and investigated composites in which ordinary Portland cement (OPC) was replaced with 10% to 50% of MK or MKmill, as well as the representative samples of paste for determination of microstructure. Compressive strengths higher than the control were obtained for composites having up to 30% of MK and up to 40% of MKmill, respectively. Increase of composite strengths with MKmill was more pronounced at lower cement replacement levels (10% and 20%). Compressive strength of composites containing agglomerated MK were satisfactory, which suggests that milling of MK, as well as purification of kaolin, may not be necessary