Influence of clay type on performance of calcined clay – Limestone portland cements

Abstract

The present work focuses on key parameters which determine the best strength performance of two calcined clay mineral additions (1:1 and 2:1 type) and mixtures with limestone. Additionally, alternative options rather than increased admixture dosage have been investigated to mitigate the rheological problems typically found when calcined clays are incorporated to cement. Isothermal heat development measurements have shown that SO3 content of blended cements can be effectively optimized based on early available alumina from clinker, but also from calcined clay. Flow measurements before mortar casting demonstrated that an adequate mixing procedure with a delayed addition of chemical admixtures can maximize their efficiency for particle dispersion. In order to reveal the influence of each main cement constituent on both workability and strength, multiple blended cements with different replacement levels, ratios of calcined clay to limestone, fineness of calcined clay and w/c ratios were tested. Results showed that the best strength development is achieved by calcined 1:1 clay for a lower ratio of calcined clay to limestone rather than calcined 2:1 clay for any given replacement level. On the other hand, workability is more strongly affected by the calcined 1:1 clay reducing the efficiency of superplasticizer (SP) required for achieving acceptable flow even at higher w/c ratio. The addition of fly ash on top of blended cements with 35% replacement of limestone and either calcined 1:1 or 2:1 clay certainly improves flow at high clinker replacements. However, strength performance of these new binders is only similar to corresponding calcined 2:1 blended cements with same replacement level, whist is significantly lowered in those containing calcined 1:1 clay. Therefore, w/c ratio may be potentially reduced to get even better strength performance with combinations of calcined 2:1 clay, limestone and fly ash.