Effects of Combining Water‐Reducing Admixtures on Workability Retention of Limestone–Metakaolin–Blended Cement Paste

ABSTRACT It is widely known that metakaolin in blended cement improves mechanical properties and durability while reducing workability. In addition to initial workability reduction, metakaolin use makes it even more challenging to retain concrete slump. In this study, six water‐reducing admixtures (WRAs), including workability modifiers, were employed in paste mixtures made with Portland limestone cement and 15 wt.% metakaolin, named limestone–metakaolin–blended cement (LMC) paste. Fresh rheological properties (i.e., yield stress, flowability, and workability retention WR) were measured. Polymer absorption, heat of hydration, and compressive strength were also investigated to understand the role and impact of the WRAs in these systems. On the basis of the test results, the dosages of the individual admixtures and their combinations that enable the mixture to achieve the neat‐cement equivalent yield stress—defined as “optimal” admixture dosages—were determined. The WR, or slump loss, of the mixtures made with the “optimal” WRA dosages was assessed. The results indicate that in an acceptable dosage range, some individual admixtures failed to achieve the neat‐cement equivalent yield stress, and at their “optimal” dosage, most individual admixtures failed to retain their mixtures’ workability when compared with the neat‐cement paste. However, some combined admixtures showed clear improvement in both flowability and WR, demonstrating the feasibility of using combined admixtures for enhancing concrete workability performance. Studies on polymer absorption, heat of hydration, and x‐ray diffraction conducted in this study further provide systematic evaluation of WRA performance as well as sanity checks for their impact on limestone–metakaolin–cement paste.