Investigation into the Preparation, Carbonation Evolution Mechanism, and Rebound Strength of High-Volume Ultrafine Mineral Admixture Concrete

Admixture ultrarefinement is an effective strategy to address the challenges of low gelling activity and slow early strength development in high-volume admixture concrete. To further enhance the low-carbon properties of concrete, an ultrafine mineral admixture with a 7-day activity index of 100% and a 28-day activity index of 114% was developed, enabling the production of high-volume ultrafine admixture mineral concrete. The carbonation behavior of this concrete was examined through phenolphthalein testing, carbonate analysis, and thermogravimetric-differential scanning calorimetry (TG-DSC), alongside an investigation of its rebound strength. The results indicated that the optimal composition of the ultrafine mineral admixture consisted of slag, fly ash, and anhydrite at a ratio of 40∶54∶6, with an ideal specific surface area of 680–720 m2/kg. This formulation enhances the consumption of Ca(OH)2, promotes the formation of hydration products, and ultimately improves mechanical strength. Carbonation test results reveal that while high pozzolanic reactivity in high-volume ultrafine mineral admixture concrete leads to a significant neutralization depth, the actual extent of carbonation is lower than that in conventional concrete. As a result, rebound strength estimates based solely on neutralization depth may be overly conservative. Experimental findings demonstrate that the rebound strength of high-volume ultrafine mineral admixture concrete can be accurately evaluated using a high-strength rebound hammer without adjustments for carbonation-induced reductions.