The use of limestone to replace physical filler of quartz powder in UHPFRC

One of the key features of ultra-high performance fiber-reinforced concrete (UHPFRC) is the inclusion of quartz powder (QP) as a filler to optimize granular packing. However, QP has not been compared against limestone powder (LP) in terms of the filler effects such as hydration characteristics, strength development, and autogenous shrinkage of the concrete. In this study, the effectiveness of LP as a filler is evaluated against that of QP in order to verify its potential for use as a new functional and human-friendly filler in UHPFRC. For this purpose, five types of fillers consisting of similar-sized QP and LP were designed while maintaining the same water-to-cement (w/c) and superplasticizer-to-cement (sppl/c) ratios. The early age strength increased by up to 12% depending on the LP content, due to the filler effect which accelerates cement hydration by providing a preferential surface for nucleation. However, the LP only acted as an inert filler, like QP; it did not participate in the formation of additional hydration products as confirmed by conducted analytical experiments. Although the 28-day strength was 4%-7% lower than that of the classical UHPFRC containing QP, the 91-day strength was 1%-4% higher. The early age shrinkage was also accelerated by the filler effect of the LP, but its duration was shortened in proportion to the LP content.