Effect of Filler Type, Fineness, and Shape on the Properties of Nonfibrous UHPC Matrix

Abstract

Fillers have been commonly used to replace a portion of cement in ultra-high-performance concrete (UHPC) for economic and eco-friendly purposes. However, mostly only the type and amount of filler are addressed, and limited efforts have been made on the size and shape effects of filler. This study used inert mineral fillers with different morphologies (approximately spherelike versus needlelike) to study the effect of the shape and different particle sizes (fineness) within the same filler type to study the effect of the size. A systematic study was carried out to investigate these effects on the workability, strength, hydration, modification of the particle-size distribution (PSD), and microstructure of the designed nonfibrous UHPC matrixes. The mixture proportion design is based on a modified Andreasen and Andersen model, and three mixtures devoid of fillers were prepared firstly to study the effect of superplasticizer dosage on the UHPC matrix to determine the saturation dosage. Results reveal that larger-sized approximately spherelike filler (limestone or quartz) results in a slightly negative effect on the strength, whereas larger-sized needlelike wollastonite enhances the strength of the mixture. The mixtures with 20% substitution of cement by the seven different fillers obtained increased workability, decreased cost, and higher strength or no more than 4% strength reduction compared with the control mixture devoid of filler. This quantification of size and shape effects of fillers will provide an effective theoretical basis, and particular fillers could be intelligently chosen in the engineering practice for the production of UHPC.