Fillers’ application in high-performance concrete

Abstract

Technological developments in building materials require the continual evaluation and upgrading of the key ingredients in cement powder paste in order to produce high-performance concrete. Replacing a percentage of the cement with multi-size fillers, from very fine powder to coarse aggregate in a properly proportioned mixture, reduces the formation of voids for a more densely packed microstructure to improve concrete properties. Moreover, the addition of fine filler tends to increase the concrete’s thirst for superplasticiser’s lubrication and dispersion effects, resulting in higher wet packing density, which can, in turn, maximise water and paste film thickness to boost concrete cohesiveness and mechanical properties. There is, however, a limit to how much filler can be added without deteriorating the concrete’s strength and durability characteristics.This study was conducted to explore the optimal quantities and qualities of filler materials, especially those having a low carbon footprint. A series of concrete mix designs using inert and pozzolanic fine fillers was examined, and their fresh and hardened properties, including workability, stability, strength, and shrinkage, were compared. The blended concrete mixtures were assessed using standard tests for flowability, passing ability, segregation stability, compressive strength, and drying shrinkage; both individual and concurrent performance comparisons were taken into account. Rheological performance governed by plastic viscosity and shear thickening of cement powder paste was studied theoretically, to verify the suitability of incorporating limestone and fly ash fillers into the structural applications of high-performance concrete.This thesis aims to highlight the factors affecting the compressive strength and the drying shrinkage of high-performance concrete when a part of the cement in the powder paste composition is replaced by limestone or fly ash filler. Further, this research seeks to confirm experimentally the potential improvement in the simultaneous rheology-strength-shrinkage performance of high-performance concrete mixes that contain fillers. Limestone fine filler in this study was found to enhance the compressive strength of concrete in a low water-to-binder ratio and reduce the shrinkage strain in higher strengths, with a tendency to allow the addition of more polycarboxylate-based superplasticiser to restore flowability.Results indicate that it is acceptable to replace up to 25% of concrete cement with a locally available natural filler or up to 35% with a recycled industrial by-product filler. This outcome was verified by local and international standard test data on slump flow, V-funnel flow rate, the L-box test, the J-ringtest, sieve segregation, compressive strength, apparent water absorption and the use of a horizontal comparator to detect length changes of concrete specimens. The modelling presented in this thesis support the use of the latest version of the Australian standard’s 3600 model for predicting the experimental drying shrinkage for concrete that includes fly ash, whereas mixes that include limestone filler demonstrated a better fit with the modified Gardner and Lockman international drying shrinkage model.The outcomes offered in this thesis contribute to the concrete knowledge and industry’s understanding of the potential improvements in flow cohesiveness, passing ability, segregation and dimensional stability, compressive strength, and sustainability to be achieved when alternative fillers replace the appropriate amount of cement powder in building materials applications. This research provides evidence that the use of high-quality fillers as a partial replacement of cement paste volume in high-performance concrete mix designs can advance the technical performance, cut construction costs, reduce powder carbon emissions and conserve binder production energy.