Microfiller effect on rheology, microstructure, and mechanical properties of high-performance concrete

The objective of this study was to develop a fundamental understanding of the microfiller effect in high-performance concrete. Ultimately, this would help in the development of blended highperformance cements containing recycled materials and industrial byproducts, offering both significant economic advantages and environmental relief. The mechanisms underlying the microfiller effect on the rheology were investigated in cement paste using a coaxial-cylinders viscometer, a mini slump test, the Marsh cone flow time, and a pressure bleed test. They were also studied in mortars using the ASTM flow-table test, and in concrete using a computer-controlled rheometer, a slump-flow test, the conventional slump test, and an induced bleeding test. It was found that microfillers enhance the superplasticizer efficiency because they increase the surface layer water and reduce the bulk water through a reduced void space in the particulate mixture. In the presence of a superplasticizer, microfillers also decrease the viscosity of concrete mixtures; the finer the particle size of the microfiller the greater the decrease. This seems to be due to a reduction of the mechanical interlocking between coarser particles. Ultrafine particles also decrease the bleed water, which reduces the occurrence of bleed channels and low density microstructural features at interfaces. As a result of the above, microfillers make the production of fluid and self-leveling concrete much easier. It was also demonstrated that triple-blended cements containing pozzolanic and non-pozzolanic fillers can achieve superior rheological properties. The microfiller effect on mechanical properties was investigated in mortars and in concrete both at early and later ages. It was discovered that this effect depends on the initial porosity of the system. At very low w/b ratios, partial replacement of cement with non-cementitious fillers would not result in lower density hydration products because the initial porosity is already very low. The hydration reactions in fact yielded denser hydration products. Thus, up to 15% replacement of cement by a non-cementitious filler caused significant increases in strength. This was even more significant in triple-blended cements containing combinations of pozzolanic and non-pozzolanic fillers for which up to 30% partial replacement of cement resulted in significant strength increases. Ultrafine carbonate fillers increased the very early age strength by about one order of magnitude, because certain microfillers appear to present energetically preferential substrates for the germination and growth of calcium hydroxide. Removal of calcium ions from the solution catalyzes the dissolution of C₃S in an attempt to regain equilibrium. This signals an earlier end of the induction period and a faster rate of the hydration reactions at early ages. Quantitative image analysis of backscattered electron micrographs was used to quantify the microfiller effect on the microstructure of high-performance concrete. Analysis was carried out on cement paste and concrete both at Id and at 28d. The acceleration of the hydration reactions at early ages due to carbonate microfillers was confirmed by this technique. Microfillers generally decreased the porosity and refined the microstructural features. This was accompanied by increased strength only when the ratio of inner hydration products to outer hydration products was increased. Densification of the paste-aggregate interface did not seem to necessarily increase the compressive strength. The microfiller effect in high-performance concrete was studied from the standpoint of the theory of particle packing. An insight into particle packing models, the effects of particle packing on rheology, and the effects of particle size distribution on hydration reactions was obtained. A new parameter, the microfiller efficiency factor was developed, based on an estimation of packing density and microfiller effect on hydration rate. A close correlation was found between the microfiller efficiency factor and compressive strength. In addition, a new model relating microstructure to strength was proposed. Most available models relate porosity to strength without accounting for the nature of the solid phase. The model proposed herein considers for the first time a quantitative value representing the nature of the hydration products to help estimate strength. This value is the ratio of the dense inner hydration products to the bulk of the rest of the hydration products. The proposed model achieved good estimations of strength. Overall, this study proposes a new approach to achieving high-strength materials. Traditionally, high strength is obtained through increased cement content, reduced w/b ratios, and high rates of hydration. This work suggests that high strength can be achieved through high initial particle packing combined with a low rate of hydration, which causes less chemical contraction, less drying shrinkage and self dessication stresses, and a higher content of inner hydration products.

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