The microstructure of synthetic aggregate produced from waste materials and its influence on the properties of concrete

This thesis examines the influence of the firing conditions on the properties and microstructure of synthetic aggregate produced using granite quarry fine waste blended with low grade ball clay as the binder. The investigation involved the use of various engineering tests and microstructural characterization techniques with special attention on the evolution of the microstructure of the aggregate pellets upon firing. The raw materials were extruded and fired in a small bench top model Trefoil rotary kiln. Various temperature profiles were simulated and the effects of firing condition on the water absorption, relative density and mineralogical and microstructural evolution of the synthetic aggregate were investigated. Powder X-ray diffraction (XRD) and electron microprobe analysis were used to study the phase transformation of the fired pellets produced using different firing conditions. Mercury intrusion porosimetry (MIP) and quantitative image analysis of backscattered electron (BSE) were employed to assess the pore structure of the synthetic aggregate. The main variables considered were the firing temperature and the duration at the highest temperature. An assessment of the microstructure at the coarse aggregate-cement paste interface is presented in the second part of this thesis. Concretes were cast with two different initial conditions of synthetic aggregate, i. e. dry and pre-wetted aggregate, and the results were compared with those obtained for the natural and Lytag lightweight aggregate concrete. This investigation was done with respect to the variables of hydration time, moisture condition of the aggregate particles and the type of coarse aggregate. Image analysis of backscattered electron image of flat and well-polished concrete samples was employed to provide quantitative information about the microstructural gradients at the coarse aggregate-cement paste interfaces. Finally, the influence of three different types of coarse aggregate on the permeability, compressive strength and elastic behavior of concrete was studied; the three aggregates were natural quartz, Lytag and the synthetic aggregate produced during this research. The results of the investigation show the significant influence of firing condition on the properties and mineralogical evolution of the synthetic aggregate. The microstructure of the synthetic aggregate was primarily controlled by two competing processes,i. e. pore growth and densification. Pore growth was observed when the aggregate was fired between 900° and 1,110°C, while densification started to overcome pore growth at about 1,110°C with a sintering time between 0 to 5 minutes. Densification of the synthetic aggregate depended on the amount of liquid phase that was associated with the melting of the feldspar mineral. The microstructural study shows that the moisture condition and type of coarse aggregate affects the microstructure of the interfacial transition zone (ITZ) between coarse aggregate and cement paste matrix. The results confirm the benefit of absorbing water from the surrounding paste by the porous aggregate on the ITZ microstructure unless too much and/or too rapid absorption of water occurs. Preferential deposition of calcium hydroxide was observed at the region close to the interface of quartz and synthetic aggregate that had a dense microstructure but very little deposition or build up was found for the porous synthetic aggregate. The Lytag and synthetic aggregate concretes showed a higher permeability than the natural quartz aggregate concrete. These results indicate the negative influence of the oven-dried technique that is commonly used in sample preparation, which might generate microcracks inside the tested samples. The 28-day compressive strengths achieved ranged from 37 to 44 N mm 2 and compared favourably with control concretes made with the natural quartz or Lytag lightweight aggregates. The elastic modulus of the synthetic and Lytag lightweight aggregate concrete appeared to be about 30 to 40 percent lower than the natural aggregate concrete.