A fundamental study of the behaviour of clay-brick fines in autoclaved calcium silicate based building products

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NO FULL TEXT AVAILABLE. This thesis contains 3rd party copyright material. ----- This research project studied the behaviour of clay-brick waste (CB) in combination with Ordinary Portland Cement (OPC) in hydrothermal environment. The aims of this study were to establish the hydrothermal chemistry involved during autoclaving when CB were used as a raw material for the production of autoclaved calcium silicate based building products and to evaluate the suitability of CB as a possible cement replacement. The compositional regions studied cover the areas 20-60 wt % SiO2 and 3.5-34 wt % Al2O3 in the CaO-Al2O3-SiO2-H2O system. Mixes were prepared with OPC and with either four different CB types or one quartz sand. The samples were cured at 180 °C for 7.5 hours or at 150 °C for 7.5 selected samples. Different curing temperatures were used to investigate the effect of curing temperature on the compressive strength and phase development. In a similar manner, different cement types were used to investigate the effect of different cement types on compressive strength, degree of carbonation and phase development. Evaluations of mechanical properties of autoclaved samples were conducted on the blends. Compressive strength testing was conducted on all blends while determination of drying shrinkage and degree of carbonation was carried out on selected blends in the region where the strength was found to be optimum. The evaluation of phase development in all mixes was carried out by employing a combination of experimental techniques including X-ray diffraction (XRD), differential thermal analysis and thermogravimetric analysis (DTA-TGA), scanning electronic microscopy (SEM) and wet chemical methods. Overall, more than 80 different blends were analysed for microstructural and macrostructural (engineering) properties. 1.1 nm tobermorite (tobermorite) along with C-S-H, hydrogarnet, portlandite (CH) and alpha-dicalcium silicate hydrate phase (α-C2SH), were established as the critical phases in relation to strength development, drying shrinkage and degree of carbonation. With regards to strength development it was concluded that tobermorite was the principal contributor since an increase in tobermorite amount corresponded to an increase in the compressive strength. Presence of α-C2SH and CH favoured strength decrease with α-C2SH reducing the strength significantly and CH delaying the tobermorite formation. There is also evidence that a decrease in hydrogarnet formation favoured strength increase. The study also established that tobermorite, despite its ability to perform as a strong binder, was susceptible to carbonation and responsible for an increase in drying shrinkage. The main phases that restrained drying shrinkage and increased resistance to CO2 attack were CH, hydrogarnet and α-C2SH, when present. From experimental data of different curing temperatures (150 °C versus 180 °C) it was established that the amount and morphology of tobermorite, hydrogarnet and CH were amenable to control through appropriate CB additions and curing temperatures. These findings are significant from the viewpoint of the durability of commercial, hydrothermally cured products and their relevance to geothermal oil-well applications. Overall, the use of CB as a cement replacement for the production of autoclaved cement- based building products was demonstrated to be a viable option. OPC:CB blends autoclaved at 180 °C exhibited comparable or better mechanical properties than their OPC: quartz counterparts that are currently used by industry.
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