Aissa Bouaissi


Geopolymer has received great attention in recent years as a new material that could replace Ordinary Portland cement (OPC) for producing concrete. Geopolymer uses the raw materials rich in aluminium and silicon, which are activated by alkaline solutions to formulate the binder. It has been proven that the geopolymer could be a material capable of providing high-quality properties and having less environmental impact. This research project aims to investigate the capability for producing a geopolymer concrete (GPC) mainly based on the combination of various by-product materials (fly ash, (FA), ground granulated blast-furnace slag (GGBS) and high-magnesium nickel slag (HMNS)) at ambient temperature. The characteristics of the precursor materials such as the chemical compositions, particle size and shape, unburned carbon content (LOI), and amorphous and crystalline phases were characterized using various technical methods. These included Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), X-ray fluorescence spectrometer (XRF), X-Ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy analysis (FTIR), to identify the different functional groups. In addition, a mix design procedure has been proposed in order to manufacture the FA-GGBS-HMNS-based geopolymer concrete, which is mainly based on the selection of the sodium silicate to sodium hydroxide (alkaline activator solution) ratio and the binder-to-liquid and binder-to-aggregate ratios. The effects of the selected materials and their characteristics on the properties of the fresh GPC at a room temperature of 25±2ºC and a relative humidity of 85-90% were later evaluated by performing workability and setting time tests. The fresh FA-GGBS-HMNS based geopolymer concrete showed excellent workability which maintained for at least 240 minutes, without any sign of setting or stiffness before starting to harden. The mechanical properties of the hardened FA-GGBS-HMNS based geopolymer concrete, e.g. the compressive strength, the splitting tensile strength and modulus of elasticity, are similar or better when compared to those of OPC concrete. A strong relationship between the compressive strength and the theoretical modulus of elasticity was shown by a true correlation with an approximate R2 ≈ 0.997. The microstructure analysis of the GPC produced exhibits the formation of an aluminosilicate amorphous phase in a three-dimensional network. The SEM images reveal a fully compact and cohesive geopolymer matrix, which explains why the mechanical properties of the FA-GGBS-HMNS based GPC are improved, both with GGBS and with HMNS. The thermal stability and durability of the designed GPC were investigated by performing both a thermal residual test at elevated temperatures up to 900ºC and a Rapid Chloride Permeability Test (RCPT) respectively. The results confirmed that the manufactured GPC showed great resistance to high temperatures, with residual strength ranged from 48.4 to 20.56 MPa. Moreover, it was found that FA-GGBS-HMNS based GPC could have the capability of resisting the migration of chloride ions and showed good behaviour against the diffusivity of chloride ions at 75 days after casting. It was also observed that, at 210 days after casting, the chloride migration coefficient increased due to the influence of different parameters such as the fineness of precursor materials, the continuation of the geopolymerization process and the pH of the pore solution of the mixture. To evaluate the dynamic behaviour of the designed FA-GGBS-HMNS GP further, an impact test was implemented using a SHPB system. The results showed that the dynamic compressive strength increased with the strain rates. A linear relationship between the dynamic increase factor (DIF) and the logarithmic strain rates experimentally, demonstrated the strain rate sensitivity of the GP paste–like material. From the aspects of damages, different failure patterns were observed under various strain rates ranging from 24.1 to 176 s-1. Furthermore, the synthesised GPC achieved good mechanical and microstructure properties at ambient curing temperature, which are sufficient for the quality of concretes, and hence it can provide the construction industry with a feasible technology which could be used for on-site and off-site applications. The main significant findings of this investigative study are: - A high compressive strength and splitting strength were achieved at 28 days with about 55.6 MPa and 4.57 MPa, respectively, with a high workable mix design. - The microstructure analysis showed the formation of dense and compact gels such as Quartz, Calcium Beryllium Praseodymium Oxide, and Magnesium Vanadium Molybdenium Oxide. - A strong relationship between the dynamic compressive strength and strain rate, which increases as the strain rate increase. - The continuity of the geopolymerization process negatively affects chloride diffusion coefficient. - The residual compressive strength increased by 3% after exposing the samples to 200 ºC, then it showed different degradation trends with the temperature up to 900 ºC.

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