Engineering properties of alkali activated geopolymer concrete using extracted sodium silicate from agro wastes / Ahmed M. E. Alnahhal

Ahmed M. E. , Alnahhal (2024) Engineering properties of alkali activated geopolymer concrete using extracted sodium silicate from agro wastes / Ahmed M. E. Alnahhal. PhD thesis, Universiti Malaya.

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Abstract

Developing alternate binders for Portland cement is essential to meet the cement demand in the building sector and maintain its sustainability. Alkali-activated materials (AAM) are becoming popular as an eco-friendly substitute for Portland cement. However, the energy-intensive manufacture of alkaline activators increases AAM system embodied energy and greenhouse gas emissions. Local industrial byproducts and wastes have been recycled into valuable activators that can replace alkali-silicate activators and minimise greenhouse gas emissions. Hydrothermally brewed silicate (HBS) from treated palm oil fuel ash (TPOFA) and rice husk ash (RHA) was produced in this innovative study. The first part of this research thesis demonstrated the synthesis of sodium silicate from RHA and TPOFA through the use of a hydrothermal process with four different silica modulus (SiO2/Na2O) ratios: 1, 1.5, 2, and 2.5; the TPOFA or RHA was mixed with NaOH powder with sufficient water and stirrer for 1 h at 80 °C. Considering that several studies have been conducted on the extraction of sodium silicate from RHA by employing various molarities of NaOH solution, the second part of this thesis research focused on the extraction of sodium silicate from TPOFA using 3 and 4 M NaOH with varying ratios of TPOFA to NaOH solution; the variables such as NaOH molarity (3 and 4 M), temperature (80 °C), and stirring time (5 hours) were followed. Finlay, the optimal sodium silicate from the RHA was utilized in the production of geopolymer foamed concrete. The palm oil fuel ash (POFA), palm oil clinker powder (POCP), coal bottom ash (CBA), fly ash (FA), and ground granulated blast-furnace slag (GGBS) were utilized as ternary binder in this geopolymer foamed concrete. The dissolving efficiency, microstructural analysis such as XRD, XRF, and FTIR were studied. Additionally, the compressive strength and modulus of rupture were investigated as part of mechanical property study. The test results of using silica modulus ratio in the extracted sodium silicate from RHA and TPOFA showed that the RHA-HBS produced about 79 MPa geopolymer mortar compressive strength while POFA-HBS produced about 52 MPa geopolymer mortar. Further, the flexural strength of 9 MPa was found for RHA–HBS–based geopolymer mortar; while the TPOFA-HBS-based geopolymer mortar produced a very close value of 8.8 MPa. It is noteworthy to mention that the density of the RHA-HBS solution was found in range of 1427 - 1550 kg/m3, while for TPOFA-HBS, it was found in range of 1350 – 1436 kg/m3. However, the using of the NaOH molar method, the test results showed that the dissolution rates for 3M and 4M of NaOH were 47% and 49% respectively. The FTIR results showed a shift in the peaks when compared with the commercial sodium silicate. In addition, a reduction was noticed in the intensity of the XRD patterns. Moreover, the SiO2 was found to be in the range of 11 – 16.8% while Na2O was about 11.5 – 24 %. Finally, the RHA-HBS-based geopolymer foamed concrete attained about 32.2 – 42.6 MPa, as compressive strength compared to the commercial sodium silicate in geopolymer foamed concrete of about 29 – 37 MPa. It is interesting to mention that the RHA-HBS-based geopolymer foamed concrete produced about 6.5 MPa as flexural strength compared to the commercial silicate of about 1.75 MPa.

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