Influence of different silica sources as an alternative to conventional sodium silicate in fly ash geopolymer concrete

Abstract

The production of Portland Cement (PC) is known for its substantial carbon emissions. Geopolymer offers an alternative to PC as a binder in concrete. Fly ash is a commonly utilised source material in geopolymers due to its extensive availability and good performance. While the elimination of lime calcination reduces the environmental impact of geopolymer binders, the use of conventional sodium silicate as an activator has been questioned by several studies due to its environmental implications. The production of sodium silicate involves high temperatures of up to 1 300 °C, which leads to an increased carbon footprint. The aim of this study was to substitute sodium silicate with silica source materials such as silica fume, rice husk ash, and glass waste powder, which are industrial and agricultural by-products. The study investigated the viability of these by-products as activators in fly ash geopolymer. The alkali-activator solution was prepared using sodium hydroxide via a hydrothermal method, while fly ash was blended with ground granulated blast-furnace slag (GGBS) to control flow workability and setting time, as well as to enhance compressive strength. The study utilised various parameters for the mixture, including the ratio of alkali-activator solution to fly ash, the ratio of silica sources to sodium hydroxide, the concentration of sodium hydroxide solution, and the GGBS ratio. In total, 70 batches of mortar and 15 batches of concrete mixtures were prepared, using mortar cubes of 50 mm and 100 mm in size, respectively. These cubes were used to assess the compressive strengths of the mortars at seven or 28 days, and of the concrete at three, seven, 28, and 90 days, following the guidelines of American Society for Testing and Materials (ASTM C109:2021). Additionally, the flow workability and setting time of fresh mortars were evaluated according to ASTM C230 (2008) and ASTM C807 (2020). Durability was assessed through durability index tests (oxygen permeability, water sorptivity, and chloride conductivity indexes), and microstructural analysis that utilised X-ray diffraction and scanning electroscope microscopy techniques. The results of the compressive strength tests, flow workability assessments, setting time measurements, durability index, and microstructural studies were analysed. In general, the compressive strength demonstrated an upward trend relative to the influence of each parameter (such as the silica source / sodium hydroxide) ratio, sodium hydroxide concentration, GGBS ratio, etc.). However, the findings also revealed that this increase could only occur to a certain extent, as a notable decline was observed upon exceeding specific ratios. Workability and setting time measurements were conducted to evaluate the fresh properties of the fly ash geopolymer when GGBS was introduced. The results for both parameters indicated a decrease with higher dosages of GGBS. To maintain acceptable workability flow, GGBS incorporation had to be limited to below 40% in the mix. Conversely, higher dosages of GGBS prolonged the setting time for this type of geopolymer, which may be advantageous in most cases, especially when working on-site. Durability index testing was performed on a mixture comprising 70% fly ash and 30% GGBS geopolymer activated with 6 M silica fume at a ratio of 1.3. The oxygen permeability index results demonstrated enhanced resistance to gas penetration overall. Water sorptivity and porosity showed favourable durability when the GGBS content remained below 30% but deteriorated when it surpassed 30% GGBS. Chloride conductivity indicated strong durability, especially with a GGBS content of 40% and higher. Microstructural analyses revealed undissolved fly ash in the samples. Nonetheless, with an increase in the molarity of the sodium hydroxide, a denser matrix and a reduced quantity of undissolved fly ash particles were observed. Furthermore, the findings indicated that incorporating more GGBS into the mixture resulted in a rougher surface of the sample. The inclusion of GGBS also led to enhanced mechanical properties of fly ash-based geopolymers. These findings highlight the potential of geopolymer concrete, activated with sustainable materials, as a viable and eco-friendly alternative to PC in the construction phase. This approach contributes to reduced carbon emissions and promotes circular economy practices.