Physico-Mechanical Evaluation of Geopolymer Concrete Activated by Sodium Hydroxide and Silica Fume-Synthesised Sodium Silicate Solution

Abstract

Commercial sodium hydroxide (NaOH) and sodium silicate (SS) have remained two of the leading alkaline activators widely used in producing geopolymer concrete, despite some identified negatives regarding their availability and additional CO2 emissions relating to the overall manufacturing process. This study reports the viability of developing geopolymer concrete using a laboratory-synthesised silica fume (SF)-derived SS solution in combination with NaOH at a molarity of 10M as an alternative binary alkali-alkaline activator to Ground Granulated Blast Furnace slag (GGBS). The use of SF in the development of geoolymer activators will pave the way for the quality usage of other high-silica content by-products from nature, industry, and agriculture. In the currently reported proof of concept, four geopolymer concrete batches were produced using different alkaline activator/precursor-A/P ratios (0.5 and 0.9) and SS to NaOH-SS/SH volume ratios (0.8/1.2 and 1.2/0.8), to establish the impact on the engineering performance. Two controls were adopted for ordinary and geopolymer concrete mixes. The engineering performance was assessed using slump and compaction index (CI) tests, while the Unconfined Compressive Strength (UCS) and tensile splitting (TS) tests were measured at different curing ages in accordance with their appropriate standards. The results indicated a reduction in slump values as the A/P ratio decreased, while the CI values showed a reversal of the identified trend in slump. Consequently, mix GC2 attained the highest UCS strength gain (62.6 MPa), displaying the superiority of the alkali activation and polymerisation process over the CSH gel. Furthermore, the impact of A/P variation on the UCS was more pronounced than SS/SH due to its vital contribution to the overall geopolymerisation process.

Mechanical Properties of a Sustainable Low-Carbon Geopolymer Concrete Using a Pumice-Derived Sodium Silicate Solution

“…Six concrete test specimens of a standard cube dimension (100 mm × 100 mm × 100 mm), two concrete cylinder of dimension 150 mm × 300 mm, and one concrete beam of dimension 100 mm × 100 mm × 500 mm were each prepared for the mix designs in accordance with BS EN 12390-2:2019 [50], for the determination of the unconfined compressive, tensile, and flexural strengths of each of the concrete design mixes [51]. The concrete test specimens were demoulded after 24 h and kept in a moist-curing tank at an ambient temperature of 20 ± 2 • C. Moist-curing was adopted over heat-curing for the geopolymer test specimens to simulate practical in situ concrete production and obtain concrete with a lower GWP (global warming potential) [30], although slag-based geopolymers perform better in compressive strength under elevated temperatures [52].…”

“…The NaOH solution was then stored in a safe, closed container for over an hour to ensure a complete mixture. Secondly, the SSA solution using PP was designed using Equation (1) according to Adeleke et al [30] and Billong et al [29] and prepared as follows: 2958 g of PP was mixed in 3500g of the initially prepared NaOH solution at an ambient temperature of 20 ± 2 • C and kept for at least 24 h before use. This ensured a thorough mixing of the PP in the NaOH solution and the dissolution of silica in an alkaline environment at ambient temperatures to produce the desired pumice-derived sodium silicate solution.…”

“…This is also because the activated GGBS geopolymer concrete entrains more air than the PC control concrete. Likewise, the highly viscous nature of the sodium silicate alkaline solution affects workability when used to activate GGBS to develop geopolymer concrete because it introduces a sticky characteristic into the fresh concrete and thus contributes to the lower workability of the geopolymer mix against the PC mix [30,60,61]. alkaline activator whilst mixing the concrete was higher with a decreasing content of GGBS, causing a decrease in workability [30].…”

“…Likewise, the highly viscous nature of the sodium silicate alkaline solution affects workability when used to activate GGBS to develop geopolymer concrete because it introduces a sticky characteristic into the fresh concrete and thus contributes to the lower workability of the geopolymer mix against the PC mix [30,60,61]. alkaline activator whilst mixing the concrete was higher with a decreasing content of GGBS, causing a decrease in workability [30]. Also, as the A/P ratio increased with the dry solid (GGBS) decreasing, the rate of dissolution of the GGBS material and the formation of reactant gel products (hydration process) increased at an early stage, causing the initial slump to decrease and resulted in an increase in slump loss [62].…”

“…Therefore, substituting the commercially produced Na 2 SiO 3 alkaline activator with a more sustainable alternatively derived from Na 2 SiO 3 leads to a lower global warming potential (GWP) in geopolymer concrete. Ongoing attempts have been made to develop geopolymer concrete using a sodium silicate alternative (SSA) alkali solution derived from industrial and agricultural waste such as rice husk and silica fumes having a high silica content [28][29][30][31][32][33]. Billong et al [29] compared the mechanical performance of metakaolin and GGBS-based geopolymer paste using commercially produced sodium silicates and an SSA derived from SFs mixed in a concentrated NaOH solution.…”