2.4.1. Effect of Different Geopolymer Precursors on Mechanical Properties of Geopolymer

Concrete for building uses geopolymer because of its great compressive strength. Its mechanical qualities, however, can vary depending on the type of geopolymer used [128–131]. Previous studies employing various geopolymer precursors are presented in Table 6.



Girish et al. [131] investigated the feasibility of employing geopolymer concrete as fine aggregate in stiff paving-grade concrete comprising quarry dust and sand. The 60/40 mixture consisted of fly ash and ground granulated blast furnace slag (GGBS), had different solid–liquid ratios, and was examined at 3, 14, and 28 days. Increasing the molar ratio of the NaOH solution from 8M to 14M increased the strength of the resulting concrete but reduced the solution's workability. The experiment used a 12M NaOH solution, and the fine aggregates included both quarry dust and sand. The maximum strength was 62.15 MPa, and it was reached after 28 days. The results of the compressive strength test as depicted in Figure 6 showed that the strength of all the mixtures had increased. The achieved compressive strength at 28 days was more than the 40 MPa minimum required for stiff pavement cement concrete. However, research needs to be undertaken to investigate whether the compressive strength of geopolymer concrete is affected by the substitution of quarry dust for sand.

**Figure 6.** Compressive strength of geopolymer concrete with different mixture compositions [131].

Girish et al. [132] investigated self-consolidating geopolymer concrete for fixed-form pavement. Optimal strength geopolymer concrete is produced with a SiO2/Al2O3 ratio between 3.0 and 3.8 and a Na2O/Al2O3 ratio of 1. Compressive strength of 40 MPa was targeted for this mixture, which also included class F fly ash, ground blast furnace slag (GGBS), NaOH particles and solution form (molar concentration: 10 and 12), Na2SiO3 (A-53 grade), fine aggregate (quarry dust and river sand), coarse aggregate (below −20 mm), retarder (Conplast SP500), sugar solution, and water. The average compressive strength of the ambient-cured M10 mix after 28 days was 56.47 MPa, which is 40% higher than the intended compressive strength. At day 56, the compressive strength had increased to a peak of 71.78 MPa. However, as highlighted in Table 7, the proposed combination lacks considerable green strength, which is essential for slip-form paving applications, due to its low viscosity and yield stress. To make the SGC more environmentally friendly and appropriate for sliding mould applications, it might be beneficial to include nanoclays and/or fibres in the material.


**Table 7.** Hardened properties of M10 mix [132].

Izzati et al. [133] evaluated the use of different levels of geopolymer. No geopolymers, 1.0 wt. % fly ash, kaolin, or slag geopolymer particles were added to Sn-0.7Cu. All the mix designs were cured for 3 days and the temperature of curing for fly ash and slag was 27 ◦C and that for kaolin was 80 ◦C. As illustrated in Figure 7, using slag geopolymer is more challenging compared to not using geopolymer and using other geopolymers. Future research can attempt at using a higher percentage of geopolymer to test the composition's hardness. This may result in higher hardness compared to 1% geopolymer. To be comparable to other geopolymers, future research needs improve its preparation procedure in terms of curing temperature.

**Figure 7.** Different compositions of composite solder hardness value [133].

Hussein and Fawzi [134] tested various geopolymer contents in mix composition. The normal composition was cement with fine aggregate and coarse aggregate and 0% and 5% copper fibre, while the geopolymer composition had varied amounts of fly ash (FA) and slag with fine aggregate and coarse aggregate and 0% and 5% copper fibre. The preparation was cured at 40 ◦C for seven to twenty-eight days to evaluate compressive strength, splitting tensile strength, and bending strength. Figure 8 demonstrates that the maximum compressive strength, splitting tensile strength, and bending strength increase when the FA to ground granulated blast furnace slag (GGBFS) ratio is 0.55:0.45 with 0.5% copper wire fibre. It indicates that the compressive strength increases as the GGBFS level rises. The maximum strength of the geopolymer content can be determined by employing longer curing times and greater FA to GGBFS ratios.

**Figure 8.** Compressive strength of geopolymer with different mixtures [134].

Hussein and Fawzi [135] analysed different contents of geopolymer by using different ratios of fly ash (FA) to ground granulated blast furnace slag (GGBFS). Cement, fine aggregate, and coarse aggregate were used in the preparation of MR0 and MR1, while fly ash to slag ratios for MG0, MG1, MG2, and MG3 were 0.75:0.25, 0.65:0.35, and 0.55:0.45 and

mixed with fine aggregate and coarse aggregate in MR1, MG1, MG2, and MG3 with 0.5% copper fibre added. The preparation was cured at 40 ◦C for seven and twenty-eight days. As depicted in Figure 9, the larger the proportion of GGBFS, the greater the compressive strength and, at ninety days, 45% GGBFS had the highest compressive strength. MG3 with a content of 45% GGBFS shows the highest split tensile strength and flexural strength. To determine the ideal fly ash to slag ratio for assessing hardness, an analysis with a higher fly ash to slag ratio could be carried out.

**Figure 9.** Different content percentages of GGBFS show different compressive strengths [135].

According to the review, mechanical qualities can be improved by utilising slag geopolymer. Research is necessary to determine whether a particular geopolymer can enhance mechanical properties. Furthermore, according to the studies mentioned, there are several preparations that would affect the strength, therefore the sample preparation procedure should be fixed, such as curing at the same temperature, to ensure that the results are unaffected. Varied drying times will result in different compressive strengths.

The mechanical characteristics of geosynthetics are affected by several geosynthetic precursors. The strength of geosynthetic polymers is improved by using varied ratios of sodium silicate/sodium hydroxide and fly ash/alkaline activators.
