The inclusion of fly ash class C, which is widely available locally, can influence the progress of strength and durability qualities of geopolymer concrete when exposed to significant environmental conditions. Alternative geopolymer combinations appropriate for curing at ambient temperatures were employed to investigate the impacts of high calcium (class C) fly ash -based geopolymer concrete. A combination of sodium hydroxide and sodium silicate alkaline activator was used to react with fly ash class C. Fly ash class C was added as 20% of the total binder without using ordinary Portland cement. The durability of fly ash class C-based geopolymer concrete in sulphate environments was tested. The changes in weight, length, and compressive strength due to exposure in magnesium sulphate solution for different periods of time were determined. The test results demonstrate that fly ash class C-based geopolymer concrete cured at normal temperatures has good resistance to sulphate attack. In general, inclusion of high calcium fly ash in geopolymer concrete improved strength and performed satisfactorily in sulphate environments when cured in ambient temperature.

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Concrete performances have weaknesses; one of those weaknesses is that it is significantly affected when put in a high sulfate and chloride environment. This study aims to investigate sulfate's effect on PCC concrete's performance. In this research, PCC concrete was immersed in water for 28 days and conducted after the casting. After being soaked in water, three immersed specimens were removed and immersed in magnesium sulfate solution for 24 days and 84 days. In contrast, three others were removed and immersed in natrium sulfate solution for 24 days and 84 days. The observation showed that PCC concrete's average compressive strength test with water immersion was 42.17 Mpa. It meets the SNI 2847:2019 Indonesian standard for sulfate classification S1. The specimens decreased to 30.74 Mpa after being soaked in the magnesium sulfate solution for 28 days and again reduced to 25.39 Mpa at 84 days. The average compressive strength of specimens bathed in the sodium sulfate solution for 28 days was 32.19 Mpa. It decreased to 28.03 Mpa at the age of 84 days. The results of this study show that the compressive strength of PCC concrete soaked in sodium sulfate meets the SNI 2847:2019 standard for sulfate classification S1, which is more than 28 Mpa. The compressive strength of PCC concrete immersed in magnesium sulfate for 1 month (28 days) is more than 28 MPa. It meets the standard as well. Yet, the compressive strength of PCC concrete soaked in magnesium sulfate for 3 months (84 days) does not meet the standard.

Geopolymer concrete has been developed to replace conventional concrete with other pozzolan materials with a high silicate alumina content. Fly ash is a material that contains a high silicate alumina of 22%. The high content of Al and Si increases the compressive strength of concrete. High-calcium fly ash is abundantly found in Indonesia. However, it has not been widely used for industry or research, and this is due to the fast-hardening time of concrete. Therefore, it has the potential to be developed. High-quality concrete has a low cement water factor that causes low workability in concrete. Self-compacting geopolymer concrete (SCGC) has been developed as a high-quality, workable concrete innovation. Concrete is produced using gravel, sand, fly ash, alkaline activator, and water materials. This study used 14 Molar levels of NaOH. The variations used were 0%, 3%, 5%, and 7% superplasticizers (SP) made from polycarboxylate. This study used a dry mixing method to overcome the setting time on concrete. The results show that the workability achieved is 645mm, and the compressive strength achieved is 41.7 MPa.

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