Influence of sepiolite addition on mechanical strength and microstructure of fly ash-metakaolin geopolymer paste

- A geopolymer composite is proposed based on fly ash and metakaolin reinforced with sepiolite.
- The optimal replacement of sepiolite in geopolymer is around 10%.
- Sepiolite is beneficial for compressive and flexural strength.
- Sepiolite improves pore refinement and leads to denser microstructure.

This work aims to verify the feasibility and broaden the application of utilizing acicular microfiber sepiolite to make reinforced geopolymer composites. Compressive strength, flexural strength and microstructure of fly ash-metakaolin (MK) geopolymer activated by sodium silicate and sodium hydroxide solutions blended with 0-20% sepiolite by mass with an interval of 5% were investigated. The experimental results reveal that sepiolite exhibits little effect on compressive strength before 3 days of curing. After 7 days, the addition of up to 10% sepiolite improves compressive strength, however, a further increase in the sepiolite leads to decrease in strength. Similarly, up to 10% sepiolite addition exhibits improving effect on flexural strength regardless of days of curing. With the increasing addition levels of sepiolite up to 10%, compact matrix can be observed compared to the reference geopolymers. Some crack can still be observed in specimen containing 20% sepiolite as compared to specimens containing 10% sepiolite. For geopolymer containing 10% sepiolite, cumulative intrusion volume corresponding to the porosity decreases. Further addition of sepiolite enlarges the cumulative intrusion volume and shifts the most probable pore diameters to higher values. The results from mechanical properties and microstructure observation are compatible. This work indicates that the optimal replacement of sepiolite in geopolymer is not exceeding 15%, and sepiolite is beneficial for compressive and flexural strength as well as pore refinement and denser microstructure. It will broaden the application of sepiolite in geopolymer composites.

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