Mechanical strength, surface abrasion resistance and microstructure of fly ash-metakaolin-sepiolite geopolymer composites

Compressive strength, flexural strength, surface abrasion resistance, Vickers-hardness of surface and microstructure evolution of fly ash-metakaolin geopolymer blended with 0-20% sepiolite by mass with an interval of 5% were investigated. The experimental results reveal that the compressive strength increases dramatically before 7 days of curing but it slows down with increasing curing ages. A pronounced increase in compressive strength can be found at longer ageing times after 7 days when sepiolite is added. Sepiolite exhibits improving effect on compressive strength especially at longer curing times of 14 days with optimal 10% addition level. Similarly, up to 10% sepiolite addition exhibits optimal improving effect on flexural strength regardless of days of curing. Sepiolite enhances the resistance to abrasion. The most pronounced decrease in weight loss was observed in 10% sepiolite added geopolymer. Vickers-hardness increases with increasing replacement levels of sepiolite, and 10% sepiolite added specimen has the highest values due to the highest strength. With the increasing addition levels of sepiolite from 5% up to 10%, compact surface can be observed. The microstructure changes of geopolymer matrix are similar to that of geopolymer surface. The mechanical properties of the geopolymer specimens are closely related to the microcrack development and the bonding between sepiolite and geopolymer matrix.