Effect of ground concrete waste as green binder on the micro-macro properties of eco-friendly metakaolin-based geopolymer mortar

Upcycling of ground concrete waste powder as green binder is an effective and high-value approach to reducing concrete waste. This study utilized ground concrete waste powder including recycled paste powder and recycled concrete powder for sustainable metakaolin-based geopolymer mortar. Concrete waste powder with irregular particle shape mainly contains hydration products, quartz and calcite. Substituting concrete waste powder for metakaolin reduces the number of alkali-activated cementitious products in blended geopolymer paste, but the nucleation and filling effects of concrete waste powder can improve paste micro-structure. Incorporating concrete waste powder raises the drying shrinkage of geopolymer mortar; recycled paste powder blended geopolymer mortar has higher drying shrinkage than recycled concrete powder blended geopolymer mortar, and the maximum drying shrinkage of geopolymer mortar with 75% recycled paste powder is 178.7% higher than that of geopolymer mortar with 75% recycled concrete powder. The compressive strength first rises and then declines with the rise of concrete waste powder content. Recycled paste powder blended geopolymer mortar has better compressive strength than recycled concrete powder blended geopolymer mortar, and the 28-d compressive strength of geopolymer mortar with 75% recycled paste powder is 34.4% larger than that of geopolymer mortar with 75% recycled concrete powder. Substituting concrete waste powder for metakaolin up to 75% declines the water and chloride ingress in blended geopolymer mortar, and recycled paste powder blended geopolymer mortar has lower water absorption and chloride migration than recycled concrete powder blended geopolymer mortar. Optimizing concrete waste powder type and content can prepare sustainable metakaolin-based geopolymer mortar having superior strength and low transport property.