As a relatively new material, geopolymer concrete offers benefits as a construction material for sustainable development. It utilizes waste materials such as recycled concrete sludge, fly ash,etc. It has a very low rate of green house gas emission when compared to ordinary Portland cement. In this study, the components of a geopolymer are concrete sludge and metakaolin, NaOH was used as an alkaline activator. The amount of water glass and the ratio of NaOH/water glass were varied to find an optimal value. To improve the mechanical properties, silica fume was added as a bonding matrix, and the specimens were cured in air, then their mechanical properties such as compressive strength and bending strength were measured and their microstructures were investigated.
In this study activated meta kaolin was developed using low graded domestic Sanchung kaolin as a new pozzolanic material for replacing the expensive silica fume. Sanchung kaolin was mainly composed of Halloysite, so it could be activated by calcination. The optimum activation temperature was about 800℃, but little affected by calcining time. And as adding this activated meta kaolin to cement mortar or concrete, 10~30% increasement of compressive strength was achieved. And pozzolanic materials such as colloidal silica, water glass, and/or silica fume were used to decrease the porosity of the recycled aggregates. Some pozzolanic materials was absorbed on the recycled aggregates and closed the open pores of surfaces. The water absorption was decreased from 6~8% to 1~2% as the amount of absorption was increased.
Compressive strength of cement mortars and concretes using surface treated recycled aggregates reaches above 95% of the strength of its natural counterparts. Investigation of the microstructures using the scanning electron micrographs showed the formation of dense surface after treated recycled aggregates.