Development of geopolymer cold-bonded artificial aggregate for UHPC applications based on fresh packing density

Abstract

Geopolymer cold-bonded aggregate (GCBA) is a new eco-friendly artificial aggregate manufactured by powder agglomeration. So far the scientific understanding of the powder particle packing within GCBA remains insufficient, which is theoretically crucial for enhancing its mechanical properties. In this study, a high-strength GCBA as the coarse aggregate for ultra-high performance concrete (UHPC) applications was developed using a high dosage of waste brick powder, and a new concept on fresh packing density was innovatively proposed to reflect the actual packing state of the precursor powders within GCBA. The effects of fresh packing density on GCBA's physico-mechanical properties and microstructure were investigated. Results show that the GCBA with the highest fresh packing density had the highest bulk crushing strength over 40 MPa at a reaction degree of only 8.1 %, and the resulting UHPC with this GCBA exhibited an impressive unconfined compressive strength of 162.9 MPa. The increase in fresh packing density led to a significant increase in the hardened GCBA's strength and density, despite it causing a reduction in the GCBA's reaction degree. The microstructural densification of GCBA with higher fresh packing density was formed by closed-packing unreacted precursor powder particles and fewer cyclic-shaped gels, resulting in a significant reduction in the required dosage of activator solution and consequently a decrease in GCBA's cost. A guideline of mixture design for GCBA was finally proposed, which differed from traditional methods by optimizing fresh pellets' performance rather than hardened pellets' performance, promising to be a more economical and efficient avenue for improving GCBA performance.