Concrete made with high-strength artificial geopolymer aggregates: Mechanical properties and failure mechanisms

Artificial geopolymer aggregates (GPA) provide an effective solution to simultaneously reduce the over-excavation of natural rock and mitigate the waste landfills. In this study, GPA with a paste compressive strength over 140 MPa were produced and used as coarse aggregates in concrete with different water-to-binder ratios (w/b = 0.3, 0.4, and 0.5). The mechanical properties and failure mechanisms of so-formed geopolymer aggregate concrete (GAC) were comprehensively investigated and compared with those of natural aggregate concrete (NAC). Although GPA showed inferior strength than natural aggregates, both the compressive and splitting tensile strengths of GAC were 8.0 % and 5.5 % higher than those of NAC, respectively, when w/b = 0.3. For the failure modes of GAC, cracks penetrated through both GPA and matrix when w/b = 0.3, while more aggregate/matrix interfacial cracks were observed as w/b increased. In comparison, major cracks propagated along the aggregate/matrix interface in NAC. From microhardness tests, the comparatively weak interfacial transition zone (ITZ) was observed in NAC, rather than in GAC. The micro-level observations demonstrated the existence of dense microstructures in GPA/matrix interfacial regions, especially when w/b is low. The findings provided a fundamental understanding of the mechanical properties and failure mechanisms of GAC, which is helpful for the future applications of GPA.