238 / 2022-06-15 13:34:07

Material design and mechanical performance of Ultra-High Performance Geopolymer Concrete

UHPC

Ultra-high performance concrete (UHPC) possesses excellent mechanical properties and durability. However, the use of high dose of Portland cement in UHPC manufacturing significantly increases the preparation cost and environmental impact. Geopolymer has good mechanical performance and durability, which is produced by chemically activating industrial solid wastes containing aluminosilicates. It is regarded as a green binder with lower production cost and carbon emission. Therefore, utilizing geopolymer to prepare UHPC, i.e. ultra-high geopolymer concrete (UHPGC), will be helpful to realize the effective combination of performance base and decarbonization goal. This study was conducted based on the two essential scientific issues: (1) Development of composition design with multiple performance requirements of UHPGC binder system and (2) Steel fiber reinforcing and toughening mechanism in UHPC.

To achieve a binder for UHPGC achieving the balance among different performances, the influences of each component on the flowability, initial and final setting times and compressive strength were quantitively studied based on the simplex-centroid design method with axial design. A binder composition range of UHPGC that meets the requirement of high workability, acceptable setting times and high compressive strength was achieved. The results showed that incorporating silica fume significantly improved the flowability and setting time when the activator modulus was greater than 1.0. Furthermore, the combination of slag, fly ash and silica fume had synergic effects on the improvement of the compressive strength and decrease of porosity. Particularly, when the activator modulus was 1.5 to 2.0, the content of slag was more than 50%, and the contents of fly ash and silica fume were less than 30%, the geopolymer mortar met the performance requirements of high flow performance (>210 mm), reasonable setting time (>120 min) and high compressive strength (>90 MPa).

The hybrid steel fiber reinforcement method was proposed and the contribution of each type of steel fiber in the steel fiber hybrid reinforcement to the compressive strength, four-point bending and direct tensile performance of UHPGC was investigated. The results showed that the different steel fiber hybrid combinations had different reinforcing and toughening efficiency on different mechanical performance of UHPGC. The straight steel fibers with lengths of 8 mm and 13 mm had significant effect on improving the compressive strength, bending and direct tensile properties of UHPGC. The hybrid reinforcement of three straight steel fibers with different lengths (6 mm, 8 mm and 13 mm) had a negative synergistic effect on the flowability, compressive strength and four-point bending property of UHPGC, but exhibited a significant positive synergistic effect on the improvement of direct tensile property. Given these results, comparing the mechanical property of UHPC, an optimized composition range of UHPGC steel fiber hybrid reinforcement with compressive strength > 150 MPa, bending strength > 18 MPa, ultimate axial tensile strength > 5 MPa and strain > 1500με was proposed.