131 / 2022-04-28 17:50:40

Multiscale Investigation on the Performance of Engineered Cementitious Composites Incorporating PE Fiber and Limestone Calcined Clay Cement (LC3)

Engineered cementitious composites; Limestone calcined clay cement; Strength; strain; Fiber dispersion.

Engineered cementitious composites (ECC) exhibits obvious strain hardening behavior overcomes the inherent drawback of brittleness in normal concrete. Nevertheless, the fabrication of ECC that consumes a high content of cement not only increases the initial material cost but also compromises the carbon and energy footprints of the composites. To promote the broader application of ECC and enhance the sustainability of ECC-made infrastructure, the usage of environmentally friendly ingredients is highly needed. Limestone calcined clay cement (LC3) promises to be one of the suitable alternatives. The embodied energy and carbon for producing calcined clay and limestone powder are much lower than those for cement production. In this paper, the mechanical performance of ECC incorporating three dosages of LC3 (0, 35%, and 50%) was investigated. The cubic compression test and uniaxial tensile test of dog-bone-shaped specimens were performed. The fiber dispersion state was studied by fluorescence technique, and the digital image analysis was carried out. The single fiber pullout test was performed to quantitatively analyze the fiber/matrix interfacial properties. Mercury intrusion porosimeter (MIP) test was used to study the influence of LC3 on the pore structure of ECC. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) tests were conducted to analyze the phase assemblage of ECC matrix incorporating LC. Through performing the life cycle assessment, the environmental advantages of using LC3 in ECC are also discussed.  

The micropore structure of the cementitious matrix is one of the key factors influencing the macroscopic mechanical performance of ECC. Fig 1 demonstrates the pore size distribution of the three kinds of binder systems. It is remarked that the distribution peak shifts towards the smaller pore size regions with the incorporation of LC3. Fig 2 shows that the total porosity is significantly reduced owing to the incorporation of LC3. The substitution of OPC by 35% LC3 demonstrates a more significant influence on the micropore structure of ECC. The chemical phase compositions of the three kinds of ECC are presented in Fig 3. For LC3-based blends, hemi-carboaluminates (Hc) are observed, which are generated due to the reaction between the aluminates, in both calcined clay and cement, and CaCO3 from limestone. For the LC3-based paste matrix, the peaks of Ca(OH)2 are remarkably reduced with the usage of calcined clay and limestone, which could be attributed to the pozzolanic reaction between Ca(OH)2 and the reactive silica and alumina phases in calcined clay. Fig 4 presents the TGA and different thermal gravimetric (DTG) curves of the three kinds of specimens. For the LC3-based matrix, the magnitude of the second peak of mass loss around 170 °C is quite modest, which corresponds to the loss of water from carboaluminates. The peak of mass loss around 460 °C is related to the dehydroxylation of Ca(OH). It is remarked that the Ca(OH)2 is largely consumed by the pozzolanic reaction and that its content is significantly reduced with the increase of LC3 dosage.  

The compressive strength of the three studied ECC types is illustrated in Fig 5. At 3 days, the strength increases with the incorporation of LC3, and the increasing ratios attain 21.74% and 10.23%, respectively, for ECC-LC3-35 and ECC- LC3-50 similar trend was observed at 7 days. While at 28 days, for ECC-LC3-35, the increasing ratio is 10.28%, and for ECC-LC3-50, the reduction ratio is 8.89%. The tensile stress-strain curves of the three kinds of ECC are presented in Fig 6. It is noted that LC3-based ECCs exhibit strain hardening behavior. The ultimate tensile strength σtu of ECC-LC3-35 attains 9.55 ± 0.59 MPa, and the ultimate tensile strain εtu of ECC-LC3-50 reaches 8.53 ± 0.30%. The results imply that the combination of calcined clay and limestone could be successfully used to fabricate ECC exhibiting both high strength and high ductility. For the three studied ECC matrix types, the force-displacement curves of the single fiber pullout test are depicted in Fig 7. The slight slip-hardening behavior was observed after the debonding between fiber and matrix. The applied external fiber load is dominantly resisted by the interfacial frictional stress between fiber and matrix. It is remarked that the frictional bond strength τ0 demonstrated firstly an increasing and then a decreasing tendency with the incorporation of LC3, which is consistent with the trend of the tensile strength at the composite scale. The fiber dispersion homogeneity is found to rise with the substitution of OPC by LC3 through the increase ratio does not show a linear relationship with the replacement ratio of OPC. The results of fluorescence analysis indicate that the combined action of calcined clay and HRDR positively alters the flocculated structure formed by cement paste and that the flowability, as well as the rheology properties of the composites are improved.  

The life cycle inventories were adopted from the database of GaBi software. Two environmental impact criteria, energy, and global warming potential (GWP) were calculated. The obtained results are presented in Fig 8 and Fig 9. It is noted that the environmental impact of cement among all the components of ECC is the highest. PE fiber ranks only second to cement although its dosage is quite low. When OPC is partly replaced by LC3, the two studied environmental impact criteria descend, especially the GWP with the reduction ratio attaining 40.31%.