Abstract
High autogenous shrinkage property is one of the disadvantages of ultra-high-performance concrete (UHPC), which may induce early age cracking and threaten the safety of concrete structure. In the present study, different dosages of calcium sulfoaluminate (CSA) cement were added in UHPC as an effective expansive binder. Hydration mechanism, autogenous shrinkage property, and compressive strength of UHPC were carried out to investigate the effect of CSA addition on the mechanical properties of UHPC. Scanning electron microscopy was also employed to characterize the intrinsic microstructural reasons relating to the changes in macroproperties. Based on the XRD diagram, increasing formation of ettringite and Ca(OH)2 can be found with increasing CSA content up to 15%. In the heat flow results of UHPC with 10% CSA addition, the maximum heat release increases to 2.6 mW/g, which is 8.3% higher than the reference UHPC, suggesting a higher degree of hydration with CSA addition. The results in autogenous shrinkage show that CSA expansion agent plays a significantly beneficial role in improving the autogenous shrinkage of UHPC. The corresponding autogenous shrinkage of UHPC is −59.66 μ ε , −131.11 μ ε , and −182.31 μ ε , respectively, at 7 d with 5%, 10%, and 15% addition, which is 108%, 117%, and 123% reduction compared to the reference specimen without CSA. In terms of compressive strength, UHPC with 5%, 10%, 15%, and 20% CSA addition has 10.5%, 17.4%, 30.2%, and 22.1% higher compressive strength than that for the reference UHPC at 28 d. Microstructural study shows that there is an extremely dense microstructure in both the bulk matrix and interfacial transition zone of UHPC with 10% CSA addition, which can be attributed to the higher autogenous shrinkage property and can therefore result in higher mechanical performance.
Highlights
With the development of super-high-rise and super-longspan building, durability and compressive strength of concrete are faced with great challenge [1, 2]
The intensities of ettringite and Ca(OH)2 peak increase with increasing CSA content up to 15%; the increment is prone to slow down with a larger CSA content of 20%. is is because, with a larger CSA content of 20%, larger proportions of water are consumed in the rapid hydration of calcium sulfoaluminate at early age, which leads to less free water available for the hydration of C3S and C2S Ca(OH)2, so the reduced CH generation can be found in CSA20
According to the XRD diagram, reduced ettringite is formed in CSA20. is phenomenon can be explained that, with the depletion of gypsum, monosulfate will be formed during the hydration of calcium sulfoaluminate as shown in equation (2), which requires less water molecule compared with that in equation (1)
Summary
With the development of super-high-rise and super-longspan building, durability and compressive strength of concrete are faced with great challenge [1, 2]. Lots of researches have been carried out to study the effect of different expansive additives on reducing the autogenous shrinkage of concrete [14, 15]. CSA cement was first produced in China in the 1970s by heating mixtures of limestone, bauxite, and gypsum at about 1250με, generating cement clinker with main compositions of calcium sulfoaluminate and belite It was characterized as an effective expansive binder owning to the massive formation of crystalline ettringite during hydration [16, 17]. With 5% CSA-CaO addition, compressive strength of UHPC increased to 97 MPa, compared to 95 MPa in the reference specimen at 3 d; corresponding flexural strength improved from 27 MPa to 30 MPa. To sum up, CSA cement is a kind of highly promising expansion agent and strength enhancer in the application of UHPC. Different dosages of CSA cement (0%, 5%, 10%, 15%, and 20%) were added in UHPC to replace part of OPC cement, aiming at producing UHPC with higher strength and durability
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