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
With the development of super-high-rise and super-long-span building, durability and compressive strength of concrete are faced with great challenge [
In order to reduce the autogenous shrinkage of UHPC, additions of expansion agent, shrinkage reducing agent (SRA), and super absorbent polymer (SAP) are often incorporated to replace part of cementitious materials at various proportions [
CSA cement was first produced in China in the 1970s by heating mixtures of limestone, bauxite, and gypsum at about 1250
To sum up, CSA cement is a kind of highly promising expansion agent and strength enhancer in the application of UHPC. But up to now, there is a lack of study on the effect of optimized CSA cement on the autogenous shrinkage, mechanical performance, and hydration mechanism of UHPC as well as the underlying microstructural changes, which is of great importance before wider application of UHPC modified by CSA cement can be made. In this study, 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.
Materials of ordinary Portland cement, CSA expansion agent, minerals of silica fume, fly ash, and slag were used to produce UHPC. Chemical compositions of each material are shown in Table
Oxide compositions of the CSA and OPC cement powder (by wt. %).
CaO | SiO2 | Al2O3 | SO3 | Fe2O3 | MgO | K2O | Na2O | TiO2 | SrO | BaO | Mn3O4 | LOI | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
CSA | 42.33 | 9.00 | 33.82 | 8.83 | 1.35 | 2.29 | 0.22 | 0.12 | 1.61 | 0.07 | 0.02 | 0.03 | 0.31 |
OPC | 62.14 | 19.42 | 4.83 | 4.81 | 1.95 | 2.13 | 0.75 | 0.24 | 0.24 | 0.07 | 0.02 | 0.07 | 3.33 |
SF | 0.11 | 97.90 | 0.52 | — | 0.17 | 0.10 | — | 0.99 | — | — | — | — | 0.21 |
FA | 3.83 | 60.84 | 23.73 | 0.63 | 6.96 | 0.55 | — | — | — | — | — | — | 3.46 |
GGBS | 37.11 | 32.91 | 15.36 | — | 0.74 | 8.52 | — | — | 1.95 | — | — | — | 3.41 |
Note: SF: silica fume; FA: fly ash; GGBS: ground granulated blast furnace slag; LOI: loss on ignition.
Mixing design of UHPC.
OPC (kg/m3) | CSA (kg/m3) | SF (kg/m3) | GGBS (kg/m3) | FA (kg/m3) | Sand (kg/m3) | SP (%) | AA (%) | ||
---|---|---|---|---|---|---|---|---|---|
CSA0 | 857.4 | 0 | 53.6 | 53.6 | 107.1 | 0.24 | 1071.7 | 0.675 | 0.25 |
CSA5 | 816.6 | 40.8 | 53.6 | 53.6 | 107.1 | 0.24 | 1071.7 | 0.675 | 0.25 |
CSA10 | 779.4 | 77.9 | 53.6 | 53.6 | 107.1 | 0.24 | 1071.7 | 0.675 | 0.25 |
CSA15 | 745.6 | 111.8 | 53.6 | 53.6 | 107.1 | 0.24 | 1071.7 | 0.675 | 0.25 |
CSA20 | 714.5 | 142.9 | 53.6 | 53.6 | 107.1 | 0.24 | 1071.7 | 0.675 | 0.25 |
Note:
In order to investigate the effect of CSA on the hydration of UHPC, corresponding pure UHPC pastes with different CSA contents (0%, 5%, 10%, 15%, and 20%) were prepared in cylinder molds. After 24 h, the specimens were cured in a water bath with the temperature set at 20
Cement hydration is an exothermic reaction, and the heat generated during the hydration may improve the autogenous shrinkage of UHPC. Therefore, investigation on the effect of CSA cement on the heat flow of UHPC is of great importance in controlling the autogenous shrinkage. In this study, multichannel isothermal calorimeter (model TAM Air, TA Instruments) was applied to investigate the heat flow in UHPC. During the tests, the isothermal temperature is 20
In this study, autogenous shrinkage of UHPC was conducted according to the Chinese standard GB/T 50082-2009 [
Testing equipment of autogenous shrinkage of UHPC.
Mechanical properties of compressive strength and bending performance were examined in this study. Compressive strength of UHPC cured at 3 d, 7 d, and 28 d was examined according to the Chinese standard GB/T 50082-2009 [
Bending performance of UHPC samples was tested by a four-point bending test using Tinius Olsen H25KS according to BS-EN 1170-5:1997 [
Experimental setup for bending performance tests of UHPC [
In order to examine the intrinsic microstructural reasons underlying mechanical performance, microstructure of fractured UHPC cured at 20°C for 28 d was characterized by the secondary electron imaging mode in SEM. Before testing, hydration of UHPC specimens was stopped by soaking them in liquid nitrogen for few minutes and then placed in a freeze drier until constant weight can be maintained. In this study, JEOL JSM-5800LV equipped with energy dispersive X-ray microanalysis (EDX) was applied for microstructural observation, operating at an accelerating voltage of 20 kV and a working distance of 12 mm. EDX was also undertaken to investigate the relevant elemental information at specific spot.
XRD diffraction diagram of UHPC paste with 0%, 5%, 10%, 15%, and 20% CSA additions cured at 20
XRD diffraction diagram of UHPC with different CSA additions at 7 d (E: ettringite; C: Ca(OH)2; A: C3S; B: C2S).
The heat flow of UHPC with 10% CSA addition up to 72 h is illustrated in Figure
Effect of CSA addition on the heat flow of UHPC up to 72 h.
The effect of CSA expansion agent on the autogenous shrinkage development of UHPC is presented in Figure
Autogenous shrinkage development of UHPC with different CSA additions.
From the results in Figure
However, when the CSA proportion increases to 20%, superior expansive effect can still be found out but there is a slight reduction in the swelling property of UHPC. The corresponding autogenous shrinkage reduced to −151.26
It can be concluded that CSA cement possesses great potential to reduce the autogenous shrinkage of UHPC, and a superior expansion property can be obtained with an optimized volume of 15%. This beneficial expansion property can be attributed to the continuous formation of expansive ettringite during hydration, and the swelling stress generated from ettringite may compensate the autogenous shrinkage developed in UHPC. The corresponding microstructural reasons are discussed in Section
Compressive strengths of UHPC with different CSA additions are illustrated in Figure
Compressive strength development of UHPC with different CSA additions.
Microstructure of UHPC with 10% CSA addition is shown in Figure
Microstructural features of UHPC with 10% CSA additions at 7 d. (a) Microstructure of bulk matrix. (b) Spot analysis on Spot “1”. (c) Microstructure at the aggregate-cement interface.
At the same time, there is a dense and intimate contact between aggregate and cement at the interfacial transition zone in Figure
XRD diagram indicates that the main hydration products of UHPC are ettringite and Ca(OH)2, both with and without CSA addition. In particular, 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. This is suggestive of an accelerated effect of CSA addition on the hydration process of UHPC at early age, and a higher degree of hydration can be achieved with CSA addition.
From the autogenous shrinkage results, it can be concluded that CSA expansion agent plays a significantly beneficial role in improving the autogenous shrinkage of UHPC, especially with an optimized addition between 5% and 15%. The autogenous shrinkage of UHPC with 5%, 10%, and 15% addition of CSA is −59.66
At the same time, addition of CSA expansion agent tends to increase the compressive strength of UHPC to a great extent. For example, compressive strength of UHPC with 5%, 10%, 15%, and 20% CSA addition is 4.2%, 9.8%, 22.5%, and 18.3%, respectively, higher than that for the reference UHPC without CSA addition. At 28 d, the corresponding increment rises to 10.5%, 17.4%, 30.2%, and 22.1%, respectively, compared to that for the reference sample.
Microstructural study shows that there is an extremely dense microstructure in UHPC with the addition of CSA, and large air voids are almost absent in the matrix. Spherical structure of silica fume is displayed in the bulk matrix, which suggests that it mainly plays a physical filler effect in a microstructural view. Prismatic-shaped ettringite can be observed in the matrix, intermixed with small quantities of C-S-H gel. At the same time, there is a dense and intimate contact between aggregate and cement at the interfacial transition zone, which can be attributed to the high autogenous shrinkage property CSA addition. High-quality interfacial transition zone with less gaps and porosity is beneficial to higher mechanical performance of the UHPC; this is in agreement with the compressive strength results in this study.
Therefore, not only is CSA cement an effective expansive agent in the application of UHPC as long as the amount of admixture is controlled within reasonable range, but it also has beneficial effect on the strength development of UHPC.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was supported by the State Key Laboratories of Silicate Materials for Architectures, Wuhan University of Technology in China (no. SYSJJ2019-17), Key Laboratory of Structure and Wind Tunnel of Guangdong Higher Education Institutes Open Fund (no. 202002), and the Natural Science Basic Research Program of Shaanxi Province (nos. 2021JQ-605 and 2020JM-536).