The chapter investigates the impact of aging conditions on the hydration and setting performance of cement paste containing triethanolamine (TEA). It begins by introducing TEA as a grinding agent and setting modifier in ordinary Portland cement, noting its high efficiency but also its sensitivity to setting and hardening performance. The study focuses on the pre-hydration process of cement, which occurs under different relative humidity levels and aging times, and how this affects the interaction between TEA and cement particles. Through extensive characterization using XRD and TGA, the chapter reveals how pre-hydration alters the mineralogical phases and hydration products, leading to changes in the cement's setting time and hydration process. Notably, the research finds that pre-hydration can increase the sensitivity of the cement hydration process to TEA, affecting both the aluminate and silicate phase reactions. The chapter concludes by highlighting the practical implications of these findings for the application of TEA in cement stored under various conditions.
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Abstract
Effect of aging conditions on the hydration and setting performance of cement paste with the addition of TEA were investigated through the combined techniques of calorimetry, Vicat test, XRD and TGA. It is found that, along with the increased aging time and RH, an obvious formation of AFt, AFm and CH can be found due to the pre-hydration of clinker. Besides, under the RH of 90%, the pre-hydration within the first day was significantly improved, and this process was continuously developed in the next 7 days through the strong carbonation of CH. Further analysis indicates that the pre-hydration can retard the cement hydration and increase the setting time of cement paste. Compared to the reference, a prolonged induction period and an increased setting time were observed for the aged samples with the addition of TEA. Besides, the commonly observed flash setting performance under the TEA dosage of 0.5 wt.-% was eliminated when the samples were pre-hydrated under 90% RH, which indicates the pre-hydration can alleviate the strongly accelerated aluminate phase hydration caused by TEA.
1 Introduction
Triethanolamine (TEA) is a grinding agent and setting modifier during the production and application of ordinary Portland cement (OPC) with the advantage of low dosage and high efficiency [1]. However, one problem that restricts its wide application is the high sensitivity of setting and hardening performance of cement paste caused by TEA [2]. Many researchers have reported that, along with the increased dosage of TEA, the setting time of cement paste was first decreased, then increased and then followed by a flash setting [3]. Beside the dosage, many other parameters could also affect the performance of TEA, such as the sulfate type and content in cement [4] and the preparation method of cement paste [5] etc. Even though many parameters affecting the setting performance of cement paste with the addition of TEA have been investigated, a huge variation in the setting time of cement paste can still be observed in our former study. Considering the only difference between these experiments was the aging of cement, it is necessary to uncover the mechanism of how the aging of the cement could affect the interaction of TEA and cement particles, which in the end substantially changes the hydration process and setting performance of cement paste.
The aging of cement can be regarded as pre-hydration process, which is a continuously physicochemical reaction of mineral phases in cement under a certain relative humidity (RH). Some studies investigate the threshold values of atmospheric humidity at which each cement mineral phase turns to sorb water vapor. Free lime reacts with water vapor at a very low 14% RH, and C3A starts to take up water vapor at 55% RH. While, the silicate phases, mainly including C2S and C3S start to sorb small amounts of water vapor at the RH of 63% and 64% [6‐8]. Vektaris [9] found that white cement is more resistant to pre-hydration compared to OPC due to the fewer amounts of C3A and free lime.
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The pre-hydration not only changes the properties of cement particles, but also affects its interaction with the chemical admixtures. M. R. Meier [10] compared the dispersing performance of three different superplasticizers (BNS, PCE and casein) on the fresh and aged cement. All three superplasticizers showed decreased dispersing performance in cement after 1 d aging and BNS exhibited the biggest reduction. Sun [11] found an increased initial fluidity in cement after being exposed at 20 ± 2 ℃ and 85%-90% RH for 4 d compared to that of fresh cement. Contrary to the many publications on the interaction of superplasticizer with aged cement, limited research focus on the performance of TEA in aged cement.
Hence, the impact of different aging conditions on the hydration process and setting time of cement paste with the addition of TEA was studied. These results, on the one hand, deepen the understanding of the impact of different aging conditions on the properties of cement with the addition of TEA. On the other hand, it can provide practical guidance on the application of TEA in cement stored under different conditions.
2 Materials and Methods
2.1 Materials
Table 1.
Chemical and mineralogical compositions of the fresh cement
Oxides
Al2O3
CaO
Fe2O3
MgO
SiO2
SO3
TiO2
LOI
Value/%
5.6
57.1
4.6
2.1
24.5
3.3
0.4
1.3
Phases
C3S
C2S
C4AF
C3A
Anhydrite
Hemihydrate
Gypsum
CaCO3
Value/%
59.6
17.7
7.3
9.2
1.6
1.8
2.1
0.2
PI 42.5 type OPC (Fushun Orcel Technology Co., Ltd.) was used in this study. The chemical and mineralogical compositions of cement are shown in Table 1. The aging process of the cement was carried out at 25 ℃ and RH of 30%, 55% and 90% in a climatic chamber (CH-150R, Dongguan Tude Environment Testing Equipment Co. Ltd.) for 1, 3, and 7 d. During the aging process, the cement particles were spread over an acrylic plate with a thickness of ~ 1mm. The cement particles were turned over and remixed every 6 h. After the aging process, the aged cement was milled into its original fitness in agate mortar by hand for further experiments. Double-deionized (DI) water was utilized to prepare cement paste, TEA was provided by Sinopharm Chemical Reagent Co., Ltd with an analytical purity exceeding 99%.
2.2 Methods
Cement paste was prepared by mixing cement, water and TEA for 1 min with a hand mixer (MFQ4080, Bosch). The TEA dosage was 0.1% and 0.5% by weight of cement. The water-to-cement ratio was 0.41. A constant ambient temperature of 25 ℃ was kept.
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The setting times of pastes were determined according to Chinese National Standard GB/T 1346–2011. The hydration process of pastes with and without TEA was monitored by an isothermal calorimeter (TAM air, TA instrument). The external mixing method was used. The heat flow was normalized based on the amount of cement. XRD and TGA were used to characterize the consumption of mineralogical phases and the formation of hydration products after aging. The radiation (Cu Kα) was generated with 40 kV and 40 mA, and the samples were scanned in a range from 5 to 40°. Phase quantification was determined by using α-Al2O3 as the internal standard. Rietveld analysis was performed with the software HighScore Plus 4.8 (PANalytical). For the TGA measurement, a heating rate of 10 ℃/min from room temperature to 1000 ℃ was applied under a N2 atmosphere with a flow rate of 40 mL/min (STA 449C, NETZSCH).
3 Results and Discussion
3.1 Characterization of the Aged Cement
Table 2.
Composition of fresh and aged cement under different aging conditions (wt.%)
Phase
Ref
30RH
55RH
90RH
1 d
7 d
1 d
7 d
1 d
7 d
C3S
59.6
59.5
59.1
59.1
58.8
56.6
47.1
C2S
17.7
17.5
17.3
17.2
17.0
13.8
9.8
C4AF
7.3
7.3
7.2
7.0
6.6
5.7
4.2
C3A
9.2
9.1
8.9
8.5
6.2
5.1
2.9
Anhydrite
1.6
1.5
1.3
1.3
0.9
-
-
Hemihydrate
1.8
1.6
1.2
1.3
0.6
-
-
Gypsum
2.1
2.3
2.9
2.2
1.4
0.9
-
CaCO3
0.2
0.3
0.4
0.5
1.4
1.6
4.6
CH
0.1
0.4
0.6
0.5
0.9
0.6
0.7
AFt
0.1
0.1
0.1
0.2
0.7
1.1
1.4
AFm
0.2
0.2
0.3
0.4
0.8
0.8
1.0
In order to verify the effect of aging on the interaction of TEA with cement, a thorough characterization of the cement after aging under different conditions was conducted. QXRD was applied for the different samples, as shown in Table 2. Under the 30% RH, a slight decrease in the mineralogical phases and an increase in hydration products were observed with the increased curing age. Along with the increased RH to 55% and 90%, more clinker phases were consumed and a higher amount of hydration products were detected. Moreover, carbonation was found for samples aging in 55% RH for 7 d and 90% RH for 1 d and 7 d.
The pre-hydration and carbonation during the aging process were also quantified by TGA (Fig. 1). The fresh cement exhibits only a minor mass loss of 1.6%, which is mainly related to the decomposition of hydration products portlandite and ettringite in the temperature range from 105 to 640 ℃, while the decomposition of CaCO3 in the temperature range from 650 to 1000 ℃ is negligible, which matches well with the QXRD data. Similar to the fresh cement, samples aged at 30% RH for 1 d and 7 d exhibit nearly no changes in mass loss, which proves again 30% RH may not induce a strong pre-hydration of cement. However, when the RH increased to 55%, the mass loss slightly increased along with the aging time. Notably, an obvious decomposition of CaCO3 was found after aging in 55% RH for 7 d, which proves the considerable carbonation as indicated by the XRD results in Table 2. Along with the increased RH to 90%, an obvious weight loss was found at around 100 ℃ after aging 1 d, which is caused by the large formation of C-S-H and AFt (Table 1). Besides, an obvious weight loss was found at around 450 ℃ for samples aged under 90% RH for 1 d, which indicates the fast pre-hydration. However, after aging 7 d, the decomposition of CH cannot be observed, but a sharp weight loss was found for the decomposition of CaCO3.
Fig. 1.
TGA of the fresh and aged cement under different aging conditions
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3.2 Hydration of the Aged Cement with the Addition of TEA
Fig. 2.
Heat flow of the fresh and aged cement with and without the addition of TEA under different aging conditions; (a) Aged for 1 d; (b) Aged for 7 d.
The hydration process of the fresh and aged cement with TEA was measured and the results are shown in Fig. 2. Without the addition of TEA, a prolonged induction period was observed in all aged samples, which is proportional to the increased aging time and RH. Moreover, except for samples aged at 30% RH, a decreased main hydration peak was found, especially for samples aged at 90% RH for 7 d.
For the samples with 0.1 wt.% TEA but aged under different conditions, a prolonged induction period to different extents was observed. Compared to the samples without TEA, the presence of TEA significantly delayed the occurrence of the main hydration peak of the fresh cement. However, nearly no difference can be found for the aged samples, regardless of the aging time of 1 d or 7 d. For the samples aged 7 d under 30% and 55% RH, the presence of TEA not only prolongs the induction period but also significantly reduces the main hydration peak. However, when the RH increased to 90%, a fast aluminate phase reaction can be observed before the hydration of silicate phase. It indicates that the sulfate balance was broken through the addition of TEA and pre-hydration can increase the sensitivity of the cement hydration process to the presence of TEA. Along with the further increased TEA dosage to 0.5 wt.%, compared to the reference without aging, the aluminate phase reaction was retarded, which in turn resulted in a faster silicate phase reaction.
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3.3 Setting Time of the Aged Cement with the Addition of TEA
Fig. 3.
Setting times of the fresh and aged cement with and without the addition of TEA under different aging conditions; (a) Aged for 1 d; (b) Aged for 7 d.
The setting times of the fresh and aged cement with and without the addition of TEA under different aging conditions were measured and the results are shown in Fig. 3. For the condition without the addition of TEA, cement aged under 30% RH for 1 d exhibits a similar setting time as fresh cement. Besides, the setting time of cement paste was prolonged along with the further increased RH during aging. Samples exhibited a strong delay in setting time for about 1.5 h after only 1 day of storage in 90% RH due to the pre-hydration of cement grains. Furthermore, the effect became pronounced with the increased aging times. Compared to the samples without the addition of TEA, the samples in the presence of TEA show a similar trend, namely the setting time was prolonged along with the increased RH and the aging time. However, two phenomena need to be noted. Firstly, the commonly observed flash setting performance under the TEA dosage of 0.5 wt.% was eliminated when the samples were pre-hydrated under 90% RH, which indicates the pre-hydration can alleviate the strongly accelerated aluminate phase hydration caused by TEA. Secondly, for the samples aged for 7 d under the RH of 90%, a reduced setting time was observed when the TEA dosage was increased from 0.1 wt.% to 0.5 wt.%. Considering the calorimetric curves shown in Fig. 2, it can be inferred that both the setting of cement paste with TEA dosage of 0.1 wt.% and 0.5 wt.% was controlled by the accelerated aluminate phase reaction. Because of the strong acceleration on the aluminate phase along with the increased TEA dosage, the sample aged for 7 d under 90 RH shows a shorter setting time.
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4 Conclusion
The combined effect of RH and aging time on the hydration process and setting time of aged cement with and without the addition of TEA was investigated. Based on the results above, the following conclusions can be obtained.
Aging of cement can lead to the pre-hydration of cement, which results in the formation of a certain amount of hydration products. Besides, with the increased RH and aging time, strong carbonation can be observed.
The pre-hydration can increase the sensitivity of the cement hydration process to the presence of TEA, which can result in a sulfate imbalance.
Both the aluminate phase and the silicate phase reactions can be suppressed through the process of pre-hydration, which results in a prolonged setting time with increased RH and aging time.
Due to the retarded aluminate phase reaction through the pre-hydration, the commonly observed flash setting performance was eliminated under the TEA dosage of 0.5 wt.%.
Acknowledgements
We sincerely thank the support from Shanxi Yuncheng Science and Technology Bureau (project "Development of specialized admixtures and auxiliary techniques for efficient shotcreting under complex and severe tunnel construction environment") and the National Natural Science Foundation of China (Project No. 52208282). In addition, the authors would like to deeply thank the Key Laboratory of Concrete Functional Materials of Yuncheng City and Shanxi Jiawei New Material Co., Ltd for their contribution to this research.
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