In diesem Kapitel wird der Einfluss von Cellulosether auf die Wasserverlustrate (WLR) von Mörteln aus Calciumsulfhoaluminat (CSAC) bei unterschiedlichen Temperaturen untersucht. Es wird untersucht, wie verschiedene Arten und Dosierungen von Cellulosether, wie Hydroxyethylcellulose (HEC) und Hydroxyethylmethylcellulose (HEMC), die Wasserrückhaltefähigkeit und Hydratationsprozesse von CSAC-Mörteln beeinflussen. Die Studie zeigt, dass die WLR im Allgemeinen mit dem CE-Gehalt steigt und dann abnimmt und dass Art und Dosierung der CE die Wasserrückhalteeigenschaften signifikant beeinflussen. Darüber hinaus untersucht das Kapitel die Auswirkungen der Temperatur auf die WLR, wobei bemerkenswert ist, dass die Gelierung von CE bei 60 ° C die Wasserspeicherkapazität verbessert. Die Forschung nutzt außerdem isothermale Kalorimetrie und 1H-Niederfeld-NMR, um den Wärmefluss und den Wassergehalt während der Hydratation zu analysieren und so ein umfassendes Verständnis der Mechanismen hinter den beobachteten Phänomenen zu liefern. Das Kapitel schließt mit der Hervorhebung der unterschiedlichen Wasserabsorptions- und Luftbindungseffekte verschiedener CE-Typen und bietet wertvolle Erkenntnisse zur Optimierung der Leistung von CSAC-Mörteln in verschiedenen Bauanwendungen.
KI-Generiert
Diese Zusammenfassung des Fachinhalts wurde mit Hilfe von KI generiert.
Abstract
Cellulose ether (CE) is widely used in cement-based materials because of its good water retention capacity that can improve the workability of the fresh mortars significantly. However, in the high temperature conditions, the CE modified cement mortars sometimes are easy to lose their good workability, which may be due to the change of the water-retention capacity of CE. This work investigates the changes in the water loss rate (WLR) of the CE modified calcium sulphoaluminate cement (CSAC) mortars at 20 ℃, 40 ℃, 60 ℃ and 80 ℃ respectively, and the effect of the types and contents of the CE was also considered. Additionally, isothermal calorimeter and 1H low-field NMR were carried out to monitor the changes of chemically bound water and CE adsorbed water content during the reaction. The results show that the WLRs of the CE modified CSAC mortars changes with temperature and the types and contents of the CE. These changes are mainly based on the fact that CE affects the state and relative contents of water molecules in mortar, and the microstructure of the CSAC mortars.
1 Introduction
Compared with the Portland cement (PC) the calcium sulphoaluminate cement (CSAC) have advantages of such as rapid setting and early strength [1‐7]. As a result, the characteristics of CSAC determine that it is better suited to mechanical construction because it is difficult to complete the operation in a short time by relying on labor [8, 9]. However, due to the high reactivity of ye’elimite in CSAC, its workability decreases significantly with time, so that the efficiency of mechanized construction has been greatly affected. In order to improve the mechanized construction efficiency, cellulose ether (CE) is usually added to cement-based materials to improve the workability of fresh mortar [10‐14]. This is mainly because the incorporation of CE can reasonably distribute the state and quantity of water in the mortar. But in high temperature conditions CE modified CSAC mortar sometimes is easy to lose its good workability owing to the change of the water retention capacities of the CE. Therefore, it is necessary to understand the water loss of the CE modified CSAC mortars to evaluate the water retention effect of CE.
The object of this work is to investigate the water loss rate (WLR) of CE modified CSAC mortar at different temperatures. The samples were placed at four curing temperatures including 20 ℃, 40 ℃, 60 ℃ and 80 ℃ to measure the water loss rate within 12 h. Moreover, isothermal calorimeter and 1H low-field NMR were used to determine the heat flow and the state and quantity of water in CSAC paste during hydration, respectively.
Anzeige
2 Experimental
2.1 Raw Materials and Mix Proportions
The cement used in this study is 42.5 grade CSAC, its detailed index can be seen from our previous paper [15]. A hydroxyethyl cellulose (HEC) and two hydroxyethyl methyl cellulose (HEMC) with high and low degree of substitution (DS) were utilized for the test, which were labeled as HEMC-H and HEMC-L. The DS of HEC, HEMC-H and HEMC-L are 2.5, 1.9 and 1.6 respectively. Tap water and the standard sand conforming to GB/T 17671–1999 were applied for the experiment. CE dosing is 0.05%, 0.1%, 0.2%, 0.3%, 0.4% and 0.5% by weight of cement respectively and the mortar without CE is a control sample. The sample was named as a combination of cellulose ether and the dosage, such as HEC-0.05%. The cement to sand ratio is 1:3, and the water to cement ratio is 0.54.
2.2 Determination of Water Loss Rate (WLR) and Air Content Measurement
The mixing method refers to Chinese standard GB/T 17671–1999. The freshly mixed mortar was immediately placed in a glass dish. After weighing the initial weight of the mortar and the glass dish, the dishes filled with mortar were placed in an environment of 20 ℃, 40 ℃, 60 ℃ or 80 ℃ until the weight test. The curing age is 15 min, 30 min, 45 min, 1 h, 2 h, 4 h, 6 h and 12 h. The WLR of the mortar at a certain time is equal to the water loss during the time divided by the initial water content of the mortar. Besides, the air content of the CSAC mortar is determined by instrument method according to the standard JGJ/T 70-2009.
2.3 Isothermal Calorimetry Test and 1H Low-Field NMR Experiment
The hydration heat measurement refers to literature [10]. The 1H low-field NMR experiment refers to literature [15]. From the test, the transverse relaxation time (T2) was obtained. It is reported that the relative content of CE adsorbed water can be expressed by the peak area of CE adsorbed water (ACE) [15]. In the same way, the ACE value of the sample is used to evaluate the relative content of CE adsorbed water in this study.
3 Results and Discussion
3.1 WLR of CE Modified CSAC Mortar
The WLR of CE modified CSAC mortar with curing time at 20 ℃ is plotted in Fig. 1. There is no obvious change in the development trend of WLR of all samples in the first 1 h, but then, the WLR of CE modified CSAC mortar is related to the content and type of CE. Specifically, for the same type of CE, the WLR of the samples broadly increases first and then decreases with the CE content at the same curing time (such as 12 h). Besides, for different types of CE, the minimum amount of CE to retain water (lower than the control sample in WLR) is different. The dosage of HEC is the least, 0.2%, followed by 0.3% of HEMC-L, and HEMC-H is the most, 0.4%. The reason may be that CE affects the hydration of CSAC paste, the water absorption of CE, and the pore structure of the sample.
Fig. 1.
The WLR of HEC modified CSAC mortar within 12 h at 20 ℃
×
Anzeige
3.2 The Change of WLR of CE Modified CSAC Mortar at Different Temperatures
Figure 2 presents the WLR of 0.5% content CE modified CSAC mortar at 20 ℃, 40 ℃, 60 ℃ and 80 ℃. Without considering 60 ℃, the WLR increases rapidly with the rise of temperature, especially in the first 1 h. However, at 60 ℃, the WLR of the CSAC mortar is lower than that at 40 ℃ in the early time. This is inconsistent with the expected results. As we all known, the gelation temperature of CE is between 50–90 ℃. Consequently, the CE used in the CSAC mortar may also undergo gelation at 60 ℃, which greatly increases the viscosity of the suspension slurry and improves the water retention capacity of the CSAC mortar in the plastic stage. Further, it can be seen from Fig. 2 that there are differences in WLR of CSAC mortar containing different types of CE at high temperature. The WLR of HEC-0.5% is still the lowest, followed by HEMC-L-0.5%, and the WLR of HEMC-H-0.5% is the highest. The result is in agreement with the above case at 20 ℃, but the difference among CEs is bigger.
Fig. 2.
The WLR of 0.5% content CE modified CSAC mortar at 20 ℃, 40 ℃, 60 ℃ and 80 ℃
×
3.3 Hydration Heat Analysis
Figure 3 presents the heat flow of CSAC paste containing different dosage HEMC-L (a) and different kinds of CE (b) within 12 h [10]. As presented, four peaks appear on the heat flow curve of the control, corresponding to the dissolution, transformation, AFt primary formation and AFt secondary formation, respectively. The effect of CE content on the first three exothermic peaks is not so significant. But with the increase of HEMC-L content, the fourth exothermic peak is advanced and the peak becomes higher. This shows that HEMC-L promotes the CSAC hydration after 1 h. Similarly, four peaks appear on each heat flow curve in Fig. 3 (b). Compared with the control, CE advances and increases the fourth exothermic peak. The results show that CE promotes the CSAC hydration. However, the promotion degree is different, HEMC-L has the strongest promotion effect, followed by HEMC-H, and HEC. In general, the promotion of hydration can lead to more free water into bound water. In this case, the free water that can evaporate in the mortar is relatively less, which may lead to a decrease in WLR.
Fig. 3.
Heat flow of CSAC paste with different dosage HEMC-L (a) and different kinds of CE (b)
×
3.4 Relative Content of CE Adsorbed Water (ACE)
The ACE alteration of HEC (a), HEMC-L (b) and HEMC-H (c) modified CSAC paste with hydration time is shown in Fig. 4. From it, the ACE value of the same type CSAC paste basically increases with the content of CE at the same time scale, but it gradually decreases with the hydration time, and the decline trend to different type of CE is distinctive. As shown in Fig. 4 (a), there is a significant decline in the ACE (HEC) value before 2 h. After that, the ACE value tends to be stable, but on the whole, it is still that the paste with high HEC content has a larger ACE value. This shows that HEC always absorbed water in the first 12 h, and the higher the content, the more water absorption. From Fig. 4 (b) and (c), the ACE value of the HEMC modified paste decreases sharply within 1 h and then approaches to 0, indicating HEMC loses water absorption after 1 h of cement hydration. In theory, the water retention capacity of CSAC mortar increases with the ACE value, because the relative quantity of internal evaporation water of the mortar becomes less.
With the increase of CE content, the CSAC hydration was promoted, and the relative content of CE adsorbed water also increased. As a result, the WLR should have decreased with CE dosage. However, the fact is that the WLR is higher than or close to that of the control at low content of CE (HEC ≤ 0.1%, HEMC-L ≤ 0.2%, HEMC-H ≤ 0.3%). The reason may be that the incorporation of CE affects the pore structure of the CSAC mortar, especially the capillary pores. It is reported that the cumulative capillary pore volume of mortar increases first and then decreases with the CE content [11]. The more capillary pores, the higher WLR should be. From this perspective, the CSAC mortar with low content of CE do not play a water retention effect compared with the control, while the water retention effect becomes stronger with the increase of CE content at high dosage.
Fig. 4.
The ACE of HEC (a), HEMC-L (b) and HEMC-H (c) modified CSAC pastes
×
3.5 Air Content
The incorporation of CE can introduce air pores into the mortar during mixing [11, 14]. Through tests, the air content of the control, HEC-0.5%, HEMC-L-0.5% and HEMC-H-0.5% is 2.2%, 11.7%, 18.2% and 21.2% respectively. It shows that the incorporation of CE has a strong air-entraining effect, and the air-entraining effects of the three CEs are different. Obviously, HEMC has better air-entraining effect than HEC, and the air-entraining effect of HEMC-H is the best. Commonly, the mortar with high air content has low bulk density and high porosity after hardening. The mortar with large porosity may be more likely to lose water.
Comprehensively, although HEC is not as strong as HEMC in promoting CSAC hydration, it has obvious water absorption effect within 12 h, and the porosity of HEC modified mortar is lower than that of HEMC modified mortar. Thus, the WLR of CSAC mortar containing HEC is lower than that containing HEMC under the same conditions. For HEMC modified CSAC mortar, both HEMC-L and HEMC-H basically lose their water absorption after the mortar hardening, but HEMC-L is superior to HEMC-H in promoting the CSAC hydration, and the porosity of HEMC-L modified mortar is lower than that of HEMC-H modified mortar. Accordingly, the WLR of CSAC mortar containing HEMC-L is lower than that containing HEMC-H.
4 Conclusions
The WLR of CSAC mortar changes regularly due to the influence of the content and type of CE and the temperature. The WLR generally increases first and then decreases with the CE content. The WLR of HEMC modified CSAC mortar is higher than that of HEC modified mortar, and the WLR of HEMC-H modified mortar is the highest. With the increase of temperature, the WLR of CE modified CSAC mortar increases rapidly. But the CE in mortar may be subjected to gelation at 60 ℃, resulting in a WLR in the plastic stage lower than that in 40 ℃.
CE affects the CSAC hydration, the CE absorbed water content, the porosity of mortar, and thus alters the WLR of the CSAC mortar. CE promotes the CSAC hydration, and in terms of promotion effect, HEMC-L > HEMC-H > HEC. With the increase of CE dosage, the content of CE adsorbed water increases. The water absorption effect of HEC is better than that of HEMC. Different types of CEs have various air-entraining effects, at the same dosage, HEMC-H > HEMC-L > HEC.
Acknowledgements
The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 51872203) and the Top Discipline Plan of Shanghai Universities-Class I (2022-3-YB-17).
Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.
The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
Die im Laufe eines Jahres in der „adhäsion“ veröffentlichten Marktübersichten helfen Anwendern verschiedenster Branchen, sich einen gezielten Überblick über Lieferantenangebote zu verschaffen.
