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GGBFS blended system
Cement and Concrete Research 123 (2019) 105773
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The microstructural change of C-S-H at elevated temperature in Portland cement/GGBFS blended system
T
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Zijian Jia, Chun Chen, Jinjie Shi, Yamei Zhang , Zhengming Sun, Peigen Zhang School of Materials Science and Engineering, Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High temperature Portland cement GGBFS Microstructure C-S-H
This investigation focuses on the change of C-S-H gels in Portland cement (PC) paste and PC/ground granulated blast-furnace slag (GGBFS) blended paste at elevated temperature up to 800 °C. Spark-like phase, which might be spurrite, can be found in the inner product of C3S in PC paste when the temperature is higher than 600 °C. The gels in PC paste show an obvious change in composition in comparison with the gels in GGBFS blended paste at elevated temperature. After 600 °C, the continuous decomposition and shrinkage of C-S-H gels results in the increase of small pores (10–40 nm) in both pastes. The nitrogen adsorption results show that some gels, which should be C-A-S-H in GGBFS blended paste, are quite stable at high temperature. The results of this study provide a comprehensive understanding of the different behavior between gels in PC paste and GGBFS blended paste at high temperature.
1. Introduction Cementitious materials suffer significant reduction in mechanical strength when they are exposed to high temperature [1–3]. It has been shown that the loss of compressive strength of concrete mainly happens at the temperature range from 400 °C to 800 °C and significant strength loss happens between 600 °C and 800 °C [4]. The performances of PC paste are closely related to its microstructure, which undergoes complex chemical and physical changes at elevated temperature. According to the researchers [5–7], owing to the deterioration of hydration products like Ca(OH)2 (hereafter called CH) and C-S-H, the average pore size in cement paste increases at elevated temperature, resulting in the loss of compressive strength [8–10]. The study from Qi Zhang [7] suggests that the structure of C-S-H remain stable when the temperature increases from 105 °C to 400 °C. When the temperature is higher than 500 °C, some gels are transformed into crystalline particles, and the volume of capillary pores increases drastically accordingly. Ground granulated blast-furnace slag (GGBFS) is the by-product of the steel industry which has already been widely applied in construction material as a kind of mineral admixture. Researches [8,11–13] demonstrate that the fire resistance of cement paste is enhanced with the replacement of 50% cement by slag at elevated temperature, because the available CH, which starts to decompose beyond 500 °C and is responsible for the decline of mechanical strength, is reduced [8]. Though the fact that the addition of slag can improve the fire resistance
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of cement paste has been proved, the mechanism of the microstructural change of PC-GGBFS paste under high temperature has not been well understood, in particular, the evolution of main hydration product C-SH under high temperature has hardly been investigated. The objective of this work is to investigate the microstructural change of C-S-H in PC paste and PC-GGBFS paste at elevated temperature. Therefore, the deterioration processes of these two systems are compared by using various techniques. The structural change mechanism of C-S-H at high temperature is discussed based on the experimental results. It is expected that the findings from this investigation will enrich the understanding of the microstructural change of slag blended cement paste under high temperature and help to develop new construction materials which perform well under high temperature. 2. Materials and experimental methods 2.1. Materials PII 52.5 Portland cement (hereafter called PC) and GGBFS were used as binding materials. The chemical compositions of the binders are summarized in Table 1. The specific surface area of PC and GGBFS were 359m2/kg and 436 m2/kg, respectively. The LOI of PC was 2.4%. The main crystalline composition of PC obtained by XRD/Rietveld was listed in Table 2.
Corresponding author. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.cemconres.2019.05.018 Received 4 May 2018; Received in revised form 3 April 2019; Accepted 23 May 2019 0008-8846/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 The chemical composition of PC and GGBFS (wt%). Oxides
PC
GGBFS
Na2O MgO Al2O3 SiO2 P2O5 SO3 CaO K2O Fe2O3 TiO2
0.09 0.63 4.69 21.0 0 2.56 64.0 0.61 3.02 0.22
0.39 7.45 15.4 32.06 0.01 1.2 41.79 0.27 0.26 0.69
Table 2 Main crystalline composition of PC by XRD/Rietveld method (wt%). C3S
C2S
C3A
C4AF
CaCO3
CaSO4·2H2O
CaSO4·0.5H2O
60.9
15.9
6.0
9.7
5.2
0.6
1.7
2.2. Mix proportion and specimen preparation In this investigation, two groups of pastes, pure cement paste (Reference group) and PC-GGBFS blended cement paste with 50% GGBFS to substitute PC (Slag group), were prepared for the experiment. The water to binder ratio for the two groups was 0.35 (Table 3). After mixing, the cement paste was poured into 40 × 40 × 160 mm molds. The specimens were demolded after 24 h and stored in the curing room with a temperature of 20 ± 3 °C and RH of 95%. In order to minimize the risk of explosion caused by the moisture in specimens and to better compare the results with those of others [5,7], 105 °C was chosen as the temperature for pre-drying. At 210d, the specimens were pre-dried for 24 h at 105 °C, then subjected to thermal treatment for 2 h at 105, 200, 400, 460, 600, 660, 720 and 800 °C separately with a heating rate of 10 °C/min. After heating, the specimens were cooled to room temperature in the furnace.
Fig. 1. Specimen appearances at elevated temperature.
agent for about 10 min. The specimens were coated with carbon to enhance the conductivity. Mercury intrusion porosimetry (MIP) and nitrogen adsorption method were used to measure the cumulative porosity and pore size distribution of specimens. MIP was used to characterize the distribution of pores larger than 3 nm, while nitrogen adsorption was used to characterize pores smaller than 3 nm. The combination of these two methods can better explain the change of microstructure of specimens at elevated temperature. The specimens were broken into a size of 0.51 cm and small pieces (< 0.05 g) for MIP test and nitrogen adsorption test, respectively. The Saito–Foley analysis method [14] was used to calculate the pore size distribution obtained by nitrogen adsorption test.
2.3. Characterization methods The compressive strength of the pastes was measured according to Chinese standard GB/T 17671–1999. Morphology of the specimens was observed by Sirion Field emission scanning electron microscopy. The specimens were coated with gold to enhance the conductivity. Thermogravimetric analysis (TGA) was applied to analyze the decomposition process of specimens at elevated temperature. The samples were heated at a rate of 10 °C/min up to 1000 °C. X-ray diffraction (XRD) was applied to analyze the phase transition of specimens at elevated temperature. The specimens were ground into powder smaller than 75 μm for test. Backscatter electrons (BSE) and Energy Dispersive Spectroscopy (EDS) were combined to analyze the composition change of hydration products in specimens at elevated temperature. The specimens were cut into about 1 cm thick. After drying, the prepared specimens were immersed in epoxy resin. When the resin got hardened, the specimens were ground with abrasive paper p400, p800, p1200 and p2500 sequentially. Then each specimen was polished with 0.25 μm polishing
3. Results 3.1. Mechanical properties Fig. 1 shows the appearances of specimens after different high temperature treatment. Clear cracks can be found on both reference group paste and slag group paste when the temperature reaches to 600 °C. With the increase of temperature, the cracking of reference group paste is rather serious, while slag group paste keeps good integrity even at 800 °C. (Reference-number stands for reference group treated at the numbered temperature; Slag-number refers to slag group treated at the numbered temperature) The compressive strength of two types of cement pastes at various high temperature is shown in Fig. 2. The initial compressive strengths of pre-dried reference group and slag group are 77 MPa and 68 MPa, respectively. Within 400 °C, the compressive strength of both groups declines mildly with the increase of temperature. After exposure to 460 °C, the strength of the reference group and slag group decreases by approximately 37% and 29%, respectively. According to other researchers [8,12], this significant strength decline is due to the decomposition of CH which causes the cracking of matrix. With the temperature goes up further, the strength of reference group continues to decrease significantly. At 720 °C, the reference specimen is in fact totally destroyed. However, the strength of slag group only decreases
Table 3 The mix proportion of pastes. Group
Water to binder ratio
PC(wt%)
GGBFS(wt%)
Reference group Slag group
0.35 0.35
100 50
0 50
2
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temperature is higher than 500 °C, and it can cause significant change in the pore structure of the paste. Therefore, the different deterioration process of gels at 720 °C also contribute a lot to the different strength loss in both groups. 3.3. XRD analysis XRD was applied to obtain a better understanding of the phase transformation in the specimens exposed to elevated temperatures, and the results are shown in Fig. 4. The amount of CH decreases in the slag group when the temperature increases from 105 °C to 200 °C. This phenomenon is probably caused by the acceleration of pozzolanic reaction between unreacted slag and CH with the increase of temperature. The peaks of CH disappeared at 600 °C in both reference group and slag group, which is attributed to the decomposition of CH phase between 460 °C to 600 °C. Both reference group and slag group have a diffuse hump at about 29°, which represents the presence of amorphous phase C-S-H, and the diffuse hump of C-S-H overlaps with those of CaCO3. The intensity of diffusion peak gradually decreases when the temperature increases to 600 °C, which may represent the transformation of C-S-H. The peak intensity of C2S increases when the temperature rises to 600 °C in both two groups, which can be attributed to the decomposition of C-S-H. A significant change in phase can be found when the temperature reaches to 800 °C, and the peak of C2S can be clearly identified in the pattern of both groups. The peaks of spurrite appear in the reference group, which may be caused by the reaction between C2S, CaO and CO2 [18].
Fig. 2. Effect of temperature on the compressive strength of reference group paste and slag group paste.
slightly when the temperature rises from 600 °C to 720 °C. The similar phenomenon was reported in [8,12] as well.
3.2. Thermal analysis The results of thermo-gravity analysis are presented in Fig. 3. The weight loss of the reference group is 3.7% higher than that of slag group when the temperature rises to 1000 °C. According to the weight loss rate curve, two small peaks can be found between 100 and 200 °C, which are mainly due to the dehydration of C-S-H [15,16]. A noticeable sharp peak between 450 °C and 500 °C, which is corresponded to the dehydration of CH, appears in both reference group and slag group [16]. Compared to the reference group, the intensity of the CH dehydration peak is, however, much lower in the slag group, attributed to the consumption of CH by the pozzolanic reaction of slag. Another peak, which is mainly caused by the decomposition of calcium carbonate [17], is found when the temperature increases to about 720 °C. In the slag group, the intensity of calcium carbonate decomposition peak is lower than that in the reference group, which suggests less calcium carbonate exist in the slag group. Compared to the reference group, the lower content of calcium carbonate may contribute to the lower strength decrease in the slag group at 720 °C. In addition, the deterioration of C-S-H has been reported [7] to start when the
3.4. BSE and EDS analysis BSE and EDS analysis were combined to understand the change of hydration products in the reference group and slag group at high temperature. 50 points were randomly selected from 3 to 5 areas to make EDS test for each phase. The element distribution of hydration products around unhydrated cement particle in both reference group and slag group are shown in Fig. 5. According to the EDS results, the Ca/Si ratios of selected unhydrated particles are around 3, indicating that part of C3S is not reacted. In the BSE images of Fig. 5, inner product (IP) can be clearly distinguished from the outer product (OP) in the reference group (Fig. 5(ad)). Clear boundaries can be found between IP, OP and unhydrated cement particles in the reference group, while the boundary between IP and OP in the slag group is not so significant (Fig. 5(e-h)). It is worth noting that when the temperature is higher than 400 °C, the gray level of IP in the reference group is higher than that observed at 105 °C. According to the EDS result in Fig. 6 (a), the Ca/Si ratios of IP in the reference group at 400, 600 and 800 °C are all higher than that at 105 °C. The Ca/Si ratios of IP in the reference group at 105, 400, 600 and 800 °C are 1.74 ± 0.08, 2.09 ± 0.04, 2.07 ± 0.1 and 2.38 ± 0.11, respectively. The Al content in OP is much higher than that in IP, which should be attributed to the existing of AFt and AFm. The high Ca/Si and Al/Si of OP also proves that OP is a mixture of C-SH, CH, AFt, etc. The Ca/Si of IP in the slag group is 1.71 ± 0.29, which is similar to that in the reference group. While the Ca/Si of OP in the slag group is 1.84 ± 0.4, which is lower than that in the reference group (2.19 ± 0.3). More Al can be found in both IP and OP in the slag group than that in the reference group. The EDS results show that, with the increase of temperature, little change can be found in the Ca/Si of OP and IP. Clear gray boundary between C3S and matrix can be found in the BSE image of Slag-800, but the EDS results show that little Al can be found in the gray boundary, which may suggest that the gray boundary does not belong to IP. As shown in Figs. 5 and 7, a kind of ‘bright’ spark-like phase can be found in IP of the reference group when the temperature reaches to 600 °C and 800 °C. The gray level of the ‘bright’ spark-like phase is
Fig. 3. Thermogravimetric curves of the reference group and the slag group at various temperatures. 3
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(a) Reference group
(b) Slag group
Fig. 4. XRD patterns of cement paste at elevated temperature.
Fig. 5. BSE images of hydration products around unhydrated cement particle (C3S) at elevated temperature. 4
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Fig. 6. The change of Ca/Si and Al/Si of IP, OP and unhydrated C3S in the reference group and slag group at elevated temperature.
higher than that of IP and close to that of C2S and C3S. In contrast, this phenomenon cannot be found in the slag group. The EDS results show that the Ca/Si of the ‘bright’ spark-like phase in Reference-600 and Reference-800 group are 1.94 ± 0.08 and 2.15 ± 0.05, respectively. The composition of IP in Reference-105 and the spark-like phase in Reference-600 and Reference-800 are listed in Table 4. Compared with IP in Reference-105, only the contents of Si and Ca have a slight change in the spark-like phase in Reference-600 and Reference-800. The Si contents of the spark-like phase in Reference-600 and Reference-800 are both lower than that of IP in Reference-105.
Table 4 Composition of IP in Reference-105, spark-like phases in Reference-600 and Reference-800. Atom (%)
Reference-105 IP
Reference-600 spark-like phase
Reference-800 spark-like phase
Na Mg Al Si S Ca Fe
0.2 ± 0.3 1.3 ± 0.4 1.6 ± 0.1 34.3 ± 1.4 1.9 ± 0.5 59.7 ± 0.7 1.1 ± 0.3
0.4 ± 0.3 2.0 ± 0.7 2.2 ± 0.4 31.7 ± 0.9 0.8 ± 0.9 61.7 ± 1.8 1.2 ± 0.3
0.3 ± 0.1 1.1 ± 0.2 1.9 ± 0.1 29.3 ± 0.9 1.8 ± 0.8 62.5 ± 0.6 0.8 ± 0.4
3.5. Micromorphology respectively. Spark-like phase can also be found in IP when the temperature changes to 600 °C (Fig. 9(a)) and 800 °C(Fig. 9(b)), which corresponds well with the spark-like phase found in BSE images (Fig. 7(c)(d)). In the slag group, the matrix is rather dense at 400 °C (Fig. 10(a)) and the unhydrated C3S can hardly be found in the matrix, maybe the unhydrated C3S is tightly wrapped in the hydration products. Similar to the results previously obtained by BSE, IP and OP are hard to be distinguished from each other in the slag group. Compared with the reference group, the C-S-H particles pack more tightly in the slag group at any temperature in this study. As shown in Fig. 10(f) and (i), IP and OP are still quite dense when the temperature is 800 °C. The reticular structure transformed by OP can also be found in the slag group when the temperature is 800 °C (Fig. 10(i)). The same to the BSE result, no spark-like phase can be found in IP of the slag group.
SEM images of hydration products in Reference-400, Reference-600 and Reference-800 are shown in Fig. 8. In the reference group, the unhydrated C3S can be clearly found in the matrix (Fig. 8(a)(b)(c)). Many small tightly packed C-S-H particles can be found in IP, which are very similar to the globule flocs described by Jennings [19]. Clear gaps can be found between the C-S-H particles in IP when the temperature changes from 400 °C to 800 °C (Fig. 8(d)(e) (f)). The gaps between C-S-H particles also increase in OP with the increase of temperature ((Fig. 8(g)(h)(i)). Many small C-S-H particles can also be found in OP when the temperature is 400 °C (Fig. 8(g)). However, the increase of temperature makes the particles in OP connect together and form a kind of reticular structure (Fig. 8(h)(i)), which may be C2S. Needle-like phase can be found in the reference group when the temperature rises to 600 °C (Fig. 8(j)) and 800 °C (Fig. 8(k)). According to the EDS analysis, the Ca/Si ratios of the needle-like phase in Reference-600 and Reference-800 are 2.01 ± 0.08 and 1.94 ± 0.25,
(a) Reference-600
(b) Reference-800
Fig. 7. The ‘bright’ spark-like phase in IP of the reference group. 5
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(a)400䉝
(d)400䉝
(g)400䉝
(b) 600䉝
(c) 800䉝
(e) 600䉝
(f) 800䉝
(h) 600䉝
(i) 800䉝
(j) 600䉝
(k) 800䉝
Fig. 8. Morphology of hydration products in the reference group at high temperature.
600 °C, a second peak appears between 10 and 40 nm in both reference group and slag group. When the temperature rises to 660 °C, significant change can be observed in the slag group. The volume of pores ranging from 10 nm to 40 nm increases remarkably. Furthermore, the second peak in the slag group shifts to the right with the increase of temperature. The pores in both groups shift to larger size when the temperature increases to 800 °C. In addition, the second peak (between 20 and 50 nm) can still be observed in the slag group at 800 °C. Pore distribution of pores between 0.5 and 4 nm obtained by
3.6. Pore distribution The distribution of pores is analyzed by MIP and nitrogen adsorption analysis, and the results are shown in Fig. 11. According to the results obtained from the MIP test, no significant change can be found in the distribution of pores in both groups when temperature changes from 105 °C to 460 °C. When temperature increases to 460 °C, the average size of pore increases significantly in both groups, which may be related to the decomposition of CH [7]. At 6
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(a) 600䉝
(b) 800䉝
Fig. 9. SEM image of the spark-like phase in IP in the reference group at high temperature.
(a)400䉝
(b) 600䉝
(c) 800䉝
(d)400䉝
(e) 600䉝
(f) 800䉝
(g)400䉝
(h) 600䉝
(i) 800䉝
Fig. 10. Morphology of hydration products in the slag group at high temperature.
reference group and slag group when the temperature rises from 600 to 800 °C. During this stage, the volume of 1-2 nm pores remains stable in the slag group. In the reference group, however, the volume of 1-2 nm pores declines remarkably when the temperature increases to 720 °C and keeps almost unchanged at similar level at 800 °C.
nitrogen adsorption analysis are shown in Fig. 11(b). It can be seen that, the pore distribution of the two groups at the temperature range from 105 °C to 460 °C are quite similar. A significant increase in the volume of 1-2 nm pores can be found in both groups when the temperature reaches to 600 °C. The distinct difference can be observed in the 7
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Fig. 11. Pore distribution of cement paste at elevated temperature (a) Pore (3–10,000 nm) distribution analyzed by MIP (b) Pore (0.5–4 nm) distribution analyzed by nitrogen adsorption.
4. Discussion
4.2. The change of C-S-H in the reference group
In this investigation, differences in the microstructure of hydration products can be found between the reference group and slag group with the increase of temperature. The morphology and pore structure of hydration products in these two groups exhibit significant change when the temperature increases from 600 °C to 800 °C, which may be closely related to the nanostructural change in C-S-H gels.
According to XRD result, C-S-H has already started to decompose into C2S at 600 °C. The appearance of peaks in the result of MIP (1040 nm) and nitrogen adsorption (1-2 nm) at 600 °C should be related to the change of C-S-H. On the one hand, part of C-S-H in IP and OP decompose at 600 °C, which causes the collapse of gel structure. On the other hand, the C-S-H gels suffer from shrinkage [23] at high temperature. These two kinds of behavior both can change the size distribution of gel pores. According to the C-S-H model of Jennings [19], the pores that exist within the gel volume can be divided into three categories, there are IGP (intraglobular pores, smaller than 1 nm), SGP (small gel pores, spaces between the packed globules, between 1 and 3 nm) and LGP (large gel pores between 3 and 12 nm). The decomposition and shrinkage of C-S-H gels broaden the spacing between C-SH particles and may increase the average size of LGP, which causes the appearance of peak (10-40 nm) in the result of MIP. This also corresponds with the decline in the packing density of C-S-H [23]. The increase of 1-2 nm pores in the reference group may be related to the change of SGP between C-S-H globules. The rise of temperature results in the shrinkage of C-S-H gels and may give rise to the decrease of contact points between C-S-H globules, which makes the SGP accessible to nitrogen [24]. The decline in the 1-2 nm pores when the temperature is higher than 720 °C can be attributed to the continuous decomposition and shrinkage of C-S-H gels. The continuous deterioration of C-S-H gels results in the collapse of gel structure and increases the average size of the gel pores. A simple diagram for the pore structure change of C-S-H gels after high temperature treatment is shown as Fig. 12, which is based on the CM-II model [19].
4.1. The spark-like phase By comparing the BSE, SEM and XRD results, we think that the spark-like phase found in the IP of reference group might be spurrite. Firstly, the spark-like phase can only be found in the reference group at high temperature (according to BSE and SEM results), so does the spurrite (according to XRD result). Secondly, the IP of C3S is normally considered as a mixture of C-S-H and nanoscale CH [20,21]. The CH in IP decomposes into CaO when the temperature is higher than 460 °C and the C-S-H into C2S at a temperature higher than 600 °C. Therefore, the appearance of CaO and C2S in IP, the high density structure of IP and the existing of CO2 in the furnace can provide the condition for the formation of spurrite at high temperature. In the slag group, the absence of spurrite can be attributed to the different phase composition of IP. The CH in IP is consumed by the pozzolanic reaction between slag and CH, leading to the lack of CaO which is necessary for the formation of spurrite. Moreover, according to other researchers [22], the incorporation of slag will cause significant reduction in the average thickness of IP around C3S. Therefore, the smaller volume fraction of IP makes the spurrite, if it could be formed, less likely to be found in the slag group. 8
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Fig. 12. A simple diagram for the pore structure change of C-S-H gels after high temperature treatment
4.3. The change of C-S-H in the slag group
5. Conclusions
Due to the incorporation of slag, the aluminum content of IP and OP in the slag group are both higher than that of the reference group, which has also been reported in other researches [25–27]. This phenomenon should be related to the pozzolanic reaction between slag and CH. The incorporation of aluminum changes the composition and structure of C-S-H [28], therefore the hydration product C-S-H in slag blended cement paste is also called C-A-S-H [22]. The change of pore size distribution in the slag group is different from that of the reference group at elevated temperature. In our research, the MIP and SEM results show that the incorporation of slag helps to densify the matrix. Moreover, the investigation of other researchers [27] shows that the incorporation of slag has changed the morphology of OP C-S-H from fibrillar to foil-like. Considering above results, it is assumed that the incorporation of slag helps to generate more tightly packed C-A-S-H gels. These tightly packed C-A-S-H gels in the slag group may have more closed LGP inside. Similar to the situation in the reference group, the increase of temperature also makes C-AS-H gels decompose and shrink, which may change the closed LGP into open pores and also increase the average size of LGP. Therefore, the second peak (10-40 nm) in the result of MIP in the slag group is higher than that in the reference group. Investigation about the thermal stability of C-S-H [29] shows that CS-H with lower Ca/Si are more stable upon heating, which could be related to the silicate structure. According to the EDS results, the C-A-SH in the slag group has lower Ca/Si but higher Al/Si comparing with the C-S-H in the reference group. The mean chain length of C-A-S-H is normally thought to be longer than that of C-S-H [30]. Therefore, the low Ca/Si and different chain structure may make the C-A-S-H gels more stable than C-S-H gels at high temperature. In the SEM observation, the C-A-S-H gels in the slag group still pack tightly at high temperature. The nitrogen adsorption result demonstrates that the SGP in the C-A-S-H gels of slag group remains high in the slag group even if the temperature reaches to 800 °C, which may suggest that part of C-A-S-H gels can still maintain the gel structure even at high temperature, further confirming the stability of C-A-S-H gels.
The aim of this investigation is to study the microstructural change of C-(A)-S-H gels in PC and PC/GGBFS blended systems at elevated temperature up to 800 °C. The morphology, chemical composition and pore distribution of cement paste were studied. The results of this study will be helpful in understanding the thermal deterioration of C-(A)-S-H gels in pure cement paste and cement paste with slag. The conclusions are summarized below: 1. C-S-H gels suffer from significant deterioration when the temperature is high than 600 °C in both groups. Spark-like phase, which seems to be spurrite, can be found in the IP of the reference group. No spark-like phase can be found in the slag group. 2. The Ca/Si ratios of IP and OP in the reference group increase with elevated temperature, while the Ca/Si ratios of IP and OP in the slag group keep almost unchanged, indicating that the gels in the slag group are more stable than that in the reference group. 3. The decomposition and shrinkage of C-S-H gels enlarges the pore size of LGP (3-12 nm), which results in the increase of small pores (10-40 nm) in both groups after 600 °C. Since the incorporation of slag helps to generate more tightly packed C-A-S-H gels with closed LGP, more small pores (10-40 nm) can be found in the slag group than in the reference group after 600 °C. 4. Compared with the reference group, the number of 1-2 nm pores in the slag group stays stable when the temperature increases from 600 °C to 800 °C, confirming the stability of C-A-S-H gels. Acknowledgements This work is supported by the National Natural Science Foundation of China (project No. 51778132) and 973 program (project No. 2015CB655100) from MOST, China. References [1] M. Fall, S.S. Samb, Effect of high temperature on strength and microstructural properties of cemented paste backfill, Fire Saf. J. 44 (2009) 642–651. [2] N. Farzadnia, A.A.A. Ali, R. Demirboga, Characterization of high strength mortars with nano alumina at elevated temperatures, Cem. Concr. Res. 54 (2013) 43–54. [3] M. Heikal, A. Ali, M. Ismail, S.A.N. Ibrahim, Behavior of composite cement pastes
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[17] P. Grattan-Bellew, Microstructural investigation of deteriorated Portland cement concretes, Constr. Build. Mater. 10 (1996) 3–16. [18] F.P. Glasser, The formation and thermal stability of spurrite, Ca5(SiO4)2CO3, Cem. Concr. Res. 3 (1973) 23–28. [19] H.M. Jennings, Refinements to colloid model of C-S-H in cement: CM-II, Cem. Concr. Res. 38 (2008) 275–289. [20] C. Hu, S. Yao, F. Zou, S. Nie, Z. Liu, F. Wang, Insights into the influencing factors on the micro-mechanical properties of calcium-silicate-hydrate gel, J. Am. Ceram. Soc. 102 (2019) 1942–1952. [21] C. Hu, Y. Han, Y. Gao, Y. Zhang, Z. Li, Property investigation of calcium–silicate–hydrate (C–S–H) gel in cementitious composites, Mater. Charact. 95 (2014) 129–139. [22] Y. Wei, X. Gao, S. Liang, A combined SPM/NI/EDS method to quantify properties of inner and outer C-S-H in OPC and slag-blended cement pastes, Cem. Concr. Compos. 85 (2018) 56–66. [23] M.J. DeJong, F.-J. Ulm, The nanogranular behavior of CSH at elevated temperatures (up to 700 C), Cem. Concr. Res. 37 (2007) 1–12. [24] P.D. Tennis, H.M. Jennings, A model for two types of calcium silicate hydrate in the microstructure of Portland cement pastes, Cem. Concr. Res. 30 (2000) 855–863. [25] J.I. Escalante-Garcia, J.H. Sharp, The chemical composition and microstructure of hydration products in blended cements, Cem. Concr. Compos. 26 (2004) 967–976. [26] I.G. Richardson, J.G. Cabrera, The nature of CeSeH in model slag-cements, Cem. Concr. Compos. 22 (2000) 259–266. [27] R. Taylor, I.G. Richardson, R.M.D. Brydson, Composition and microstructure of 20year-old ordinary Portland cement–ground granulated blast-furnace slag blends containing 0 to 100% slag, Cem. Concr. Res. 40 (2010) 971–983. [28] P. Faucon, A. Delagrave, J. Petit, C. Richet, J. Marchand, H. Zanni, Aluminum incorporation in calcium silicate hydrates (C− S− H) depending on their ca/Si ratio, J. Phys. Chem. B 103 (1999) 7796–7802. [29] E. Tajuelo Rodriguez, K. Garbev, D. Merz, L. Black, I.G. Richardson, Thermal stability of C-S-H phases and applicability of Richardson and Groves' and Richardson C-(A)-S-H(I) models to synthetic C-S-H, Cem. Concr. Res. 93 (2017) 45–56. [30] F. Puertas, M. Palacios, H. Manzano, J.S. Dolado, A. Rico, J. Rodríguez, A model for the C-A-S-H gel formed in alkali-activated slag cements, J. Eur. Ceram. Soc. 31 (2011) 2043–2056.
containing silica nano-particles at elevated temperature, Constr. Build. Mater. 70 (2014) 339–350. Y. Chan, G. Peng, M. Anson, Residual strength and pore structure of high-strength concrete and normal strength concrete after exposure to high temperatures, Cem. Concr. Compos. 21 (1999) 23–27. Q. Zhang, G. Ye, Microstructure analysis of heated Portland cement paste, Procedia Eng. 14 (2011) 830–836. G. Ye, X. Liu, G. De Schutter, L. Taerwe, P. Vandevelde, Phase distribution and microstructural changes of self-compacting cement paste at elevated temperature, Cem. Concr. Res. 37 (2007) 978–987. Q. Zhang, G. Ye, E. Koenders, Investigation of the structure of heated Portland cement paste by using various techniques, Constr. Build. Mater. 38 (2013) 1040–1050. A. Mendes, J. Sanjayan, F. Collins, Phase transformations and mechanical strength of OPC/slag pastes submitted to high temperatures, Mater. Struct. 41 (2008) 345–350. Q. Ma, R. Guo, Z. Zhao, Z. Lin, K. He, Mechanical properties of concrete at high temperature—a review, Constr. Build. Mater. 93 (2015) 371–383. C. Alonso, L. Fernandez, Dehydration and rehydration processes of cement paste exposed to high temperature environments, J. Mater. Sci. 39 (2004) 3015–3024. M. Heikal, O.K. Al-Duaij, N.S. Ibrahim, Microstructure of composite cements containing blast-furnace slag and silica nano-particles subjected to elevated thermally treatment temperature, Constr. Build. Mater. 93 (2015) 1067–1077. H.Y. Wang, The effects of elevated temperature on cement paste containing GGBFS, Cem. Concr. Compos. 30 (2008) 992–999. A. Mendes, J.G. Sanjayan, F. Collins, Long-term progressive deterioration following fire exposure of OPC versus slag blended cement pastes, Mater. Struct. 42 (2009) 95. A. Saito, H. Foley, Curvature and parametric sensitivity in models for adsorption in micropores, AICHE J. 37 (1991) 429–436. H. Fares, S. Remond, A. Noumowe, A. Cousture, High temperature behaviour of self-consolidating concrete, Cem. Concr. Res. 40 (2010) 488–496. L. Alarcon-Ruiz, G. Platret, E. Massieu, A. Ehrlacher, The use of thermal analysis in assessing the effect of temperature on a cement paste, Cem. Concr. Res. 35 (2005) 609–613.
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Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres
The microstructural change of C-S-H at elevated temperature in Portland cement/GGBFS blended system
T
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Zijian Jia, Chun Chen, Jinjie Shi, Yamei Zhang , Zhengming Sun, Peigen Zhang School of Materials Science and Engineering, Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High temperature Portland cement GGBFS Microstructure C-S-H
This investigation focuses on the change of C-S-H gels in Portland cement (PC) paste and PC/ground granulated blast-furnace slag (GGBFS) blended paste at elevated temperature up to 800 °C. Spark-like phase, which might be spurrite, can be found in the inner product of C3S in PC paste when the temperature is higher than 600 °C. The gels in PC paste show an obvious change in composition in comparison with the gels in GGBFS blended paste at elevated temperature. After 600 °C, the continuous decomposition and shrinkage of C-S-H gels results in the increase of small pores (10–40 nm) in both pastes. The nitrogen adsorption results show that some gels, which should be C-A-S-H in GGBFS blended paste, are quite stable at high temperature. The results of this study provide a comprehensive understanding of the different behavior between gels in PC paste and GGBFS blended paste at high temperature.
1. Introduction Cementitious materials suffer significant reduction in mechanical strength when they are exposed to high temperature [1–3]. It has been shown that the loss of compressive strength of concrete mainly happens at the temperature range from 400 °C to 800 °C and significant strength loss happens between 600 °C and 800 °C [4]. The performances of PC paste are closely related to its microstructure, which undergoes complex chemical and physical changes at elevated temperature. According to the researchers [5–7], owing to the deterioration of hydration products like Ca(OH)2 (hereafter called CH) and C-S-H, the average pore size in cement paste increases at elevated temperature, resulting in the loss of compressive strength [8–10]. The study from Qi Zhang [7] suggests that the structure of C-S-H remain stable when the temperature increases from 105 °C to 400 °C. When the temperature is higher than 500 °C, some gels are transformed into crystalline particles, and the volume of capillary pores increases drastically accordingly. Ground granulated blast-furnace slag (GGBFS) is the by-product of the steel industry which has already been widely applied in construction material as a kind of mineral admixture. Researches [8,11–13] demonstrate that the fire resistance of cement paste is enhanced with the replacement of 50% cement by slag at elevated temperature, because the available CH, which starts to decompose beyond 500 °C and is responsible for the decline of mechanical strength, is reduced [8]. Though the fact that the addition of slag can improve the fire resistance
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of cement paste has been proved, the mechanism of the microstructural change of PC-GGBFS paste under high temperature has not been well understood, in particular, the evolution of main hydration product C-SH under high temperature has hardly been investigated. The objective of this work is to investigate the microstructural change of C-S-H in PC paste and PC-GGBFS paste at elevated temperature. Therefore, the deterioration processes of these two systems are compared by using various techniques. The structural change mechanism of C-S-H at high temperature is discussed based on the experimental results. It is expected that the findings from this investigation will enrich the understanding of the microstructural change of slag blended cement paste under high temperature and help to develop new construction materials which perform well under high temperature. 2. Materials and experimental methods 2.1. Materials PII 52.5 Portland cement (hereafter called PC) and GGBFS were used as binding materials. The chemical compositions of the binders are summarized in Table 1. The specific surface area of PC and GGBFS were 359m2/kg and 436 m2/kg, respectively. The LOI of PC was 2.4%. The main crystalline composition of PC obtained by XRD/Rietveld was listed in Table 2.
Corresponding author. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.cemconres.2019.05.018 Received 4 May 2018; Received in revised form 3 April 2019; Accepted 23 May 2019 0008-8846/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 The chemical composition of PC and GGBFS (wt%). Oxides
PC
GGBFS
Na2O MgO Al2O3 SiO2 P2O5 SO3 CaO K2O Fe2O3 TiO2
0.09 0.63 4.69 21.0 0 2.56 64.0 0.61 3.02 0.22
0.39 7.45 15.4 32.06 0.01 1.2 41.79 0.27 0.26 0.69
Table 2 Main crystalline composition of PC by XRD/Rietveld method (wt%). C3S
C2S
C3A
C4AF
CaCO3
CaSO4·2H2O
CaSO4·0.5H2O
60.9
15.9
6.0
9.7
5.2
0.6
1.7
2.2. Mix proportion and specimen preparation In this investigation, two groups of pastes, pure cement paste (Reference group) and PC-GGBFS blended cement paste with 50% GGBFS to substitute PC (Slag group), were prepared for the experiment. The water to binder ratio for the two groups was 0.35 (Table 3). After mixing, the cement paste was poured into 40 × 40 × 160 mm molds. The specimens were demolded after 24 h and stored in the curing room with a temperature of 20 ± 3 °C and RH of 95%. In order to minimize the risk of explosion caused by the moisture in specimens and to better compare the results with those of others [5,7], 105 °C was chosen as the temperature for pre-drying. At 210d, the specimens were pre-dried for 24 h at 105 °C, then subjected to thermal treatment for 2 h at 105, 200, 400, 460, 600, 660, 720 and 800 °C separately with a heating rate of 10 °C/min. After heating, the specimens were cooled to room temperature in the furnace.
Fig. 1. Specimen appearances at elevated temperature.
agent for about 10 min. The specimens were coated with carbon to enhance the conductivity. Mercury intrusion porosimetry (MIP) and nitrogen adsorption method were used to measure the cumulative porosity and pore size distribution of specimens. MIP was used to characterize the distribution of pores larger than 3 nm, while nitrogen adsorption was used to characterize pores smaller than 3 nm. The combination of these two methods can better explain the change of microstructure of specimens at elevated temperature. The specimens were broken into a size of 0.51 cm and small pieces (< 0.05 g) for MIP test and nitrogen adsorption test, respectively. The Saito–Foley analysis method [14] was used to calculate the pore size distribution obtained by nitrogen adsorption test.
2.3. Characterization methods The compressive strength of the pastes was measured according to Chinese standard GB/T 17671–1999. Morphology of the specimens was observed by Sirion Field emission scanning electron microscopy. The specimens were coated with gold to enhance the conductivity. Thermogravimetric analysis (TGA) was applied to analyze the decomposition process of specimens at elevated temperature. The samples were heated at a rate of 10 °C/min up to 1000 °C. X-ray diffraction (XRD) was applied to analyze the phase transition of specimens at elevated temperature. The specimens were ground into powder smaller than 75 μm for test. Backscatter electrons (BSE) and Energy Dispersive Spectroscopy (EDS) were combined to analyze the composition change of hydration products in specimens at elevated temperature. The specimens were cut into about 1 cm thick. After drying, the prepared specimens were immersed in epoxy resin. When the resin got hardened, the specimens were ground with abrasive paper p400, p800, p1200 and p2500 sequentially. Then each specimen was polished with 0.25 μm polishing
3. Results 3.1. Mechanical properties Fig. 1 shows the appearances of specimens after different high temperature treatment. Clear cracks can be found on both reference group paste and slag group paste when the temperature reaches to 600 °C. With the increase of temperature, the cracking of reference group paste is rather serious, while slag group paste keeps good integrity even at 800 °C. (Reference-number stands for reference group treated at the numbered temperature; Slag-number refers to slag group treated at the numbered temperature) The compressive strength of two types of cement pastes at various high temperature is shown in Fig. 2. The initial compressive strengths of pre-dried reference group and slag group are 77 MPa and 68 MPa, respectively. Within 400 °C, the compressive strength of both groups declines mildly with the increase of temperature. After exposure to 460 °C, the strength of the reference group and slag group decreases by approximately 37% and 29%, respectively. According to other researchers [8,12], this significant strength decline is due to the decomposition of CH which causes the cracking of matrix. With the temperature goes up further, the strength of reference group continues to decrease significantly. At 720 °C, the reference specimen is in fact totally destroyed. However, the strength of slag group only decreases
Table 3 The mix proportion of pastes. Group
Water to binder ratio
PC(wt%)
GGBFS(wt%)
Reference group Slag group
0.35 0.35
100 50
0 50
2
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temperature is higher than 500 °C, and it can cause significant change in the pore structure of the paste. Therefore, the different deterioration process of gels at 720 °C also contribute a lot to the different strength loss in both groups. 3.3. XRD analysis XRD was applied to obtain a better understanding of the phase transformation in the specimens exposed to elevated temperatures, and the results are shown in Fig. 4. The amount of CH decreases in the slag group when the temperature increases from 105 °C to 200 °C. This phenomenon is probably caused by the acceleration of pozzolanic reaction between unreacted slag and CH with the increase of temperature. The peaks of CH disappeared at 600 °C in both reference group and slag group, which is attributed to the decomposition of CH phase between 460 °C to 600 °C. Both reference group and slag group have a diffuse hump at about 29°, which represents the presence of amorphous phase C-S-H, and the diffuse hump of C-S-H overlaps with those of CaCO3. The intensity of diffusion peak gradually decreases when the temperature increases to 600 °C, which may represent the transformation of C-S-H. The peak intensity of C2S increases when the temperature rises to 600 °C in both two groups, which can be attributed to the decomposition of C-S-H. A significant change in phase can be found when the temperature reaches to 800 °C, and the peak of C2S can be clearly identified in the pattern of both groups. The peaks of spurrite appear in the reference group, which may be caused by the reaction between C2S, CaO and CO2 [18].
Fig. 2. Effect of temperature on the compressive strength of reference group paste and slag group paste.
slightly when the temperature rises from 600 °C to 720 °C. The similar phenomenon was reported in [8,12] as well.
3.2. Thermal analysis The results of thermo-gravity analysis are presented in Fig. 3. The weight loss of the reference group is 3.7% higher than that of slag group when the temperature rises to 1000 °C. According to the weight loss rate curve, two small peaks can be found between 100 and 200 °C, which are mainly due to the dehydration of C-S-H [15,16]. A noticeable sharp peak between 450 °C and 500 °C, which is corresponded to the dehydration of CH, appears in both reference group and slag group [16]. Compared to the reference group, the intensity of the CH dehydration peak is, however, much lower in the slag group, attributed to the consumption of CH by the pozzolanic reaction of slag. Another peak, which is mainly caused by the decomposition of calcium carbonate [17], is found when the temperature increases to about 720 °C. In the slag group, the intensity of calcium carbonate decomposition peak is lower than that in the reference group, which suggests less calcium carbonate exist in the slag group. Compared to the reference group, the lower content of calcium carbonate may contribute to the lower strength decrease in the slag group at 720 °C. In addition, the deterioration of C-S-H has been reported [7] to start when the
3.4. BSE and EDS analysis BSE and EDS analysis were combined to understand the change of hydration products in the reference group and slag group at high temperature. 50 points were randomly selected from 3 to 5 areas to make EDS test for each phase. The element distribution of hydration products around unhydrated cement particle in both reference group and slag group are shown in Fig. 5. According to the EDS results, the Ca/Si ratios of selected unhydrated particles are around 3, indicating that part of C3S is not reacted. In the BSE images of Fig. 5, inner product (IP) can be clearly distinguished from the outer product (OP) in the reference group (Fig. 5(ad)). Clear boundaries can be found between IP, OP and unhydrated cement particles in the reference group, while the boundary between IP and OP in the slag group is not so significant (Fig. 5(e-h)). It is worth noting that when the temperature is higher than 400 °C, the gray level of IP in the reference group is higher than that observed at 105 °C. According to the EDS result in Fig. 6 (a), the Ca/Si ratios of IP in the reference group at 400, 600 and 800 °C are all higher than that at 105 °C. The Ca/Si ratios of IP in the reference group at 105, 400, 600 and 800 °C are 1.74 ± 0.08, 2.09 ± 0.04, 2.07 ± 0.1 and 2.38 ± 0.11, respectively. The Al content in OP is much higher than that in IP, which should be attributed to the existing of AFt and AFm. The high Ca/Si and Al/Si of OP also proves that OP is a mixture of C-SH, CH, AFt, etc. The Ca/Si of IP in the slag group is 1.71 ± 0.29, which is similar to that in the reference group. While the Ca/Si of OP in the slag group is 1.84 ± 0.4, which is lower than that in the reference group (2.19 ± 0.3). More Al can be found in both IP and OP in the slag group than that in the reference group. The EDS results show that, with the increase of temperature, little change can be found in the Ca/Si of OP and IP. Clear gray boundary between C3S and matrix can be found in the BSE image of Slag-800, but the EDS results show that little Al can be found in the gray boundary, which may suggest that the gray boundary does not belong to IP. As shown in Figs. 5 and 7, a kind of ‘bright’ spark-like phase can be found in IP of the reference group when the temperature reaches to 600 °C and 800 °C. The gray level of the ‘bright’ spark-like phase is
Fig. 3. Thermogravimetric curves of the reference group and the slag group at various temperatures. 3
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(a) Reference group
(b) Slag group
Fig. 4. XRD patterns of cement paste at elevated temperature.
Fig. 5. BSE images of hydration products around unhydrated cement particle (C3S) at elevated temperature. 4
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Fig. 6. The change of Ca/Si and Al/Si of IP, OP and unhydrated C3S in the reference group and slag group at elevated temperature.
higher than that of IP and close to that of C2S and C3S. In contrast, this phenomenon cannot be found in the slag group. The EDS results show that the Ca/Si of the ‘bright’ spark-like phase in Reference-600 and Reference-800 group are 1.94 ± 0.08 and 2.15 ± 0.05, respectively. The composition of IP in Reference-105 and the spark-like phase in Reference-600 and Reference-800 are listed in Table 4. Compared with IP in Reference-105, only the contents of Si and Ca have a slight change in the spark-like phase in Reference-600 and Reference-800. The Si contents of the spark-like phase in Reference-600 and Reference-800 are both lower than that of IP in Reference-105.
Table 4 Composition of IP in Reference-105, spark-like phases in Reference-600 and Reference-800. Atom (%)
Reference-105 IP
Reference-600 spark-like phase
Reference-800 spark-like phase
Na Mg Al Si S Ca Fe
0.2 ± 0.3 1.3 ± 0.4 1.6 ± 0.1 34.3 ± 1.4 1.9 ± 0.5 59.7 ± 0.7 1.1 ± 0.3
0.4 ± 0.3 2.0 ± 0.7 2.2 ± 0.4 31.7 ± 0.9 0.8 ± 0.9 61.7 ± 1.8 1.2 ± 0.3
0.3 ± 0.1 1.1 ± 0.2 1.9 ± 0.1 29.3 ± 0.9 1.8 ± 0.8 62.5 ± 0.6 0.8 ± 0.4
3.5. Micromorphology respectively. Spark-like phase can also be found in IP when the temperature changes to 600 °C (Fig. 9(a)) and 800 °C(Fig. 9(b)), which corresponds well with the spark-like phase found in BSE images (Fig. 7(c)(d)). In the slag group, the matrix is rather dense at 400 °C (Fig. 10(a)) and the unhydrated C3S can hardly be found in the matrix, maybe the unhydrated C3S is tightly wrapped in the hydration products. Similar to the results previously obtained by BSE, IP and OP are hard to be distinguished from each other in the slag group. Compared with the reference group, the C-S-H particles pack more tightly in the slag group at any temperature in this study. As shown in Fig. 10(f) and (i), IP and OP are still quite dense when the temperature is 800 °C. The reticular structure transformed by OP can also be found in the slag group when the temperature is 800 °C (Fig. 10(i)). The same to the BSE result, no spark-like phase can be found in IP of the slag group.
SEM images of hydration products in Reference-400, Reference-600 and Reference-800 are shown in Fig. 8. In the reference group, the unhydrated C3S can be clearly found in the matrix (Fig. 8(a)(b)(c)). Many small tightly packed C-S-H particles can be found in IP, which are very similar to the globule flocs described by Jennings [19]. Clear gaps can be found between the C-S-H particles in IP when the temperature changes from 400 °C to 800 °C (Fig. 8(d)(e) (f)). The gaps between C-S-H particles also increase in OP with the increase of temperature ((Fig. 8(g)(h)(i)). Many small C-S-H particles can also be found in OP when the temperature is 400 °C (Fig. 8(g)). However, the increase of temperature makes the particles in OP connect together and form a kind of reticular structure (Fig. 8(h)(i)), which may be C2S. Needle-like phase can be found in the reference group when the temperature rises to 600 °C (Fig. 8(j)) and 800 °C (Fig. 8(k)). According to the EDS analysis, the Ca/Si ratios of the needle-like phase in Reference-600 and Reference-800 are 2.01 ± 0.08 and 1.94 ± 0.25,
(a) Reference-600
(b) Reference-800
Fig. 7. The ‘bright’ spark-like phase in IP of the reference group. 5
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(a)400䉝
(d)400䉝
(g)400䉝
(b) 600䉝
(c) 800䉝
(e) 600䉝
(f) 800䉝
(h) 600䉝
(i) 800䉝
(j) 600䉝
(k) 800䉝
Fig. 8. Morphology of hydration products in the reference group at high temperature.
600 °C, a second peak appears between 10 and 40 nm in both reference group and slag group. When the temperature rises to 660 °C, significant change can be observed in the slag group. The volume of pores ranging from 10 nm to 40 nm increases remarkably. Furthermore, the second peak in the slag group shifts to the right with the increase of temperature. The pores in both groups shift to larger size when the temperature increases to 800 °C. In addition, the second peak (between 20 and 50 nm) can still be observed in the slag group at 800 °C. Pore distribution of pores between 0.5 and 4 nm obtained by
3.6. Pore distribution The distribution of pores is analyzed by MIP and nitrogen adsorption analysis, and the results are shown in Fig. 11. According to the results obtained from the MIP test, no significant change can be found in the distribution of pores in both groups when temperature changes from 105 °C to 460 °C. When temperature increases to 460 °C, the average size of pore increases significantly in both groups, which may be related to the decomposition of CH [7]. At 6
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(a) 600䉝
(b) 800䉝
Fig. 9. SEM image of the spark-like phase in IP in the reference group at high temperature.
(a)400䉝
(b) 600䉝
(c) 800䉝
(d)400䉝
(e) 600䉝
(f) 800䉝
(g)400䉝
(h) 600䉝
(i) 800䉝
Fig. 10. Morphology of hydration products in the slag group at high temperature.
reference group and slag group when the temperature rises from 600 to 800 °C. During this stage, the volume of 1-2 nm pores remains stable in the slag group. In the reference group, however, the volume of 1-2 nm pores declines remarkably when the temperature increases to 720 °C and keeps almost unchanged at similar level at 800 °C.
nitrogen adsorption analysis are shown in Fig. 11(b). It can be seen that, the pore distribution of the two groups at the temperature range from 105 °C to 460 °C are quite similar. A significant increase in the volume of 1-2 nm pores can be found in both groups when the temperature reaches to 600 °C. The distinct difference can be observed in the 7
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Fig. 11. Pore distribution of cement paste at elevated temperature (a) Pore (3–10,000 nm) distribution analyzed by MIP (b) Pore (0.5–4 nm) distribution analyzed by nitrogen adsorption.
4. Discussion
4.2. The change of C-S-H in the reference group
In this investigation, differences in the microstructure of hydration products can be found between the reference group and slag group with the increase of temperature. The morphology and pore structure of hydration products in these two groups exhibit significant change when the temperature increases from 600 °C to 800 °C, which may be closely related to the nanostructural change in C-S-H gels.
According to XRD result, C-S-H has already started to decompose into C2S at 600 °C. The appearance of peaks in the result of MIP (1040 nm) and nitrogen adsorption (1-2 nm) at 600 °C should be related to the change of C-S-H. On the one hand, part of C-S-H in IP and OP decompose at 600 °C, which causes the collapse of gel structure. On the other hand, the C-S-H gels suffer from shrinkage [23] at high temperature. These two kinds of behavior both can change the size distribution of gel pores. According to the C-S-H model of Jennings [19], the pores that exist within the gel volume can be divided into three categories, there are IGP (intraglobular pores, smaller than 1 nm), SGP (small gel pores, spaces between the packed globules, between 1 and 3 nm) and LGP (large gel pores between 3 and 12 nm). The decomposition and shrinkage of C-S-H gels broaden the spacing between C-SH particles and may increase the average size of LGP, which causes the appearance of peak (10-40 nm) in the result of MIP. This also corresponds with the decline in the packing density of C-S-H [23]. The increase of 1-2 nm pores in the reference group may be related to the change of SGP between C-S-H globules. The rise of temperature results in the shrinkage of C-S-H gels and may give rise to the decrease of contact points between C-S-H globules, which makes the SGP accessible to nitrogen [24]. The decline in the 1-2 nm pores when the temperature is higher than 720 °C can be attributed to the continuous decomposition and shrinkage of C-S-H gels. The continuous deterioration of C-S-H gels results in the collapse of gel structure and increases the average size of the gel pores. A simple diagram for the pore structure change of C-S-H gels after high temperature treatment is shown as Fig. 12, which is based on the CM-II model [19].
4.1. The spark-like phase By comparing the BSE, SEM and XRD results, we think that the spark-like phase found in the IP of reference group might be spurrite. Firstly, the spark-like phase can only be found in the reference group at high temperature (according to BSE and SEM results), so does the spurrite (according to XRD result). Secondly, the IP of C3S is normally considered as a mixture of C-S-H and nanoscale CH [20,21]. The CH in IP decomposes into CaO when the temperature is higher than 460 °C and the C-S-H into C2S at a temperature higher than 600 °C. Therefore, the appearance of CaO and C2S in IP, the high density structure of IP and the existing of CO2 in the furnace can provide the condition for the formation of spurrite at high temperature. In the slag group, the absence of spurrite can be attributed to the different phase composition of IP. The CH in IP is consumed by the pozzolanic reaction between slag and CH, leading to the lack of CaO which is necessary for the formation of spurrite. Moreover, according to other researchers [22], the incorporation of slag will cause significant reduction in the average thickness of IP around C3S. Therefore, the smaller volume fraction of IP makes the spurrite, if it could be formed, less likely to be found in the slag group. 8
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Fig. 12. A simple diagram for the pore structure change of C-S-H gels after high temperature treatment
4.3. The change of C-S-H in the slag group
5. Conclusions
Due to the incorporation of slag, the aluminum content of IP and OP in the slag group are both higher than that of the reference group, which has also been reported in other researches [25–27]. This phenomenon should be related to the pozzolanic reaction between slag and CH. The incorporation of aluminum changes the composition and structure of C-S-H [28], therefore the hydration product C-S-H in slag blended cement paste is also called C-A-S-H [22]. The change of pore size distribution in the slag group is different from that of the reference group at elevated temperature. In our research, the MIP and SEM results show that the incorporation of slag helps to densify the matrix. Moreover, the investigation of other researchers [27] shows that the incorporation of slag has changed the morphology of OP C-S-H from fibrillar to foil-like. Considering above results, it is assumed that the incorporation of slag helps to generate more tightly packed C-A-S-H gels. These tightly packed C-A-S-H gels in the slag group may have more closed LGP inside. Similar to the situation in the reference group, the increase of temperature also makes C-AS-H gels decompose and shrink, which may change the closed LGP into open pores and also increase the average size of LGP. Therefore, the second peak (10-40 nm) in the result of MIP in the slag group is higher than that in the reference group. Investigation about the thermal stability of C-S-H [29] shows that CS-H with lower Ca/Si are more stable upon heating, which could be related to the silicate structure. According to the EDS results, the C-A-SH in the slag group has lower Ca/Si but higher Al/Si comparing with the C-S-H in the reference group. The mean chain length of C-A-S-H is normally thought to be longer than that of C-S-H [30]. Therefore, the low Ca/Si and different chain structure may make the C-A-S-H gels more stable than C-S-H gels at high temperature. In the SEM observation, the C-A-S-H gels in the slag group still pack tightly at high temperature. The nitrogen adsorption result demonstrates that the SGP in the C-A-S-H gels of slag group remains high in the slag group even if the temperature reaches to 800 °C, which may suggest that part of C-A-S-H gels can still maintain the gel structure even at high temperature, further confirming the stability of C-A-S-H gels.
The aim of this investigation is to study the microstructural change of C-(A)-S-H gels in PC and PC/GGBFS blended systems at elevated temperature up to 800 °C. The morphology, chemical composition and pore distribution of cement paste were studied. The results of this study will be helpful in understanding the thermal deterioration of C-(A)-S-H gels in pure cement paste and cement paste with slag. The conclusions are summarized below: 1. C-S-H gels suffer from significant deterioration when the temperature is high than 600 °C in both groups. Spark-like phase, which seems to be spurrite, can be found in the IP of the reference group. No spark-like phase can be found in the slag group. 2. The Ca/Si ratios of IP and OP in the reference group increase with elevated temperature, while the Ca/Si ratios of IP and OP in the slag group keep almost unchanged, indicating that the gels in the slag group are more stable than that in the reference group. 3. The decomposition and shrinkage of C-S-H gels enlarges the pore size of LGP (3-12 nm), which results in the increase of small pores (10-40 nm) in both groups after 600 °C. Since the incorporation of slag helps to generate more tightly packed C-A-S-H gels with closed LGP, more small pores (10-40 nm) can be found in the slag group than in the reference group after 600 °C. 4. Compared with the reference group, the number of 1-2 nm pores in the slag group stays stable when the temperature increases from 600 °C to 800 °C, confirming the stability of C-A-S-H gels. Acknowledgements This work is supported by the National Natural Science Foundation of China (project No. 51778132) and 973 program (project No. 2015CB655100) from MOST, China. References [1] M. Fall, S.S. Samb, Effect of high temperature on strength and microstructural properties of cemented paste backfill, Fire Saf. J. 44 (2009) 642–651. [2] N. Farzadnia, A.A.A. Ali, R. Demirboga, Characterization of high strength mortars with nano alumina at elevated temperatures, Cem. Concr. Res. 54 (2013) 43–54. [3] M. Heikal, A. Ali, M. Ismail, S.A.N. Ibrahim, Behavior of composite cement pastes
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