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Mechanical and hydration properties of low clinker cement containing high volume superfine blast furnace slag and nano silica
Construction and Building Materials 238 (2020) 117683
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Mechanical and hydration properties of low clinker cement containing high volume superfine blast furnace slag and nano silica Wenguang Jiang, Xiangguo Li ⇑, Yang Lv ⇑, Dongbing Jiang, Zhuolin Liu, Chenhao He State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China
h i g h l i g h t s Low clinker cement with 70% superfine blast furnace slag shows superior strength. Superfine blast furnace slag decreases the hydration heat of low clinker cement. Superfine blast furnace slag modifies the pore structure of low clinker cement paste. Nano silica improves the mechanical and hydration properties of low clinker cement.
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Article history: Received 13 June 2019 Received in revised form 13 November 2019 Accepted 22 November 2019
Keywords: Low clinker cement Superfine blast furnace slag Nano silica Mechanical strength Hydration Pore structure
a b s t r a c t Substituting part of cement clinker with superfine blast furnace slag (SFBFS) is a wonderful way to solve the problems of environmental pollution and resource consumption caused by cement manufacture. In this study, a low clinker cement containing high volume SFBFS was prepared with nano silica (NS) for property improving. The low clinker cement containing no more than 70% SFBFS exhibited higher mechanical strength than plain cement. At 90 d, the compressive and flexural strength of low clinker cement containing 70% SFBFS were 9.8% and 27.3% higher than that of plain cement. The hydration exothermic action of low clinker cement were significantly decreased, in comparison to plain cement. The results of X-ray powder diffraction and thermogravimetric analysis revealed that SFBFS was involved in the pozzolanic reaction, resulting in the consumption of CH and the production of C-S-H. The result of mercury intrusion porosimetry demonstrated that the incorporation of SFBFS refined the pore structure of blended cement pastes. In addition, NS played an important role in improving the mechanical strength, promoting the hydration and modifying the pore structure of low clinker cement blended with SFBFS. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction It is estimated that the world’s production of cement is 3–5 Gt per year and the number is growing continuously [1]. Concerning the environment issues, to manufacture 1 t of cement clinker discharges approximately 1 t of CO2, 0.74 kg of SO2, 1.15 kg of NOx and 20 kg of dust [2–5], which may cause greenhouse effect, acid rain and haze. Moreover, the cement industry consumes numerous energy, the cost on energy consumption accounts for 40–60% of the total cost of cement production [6,7]. Cement industry is under tremendous pressure to make great efforts to reduce both environmental pollution and energy consumption [8,9]. To react to the above problems, improving cement production process is not sufficient [10]. An effective solution is to use supplementary cementi-
⇑ Corresponding authors. E-mail addresses: [email protected] (X. Li), [email protected] (Y. Lv). https://doi.org/10.1016/j.conbuildmat.2019.117683 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
tious materials (SCM) to partially substitute clinker, thus to reduce the pollutant discharge as well as energy consumption caused by cement clinker manufacture [11,12]. Blast furnace slag (BFS), due to its amorphous nature and high hydraulic activity as well as pozzolanic activity, has been used as SCM in cement industry for many years [13,14]. Flower and Sanjayan [15] have demonstrated that substituting 40% Portland cement with BFS can decrease the CO2 discharge of typical concrete mixtures by 22%. Moreover, the use of moderate BFS showed positive effect on mechanical strength and durability of concrete [16– 19]. However, there was a limitation on the substitution of BFS in blended cement due to the slow hydration rate and low early mechanical strength [20,21]. Li et al. [22] suggested that the ideal dosage of BFS in cement based materials should not exceed 30%. Oner and Akyuz [23] revealed that when the amount of BFS was higher than 55%, the strength development of concrete was restricted.
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Breaking through the limitation and improving the substitution of BFS in blended cement is of great significance to cement industry. Wu et al. [24] pointed out that the pozzolanic reaction of BFS occurred only on the surface. Zhu et al. [25] showed that the hydration activity of BFS was closely related to the fineness. Wang et al. [26] indicated that BFS with larger particle size (>20 lm) showed much lower reactivity, while the BFS with particle size smaller than 5 lm made great contribution in the hydration process. Therefore, theoretically, BFS with finer particle size could be more fully hydrated and was expected to substitute a larger fraction of cement clinker in blended cement [14]. In addition, the utilization of nanomaterials in cement-based composites is one of the most concerned research hotspots in recent years [27,28]. Among the nanomaterials, nano silica (NS) was more widely used as it can obviously improve the mechanical strength of cement-based composites [29–31]. Zhang et al. [32] found that the incorporation of NS tended to shorten setting time and to increase the early stage compressive strength of highvolume slag concrete. Moreover, NS could improve the pozzolanic activity of fly ash by acting as the crystallization center of hydration products [33–35]. However, the effect of NS addition on the mechanical strength and hydration properties of cementitious materials blended with BFS was relatively rare [32]. Thus, the use of NS may be an alternative way to increase the substitution of BFS in blended cement. In order to make more effective utilization of BFS to develop low clinker cement, a superfine blast furnace slag (SFBFS) was used for cement clinker substitution to prepare low clinker cement, and NS was used for property improving. To evaluate the appropriate content of SFBFS in low clinker cement, the mechanical properties of low clinker cement blended with high volume SFBFS was studied. In addition, the hydration heat, hydration products and microstructure were characterized by means of isothermal microcalorimetry, X-ray powder diffraction (XRD), thermogravimetric analysis (TG) and mercury intrusion porosimetry (MIP). The resulting data are expected to improve the familiarity of the characteristics of SFBFS and provide a valuable reference for further utilization of SFBFS in cement production with high efficiency and low pollution.
2. Experimental 2.1. Materials Ordinary Portland Cement (OPC, 42.5) from China Resources Cement Holdings Limited (Wuhan, China), SFBFS from Wuxin New Building Materials Co., Ltd. (Wuhan, China) and NS (hydrophilic-150, SiO2 > 99.8%) from Aladdin Industrial Corporation (Shanghai, China) were used as binder in this study. The Blaine surface area of OPC, SFBFS and NS were 3200 cm2/g, 10280 cm2/g and 150 m2/g, respectively. The particle size of NS provided by supplier was in the range of 7–40 nm. The chemical composition of OPC and SFBFS measured by X-Ray Fluorescence (XRF) was listed in Table 1. The particle size distributions of OPC and SFBFS determined by Laser Particle Size Analyzer was summarized in Fig. 1. The D10, D50 and D90 were 1.086 lm, 10.635 lm, 33.561 lm for OPC and 1.073 lm, 3.364 lm, 6.906 lm for SFBFS, respectively.
Fig. 1. Particle size distribution of OPC and SFBFS.
The standard sand was from Xiamen China ISO Standard Sand Co. Ltd. (Wuhan, China). The powder polycarboxylate (PC) superplasticizer from Shanghai Sanrui Chemical Co. Ltd. (Shanghai, China) was used to modify the workability of low clinker cement mixtures. 2.2. Experimental procedure 2.2.1. Specimens preparation To investigate the influence of SFBFS content and NS addition on the performance of low clinker cement, the control sample (binder without SFBFS and NS), S-series samples (binder with SFBFS but without NS) and S-NS-series samples (binder with SFBFS and NS) were prepared, the compositions for all the mixtures were presented in Table 2. To attain similar workability of mortar (flow diameter of around 95–105 mm), different amounts of PC superplasticizer were added for specimens preparation. Prior to mixing, the powder PC superplasticizer was pre-dissolved in deionized water. In the case of S-NS-series specimens, in order to avoid agglomeration of NS, the weighed NS was dissolved in the PC superplasticizer solution by ultrasonic dispersion for 10 min (250 W, 40 KHz). The mixing and moulding procedure of mortar specimens were performed in accordance with Chinese standard (GB/T17671-1999). After 1 d, the samples were demolded and then cured in a fog room at 20 ± 2 °C and 95% relative humidity to designed age. In addition, cement pastes were also prepared in accordance with mixture design listed in Table 2 with exclusive of sand. The paste samples were cast into plastic bottles sealed with caps and cured under the same curing conditions as mortar samples to designed age. 2.2.2. Mechanical property The mechanical property (compressive and flexural strength) of cement mortar samples were measured by referring to Chinese standard (GB/T17671-1999). The loading rates was 2400 ± 200 N/s for compressive strength test and 50 ± 10 N/s for flexural strength test.
Table 1 Chemical composition of OPC and SFBFS (mass, %).
a
Materials
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
SO3
LOIa
OPC SFBFS
21.99 34.39
5.92 13.78
3.26 0.19
58.64 40.26
1.98 7.43
0.74 0.44
0.27 0.3
2.6 1.92
3.5 0
LOI: Loss on ignition.
W. Jiang et al. / Construction and Building Materials 238 (2020) 117683 Table 2 Mix proportions of samples (by mass). Samples
Control S-30 S-50 S-70 S-90 S-NS-30 S-NS-50 S-NS-70 S-NS-90
Binder (B) OPC
SFBFS
NS
100 70 50 30 10 70 50 30 10
0 30 50 70 90 29 49 69 89
0 0 0 0 0 1 1 1 1
Sand/B
Water/B
PC/B (%)
3 3 3 3 3 3 3 3 3
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
0.12 0.12 0.12 0.12 0.12 0.22 0.22 0.22 0.22
2.2.3. Heat of hydration The hydration heat of cement paste samples was determined by a TAM air isothermal calorimeter (TA Instruments, USA) at 20 . Approximately 7 g freshly mixed paste was weighed and enclosed in a plastic bottle for each sample. Afterwards, the bottle was sealed and placed into the isothermal calorimeter immediately. The cumulative hydration heat and heat release rate of samples were automatically recorded till 96 h. 2.2.4. X-ray powder diffraction (XRD) The crystalline mineral phases in hydration products of cement paste samples were identified by X-ray powder diffraction (XRD) which were conducted on an Empyrean diffractometer with Cu Ka radiation (40 kV, 30 mA), step-scanned at a scanning speed of 0.4 s/step with a step size of 0.02°(2h)/step from 5° to 70°. For the measurement, small pieces of samples were extracted from the center of the broken blended cement pastes and immersed in ethanol to stop hydration. Afterwards, the sample pieces were oven-dried at 40 for 48 h and then ground into fine powder with particle size less than 75 lm.
mechanical strength of samples with different SFBFS content were increased at different rates. At 90 d, the compressive strength of Sseries samples decreased with the SFBFS content increased from 30% to 70%, while the flexural strength showed an opposite trend. In comparison to the control sample, the compressive strength of S-30, S-50 and S-70 was increased by 22.4%, 11.4% and 9.8%, the flexural strength was increased by 17.3%, 24.5% and 27.3%, respectively, at 90 d. When the content of SFBFS was up to 90%, the compressive strength of S-90 was decreased sharply, it was more than 20% lower than that of the control sample at all testing age. When the addition amount of SFBFS was less than 70%, the incorporation of SFBFS improved the matrix compactness and enhanced the interfacial adherence between paste and sand [13]. However, in the case of mixture with 90% SFBFS, the amount of CH produced by the hydration of 10% cement clinker might be not enough to react with SFBFS, resulting in low hydration degree of SFBFS [36]. The development of compressive and flexural strength of S-NSseries samples was presented in Fig. 3. The compressive and flexural strength of S-NS-series samples were higher than those of Sseries samples at the same curing age, indicating that NS can be used as an effective reinforcing material to improve the mechanical strength of the low clinker cement. For example, the compressive strength of S-NS-70 was 51.4 and 75.8 MPa at 3 d and 28 d, respectively, which was 15.5% and 17.2% higher than those of S-70. The results could be explained as follow. On the one hand, NS could hydrate rapidly and adequately. On the other hand, NS may act
2.2.5. Thermal analysis Thermogravimetric analysis (TG) was conducted on cement paste samples by using a simultaneous thermal analysis (STA449F3, Germany). The tests were carried out from ambient temperature (23 ± 2 ) to 1000 at a heating rate of 10 /min under N2 atmosphere. The samples were prepared in accordance with XRD test. 2.2.6. Mercury intrusion porosimetry test The pore structure of hardened cement pastes were characterized by mercury intrusion porosimetry (MIP) tests using a Quanta Chrome Pore Master GT60 mercury intrusion porosimeter. For the measurement, the samples were immersed in ethanol to stop hydration and then oven-dried at 40 for 48 h. Subsequently, they were processed into 3–5 mm pieces. 3. Results and discussion 3.1. Mechanical performances The influence of SFBFS content on the compressive and flexural strength of low clinker cement mortar at different testing ages was summarized in Fig. 2(a) and (b), respectively. Fig. 2 shows that the compressive and flexural strength of samples varied significantly with hydration age and SFBFS content. At 3 d, the S-50 showed highest value of compressive and flexural strength among the mixtures under investigated. The 3 d compressive and flexural strengths of S-30 were comparable to those of S70 sample, which were about 20% and 25% higher than that of control sample, respectively. With hydration processing, the
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Fig. 2. Mechanical strength of S-series samples.
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The evolution curves of heat release rate and cumulative hydration heat of samples were summarized in Fig. 4. The hydration exothermic process of cement was usually divided into initial stage, induction stage, accelerating stage, and decelerating stage according to previous literature [24]. As shown in Fig. 4(a), there were negligible difference in the induction stage between the control sample and S-series samples with no more than 50% SFBFS, however, when the SFBFS content in low clinker cement reached 70% and higher, the induction stage was visibly prolonged. For example, the ending time of induction stage of control sample, S-30 and S-50 was nearly 5.5 h, but it prolonged to 9.75 h for S-70 and 10.54 h for S-90. This prolongation may be due to the absorption of Ca2+ of excessive SFBFS resulted in a too low Ca2+concentration in solution, thus delaying the
nucleation and growth of hydration products of cement [37]. Moreover, in the accelerating stage, the heat release rate of the main exothermic peak of low clinker cement was obviously decreased with the increase of SFBFS content, which was because the exothermic peak was mainly resulted from cement hydration while SFBFS diluted cement. Furthermore, in the decelerating period, there was a prominent exothermic peak for S-70 and S-90, meanwhile the heat release rate of S-30 and S-50 was higher than that of control sample. The phenomenon was caused by the pozzolanic reaction of SFBFS. In addition, compared to S-70, the exothermic peak of S-NS-70 was shifted to an earlier stage. It revealed that the incorporation of NS promoted the hydration reaction of cement and the pozzolanic reaction of SFBFS. As shown in Fig. 4(b), the incorporation of SFBFS effectively decreased the cumulative heat of hydration of low clinker cement. The total hydration heat was decreased from 243.76 J/g for control sample to 224.23 J/g for S-30, 205.68 J/g for S-50, 168.90 J/g for S70 and 101.76 J/g for S-90, decreased by 8.01%, 15.62%, 30.71% and 58.25%, respectively. The result indicated that the heat released from pozzolanic reaction of SFBFS was much lower than cement hydration, therefore, the utilization of SFBFS was potential to prevent the generation of thermal cracking in low clinker cement. The total hydration heat of S-NS-70 was 176.68 J/g, which was higher than that of S-70. It demonstrated that NS can promote the hydration of low clinker cement. However, the total hydration heat of SNS-70 was still 27.52% lower than that of the control sample, indicating that NS would not cause tremendous rise in hydration heat
Fig. 3. Mechanical strength of S-NS-series samples.
Fig. 4. Hydration heat of samples.
as the crystallization center for nucleation and growth of hydration products of cement and SFBFS, thus improving their hydration degree. However, although the compressive strength of S-NS-90 was increased by the addition of NS, it was still much lower than that of control sample at all testing ages. Consequently, considering the mechanical strength, the maximum content of SFBFS in low clinker cement should no more than 90%, and 70% was an appropriate proportion. In the following research, the hydration properties and microstructure of sample with 70% SFBFS content were emphatically studied. 3.2. Hydration heat
W. Jiang et al. / Construction and Building Materials 238 (2020) 117683
of low clinker cement containing 70% SFBFS leading to risk of thermal cracking. The similar phenomenon was reported by Zhang [32]. 3.3. XRD analysis The XRD diffraction patterns of paste samples cured for 28 d were shown in Fig. 5. As can be found in Fig. 5 that the main mineral phases in paste samples included calcium hydroxide (CH), calcium silicate hydrate (C-S-H), hydrotalcite, hydrotalcite-like phase and unhydrated alite (C3S) and belite (C2S). The hydrotalcite and hydrotalcite-like phase were the hydration products of slag when the concentration of magnesium reached a specific point [38,39]. There was a decrease in diffraction peaks of C3S and C2S phases with the increase of SFBFS. This can be attributed to the dilution effect of SFBFS which resulted in the decrease of particle concentration of unhydrated cement clinker. Meanwhile, a more visible variation was that the intensity of CH peaks of the samples remarkably decreased with the increasing of SFBFS. That was because the CH generated form the hydration of clinker was partially involved in the pozzolanic reaction of SFBFS. Thus, the higher the SFBFS content, the more the CH consumption. C-S-H was the product of hydration reaction of cement clinker and pozzolanic reaction of SFBFS. The diffraction peak of C-S-H at around 29.5° with tiny difference was detected in all samples. It is indicating that certain amount of SFBFS was involved in the pozzolanic reaction, resulting in the consumption of CH and the production of C-S-H.
where WLCH is the mass loss in the second stage; WMCH and WMH2 O represent the relative molecular mass of CH and H2O, respectively. The calculation results were shown in Table 3. The third stage in the range of 600–750 was originated from the further degradation of C-S-H gel and hydrated aluminate phases [41]. The mass loss of cement pastes in different stages were also summarized in Table 3. From Table 3, the content of CH was reduced with the increase of SFBFS content in low clinker cement, which was in good correspondence to the XRD results. The content of CH was a balance of the following factors. First, the addition of SFBFS diluted cement clinker leading to less CH generation when the hydration degree of cement maintained the same. Second, the available water to cement ratio was increased, which improved the hydration degree of cement, thus promoting the formation of CH. Third, the pozzolanic reaction of SFBFS consumed CH. Moreover, compared S-70 with S-NS-70, it can be found that the addition of NS decreased the CH content in cement pastes, and the mass loss of S-NS-70 in the first mass loss stage was higher. It proved that NS was beneficial to improve the pozzolanic reaction of SFBFS.
3.5. Pore structure The cumulative porosity and pore size distribution of hardened cement pastes measured by MIP at 28 d were summarized in Fig. 7 (a) and (b), respectively. According to the literature [42,43], the pores were divided into large capillary pores (>0.05 lm), medium
3.4. Thermal analysis Fig. 6(a) and (b) shows the TG and DTG curves of paste samples at 28 d, respectively. As shown in Fig. 6(a), SFBFS significantly affected the total mass loss of cement pastes. A higher SFBFS content led to a lower mass loss, which indicated that with the increase of SFBFS, the total hydration products of low clinker cement pastes were decreased. In addition, from the TG and DTG curves, there were three major mass loss stages during the heating process of the cement pastes. The first stage between ambient to 300 corresponding to the evaporation of free water as well as the dehydration of C-S-H, ettringite and AFm. The second stage at around 450 was attributed to the decomposition of CH. The amount of CH can be calculated according to the equation [40]:
CHð%Þ ¼ WLCH ð%Þ
WMCH WMH2 O
Fig. 5. XRD analysis of cement pastes at 28 d.
5
ð1Þ
Fig. 6. TG and DTG analysis of cement pastes.
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W. Jiang et al. / Construction and Building Materials 238 (2020) 117683
Table 3 CH content and mass loss of paste samples at different stages. Sample
CH (%)
Control S-30 S-50 S-70 S-NS-70 S-90
17.69 11.68 8.75 5.77 5.60 3.00
Mass loss (%) <300
400–500
600–750
10.73 11.17 10.27 8.70 9.38 6.05
4.30 2.84 2.13 1.40 1.36 0.73
2.65 2.36 2.24 2.09 1.49 1.08
capillary pores (0.05 ~ 0.01 lm) and gel pores (<0.01 lm). The volumes of the three types of pores were presented in Fig. 8. The incorporation of SFBFS had significant effects on the pore structure of hardened cement pastes. From Fig. 7 (a), the cumulative porosity of control sample was 11.57 102 cm3/g. With the incorporation of SFBFS, the cumulative porosity decreased to 6.32 102 cm3/g for S-30 and 4.56 102 cm3/g for S-50, which decreased by 45.38% and 60.59%, respectively, compared to control sample. Then, when the SFBFS content beyond 50% in low clinker cement, the cumulative porosity increased with the increase of SFBFS, that of S-90 was 14.50 cm3/g, which was 25.32% higher than the control sample. Moreover, from Figs. 7(b) and 8, the pore structure of control sample was mainly composed of medium capillary pores and large capillary pores which account for 50.39% and 31.46%, respectively. After the addition of SFBFS, the pore size distribution of S-series
Fig. 8. Volumes of the three types of pores in the samples.
samples was refined, the large capillary pores sharply reduced to less than 10%, and the medium capillary pores or gel pores increased correspondingly. For example, the pore size distribution of S-30 mainly included 66.30% of medium capillary pores and 25.47% of gel pores, while the large capillary pores only accounted for 8.23%. The pore size distribution of S-50 and S-70 was similar, it was dominated by gel pores whose proportion was nearly 60%, besides, the medium capillary pores were almost 40%. Uniquely, the pore size distribution of S-90 was converged in medium capillary pores and the fraction was as high as 88.07%. The variation of pore structure between samples with different SFBFS contents may be due to the following factors. Firstly, due to the much finer particle size nature of SFBFS, the SFBFS particles can be filled in the gap of cement particles, refining the pore size distribution of low clinker cement pastes and thus reducing the porosity. It also explains why the S-series samples showed significantly lower proportion of large capillary pores compared with control sample. Secondly, the pore structure were modified by the C-S-H phases which generated from the hydration of cement and SFBFS [44]. Thirdly, an excessive SFBFS content seriously reduced the amount of hydration products of low clinker cement, resulting in an increase in porosity. As for the effect of NS on the pore structure of low clinker cement pastes, the cumulative porosity decreased from 11.19 102 cm3/g for S-70 to 7.81 102 cm3/g for S-NS-70, decreased by 30.21%. Moreover, S-NS-70 showed higher gel pores fraction and lower medium capillary pores fraction compared with S-70. NS can be filled in the gap between SFBFS and cement. Meanwhile, NS could hydrated rapidly to generate C-S-H gel. Besides, as mentioned previously, NS may act as the crystallization center to promote the hydration and pozzolanic reaction process of cement and SFBFS, thereby increasing the amount of hydration products. All of these made great contribution to improve the pore structure of cement paste. However, the partial agglomeration of NS may also cause voids in blended cement, as reported by Kong [45], so that there was an abnormal increase in large capillary pores fraction of S-NS-70.
4. Conclusions
Fig. 7. Pore structure of samples.
The study synthetically investigated the mechanical strength, hydration properties and pore structure of low clinker cement containing high volume superfine blast furnace slag. NS was used for improving the properties. The following conclusions can be drawn according to the test results:
W. Jiang et al. / Construction and Building Materials 238 (2020) 117683
(1) The mechanical strength of the low clinker cement containing 70% SFBFS was superior to plain cement at all testing ages. Moreover, NS can be used as an effective reinforcing material to improve the mechanical properties. (2) SFBFS can effectively decrease the heat release rate and reduce the cumulative hydration heat of blended cement. The cumulative hydration heat of low clinker cement containing 70% SFS was decreased by 30.71% compared to control sample, indicating that SFS is potential to prevent the generation of thermal cracking. NS can promote the hydration but will not cause tremendous rise in total hydration heat of low clinker cement containing 70% SFBFS leading to risk of thermal cracking. (3) XRD and TG-DTG results confirmed that SFBFS was involved in the pozzolanic reaction, leading to consumption of CH and production of C-S-H. NS was beneficial to improve the pozzolanic reaction of SFBFS. (4) SFBFS can play an important role in improving the pore structure of blended cement pastes. The pore size distribution of low clinker cement containing 70% SFBFS was dominated by nearly 60% gel pores, whereas the large capillary pores only accounted for 2.77%. Besides, NS made significant contribution in reducing porosity of low clinker cement. CRediT authorship contribution statement Wenguang Jiang: Investigation, Data curation, Writing - original draft, Writing - review & editing. Xiangguo Li: Conceptualization, Funding acquisition, Project administration, Supervision, Writing - review & editing. Yang Lv: Data curation, Funding acquisition, Writing - review & editing. Dongbing Jiang: Investigation. Zhuolin Liu: Methodology. Chenhao He: Investigation. Acknowledgments The authors are grateful for the financial support provided by the National Natural Science Foundation of China (51572207, 51808420) and the Fundamental Research Funds for the Central Universities (WUT: 2019IVB058). Declaration of Competing Interest We declare that there is no conflict of interest in this work. References [1] J. Li, W. Zhang, C. Li, P.J.M. Monteiro, Green concrete containing diatomaceous earth and limestone: workability, mechanical properties, and life-cycle assessment, J. Clean. Prod. 223 (2019) 662–679. [2] B. Ma, X. Li, X. Wang, R. Dong, H. Zhu, Study on environmental load and the control approach in cement industry, Cem. Eng. 2 (2005) 79–82. [3] A. Schoeler, B. Lothenbach, F. Winnefeld, M. Zajac, Hydration of quaternary Portland cement blends containing blast-furnace slag, siliceous fly ash and limestone powder, Cem. Concr. Compos. 55 (2015) 374–382. [4] Y.C. Díaz, S.S. Berriel, U. Heierli, A.R. Favier, I.R.S. Machado, K.L. Scrivener, J.F.M. Hernández, G. Habert, Limestone calcined clay cement as a low-carbon solution to meet expanding cement demand in emerging economies, Dev. Eng. 2 (2017) 82–91. [5] N. Mahasenan, S. Smith, K. Humphreys, The cement industry and global climate change: current and potential future cement industry CO2 emissions, in: Y. Kaya (Ed.), Greenhouse Gas Control Technologies – 6th International Conference, Pergamon, Oxford, 2003, pp. 995–1000. [6] D. Xu, Y. Cui, H. Li, K. Yang, W. Xu, Y. Chen, On the future of Chinese cement industry, Cem. Concr. Res. 78 (2015) 2–13. [7] X. Chai, H. Kao, T. Guo, Q. Zhang, Study on thermal energy recovery methods of cement industry basing on the energy balance methods, J. Bull. Chin. Ceram. 32 (4) (2013) 692–698. [8] S.A. Ishak, H. Hashim, Low carbon measures for cement plant a review, J. Clean. Prod. 103 (2015) 260–274.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Mechanical and hydration properties of low clinker cement containing high volume superfine blast furnace slag and nano silica Wenguang Jiang, Xiangguo Li ⇑, Yang Lv ⇑, Dongbing Jiang, Zhuolin Liu, Chenhao He State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China
h i g h l i g h t s Low clinker cement with 70% superfine blast furnace slag shows superior strength. Superfine blast furnace slag decreases the hydration heat of low clinker cement. Superfine blast furnace slag modifies the pore structure of low clinker cement paste. Nano silica improves the mechanical and hydration properties of low clinker cement.
a r t i c l e
i n f o
Article history: Received 13 June 2019 Received in revised form 13 November 2019 Accepted 22 November 2019
Keywords: Low clinker cement Superfine blast furnace slag Nano silica Mechanical strength Hydration Pore structure
a b s t r a c t Substituting part of cement clinker with superfine blast furnace slag (SFBFS) is a wonderful way to solve the problems of environmental pollution and resource consumption caused by cement manufacture. In this study, a low clinker cement containing high volume SFBFS was prepared with nano silica (NS) for property improving. The low clinker cement containing no more than 70% SFBFS exhibited higher mechanical strength than plain cement. At 90 d, the compressive and flexural strength of low clinker cement containing 70% SFBFS were 9.8% and 27.3% higher than that of plain cement. The hydration exothermic action of low clinker cement were significantly decreased, in comparison to plain cement. The results of X-ray powder diffraction and thermogravimetric analysis revealed that SFBFS was involved in the pozzolanic reaction, resulting in the consumption of CH and the production of C-S-H. The result of mercury intrusion porosimetry demonstrated that the incorporation of SFBFS refined the pore structure of blended cement pastes. In addition, NS played an important role in improving the mechanical strength, promoting the hydration and modifying the pore structure of low clinker cement blended with SFBFS. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction It is estimated that the world’s production of cement is 3–5 Gt per year and the number is growing continuously [1]. Concerning the environment issues, to manufacture 1 t of cement clinker discharges approximately 1 t of CO2, 0.74 kg of SO2, 1.15 kg of NOx and 20 kg of dust [2–5], which may cause greenhouse effect, acid rain and haze. Moreover, the cement industry consumes numerous energy, the cost on energy consumption accounts for 40–60% of the total cost of cement production [6,7]. Cement industry is under tremendous pressure to make great efforts to reduce both environmental pollution and energy consumption [8,9]. To react to the above problems, improving cement production process is not sufficient [10]. An effective solution is to use supplementary cementi-
⇑ Corresponding authors. E-mail addresses: [email protected] (X. Li), [email protected] (Y. Lv). https://doi.org/10.1016/j.conbuildmat.2019.117683 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
tious materials (SCM) to partially substitute clinker, thus to reduce the pollutant discharge as well as energy consumption caused by cement clinker manufacture [11,12]. Blast furnace slag (BFS), due to its amorphous nature and high hydraulic activity as well as pozzolanic activity, has been used as SCM in cement industry for many years [13,14]. Flower and Sanjayan [15] have demonstrated that substituting 40% Portland cement with BFS can decrease the CO2 discharge of typical concrete mixtures by 22%. Moreover, the use of moderate BFS showed positive effect on mechanical strength and durability of concrete [16– 19]. However, there was a limitation on the substitution of BFS in blended cement due to the slow hydration rate and low early mechanical strength [20,21]. Li et al. [22] suggested that the ideal dosage of BFS in cement based materials should not exceed 30%. Oner and Akyuz [23] revealed that when the amount of BFS was higher than 55%, the strength development of concrete was restricted.
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Breaking through the limitation and improving the substitution of BFS in blended cement is of great significance to cement industry. Wu et al. [24] pointed out that the pozzolanic reaction of BFS occurred only on the surface. Zhu et al. [25] showed that the hydration activity of BFS was closely related to the fineness. Wang et al. [26] indicated that BFS with larger particle size (>20 lm) showed much lower reactivity, while the BFS with particle size smaller than 5 lm made great contribution in the hydration process. Therefore, theoretically, BFS with finer particle size could be more fully hydrated and was expected to substitute a larger fraction of cement clinker in blended cement [14]. In addition, the utilization of nanomaterials in cement-based composites is one of the most concerned research hotspots in recent years [27,28]. Among the nanomaterials, nano silica (NS) was more widely used as it can obviously improve the mechanical strength of cement-based composites [29–31]. Zhang et al. [32] found that the incorporation of NS tended to shorten setting time and to increase the early stage compressive strength of highvolume slag concrete. Moreover, NS could improve the pozzolanic activity of fly ash by acting as the crystallization center of hydration products [33–35]. However, the effect of NS addition on the mechanical strength and hydration properties of cementitious materials blended with BFS was relatively rare [32]. Thus, the use of NS may be an alternative way to increase the substitution of BFS in blended cement. In order to make more effective utilization of BFS to develop low clinker cement, a superfine blast furnace slag (SFBFS) was used for cement clinker substitution to prepare low clinker cement, and NS was used for property improving. To evaluate the appropriate content of SFBFS in low clinker cement, the mechanical properties of low clinker cement blended with high volume SFBFS was studied. In addition, the hydration heat, hydration products and microstructure were characterized by means of isothermal microcalorimetry, X-ray powder diffraction (XRD), thermogravimetric analysis (TG) and mercury intrusion porosimetry (MIP). The resulting data are expected to improve the familiarity of the characteristics of SFBFS and provide a valuable reference for further utilization of SFBFS in cement production with high efficiency and low pollution.
2. Experimental 2.1. Materials Ordinary Portland Cement (OPC, 42.5) from China Resources Cement Holdings Limited (Wuhan, China), SFBFS from Wuxin New Building Materials Co., Ltd. (Wuhan, China) and NS (hydrophilic-150, SiO2 > 99.8%) from Aladdin Industrial Corporation (Shanghai, China) were used as binder in this study. The Blaine surface area of OPC, SFBFS and NS were 3200 cm2/g, 10280 cm2/g and 150 m2/g, respectively. The particle size of NS provided by supplier was in the range of 7–40 nm. The chemical composition of OPC and SFBFS measured by X-Ray Fluorescence (XRF) was listed in Table 1. The particle size distributions of OPC and SFBFS determined by Laser Particle Size Analyzer was summarized in Fig. 1. The D10, D50 and D90 were 1.086 lm, 10.635 lm, 33.561 lm for OPC and 1.073 lm, 3.364 lm, 6.906 lm for SFBFS, respectively.
Fig. 1. Particle size distribution of OPC and SFBFS.
The standard sand was from Xiamen China ISO Standard Sand Co. Ltd. (Wuhan, China). The powder polycarboxylate (PC) superplasticizer from Shanghai Sanrui Chemical Co. Ltd. (Shanghai, China) was used to modify the workability of low clinker cement mixtures. 2.2. Experimental procedure 2.2.1. Specimens preparation To investigate the influence of SFBFS content and NS addition on the performance of low clinker cement, the control sample (binder without SFBFS and NS), S-series samples (binder with SFBFS but without NS) and S-NS-series samples (binder with SFBFS and NS) were prepared, the compositions for all the mixtures were presented in Table 2. To attain similar workability of mortar (flow diameter of around 95–105 mm), different amounts of PC superplasticizer were added for specimens preparation. Prior to mixing, the powder PC superplasticizer was pre-dissolved in deionized water. In the case of S-NS-series specimens, in order to avoid agglomeration of NS, the weighed NS was dissolved in the PC superplasticizer solution by ultrasonic dispersion for 10 min (250 W, 40 KHz). The mixing and moulding procedure of mortar specimens were performed in accordance with Chinese standard (GB/T17671-1999). After 1 d, the samples were demolded and then cured in a fog room at 20 ± 2 °C and 95% relative humidity to designed age. In addition, cement pastes were also prepared in accordance with mixture design listed in Table 2 with exclusive of sand. The paste samples were cast into plastic bottles sealed with caps and cured under the same curing conditions as mortar samples to designed age. 2.2.2. Mechanical property The mechanical property (compressive and flexural strength) of cement mortar samples were measured by referring to Chinese standard (GB/T17671-1999). The loading rates was 2400 ± 200 N/s for compressive strength test and 50 ± 10 N/s for flexural strength test.
Table 1 Chemical composition of OPC and SFBFS (mass, %).
a
Materials
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
SO3
LOIa
OPC SFBFS
21.99 34.39
5.92 13.78
3.26 0.19
58.64 40.26
1.98 7.43
0.74 0.44
0.27 0.3
2.6 1.92
3.5 0
LOI: Loss on ignition.
W. Jiang et al. / Construction and Building Materials 238 (2020) 117683 Table 2 Mix proportions of samples (by mass). Samples
Control S-30 S-50 S-70 S-90 S-NS-30 S-NS-50 S-NS-70 S-NS-90
Binder (B) OPC
SFBFS
NS
100 70 50 30 10 70 50 30 10
0 30 50 70 90 29 49 69 89
0 0 0 0 0 1 1 1 1
Sand/B
Water/B
PC/B (%)
3 3 3 3 3 3 3 3 3
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
0.12 0.12 0.12 0.12 0.12 0.22 0.22 0.22 0.22
2.2.3. Heat of hydration The hydration heat of cement paste samples was determined by a TAM air isothermal calorimeter (TA Instruments, USA) at 20 . Approximately 7 g freshly mixed paste was weighed and enclosed in a plastic bottle for each sample. Afterwards, the bottle was sealed and placed into the isothermal calorimeter immediately. The cumulative hydration heat and heat release rate of samples were automatically recorded till 96 h. 2.2.4. X-ray powder diffraction (XRD) The crystalline mineral phases in hydration products of cement paste samples were identified by X-ray powder diffraction (XRD) which were conducted on an Empyrean diffractometer with Cu Ka radiation (40 kV, 30 mA), step-scanned at a scanning speed of 0.4 s/step with a step size of 0.02°(2h)/step from 5° to 70°. For the measurement, small pieces of samples were extracted from the center of the broken blended cement pastes and immersed in ethanol to stop hydration. Afterwards, the sample pieces were oven-dried at 40 for 48 h and then ground into fine powder with particle size less than 75 lm.
mechanical strength of samples with different SFBFS content were increased at different rates. At 90 d, the compressive strength of Sseries samples decreased with the SFBFS content increased from 30% to 70%, while the flexural strength showed an opposite trend. In comparison to the control sample, the compressive strength of S-30, S-50 and S-70 was increased by 22.4%, 11.4% and 9.8%, the flexural strength was increased by 17.3%, 24.5% and 27.3%, respectively, at 90 d. When the content of SFBFS was up to 90%, the compressive strength of S-90 was decreased sharply, it was more than 20% lower than that of the control sample at all testing age. When the addition amount of SFBFS was less than 70%, the incorporation of SFBFS improved the matrix compactness and enhanced the interfacial adherence between paste and sand [13]. However, in the case of mixture with 90% SFBFS, the amount of CH produced by the hydration of 10% cement clinker might be not enough to react with SFBFS, resulting in low hydration degree of SFBFS [36]. The development of compressive and flexural strength of S-NSseries samples was presented in Fig. 3. The compressive and flexural strength of S-NS-series samples were higher than those of Sseries samples at the same curing age, indicating that NS can be used as an effective reinforcing material to improve the mechanical strength of the low clinker cement. For example, the compressive strength of S-NS-70 was 51.4 and 75.8 MPa at 3 d and 28 d, respectively, which was 15.5% and 17.2% higher than those of S-70. The results could be explained as follow. On the one hand, NS could hydrate rapidly and adequately. On the other hand, NS may act
2.2.5. Thermal analysis Thermogravimetric analysis (TG) was conducted on cement paste samples by using a simultaneous thermal analysis (STA449F3, Germany). The tests were carried out from ambient temperature (23 ± 2 ) to 1000 at a heating rate of 10 /min under N2 atmosphere. The samples were prepared in accordance with XRD test. 2.2.6. Mercury intrusion porosimetry test The pore structure of hardened cement pastes were characterized by mercury intrusion porosimetry (MIP) tests using a Quanta Chrome Pore Master GT60 mercury intrusion porosimeter. For the measurement, the samples were immersed in ethanol to stop hydration and then oven-dried at 40 for 48 h. Subsequently, they were processed into 3–5 mm pieces. 3. Results and discussion 3.1. Mechanical performances The influence of SFBFS content on the compressive and flexural strength of low clinker cement mortar at different testing ages was summarized in Fig. 2(a) and (b), respectively. Fig. 2 shows that the compressive and flexural strength of samples varied significantly with hydration age and SFBFS content. At 3 d, the S-50 showed highest value of compressive and flexural strength among the mixtures under investigated. The 3 d compressive and flexural strengths of S-30 were comparable to those of S70 sample, which were about 20% and 25% higher than that of control sample, respectively. With hydration processing, the
3
Fig. 2. Mechanical strength of S-series samples.
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The evolution curves of heat release rate and cumulative hydration heat of samples were summarized in Fig. 4. The hydration exothermic process of cement was usually divided into initial stage, induction stage, accelerating stage, and decelerating stage according to previous literature [24]. As shown in Fig. 4(a), there were negligible difference in the induction stage between the control sample and S-series samples with no more than 50% SFBFS, however, when the SFBFS content in low clinker cement reached 70% and higher, the induction stage was visibly prolonged. For example, the ending time of induction stage of control sample, S-30 and S-50 was nearly 5.5 h, but it prolonged to 9.75 h for S-70 and 10.54 h for S-90. This prolongation may be due to the absorption of Ca2+ of excessive SFBFS resulted in a too low Ca2+concentration in solution, thus delaying the
nucleation and growth of hydration products of cement [37]. Moreover, in the accelerating stage, the heat release rate of the main exothermic peak of low clinker cement was obviously decreased with the increase of SFBFS content, which was because the exothermic peak was mainly resulted from cement hydration while SFBFS diluted cement. Furthermore, in the decelerating period, there was a prominent exothermic peak for S-70 and S-90, meanwhile the heat release rate of S-30 and S-50 was higher than that of control sample. The phenomenon was caused by the pozzolanic reaction of SFBFS. In addition, compared to S-70, the exothermic peak of S-NS-70 was shifted to an earlier stage. It revealed that the incorporation of NS promoted the hydration reaction of cement and the pozzolanic reaction of SFBFS. As shown in Fig. 4(b), the incorporation of SFBFS effectively decreased the cumulative heat of hydration of low clinker cement. The total hydration heat was decreased from 243.76 J/g for control sample to 224.23 J/g for S-30, 205.68 J/g for S-50, 168.90 J/g for S70 and 101.76 J/g for S-90, decreased by 8.01%, 15.62%, 30.71% and 58.25%, respectively. The result indicated that the heat released from pozzolanic reaction of SFBFS was much lower than cement hydration, therefore, the utilization of SFBFS was potential to prevent the generation of thermal cracking in low clinker cement. The total hydration heat of S-NS-70 was 176.68 J/g, which was higher than that of S-70. It demonstrated that NS can promote the hydration of low clinker cement. However, the total hydration heat of SNS-70 was still 27.52% lower than that of the control sample, indicating that NS would not cause tremendous rise in hydration heat
Fig. 3. Mechanical strength of S-NS-series samples.
Fig. 4. Hydration heat of samples.
as the crystallization center for nucleation and growth of hydration products of cement and SFBFS, thus improving their hydration degree. However, although the compressive strength of S-NS-90 was increased by the addition of NS, it was still much lower than that of control sample at all testing ages. Consequently, considering the mechanical strength, the maximum content of SFBFS in low clinker cement should no more than 90%, and 70% was an appropriate proportion. In the following research, the hydration properties and microstructure of sample with 70% SFBFS content were emphatically studied. 3.2. Hydration heat
W. Jiang et al. / Construction and Building Materials 238 (2020) 117683
of low clinker cement containing 70% SFBFS leading to risk of thermal cracking. The similar phenomenon was reported by Zhang [32]. 3.3. XRD analysis The XRD diffraction patterns of paste samples cured for 28 d were shown in Fig. 5. As can be found in Fig. 5 that the main mineral phases in paste samples included calcium hydroxide (CH), calcium silicate hydrate (C-S-H), hydrotalcite, hydrotalcite-like phase and unhydrated alite (C3S) and belite (C2S). The hydrotalcite and hydrotalcite-like phase were the hydration products of slag when the concentration of magnesium reached a specific point [38,39]. There was a decrease in diffraction peaks of C3S and C2S phases with the increase of SFBFS. This can be attributed to the dilution effect of SFBFS which resulted in the decrease of particle concentration of unhydrated cement clinker. Meanwhile, a more visible variation was that the intensity of CH peaks of the samples remarkably decreased with the increasing of SFBFS. That was because the CH generated form the hydration of clinker was partially involved in the pozzolanic reaction of SFBFS. Thus, the higher the SFBFS content, the more the CH consumption. C-S-H was the product of hydration reaction of cement clinker and pozzolanic reaction of SFBFS. The diffraction peak of C-S-H at around 29.5° with tiny difference was detected in all samples. It is indicating that certain amount of SFBFS was involved in the pozzolanic reaction, resulting in the consumption of CH and the production of C-S-H.
where WLCH is the mass loss in the second stage; WMCH and WMH2 O represent the relative molecular mass of CH and H2O, respectively. The calculation results were shown in Table 3. The third stage in the range of 600–750 was originated from the further degradation of C-S-H gel and hydrated aluminate phases [41]. The mass loss of cement pastes in different stages were also summarized in Table 3. From Table 3, the content of CH was reduced with the increase of SFBFS content in low clinker cement, which was in good correspondence to the XRD results. The content of CH was a balance of the following factors. First, the addition of SFBFS diluted cement clinker leading to less CH generation when the hydration degree of cement maintained the same. Second, the available water to cement ratio was increased, which improved the hydration degree of cement, thus promoting the formation of CH. Third, the pozzolanic reaction of SFBFS consumed CH. Moreover, compared S-70 with S-NS-70, it can be found that the addition of NS decreased the CH content in cement pastes, and the mass loss of S-NS-70 in the first mass loss stage was higher. It proved that NS was beneficial to improve the pozzolanic reaction of SFBFS.
3.5. Pore structure The cumulative porosity and pore size distribution of hardened cement pastes measured by MIP at 28 d were summarized in Fig. 7 (a) and (b), respectively. According to the literature [42,43], the pores were divided into large capillary pores (>0.05 lm), medium
3.4. Thermal analysis Fig. 6(a) and (b) shows the TG and DTG curves of paste samples at 28 d, respectively. As shown in Fig. 6(a), SFBFS significantly affected the total mass loss of cement pastes. A higher SFBFS content led to a lower mass loss, which indicated that with the increase of SFBFS, the total hydration products of low clinker cement pastes were decreased. In addition, from the TG and DTG curves, there were three major mass loss stages during the heating process of the cement pastes. The first stage between ambient to 300 corresponding to the evaporation of free water as well as the dehydration of C-S-H, ettringite and AFm. The second stage at around 450 was attributed to the decomposition of CH. The amount of CH can be calculated according to the equation [40]:
CHð%Þ ¼ WLCH ð%Þ
WMCH WMH2 O
Fig. 5. XRD analysis of cement pastes at 28 d.
5
ð1Þ
Fig. 6. TG and DTG analysis of cement pastes.
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W. Jiang et al. / Construction and Building Materials 238 (2020) 117683
Table 3 CH content and mass loss of paste samples at different stages. Sample
CH (%)
Control S-30 S-50 S-70 S-NS-70 S-90
17.69 11.68 8.75 5.77 5.60 3.00
Mass loss (%) <300
400–500
600–750
10.73 11.17 10.27 8.70 9.38 6.05
4.30 2.84 2.13 1.40 1.36 0.73
2.65 2.36 2.24 2.09 1.49 1.08
capillary pores (0.05 ~ 0.01 lm) and gel pores (<0.01 lm). The volumes of the three types of pores were presented in Fig. 8. The incorporation of SFBFS had significant effects on the pore structure of hardened cement pastes. From Fig. 7 (a), the cumulative porosity of control sample was 11.57 102 cm3/g. With the incorporation of SFBFS, the cumulative porosity decreased to 6.32 102 cm3/g for S-30 and 4.56 102 cm3/g for S-50, which decreased by 45.38% and 60.59%, respectively, compared to control sample. Then, when the SFBFS content beyond 50% in low clinker cement, the cumulative porosity increased with the increase of SFBFS, that of S-90 was 14.50 cm3/g, which was 25.32% higher than the control sample. Moreover, from Figs. 7(b) and 8, the pore structure of control sample was mainly composed of medium capillary pores and large capillary pores which account for 50.39% and 31.46%, respectively. After the addition of SFBFS, the pore size distribution of S-series
Fig. 8. Volumes of the three types of pores in the samples.
samples was refined, the large capillary pores sharply reduced to less than 10%, and the medium capillary pores or gel pores increased correspondingly. For example, the pore size distribution of S-30 mainly included 66.30% of medium capillary pores and 25.47% of gel pores, while the large capillary pores only accounted for 8.23%. The pore size distribution of S-50 and S-70 was similar, it was dominated by gel pores whose proportion was nearly 60%, besides, the medium capillary pores were almost 40%. Uniquely, the pore size distribution of S-90 was converged in medium capillary pores and the fraction was as high as 88.07%. The variation of pore structure between samples with different SFBFS contents may be due to the following factors. Firstly, due to the much finer particle size nature of SFBFS, the SFBFS particles can be filled in the gap of cement particles, refining the pore size distribution of low clinker cement pastes and thus reducing the porosity. It also explains why the S-series samples showed significantly lower proportion of large capillary pores compared with control sample. Secondly, the pore structure were modified by the C-S-H phases which generated from the hydration of cement and SFBFS [44]. Thirdly, an excessive SFBFS content seriously reduced the amount of hydration products of low clinker cement, resulting in an increase in porosity. As for the effect of NS on the pore structure of low clinker cement pastes, the cumulative porosity decreased from 11.19 102 cm3/g for S-70 to 7.81 102 cm3/g for S-NS-70, decreased by 30.21%. Moreover, S-NS-70 showed higher gel pores fraction and lower medium capillary pores fraction compared with S-70. NS can be filled in the gap between SFBFS and cement. Meanwhile, NS could hydrated rapidly to generate C-S-H gel. Besides, as mentioned previously, NS may act as the crystallization center to promote the hydration and pozzolanic reaction process of cement and SFBFS, thereby increasing the amount of hydration products. All of these made great contribution to improve the pore structure of cement paste. However, the partial agglomeration of NS may also cause voids in blended cement, as reported by Kong [45], so that there was an abnormal increase in large capillary pores fraction of S-NS-70.
4. Conclusions
Fig. 7. Pore structure of samples.
The study synthetically investigated the mechanical strength, hydration properties and pore structure of low clinker cement containing high volume superfine blast furnace slag. NS was used for improving the properties. The following conclusions can be drawn according to the test results:
W. Jiang et al. / Construction and Building Materials 238 (2020) 117683
(1) The mechanical strength of the low clinker cement containing 70% SFBFS was superior to plain cement at all testing ages. Moreover, NS can be used as an effective reinforcing material to improve the mechanical properties. (2) SFBFS can effectively decrease the heat release rate and reduce the cumulative hydration heat of blended cement. The cumulative hydration heat of low clinker cement containing 70% SFS was decreased by 30.71% compared to control sample, indicating that SFS is potential to prevent the generation of thermal cracking. NS can promote the hydration but will not cause tremendous rise in total hydration heat of low clinker cement containing 70% SFBFS leading to risk of thermal cracking. (3) XRD and TG-DTG results confirmed that SFBFS was involved in the pozzolanic reaction, leading to consumption of CH and production of C-S-H. NS was beneficial to improve the pozzolanic reaction of SFBFS. (4) SFBFS can play an important role in improving the pore structure of blended cement pastes. The pore size distribution of low clinker cement containing 70% SFBFS was dominated by nearly 60% gel pores, whereas the large capillary pores only accounted for 2.77%. Besides, NS made significant contribution in reducing porosity of low clinker cement. CRediT authorship contribution statement Wenguang Jiang: Investigation, Data curation, Writing - original draft, Writing - review & editing. Xiangguo Li: Conceptualization, Funding acquisition, Project administration, Supervision, Writing - review & editing. Yang Lv: Data curation, Funding acquisition, Writing - review & editing. Dongbing Jiang: Investigation. Zhuolin Liu: Methodology. Chenhao He: Investigation. Acknowledgments The authors are grateful for the financial support provided by the National Natural Science Foundation of China (51572207, 51808420) and the Fundamental Research Funds for the Central Universities (WUT: 2019IVB058). Declaration of Competing Interest We declare that there is no conflict of interest in this work. References [1] J. Li, W. Zhang, C. Li, P.J.M. Monteiro, Green concrete containing diatomaceous earth and limestone: workability, mechanical properties, and life-cycle assessment, J. Clean. Prod. 223 (2019) 662–679. [2] B. Ma, X. Li, X. Wang, R. Dong, H. Zhu, Study on environmental load and the control approach in cement industry, Cem. Eng. 2 (2005) 79–82. [3] A. Schoeler, B. Lothenbach, F. Winnefeld, M. Zajac, Hydration of quaternary Portland cement blends containing blast-furnace slag, siliceous fly ash and limestone powder, Cem. Concr. Compos. 55 (2015) 374–382. [4] Y.C. Díaz, S.S. Berriel, U. Heierli, A.R. Favier, I.R.S. Machado, K.L. Scrivener, J.F.M. Hernández, G. Habert, Limestone calcined clay cement as a low-carbon solution to meet expanding cement demand in emerging economies, Dev. Eng. 2 (2017) 82–91. [5] N. Mahasenan, S. Smith, K. Humphreys, The cement industry and global climate change: current and potential future cement industry CO2 emissions, in: Y. Kaya (Ed.), Greenhouse Gas Control Technologies – 6th International Conference, Pergamon, Oxford, 2003, pp. 995–1000. [6] D. Xu, Y. Cui, H. Li, K. Yang, W. Xu, Y. Chen, On the future of Chinese cement industry, Cem. Concr. Res. 78 (2015) 2–13. [7] X. Chai, H. Kao, T. Guo, Q. Zhang, Study on thermal energy recovery methods of cement industry basing on the energy balance methods, J. Bull. Chin. Ceram. 32 (4) (2013) 692–698. [8] S.A. Ishak, H. Hashim, Low carbon measures for cement plant a review, J. Clean. Prod. 103 (2015) 260–274.
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