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Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials
Journal of Cleaner Production xxx (xxxx) xxx
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Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials Zhaofeng Li*, Jian Zhang, Shucai Li, Yifan Gao, Chao Liu, Yanhai Qi Geotechnical and Structural Engineering Research Center, Shandong University, Jinan, Shandong, 250061, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 June 2019 Received in revised form 20 August 2019 Accepted 5 October 2019 Available online xxx
Grouting technology is widely used in the field of disaster prevention and in the control of underground engineering. Ordinary Portland cement based grouts are the most widely used grouting material at present. But it has the shortcomings of high energy consumption and high engineering cost. The development of a solid waste grouting material has become a current research effort in the field. This study examined the effect of incorporating gypsum dihydrate (NG), flue gas desulfurization (FGD) gypsum, and phosphogypsum (PG) on the workability and mechanical properties of red mud-slag grouting materials with different gypsum contents. Test results indicate that the incorporation of gypsums can decrease the fluidity and shorten the setting time of red mud-slag grouting materials. All of the specimens tested have rheological characteristics which follow the Herschel-Buckley model. All of the different gypsums significantly increased the compressive strength of the grout samples. Analysis of the hydration process, X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), infrared spectroscopy (IR), and scanning electron microscopy - energy dispersive spectrometry (SEM-EDS) showed that the increase in the compressive strength was due to the increase in the concentration of Al3þ and Si4þ 3þ leached from red mud and slag in the presence of SO2 4 and the generation of additional CeSeH. Fe was shown to participate in the hydration process of the grout in the presence of SO2 4 . These results could lead to high-performance, low-cost grouting materials for grouting engineering, and promote the synergistic utilization of slag, red mud, and solid waste gypsum, to protect the ecological environment. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Zhen Leng Keywords: Gypsum Red mud Grouting material Rheological characteristics Hardening process Mechanical property
1. Introduction The 21st century is the age of underground space. The scale of underground buildings, such as tunnels and metros, is increasing and the scale and quantity of geological hazards are increasing at the same time. Grouting, as the main method of disaster control for these underground buildings, has been widely used and achieved good results (Li et al., 2016; Paul et al., 2015). Ordinary Portland cement slurry is currently one of the most commonly used grouting materials (Zilong Zhou et al., 2019). Even though cement slurry is commonly used, it has disadvantages such as long setting times, high cost, and high energy consumption in preparation (C. Chen et al., 2010; M.Schneider et al., 2011), which does not meet the requirements for green, ecological engineering construction. A green, environmental grouting material is needed in order to realize the environment-friendly and low cost construction of
* Corresponding author. E-mail address: [email protected] (Z. Li).
underground engineering projects. Red mud is a solid powder waste discharged during the production of alumina (M. A. Khairul et al., 2019; Emile Mukiza et al., 2019). Approximately 120 million tons of red mud are produced annually around the world. Red mud has a harmful effect on human beings, animals, plants, and the ecological environment, and it is difficult to disposal (J. He et al., 2014). Geopolymer is a term of three dimensional networks which are considered as potential substitutes for ordinary Portland cement (OPC) because it has high compressive strength, short setting times, anti-corrosion proper_ ties, and low carbon emission (J. Davidovits, 1989, 2019; Piotr Rozek et al., 2019). Geopolymers are considered to be an effective way to prepare cementitious materials from solid waste resources (Wang et al., 2017; Peng Zhang et al., 2018; Rafia Firdous et al., 2018). Red mud mainly consists of Al2O3, SiO2, Fe2O3, and Na2O, and it has potential to be used in geopolymers. In recent years, much work has been carried out on red mud-based geopolymers (Arie van Riessen et al., 2013; N. Ye et al., 2016). Kardelen Kaya and SoyerUzun (2016) investigated the microstructure and mechanical
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Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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properties of red mud-metakaolin based geopolymers with different red mud content. Singh et al. (2018) identified the effect of mechanical activation of the red mud and curing methods on the strength of a red mud-fly ash geopolymer. Cui et al. (2017) investigated the electrical conductance, setting time, shrinkage, and mechanical properties of red mud-slag geopolymer. Nie et al. (2016) investigated the feasibility of using Bayer red mud to make geopolymers. Ye et al. (2016) investigated the strength evolution of a red mud-slag geopolymer over 6 years, the results showed that this geopolymer is susceptible to carbonation in an atmospheric environment. The existing body of research, including our previous research results, shows that red mud is mainly used as filling material in the preparation of geopolymers with metakaolin, blast-furnace slag, or fly ash, because of its low activity. The amount of red mud which is participating in the formation of the geopolymer is limited (Rafia Firdous et al., 2018).In order to use red mud in the field of cementitious materials, red mud must be activated and the mechanical properties of red mud-based geopolymers must be improved. Many researchers have shown that blended solid waste can prepare high cementitious materials with high mechanical properties, due to chemical, physical, and mineralogical synergistic effects (Kumar and Kumar, 2013; M. Zhang et al., 2014; Siyu Duan et al., 2018; Wang et al., 2017). As typical solid wastes, flue gas desulfurization (FGD) gypsum (J.I. Escalante-García et al., 2009) and phosphogypsum (PG) have received a lot of attention for use in cementitious materials (Vaiciukyniene et al., 2018). Presently, FGD is mainly used in the cement and building materials industry, as pavement base, and in cementing systems (Caillahua and Moura, 2018). PG is mainly used in building materials preparation and agricultural production. The chemical composition of FGD and PG are mainly CaO and sulfate. In theory, gypsums are expected to accelerate the geopolymerization of red mud and blast-furnace slag with a synergistic effect and are expected to improve the mechanical properties of the resulting red mud-based geopolymer. This present study explored the feasibility of preparing grouting material from red mud and investigated the synergistic effects in the setting time, hardening process, mechanical properties of red mud-slag grouting materials with different gypsums. Gypsum dehydrate (NG), FGD, PG were used to study the effect of the gypsum on the properties of red mud-slag grouting materials. XRD, IR, and SEM-EDS were used to analyze the mineral composition and microstructure characteristics of the red mud-slag-gypsum composites. 2. Experimental 2.1. Raw materials The Bayer red mud and FGD were collected from the Xinfa group, Shandong, China. PG was obtained from the Shikefeng Chemical Co., Ltd. The blast furnace slag (slag) was brought from the Luxin group, Shandong, China. The alkali activator, sodium hydroxide, and high purity natural gypsum (NG) were bought from
the Tianjin Dengke Chemical Reagent Co., Ltd. The chemical compositions and mineral components of red mud, BFS, and different gypsums were analyzed by X-ray fluorescence (XRF) and X-ray diffraction (XRD) and the results are shown in Table 1 and Fig. 1. NG contains impurities such as carbonate and clay minerals. FGD also contains impurities such as Fe2O3 and fly ash due to the desulfurization process. PG is an acidic material with a pH value of 3.2. Apart from the main components listed in Table 1, PG contains small amounts of impurities such as phosphoric acid, phosphate, and acid insoluble substances. The main mineral components of red mud are katoite, hematite, cancrinite, gibbsite, and muscovite. The main mineral components of NG, FGD, and PG are CaSO4$2H2O, and CaSO4. FGD and PG also contains a small amount of CaSO4$1/2H2O. As can be seen in Fig. 2, the morphology of three kinds of gypsum are mainly plate and sheet, NG has the smallest particle size and PG has the largest particle size.
2.2. Experimental procedures In the experiments, the mass ratio of red mud to slag was 1:1 and the water to binder ratio was designed as 1.0, which is widely used in grouting engineering applications. Red mud-slag grout with different gypsums are denoted as RM-control, RM-NG, RM-FGD, and RM-PG. The amount of different gypsums in the grout samples are 0 wt %, 2 wt %, 4 wt %, 6 wt %, 8 wt %, and 10 wt %. An alkali activator solution with the concentration of 8 wt % was prepared and cooled to room temperature before sample preparation. Red mud, slag, gypsums, and the alkali activator solution were stirred uniformly according to the design ratio according to the operation of cement paste, and then poured into 40 mm 40 mm 40 mm molds to prepare samples. The samples were demolded after 24 h, and then kept in water at 20 C. The compressive strengths were tested at different curing time by the uniaxial compressive strength test according to JGJ/T70-2009 (3 samples were tested at each curing age, and the average values were obtained). The fluidity was tested according to Chinese standard of GB/ T8077-2000. The rheological characteristics were determined by HAAKE MARS 40 from Thermo Fisher Scientific Inc, USA. The setting time of red mud-slag grout was measured by the Vicat test. Based on ASTM Standard C191, the initial and final setting time was recorded when the penetration height of the Vicat needle was 25 mm and less than 1 mm. The hardening process was recorded by measuring the temperature evolution during the first 3 days using a hydration heat meter (Tianjin Gangyuan Instrument Co., Ltd.). The total mass of the tested grout paste was constant at 800 g. IR analysis was performed using a Nicolet iS50 spectrometer from Thermo Fisher Scientific Inc, USA. The mineralogical morphology of the red mud-slag grout was investigated by scanning electron microscopy (SEM, Thermo Fisher Quattro S). All specimens analyzed for microstructure were firstly crushed into pieces and stored in alcohol for at least 24 h, and then dried at 60 C for 24 h.
Table 1 Chemical compositions of the raw materials. Material
SiO2 (%)
Al2O3 (%)
Fe2O3 (%)
CaO (%)
MgO (%)
SO3 (%)
Na2O (%)
K2O (%)
Ti (%)
Red mud slag NG FGD PG
14.4 20.5 3.50 1.01 5.87
22.2 12.1 0.49 0.39 1.05
40.2 0.55 0.14 0.10 0.39
2 57.2 35.18 35.13 31.0
0.17 5.05 1.68 0.20 0.01
0.28 0.83 38.84 42.74 43.0
12.7 0.36 0.69 0.90 -
0.11 0.58 0.09 0.10 -
6.19 1.6 -
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 1. XRD patterns of red mud and different gypsums.
Fig. 2. SEM images of different gypsums.
3. Results and discussion 3.1. Flow characteristic The flow characteristics of a grouting material directly determine its pumpability, the fluidity, and rheological characteristics. The flow characteristics of red mud-slag grout with different gypsums are shown in Fig. 3 and Fig. 4. Fig. 3 shows that the flow diameter of the red mud-slag based grout slurry was 33 cm. The flow diameter decreased when gypsums were added to the composite, and the reduction of the flow diameter increased with the increase of the gypsum content. This phenomenon is related to the fact that the main component of gypsum is CaSO4$2H2O, which has a monoclinic structure. The CaSO4$2H2O particles are mostly acicular, prismatic or hexagonal plate-like, so in the presence of water, increasing the content of gypsums will reduce the fluidity. We can also conclude from Fig. 3 that the NG has the greatest effect on the flowability, followed by PG, and FGD has the least influence. This is likely caused by the finer particle size and clay mineral impurities in NG. In order to ensure the fluidity of red mud-slag paste, solid waste gypsum and its content should be properly selected. Rheology can characterize the intrinsic relationship between the internal structure and macroscopic properties of materials. The rheological properties of grout can guide the design of grouting parameters. The rheological characteristics of red mud-slag grout with different gypsums are shown in Fig. 4. The Herschel-Buckley
Fig. 3. Effect of different gypsums on the fluidity of red mud-slag grout. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
model can be used to describe the rheology of the specimens. The curves were fit to the Herschel-Buckley model and the fitting parameters are shown in Table 2. The flow coefficient, n, was larger than 1 and a shear thickening behavior is found for all specimens. The flow coefficient increased with the amount of gypsum, which is consistent with the trend seen in the flowability measurements.
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 4. Effect of different gypsums on the rheological characteristics of red mud-slag grout. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 2 Fitting results of the Herschel-Buckley model. Specimen
Fitting equation
R
RM-control RM-NG-2 wt. % RM-FGD-2 wt. % RM-PG-2 wt. % RM-NG-6 wt. % RM-FGD-6 wt. % RM-PG-6 wt. % RM-NG-10 wt % RM- FGD-10 wt % RM-PG-10 wt %
t ¼ 14.15 þ 0.001g t ¼ 9.88 þ 0.008g2.07 t ¼ 13.36 þ 0.007g2.56 t ¼ 14.7 þ 0.002g2.84 t ¼ 28.4 þ 0.00003g3.99 t ¼ 23.6 þ 0.00005g3.04 t ¼ 38.6 þ 0.00004g3.05 t ¼ 42.3 þ 0.000004g4.28 t ¼ 27.6 þ 0.000002g3.81 t ¼ 33.7 þ 0.000026g3.55 2.27
0.9957 0.9930 0.9795 0.9872 0.9928 0.9958 0.9649 0.9748 0.9910 0.9644
The curves of yield stress are consistent with the result of flow diameter. The incorporation of different gypsums all increased the yield stress, which indicates that the grouting pressure should be increased to ensure better fluidity of the slurry. 3.2. Setting time Setting time is an important technical parameter for grouting materials, which has influence on the pumpability of grout and grouting effect. The initial and final setting times of red mud-slag grout with different gypsums is shown in Fig. 5. The incorporation of different gypsums in the red mud-slag grout shortened their setting times significantly. The results show that the setting time decreased at first and then increased with the amount of gypsum in the grout. This effect can be explained by the fact that the Ca2þ in gypsum can react with silicate and more CeSeH would be formed, which would promote slurry condensation (Rashad, 2017), Also, the SO2 4 in gypsum can promote the leaching reaction of Al3þ and Si4þ ions in BFS and red mud, similar to the mechanism of Na2SO4 (Peng Rao et al., 2019). Fig. 4 also shows that NG has the most significant effect on the setting time of red mud-slag grout, while the PG has the largest setting times. This is because FGD and PG have a small amount of impurities which hinder the geopolymerization process, and the acid impurities in PG weaken the alkalinity of the system. It was observed that the initial and final setting time have an optimum value of gypsum dosage, which were 6 wt % and 8 wt %, respectively. This is probably because when the dosage of gypsum is less
than the optimum value, gypsum has a positive effect on the stimulation of the geopolymerization reaction. While when the dosage is larger than the optimum value, the hydration products are attached to the surface of the raw material particles, which prevents further hydration, which increases the setting time. But on the whole, they still had the function of coagulation promotion. 3.3. Mechanical properties The compressive strength of red mud-slag grout pastes with different gypsum content are shown in Fig. 5. The 3 d and 28 d compressive strengths of red mud-slag grout paste are 5.2 MPa and 9.3 MPa, respectively. Fig. 6 shows that NG and FGD improve the mechanical properties of red mud-slag grout significantly, and PG has a threshold concentration before the mechanical enhancement is observed (higher than 4 wt %). This increase in the compressive strength was attributed to the reaction between the Ca2þ in the gypsum and silicate, forming CeSeH. SO2 4 in the gypsum can also react with CeSeH and generate ettringite, which will result in a compaction of the paste matrix and also improve the compressive strength (Kornkanok Boonserm et al., 2012) In addition, SO2 4 ions can also promote the dissolution of Al3þ and Si4þ ions from BFS and red mud (Peng Rao et al., 2019) forming more geopolymer gel, thus improving the compressive strength. It can also be seen that NG and FGD have a more significant effect on the enhancement of mechanical properties than PG. This is probably because the NG has the highest purity, and the impurities of PG decrease the mechanical properties. The enhancement of the mechanical properties by NG and PG both increased at first and then decreased with increasing content, while the compressive strength of RM-FGD decreased firstly and then increased with the increment of FGD content. As shown in Fig. 7, red mud-slag grout has a high early strength growth and the compressive strength increases slowly after curing during the first 7 days. It can be observed that, RM-NG has the highest mechanical strength and growth rate, which is related to the higher purity of this gypsum. RM-PG has the slowest strength growth rate of the samples with gypsum, which is due to its acid impurities which slowed the geopolymerization reaction in the alkaline environment. The growth rate of RM-PG was still slightly higher than the control group.
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 5. Effect of different gypsums on the setting time of red mud-slag geopolymer pastes. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Effect of different gypsums on the mechanical properties of red mud-based grout paste (horizontal line in the figure is the compressive strength of control group).
3.4. Hardening process
Fig. 7. The mechanical strength evolution of red mud-slag grout with different gypsums. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
The hardening process of the red mud-slag grout with different gypsums was studied at 8.0 wt % gypsum. Fig. 8 shows that the control group and red mud-slag with different gypsums have similar patterns of temperature evolution, which includes a dissolution period, an introduction period, an acceleration period, and a stable period. The dissolution period indicates Si4þ and Al3þ were leached from the raw materials and an intermediate complex was formed. The introduction period implies that the formed intermediate complex covers the surface of the red mud and slag particles, which prevents further dissolution of Si4þ and Al3þ, thus slowing down the reaction rate. In the acceleration period, a large quantity of N-A-S-H and CeSeH was formed. As more geopolymer gel was formed, the gradual decreasing of the rate of temperature growth corresponds to slow reactions, because unreacted particles have more difficulty moving through the hydration products, bringing about the start of the stable period. It can be seen from Fig. 8 that in the dissolution and introduction
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 8. Heat evolution of red mud-based grout pastes prepared with different gypsums. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
period, RM-FGD has the highest temperature growth rate, followed by RM-PG, RM-NG, and the control group has the lowest growth rate. The order of temperature peak values is RM-PG, RM-FGD, RMNG and the control group. The peak value of temperature appears at 13.8 h, 12.6 h, 11.2 h, and 15.3 h for the specimens of RM-NG, RMFGD, RM-PG, and the control group, respectively. This further proves that the SO2 4 in the gypsum can change the charge distribution of the system, and accelerate the leaching of Si4þ and Al3þ. The peak temperature values of RM-FGD and RM-PG are higher than that of the control group, which is related to the presence of CaSO4$1/2 H2O and CaSO4 in FGD and PG. The highest temperature of RM-PG may be attributed to the neutralization reaction between the alkali activator and acidic impurity. The 3 d total heat release of red mud-slag grout with different gypsums is shown in Fig. 8. It was observed that the total heat released for the control group was 39.0 J/g. The total heat released from RM-NG, RM-FGD, RM-PG were 53.6 J/g, 40.1 J/g, 68.2 J/g, respectively.
3.5. Microstructure analysis 3.5.1. Pore structure The pore structures of grouting materials directly affect its impermeability and service safety. The pore structures of red mudslag grout with different gypsums were analyzed using mercury intrusion porosimetry (MIP), and the resulting pore size distributions and porosities are shown in Fig. 9. As can be seen, the main pore sizes of RM-control, RM-NG, RM-FGD, RM-PG are 707 nm, 27 nm, 47 nm, and 59 nm, respectively, and the corresponding porosities are 35.4%, 23.98%, 24.69%, and 28.13%. According to previous works, pore sizes can be divided into three classes: small gel pores (3.5e10 nm), large gel pores (10e100 nm), and capillary pores (0.1 mm to several micrometers) (Zuhua Zhang et al., 2016). It is noted that RM-control has capillary pores, while when the gypsums were added in to the red mud-slag grout, the pore size decreased and became small gel pores. This phenomenon illustrates that more hydration products were formed in the presence of gypsum. This further confirms the conclusions drawn from the setting time, compressive strength, and hardening process results. In addition, the variation of the porosity is consistent the mechanical strength, because the grout paste matrix is a multi-phase
and heterogeneous porous material. After hardening, many holes of different sizes and shapes will be produced, which seriously affects the mechanical strength of the grout. The compressive strength of cementitious materials increases linearly with density (Ali Ugur Ozturk and Bulent Baradan, 2008). 3.5.2. XRD analysis Fig. 10 shows the XRD pattern of red mud-control group and the composites with different kinds of gypsums. For the sample from the red mud-control group, diffusion peaks around 2q values of 20e40 indicate the existence of the geopolymer gel and the main hydration products were unnamed zeolite (PDF no. 31e1271), geopolymer gel, CeSeH, and unreacted minerals, like hematite (PDF no. 33e0664). It can be observed from Fig. 10 that the addition of 8 wt % gypsum to the red mud-slag grout produced the additional hydration products of ferrogerite (PDF no. 31e0617), calcium aluminum oxide carbonate hydrate. The intensity of the peaks from CeSeH increased while the peaks from hematite decreased in the presence of gypsums. The improvement in the compressive strength was attributed to the presence of these crystalline phases (Cho and Choi, 2016) This further proves that the Ca2þ in gypsum can react with silicate and form CeSeH and NeS-A-C-H. In addition, the SO2 in gypsum changed the charge distribution and 4 promoted the dissolution of Al3þ and Si4þ ions from the BFS and red mud, leading to a stronger geopolymer network. The presence of ferrogerite indicated that hematite can participate in the geopolymerization process in the presence of gypsum, which can also enhance the mechanical properties of the grout samples. 3.5.3. IR analysis The IR spectra of red mud-slag based grout pastes with different gypsums are shown in Fig. 11. The peaks between 442 and 712 cm1 are the bending vibrations of SieO-T (T ¼ Si, Al, Fe) or FeeO, and the peak at 994 cm1 is the symmetric stretching vibration of SieOeSi or SieOeAl, which indicates that the geopolymerization took place and the geopolymer gel was generated (Mohamed R. El-Naggar and Mohamed I. El-Dessouky, 2017). The peaks at 1110 cm1 proved the presence of CeSeH or NeC-A-S-H. The peak at approximately 1427 cm1 is the stretching vibration of OeCeO. The peak at 1646 cm1 is the bending vibration of OH or H2O. In addition, the 1 peaks of SO2 4 were covered by other peaks. The peaks at 873 cm
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 9. Effect of different gypsums on the setting time of red mud-slag grout paste. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 10. XRD patterns of red mud-slag grout paste prepared with different gypsums. (FFerrogerite, H-Hematite, C-Calcium aluminium oxide carbonate hydrate, S-Silicate calcium hydrate, Z-Unnamed zeolite). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 11. IR spectra of red mud-slag based grout pastes prepared by different gypsums. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
are the anti-symmetric stretching and out-of-plane bending modes of CO2 3 ions, which indicates the presence sodium carbonate, and is attributed to the carbonation process of unreacted Na2O and CO2 (Xiao Xue et al., 2018; Rattanasak and Chindaprasirt, 2009). This also proved that the hydration product, Ca(OH)2, reacted with CO2 and CaCO3 was formed. As can be seen from Fig. 11, the peaks around 994 cm1 of RMNG, RM-FGD and RM-PG are all wider than in the RM-control, which proves that there is more geopolymer gel formed in red mud-slag based grout in the presence of different gypsums. This illustrates that gypsum can improve the mechanical properties of 3þ the grout, because the SO2 and 4 ions promote the leaching of Al 4þ Si , forming more geopolymer gel and enhancing the compressive strength. The peaks at 873 cm1 and 1110 cm1 indicate that there are more CeSeH and NeC-A-S-H when gypsum is added into the red mud-slag grout. This is because the Ca2þ reacted with silicate and formed additional CeSeH and it can also took part in the geopolymerization process. The intensity of the peaks related to the FeeO bonds decreased when gypsums were added in to the red mud-slag based grout. The variation of FeeO bands suggest that hematite participated in the geopolymerization process in the presence of gypsums (Smita Singh et al., 2018). 3.5.4. SEM-EDS analysis The SEM of red mud-slag grout pastes with different gypsums are shown in Fig. 11. The incorporation of gypsums can improve the homogeneity and denseness of the paste matrix, because more hydration products were formed with the presence of gypsums. Fig. 12 and Table 3 show that in the control group, the main hydration products are geopolymer gel and CeSeH. When NG, FGD, or PG was added to the red mud-slag grout paste, the paste matrixpaste matrix became compact and more CeSeH, NeC-A-S-H were formed. This further proves the conclusions which were determined from the analysis of the XRD and IR results. There is more C-A-S-H formed in the samples of RM-NG and RM-FGD, because the Ca2þ ions in the gypsum react with aluminosilicate gel and silicate in the composite. This also proves that the compressive strengths of RM-NG and RM-FGD are higher than that of RM-PG. We also observe from the SEM-EDS analysis that regarding the three EDS analysis point of control group, the main hydration product was N-(C)eS-A-H, while RM-NG, RM-FGD, and RM-PG have more Fe ions doped into the structure of the geopoplymer gel, which were highlighted with blue circle in Table 3. This
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 12. SEM photography of red mud-slag grout ( 2500). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Table 3 The elements in the areas of the specimens scanned by EDS. Points 1, 2, and 3 refer to the positions noted in the SEM imaged in Fig. 11. RM-control
O Si Al Fe Ca Na
RM-NG
RM-FGD
RM-PG
Point 1(%)
Point 2(%)
Point 3(%)
Point 1(%)
Point 2(%)
Point 3(%)
Point 1 (%)
Point 2(%)
Point 3(%)
Point 1(%)
Point 2(%)
Point 3(%）
67.08 6.00 5.62 1.68 5.33 6.72
67.58 6.13 4.80 1.21 3.51 8.48
68.46 4.75 5.75 1.24 8.61 4.74
43.7 3.78 5.77 1.41 18.53 2.29
39.07 9.71 11.13
49.5 6.67 6.56 2.29 6.40 6.03
46.37 4.99 5.77 1.68 11.59 3.46
53.37 6.81 6.47 1.41 7.48 5.42
44.78 6.77 8.79
47.85 5.84 8.70 1.25 5.43 2.15
45.31 6.19 8.26 1.61 7.11 2.08
41.20 6.94 10.29
8.00 7.78
indicates that hematite has participated in the geopolymerization reaction in the presence of gypsums. This result is consistent with the XRD and FTIR analysis, and this was shown previously by Smita Singh et al. (2018). 4. Conclusions The results showed that the properties of red mud-slag grout were enhanced with the addition of gypsum dihydrate (NG), flue gas desulfurized gypsum (FGD), and phosphogypsum (PG). The incorporation of different gypsums can enhance the mechanical properties of red mud-slag grout. The improvement in the compressive strength increases at first and then decreases as the amount of NG and PG increases, and decreases at first and then increases when the concentration of FGD goes from 2 wt % to 10 wt %. XRD, FTIR, and SEM showed that the presence of SO2 4 increased the dissolution of Al3þ and Si4þ from the red mud and BFS, and also promoted Fe3þ to participate in the geopolymerization reaction, thus enhancing the strength of red mud/slag geopolymer. In addition, Ca2þ in the gypsum can react with silicate and formed additional CeSeH or with aluminosilicate groups and improve the compressive strength of the paste matrix. The incorporation of gypsums can improve the homogeneity and the denseness the paste matrix of the red mud-slag grout. Acknowledgements This study was financially supported by the Young Scientists Funds of National Natural Science Foundation of China (Grant No. 51709158), the Major Basic Project of Shandong Provincial Natural Science Foundation of China (Project No. ZR2017ZC0734), the China Postdoctoral Science Foundation Funded Project (No. 2018M632676). References Arie van Riessen, Jamieson, Evan, Kealley, Catherine S., Hart, Robert D., Williams, Ross P., 2013. Bayer-geopolymers: an exploration of synergy between the alumina and geopolymer industries. Cement Concr. Compos. 41, 29e33. Boonserm, Kornkanok, Sata, Vanchai, Pimraksa, Kedsarin, Chindaprasirt, Prinya, 2012. Improved geopolymerization of bottom ash by incorporating fly ash and using waste gypsum as additive. Cement Concr. Compos. 34, 819e824. , 2018. Technical feasibility for use Caillahua, Mariella Cortez, Moura, Francisco Jose of FGD gypsum as an additive setting time retarder for Portland cement. J. Mater. Res. Technol. 7 (2), 190e197. Chen, C., Habert, G., Bouzidi, Y., Jullien, A., 2010. Environmental impact of cement production: detail of the different processes and cement plant variability evaluation. Clean. Prod. 18 (5), 478e485. Cho, Bongsuk, Choi, Hyeonggil, 2016. Physical and chemical properties of concrete using GGBFS-KRslag-gypsum binder. Constr. Build. Mater. 123, 436e443. Davidovits, J., 1989. Geopolymers and geopolymeric new materials. J. Therm. Anal. 35 (2), 429e441. Davidovits, Joseph, Huaman, Luis, Davidovits, Ralph, 2019. Ancient geopolymer in south-American monument. SEM and petrographic evidence. Mater. Lett. 235, 120e124.
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2.90 7.47
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials Zhaofeng Li*, Jian Zhang, Shucai Li, Yifan Gao, Chao Liu, Yanhai Qi Geotechnical and Structural Engineering Research Center, Shandong University, Jinan, Shandong, 250061, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 June 2019 Received in revised form 20 August 2019 Accepted 5 October 2019 Available online xxx
Grouting technology is widely used in the field of disaster prevention and in the control of underground engineering. Ordinary Portland cement based grouts are the most widely used grouting material at present. But it has the shortcomings of high energy consumption and high engineering cost. The development of a solid waste grouting material has become a current research effort in the field. This study examined the effect of incorporating gypsum dihydrate (NG), flue gas desulfurization (FGD) gypsum, and phosphogypsum (PG) on the workability and mechanical properties of red mud-slag grouting materials with different gypsum contents. Test results indicate that the incorporation of gypsums can decrease the fluidity and shorten the setting time of red mud-slag grouting materials. All of the specimens tested have rheological characteristics which follow the Herschel-Buckley model. All of the different gypsums significantly increased the compressive strength of the grout samples. Analysis of the hydration process, X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), infrared spectroscopy (IR), and scanning electron microscopy - energy dispersive spectrometry (SEM-EDS) showed that the increase in the compressive strength was due to the increase in the concentration of Al3þ and Si4þ 3þ leached from red mud and slag in the presence of SO2 4 and the generation of additional CeSeH. Fe was shown to participate in the hydration process of the grout in the presence of SO2 4 . These results could lead to high-performance, low-cost grouting materials for grouting engineering, and promote the synergistic utilization of slag, red mud, and solid waste gypsum, to protect the ecological environment. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Zhen Leng Keywords: Gypsum Red mud Grouting material Rheological characteristics Hardening process Mechanical property
1. Introduction The 21st century is the age of underground space. The scale of underground buildings, such as tunnels and metros, is increasing and the scale and quantity of geological hazards are increasing at the same time. Grouting, as the main method of disaster control for these underground buildings, has been widely used and achieved good results (Li et al., 2016; Paul et al., 2015). Ordinary Portland cement slurry is currently one of the most commonly used grouting materials (Zilong Zhou et al., 2019). Even though cement slurry is commonly used, it has disadvantages such as long setting times, high cost, and high energy consumption in preparation (C. Chen et al., 2010; M.Schneider et al., 2011), which does not meet the requirements for green, ecological engineering construction. A green, environmental grouting material is needed in order to realize the environment-friendly and low cost construction of
* Corresponding author. E-mail address: [email protected] (Z. Li).
underground engineering projects. Red mud is a solid powder waste discharged during the production of alumina (M. A. Khairul et al., 2019; Emile Mukiza et al., 2019). Approximately 120 million tons of red mud are produced annually around the world. Red mud has a harmful effect on human beings, animals, plants, and the ecological environment, and it is difficult to disposal (J. He et al., 2014). Geopolymer is a term of three dimensional networks which are considered as potential substitutes for ordinary Portland cement (OPC) because it has high compressive strength, short setting times, anti-corrosion proper_ ties, and low carbon emission (J. Davidovits, 1989, 2019; Piotr Rozek et al., 2019). Geopolymers are considered to be an effective way to prepare cementitious materials from solid waste resources (Wang et al., 2017; Peng Zhang et al., 2018; Rafia Firdous et al., 2018). Red mud mainly consists of Al2O3, SiO2, Fe2O3, and Na2O, and it has potential to be used in geopolymers. In recent years, much work has been carried out on red mud-based geopolymers (Arie van Riessen et al., 2013; N. Ye et al., 2016). Kardelen Kaya and SoyerUzun (2016) investigated the microstructure and mechanical
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Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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properties of red mud-metakaolin based geopolymers with different red mud content. Singh et al. (2018) identified the effect of mechanical activation of the red mud and curing methods on the strength of a red mud-fly ash geopolymer. Cui et al. (2017) investigated the electrical conductance, setting time, shrinkage, and mechanical properties of red mud-slag geopolymer. Nie et al. (2016) investigated the feasibility of using Bayer red mud to make geopolymers. Ye et al. (2016) investigated the strength evolution of a red mud-slag geopolymer over 6 years, the results showed that this geopolymer is susceptible to carbonation in an atmospheric environment. The existing body of research, including our previous research results, shows that red mud is mainly used as filling material in the preparation of geopolymers with metakaolin, blast-furnace slag, or fly ash, because of its low activity. The amount of red mud which is participating in the formation of the geopolymer is limited (Rafia Firdous et al., 2018).In order to use red mud in the field of cementitious materials, red mud must be activated and the mechanical properties of red mud-based geopolymers must be improved. Many researchers have shown that blended solid waste can prepare high cementitious materials with high mechanical properties, due to chemical, physical, and mineralogical synergistic effects (Kumar and Kumar, 2013; M. Zhang et al., 2014; Siyu Duan et al., 2018; Wang et al., 2017). As typical solid wastes, flue gas desulfurization (FGD) gypsum (J.I. Escalante-García et al., 2009) and phosphogypsum (PG) have received a lot of attention for use in cementitious materials (Vaiciukyniene et al., 2018). Presently, FGD is mainly used in the cement and building materials industry, as pavement base, and in cementing systems (Caillahua and Moura, 2018). PG is mainly used in building materials preparation and agricultural production. The chemical composition of FGD and PG are mainly CaO and sulfate. In theory, gypsums are expected to accelerate the geopolymerization of red mud and blast-furnace slag with a synergistic effect and are expected to improve the mechanical properties of the resulting red mud-based geopolymer. This present study explored the feasibility of preparing grouting material from red mud and investigated the synergistic effects in the setting time, hardening process, mechanical properties of red mud-slag grouting materials with different gypsums. Gypsum dehydrate (NG), FGD, PG were used to study the effect of the gypsum on the properties of red mud-slag grouting materials. XRD, IR, and SEM-EDS were used to analyze the mineral composition and microstructure characteristics of the red mud-slag-gypsum composites. 2. Experimental 2.1. Raw materials The Bayer red mud and FGD were collected from the Xinfa group, Shandong, China. PG was obtained from the Shikefeng Chemical Co., Ltd. The blast furnace slag (slag) was brought from the Luxin group, Shandong, China. The alkali activator, sodium hydroxide, and high purity natural gypsum (NG) were bought from
the Tianjin Dengke Chemical Reagent Co., Ltd. The chemical compositions and mineral components of red mud, BFS, and different gypsums were analyzed by X-ray fluorescence (XRF) and X-ray diffraction (XRD) and the results are shown in Table 1 and Fig. 1. NG contains impurities such as carbonate and clay minerals. FGD also contains impurities such as Fe2O3 and fly ash due to the desulfurization process. PG is an acidic material with a pH value of 3.2. Apart from the main components listed in Table 1, PG contains small amounts of impurities such as phosphoric acid, phosphate, and acid insoluble substances. The main mineral components of red mud are katoite, hematite, cancrinite, gibbsite, and muscovite. The main mineral components of NG, FGD, and PG are CaSO4$2H2O, and CaSO4. FGD and PG also contains a small amount of CaSO4$1/2H2O. As can be seen in Fig. 2, the morphology of three kinds of gypsum are mainly plate and sheet, NG has the smallest particle size and PG has the largest particle size.
2.2. Experimental procedures In the experiments, the mass ratio of red mud to slag was 1:1 and the water to binder ratio was designed as 1.0, which is widely used in grouting engineering applications. Red mud-slag grout with different gypsums are denoted as RM-control, RM-NG, RM-FGD, and RM-PG. The amount of different gypsums in the grout samples are 0 wt %, 2 wt %, 4 wt %, 6 wt %, 8 wt %, and 10 wt %. An alkali activator solution with the concentration of 8 wt % was prepared and cooled to room temperature before sample preparation. Red mud, slag, gypsums, and the alkali activator solution were stirred uniformly according to the design ratio according to the operation of cement paste, and then poured into 40 mm 40 mm 40 mm molds to prepare samples. The samples were demolded after 24 h, and then kept in water at 20 C. The compressive strengths were tested at different curing time by the uniaxial compressive strength test according to JGJ/T70-2009 (3 samples were tested at each curing age, and the average values were obtained). The fluidity was tested according to Chinese standard of GB/ T8077-2000. The rheological characteristics were determined by HAAKE MARS 40 from Thermo Fisher Scientific Inc, USA. The setting time of red mud-slag grout was measured by the Vicat test. Based on ASTM Standard C191, the initial and final setting time was recorded when the penetration height of the Vicat needle was 25 mm and less than 1 mm. The hardening process was recorded by measuring the temperature evolution during the first 3 days using a hydration heat meter (Tianjin Gangyuan Instrument Co., Ltd.). The total mass of the tested grout paste was constant at 800 g. IR analysis was performed using a Nicolet iS50 spectrometer from Thermo Fisher Scientific Inc, USA. The mineralogical morphology of the red mud-slag grout was investigated by scanning electron microscopy (SEM, Thermo Fisher Quattro S). All specimens analyzed for microstructure were firstly crushed into pieces and stored in alcohol for at least 24 h, and then dried at 60 C for 24 h.
Table 1 Chemical compositions of the raw materials. Material
SiO2 (%)
Al2O3 (%)
Fe2O3 (%)
CaO (%)
MgO (%)
SO3 (%)
Na2O (%)
K2O (%)
Ti (%)
Red mud slag NG FGD PG
14.4 20.5 3.50 1.01 5.87
22.2 12.1 0.49 0.39 1.05
40.2 0.55 0.14 0.10 0.39
2 57.2 35.18 35.13 31.0
0.17 5.05 1.68 0.20 0.01
0.28 0.83 38.84 42.74 43.0
12.7 0.36 0.69 0.90 -
0.11 0.58 0.09 0.10 -
6.19 1.6 -
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 1. XRD patterns of red mud and different gypsums.
Fig. 2. SEM images of different gypsums.
3. Results and discussion 3.1. Flow characteristic The flow characteristics of a grouting material directly determine its pumpability, the fluidity, and rheological characteristics. The flow characteristics of red mud-slag grout with different gypsums are shown in Fig. 3 and Fig. 4. Fig. 3 shows that the flow diameter of the red mud-slag based grout slurry was 33 cm. The flow diameter decreased when gypsums were added to the composite, and the reduction of the flow diameter increased with the increase of the gypsum content. This phenomenon is related to the fact that the main component of gypsum is CaSO4$2H2O, which has a monoclinic structure. The CaSO4$2H2O particles are mostly acicular, prismatic or hexagonal plate-like, so in the presence of water, increasing the content of gypsums will reduce the fluidity. We can also conclude from Fig. 3 that the NG has the greatest effect on the flowability, followed by PG, and FGD has the least influence. This is likely caused by the finer particle size and clay mineral impurities in NG. In order to ensure the fluidity of red mud-slag paste, solid waste gypsum and its content should be properly selected. Rheology can characterize the intrinsic relationship between the internal structure and macroscopic properties of materials. The rheological properties of grout can guide the design of grouting parameters. The rheological characteristics of red mud-slag grout with different gypsums are shown in Fig. 4. The Herschel-Buckley
Fig. 3. Effect of different gypsums on the fluidity of red mud-slag grout. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
model can be used to describe the rheology of the specimens. The curves were fit to the Herschel-Buckley model and the fitting parameters are shown in Table 2. The flow coefficient, n, was larger than 1 and a shear thickening behavior is found for all specimens. The flow coefficient increased with the amount of gypsum, which is consistent with the trend seen in the flowability measurements.
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 4. Effect of different gypsums on the rheological characteristics of red mud-slag grout. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 2 Fitting results of the Herschel-Buckley model. Specimen
Fitting equation
R
RM-control RM-NG-2 wt. % RM-FGD-2 wt. % RM-PG-2 wt. % RM-NG-6 wt. % RM-FGD-6 wt. % RM-PG-6 wt. % RM-NG-10 wt % RM- FGD-10 wt % RM-PG-10 wt %
t ¼ 14.15 þ 0.001g t ¼ 9.88 þ 0.008g2.07 t ¼ 13.36 þ 0.007g2.56 t ¼ 14.7 þ 0.002g2.84 t ¼ 28.4 þ 0.00003g3.99 t ¼ 23.6 þ 0.00005g3.04 t ¼ 38.6 þ 0.00004g3.05 t ¼ 42.3 þ 0.000004g4.28 t ¼ 27.6 þ 0.000002g3.81 t ¼ 33.7 þ 0.000026g3.55 2.27
0.9957 0.9930 0.9795 0.9872 0.9928 0.9958 0.9649 0.9748 0.9910 0.9644
The curves of yield stress are consistent with the result of flow diameter. The incorporation of different gypsums all increased the yield stress, which indicates that the grouting pressure should be increased to ensure better fluidity of the slurry. 3.2. Setting time Setting time is an important technical parameter for grouting materials, which has influence on the pumpability of grout and grouting effect. The initial and final setting times of red mud-slag grout with different gypsums is shown in Fig. 5. The incorporation of different gypsums in the red mud-slag grout shortened their setting times significantly. The results show that the setting time decreased at first and then increased with the amount of gypsum in the grout. This effect can be explained by the fact that the Ca2þ in gypsum can react with silicate and more CeSeH would be formed, which would promote slurry condensation (Rashad, 2017), Also, the SO2 4 in gypsum can promote the leaching reaction of Al3þ and Si4þ ions in BFS and red mud, similar to the mechanism of Na2SO4 (Peng Rao et al., 2019). Fig. 4 also shows that NG has the most significant effect on the setting time of red mud-slag grout, while the PG has the largest setting times. This is because FGD and PG have a small amount of impurities which hinder the geopolymerization process, and the acid impurities in PG weaken the alkalinity of the system. It was observed that the initial and final setting time have an optimum value of gypsum dosage, which were 6 wt % and 8 wt %, respectively. This is probably because when the dosage of gypsum is less
than the optimum value, gypsum has a positive effect on the stimulation of the geopolymerization reaction. While when the dosage is larger than the optimum value, the hydration products are attached to the surface of the raw material particles, which prevents further hydration, which increases the setting time. But on the whole, they still had the function of coagulation promotion. 3.3. Mechanical properties The compressive strength of red mud-slag grout pastes with different gypsum content are shown in Fig. 5. The 3 d and 28 d compressive strengths of red mud-slag grout paste are 5.2 MPa and 9.3 MPa, respectively. Fig. 6 shows that NG and FGD improve the mechanical properties of red mud-slag grout significantly, and PG has a threshold concentration before the mechanical enhancement is observed (higher than 4 wt %). This increase in the compressive strength was attributed to the reaction between the Ca2þ in the gypsum and silicate, forming CeSeH. SO2 4 in the gypsum can also react with CeSeH and generate ettringite, which will result in a compaction of the paste matrix and also improve the compressive strength (Kornkanok Boonserm et al., 2012) In addition, SO2 4 ions can also promote the dissolution of Al3þ and Si4þ ions from BFS and red mud (Peng Rao et al., 2019) forming more geopolymer gel, thus improving the compressive strength. It can also be seen that NG and FGD have a more significant effect on the enhancement of mechanical properties than PG. This is probably because the NG has the highest purity, and the impurities of PG decrease the mechanical properties. The enhancement of the mechanical properties by NG and PG both increased at first and then decreased with increasing content, while the compressive strength of RM-FGD decreased firstly and then increased with the increment of FGD content. As shown in Fig. 7, red mud-slag grout has a high early strength growth and the compressive strength increases slowly after curing during the first 7 days. It can be observed that, RM-NG has the highest mechanical strength and growth rate, which is related to the higher purity of this gypsum. RM-PG has the slowest strength growth rate of the samples with gypsum, which is due to its acid impurities which slowed the geopolymerization reaction in the alkaline environment. The growth rate of RM-PG was still slightly higher than the control group.
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 5. Effect of different gypsums on the setting time of red mud-slag geopolymer pastes. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Effect of different gypsums on the mechanical properties of red mud-based grout paste (horizontal line in the figure is the compressive strength of control group).
3.4. Hardening process
Fig. 7. The mechanical strength evolution of red mud-slag grout with different gypsums. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
The hardening process of the red mud-slag grout with different gypsums was studied at 8.0 wt % gypsum. Fig. 8 shows that the control group and red mud-slag with different gypsums have similar patterns of temperature evolution, which includes a dissolution period, an introduction period, an acceleration period, and a stable period. The dissolution period indicates Si4þ and Al3þ were leached from the raw materials and an intermediate complex was formed. The introduction period implies that the formed intermediate complex covers the surface of the red mud and slag particles, which prevents further dissolution of Si4þ and Al3þ, thus slowing down the reaction rate. In the acceleration period, a large quantity of N-A-S-H and CeSeH was formed. As more geopolymer gel was formed, the gradual decreasing of the rate of temperature growth corresponds to slow reactions, because unreacted particles have more difficulty moving through the hydration products, bringing about the start of the stable period. It can be seen from Fig. 8 that in the dissolution and introduction
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 8. Heat evolution of red mud-based grout pastes prepared with different gypsums. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
period, RM-FGD has the highest temperature growth rate, followed by RM-PG, RM-NG, and the control group has the lowest growth rate. The order of temperature peak values is RM-PG, RM-FGD, RMNG and the control group. The peak value of temperature appears at 13.8 h, 12.6 h, 11.2 h, and 15.3 h for the specimens of RM-NG, RMFGD, RM-PG, and the control group, respectively. This further proves that the SO2 4 in the gypsum can change the charge distribution of the system, and accelerate the leaching of Si4þ and Al3þ. The peak temperature values of RM-FGD and RM-PG are higher than that of the control group, which is related to the presence of CaSO4$1/2 H2O and CaSO4 in FGD and PG. The highest temperature of RM-PG may be attributed to the neutralization reaction between the alkali activator and acidic impurity. The 3 d total heat release of red mud-slag grout with different gypsums is shown in Fig. 8. It was observed that the total heat released for the control group was 39.0 J/g. The total heat released from RM-NG, RM-FGD, RM-PG were 53.6 J/g, 40.1 J/g, 68.2 J/g, respectively.
3.5. Microstructure analysis 3.5.1. Pore structure The pore structures of grouting materials directly affect its impermeability and service safety. The pore structures of red mudslag grout with different gypsums were analyzed using mercury intrusion porosimetry (MIP), and the resulting pore size distributions and porosities are shown in Fig. 9. As can be seen, the main pore sizes of RM-control, RM-NG, RM-FGD, RM-PG are 707 nm, 27 nm, 47 nm, and 59 nm, respectively, and the corresponding porosities are 35.4%, 23.98%, 24.69%, and 28.13%. According to previous works, pore sizes can be divided into three classes: small gel pores (3.5e10 nm), large gel pores (10e100 nm), and capillary pores (0.1 mm to several micrometers) (Zuhua Zhang et al., 2016). It is noted that RM-control has capillary pores, while when the gypsums were added in to the red mud-slag grout, the pore size decreased and became small gel pores. This phenomenon illustrates that more hydration products were formed in the presence of gypsum. This further confirms the conclusions drawn from the setting time, compressive strength, and hardening process results. In addition, the variation of the porosity is consistent the mechanical strength, because the grout paste matrix is a multi-phase
and heterogeneous porous material. After hardening, many holes of different sizes and shapes will be produced, which seriously affects the mechanical strength of the grout. The compressive strength of cementitious materials increases linearly with density (Ali Ugur Ozturk and Bulent Baradan, 2008). 3.5.2. XRD analysis Fig. 10 shows the XRD pattern of red mud-control group and the composites with different kinds of gypsums. For the sample from the red mud-control group, diffusion peaks around 2q values of 20e40 indicate the existence of the geopolymer gel and the main hydration products were unnamed zeolite (PDF no. 31e1271), geopolymer gel, CeSeH, and unreacted minerals, like hematite (PDF no. 33e0664). It can be observed from Fig. 10 that the addition of 8 wt % gypsum to the red mud-slag grout produced the additional hydration products of ferrogerite (PDF no. 31e0617), calcium aluminum oxide carbonate hydrate. The intensity of the peaks from CeSeH increased while the peaks from hematite decreased in the presence of gypsums. The improvement in the compressive strength was attributed to the presence of these crystalline phases (Cho and Choi, 2016) This further proves that the Ca2þ in gypsum can react with silicate and form CeSeH and NeS-A-C-H. In addition, the SO2 in gypsum changed the charge distribution and 4 promoted the dissolution of Al3þ and Si4þ ions from the BFS and red mud, leading to a stronger geopolymer network. The presence of ferrogerite indicated that hematite can participate in the geopolymerization process in the presence of gypsum, which can also enhance the mechanical properties of the grout samples. 3.5.3. IR analysis The IR spectra of red mud-slag based grout pastes with different gypsums are shown in Fig. 11. The peaks between 442 and 712 cm1 are the bending vibrations of SieO-T (T ¼ Si, Al, Fe) or FeeO, and the peak at 994 cm1 is the symmetric stretching vibration of SieOeSi or SieOeAl, which indicates that the geopolymerization took place and the geopolymer gel was generated (Mohamed R. El-Naggar and Mohamed I. El-Dessouky, 2017). The peaks at 1110 cm1 proved the presence of CeSeH or NeC-A-S-H. The peak at approximately 1427 cm1 is the stretching vibration of OeCeO. The peak at 1646 cm1 is the bending vibration of OH or H2O. In addition, the 1 peaks of SO2 4 were covered by other peaks. The peaks at 873 cm
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 9. Effect of different gypsums on the setting time of red mud-slag grout paste. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 10. XRD patterns of red mud-slag grout paste prepared with different gypsums. (FFerrogerite, H-Hematite, C-Calcium aluminium oxide carbonate hydrate, S-Silicate calcium hydrate, Z-Unnamed zeolite). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 11. IR spectra of red mud-slag based grout pastes prepared by different gypsums. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
are the anti-symmetric stretching and out-of-plane bending modes of CO2 3 ions, which indicates the presence sodium carbonate, and is attributed to the carbonation process of unreacted Na2O and CO2 (Xiao Xue et al., 2018; Rattanasak and Chindaprasirt, 2009). This also proved that the hydration product, Ca(OH)2, reacted with CO2 and CaCO3 was formed. As can be seen from Fig. 11, the peaks around 994 cm1 of RMNG, RM-FGD and RM-PG are all wider than in the RM-control, which proves that there is more geopolymer gel formed in red mud-slag based grout in the presence of different gypsums. This illustrates that gypsum can improve the mechanical properties of 3þ the grout, because the SO2 and 4 ions promote the leaching of Al 4þ Si , forming more geopolymer gel and enhancing the compressive strength. The peaks at 873 cm1 and 1110 cm1 indicate that there are more CeSeH and NeC-A-S-H when gypsum is added into the red mud-slag grout. This is because the Ca2þ reacted with silicate and formed additional CeSeH and it can also took part in the geopolymerization process. The intensity of the peaks related to the FeeO bonds decreased when gypsums were added in to the red mud-slag based grout. The variation of FeeO bands suggest that hematite participated in the geopolymerization process in the presence of gypsums (Smita Singh et al., 2018). 3.5.4. SEM-EDS analysis The SEM of red mud-slag grout pastes with different gypsums are shown in Fig. 11. The incorporation of gypsums can improve the homogeneity and denseness of the paste matrix, because more hydration products were formed with the presence of gypsums. Fig. 12 and Table 3 show that in the control group, the main hydration products are geopolymer gel and CeSeH. When NG, FGD, or PG was added to the red mud-slag grout paste, the paste matrixpaste matrix became compact and more CeSeH, NeC-A-S-H were formed. This further proves the conclusions which were determined from the analysis of the XRD and IR results. There is more C-A-S-H formed in the samples of RM-NG and RM-FGD, because the Ca2þ ions in the gypsum react with aluminosilicate gel and silicate in the composite. This also proves that the compressive strengths of RM-NG and RM-FGD are higher than that of RM-PG. We also observe from the SEM-EDS analysis that regarding the three EDS analysis point of control group, the main hydration product was N-(C)eS-A-H, while RM-NG, RM-FGD, and RM-PG have more Fe ions doped into the structure of the geopoplymer gel, which were highlighted with blue circle in Table 3. This
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Fig. 12. SEM photography of red mud-slag grout ( 2500). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759
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Table 3 The elements in the areas of the specimens scanned by EDS. Points 1, 2, and 3 refer to the positions noted in the SEM imaged in Fig. 11. RM-control
O Si Al Fe Ca Na
RM-NG
RM-FGD
RM-PG
Point 1(%)
Point 2(%)
Point 3(%)
Point 1(%)
Point 2(%)
Point 3(%)
Point 1 (%)
Point 2(%)
Point 3(%)
Point 1(%)
Point 2(%)
Point 3(%）
67.08 6.00 5.62 1.68 5.33 6.72
67.58 6.13 4.80 1.21 3.51 8.48
68.46 4.75 5.75 1.24 8.61 4.74
43.7 3.78 5.77 1.41 18.53 2.29
39.07 9.71 11.13
49.5 6.67 6.56 2.29 6.40 6.03
46.37 4.99 5.77 1.68 11.59 3.46
53.37 6.81 6.47 1.41 7.48 5.42
44.78 6.77 8.79
47.85 5.84 8.70 1.25 5.43 2.15
45.31 6.19 8.26 1.61 7.11 2.08
41.20 6.94 10.29
8.00 7.78
indicates that hematite has participated in the geopolymerization reaction in the presence of gypsums. This result is consistent with the XRD and FTIR analysis, and this was shown previously by Smita Singh et al. (2018). 4. Conclusions The results showed that the properties of red mud-slag grout were enhanced with the addition of gypsum dihydrate (NG), flue gas desulfurized gypsum (FGD), and phosphogypsum (PG). The incorporation of different gypsums can enhance the mechanical properties of red mud-slag grout. The improvement in the compressive strength increases at first and then decreases as the amount of NG and PG increases, and decreases at first and then increases when the concentration of FGD goes from 2 wt % to 10 wt %. XRD, FTIR, and SEM showed that the presence of SO2 4 increased the dissolution of Al3þ and Si4þ from the red mud and BFS, and also promoted Fe3þ to participate in the geopolymerization reaction, thus enhancing the strength of red mud/slag geopolymer. In addition, Ca2þ in the gypsum can react with silicate and formed additional CeSeH or with aluminosilicate groups and improve the compressive strength of the paste matrix. The incorporation of gypsums can improve the homogeneity and the denseness the paste matrix of the red mud-slag grout. Acknowledgements This study was financially supported by the Young Scientists Funds of National Natural Science Foundation of China (Grant No. 51709158), the Major Basic Project of Shandong Provincial Natural Science Foundation of China (Project No. ZR2017ZC0734), the China Postdoctoral Science Foundation Funded Project (No. 2018M632676). References Arie van Riessen, Jamieson, Evan, Kealley, Catherine S., Hart, Robert D., Williams, Ross P., 2013. Bayer-geopolymers: an exploration of synergy between the alumina and geopolymer industries. Cement Concr. Compos. 41, 29e33. Boonserm, Kornkanok, Sata, Vanchai, Pimraksa, Kedsarin, Chindaprasirt, Prinya, 2012. Improved geopolymerization of bottom ash by incorporating fly ash and using waste gypsum as additive. Cement Concr. Compos. 34, 819e824. , 2018. Technical feasibility for use Caillahua, Mariella Cortez, Moura, Francisco Jose of FGD gypsum as an additive setting time retarder for Portland cement. J. Mater. Res. Technol. 7 (2), 190e197. Chen, C., Habert, G., Bouzidi, Y., Jullien, A., 2010. Environmental impact of cement production: detail of the different processes and cement plant variability evaluation. Clean. Prod. 18 (5), 478e485. Cho, Bongsuk, Choi, Hyeonggil, 2016. Physical and chemical properties of concrete using GGBFS-KRslag-gypsum binder. Constr. Build. Mater. 123, 436e443. Davidovits, J., 1989. Geopolymers and geopolymeric new materials. J. Therm. Anal. 35 (2), 429e441. Davidovits, Joseph, Huaman, Luis, Davidovits, Ralph, 2019. Ancient geopolymer in south-American monument. SEM and petrographic evidence. Mater. Lett. 235, 120e124.
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Please cite this article as: Li, Z et al., Effect of different gypsums on the workability and mechanical properties of red mud-slag based grouting materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118759