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Study on the development of inorganic polymers from red mud and slag system: Application in mortar and lightweight materials
Construction and Building Materials 156 (2017) 486–495
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Study on the development of inorganic polymers from red mud and slag system: Application in mortar and lightweight materials Patrick N. Lemougna a,b,⇑, Kai-tuo Wang a, Qing Tang a, Xue-min Cui a,⇑ a School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China b Local Materials Promotion Authority, MINRESI/MIPROMALO, P.O. Box 2396, Yaounde, Cameroon
h i g h l i g h t s Inorganic polymers from red mud (RM) and slag system were investigated. Better reactivity and faster setting were obtained for higher contents of slag. Up to 50% of RM in the system led to a 7 days strength of 54 MPa at 25 °C. Sand and H2O2 addition led to lightweight and mortar products of 10–70 MPa. The results are of interest for the conversion of red mud and slag by-products.
a r t i c l e
i n f o
Article history: Received 10 July 2017 Received in revised form 31 August 2017 Accepted 5 September 2017
Keywords: Geopolymer Red mud Slag Sustainability Building applications
a b s t r a c t The conversion of red mud into valuable products remains an important issue to be addressed. Inorganic polymers from red mud and slag system were investigated. Slag was substituted at 25, 50 and 75% by red mud in the system. Sodium silicate solutions of modulus ranging from 1.6 to 2.2 were used and the samples were cured at 25, 40 and 60 °C. Slag was amorphous phase while red mud contained some crystalline phases, mainly hematite, crancrinite and katoite. A better reactivity, faster setting and lower shrinkage were obtained for higher contents of slag, with a modulus of sodium silicate solution of 2.0. The composition made of 50% red mud and 50% slag achieved a 7 days compressive strength of 54 MPa at 25 °C. The use of standard sand and hydrogen peroxide allowed the formation of mortar and lightweight materials with reduced mechanical properties, but still of interest to address both issues of sustainable use of resources and CO2 reduction in construction. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The term ‘‘geopolymer” was coined by Davidovits in 1978 to describe inorganic polymers with tridimensional structures formed by low temperature polycondensation of aluminosilicates [1,2]. These materials have received a burgeoning interest during the last decades due to their excellent physical, mechanical and thermal properties [2–9]. The development of geopolymer materials for building applications was reported to be less CO2 footprint in comparison to the production of Ordinary Portland Cement (OPC). Actually, the production of one ton of OPC releases about
⇑ Corresponding authors at: School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China (P.N. Lemougna). E-mail addresses: [email protected] (P.N. Lemougna), [email protected]. cn (X.-m. Cui). http://dx.doi.org/10.1016/j.conbuildmat.2017.09.015 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
1 ton of CO2 and the total OPC production is currently contributing to about 5–7% of global anthropogenic CO2 [7–9]. Besides, optimally formulated geopolymer cement was found to be able to reduce by 80% the CO2 associated to OPC cement production industry [7]. Another important advantage of geopolymers is the opportunity to valorize a wide range of secondary raw materials, including slag and red mud, the usage of which can contribute to the sustainability of non-renewable natural resources such as limestone and clay for building industry. Additional environmental benefits associated to the absence of surface-excavated operations required in the case of natural resources were also reported [10– 12]. Hence, the successful development and use of building materials from industrial by-products such as slag and red mud is expected to contribute to both sustainable development and CO2 reduction of OPC industry, the production of which has passed 4 billion tons in 2015 [13].
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Slag (ground granulated blast furnace slag) is an industrial byproduct material generated from the manufacturing of pig iron. It is well known for its high long-term strength and resistance to deterioration under severe environmental conditions as well as its environmental advantage due to lower CO2 emissions and energy consumption in the development of cementitious materials [12,14]. Beside slag, red mud (RM) is a waste product produced in huge amounts from aluminum industries. It is reported that 1.5– 1.6 tons of RM is generated per ton of alumina produced, most of it stored in landfill areas or on-site waste lakes [11]. The mineral and chemical compositions of red mud depend on the quality of bauxite, and to a lesser degree, the processing parameters [15,16]. The treatment and utilization of huge red mud wastes has been a major challenge for the alumina industry throughout the world. Among the valorization options, the development of structural materials has been reported as a potential area of interest to be explored [17–20]. However, despite the difference in chemical and mineralogical composition of red mud from different origins, many types of red mud were found not to be reactive enough for geopolymer synthesis and as consequence, were generally mixed with more reactive aluminosilicate materials, mainly fly ashes and metakaolin [11,15,21–23]. Potential structural materials based on red mud combined with silica fume, metakaolin or fly ash were reported [11,22–26], but very little has been reported on the development of red mud/slag system. Pan et al. [27] observed that a poorly rich Al red mud, blended with slag, can present acceptable strength and chemical resistance after alkali activation. Considering the divergence of red muds from different locations [16] and the limited information on the mix proportioning or curing temperature in this previous report, further research is needed to explore the possibilities of red mud valorization. Furthermore, an economical widely accepted technology for the recycle and reuse of red mud has yet to be developed, despite the increasing environmental pressure associated to its worldwide annual generation, now surpassing 150 106 t [19,21]. In building engineering, the use of binders or concretes of lower densities is beneficial in terms of structural load-bearing, acoustic and thermal insulation. Adding foaming agents is a common way of reducing the self-weight of binder pastes. Foaming agents generate artificial pores in hardened pastes and reduce their specific gravities. However, the foamed binder pastes are not suitable as load bearing building materials, due to their lower strength, but are efficient for nonstructural applications such as thermal insulation or soundproofing [28,29]. The present study investigated the conversion of red mud and slag into geopolymer materials for potential building applications. Many compositions were prepared by combining slag and red mud with an introduction of up to 75% of red mud in the system. Sodium silicate solutions with a silica modulus ranging from 1.6 to 2.2 were used and the geopolymer slurry was cured at 25– 60 °C. The resulting geopolymers were characterized by X-ray diffraction, Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, Electric conductance, setting time, linear shrinkage and compressive strength. Finally, an application in the development of mortar and lightweight materials was assessed.
487
1.44 m2/g. The oxide composition of the slag and red mud determined by X-ray fluorescence and their particle size information are respectively reported in Tables 1 and 2. The alkaline activating solutions with silica modulus (R = SiO2/Na2O) of 1.6–2.2 with 0.2 interval were prepared by dissolving solid sodium hydroxide in a commercial sodium water glass with R = 3.3. The alkaline activating solutions were sealed and stored for a minimum of 24 h prior to use. 2.2. Specimen preparation The preparation of the fresh mixtures was performed by mixing slag, red mud, water glass of different moduli (R = 1.6; 1.8; 2.0 and 2.2), and some amount of deionized water. The mixing process was performed for about 10 min, using an electric mixer at 600 rpm. The homogenous pastes were then casted in cubic alloy molds of 20 20 20 mm3, covered with a thin layer of plastic to facilitate the removal of the hardened pastes upon curing. The alloy molds were vibrated on a vibration table for 2 min to remove air bubbles and sealed afterwards. The specimens were cured either at 25 °C, 40 °C and 60 °C for 7 days prior to characterization. Some specimens were maintained at 25 °C for 28 days for compressive strength test. For the preparation of slag/red mud/sand geopolymer composites, some amount of standard sand ISO 679: 1989 was added (1/4, 2/4, 3/4 ratios of sand mass/slag + red mud mass). Lightweight materials were obtained by adding H2O2 (AR grade; 30% H2O2) in the mixture made with 50% slag and 50% red mud. The percentages of H2O2 were 0.5; 0.75, 1.00 and 1.25% of the dry mass of red mud and slag. The details on the mix proportioning are presented in Table 3. 2.3. Characterization method 2.3.1. XRD and FTIR analyses The samples were powdered and examined by X-ray diffraction with a Rigaku Mini Flex 600 instrument with Ni-filtered Cu (Ka) radiation, a step size of 0.02°, operated at 40 kV and 15 mA, with a dwell time of 0.5 s and a 2h range of 5–80°. The powdered samples were also pressed into KBr pellets for FTIR analysis using a Thermo Scientific FTIR spectrometer. 2.3.2. SEM/EDS and optical analysis Scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were performed to analyze the polished surfaces of the specimens with an S3400N device (Japan Hitachi Limited Company). Specimens were impregnated using absolute ethyl alcohol, polished with SiC paper, and then coated with gold. Optical analysis of the composite slag red mud geopolymer containing sand was performed using an Olympus optical microscope of type SZ61. 2.3.3. Electric conductance and setting time analysis Geopolymer slurry was prepared and casted into a 40-mm diameter 20 mm height cylindrical mold. Two parallel copper electrodes were placed in the slurry and connected to an impedance equipment to record the electric conductance change of the slurry. The experiment was performed at 25 °C and frequency: 1.0 kHz; voltage: 1.0 V. The change on the electric conductance of the slurry was recorded for 5 h as follows: 1.0 min interval during the first hour, 5 min interval during the second hour and 15 min interval during the last 3 h. The setting time test was performed according to CNS786 and ASTM C191-01 standard test methods, using the Vicat apparatus. 2.3.4. Compressive strength, linear shrinkage and bulk density Compressive strength testing was performed on 20 20 20 mm3 specimens using a DNS100 universal testing machine. The displacement rate used was 0.5 mm/min. The test was performed on specimens cured for 7 and 28 days. For the wet compressive strength, the specimens were immersed for 24 h in water prior to testing. The values were determined as the average of three (03) samples of each composition, and the standard deviation of the three replicate specimens was used as the error bar in Figures. The linear shrinkage was expressed as the percentage of sample size reduction after 3, 7 and 28 days. The bulk density of the cubic samples was determined using the Archimedes principle.
3. Results and discussion 2. Experimental 2.1. Materials The red mud used in this study was from Guangxi Province, China. The material was dried at 105 °C for 24 h and sieved in a 144 lm sieve. The specific surface area determined by the BET method was 8.04 m2/g. The ground granulated blast furnace slag used in this study was provided by the Chengde Group Company, Beihai Guangxi, PR. China. The specific surface area determined by the BET method was
3.1. Phases composition The phase composition of the synthetized products was investigated using XRD and FTIR analyses. Fig. 1 presents the XRD results of the pure slag (a), alkali silicate activated slag (b), pure red mud (c) and alkali activated red mud/ slag with 50% red mud and 50% slag (c), prepared with a sodium silicate solution of R = 2.0. Slag was almost amorphous while the
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Table 1 Chemical composition of slag and red mud.
Slag Red mud
Fe2O3
Al2O3
CaO
SiO2
TiO2
Na2O
MgO
SO2
K2O
MnO
0.63 33.99
15.32 18.47
37.15 14.19
34.21 9.39
0.80 5.42
0.39 5.11
9.34 0.32
1.82 0.33
0.41 0.10
0.59 0.091
Table 2 Particle size information of slag and red mud.
Slag Red mud
D10 (lm)
D25 (lm)
D50 (lm)
D75 (lm)
D90 (lm)
3.26 0.16
7.56 0.24
17.08 0.76
26.22 4.88
33.09 11.53
Table 3 Mixture compositions.
*
Composition N°
Modulus of the liquid water glass
Na2O/Al2O3 (molar)
SiO2/Al2O3 (molar)
Slag (g)
Red mud (g)
Liquid water glass (g)
Water (g)
H2O2 (%)
Standard sand (g)*
1 2 3 4 5 6 7 8
2.0 2.0 2.0 2.0 1.6 1.8 2.2 2.0
0.68 0.77 0.84 0.92 0.96 0.90 0.80 0.84
5.07 4.17 3.35 2.61 3.30 3.33 3.37 3.35
90 67.5 45 22.5 45 45 45 45
– 22.5 45 67.5 45 45 45 45
38 38 38 38 38 38 38 38
6 6 6 6 6 6 6 6
– – – – – – – –
9 10 11
2.0 2.0 2.0
0.77 0.84 0.92
4.17 3.35 2.61
67.5 45 22.5
22.5 45 67.5
38 38 38
6 6 6
– – – – – – – 0.5; 0.75; 1.00; 1.25. – – –
30; 90; 270 30; 90; 270 30; 90; 270
The SiO2/Al2O3 molar ratio is not taking into account the silica from the standard sand.
(d)
(c)
(b) (a)
2 θ (°) Key:
= Katoite, Si-rich, Ca3Al2(SiO4)(OH)8 , PDF n° 38-0368 = Hemate, Fe2O3, PDF n° 33-0664 = Cancrinite, Na6Ca1.5 Al6Si6O24 (CO3)1.6, PDF n° 34-0176 = Diaspore, AlO(OH) , PDF n° 05-0355
Fig. 1. XRD spectra of slag (a), geopolymer from slag (b), red mud (c) geopolymer from 50% slag and 50% red mud (d).
main crystalline phases observed in red mud were hematite, Fe2O3, PDF 33-0664 and cancrinite, Na6Ca1.5 Al6Si6O24 (CO3)1.6, PDF 340176. Few amounts of katoite, Si-rich, Ca3Al2(SiO4)(OH)8, PDF no. 38-0368 and diaspore, AlO(OH), PDF 05-0355 were also observed. Slag did not present a great change on the XRD pattern after alkali activation. The crystalline components of red mud persisted on the XRD pattern of the geopolymer resulting from the mixture of slag and red mud, with the apparent crystalline reflections of hematite, cancrinite and katoite, suggesting an incomplete dissolution of red mud after alkali activation.
The main reaction product formed by alkaline activation of slag is generally an aluminum- substituted C-A-S-H type gel, with a disordered tobermorite-like type structure [30,31], hence, difficult to be characterized by XRD analysis. This is in agreement with the XRD pattern of the alkali activated slag, which was almost completely amorphous. The formation of C-A-S-H gel in the mixture of slag with sodium silicate solution involves the reaction of silicate species supplied by sodium silicate solution with calcium, aluminum and additional silicon supplied by slag dissolution. The process is affected by slag chemistry and as the reaction progress,
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C-A-S-H gel formation removes calcium from the solution phase and so further drives slag dissolution and enhances the overall extent of reaction, forming the solid binder product [30,31]. The formation of few amount of secondary reaction product is not excluded and could explain the presence of some trace of crystalline phases which were difficult to ascribe to a specific mineral. The presence of the starting crystalline phases in the inorganic polymer prepared from red mud, mainly hematite and cancrinite, is in agreement with previously reported studies [11,26]. This suggests the common absence or low dissolution of these minerals in the alkaline condition of the inorganic polymer synthesis. The infrared spectra of red mud (a), slag (b), inorganic polymer resulting from the mixture of 50% red mud and 50% slag (c), and inorganic polymer from slag (d) are presented in Fig. 2. Globally, the large bands at 900–1200 cm1 are assigned to the stretching vibrations of Si(Al)AO groups and are sensitive to the content of structural Si and Al [25,30,32]. The bands around 1400 to 1500 cm1 indicated the presence of OACAO [19,33]. The bands around 1600–1700 cm1 are attributed to water, since they correspond to the characteristic regions of OAH stretching and HAOAH bending in H2O [19,34]. In the case of red mud (Fig. 2a), the stretching vibrations of the FeAO bands of the hematite structure is well observed around 450 and 550 cm1. The band around 1100 cm1 arose from the presence of SiO2 [19]. After alkali activation, this band disappeared in the spectrum of the inorganic polymer resulting from the mixture of 50% slag and 50% red mud (Fig. 2c), suggesting the participation of SiO2 of the initial red mud in the formation of the inorganic polymer network. The SiAO stretching vibration band in alkali activated slag is narrow in comparison to the one in pure slag, indicating the presence of shortrange order in the structure of the gel formed in the alkali activated slag paste [30]. 3.2. Microstructural characterization The images obtained from Scanning Electron Microscopy analysis are presented in Fig. 3. Red mud particles were smaller than slag particles (Fig. 3a and a0 respectively), with a large fraction of particles below 2 mm. Besides, slag presented larger particles ranging from about 20 mm to less that 1 mm. This is in agreement with the particles size information of slag and red mud reported in the experimental section. Polished samples of inorganic polymer
Si(Al)–O
O-C-O
(d)
H–O–H (c) (b) SiO2
(a) Fe-O
Wavenumbers (cm-1) Fig. 2. Infra red spectra of red mud (a), slag (b), geopolymer from 50% slag and 50% red mud (c), geopolymer from slag (d).
489
from 50% slag and 50% red mud and inorganic polymer from slag (Fig. 3B and C respectively) showed a more or less homogenous microstructure at lower magnification, with a relatively homogeneous distribution of Si, Al, Ca and Na (element maps). Higher magnifications revealed a dense microstructure with the presence of some cracks in the inorganic polymers resulting from both slag (Fig. 3d0 ) and 50% red mud + 50% slag (Fig. 3d). It is also observed that the dissolution of the starting aluminosilicate precursor is not complete after alkali activation. Relics of slag particles are more obviously distinctive in the geopolymer gel, possibly due to their coarser particles. Hence, possible transformation of slag into more fine particles could have led to a better dissolution and reactivity. The presence of cracks on the microstructure of geopolymer resulting from slag has been previously reported and was suggested to arise from the shrinkage occurring during the gel formation or to the possible difference in composition of different gel layers surrounding the relic of slag particles [30,35]. However, the overall microstructure remained relatively dense to provide high strength materials as shown in the compressive strength section. The atomic composition of the mixture made of 50% red mud and 50% slag (Fig. 4A) revealed the presence of Si (31 At%), Ca (24 At%), Na (14 At%), Al (17 At%), Fe (7% At), Mg (6% At%). For the case of the mixture made with only slag (Fig. 4B), the atomic composition revealed the presence of Si (38 At%), Ca (27 At%), Na (13 At%), Al (12 At%) and Mg (9% At%). These compositions are globally in agreement with the chemical composition of the starting materials and the mixtures proportioning. It is noted that in both cases, the amount of Ca, Na, Si, and Al elements were sufficient for the formation of C-N-A-S-H gel (Fig. 4). This type of gel in alkali activated materials was suggested to be highly stable as C-S-H based OPC systems, provided that proper charge balancing and limited structural disorder take place during their synthesis [36]. Based on microstructure analysis, it could be stated that the synthesized slag–red mud geopolymers are a kind of composite consisting of C-N-A-S-H gel and other phases as fillers, mainly from incomplete dissolution of the starting materials. 3.3. Electric conductance and setting time of the geopolymer slurry The electric conductance curves of the fresh slurry of inorganic polymer from slag and slag/red mud mixtures with 25, 50 and 75% slag, at 25 °C, are presented in Fig. 5. The electric conductance of all the compositions first increased and then decreased with the increment of the reaction time. This could be explained by the fact that at the beginning of the reaction, the concentrations of ions such as Ca2+, Si4+, Mg2+and Al3+ are increasing in the fresh slurry due to the dissolution of the starting aluminosilicate material, thus, contributing to an increase of the electric conductance. This first trend is similar to the one observed in the geopolymerization of metakaolin at 40–100 °C [37], although the experiment was conducted at lower temperature, suggesting a higher tendency of slag/red mud system to dissolve in alkaline medium at lower temperature. As the reaction time went on, mobile ions were consumed by the formation of the geopolymer network and this led to a decrease of electric conductance. The maximum values of electric conductance were observed to decrease with the increase of red mud in the slag/red mud geopolymer system, suggesting a higher susceptibility to dissolve slag in comparison to red mud, despite the coarser size of slag particles observed by BET and particle size analysis. In the same direction, the geopolymer from slag also presented the lowest electric conductance after 5 h of reaction. This suggests that after the dissolution step, the solidification step also occurred faster for slag and was delayed with the increase of red mud in the system, due to the relatively lower reactivity of red mud in comparison to slag. This is in agreement with previously reported studies on alkali activated slag, where fast setting
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a'
a
A) 20 μm
20 μm Al
Si
Fe 500 μm B)
Ca
Na
Mg
Si
Al
Fe
500 μm C) Ca
Mg
Na
d'
d
Geopolymer gel
Undisolved D)
Geopolymer gel
Micro cracks
Micro cracks
20μm
20μm
Fig.3. A) SEM images of red mud powder (a), slag powder (a0 ); B) geopolymer from 50% slag and 50% red mud; C) geopolymer from slag; D) higher magnification of geopolymer from 50% slag and 50% red mud (d), and 100% slag (d0 ).
was often commented as a possible limiting factor for their practical application as cements [33]. Hence, red mud can be used as a setting time retardant in slag-red mud geopolymer binders as shown in Fig. 6, although the highest values of initial and final setting time remained relatively low (57 and 146 min respectively). 3.4. Compressive strength and linear shrinkage of slag and slag/red mud geopolymer The 7 and 28 days compressive strength of slag/red mud system (50% slag and 50% red mud) with different moduli of the activating solution are presented in Fig. 7. When the modulus of the activating solution varied from 1.6 to 2.2, the compressive strength of the geopolymer samples slightly increased up to an optimal value at
R = 2.0 with the values of about 55 and 85 MPa for the 7 and 28 days compressive strength respectively. Since the activators were prepared with the same initial silicate solution by varying the concentrations of Na2O in the silicate solution, these data imply that the modulus of the activating solution has a strong effect on the slag/red mud geopolymerization and strength development. Actually, increasing alkalinity (low silica modulus) tends to promote faster reaction progression during the early reaction stages of geopolymerization of high calcium aluminosilicates such as slag. However, after the initial stages of hydration, higher silica content and lower pH tend to favor the dissociation of calcium [35,38]. Since both phases are important for the overall strength development, it is thus likely that the modulus 2.0 is the optimum that
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491
Si Ca Al A) Na
Fe
Si Ca
Al B)
Fig. 6. Setting time of slag/red mud geopolymers at 25 °C, prepared with sodium silicate solution of R = 2.0.
Na
Fig. 4. Element composition of geopolymer from 50% slag and 50% red mud (A), and geopolymer from slag (B).
Fig. 7. Effect of the modulus of the activating solution on the 7 and 28 days compressive strength of geopolymers from 50% slag and 50% red mud, cured at 25 °C.
Fig. 5. Electric conductance of geopolymers from slag and slag/red mud prepared with sodium silicate solution of R = 2.0.
allowed the highest amount of reaction product, resulting in highest compressive strength. The effects of red mud percentage and curing temperature on the compressive strength of geopolymer prepared with the activating solution of modulus 2.0 are presented in Figs. 8 and 9. In Fig. 8, the 7 days compressive strength of the slag/red mud system at 25 °C is varying from about 10–85 MPa, the strength reducing with the increase of red mud in the system. It is noticed that up to 50% of red mud in the system did not lead to a significant strength reduction, suggesting the possibility of preparing relatively good strength materials by integrating up to 50% of red mud in the mix-
ture. Another important observation is the need of long curing time to achieve good strength for samples containing the highest amount of red mud (75%). For these samples, the compressive strength increased from 10 MPa at 7 days of curing at 25 °C to about 30 MPa after 28 days, hence an increase of about 200%. It can thus be concluded that slag is more reactive than red mud at ambient temperature (25 °C) and is the main driver for strength development in the system at this temperature. When varying the curing temperature from 25 to 60 °C for one week, the optimum curing temperature is 40 °C (Fig. 9) when the system is containing up to 50% of red mud. Actually, heat is beneficial to increase the reaction rate by accelerating the dissolution of silica and alumina species from aluminosilicates, facilitating the polycondensation process and hardening of geopolymer matrix. However, for any geopolymer paste composition, there is a threshold temperature beyond which no strength increase is observed. Beyond this temperature, the formation of geopolymer gel on the particle surface is too rapid and hinders further dissolution of aluminosilicates
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Fig. 8. Effect of the percentage of slag (25–100%) on the 7 and 28 days compressive strength of geopolymers cured at 25 °C.
of red mud from different locations, most of these studies have shown compressive strength below 30 MPa for geopolymer composites containing less than 50% of red mud at 25 °C. Hence, the results of this study suggests slag to be one of the most suitable aluminosilicate for the development of composite geopolymers containing red mud at ambient temperature (25 °C), with a compressive strength satisfying the minimum requirement for many building materials [39,41]. Fig. 10 shows the drying shrinkage at 3, 7 and 28 days of geopolymers with different percentages of red mud at 25 °C. From this figure, it is observed that increasing red mud in the system led to an increase of the linear shrinkage. However, red mud delayed the drying shrinkage at the early stage of drying. Up to 25% of red mud did not affect the drying shrinkage. However, the shrinkage of the composition containing 75% red mud was significant, with potential to lead to some cracks. The drying shrinkage is an important parameter for geopolymer cement and concretes and relies on many factors including the type of starting material, the activating solution and the water binder ratio [42–44]. Albeit the 28 days drying shrinkage obtained here (0.8–2.5%) was higher than the general specifications for normal OPC concretes [42], the materials developed could still find many applications in the building industry sector. Furthermore, composite geopolymers prepared with sand addition are expected to have much lower shrinkage as sand itself is not shrinking during the curing regime.
3.5. Application in the development of mortar and lightweight materials The wet and dry compressive strength of the composite slag/red mud/sand geopolymer with ¼, 2/4 and ¾ sand additions are presented in Fig. 11. Dry compressive strength values were higher than the wet ones. This was suggested to be linked to the transformation of some Si-O bonds into Si-OH bonds after water immersion, weakening the structure. Sand addition globally contributed to the reduction of compressive strength which was in the range 15–80 MPa for the dry strength, depending on the mix proportioning. This behavior likely arose from the absence of solid interfacial bonding between sand particles and geopolymer matrix as in the case of the addition of fused silica or more reactive fine fillers in some geopolymer slurry [6,45,46]. From the photo and optical microscope image of slag/red mud/sand geopolymer composite Fig. 9. Effect of temperature (25, 40 and 60 °C) on the 7 and 28 days compressive strength of geopolymers from slag and slag/red mud.
and/or the time for a better dissolution of aluminosilicates becomes insufficient due to rapid setting of the geopolymer paste, leading to lower reaction product and lower strength [32,39]. This explains why slag system was less sensitive to the increase of the curing temperature from 25 to 40 °C, with an observed increase of 10% of strength, at variance to slag/red mud system (50% slag and 50% red mud) which increased for 37%. For both slag and slag red mud system (up to 50% of red mud), curing above 40 °C was not beneficial for strength improvement. However, at 75% of red mud in the system, an increase in the compressive strength with the increase of the curing temperature from 25 to 60 °C was observed. The compressive strength observed here is higher than that of previous studies on slag/red mud [27]. This may be due to the difference in chemical and mineral composition of the two red muds, the better performance observed here likely arose from the higher content of aluminum in the current red mud. Many studies have attempted to produce composite geopolymers from red mud and other aluminosilicate materials such as metakaolin, rice husk or fly ash [11,22–24,40]. Despite the variability in physical properties
Fig. 10. 3, 7 and 28 days drying shrinkage of geopolymer with different percentage of red mud.
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Wet strength
Dry strength
Fig. 11. 28 days dry and wet compressive strength of slag/red mud/sand geopolymer composites cured at 25 °C.
2 mm
Si
Micro sand parcles
Sand parcle Binding phase
500μm
20 mm-
0% sand
1/4 sand
2/4 sand
3/4 sand
Fig. 12. Photos, optical microscope and SEM images of broken samples of geopolymer composites (75% slag and 25% red mud) at indicated amount of sand.
(Fig. 12), it is seen that the alkalinity of the geopolymer mixture did not allow a partial dissolution at the surface of coarser sand aggregates, that would have led to a more intimate interfacial bonding. Consequently, a reduction of strength was observed with the increase of sand proportion in the mixture. However, micro sand aggregates were more closely mixed with the binding phase (SEM images) and were only be able to be identified using Si map and, better strength was observed with the increase of the content of slag in the system. A similar result was observed in a study of metakaolin/slag geopolymer, where slag addition was found to provide better strength, due to a denser microstructure created in the matrix [47]. It is noteworthy that the use of sand aggregate to produce structural materials from slag/red mud geopolymer is expected to further reduce the production cost and can easily lead to application of products such as paving blocks or building bricks. The photo and SEM images of the lightweight materials prepared by incorporating 0.5–1.25% H2O2 in the geopolymer made of 50% slag and 50% red mud are presented in Fig. 13. The number and size of pores increased with the increase of the content of H2O2 in the system. This led to a reduction of the bulk density of the
materials from about 1.7 g/cm3 at 0.5% H2O2 to 1.0 g/cm3 at 1.25% H2O2 (Fig. 14). Despite the simultaneous reduction in the compressive strength, the residual strength coupled with the lightweight (low density) of the materials could find many applications in the field of civil engineering. Future investigation will be done on their thermal properties in order to assess their potential to mitigate the energy losses inside buildings. In summary, the overall results are significant to contribute in addressing both issues of CO2 reduction in the building sector and the sustainable use of resources by the conversion of industrial by-products into useful and valuable products, using geopolymer technology.
4. Conclusions Inorganic polymers from slag and red mud were prepared. The end products were characterized by X- ray diffraction, scanning electron microscopy, setting time and compressive strength. The modulus of the sodium silicate solution of 2.0 was the optimum for better strength development for both slag and red mud/slag
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Fig. 13. Photos and SEM images of broken geopolymers made of 50% red mud and 50% slag, at indicated amount of H2O2.
Fig. 14. Effect of H2O2 on the 7 days compressive strength and bulk density of the geopolymer.made of 50% red mud and 50% slag at 25 °C.
system. A higher reactivity, faster setting and lower shrinkage were observed for slag, with a 7 days compressive strength of about 90 MPa at 25 °C. However, up to 25% of red mud did not affect the drying shrinkage and a 7 days compressive strength of 54 MPa was obtained at 25 °C with geopolymer composite containing 50% red mud and 50% slag. High temperature and long curing were increasingly significant for strength development as the percentage of red mud increased in the system. The addition of ¼, 2/4 and ¾ sand in the geopolymers made of slag and red mud has led to mortars with reduced strength, but still in the useful range for building application. Potential lightweight materials with bulk density of about 1.0 g/cm3 were also obtained by the addition of H2O2. The overall results are significant to contribute in addressing both issues of CO2 reduction in the building sector and the sustainable use of resources by the conversion of industrial by-products into useful and valuable products, using geopolymer technology. Acknowledgments This work was supported by the Chinese Natural Science Fund (grant: 51262002, 21566006 and 51561135012) and Postdoctoral Project from Guangxi University. References [1] J. Davidovits, Geopolymers: inorganic polymeric new materials, J. Therm. Anal. 37 (1991) 1633–1656.
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Study on the development of inorganic polymers from red mud and slag system: Application in mortar and lightweight materials Patrick N. Lemougna a,b,⇑, Kai-tuo Wang a, Qing Tang a, Xue-min Cui a,⇑ a School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China b Local Materials Promotion Authority, MINRESI/MIPROMALO, P.O. Box 2396, Yaounde, Cameroon
h i g h l i g h t s Inorganic polymers from red mud (RM) and slag system were investigated. Better reactivity and faster setting were obtained for higher contents of slag. Up to 50% of RM in the system led to a 7 days strength of 54 MPa at 25 °C. Sand and H2O2 addition led to lightweight and mortar products of 10–70 MPa. The results are of interest for the conversion of red mud and slag by-products.
a r t i c l e
i n f o
Article history: Received 10 July 2017 Received in revised form 31 August 2017 Accepted 5 September 2017
Keywords: Geopolymer Red mud Slag Sustainability Building applications
a b s t r a c t The conversion of red mud into valuable products remains an important issue to be addressed. Inorganic polymers from red mud and slag system were investigated. Slag was substituted at 25, 50 and 75% by red mud in the system. Sodium silicate solutions of modulus ranging from 1.6 to 2.2 were used and the samples were cured at 25, 40 and 60 °C. Slag was amorphous phase while red mud contained some crystalline phases, mainly hematite, crancrinite and katoite. A better reactivity, faster setting and lower shrinkage were obtained for higher contents of slag, with a modulus of sodium silicate solution of 2.0. The composition made of 50% red mud and 50% slag achieved a 7 days compressive strength of 54 MPa at 25 °C. The use of standard sand and hydrogen peroxide allowed the formation of mortar and lightweight materials with reduced mechanical properties, but still of interest to address both issues of sustainable use of resources and CO2 reduction in construction. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The term ‘‘geopolymer” was coined by Davidovits in 1978 to describe inorganic polymers with tridimensional structures formed by low temperature polycondensation of aluminosilicates [1,2]. These materials have received a burgeoning interest during the last decades due to their excellent physical, mechanical and thermal properties [2–9]. The development of geopolymer materials for building applications was reported to be less CO2 footprint in comparison to the production of Ordinary Portland Cement (OPC). Actually, the production of one ton of OPC releases about
⇑ Corresponding authors at: School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China (P.N. Lemougna). E-mail addresses: [email protected] (P.N. Lemougna), [email protected]. cn (X.-m. Cui). http://dx.doi.org/10.1016/j.conbuildmat.2017.09.015 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
1 ton of CO2 and the total OPC production is currently contributing to about 5–7% of global anthropogenic CO2 [7–9]. Besides, optimally formulated geopolymer cement was found to be able to reduce by 80% the CO2 associated to OPC cement production industry [7]. Another important advantage of geopolymers is the opportunity to valorize a wide range of secondary raw materials, including slag and red mud, the usage of which can contribute to the sustainability of non-renewable natural resources such as limestone and clay for building industry. Additional environmental benefits associated to the absence of surface-excavated operations required in the case of natural resources were also reported [10– 12]. Hence, the successful development and use of building materials from industrial by-products such as slag and red mud is expected to contribute to both sustainable development and CO2 reduction of OPC industry, the production of which has passed 4 billion tons in 2015 [13].
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Slag (ground granulated blast furnace slag) is an industrial byproduct material generated from the manufacturing of pig iron. It is well known for its high long-term strength and resistance to deterioration under severe environmental conditions as well as its environmental advantage due to lower CO2 emissions and energy consumption in the development of cementitious materials [12,14]. Beside slag, red mud (RM) is a waste product produced in huge amounts from aluminum industries. It is reported that 1.5– 1.6 tons of RM is generated per ton of alumina produced, most of it stored in landfill areas or on-site waste lakes [11]. The mineral and chemical compositions of red mud depend on the quality of bauxite, and to a lesser degree, the processing parameters [15,16]. The treatment and utilization of huge red mud wastes has been a major challenge for the alumina industry throughout the world. Among the valorization options, the development of structural materials has been reported as a potential area of interest to be explored [17–20]. However, despite the difference in chemical and mineralogical composition of red mud from different origins, many types of red mud were found not to be reactive enough for geopolymer synthesis and as consequence, were generally mixed with more reactive aluminosilicate materials, mainly fly ashes and metakaolin [11,15,21–23]. Potential structural materials based on red mud combined with silica fume, metakaolin or fly ash were reported [11,22–26], but very little has been reported on the development of red mud/slag system. Pan et al. [27] observed that a poorly rich Al red mud, blended with slag, can present acceptable strength and chemical resistance after alkali activation. Considering the divergence of red muds from different locations [16] and the limited information on the mix proportioning or curing temperature in this previous report, further research is needed to explore the possibilities of red mud valorization. Furthermore, an economical widely accepted technology for the recycle and reuse of red mud has yet to be developed, despite the increasing environmental pressure associated to its worldwide annual generation, now surpassing 150 106 t [19,21]. In building engineering, the use of binders or concretes of lower densities is beneficial in terms of structural load-bearing, acoustic and thermal insulation. Adding foaming agents is a common way of reducing the self-weight of binder pastes. Foaming agents generate artificial pores in hardened pastes and reduce their specific gravities. However, the foamed binder pastes are not suitable as load bearing building materials, due to their lower strength, but are efficient for nonstructural applications such as thermal insulation or soundproofing [28,29]. The present study investigated the conversion of red mud and slag into geopolymer materials for potential building applications. Many compositions were prepared by combining slag and red mud with an introduction of up to 75% of red mud in the system. Sodium silicate solutions with a silica modulus ranging from 1.6 to 2.2 were used and the geopolymer slurry was cured at 25– 60 °C. The resulting geopolymers were characterized by X-ray diffraction, Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, Electric conductance, setting time, linear shrinkage and compressive strength. Finally, an application in the development of mortar and lightweight materials was assessed.
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1.44 m2/g. The oxide composition of the slag and red mud determined by X-ray fluorescence and their particle size information are respectively reported in Tables 1 and 2. The alkaline activating solutions with silica modulus (R = SiO2/Na2O) of 1.6–2.2 with 0.2 interval were prepared by dissolving solid sodium hydroxide in a commercial sodium water glass with R = 3.3. The alkaline activating solutions were sealed and stored for a minimum of 24 h prior to use. 2.2. Specimen preparation The preparation of the fresh mixtures was performed by mixing slag, red mud, water glass of different moduli (R = 1.6; 1.8; 2.0 and 2.2), and some amount of deionized water. The mixing process was performed for about 10 min, using an electric mixer at 600 rpm. The homogenous pastes were then casted in cubic alloy molds of 20 20 20 mm3, covered with a thin layer of plastic to facilitate the removal of the hardened pastes upon curing. The alloy molds were vibrated on a vibration table for 2 min to remove air bubbles and sealed afterwards. The specimens were cured either at 25 °C, 40 °C and 60 °C for 7 days prior to characterization. Some specimens were maintained at 25 °C for 28 days for compressive strength test. For the preparation of slag/red mud/sand geopolymer composites, some amount of standard sand ISO 679: 1989 was added (1/4, 2/4, 3/4 ratios of sand mass/slag + red mud mass). Lightweight materials were obtained by adding H2O2 (AR grade; 30% H2O2) in the mixture made with 50% slag and 50% red mud. The percentages of H2O2 were 0.5; 0.75, 1.00 and 1.25% of the dry mass of red mud and slag. The details on the mix proportioning are presented in Table 3. 2.3. Characterization method 2.3.1. XRD and FTIR analyses The samples were powdered and examined by X-ray diffraction with a Rigaku Mini Flex 600 instrument with Ni-filtered Cu (Ka) radiation, a step size of 0.02°, operated at 40 kV and 15 mA, with a dwell time of 0.5 s and a 2h range of 5–80°. The powdered samples were also pressed into KBr pellets for FTIR analysis using a Thermo Scientific FTIR spectrometer. 2.3.2. SEM/EDS and optical analysis Scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were performed to analyze the polished surfaces of the specimens with an S3400N device (Japan Hitachi Limited Company). Specimens were impregnated using absolute ethyl alcohol, polished with SiC paper, and then coated with gold. Optical analysis of the composite slag red mud geopolymer containing sand was performed using an Olympus optical microscope of type SZ61. 2.3.3. Electric conductance and setting time analysis Geopolymer slurry was prepared and casted into a 40-mm diameter 20 mm height cylindrical mold. Two parallel copper electrodes were placed in the slurry and connected to an impedance equipment to record the electric conductance change of the slurry. The experiment was performed at 25 °C and frequency: 1.0 kHz; voltage: 1.0 V. The change on the electric conductance of the slurry was recorded for 5 h as follows: 1.0 min interval during the first hour, 5 min interval during the second hour and 15 min interval during the last 3 h. The setting time test was performed according to CNS786 and ASTM C191-01 standard test methods, using the Vicat apparatus. 2.3.4. Compressive strength, linear shrinkage and bulk density Compressive strength testing was performed on 20 20 20 mm3 specimens using a DNS100 universal testing machine. The displacement rate used was 0.5 mm/min. The test was performed on specimens cured for 7 and 28 days. For the wet compressive strength, the specimens were immersed for 24 h in water prior to testing. The values were determined as the average of three (03) samples of each composition, and the standard deviation of the three replicate specimens was used as the error bar in Figures. The linear shrinkage was expressed as the percentage of sample size reduction after 3, 7 and 28 days. The bulk density of the cubic samples was determined using the Archimedes principle.
3. Results and discussion 2. Experimental 2.1. Materials The red mud used in this study was from Guangxi Province, China. The material was dried at 105 °C for 24 h and sieved in a 144 lm sieve. The specific surface area determined by the BET method was 8.04 m2/g. The ground granulated blast furnace slag used in this study was provided by the Chengde Group Company, Beihai Guangxi, PR. China. The specific surface area determined by the BET method was
3.1. Phases composition The phase composition of the synthetized products was investigated using XRD and FTIR analyses. Fig. 1 presents the XRD results of the pure slag (a), alkali silicate activated slag (b), pure red mud (c) and alkali activated red mud/ slag with 50% red mud and 50% slag (c), prepared with a sodium silicate solution of R = 2.0. Slag was almost amorphous while the
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Table 1 Chemical composition of slag and red mud.
Slag Red mud
Fe2O3
Al2O3
CaO
SiO2
TiO2
Na2O
MgO
SO2
K2O
MnO
0.63 33.99
15.32 18.47
37.15 14.19
34.21 9.39
0.80 5.42
0.39 5.11
9.34 0.32
1.82 0.33
0.41 0.10
0.59 0.091
Table 2 Particle size information of slag and red mud.
Slag Red mud
D10 (lm)
D25 (lm)
D50 (lm)
D75 (lm)
D90 (lm)
3.26 0.16
7.56 0.24
17.08 0.76
26.22 4.88
33.09 11.53
Table 3 Mixture compositions.
*
Composition N°
Modulus of the liquid water glass
Na2O/Al2O3 (molar)
SiO2/Al2O3 (molar)
Slag (g)
Red mud (g)
Liquid water glass (g)
Water (g)
H2O2 (%)
Standard sand (g)*
1 2 3 4 5 6 7 8
2.0 2.0 2.0 2.0 1.6 1.8 2.2 2.0
0.68 0.77 0.84 0.92 0.96 0.90 0.80 0.84
5.07 4.17 3.35 2.61 3.30 3.33 3.37 3.35
90 67.5 45 22.5 45 45 45 45
– 22.5 45 67.5 45 45 45 45
38 38 38 38 38 38 38 38
6 6 6 6 6 6 6 6
– – – – – – – –
9 10 11
2.0 2.0 2.0
0.77 0.84 0.92
4.17 3.35 2.61
67.5 45 22.5
22.5 45 67.5
38 38 38
6 6 6
– – – – – – – 0.5; 0.75; 1.00; 1.25. – – –
30; 90; 270 30; 90; 270 30; 90; 270
The SiO2/Al2O3 molar ratio is not taking into account the silica from the standard sand.
(d)
(c)
(b) (a)
2 θ (°) Key:
= Katoite, Si-rich, Ca3Al2(SiO4)(OH)8 , PDF n° 38-0368 = Hemate, Fe2O3, PDF n° 33-0664 = Cancrinite, Na6Ca1.5 Al6Si6O24 (CO3)1.6, PDF n° 34-0176 = Diaspore, AlO(OH) , PDF n° 05-0355
Fig. 1. XRD spectra of slag (a), geopolymer from slag (b), red mud (c) geopolymer from 50% slag and 50% red mud (d).
main crystalline phases observed in red mud were hematite, Fe2O3, PDF 33-0664 and cancrinite, Na6Ca1.5 Al6Si6O24 (CO3)1.6, PDF 340176. Few amounts of katoite, Si-rich, Ca3Al2(SiO4)(OH)8, PDF no. 38-0368 and diaspore, AlO(OH), PDF 05-0355 were also observed. Slag did not present a great change on the XRD pattern after alkali activation. The crystalline components of red mud persisted on the XRD pattern of the geopolymer resulting from the mixture of slag and red mud, with the apparent crystalline reflections of hematite, cancrinite and katoite, suggesting an incomplete dissolution of red mud after alkali activation.
The main reaction product formed by alkaline activation of slag is generally an aluminum- substituted C-A-S-H type gel, with a disordered tobermorite-like type structure [30,31], hence, difficult to be characterized by XRD analysis. This is in agreement with the XRD pattern of the alkali activated slag, which was almost completely amorphous. The formation of C-A-S-H gel in the mixture of slag with sodium silicate solution involves the reaction of silicate species supplied by sodium silicate solution with calcium, aluminum and additional silicon supplied by slag dissolution. The process is affected by slag chemistry and as the reaction progress,
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C-A-S-H gel formation removes calcium from the solution phase and so further drives slag dissolution and enhances the overall extent of reaction, forming the solid binder product [30,31]. The formation of few amount of secondary reaction product is not excluded and could explain the presence of some trace of crystalline phases which were difficult to ascribe to a specific mineral. The presence of the starting crystalline phases in the inorganic polymer prepared from red mud, mainly hematite and cancrinite, is in agreement with previously reported studies [11,26]. This suggests the common absence or low dissolution of these minerals in the alkaline condition of the inorganic polymer synthesis. The infrared spectra of red mud (a), slag (b), inorganic polymer resulting from the mixture of 50% red mud and 50% slag (c), and inorganic polymer from slag (d) are presented in Fig. 2. Globally, the large bands at 900–1200 cm1 are assigned to the stretching vibrations of Si(Al)AO groups and are sensitive to the content of structural Si and Al [25,30,32]. The bands around 1400 to 1500 cm1 indicated the presence of OACAO [19,33]. The bands around 1600–1700 cm1 are attributed to water, since they correspond to the characteristic regions of OAH stretching and HAOAH bending in H2O [19,34]. In the case of red mud (Fig. 2a), the stretching vibrations of the FeAO bands of the hematite structure is well observed around 450 and 550 cm1. The band around 1100 cm1 arose from the presence of SiO2 [19]. After alkali activation, this band disappeared in the spectrum of the inorganic polymer resulting from the mixture of 50% slag and 50% red mud (Fig. 2c), suggesting the participation of SiO2 of the initial red mud in the formation of the inorganic polymer network. The SiAO stretching vibration band in alkali activated slag is narrow in comparison to the one in pure slag, indicating the presence of shortrange order in the structure of the gel formed in the alkali activated slag paste [30]. 3.2. Microstructural characterization The images obtained from Scanning Electron Microscopy analysis are presented in Fig. 3. Red mud particles were smaller than slag particles (Fig. 3a and a0 respectively), with a large fraction of particles below 2 mm. Besides, slag presented larger particles ranging from about 20 mm to less that 1 mm. This is in agreement with the particles size information of slag and red mud reported in the experimental section. Polished samples of inorganic polymer
Si(Al)–O
O-C-O
(d)
H–O–H (c) (b) SiO2
(a) Fe-O
Wavenumbers (cm-1) Fig. 2. Infra red spectra of red mud (a), slag (b), geopolymer from 50% slag and 50% red mud (c), geopolymer from slag (d).
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from 50% slag and 50% red mud and inorganic polymer from slag (Fig. 3B and C respectively) showed a more or less homogenous microstructure at lower magnification, with a relatively homogeneous distribution of Si, Al, Ca and Na (element maps). Higher magnifications revealed a dense microstructure with the presence of some cracks in the inorganic polymers resulting from both slag (Fig. 3d0 ) and 50% red mud + 50% slag (Fig. 3d). It is also observed that the dissolution of the starting aluminosilicate precursor is not complete after alkali activation. Relics of slag particles are more obviously distinctive in the geopolymer gel, possibly due to their coarser particles. Hence, possible transformation of slag into more fine particles could have led to a better dissolution and reactivity. The presence of cracks on the microstructure of geopolymer resulting from slag has been previously reported and was suggested to arise from the shrinkage occurring during the gel formation or to the possible difference in composition of different gel layers surrounding the relic of slag particles [30,35]. However, the overall microstructure remained relatively dense to provide high strength materials as shown in the compressive strength section. The atomic composition of the mixture made of 50% red mud and 50% slag (Fig. 4A) revealed the presence of Si (31 At%), Ca (24 At%), Na (14 At%), Al (17 At%), Fe (7% At), Mg (6% At%). For the case of the mixture made with only slag (Fig. 4B), the atomic composition revealed the presence of Si (38 At%), Ca (27 At%), Na (13 At%), Al (12 At%) and Mg (9% At%). These compositions are globally in agreement with the chemical composition of the starting materials and the mixtures proportioning. It is noted that in both cases, the amount of Ca, Na, Si, and Al elements were sufficient for the formation of C-N-A-S-H gel (Fig. 4). This type of gel in alkali activated materials was suggested to be highly stable as C-S-H based OPC systems, provided that proper charge balancing and limited structural disorder take place during their synthesis [36]. Based on microstructure analysis, it could be stated that the synthesized slag–red mud geopolymers are a kind of composite consisting of C-N-A-S-H gel and other phases as fillers, mainly from incomplete dissolution of the starting materials. 3.3. Electric conductance and setting time of the geopolymer slurry The electric conductance curves of the fresh slurry of inorganic polymer from slag and slag/red mud mixtures with 25, 50 and 75% slag, at 25 °C, are presented in Fig. 5. The electric conductance of all the compositions first increased and then decreased with the increment of the reaction time. This could be explained by the fact that at the beginning of the reaction, the concentrations of ions such as Ca2+, Si4+, Mg2+and Al3+ are increasing in the fresh slurry due to the dissolution of the starting aluminosilicate material, thus, contributing to an increase of the electric conductance. This first trend is similar to the one observed in the geopolymerization of metakaolin at 40–100 °C [37], although the experiment was conducted at lower temperature, suggesting a higher tendency of slag/red mud system to dissolve in alkaline medium at lower temperature. As the reaction time went on, mobile ions were consumed by the formation of the geopolymer network and this led to a decrease of electric conductance. The maximum values of electric conductance were observed to decrease with the increase of red mud in the slag/red mud geopolymer system, suggesting a higher susceptibility to dissolve slag in comparison to red mud, despite the coarser size of slag particles observed by BET and particle size analysis. In the same direction, the geopolymer from slag also presented the lowest electric conductance after 5 h of reaction. This suggests that after the dissolution step, the solidification step also occurred faster for slag and was delayed with the increase of red mud in the system, due to the relatively lower reactivity of red mud in comparison to slag. This is in agreement with previously reported studies on alkali activated slag, where fast setting
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a'
a
A) 20 μm
20 μm Al
Si
Fe 500 μm B)
Ca
Na
Mg
Si
Al
Fe
500 μm C) Ca
Mg
Na
d'
d
Geopolymer gel
Undisolved D)
Geopolymer gel
Micro cracks
Micro cracks
20μm
20μm
Fig.3. A) SEM images of red mud powder (a), slag powder (a0 ); B) geopolymer from 50% slag and 50% red mud; C) geopolymer from slag; D) higher magnification of geopolymer from 50% slag and 50% red mud (d), and 100% slag (d0 ).
was often commented as a possible limiting factor for their practical application as cements [33]. Hence, red mud can be used as a setting time retardant in slag-red mud geopolymer binders as shown in Fig. 6, although the highest values of initial and final setting time remained relatively low (57 and 146 min respectively). 3.4. Compressive strength and linear shrinkage of slag and slag/red mud geopolymer The 7 and 28 days compressive strength of slag/red mud system (50% slag and 50% red mud) with different moduli of the activating solution are presented in Fig. 7. When the modulus of the activating solution varied from 1.6 to 2.2, the compressive strength of the geopolymer samples slightly increased up to an optimal value at
R = 2.0 with the values of about 55 and 85 MPa for the 7 and 28 days compressive strength respectively. Since the activators were prepared with the same initial silicate solution by varying the concentrations of Na2O in the silicate solution, these data imply that the modulus of the activating solution has a strong effect on the slag/red mud geopolymerization and strength development. Actually, increasing alkalinity (low silica modulus) tends to promote faster reaction progression during the early reaction stages of geopolymerization of high calcium aluminosilicates such as slag. However, after the initial stages of hydration, higher silica content and lower pH tend to favor the dissociation of calcium [35,38]. Since both phases are important for the overall strength development, it is thus likely that the modulus 2.0 is the optimum that
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491
Si Ca Al A) Na
Fe
Si Ca
Al B)
Fig. 6. Setting time of slag/red mud geopolymers at 25 °C, prepared with sodium silicate solution of R = 2.0.
Na
Fig. 4. Element composition of geopolymer from 50% slag and 50% red mud (A), and geopolymer from slag (B).
Fig. 7. Effect of the modulus of the activating solution on the 7 and 28 days compressive strength of geopolymers from 50% slag and 50% red mud, cured at 25 °C.
Fig. 5. Electric conductance of geopolymers from slag and slag/red mud prepared with sodium silicate solution of R = 2.0.
allowed the highest amount of reaction product, resulting in highest compressive strength. The effects of red mud percentage and curing temperature on the compressive strength of geopolymer prepared with the activating solution of modulus 2.0 are presented in Figs. 8 and 9. In Fig. 8, the 7 days compressive strength of the slag/red mud system at 25 °C is varying from about 10–85 MPa, the strength reducing with the increase of red mud in the system. It is noticed that up to 50% of red mud in the system did not lead to a significant strength reduction, suggesting the possibility of preparing relatively good strength materials by integrating up to 50% of red mud in the mix-
ture. Another important observation is the need of long curing time to achieve good strength for samples containing the highest amount of red mud (75%). For these samples, the compressive strength increased from 10 MPa at 7 days of curing at 25 °C to about 30 MPa after 28 days, hence an increase of about 200%. It can thus be concluded that slag is more reactive than red mud at ambient temperature (25 °C) and is the main driver for strength development in the system at this temperature. When varying the curing temperature from 25 to 60 °C for one week, the optimum curing temperature is 40 °C (Fig. 9) when the system is containing up to 50% of red mud. Actually, heat is beneficial to increase the reaction rate by accelerating the dissolution of silica and alumina species from aluminosilicates, facilitating the polycondensation process and hardening of geopolymer matrix. However, for any geopolymer paste composition, there is a threshold temperature beyond which no strength increase is observed. Beyond this temperature, the formation of geopolymer gel on the particle surface is too rapid and hinders further dissolution of aluminosilicates
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Fig. 8. Effect of the percentage of slag (25–100%) on the 7 and 28 days compressive strength of geopolymers cured at 25 °C.
of red mud from different locations, most of these studies have shown compressive strength below 30 MPa for geopolymer composites containing less than 50% of red mud at 25 °C. Hence, the results of this study suggests slag to be one of the most suitable aluminosilicate for the development of composite geopolymers containing red mud at ambient temperature (25 °C), with a compressive strength satisfying the minimum requirement for many building materials [39,41]. Fig. 10 shows the drying shrinkage at 3, 7 and 28 days of geopolymers with different percentages of red mud at 25 °C. From this figure, it is observed that increasing red mud in the system led to an increase of the linear shrinkage. However, red mud delayed the drying shrinkage at the early stage of drying. Up to 25% of red mud did not affect the drying shrinkage. However, the shrinkage of the composition containing 75% red mud was significant, with potential to lead to some cracks. The drying shrinkage is an important parameter for geopolymer cement and concretes and relies on many factors including the type of starting material, the activating solution and the water binder ratio [42–44]. Albeit the 28 days drying shrinkage obtained here (0.8–2.5%) was higher than the general specifications for normal OPC concretes [42], the materials developed could still find many applications in the building industry sector. Furthermore, composite geopolymers prepared with sand addition are expected to have much lower shrinkage as sand itself is not shrinking during the curing regime.
3.5. Application in the development of mortar and lightweight materials The wet and dry compressive strength of the composite slag/red mud/sand geopolymer with ¼, 2/4 and ¾ sand additions are presented in Fig. 11. Dry compressive strength values were higher than the wet ones. This was suggested to be linked to the transformation of some Si-O bonds into Si-OH bonds after water immersion, weakening the structure. Sand addition globally contributed to the reduction of compressive strength which was in the range 15–80 MPa for the dry strength, depending on the mix proportioning. This behavior likely arose from the absence of solid interfacial bonding between sand particles and geopolymer matrix as in the case of the addition of fused silica or more reactive fine fillers in some geopolymer slurry [6,45,46]. From the photo and optical microscope image of slag/red mud/sand geopolymer composite Fig. 9. Effect of temperature (25, 40 and 60 °C) on the 7 and 28 days compressive strength of geopolymers from slag and slag/red mud.
and/or the time for a better dissolution of aluminosilicates becomes insufficient due to rapid setting of the geopolymer paste, leading to lower reaction product and lower strength [32,39]. This explains why slag system was less sensitive to the increase of the curing temperature from 25 to 40 °C, with an observed increase of 10% of strength, at variance to slag/red mud system (50% slag and 50% red mud) which increased for 37%. For both slag and slag red mud system (up to 50% of red mud), curing above 40 °C was not beneficial for strength improvement. However, at 75% of red mud in the system, an increase in the compressive strength with the increase of the curing temperature from 25 to 60 °C was observed. The compressive strength observed here is higher than that of previous studies on slag/red mud [27]. This may be due to the difference in chemical and mineral composition of the two red muds, the better performance observed here likely arose from the higher content of aluminum in the current red mud. Many studies have attempted to produce composite geopolymers from red mud and other aluminosilicate materials such as metakaolin, rice husk or fly ash [11,22–24,40]. Despite the variability in physical properties
Fig. 10. 3, 7 and 28 days drying shrinkage of geopolymer with different percentage of red mud.
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Wet strength
Dry strength
Fig. 11. 28 days dry and wet compressive strength of slag/red mud/sand geopolymer composites cured at 25 °C.
2 mm
Si
Micro sand parcles
Sand parcle Binding phase
500μm
20 mm-
0% sand
1/4 sand
2/4 sand
3/4 sand
Fig. 12. Photos, optical microscope and SEM images of broken samples of geopolymer composites (75% slag and 25% red mud) at indicated amount of sand.
(Fig. 12), it is seen that the alkalinity of the geopolymer mixture did not allow a partial dissolution at the surface of coarser sand aggregates, that would have led to a more intimate interfacial bonding. Consequently, a reduction of strength was observed with the increase of sand proportion in the mixture. However, micro sand aggregates were more closely mixed with the binding phase (SEM images) and were only be able to be identified using Si map and, better strength was observed with the increase of the content of slag in the system. A similar result was observed in a study of metakaolin/slag geopolymer, where slag addition was found to provide better strength, due to a denser microstructure created in the matrix [47]. It is noteworthy that the use of sand aggregate to produce structural materials from slag/red mud geopolymer is expected to further reduce the production cost and can easily lead to application of products such as paving blocks or building bricks. The photo and SEM images of the lightweight materials prepared by incorporating 0.5–1.25% H2O2 in the geopolymer made of 50% slag and 50% red mud are presented in Fig. 13. The number and size of pores increased with the increase of the content of H2O2 in the system. This led to a reduction of the bulk density of the
materials from about 1.7 g/cm3 at 0.5% H2O2 to 1.0 g/cm3 at 1.25% H2O2 (Fig. 14). Despite the simultaneous reduction in the compressive strength, the residual strength coupled with the lightweight (low density) of the materials could find many applications in the field of civil engineering. Future investigation will be done on their thermal properties in order to assess their potential to mitigate the energy losses inside buildings. In summary, the overall results are significant to contribute in addressing both issues of CO2 reduction in the building sector and the sustainable use of resources by the conversion of industrial by-products into useful and valuable products, using geopolymer technology.
4. Conclusions Inorganic polymers from slag and red mud were prepared. The end products were characterized by X- ray diffraction, scanning electron microscopy, setting time and compressive strength. The modulus of the sodium silicate solution of 2.0 was the optimum for better strength development for both slag and red mud/slag
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Fig. 13. Photos and SEM images of broken geopolymers made of 50% red mud and 50% slag, at indicated amount of H2O2.
Fig. 14. Effect of H2O2 on the 7 days compressive strength and bulk density of the geopolymer.made of 50% red mud and 50% slag at 25 °C.
system. A higher reactivity, faster setting and lower shrinkage were observed for slag, with a 7 days compressive strength of about 90 MPa at 25 °C. However, up to 25% of red mud did not affect the drying shrinkage and a 7 days compressive strength of 54 MPa was obtained at 25 °C with geopolymer composite containing 50% red mud and 50% slag. High temperature and long curing were increasingly significant for strength development as the percentage of red mud increased in the system. The addition of ¼, 2/4 and ¾ sand in the geopolymers made of slag and red mud has led to mortars with reduced strength, but still in the useful range for building application. Potential lightweight materials with bulk density of about 1.0 g/cm3 were also obtained by the addition of H2O2. The overall results are significant to contribute in addressing both issues of CO2 reduction in the building sector and the sustainable use of resources by the conversion of industrial by-products into useful and valuable products, using geopolymer technology. Acknowledgments This work was supported by the Chinese Natural Science Fund (grant: 51262002, 21566006 and 51561135012) and Postdoctoral Project from Guangxi University. References [1] J. Davidovits, Geopolymers: inorganic polymeric new materials, J. Therm. Anal. 37 (1991) 1633–1656.
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