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Preparation and characterization of foamed concrete with Ti-extracted residues and red gypsum
Construction and Building Materials 171 (2018) 109–119
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preparation and characterization of foamed concrete with Ti-extracted residues and red gypsum Jiufu Zhang, Yun Yan ⇑, Zhihua Hu School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, SiChuan 621010, PR China
h i g h l i g h t s Large stockpile of TRs and RG were co-treated as main binding materials to prepare FC. Hydration kinetics of blended binder was evaluated. Effects of technological parameters on the bulk density and compressive strength of FC were investigated. Compressive strength and thermal conductivity of FC met the requirements of Chinese standard.
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Article history: Received 14 December 2017 Received in revised form 7 March 2018 Accepted 8 March 2018
Keywords: Foamed concrete Ti-extracted residues Red gypsum Hydration kinetics Mechanical characteristics Microstructure
a b s t r a c t The recycling of Ti-extracted residues (TRs) and red gypsum (RG), which are the industrial wastes from titanium dioxide manufacturing industry, will avoid the increasing impact on the environment due to their difficulty for disposal. In this paper, the ternary blended binder mainly consisting of ordinary Portland cement (OPC), TRs and RG for the preparation of foamed concrete (FC) was formulated in the laboratory. The hydration kinetics of blended cement, mechanical characteristics and microstructure of FC were investigated. The results indicated that quick lime and sulfoaluminate cement (SAC) evidently reduced the setting time of the complex binder, but mechanical activation for TRs could not substantially shorten those of blended binder. In the case of FC, the optimum technological parameters were proposed as follows: OPC: TRs: RG = 10:45:45 (by mass, the same below), quick lime 2%, SAC 4%, Na2SO4 0.4%, water reducer 0.2%, foam 4.6% and W/B 0.60. The maximum compressive strength and specific strength of FC with density of 437 kg/m3 reached up to 2.14 MPa and 4.91 kNm/kg, respectively. Microstructure analysis revealed that the interconnected assemblages in FC are floccular and petal-like hydrated gels, need-like ettringite and unreactive plate-like gypsum. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Since its specific characteristics discovered by many researchers all over the world, titanium dioxide (TiO2) in various structures has been widely applied in energy and environmental analysis systems [1], pesticides’ degradation [2], dye-sensitized solar cells [3,4] and biomedical systems [5] etc. The well-accepted processes for the manufacture of titanium dioxide are chloride process and sulfate process. In the chloride process, titanium dioxide is generally produced from titanium-rich ilmenite through a TiCl4 stage at elevated temperature. Due to the advantages of this process such as high-quality product and small amount of wastes, it is more beneficial for the manufacture of TiO2 pigment. However, the requirement of this process for the qualification of the feedstock that ⇑ Corresponding author. E-mail address: [email protected] (Y. Yan). https://doi.org/10.1016/j.conbuildmat.2018.03.072 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
the content of MgO + CaO is less than 1.5 wt%, limits the reusage of low-grade TiO2-bearing materials with high content of CaO and/or MgO. High-titanium slag (HTS), a solid by-product from the blast furnace iron-making process using vanadium titano-magnetite ore from Panxi region of Sichuan Province (China), contains valuable 20–25% TiO2 (by weight) and calcium-alumina-silicon composites. It was once utilized as aggregates or supplementary cementitious materials in civil engineering due to its voluminous accumulation [6,7]. To make full use of the valuable TiO2 in HTS, an innovative process of so-called carbonization at elevated temperature and low temperature chlorination is proposed by Ansteel Research Institute of Vanadium and Titanium (China) in which ground HTS is heated at 925–1010 °C in the presence of coke and thus titanium carbide (TiC) is produced [8]. After that, 91.77% TiC is transformed into titanium tetrachloride (TiCl4) at 580 °C in the furnace filled with chlorine gas. In this novel process, the TiO2-bearing feedstock
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with high content of CaO and/or MgO can be utilized to yield TiO2. However, a large amount of Ti-extracted residues (TRs) is simultaneously produced in this process and one ton of titanium slag can produce about 0.8 ton of TRs. At the end of this procedure, the asreceived TRs are rapidly cooled by water to a vitreous state. TRs incorporate 3–5% (wt%) alkaline earth metal chlorides, e.g. MgCl2 and CaCl2 as well, which limits its large-scale applications [8,9]. In view of these problems, a new method for managing TRs is critical for high value-added utilization of HTS and environmental protection. Red gypsum (RG) is largely generated in the sulfate process which utilizes the lime or limestone for managing waste sulphuric acid. In China, about 15 million tons of RG are generated yearly in which production of one ton of TiO2 can generate five to six tons of RG. It is a fine powder composed of crystalline gypsum (CaSO42H2O) and amorphous ferric hydroxide (Fe(OH)3) [10]. The common treating method for RG is outdoor stacking, which may cause soil and/or river contamination. Although recycling of RG in cement production [11], gypsum blocks [12] and light-weight wall [13] etc have been studied by many researchers, the consumption rate of RG is still considerably low that cannot achieve the goal of voluminous depletion due to their high transportation cost and high energy demand for treating RG. In light of these problems of TRs and RG, it is a promising approach that the TRs and RG are used as raw materials for preparing construction and building materials. If the TRs and RG were simultaneously utilized as cementitious materials to prepare foamed concrete (FC), this co-treatment method is propitious to waste recycling and energy-savings because FC do not directly contact steel fiber or reinforcement. As all known, FC is considered to be a binary system with pores and solid matrices. Previous studies have shown that the air bubbles in FC slurry are more prone to become unstable because of high gas-liquid interface energy [14,15] and prolonging setting time of fresh FC [16,17]. Desirable FC products can be synthesized through controlling the balance between the hydration rate of solid pastes and stabilizing time of air bubbles [18]. Owing to the low hydration rate of aforementioned ternary blended binder composed of OPC, RG and TRs, the mechanical activation for TRs and the addition of quick lime and sulphoaluminate cement in blended mixture were conducted for accelerating hydration reactions but their influences on the early hydration mechanism need further investigation. In the case of FC, both the technological parameters on bulk density, compressive strength and microstructure were also studied for optimizing FC’s performance.
2. Materials and methods 2.1. Raw materials Foamed concrete (FC) in this experiment were prepared using original red gypsum (RG), finely ground titanium-extracted residues (TRs), ordinary Portland cement (OPC), quick lime, sulfoaluminate cement (SAC) and silica fume (SF), water, superplasticizer and foaming agent. RG and TRs were collected from titanium dioxide plant in Sichuan Province, China. The water content of RG and TRs was 6.4% and 12.3%, respectively. The pH value of RG was determined as 5.5. OPC used in this work was bought from the Lafarge Shuangma cement plant (Jiangyou, Sichuan Province, China), which complied with Chinese standard: GB1752007 [19]. Additionally, the specific gravity of RG, TRs and OPC was 2.33, 2.96 and 3.10. Quick lime was obtained from the local market, and its effective CaO content was 84.2%, additionally, the slake-time and slake-temperature were determined to be 4 min and 92 °C, respectively. Introduction of SF into FC slurries was treated as supplementary cementing materials. Sodium sulfate and sodium hydroxide as chemical activators were also bought from the market. Animal protein foaming agent was diluted and stirred to generate foam and the dilution ratio was kept constant at 1:10 by weight and the obtained foam density was measured as 25 kg/m3.
2.2. Mix proportions of blended binder and preparation of foamed concrete specimens The mix proportions of blended cement are presented in Table 1. For the manufacture of FC, the raw materials were weighed in predetermined proportions (dry basis) and mixed homogeneously in a laboratory mixer for 1 min. The slurries in a pan mixer were then obtained through high-speedily stirring the aforementioned dry mixture with water for 3 min, into which superplasticizer and/or chemical activators were added at the same time. The foam was then added into the slurries and subsequently mixed again for 1 min. Afterwards, the FC slurries were molded in a steel mould with dimensions of 70.7 mm 70.7 mm 70.7 mm. A plastic film was used to cover the fresh FC to prevent from losing moisture and contacting CO2. After being cured in a standard curing chamber (20 ± 1 °C, relative humidity 90%) for 48 h, FC was de-molded and subjected to standard curing conditions until the designated age.
Table 1 Mix proportions of blended cement. Sample number
OPC/%
TRs/%
RG/%
Quick lime/%
SAC/%
W/B
A0 A1 A2 (B4, C90) A3 A4 A5 B0 B2 B6 B8 B10 C60 C70 C80
9.6 9.5 9.4 9.3 9.2 9.1 9.8 9.6 9.2 9.0 8.8 9.4 9.4 9.4
43.2 42.75 42.3 41.85 41.4 40.95 44.1 43.2 41.4 40.5 39.6 42.3 42.3 42.3
43.2 42.75 42.3 41.85 41.4 40.95 44.1 43.2 41.4 40.5 39.6 42.3 42.3 42.3
0 1 2 3 4 5 2 2 2 2 2 2 2 2
4 4 4 4 4 4 0 2 6 8 10 4 4 4
0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6
Note: TRs of Sample A0–A5, B0–B10 were ball milled for 90 min, C60, C70, C80, C90 were ground in a ball mill for 60 min, 70 min, 80 min, 90 min, respectively.
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2.3. Test methods
3. Results and discussion
The chemical components of RG, OPC and TRs were performed by X-ray fluorescence (XRF, Axios, PANalytical, Holland), and the mineralogy of raw materials is conducted via a X-ray diffractometer (XRD, RIGAKU, D/max-1400) using Cu-Ka radiation (step: 0.02°, 40 kV, 70 mA) at various two-theta interval. Particle size distributions of raw materials were conducted via a laser dispersing equipment (Mastersizer 2000, Malven, Worcestershire, UK). The reactivity index test at 7 and 28 days for TRs milled at 60, 70, 80, 90 min was complied with GB/T 18046-2008 [20]. In addition, the methods for measuring the water requirement of normal consistency and setting time of blended cement were in accordance with GB/T 1346-2011 [21]. The investigation of hydration kinetics with regard to the ternary blended binder was conducted at 20 °C through an eight channel TAM Air Isothermal Calorimeter from TA Instruments, USA. The mass of investigated sample and water was set as 4.34 g and 2.60 g, respectively, and then the sample and water were mixed externally for one minute. Better understanding the microstructure of FC was also achieved by XRD (RIGAKU, D/max-1400) and scanning electron microscope (SEM, Ultra 55, Carl Zeiss AG., Germany) after the FC samples were immersed in absolute ethyl alcohol for three days and dried at 40 °C in a vacuum oven for six hours. The tests of compressive strength and dry bulk density were performed in accordance with Chinese standard (JC/T 266-2011) [22]. In order to compare the compressive strengths of specimens with different bulk density, a formula named specific strength was given as S ¼ , (MPa)/D (kg/m3) in which , and D was the average value from triplicate compressive strength and bulk density of FC, respectively. The thermal conductivity of foamed concrete with the dimensions (70.7 mm 70.7 mm 10.0 mm) was determined from three results of each sample by a thermal conductivity analyzer (TCiTM, C-Therm Technologies Ltd., Canada).
3.1. Characterization of raw materials 3.1.1. Chemical and mineral compositions analysis The chemical compositions of raw materials analyzed by XRF are shown in Table 2. It can be seen that the CaO (28.1%) and SO3 (33.39%) are predominantly existed in RG, which contains 11.62% Fe2O3 as well. For the TRs, the main compositions are CaO (26.81%), SiO2 (24.8%), Al2O3 (12.59%) and TiO2 (10.69%). The loss of ignition for RG and TRs are 20.88% and 8.34%, respectively. The mineralogy of RG, OPC and TRs is also shown in Fig. 1. As can be seen, the mineral component in RG is gypsum (CaSO42H2O), however, there are no characteristic peaks of ironrich compounds, which is mainly attributed to the disordered Fe (OH)3 gel resulting from the hydrolyzation of Fe3+ ions. The major phases of the OPC consist of alite (C3S) and belite (C2S), aluminate phase (C3A) and ferrite phase (C4AF), all contributing to the formation of hydrated assemblages and providing a high alkalinity in pore solution (pH 12.5) when OPC contacts water [23]. For the TRs, the main mineral phase is the vitreous body composed of calcium-aluminum-silicon-rich phases (Fig. 1b), as a result, it could exhibit latent hydraulic property in the presence of alkaline activator [24]. There are also observations of hematite (Fe2O3), khamrabaevite (TiC), perovskite (CaTiO3) and wollastonite (CS) in TRs. 3.1.2. Particle size distribution The grain size distribution analysis of RG, OPC was performed using a laser dispersing equipment, as shown in Fig. 2a. The results show that the median particle size (median diameter, 50% of cumulative distribution of particle, D50) of the OPC is similar to that of RG, whereas the big difference of D90 (90% of cumulative distribution of particle) between OPC and RG is also observed.
Table 2 Chemical compositions of raw materials (wt%). Compositions (%)
CaO
SiO2
SO3
Al2O3
Fe2O3
MgO
TiO2
Cl
LOI
RG TRs OPC SF
28.10 26.81 60.85 0.32
1.99 24.80 20.02 94.30
33.39 1.30 4.28 0.38
0.96 12.59 5.20 0.30
11.62 4.00 3.24 0.07
0.92 6.76 1.62 0.10
1.62 10.69 0.37 –
0.06 3.34 0.04 0.09
20.88 8.34 3.27 3.64
Fig. 1. XRD patterns of (a) original RG and OPC and (b) TRs.
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Fig. 2. Particle size distribution of (a) RG and OPC and (b) TRs milled at different time.
Table 3 Reactivity index at 7, 28 days of TRs with different fineness. Milling time
60 min
70 min
80 min
90 min
Reactivity index at 7 d Reactivity index at 28 d
64.5 75.3
70.2 85.7
80.6 90.4
87.5 98.7
Ti-extracted residues are ground in a laboratory ball mill. The particle size distributions of finely milled TRs obtained at different milling time are presented in Fig. 2b and the corresponding reactivity index of TRs at 7 days and 28 days are shown in Table 3. As shown in Fig. 2b, the D50 slightly decreased and the reactivity index of TRs at 7, 28 days showed an evidently increasing tendency as the milling time increasing. This is mainly attributed to the number of imperfections or active centers existing at the edges of the TRs particles can be increased at prolonged milling time [24]. 3.2. Fresh properties of blended cement 3.2.1. Water requirement of normal consistency The effects of quick lime, sulfoaluminate cement (SAC) and TRs’ fineness on the water requirement of normal consistency of blended cement pastes are shown in Table 4. As can be seen, the water requirement of normal consistency of OPC (27.2%) was lower than that of blended cement pastes, fluctuating from 29.1% to 30.4% with no significant tendency. This is mainly because the high water absorption of amorphous gel with high specific area consisting of Fe(OH)3 in red gypsum [10,25]. In Table 4, water demand for normal consistency of blended cement containing more SAC was lower than those of pastes with lower SAC content. This result is possibly attributed to the lower amount of RG in binding system with higher SAC content and particle grading in reference [26]. In addition, the water demand slightly increases with the increase of milling time. 3.2.2. Setting time The results in Table 4 show that the setting time shortened as the content of quick lime increased. When the quick lime content increased from 0 to 5%, the initial and final setting time decreased from 205 to 127 min and 375 to 237 min, respectively. Research has shown a reduction in both initial and final setting time of blended binder when the dosage of SAC increased. When the extent to which SAC was utilized to replace the cementitious binder consisting of OPC, TRs, RG and quick lime was increased from 0 to 10%, the initial and final setting times rapidly decreased from 222 to 99 min and from 330 to 179 min, respectively. The effect of SAC on setting time is apparent, being mainly attributed to the
Table 4 Initial and the final setting times of the ternary blended binder. Sample number
Water requirement of (%)
Initial setting time (min)
Final setting time (min)
A0 A1 A2 (B4, C90) A3 A4 A5 B0 B2 B6 B8 B10 C60 C70 C80
30.4 29.3 29.1 29.2 29.2 29.4 30.4 29.7 28.8 28.7 28.6 28.5 28.7 28.8
205 181 160 154 136 127 222 179 150 134 99 203 194 180
375 333 310 276 254 237 330 312 245 226 179 370 355 333
rapid hydration of yeelimite in SAC [27]. According to She et al. [16], mixes with short setting time are capable to stabilize the air bubbles and narrow the pore size distribution, resulting in good mechanical strength and durability [28]. Work has also shown that with the increasing milling time, initial and final setting times slightly decrease from 203 to 160 min and from 370 min to 310 min, respectively. These results suggest that at early ages, the dissolving rate of Ca2+, silicate and/or aluminosilicate components in TRs could not substantially increase with milling time of TRs. 3.3. Hydration kinetics of blended cement 3.3.1. Effect of quick lime on hydration process of blended cement The influences of the quick lime on the heat evolution of blended cement within the first 120 h (assigned as A0–A5) are presented in Fig. 3a and b. It can be seen from Fig. 3a that there are two peaks in A0 and A1, from the time when the mix contact with water. However, three peaks of the rate of heat evolution are visible in A2-A5 samples. The first peak in all systems is very high, which corresponds to the initial reaction of the C3A in OPC particles and/or digesting process of quick lime. The released heat of initial hydration of yeelimite in SAC contributes to the first peak as well
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Fig. 3. Effect of quick lime content on (a) heat flow; (b) cumulative heat.
[23,27]. Following the first peak, the curves of heat flow rate slow down over a short period and there follow a dormant period or induction period during which higher heat flow rate is observed at higher quick lime content. In such a condition, Ca(OH)2 is mainly liberated by the hydrolysis of quick lime and calcium silicates and the pH in pore solution can be rapidly promoted to a value higher than 12.5 at 20 °C [23,24]. Therefore, the network modifiers in glassy body of TRs such as Ca2+ are rapidly dissolved into the solution due to their low bond energies with oxygen atoms (less than 210 kJ/mol). Under the attack of OH, silicate and/or aluminosilicate components in TRs are gradually depolymerized and dissolved into the solution. Due to the lower solubility of calcium hydroxide than CASAH and/or CAAASAH, a thin hydrated products precipitates rapidly in the solution, resulting in the second heat evolution peak at about 10 h in samples A2–A5. However, the free lime content in A0 and A1 are not enough to substantially accelerate the dissolving rate of silicate and/or aluminosilicate components, so the hydration profile in these two samples are different from those of A2–A5. With the prolonging time, the intense main second (A0, A1) or third hydration peaks (A2–A5) are shifted to later ages with the increasing quick lime. This retarding effect is probably attributed to a coating formed on the surface of TRs composed of hydrated products and hydrated lime. Therefore, the dissolving rate of strength-giving components in TRs reduced in this condition and meanwhile, the migration rate of ions through the coating decreased. Research in Fig. 3b has also shown that the cumulative heat in A3–A5 are higher than A0–A2, which mainly arises from the combination of heat released from digestion of higher quick lime amount and reaction concerning the formation of hydrated
products. It can also be observed that the increasing degrees among A3, A4 and A5 are lower than those of A0, A1 and A2, which probably is ascribed to the retarding effect of high content of quick lime as well. 3.3.2. Effect of SAC on hydration process of blended cement Fig. 4a shows a plot of the heat evolution of samples with different sulfoaluminate cement content. While there is no significant observation concerning the onset of acceleration, the minimum value of heat flow rate in all systems is increasing with an increase of SAC content. In addition the corresponding time at the second peak in all samples is shifted to short ages. This behavior reveals an significant influence of SAC content on the precipitation of hydrated gels i.e. CASAH and/or CA(A)ASAH. Trauchessec et al. [29] have reported that the hydration mechanisms and hardening speed of OPC are modified by the yeelimite and free lime amount. OPC reacts during the first days and the hydration of yeelimite occurs in the presence of lime or portlandite, which consumes the Ca2+ and/or Ca(OH)2 to form ettringite [27], therefore, the dissolving rate of OPC and TRs can be accelerated in samples with adequate SAC. It can also be observed that the onset of third peaks at around 35 h–55 h is shifted to short time with an increase of SAC. This result may be related to the channel of needle-like ettringite on the vitreous body surface of TRs in systems with high SAC, so the ions controlling the hydration of TRs is easy to transfer. Apart from this, the heat released during early ages may accelerate the ongoing hydration at later ages. As can be seen from Fig. 4b, while the cumulative heat of samples before 72 h is increasing with SAC, that of samples after 72 h exhibits no significant tendency. It indicates that the hydration
Fig. 4. Effect of SAC content on (a) heat flow; (b) cumulative heat.
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rate during the first three days is proportional to SAC content and the hydration process later becomes slow, which is mainly attributed to the low diffusing rate of ions with respect to the generation of CASAH and/or CA(A)ASAH. 3.3.3. Effect of milling time of TRs on hydration process of blended cement Fig. 5a has illustrated that the heat flow rate curves are nearly overlapping prior to 10 h and two main exothermic peaks occur in all samples (C60-C90). It can be seen that at around 20 h, the heat flow curve of C90 is higher than that of other three samples, but the curve of C70 lower than that of C60. Approximately at 50 h, the time of corresponding peaks shift to short period with an increase of milling time, meaning that mechanical treatment for TRs can accelerate the hydration process and achieve high early strength. This is predominantly ascribed to the higher amount of active particles at higher specific area and thus dissolving rate of specific components controlling the generation of hydration products can be accelerated [24,30]. The cumulative heat profile slightly increases with an increase of milling time for TRs prior to about 72 h, as shown red rim A (Fig. 5b). However, red rim B shows that higher cumulative heat was achieved at higher milling time after 72 h, which indicates the lower hydration rate at longer period. This is because more hydrated gels are precipitated in samples with finer TRs and the migration rate of ions decreased. 3.4. Effect of various parameters on properties of FC 3.4.1. Effect of quick lime on properties of FC The effect of quick lime on bulk density, compressive strength and specific strength of foamed concrete is presented in Fig. 6. The total weight of dry mixtures was kept constant, and the content of superplasticizer and foam was 0.2%, 4.6% by the mass of dry mix, respectively. The milling time of TRs was 90 min. Water to binder ratio (W/B) in this part was set as 0.6. Results from Fig. 6 show that the bulk density increased from 388 to 473 kg/ m3 when quick lime dosage was added from 0 to 1%, however, the bulk density later almost linearly reduced from 473 to 426 kg/m3 when the content of quick lime increased from 1% to 5%. This is because the hydrated lime (Ca(OH)2) neutralizes the acid remained in RG and immediately promotes the alkalinity of fresh mixes [31]. Even so, the amount of quick lime at 1% cannot be enough to stabilize the air bubbles. With the increase of quick lime from 1% to 5%, the stabilizing process of air bubbles in FC mixtures is realized by the reduction of setting time as shown in Table 4, so the bulk density later decreased. Fig. 6 has also shown that the content of quick lime up to 2% achieves the maximum strength (1.93 MPa) and specific strength
Fig. 6. Effect of quick lime on properties of FC.
(4.23 kNm/kg). The results indicate that the quick lime content at a suitable percentage is favorable for achieving optimum mechanical properties. This is mainly arising from two aspects: (i) the heat release from the digestion of quick lime can accelerate the hydration rate of blended system; (ii) the Ca(OH)2 in liquid slurry also activates the TRs, and thus form more hydrated calcium silicates (CASAH), which leads to enhance the mechanical properties. Nevertheless, both compressive strength and specific strength later go down when quick lime exceeds 2%. According to Tian et al. [31] and Chen et al. [32], more ettringite in hardened binders will be formed and decrease the strength of foamed concrete in such a condition. Apart from this, the reduction of polymerization degree occurs in CA(A)ASAH due to its excessive Ca/Si ratio [31,32]. Based on the results, the optimum dosage of quick lime was determined as 2%.
3.4.2. Effect of sulfoaluminate cement on properties of FC Fig. 7 shows the effect of sulfoaluminate cement (SAC) on bulk density, compressive strength and specific strength of FC. The mix proportions of foamed concrete were set as follows: OPC: RG: TRs = 10:45:45 in which TRs was milled for 90 min, quick lime 2% (mass percentage), foam 4.6% and water reducer 0.2% by the mass of total dry materials. The density from Fig. 7 shows a similar trend to that from Fig. 6. It increases from 458 to 472 kg/m3 and then decreases to 426 kg/m3 at 6% SAC. With an increase of SAC from 6% to 10%, the density keeps almost constant at 427 kg/m3. It can be observed that the SAC at a certain percentage is able to improve the mechanical strength of specimens. The maximum compressive strength (1.93 MPa) from Fig. 7 is achieved at a content of SAC up to 4%, which is 15% higher than that of the specimen (1.64 MPa)
Fig. 5. Effect of milling time of TRs on (a) heat flow; (b) cumulative heat.
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explained ascendant mechanical property of FC with increasing milling time. With regard to the mechanical strength, the milling time of TRs utilized for preparation of FC is 90 min.
Fig. 7. Effect of sulfoaluminate cement (SAC) on properties of FC.
with no SAC. The specific strength also has a similar trend with compressive strength and its highest value reaches up to 4.23 kNm/kg. However, an increase of SAC results in a reduction in strength. Both compressive strength and specific strength dramatically decreases to 0.95 MPa and 2.27 kNm/kg. Afterwards, these two strengths nearly keep a approximate value with the excessive addition of SAC. As can be seen, the appropriate content of SAC in FC specimens is favorable for achieving a desirable strength. As the addition of SAC produced ettringite which compensated the shrinkage of blends such as OPC-SAC based binder [29], the solid phases of FC became dense at 4% SAC. However, the excessive ettringite led to the weak of mechanical characteristic. From the results, adding 4% SAC into blended cement is beneficial for desirable property of FC.
3.4.3. Effect of TRs fineness on properties of FC The influence of TRs fineness on the bulk density and mechanical property is depicted in Fig. 8. The mix compositions for this section are as follows: OPC: RG: TRs = 10:45:45 in which TRs was milled for various time, quick lime 2% (mass percentage), SAC 4%, foam 4.6% and water reducer 0.2% by the mass of total dry materials. It can be seen from Fig. 8 that the bulk density exhibited ascendant tendency except the specimen in which TRs was grounded for 80 min and fluctuated from 414 to 456 kg/m3. This is probably attributed to the slight increase of water demand in Table 4. For the compressive strength and specific strength, they increased from 0.93 to 1.93 MPa and from 2.50 to 4.23 kNm/kg, respectively, which mainly is ascribed to number of imperfections or active centers existing at the edges of the TRs particles can be increased at prolonged milling time [24]. The reactivity index in Table 3 also
3.4.4. Effect of additional chemical activators on properties of FC Sodium hydroxide (NaOH) and sodium sulfate (Na2SO4) are the common chemical activators for activating the latent hydraulic property of calcium-siliceous based materials, such as GGBS [30]. In this study, these two activators were used to activate the reactivity of TRs. Effect of Na2SO4 and NaOH on the bulk density and mechanical property of FC specimens based on the above optimum parameters is presented in Fig. 9a and b. It is interesting to note that the bulk density of specimens decreased with an increase of Na2SO4 and the optimum mechanical property was achieved when the content of Na2SO4 was 0.4%, while the addition of NaOH into FC slurry leads to the reduction of mechanical strength and increase of bulk density. The maximum compressive strength and specific strength reached up to 2.14 MPa and 4.91 kNm/kg, respectively. The change profile of bulk density of specimens adding Na2SO4 is due to the accelerating hardening speed of slurry but that of specimens adding NaOH is mainly attributed to the negative compatibility between NaOH and foam generated. Owing to the presence of Na2SO4, the alkalinity in wall section was gradually promoted and thus the dissolving rate of silicate and/or aluminosilicate components in TRs increased. However, the addition of NaOH can immediately increase the pH of FC and thus the thickening rate of slurry will be rapidly accelerated [24]. With the increasing curing time, the hydrated products consisting of CA(A)ASAH and ettringite can break the interconnected structure at early ages. Therefore, the profile strength evolution is different from the specimens in the presence of NaOH and Na2SO4. 3.4.5. Effect of silica fume on properties of FC Silica fume (SF) as pozzolanic material is utilized to hinder the collapse of fresh high-porosity (HPFC) foamed concrete during the hardening process and improve the compressive strength of HPFC [18]. The mix proportions are as follows: OPC: RG: TRs = 10:45:45 in which TRs was replaced by SF (0–12%), quick lime 2% (mass percentage), SAC 4%, foam 4.6% and water reducer 0.2% by the mass of total dry materials. The effect of SF on the bulk density and mechanical property of FC is shown in Fig. 10. It can be noted that the bulk density increased with an increase of SF but both compressive strength and specific strength were reversely in proportion to the bulk density. The viscosity of slurry increased with the increasing content of SF, the air bubbles could be broken at higher SF content, therefore, the bulk density of specimens increased. The silica fume in blended mixtures react with portlandite (Ca(OH)2) to produce CASAH gel and thus lower the alkalinity, which results in low dissolving rate of TRs. In such a condition, the reduction of mechanical property occurs. 3.4.6. Effect of foam content on properties of FC Effect of foam content on density and mechanical property of specimens is presented in Fig. 11. The bulk density and mechanical property of specimens go down with an increase of foam content. The relational expressions of foam content (X) and bulk density (Y1) and compressive strength (Y2) are shown in Fig. 11. It is achieved by linearly fitting the two curves and the value of R2 comes up to 0.9704 and 0.9578 respectively, indicating the good matching degree between Y1, Y2 and X. 3.5. Microstructure of foamed concrete
Fig. 8. Effect of milling time of TRs on properties of FC.
3.5.1. X-ray diffraction (XRD) analysis X-ray diffraction (XRD) analysis is performed to investigate the phase change and hardening mechanism of FC. The XRD patterns of
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Fig. 9. Effect of (a) Na2SO4 and (b) NaOH on properties of FC.
Fig. 10. Effect of SF on properties of FC.
Fig. 12. XRD patterns of foamed concrete.
Fig. 11. Effect of foam content on properties of FC.
FC mixtures was a function of time are presented in Fig. 12. As can be seen, the predominant crystalline phases are identified to be gypsum (PDF# 70-0792), ettringite (PDF# 41-1451), calcite (PDF# 47-1743) and khamrabaevite (PDF# 89-3828). However, there are no crystalline peaks or diffused peaks at 18° and 34° of 2h of crystalline portlandite (Ca(OH)2) in Fig. 12, which may be transformed into calcite. The existence of calcite may also be obtained from the carbonization of ettringite [23]. It can also be seen that little change in the relative intensities of the main three characteristic peaks of khamrabaevite, i.e. (d, 2h) = (2.50 Å, 36.0°), (2.16 Å, 41.8°), (1.53 Å, 60.6°) was observed. Reflections for unreacted gypsum were detected in two samples but the intensities of the gypsum were not reduced with curing time, which suggests
that large amount of gypsum merely play a role as filler. However, a small amount of sulfate in waste gypsum has been demonstrated to stimulate the reactivity of silico-calcium components and thus improve the mechanical properties [31,33]. For the ettringite, the intensities of three specific peaks exhibits no significant tendency because the relative intensity are closely related with amount and crystallinity. The broad band at around 20°–35° of two-theta revealed the existence of CA(A)ASAH [34] and unreacted disordered material in TRs. The corresponding main reaction with respect to formation of CA(A)ASAH and ettringite are given by the following Eqs. (1)–(11): When quick lime, alite and belite hydrates, Ca(OH)2 and C3S2H3 (CASAH) are generated as follows:
CaO þ H2 O ! CaðOHÞ2
ð1Þ
2C3 S þ 6H2 O ! C3 S2 H3 þ 3CaðOHÞ2
ð2Þ
2C2 S þ 4H2 O ! C3 S2 H3 þ CaðOHÞ2
ð3Þ
CaðOHÞ2 ! Ca2þ þ 2OH
ð4Þ
2+
Under the adequate attack of OH in such a condition, Ca and silicate, aluminosilicate network formers in TRs are gradually depolymerized and dissolved into the solution, resulting in more CA(A)ASAH and 3CaOAl2O33CaSO432H2O (ettringite) [24] as follows:
Si O Si þ3OH ! ðSiOðOHÞ3 Þ
ð5Þ
J. Zhang et al. / Construction and Building Materials 171 (2018) 109–119
Si O Al þ7OH ! ðSiOðOHÞ3 Þ þ ðAlðOHÞ4 Þ
ð6Þ
XðSiOðOHÞ3 Þ þ YCa2þ þ ðZ X YÞH2 O þ ð2Y XÞOH ! CY SX HZ
ð7Þ
2þ þ 4OH þ 26H2 O 2ðAlðOHÞ4 Þ þ 3SO2 4 þ 6Ca
! 3CaO Al2 O3 3CaSO4 32H2 O
ð8Þ
The ettringite also comes from the hydration of yeelimite and C3A in SAC and OPC [23,27]. In the presence of calcium hydroxide,
3CaO 3Al2 O3 CaSO4 ðyeelimiteÞ þ 8ðCaSO4 2H2 OÞ þ 6CaðOHÞ2 þ 74H2 O ! 3ð3CaO Al2 O3 3CaSO4 32H2 OÞ
ð9Þ
C3 A þ 3CaSO4 2H2 O þ 28H2 O
117
3.5.2. Morphology and EDX analysis The micro-morphology of foamed concrete is shown in Fig. 13. In wall section of FC, the plate-like material in Fig. 13a (A1) and Fig. 13b (B1) is believed to be red gypsum partly consisting of Si and Al (Fig. 14a), indicating that a little hydrated gel i.e. CA(A)A SAH precipitated on the surface of RG. From Fig. 13b (B1), it can be seen that part of gypsum was wrapped by petal-like hydrated gels (CA(A)ASAH), which is beneficial for the better water resistance [35]. There are also long-strip shaped and bulk CA(A)A SAH in Fig. 13a (A2) and Fig. 13 (B2), and needle-like AFt (ettringite) in clusters under the CA(A)ASAH was observed in FC cured for 3 and 28 days. All hydrated products and gypsum linked each other, giving the mechanical strength of FC. However, the cracks around ettringite in Fig. 13b (B2) indicated that excessive ettringite lead to high porosity in wall section and thus result in low mechanical strength. 3.6. Performances of the FC products
! 3CaO Al2 O3 3CaSO4 32H2 O
ð10Þ
In the absence of calcium hydroxide,
3CaO 3Al2 O3 CaSO4 þ 2CaSO4 2H2 O þ 34H2 O ! 3ð3CaO Al2 O3 3CaSO4 32H2OÞ þ 4AlðOHÞ3
ð11Þ
FC has excellent thermal insulation properties because of its cellular character [28,36]. The low thermal conductivity and high specific strength make it valuable for its application in wall structure. According to previous studies, the thermal insulating characteristics of FC are reversely proportional to the density [17,18], but
Fig. 13. SEM micrographs of foamed concrete cured for (a) 3 days; (b) 28 days.
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J. Zhang et al. / Construction and Building Materials 171 (2018) 109–119
Fig. 14. EDX analysis of hydration products.
Table 5 Performances of the foamed concrete and the demand of standard. Density grade
A04 A08 A10
Demand of standard
Test results
Density (kg/m3)
Compressive strength (MPa)
Thermal conductivity (W/mK)
Density (kg/m3)
Compressive strength (MPa)
Thermal conductivity (W/mK)
420 840 1050
0.5–1.0 1.8–3.0 3.5–5.0
0.10 0.21 0.27
407 798 1020
1.4 5.8 7.9
0.08 0.18 0.21
the compressive strength are in proportion to the density. In this section, the density grade of FC and its corresponding compressive strength and thermal conductivity of the FC products based on optimum parameters are shown in Table 5. It is evident that the compressive strength and thermal conductivity of FC increase with the increasing bulk density. Among the control FC specimens, the highest value of compressive strength (7.9 MPa) and thermal conductivity (0.21 W/mK) was found for A10 with density of 1020 kg/m3, which are 464% and 162% higher than those of A04 with density of 407 kg/m3. This is because with the decrease of bulk density, the porosity volume of FC increases and amount of solid phases decrease. In summary, the compressive strength and thermal conductivity of different density grade FC meet the Chinese standard (JG/T 266-2011) [22]. 4. Conclusions Utilization of TRs and RG as the main starting materials for the preparation of foamed concrete is feasible and favorable for environment-protecting and energy-savings. In this paper, the addition of quick lime and sulfoaluminate cement (SAC) shortened the setting times of ternary blended cement, but TRs milled for a prolong time cannot substantially shorten those of blended
cement. In addition, the redundant content of quick lime retards the later hydration of blended binder. The optimum technological parameters for the preparation of FC were as follows: OPC: TRs: RG = 10:45:45, quick lime 2%, SAC 4%, Na2SO4 0.4%, superplasticizer 0.2%, foam 4.6% and W/B 0.60. The maximum compressive strength and specific strength of FC with density of 437 kg/m3 can reach up to 2.14 MPa and 4.91 kNm/kg, respectively. However, the presence of NaOH and silica fume decreased the mechanical strength of FC. Microstructure analysis revealed that the main mineral phases in wall section of FC are demonstrated as floccular and petal-like CA(A)ASAH, need-like ettringite and unreactive platelike gypsum, which are the strength-given components in foamed concrete. In the case of FC products with various bulk densities, both the compressive strength and thermal conductivity of FC specimens meet the demand of a Chinese standard (JG/T 2662011) [22]. Acknowledgement The authors would like to thank for the financial and technological help of the State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials and Southwest University of Science and Technology.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preparation and characterization of foamed concrete with Ti-extracted residues and red gypsum Jiufu Zhang, Yun Yan ⇑, Zhihua Hu School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, SiChuan 621010, PR China
h i g h l i g h t s Large stockpile of TRs and RG were co-treated as main binding materials to prepare FC. Hydration kinetics of blended binder was evaluated. Effects of technological parameters on the bulk density and compressive strength of FC were investigated. Compressive strength and thermal conductivity of FC met the requirements of Chinese standard.
a r t i c l e
i n f o
Article history: Received 14 December 2017 Received in revised form 7 March 2018 Accepted 8 March 2018
Keywords: Foamed concrete Ti-extracted residues Red gypsum Hydration kinetics Mechanical characteristics Microstructure
a b s t r a c t The recycling of Ti-extracted residues (TRs) and red gypsum (RG), which are the industrial wastes from titanium dioxide manufacturing industry, will avoid the increasing impact on the environment due to their difficulty for disposal. In this paper, the ternary blended binder mainly consisting of ordinary Portland cement (OPC), TRs and RG for the preparation of foamed concrete (FC) was formulated in the laboratory. The hydration kinetics of blended cement, mechanical characteristics and microstructure of FC were investigated. The results indicated that quick lime and sulfoaluminate cement (SAC) evidently reduced the setting time of the complex binder, but mechanical activation for TRs could not substantially shorten those of blended binder. In the case of FC, the optimum technological parameters were proposed as follows: OPC: TRs: RG = 10:45:45 (by mass, the same below), quick lime 2%, SAC 4%, Na2SO4 0.4%, water reducer 0.2%, foam 4.6% and W/B 0.60. The maximum compressive strength and specific strength of FC with density of 437 kg/m3 reached up to 2.14 MPa and 4.91 kNm/kg, respectively. Microstructure analysis revealed that the interconnected assemblages in FC are floccular and petal-like hydrated gels, need-like ettringite and unreactive plate-like gypsum. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Since its specific characteristics discovered by many researchers all over the world, titanium dioxide (TiO2) in various structures has been widely applied in energy and environmental analysis systems [1], pesticides’ degradation [2], dye-sensitized solar cells [3,4] and biomedical systems [5] etc. The well-accepted processes for the manufacture of titanium dioxide are chloride process and sulfate process. In the chloride process, titanium dioxide is generally produced from titanium-rich ilmenite through a TiCl4 stage at elevated temperature. Due to the advantages of this process such as high-quality product and small amount of wastes, it is more beneficial for the manufacture of TiO2 pigment. However, the requirement of this process for the qualification of the feedstock that ⇑ Corresponding author. E-mail address: [email protected] (Y. Yan). https://doi.org/10.1016/j.conbuildmat.2018.03.072 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
the content of MgO + CaO is less than 1.5 wt%, limits the reusage of low-grade TiO2-bearing materials with high content of CaO and/or MgO. High-titanium slag (HTS), a solid by-product from the blast furnace iron-making process using vanadium titano-magnetite ore from Panxi region of Sichuan Province (China), contains valuable 20–25% TiO2 (by weight) and calcium-alumina-silicon composites. It was once utilized as aggregates or supplementary cementitious materials in civil engineering due to its voluminous accumulation [6,7]. To make full use of the valuable TiO2 in HTS, an innovative process of so-called carbonization at elevated temperature and low temperature chlorination is proposed by Ansteel Research Institute of Vanadium and Titanium (China) in which ground HTS is heated at 925–1010 °C in the presence of coke and thus titanium carbide (TiC) is produced [8]. After that, 91.77% TiC is transformed into titanium tetrachloride (TiCl4) at 580 °C in the furnace filled with chlorine gas. In this novel process, the TiO2-bearing feedstock
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with high content of CaO and/or MgO can be utilized to yield TiO2. However, a large amount of Ti-extracted residues (TRs) is simultaneously produced in this process and one ton of titanium slag can produce about 0.8 ton of TRs. At the end of this procedure, the asreceived TRs are rapidly cooled by water to a vitreous state. TRs incorporate 3–5% (wt%) alkaline earth metal chlorides, e.g. MgCl2 and CaCl2 as well, which limits its large-scale applications [8,9]. In view of these problems, a new method for managing TRs is critical for high value-added utilization of HTS and environmental protection. Red gypsum (RG) is largely generated in the sulfate process which utilizes the lime or limestone for managing waste sulphuric acid. In China, about 15 million tons of RG are generated yearly in which production of one ton of TiO2 can generate five to six tons of RG. It is a fine powder composed of crystalline gypsum (CaSO42H2O) and amorphous ferric hydroxide (Fe(OH)3) [10]. The common treating method for RG is outdoor stacking, which may cause soil and/or river contamination. Although recycling of RG in cement production [11], gypsum blocks [12] and light-weight wall [13] etc have been studied by many researchers, the consumption rate of RG is still considerably low that cannot achieve the goal of voluminous depletion due to their high transportation cost and high energy demand for treating RG. In light of these problems of TRs and RG, it is a promising approach that the TRs and RG are used as raw materials for preparing construction and building materials. If the TRs and RG were simultaneously utilized as cementitious materials to prepare foamed concrete (FC), this co-treatment method is propitious to waste recycling and energy-savings because FC do not directly contact steel fiber or reinforcement. As all known, FC is considered to be a binary system with pores and solid matrices. Previous studies have shown that the air bubbles in FC slurry are more prone to become unstable because of high gas-liquid interface energy [14,15] and prolonging setting time of fresh FC [16,17]. Desirable FC products can be synthesized through controlling the balance between the hydration rate of solid pastes and stabilizing time of air bubbles [18]. Owing to the low hydration rate of aforementioned ternary blended binder composed of OPC, RG and TRs, the mechanical activation for TRs and the addition of quick lime and sulphoaluminate cement in blended mixture were conducted for accelerating hydration reactions but their influences on the early hydration mechanism need further investigation. In the case of FC, both the technological parameters on bulk density, compressive strength and microstructure were also studied for optimizing FC’s performance.
2. Materials and methods 2.1. Raw materials Foamed concrete (FC) in this experiment were prepared using original red gypsum (RG), finely ground titanium-extracted residues (TRs), ordinary Portland cement (OPC), quick lime, sulfoaluminate cement (SAC) and silica fume (SF), water, superplasticizer and foaming agent. RG and TRs were collected from titanium dioxide plant in Sichuan Province, China. The water content of RG and TRs was 6.4% and 12.3%, respectively. The pH value of RG was determined as 5.5. OPC used in this work was bought from the Lafarge Shuangma cement plant (Jiangyou, Sichuan Province, China), which complied with Chinese standard: GB1752007 [19]. Additionally, the specific gravity of RG, TRs and OPC was 2.33, 2.96 and 3.10. Quick lime was obtained from the local market, and its effective CaO content was 84.2%, additionally, the slake-time and slake-temperature were determined to be 4 min and 92 °C, respectively. Introduction of SF into FC slurries was treated as supplementary cementing materials. Sodium sulfate and sodium hydroxide as chemical activators were also bought from the market. Animal protein foaming agent was diluted and stirred to generate foam and the dilution ratio was kept constant at 1:10 by weight and the obtained foam density was measured as 25 kg/m3.
2.2. Mix proportions of blended binder and preparation of foamed concrete specimens The mix proportions of blended cement are presented in Table 1. For the manufacture of FC, the raw materials were weighed in predetermined proportions (dry basis) and mixed homogeneously in a laboratory mixer for 1 min. The slurries in a pan mixer were then obtained through high-speedily stirring the aforementioned dry mixture with water for 3 min, into which superplasticizer and/or chemical activators were added at the same time. The foam was then added into the slurries and subsequently mixed again for 1 min. Afterwards, the FC slurries were molded in a steel mould with dimensions of 70.7 mm 70.7 mm 70.7 mm. A plastic film was used to cover the fresh FC to prevent from losing moisture and contacting CO2. After being cured in a standard curing chamber (20 ± 1 °C, relative humidity 90%) for 48 h, FC was de-molded and subjected to standard curing conditions until the designated age.
Table 1 Mix proportions of blended cement. Sample number
OPC/%
TRs/%
RG/%
Quick lime/%
SAC/%
W/B
A0 A1 A2 (B4, C90) A3 A4 A5 B0 B2 B6 B8 B10 C60 C70 C80
9.6 9.5 9.4 9.3 9.2 9.1 9.8 9.6 9.2 9.0 8.8 9.4 9.4 9.4
43.2 42.75 42.3 41.85 41.4 40.95 44.1 43.2 41.4 40.5 39.6 42.3 42.3 42.3
43.2 42.75 42.3 41.85 41.4 40.95 44.1 43.2 41.4 40.5 39.6 42.3 42.3 42.3
0 1 2 3 4 5 2 2 2 2 2 2 2 2
4 4 4 4 4 4 0 2 6 8 10 4 4 4
0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6
Note: TRs of Sample A0–A5, B0–B10 were ball milled for 90 min, C60, C70, C80, C90 were ground in a ball mill for 60 min, 70 min, 80 min, 90 min, respectively.
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2.3. Test methods
3. Results and discussion
The chemical components of RG, OPC and TRs were performed by X-ray fluorescence (XRF, Axios, PANalytical, Holland), and the mineralogy of raw materials is conducted via a X-ray diffractometer (XRD, RIGAKU, D/max-1400) using Cu-Ka radiation (step: 0.02°, 40 kV, 70 mA) at various two-theta interval. Particle size distributions of raw materials were conducted via a laser dispersing equipment (Mastersizer 2000, Malven, Worcestershire, UK). The reactivity index test at 7 and 28 days for TRs milled at 60, 70, 80, 90 min was complied with GB/T 18046-2008 [20]. In addition, the methods for measuring the water requirement of normal consistency and setting time of blended cement were in accordance with GB/T 1346-2011 [21]. The investigation of hydration kinetics with regard to the ternary blended binder was conducted at 20 °C through an eight channel TAM Air Isothermal Calorimeter from TA Instruments, USA. The mass of investigated sample and water was set as 4.34 g and 2.60 g, respectively, and then the sample and water were mixed externally for one minute. Better understanding the microstructure of FC was also achieved by XRD (RIGAKU, D/max-1400) and scanning electron microscope (SEM, Ultra 55, Carl Zeiss AG., Germany) after the FC samples were immersed in absolute ethyl alcohol for three days and dried at 40 °C in a vacuum oven for six hours. The tests of compressive strength and dry bulk density were performed in accordance with Chinese standard (JC/T 266-2011) [22]. In order to compare the compressive strengths of specimens with different bulk density, a formula named specific strength was given as S ¼ , (MPa)/D (kg/m3) in which , and D was the average value from triplicate compressive strength and bulk density of FC, respectively. The thermal conductivity of foamed concrete with the dimensions (70.7 mm 70.7 mm 10.0 mm) was determined from three results of each sample by a thermal conductivity analyzer (TCiTM, C-Therm Technologies Ltd., Canada).
3.1. Characterization of raw materials 3.1.1. Chemical and mineral compositions analysis The chemical compositions of raw materials analyzed by XRF are shown in Table 2. It can be seen that the CaO (28.1%) and SO3 (33.39%) are predominantly existed in RG, which contains 11.62% Fe2O3 as well. For the TRs, the main compositions are CaO (26.81%), SiO2 (24.8%), Al2O3 (12.59%) and TiO2 (10.69%). The loss of ignition for RG and TRs are 20.88% and 8.34%, respectively. The mineralogy of RG, OPC and TRs is also shown in Fig. 1. As can be seen, the mineral component in RG is gypsum (CaSO42H2O), however, there are no characteristic peaks of ironrich compounds, which is mainly attributed to the disordered Fe (OH)3 gel resulting from the hydrolyzation of Fe3+ ions. The major phases of the OPC consist of alite (C3S) and belite (C2S), aluminate phase (C3A) and ferrite phase (C4AF), all contributing to the formation of hydrated assemblages and providing a high alkalinity in pore solution (pH 12.5) when OPC contacts water [23]. For the TRs, the main mineral phase is the vitreous body composed of calcium-aluminum-silicon-rich phases (Fig. 1b), as a result, it could exhibit latent hydraulic property in the presence of alkaline activator [24]. There are also observations of hematite (Fe2O3), khamrabaevite (TiC), perovskite (CaTiO3) and wollastonite (CS) in TRs. 3.1.2. Particle size distribution The grain size distribution analysis of RG, OPC was performed using a laser dispersing equipment, as shown in Fig. 2a. The results show that the median particle size (median diameter, 50% of cumulative distribution of particle, D50) of the OPC is similar to that of RG, whereas the big difference of D90 (90% of cumulative distribution of particle) between OPC and RG is also observed.
Table 2 Chemical compositions of raw materials (wt%). Compositions (%)
CaO
SiO2
SO3
Al2O3
Fe2O3
MgO
TiO2
Cl
LOI
RG TRs OPC SF
28.10 26.81 60.85 0.32
1.99 24.80 20.02 94.30
33.39 1.30 4.28 0.38
0.96 12.59 5.20 0.30
11.62 4.00 3.24 0.07
0.92 6.76 1.62 0.10
1.62 10.69 0.37 –
0.06 3.34 0.04 0.09
20.88 8.34 3.27 3.64
Fig. 1. XRD patterns of (a) original RG and OPC and (b) TRs.
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Fig. 2. Particle size distribution of (a) RG and OPC and (b) TRs milled at different time.
Table 3 Reactivity index at 7, 28 days of TRs with different fineness. Milling time
60 min
70 min
80 min
90 min
Reactivity index at 7 d Reactivity index at 28 d
64.5 75.3
70.2 85.7
80.6 90.4
87.5 98.7
Ti-extracted residues are ground in a laboratory ball mill. The particle size distributions of finely milled TRs obtained at different milling time are presented in Fig. 2b and the corresponding reactivity index of TRs at 7 days and 28 days are shown in Table 3. As shown in Fig. 2b, the D50 slightly decreased and the reactivity index of TRs at 7, 28 days showed an evidently increasing tendency as the milling time increasing. This is mainly attributed to the number of imperfections or active centers existing at the edges of the TRs particles can be increased at prolonged milling time [24]. 3.2. Fresh properties of blended cement 3.2.1. Water requirement of normal consistency The effects of quick lime, sulfoaluminate cement (SAC) and TRs’ fineness on the water requirement of normal consistency of blended cement pastes are shown in Table 4. As can be seen, the water requirement of normal consistency of OPC (27.2%) was lower than that of blended cement pastes, fluctuating from 29.1% to 30.4% with no significant tendency. This is mainly because the high water absorption of amorphous gel with high specific area consisting of Fe(OH)3 in red gypsum [10,25]. In Table 4, water demand for normal consistency of blended cement containing more SAC was lower than those of pastes with lower SAC content. This result is possibly attributed to the lower amount of RG in binding system with higher SAC content and particle grading in reference [26]. In addition, the water demand slightly increases with the increase of milling time. 3.2.2. Setting time The results in Table 4 show that the setting time shortened as the content of quick lime increased. When the quick lime content increased from 0 to 5%, the initial and final setting time decreased from 205 to 127 min and 375 to 237 min, respectively. Research has shown a reduction in both initial and final setting time of blended binder when the dosage of SAC increased. When the extent to which SAC was utilized to replace the cementitious binder consisting of OPC, TRs, RG and quick lime was increased from 0 to 10%, the initial and final setting times rapidly decreased from 222 to 99 min and from 330 to 179 min, respectively. The effect of SAC on setting time is apparent, being mainly attributed to the
Table 4 Initial and the final setting times of the ternary blended binder. Sample number
Water requirement of (%)
Initial setting time (min)
Final setting time (min)
A0 A1 A2 (B4, C90) A3 A4 A5 B0 B2 B6 B8 B10 C60 C70 C80
30.4 29.3 29.1 29.2 29.2 29.4 30.4 29.7 28.8 28.7 28.6 28.5 28.7 28.8
205 181 160 154 136 127 222 179 150 134 99 203 194 180
375 333 310 276 254 237 330 312 245 226 179 370 355 333
rapid hydration of yeelimite in SAC [27]. According to She et al. [16], mixes with short setting time are capable to stabilize the air bubbles and narrow the pore size distribution, resulting in good mechanical strength and durability [28]. Work has also shown that with the increasing milling time, initial and final setting times slightly decrease from 203 to 160 min and from 370 min to 310 min, respectively. These results suggest that at early ages, the dissolving rate of Ca2+, silicate and/or aluminosilicate components in TRs could not substantially increase with milling time of TRs. 3.3. Hydration kinetics of blended cement 3.3.1. Effect of quick lime on hydration process of blended cement The influences of the quick lime on the heat evolution of blended cement within the first 120 h (assigned as A0–A5) are presented in Fig. 3a and b. It can be seen from Fig. 3a that there are two peaks in A0 and A1, from the time when the mix contact with water. However, three peaks of the rate of heat evolution are visible in A2-A5 samples. The first peak in all systems is very high, which corresponds to the initial reaction of the C3A in OPC particles and/or digesting process of quick lime. The released heat of initial hydration of yeelimite in SAC contributes to the first peak as well
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Fig. 3. Effect of quick lime content on (a) heat flow; (b) cumulative heat.
[23,27]. Following the first peak, the curves of heat flow rate slow down over a short period and there follow a dormant period or induction period during which higher heat flow rate is observed at higher quick lime content. In such a condition, Ca(OH)2 is mainly liberated by the hydrolysis of quick lime and calcium silicates and the pH in pore solution can be rapidly promoted to a value higher than 12.5 at 20 °C [23,24]. Therefore, the network modifiers in glassy body of TRs such as Ca2+ are rapidly dissolved into the solution due to their low bond energies with oxygen atoms (less than 210 kJ/mol). Under the attack of OH, silicate and/or aluminosilicate components in TRs are gradually depolymerized and dissolved into the solution. Due to the lower solubility of calcium hydroxide than CASAH and/or CAAASAH, a thin hydrated products precipitates rapidly in the solution, resulting in the second heat evolution peak at about 10 h in samples A2–A5. However, the free lime content in A0 and A1 are not enough to substantially accelerate the dissolving rate of silicate and/or aluminosilicate components, so the hydration profile in these two samples are different from those of A2–A5. With the prolonging time, the intense main second (A0, A1) or third hydration peaks (A2–A5) are shifted to later ages with the increasing quick lime. This retarding effect is probably attributed to a coating formed on the surface of TRs composed of hydrated products and hydrated lime. Therefore, the dissolving rate of strength-giving components in TRs reduced in this condition and meanwhile, the migration rate of ions through the coating decreased. Research in Fig. 3b has also shown that the cumulative heat in A3–A5 are higher than A0–A2, which mainly arises from the combination of heat released from digestion of higher quick lime amount and reaction concerning the formation of hydrated
products. It can also be observed that the increasing degrees among A3, A4 and A5 are lower than those of A0, A1 and A2, which probably is ascribed to the retarding effect of high content of quick lime as well. 3.3.2. Effect of SAC on hydration process of blended cement Fig. 4a shows a plot of the heat evolution of samples with different sulfoaluminate cement content. While there is no significant observation concerning the onset of acceleration, the minimum value of heat flow rate in all systems is increasing with an increase of SAC content. In addition the corresponding time at the second peak in all samples is shifted to short ages. This behavior reveals an significant influence of SAC content on the precipitation of hydrated gels i.e. CASAH and/or CA(A)ASAH. Trauchessec et al. [29] have reported that the hydration mechanisms and hardening speed of OPC are modified by the yeelimite and free lime amount. OPC reacts during the first days and the hydration of yeelimite occurs in the presence of lime or portlandite, which consumes the Ca2+ and/or Ca(OH)2 to form ettringite [27], therefore, the dissolving rate of OPC and TRs can be accelerated in samples with adequate SAC. It can also be observed that the onset of third peaks at around 35 h–55 h is shifted to short time with an increase of SAC. This result may be related to the channel of needle-like ettringite on the vitreous body surface of TRs in systems with high SAC, so the ions controlling the hydration of TRs is easy to transfer. Apart from this, the heat released during early ages may accelerate the ongoing hydration at later ages. As can be seen from Fig. 4b, while the cumulative heat of samples before 72 h is increasing with SAC, that of samples after 72 h exhibits no significant tendency. It indicates that the hydration
Fig. 4. Effect of SAC content on (a) heat flow; (b) cumulative heat.
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rate during the first three days is proportional to SAC content and the hydration process later becomes slow, which is mainly attributed to the low diffusing rate of ions with respect to the generation of CASAH and/or CA(A)ASAH. 3.3.3. Effect of milling time of TRs on hydration process of blended cement Fig. 5a has illustrated that the heat flow rate curves are nearly overlapping prior to 10 h and two main exothermic peaks occur in all samples (C60-C90). It can be seen that at around 20 h, the heat flow curve of C90 is higher than that of other three samples, but the curve of C70 lower than that of C60. Approximately at 50 h, the time of corresponding peaks shift to short period with an increase of milling time, meaning that mechanical treatment for TRs can accelerate the hydration process and achieve high early strength. This is predominantly ascribed to the higher amount of active particles at higher specific area and thus dissolving rate of specific components controlling the generation of hydration products can be accelerated [24,30]. The cumulative heat profile slightly increases with an increase of milling time for TRs prior to about 72 h, as shown red rim A (Fig. 5b). However, red rim B shows that higher cumulative heat was achieved at higher milling time after 72 h, which indicates the lower hydration rate at longer period. This is because more hydrated gels are precipitated in samples with finer TRs and the migration rate of ions decreased. 3.4. Effect of various parameters on properties of FC 3.4.1. Effect of quick lime on properties of FC The effect of quick lime on bulk density, compressive strength and specific strength of foamed concrete is presented in Fig. 6. The total weight of dry mixtures was kept constant, and the content of superplasticizer and foam was 0.2%, 4.6% by the mass of dry mix, respectively. The milling time of TRs was 90 min. Water to binder ratio (W/B) in this part was set as 0.6. Results from Fig. 6 show that the bulk density increased from 388 to 473 kg/ m3 when quick lime dosage was added from 0 to 1%, however, the bulk density later almost linearly reduced from 473 to 426 kg/m3 when the content of quick lime increased from 1% to 5%. This is because the hydrated lime (Ca(OH)2) neutralizes the acid remained in RG and immediately promotes the alkalinity of fresh mixes [31]. Even so, the amount of quick lime at 1% cannot be enough to stabilize the air bubbles. With the increase of quick lime from 1% to 5%, the stabilizing process of air bubbles in FC mixtures is realized by the reduction of setting time as shown in Table 4, so the bulk density later decreased. Fig. 6 has also shown that the content of quick lime up to 2% achieves the maximum strength (1.93 MPa) and specific strength
Fig. 6. Effect of quick lime on properties of FC.
(4.23 kNm/kg). The results indicate that the quick lime content at a suitable percentage is favorable for achieving optimum mechanical properties. This is mainly arising from two aspects: (i) the heat release from the digestion of quick lime can accelerate the hydration rate of blended system; (ii) the Ca(OH)2 in liquid slurry also activates the TRs, and thus form more hydrated calcium silicates (CASAH), which leads to enhance the mechanical properties. Nevertheless, both compressive strength and specific strength later go down when quick lime exceeds 2%. According to Tian et al. [31] and Chen et al. [32], more ettringite in hardened binders will be formed and decrease the strength of foamed concrete in such a condition. Apart from this, the reduction of polymerization degree occurs in CA(A)ASAH due to its excessive Ca/Si ratio [31,32]. Based on the results, the optimum dosage of quick lime was determined as 2%.
3.4.2. Effect of sulfoaluminate cement on properties of FC Fig. 7 shows the effect of sulfoaluminate cement (SAC) on bulk density, compressive strength and specific strength of FC. The mix proportions of foamed concrete were set as follows: OPC: RG: TRs = 10:45:45 in which TRs was milled for 90 min, quick lime 2% (mass percentage), foam 4.6% and water reducer 0.2% by the mass of total dry materials. The density from Fig. 7 shows a similar trend to that from Fig. 6. It increases from 458 to 472 kg/m3 and then decreases to 426 kg/m3 at 6% SAC. With an increase of SAC from 6% to 10%, the density keeps almost constant at 427 kg/m3. It can be observed that the SAC at a certain percentage is able to improve the mechanical strength of specimens. The maximum compressive strength (1.93 MPa) from Fig. 7 is achieved at a content of SAC up to 4%, which is 15% higher than that of the specimen (1.64 MPa)
Fig. 5. Effect of milling time of TRs on (a) heat flow; (b) cumulative heat.
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explained ascendant mechanical property of FC with increasing milling time. With regard to the mechanical strength, the milling time of TRs utilized for preparation of FC is 90 min.
Fig. 7. Effect of sulfoaluminate cement (SAC) on properties of FC.
with no SAC. The specific strength also has a similar trend with compressive strength and its highest value reaches up to 4.23 kNm/kg. However, an increase of SAC results in a reduction in strength. Both compressive strength and specific strength dramatically decreases to 0.95 MPa and 2.27 kNm/kg. Afterwards, these two strengths nearly keep a approximate value with the excessive addition of SAC. As can be seen, the appropriate content of SAC in FC specimens is favorable for achieving a desirable strength. As the addition of SAC produced ettringite which compensated the shrinkage of blends such as OPC-SAC based binder [29], the solid phases of FC became dense at 4% SAC. However, the excessive ettringite led to the weak of mechanical characteristic. From the results, adding 4% SAC into blended cement is beneficial for desirable property of FC.
3.4.3. Effect of TRs fineness on properties of FC The influence of TRs fineness on the bulk density and mechanical property is depicted in Fig. 8. The mix compositions for this section are as follows: OPC: RG: TRs = 10:45:45 in which TRs was milled for various time, quick lime 2% (mass percentage), SAC 4%, foam 4.6% and water reducer 0.2% by the mass of total dry materials. It can be seen from Fig. 8 that the bulk density exhibited ascendant tendency except the specimen in which TRs was grounded for 80 min and fluctuated from 414 to 456 kg/m3. This is probably attributed to the slight increase of water demand in Table 4. For the compressive strength and specific strength, they increased from 0.93 to 1.93 MPa and from 2.50 to 4.23 kNm/kg, respectively, which mainly is ascribed to number of imperfections or active centers existing at the edges of the TRs particles can be increased at prolonged milling time [24]. The reactivity index in Table 3 also
3.4.4. Effect of additional chemical activators on properties of FC Sodium hydroxide (NaOH) and sodium sulfate (Na2SO4) are the common chemical activators for activating the latent hydraulic property of calcium-siliceous based materials, such as GGBS [30]. In this study, these two activators were used to activate the reactivity of TRs. Effect of Na2SO4 and NaOH on the bulk density and mechanical property of FC specimens based on the above optimum parameters is presented in Fig. 9a and b. It is interesting to note that the bulk density of specimens decreased with an increase of Na2SO4 and the optimum mechanical property was achieved when the content of Na2SO4 was 0.4%, while the addition of NaOH into FC slurry leads to the reduction of mechanical strength and increase of bulk density. The maximum compressive strength and specific strength reached up to 2.14 MPa and 4.91 kNm/kg, respectively. The change profile of bulk density of specimens adding Na2SO4 is due to the accelerating hardening speed of slurry but that of specimens adding NaOH is mainly attributed to the negative compatibility between NaOH and foam generated. Owing to the presence of Na2SO4, the alkalinity in wall section was gradually promoted and thus the dissolving rate of silicate and/or aluminosilicate components in TRs increased. However, the addition of NaOH can immediately increase the pH of FC and thus the thickening rate of slurry will be rapidly accelerated [24]. With the increasing curing time, the hydrated products consisting of CA(A)ASAH and ettringite can break the interconnected structure at early ages. Therefore, the profile strength evolution is different from the specimens in the presence of NaOH and Na2SO4. 3.4.5. Effect of silica fume on properties of FC Silica fume (SF) as pozzolanic material is utilized to hinder the collapse of fresh high-porosity (HPFC) foamed concrete during the hardening process and improve the compressive strength of HPFC [18]. The mix proportions are as follows: OPC: RG: TRs = 10:45:45 in which TRs was replaced by SF (0–12%), quick lime 2% (mass percentage), SAC 4%, foam 4.6% and water reducer 0.2% by the mass of total dry materials. The effect of SF on the bulk density and mechanical property of FC is shown in Fig. 10. It can be noted that the bulk density increased with an increase of SF but both compressive strength and specific strength were reversely in proportion to the bulk density. The viscosity of slurry increased with the increasing content of SF, the air bubbles could be broken at higher SF content, therefore, the bulk density of specimens increased. The silica fume in blended mixtures react with portlandite (Ca(OH)2) to produce CASAH gel and thus lower the alkalinity, which results in low dissolving rate of TRs. In such a condition, the reduction of mechanical property occurs. 3.4.6. Effect of foam content on properties of FC Effect of foam content on density and mechanical property of specimens is presented in Fig. 11. The bulk density and mechanical property of specimens go down with an increase of foam content. The relational expressions of foam content (X) and bulk density (Y1) and compressive strength (Y2) are shown in Fig. 11. It is achieved by linearly fitting the two curves and the value of R2 comes up to 0.9704 and 0.9578 respectively, indicating the good matching degree between Y1, Y2 and X. 3.5. Microstructure of foamed concrete
Fig. 8. Effect of milling time of TRs on properties of FC.
3.5.1. X-ray diffraction (XRD) analysis X-ray diffraction (XRD) analysis is performed to investigate the phase change and hardening mechanism of FC. The XRD patterns of
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Fig. 9. Effect of (a) Na2SO4 and (b) NaOH on properties of FC.
Fig. 10. Effect of SF on properties of FC.
Fig. 12. XRD patterns of foamed concrete.
Fig. 11. Effect of foam content on properties of FC.
FC mixtures was a function of time are presented in Fig. 12. As can be seen, the predominant crystalline phases are identified to be gypsum (PDF# 70-0792), ettringite (PDF# 41-1451), calcite (PDF# 47-1743) and khamrabaevite (PDF# 89-3828). However, there are no crystalline peaks or diffused peaks at 18° and 34° of 2h of crystalline portlandite (Ca(OH)2) in Fig. 12, which may be transformed into calcite. The existence of calcite may also be obtained from the carbonization of ettringite [23]. It can also be seen that little change in the relative intensities of the main three characteristic peaks of khamrabaevite, i.e. (d, 2h) = (2.50 Å, 36.0°), (2.16 Å, 41.8°), (1.53 Å, 60.6°) was observed. Reflections for unreacted gypsum were detected in two samples but the intensities of the gypsum were not reduced with curing time, which suggests
that large amount of gypsum merely play a role as filler. However, a small amount of sulfate in waste gypsum has been demonstrated to stimulate the reactivity of silico-calcium components and thus improve the mechanical properties [31,33]. For the ettringite, the intensities of three specific peaks exhibits no significant tendency because the relative intensity are closely related with amount and crystallinity. The broad band at around 20°–35° of two-theta revealed the existence of CA(A)ASAH [34] and unreacted disordered material in TRs. The corresponding main reaction with respect to formation of CA(A)ASAH and ettringite are given by the following Eqs. (1)–(11): When quick lime, alite and belite hydrates, Ca(OH)2 and C3S2H3 (CASAH) are generated as follows:
CaO þ H2 O ! CaðOHÞ2
ð1Þ
2C3 S þ 6H2 O ! C3 S2 H3 þ 3CaðOHÞ2
ð2Þ
2C2 S þ 4H2 O ! C3 S2 H3 þ CaðOHÞ2
ð3Þ
CaðOHÞ2 ! Ca2þ þ 2OH
ð4Þ
2+
Under the adequate attack of OH in such a condition, Ca and silicate, aluminosilicate network formers in TRs are gradually depolymerized and dissolved into the solution, resulting in more CA(A)ASAH and 3CaOAl2O33CaSO432H2O (ettringite) [24] as follows:
Si O Si þ3OH ! ðSiOðOHÞ3 Þ
ð5Þ
J. Zhang et al. / Construction and Building Materials 171 (2018) 109–119
Si O Al þ7OH ! ðSiOðOHÞ3 Þ þ ðAlðOHÞ4 Þ
ð6Þ
XðSiOðOHÞ3 Þ þ YCa2þ þ ðZ X YÞH2 O þ ð2Y XÞOH ! CY SX HZ
ð7Þ
2þ þ 4OH þ 26H2 O 2ðAlðOHÞ4 Þ þ 3SO2 4 þ 6Ca
! 3CaO Al2 O3 3CaSO4 32H2 O
ð8Þ
The ettringite also comes from the hydration of yeelimite and C3A in SAC and OPC [23,27]. In the presence of calcium hydroxide,
3CaO 3Al2 O3 CaSO4 ðyeelimiteÞ þ 8ðCaSO4 2H2 OÞ þ 6CaðOHÞ2 þ 74H2 O ! 3ð3CaO Al2 O3 3CaSO4 32H2 OÞ
ð9Þ
C3 A þ 3CaSO4 2H2 O þ 28H2 O
117
3.5.2. Morphology and EDX analysis The micro-morphology of foamed concrete is shown in Fig. 13. In wall section of FC, the plate-like material in Fig. 13a (A1) and Fig. 13b (B1) is believed to be red gypsum partly consisting of Si and Al (Fig. 14a), indicating that a little hydrated gel i.e. CA(A)A SAH precipitated on the surface of RG. From Fig. 13b (B1), it can be seen that part of gypsum was wrapped by petal-like hydrated gels (CA(A)ASAH), which is beneficial for the better water resistance [35]. There are also long-strip shaped and bulk CA(A)A SAH in Fig. 13a (A2) and Fig. 13 (B2), and needle-like AFt (ettringite) in clusters under the CA(A)ASAH was observed in FC cured for 3 and 28 days. All hydrated products and gypsum linked each other, giving the mechanical strength of FC. However, the cracks around ettringite in Fig. 13b (B2) indicated that excessive ettringite lead to high porosity in wall section and thus result in low mechanical strength. 3.6. Performances of the FC products
! 3CaO Al2 O3 3CaSO4 32H2 O
ð10Þ
In the absence of calcium hydroxide,
3CaO 3Al2 O3 CaSO4 þ 2CaSO4 2H2 O þ 34H2 O ! 3ð3CaO Al2 O3 3CaSO4 32H2OÞ þ 4AlðOHÞ3
ð11Þ
FC has excellent thermal insulation properties because of its cellular character [28,36]. The low thermal conductivity and high specific strength make it valuable for its application in wall structure. According to previous studies, the thermal insulating characteristics of FC are reversely proportional to the density [17,18], but
Fig. 13. SEM micrographs of foamed concrete cured for (a) 3 days; (b) 28 days.
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Fig. 14. EDX analysis of hydration products.
Table 5 Performances of the foamed concrete and the demand of standard. Density grade
A04 A08 A10
Demand of standard
Test results
Density (kg/m3)
Compressive strength (MPa)
Thermal conductivity (W/mK)
Density (kg/m3)
Compressive strength (MPa)
Thermal conductivity (W/mK)
420 840 1050
0.5–1.0 1.8–3.0 3.5–5.0
0.10 0.21 0.27
407 798 1020
1.4 5.8 7.9
0.08 0.18 0.21
the compressive strength are in proportion to the density. In this section, the density grade of FC and its corresponding compressive strength and thermal conductivity of the FC products based on optimum parameters are shown in Table 5. It is evident that the compressive strength and thermal conductivity of FC increase with the increasing bulk density. Among the control FC specimens, the highest value of compressive strength (7.9 MPa) and thermal conductivity (0.21 W/mK) was found for A10 with density of 1020 kg/m3, which are 464% and 162% higher than those of A04 with density of 407 kg/m3. This is because with the decrease of bulk density, the porosity volume of FC increases and amount of solid phases decrease. In summary, the compressive strength and thermal conductivity of different density grade FC meet the Chinese standard (JG/T 266-2011) [22]. 4. Conclusions Utilization of TRs and RG as the main starting materials for the preparation of foamed concrete is feasible and favorable for environment-protecting and energy-savings. In this paper, the addition of quick lime and sulfoaluminate cement (SAC) shortened the setting times of ternary blended cement, but TRs milled for a prolong time cannot substantially shorten those of blended
cement. In addition, the redundant content of quick lime retards the later hydration of blended binder. The optimum technological parameters for the preparation of FC were as follows: OPC: TRs: RG = 10:45:45, quick lime 2%, SAC 4%, Na2SO4 0.4%, superplasticizer 0.2%, foam 4.6% and W/B 0.60. The maximum compressive strength and specific strength of FC with density of 437 kg/m3 can reach up to 2.14 MPa and 4.91 kNm/kg, respectively. However, the presence of NaOH and silica fume decreased the mechanical strength of FC. Microstructure analysis revealed that the main mineral phases in wall section of FC are demonstrated as floccular and petal-like CA(A)ASAH, need-like ettringite and unreactive platelike gypsum, which are the strength-given components in foamed concrete. In the case of FC products with various bulk densities, both the compressive strength and thermal conductivity of FC specimens meet the demand of a Chinese standard (JG/T 2662011) [22]. Acknowledgement The authors would like to thank for the financial and technological help of the State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials and Southwest University of Science and Technology.
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