Performance investigation and environmental application of basic oxygen furnace slag – Rice husk ash based composite cementitious materials

Construction and Building Materials 123 (2016) 493–500

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Performance investigation and environmental application of basic oxygen furnace slag – Rice husk ash based composite cementitious materials Wenfeng Yang a,b, Yongjie Xue b,⇑, Shaopeng Wu b, Yue Xiao b, Min Zhou c a b c

Aviation Engineering Institute, Civil Aviation University of China, 618307 Sichuan, Guanghan, China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 430070 Hubei, Wuhan, China School of Resource and Environment Science, Wuhan University, 430070 Hubei, Wuhan, China

h i g h l i g h t s  Basic oxygen furnace slag and rice husk ash based composite cementitious materials.  Investigation on strength and durable properties.  Preparation of cement mortar and concrete.  Stabilization and solidification in contaminated soils and TCLP experiments for evaluation of environmental application.

a r t i c l e

i n f o

Article history: Received 20 January 2015 Received in revised form 23 June 2016 Accepted 15 July 2016 Available online 20 July 2016 Keywords: Basic oxygen furnace slag Rice husk ash Cementitious materials Performance Solidification stabilization

a b s t r a c t In this present work, a novel composite cementitious material was prepared by mechanical activation of basic oxygen furnace slag (BOFs) – rice husk ash (RHA) and admixtures composite system. The basic, mechanical and durable properties of this cementitious blend mortars and concrete were investigated. Then a solidification/stabilization (S/S) process and toxicity characteristic leaching procedure (TCLP) for simulated contaminated soils was utilized to show the engineering and environment application of this cementitious material. Test results showed that ground BOFs and RHA can be used as the supplementary materials for Portland cement. The workability, strength, sulphate attack resistance, dry shrinkage resistance and freezing-thawing resistance performance of this composite cementitious was close or partly superior to those of Portland cement. The maximum compressive strength of concrete at 28 days by using this composite cementitious material was 52.51 MPa. S/S and TCLP test results indicated that BOFs and RHA based composite cementitious materials can be used as stabilizers as cement in fix of Pb, Cd and Cr contaminated soils. 28 days unconfined compressive strength of stabilized soils ranged from 6.65 MPa to 7.45 MPa at 8% of stabilizer content level. Concentration of leached Pb, Cd and total Cr was below the standard level limitation. Ó 2016 Published by Elsevier Ltd.

1. Introduction The annual production of rice from across the globe is around 700 million tons per year [1]. China alone produces more than 200 million tons in 2013 [1], about 20% of which is rice husk which is a well-known agro-industrial by-product in many parts of the world [2]. A large amount of rice husk has been used as bio fuel in power boilers. Even after its incineration, 20% of rice husk’s weight remains as a waste material in the form of rice husk ash ⇑ Corresponding author. E-mail address: [email protected] (Y. Xue). http://dx.doi.org/10.1016/j.conbuildmat.2016.07.051 0950-0618/Ó 2016 Published by Elsevier Ltd.

(RHA) [3]. Hence, resource utilization of RHA is a worldwide interesting issue. Under controlled combustion conditions, rice husk can be burnt into RHA that fulfils the physical characteristics and chemical composition of mineral admixtures [4]. Nowadays the use of mineral additions in the construction industry is constantly increasing mainly due to their environmental and sustainability implications. RHA appears as a specific option for utilization in cement concrete due to its non-crystalline silica structure and highly reactive pozzolanic activity which is obtained during combustion [4,5]. Significant research has been directed towards the utilization of RHA as supplementary cementitious materials [6–8]. For several decades,

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the use of RHA as a highly reactive pozzolanic material in cement paste, mortar and concrete production has been researched [9–20]. Using it provides several advantages, such as improved strength and durability properties, and environmental benefits related to the disposal of waste materials, and to reduced carbon dioxide emissions and to save natural resource. Basic oxygen furnace slag (BOFs) is a final waste material in the basic oxygen furnace steel making process [21]. Management of BOFs has become a significant issue in environmental engineering due to the enormous quantities generated and the associated disposal costs and constraints. In the most recent years, several researchers have studied BOFs and pozzolanic materials system and prepared composite cementitious materials [22,23]. All of researches have agreed that effect of alkali activation from BOFs to pozzolanic materials such as blast furnace slag and coal fly ash dominates the process of hydration reaction. The main mineralogical composition of BOFs is dicalciumsilicate (C2S) and tricalciumsilicate (C3S) which is same to cement clinker. A specific percentage of C2S and C3S can yield a specific amount of Ca(OH)2 and it means BOFs can provide base environment during its hydration process. Therefore, a specific amount of RHA which contains high volume amorphous silica reacts chemically with a specific amount of Ca(OH)2 to form the secondary type of Calcium– Silicate–Hydrated (C–S–H) gel [24–28]. Up to now, little research has been done to investigate the preparation and properties investigation of basic oxygen furnace slag – rice husk ash based cementitious materials. Therefore, this study aims to prepare a novel cementitious material based BOFs and RHA. For this reason, the strength activity index, workability, durability of mortar which contains oxygen furnace slag – rice husk ash based cementitious materials was investigated. Furthermore this cementitious material was used as stabilizer in remediation of sewage sludge to evaluate effect of stabilization and solidification. The knowledge of experimental results is fundamental for possible applications of BOFs and RHA in civil engineering and environmental protection. 2. Materials and methods 2.1. Materials RHA and BOFs used in this study were derived from a local biomass power plant and steel-making factory respectively. The RHA, of high graphitic carbon content and loss on ignition, was obtained by controlled burning, which was performed at a biomass power generation plant and produced ash with milling treatment that was clear in black colour as shown in Fig 1a. The sieve size of BOF slag with two years setting time was controlled under 0.6 mm as original materials for the sequent milling process to produce BOFs powder as shown in Fig. 1b. As can be seen, RHA is mostly amorphous silica and partially crystalline silica. It can be seen from Fig. 2a that RHA has a porous cellular structure and consists of irregular-shaped particles. More porous material

Fig. 1. Image of a) ground RHA; b) ground BOFs.

microstructure of BOFs after been ground can be seen from Fig. 2b. Fig. 3a provides the X-ray powder diffraction (XRD) pattern of RHA. As can be seen, RHA is mostly amorphous silica and partially crystalline silica. Fig 3b provides that BOFs are complex in spectrum and some of them overlapped. Major miner products were RO phase (divalent metal oxides solid solution), dicalcium silicate and tricalcium silicate. The chemical composition of RHA and BOFs by X Ray Fluorescence (XRF) are listed in Table 1. The ASTM type I ordinary Portland cement (OPC) was produced by Huaxin Cement Company. The physical and chemical characteristics of Portland cement are given in Table 1. Natural sand (modulus of fineness 2.5, density 2.65 and absorption capacity 1.1%) were provided from local quarries. The mixing water was local tap water. Sulphonated Napthalene Polymers based superplast (SP) with the specific gravity of 1.220–1.225 was used as a high range water reducer. Heavy metal contaminated soils used in this work were simulated in laboratory by adding cadmium nitrate, chromic nitrate and lead nitrate into clay and sand compound. The concentration of three kinds of chemical contaminants in soils is listed in Table 2. To avoid the effect of dilution, chemical contaminants were added into soils in the beginning, and then mixing for 300 s to make sure chemicals well-distributed; finally, keeping the water content of simulated contaminated soils samples at 10% and putting into glass desiccators for stabilization and solidification test. 2.2. Methods 2.2.1. Strength and durability test program The ground RHA was used as a pozzolanic material while BOFs was used as alkali activator with cement admixture in mortar. The mortar was tested for compressive strength and durability properties. Mixture proportions of mortar are listed in Table 3. Two mixtures scheme were provided. 60–80% of content of total RHA and BOFs used as supplementary cementitious materials in composite cement is remarked as high substitution-rate composite cement (HSCC) while 20–60% of total RHA and BOFs means low substitution-rate composite cement (LSCC). Water–cementitious materials ratio is 0.5; SP content is 0.5% by weight in composite cementitious materials and sand to cementitious materials ratio is 2.75. Control Portland cement mortar mix, without RHA and BOFs, were also included for comparison under same condition. According to Table 3, Ground RHA and BOFs powder dry-mixed in a mortar mixer for 30 s; second, cement (if needed), mixing for another 30 s; thirdly, sand mixing for 20 s; finally, adding a specific water into with SP into mixer and mixing for 120 s. Total 200 s mixing time could make sure RHA and BOFs well distributed and good workability of fresh mortar. Then, the mortar mixtures were cast into plastic moulds and compacted with a plastic rod until no air bubbles observed. After casting, all of the specimens were left in their casts for 24 h and then they were unmolded and immersed in a water curing tank until they were required for the sequent tests. The physical properties of the fresh and hardened mortars, such as the setting time, rate of water adsorption and fluidity were determined in accordance to ASTM C807, C1403 and C1437 respectively. Mechanical testing of hardened mortar samples after 3, 7 and 28 days of curing tests was performed in triplicate. 100 mm  100 mm  100 mm cubic specimens were cast for a compressive strength test according to ASTM C109 wile 40 mm  40 mm  160 mm specimens were cast for a flexural strength test according to ASTM C293. The sulphate resistance of the mortar bars was determined according to ASTM C 1012. The mortar bars were cured in lime water and then immersed in sodium sulphate solution. The length changes were measured using length comparators in the durations. Six replicate specimens per mix were subjected to durability testing for a period of up to 4 months. Rapid freezing–thawing test was basically conducted

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Fig. 2. SEM image of a) ground RHA; b) ground BOFs.

Table 3 Mix proportions of mortars. Serial number

Fig. 3. XRD spectrum of a) ground RHA; b) ground BOFs.

Table 1 Physical and chemical composition analysis of RHA and BOFs. RHA

BOFs

Cement

Physical properties Specific gravity Specific surface, Blaine, m2/kg Nitrogen adsorption, m2/g 80 lm sieve passing percentage,% 45 lm sieve passing percentage,%

2.18 — 25 99.48 97.10

2.95 405 — 94.50 80.25

3.14 350 — 92.25 76.80

Chemical composition Silica (as SiO2) Calcium (as CaO) Iron (as Fe2O3 and FeO) Magnesium (as MgO) Alumina (as Al2O3) Phosphorus (as P2O5) Titanium (as TiO2) Potassium (as K2O) Sulphur (as SO3) Loss on ignition

85.93 2.87 1.12 0.73 2.20 1.37 0.13 3.76 0.26 1.55

13.54 43.38 28.60 5.79 3.27 1.51 0.62 0.05 0.37 2.66

21.48 60.95 2.25 4.22 4.65 — — 1.04 2.15 2.02

Activity index ASTM C311-98b

94.5

88.6

100

Table 2 Heavy metal concentration in simulated contaminated soils. Chemicals

Contaminant

Concentration in soils/mg/kg

Pb(NO3)2 Cd(NO3)2 Cr(NO3)3

Pb Cd Cr

7000 4000 4000

according to ASTM C666. This method was slightly adapted to well understand the behavior of the mortars in the face of detrimental frost conditions. Three samples of each of the mixes were tested after a curing time of 28 days in the wet chamber. The test

BOFs (%)

RHA (%)

Cement (%)

SP (%)

Contrast sample with SP

OPC1

0

0

100

0.5

HSCC (with 60%–80% of cement substituted by RHA and BOFs)

HS1 HS2 HS3 HS4 HS5 HS6 HS7 HS8 HS9

20 20 20 30 30 30 40 40 40

40 50 60 30 40 50 20 30 40

40 30 20 40 30 20 40 30 20

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

Contrast sample without SP

OPC2

0

0

100

0

LSCC (with 20%–60% of cement substituted by RHA and BOFs)

LS1 LS2 LS3 LS4 LS5 LS6 LS7 LS8 LS9

10 10 10 20 20 20 30 30 30

10 20 30 10 20 30 10 20 30

80 70 60 70 60 50 60 50 40

0 0 0.3 0 0.2 0.3 0 0.2 0.3

comprised 25 cycles, each lasting for 1 day; after the test, the sample weights were measured, and the surface appearance of the samples was observed. A full 24-h cycle involved 18 h of storage in a freezer at 20 °C to 15 °C followed by 6 h of immersion in water at a room temperature of 15 °C to 20 °C. Drying shrinkage test was in conformity with ASTM C531. The drying shrinkage of samples with a size of 25 mm  15 mm  280 mm was recorded each day until constant value was observed for at least two successive days. An average of four readings in each batch is considered as the shrinkage value for that particular batch. In concrete program, the preparation of concrete specimens for cylinder compressive strength, the concrete cylinders with dimension of U 100 mm  200 mm were tested for compressive strength according to ASTM C39. 2.2.2. Stabilization solidification and leaching program Several stabilization/solidification (SS) technologies have been applied to treat the waste, even the hazardous materials [29–33]. In this work, a solidification/stabilization process for sewage sludge was utilized to show the engineering and environment application of RHA-BOFs based composite cementitious materials which were used in the S/S process for simulated heavy metal contaminated soils. The volume of stabilizer ranged from 5% to 8% by weight. Water to binder ratio was 10%. All the mixtures were mixed for 10 min in a mechanical mixer at room temperature and were cast into U50 mm  100 mm cylinder moulds and compacted with a steel rod. Water was added at the first 1 min. All the prepared specimens were placed into a fog tank at 25 °C.

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The plastic vials were stripped and the specimens were removed after the curing ages of 7 and 28 days. Unconfined compressive strength was measured according to ASTM D1633 by applying a vertical load axially at a constant strain rate of 0.045 in./min until failure of the cylindrical specimen. For each stabilized procedure, 3 samples were prepared for compressive strength test before leaching test. Testing results were established from these samples for triplicate analyses, and the average value was reported to ensure the reproducibility of the data. After strength test, solidified forms of samples were crushed for Toxicity Characteristic Leaching Procedure (TCLP) test analysis. Several leaching test methods have been used to evaluate leachability of test samples. The TCLP-1311 standard, which has been widely used [34,35], comprises a batch leaching test performed at an L/S (liquid-to-solid) ratio of 20. About 10 g of crushed samples was weighted into polypropylene bottles. About 200 mL of the TCLP leachant (0.1 M HAc, pH = 2.88) was added. The bottles were tumbled at 30 rpm in a rotary extractor at room temperature for 18 h. At the end of the extraction, the leachate was filtered with glass fiber filter paper. The pH of the filtrate was measured and the leachate was acidified by a small amount of concentrated nitric acid to pH < 2 before being analyzed by Atomic Absorption Spectroscopy (AAS) or Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES). 3. Results and discussion 3.1. Workability of mortars The results of water demand, fluidity, setting time and soundness of fresh mortar mixtures are listed in Table 4. It can be seen that different composite cementitious composition leads to different workability results of mortars. Water demand of neat cement mortar is higher than that of composite cementitious mortars in either HSCC or LSCC mixtures. Increasing supplementary materials, such as RHA and BOFs, leads to an increase of water demand. Results of water demand show little change while remaining the same content of RHA in mortars. This indicates that water demand of BOFs is same to that of cement due to similar specific surface area. However, remaining the same content of BOFs in mortars, water demand obviously increases with an increase of RHA. This may be contributed to the broken particles of RHA generated during milling process [20,28]. Increasing

supplementary materials in composite cementitious materials leads to decrease the fluidity (flow table test) of fresh mortar mixtures. This changing rule is corresponding to that of water demand. It can be indicated that RHA influence the fluidity more easily than BOFs. Initial setting time and final setting time of composite cementitious mortars is earlier and later than that of cement mortar respectively. The higher water demand of supplementary materials leads to the higher water adsorption at the beginning of hydration. This is useful to early hydration and initial setting time shifts to an earlier time. During the process of hydration, final setting time delays due to the lower hydration activity of the supplementary materials. It can be concluded that soundness of all kinds of mortar mixtures are good. 3.2. Compressive and flexural strength Strength is usually considered as one of the most important properties of concrete and a major indicator of general quality control. Factors influencing the strength of mortar include the types and quality of materials, the mixture proportion, the construction methods, the curing condition, and the test method. Compressive and flexural strength test results are shown in Table 5, which shows the relationship between testing age and compressive strength of mortar samples with various ground RHA and BOFs content at the same ratio of water to binder and different SP content. In the early phase (7 days of curing time), the addition of ground RHA and BOFs reduces the amount of cement by 20–80%, the volume of capillary pores then increases, accumulating calcium hydroxide (CH) on the interface. As a result, the structure is less compact, causing the strength to be lower than that of the reference specimen. After 28 days, pozzolanic reaction starts to proceed, decreasing the amount of CH and improving the densification. Consequently, the compressive and flexural strength is enhanced in the later phase. Compared to cement, both HSCC and LSCC type cementitious materials decrease the strength of mortar specimens due to supplement of cement by low reactive materials. Remaining same content of RHA in mixtures, increasing content of BOFs leads to obviously decrease the early age strength. This strength difference reduces with the development of hydration. Remaining same content of BOFs in mixtures, strength result shows an obvious decrease in HSCC specimens, while a slight decrease of strength is observed in LSCC specimens. This indicates Table 5 Compressive and flexural strength test results of mortar specimens.

Table 4 Test results of workability of fresh mortar mixtures.

Serial number

Serial number

Water demand (g)

Fluidity (mm)

Initial setting time (min)

Final setting time (min)

Soundness

OPC1 HS1 HS2 HS3 HS4 HS5 HS6 HS7 HS8 HS9 OPC2 LS1 LS2 LS3 LS4 LS5 LS6 LS7 LS8 LS9

140 187 200 211.5 168 184.5 197 153.5 168 183 137 148 157 168 146 154 167 146 155 168

202 181 162 140 192 165 148 195 170 158 175 160 154 128 182 155 138 180 165 140

158 127 133 147 155 147 157 136 146 161 155 136 138 141 150 145 144 158 143 141

256 270 275 284 263 271 276 258 267 283 251 257 265 269 269 271 276 271 279 280

pass pass pass pass pass pass pass pass pass pass pass pass pass pass pass pass pass pass pass pass

OPC1 HS1 HS2 HS3 HS4 HS5 HS6 HS7 HS8 HS9 OPC2 LS1 LS2 LS3 LS4 LS5 LS6 LS7 LS8 LS9

Compressive strength (MPa)

Flexural strength (MPa)

3 days

7 days

28 days

3 days

7 days

28 days

30.5 16.8 13.8 9.8 12.2 10.5 8.8 11.2 9.2 7.2 26.5 26.5 24.6 21.2 24.7 20.8 17.5 20.9 16.8 12.4

37.2 27.8 23.9 17.5 19.1 19.8 18.7 18.4 18.0 16.8 35.4 33.8 31.5 29.8 31.2 30.2 23.1 29.1 25.0 19.6

43.6 32.1 29.8 26.8 28.2 26.2 22.4 25.8 25.0 24.5 46.8 45.2 42.3 40.1 41.8 41.2 41.6 37.2 36.2 30.1

6.5 4.8 3.9 3.5 4.1 3.7 3.6 4.3 3.8 2.5 7.5 6.1 5.6 4.9 5.5 4.7 3.9 5.3 4.8 3.5

8.0 9.0 8.5 5.8 6.3 6.5 6.8 6.6 6.2 5.5 7.8 7.4 6.4 6.2 6.3 6.1 6.0 5.8 5.6 5.5

9.8 8.8 8.2 6.2 7.0 7.1 7.2 7.5 6.4 5.8 9.8 9.5 9.0 8.7 8.8 9.7 9.4 9.0 8.8 7.6

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Fig. 4. Water absorption test results of mortar specimens.

that higher substitution of cement (P30%) by supplementary materials such as RHA will lead to decrease strength due to lower early activation and incompletely pozzolanic reaction. Based on the results of strength, HSCC seems to be more suitable in controlled low strength materials (CLSM) due to better workability, low strength and economic factor, while LSCC can be attempted to be used in cement concrete. For further investigation on durability and potential utilization of BOFs-RHA composite cementitious mortar, water absorption rate, sulphate resistance, rapid freezing–thawing and dry shrinkage of HS1, HS4, HS5, HS7 and HS8 mortar specimens are tested for evaluation of HSCC. Specimens containing LS1, LS2, LS4 and LS5 are tested for evaluation of LSCC potential utilization in cement concrete. 3.3. Water absorption rate Results for permeability properties (water absorption) of mortar specimens are presented in Fig. 4. When BOFs and RHA are used to partially replace OPC, the water absorption increased considerably (up to a range of 30% and 40%, respectively) and was higher than that for reference specimen (100% OPC). However, HSCC mortar specimens with RHA and BOFs tend to show adverse performance with respect to water absorption when the substitution level exceeded 60% by weight of binder. This adverse performance is attributable to supplementary materials’ higher surface area, higher fineness, and subsequent water demand in the mixing process in comparison with neat OPC reference specimen. Especially with fresh state HSCC mortar mixtures blended with RHA, the workability gradually reduced when RHA exceeded 20%. This reduction in the workability of RHA and BOFs blended cement creates voids inside the mortar, and may be responsible for subsequent poor resistance to water absorption. 3.4. Sulphate attack resistance The results from the specimens exposed to 5% sodium sulphate solution are listed in Table 6. Compressive strength of

OPC1 specimen in sodium sulphate solution is lower than that in water, while the opposite rules can be observed that strength of HSCC specimens increases in sodium sulphate solution. Almost all specimens weight increase either in sodium sulphate solution or in water except both the 2.884% of reduction of OPC1 and the 0.026% of reduction of HS8 in sodium sulphate solution. This indicates that BOFs and RHA composite cementitious materials is superior to cement in sulphate attack resistance performance. Compared HS1, HS4 and HS7 with OPC1, an increase of coefficient of sulphate attack resistance indicate clearly that BOFs is more contributable to sulphate attack resistance than RHA with same content substitution of RHA by BOFs. On the other hand, with same content substitution of cement by RHA (HS4 and HS5, HS7 and HS8), an increase of coefficient of sulphate attack resistance also indicate that sulphate attack resistance performance is improved by adding RHA in cement blended mortar. Well-tended appearance of HSCC mortar specimens was observed. It is known that expansion cracking leads to mass loss during a process of saturation. Hence, it can be concluded that a larger expansion reduction was obtained due to decrease of reduction of mass loss. This good effect of BOFs and RHA on sulphate attack resistance can be attributed to larger specific surface area, fineness and pozzolanic activity [13].

3.5. Dry shrinkage Dry shrinkage results for the mortar specimens under investigation are listed in Table 7. These results correspond to the average micro deformations (10 6 mm/mm) of the six specimens were tested for HSCC and OPC under investigation for 90 days. The shrinkage of all the specimens ranges from 142.67 to 702.55  10 6 mm/mm. It can be observed that with an increase of curing days the shrinkage values of all specimens clearly increase and rate of shrinkage changes greatly before a curing time of 56 days. From a curing time of 56 days to 90 days, value of shrinkage has little change. All shrinkage values of HSCC specimens are higher than that of OPC1. Compared to HS1, HS5 and HS4, HS8, increasing content of BOFs while remaining RHA content level leads to reduce shrinkage. It indicates that dry shrinkage resistance performance of BOFs is superior to cement. This may be due to expansion of BOFs produced by free calcium oxide and magnesium oxide during the hydration process. This expansion can offset shrinkage effect from cement. Compared to HS4, HS5 and HS7, HS8, increasing content of RHA while remaining BOFs content level also leads to reduce shrinkage. It is known that silica fume can reduce dry shrinkage of cement [6]. RHA which shows a similar chemical composition and physical properties, especially the specific surface area and fineness can lead to such reduction of dry shrinkage. Result from HS1, HS4 and HS5 shows that a reduction of shrinkage value is observed by increasing content of BOFs, reducing same content of RHA while remaining cement content level.

Table 6 Sulphate attack resistance test results of mortar specimens. Mix

Compressive strength @water RW (MPa)

Compressive strength @sodium sulphate solution RS (MPa)

Coefficient of sulphate attack resistance Rs/RW (%)

OPC1 HS1 HS4 HS5 HS7 HS8

51.25 35.35 28.4 29.2 26.05 21.4

44.53 35.83 29.05 29.98 26.95 25.98

86.89 101.34 102.28 102.67 103.45 121.38

Mass loss rate @water (%) 0.213 0.245 0.323 0.357 0.377 0.480

Mass loss rate @ sodium sulphate solution (%) 2.884 0.042 0.066 0.015 0.072 0.026

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Table 7 Dry shrinkage test results of mortar specimens. Mix

OPC1 HS1 HS4 HS5 HS7 HS8

Rate of dry shrinkage (10

6

mm/mm)

1 day

3 days

5 days

7 days

14 days

21 days

28 days

56 days

90 days

205.33 174.33 150.33 142.67 152.33 146.14

291.33 252.67 238.67 214.33 227.52 213.52

368.33 309.33 291.33 274.33 287.33 287.00

403.83 346.17 334.17 301.17 312.05 306.17

541.83 437.17 447.83 386.17 389.50 375.00

606.87 509.33 495.67 442.33 437.00 425.53

679.24 560.33 526.30 465.67 460.50 449.50

697.15 583.03 558.67 486.67 489.67 ‘475.67

702.55 585.68 572.15 498.62 501.25 488.33

lower air content in RHA specimen which shows better tolerance to rapid freeze–thaw cycles.

3.6. Freezing–thawing resistance The appearance, loss of mass and compressive strength of mortar cubes subjected to repeated freeze–thaw cycles are shown in Table 8. Upon visual inspection (Fig. 5), it is clear that the surface of the specimens in which BOFs and RHA blended cement keeps intact and starts to peel off after 25 cycles and 50 cycles respectively; the level of damage gradually increased from 25 to 50 cycles of freezing thawing test. Significant differences in freeze–thaw resistance are observed between HS1, HS7 and OPC1. It also can be concluded from Table 8 that mass loss and strength loss of HS7 specimen is the highest among all of specimens, while such indexes of HS1 which contains 20% of BOFs is lower than that of OPC1. This indicates that RHA can improve the performance of freeze–thaw resistance while BOFs seems to weak such performance. Compared to HS4 with HS5, the increase of RHA leads to a decrease of loss rate in both cycles. It can be concluded that specimens with less than 40% of BOFs and more than 30% RHA under this investigation is superior to reference specimen in freeze–thaw resistance performance, presumably because of the

3.7. Concrete design and compressive strength LS1, LS2, LS4 and LS5 of LSCC were used in concrete for replacement of cement. Concrete mix proportion and compressive strength of specimens are listed in Table 9. Concrete mixtures were designed at the same water to binder ratio, sand ratio and content of SP for evaluation of effect of LSCC on concrete. With an increase of BOFs and RHA, slump of fresh concrete mixtures decrease obviously due to higher water absorption rate. In the early phase, the addition of BOFs and RHA reduces the amount of cement by 20–30%, the volume of capillary pores then increases, accumulating CH on the interface during the process of hydration. As a result, the structure is less compact, causing the strength to be lower than that of the OPC specimen. After 28 days, pozzolanic reaction starts to proceed, decreasing the amount of CH and improving the densification. Consequently, the compressive strength is enhanced in the later phase. Comparison of the data for 7 and 28 days of curing

Table 8 Freezing and thawing test results of mortar specimens. Mix

OPC1 HS1 HS4 HS5 HS7 HS8

25 cycles

50 cycles

Mass loss

Compressive strength

Mass loss

Compressive strength

Test (%)

Test (MPa)

Blank (MPa)

Loss rate (%)

Test (%)

Test (MPa)

Blank (MPa)

Loss rate (%)

0.94 0.63 0.70 0.72 0.85 0.83

41.94 29.81 22.11 21.42 21.32 20.01

45.61 31.20 23.16 22.31 23.04 21.41

8.05 4.46 4.52 3.98 7.46 6.54

4.47 2.70 3.45 3.19 4.50 3.78

34.81 24.35 20.68 21.60 17.18 18.20

50.35 30.62 29.62 29.45 26.41 24.94

30.87 20.48 30.18 26.66 34.95 27.03

Fig. 5. Deterioration of (a) HS7; (b) HS1; (c) OPC1 specimens after 50 cycles of freezing and thawing.

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W. Yang et al. / Construction and Building Materials 123 (2016) 493–500 Table 9 Concrete mix proportion and compressive strength results. Mix

OPC1 LS1 LS2 LS4 LS5

BOFs (kg m

3

)

0 42 42 84 84

RHA (kg m

0 42 84 42 84

3

)

Cement (kg m 3)

Sand (kg m

420 336 294 294 252

707.6

3

)

Aggregate (kg m 3)

Water (kg m 3)

SP (kg m

1154.4

168

2.1

Slump 3

)

213 187 171 192 174

Compressive strength (MPa) 3 days

7 days

28 days

39.37 32.84 31.59 29.38 29.37

41.64 38.08 36.72 35.16 35.45

51.80 52.51 51.13 50.64 48.15

ages shows that the compressive strength of LSCC concretes increases 13 MPa to 15 MPa which is higher than 10 MPa from OPC specimen. With 0.4 of water to binder ratio and 0.38 of sand to binder ratio, compressive strength at 28 days of LSCC concrete in the 48.15 MPa to 52.51 MPa range is obtained in this investigation. 3.8. Strength of S/S for simulated contaminated soils The solidification of the stabilized simulated contaminated soils by Cd, Pb and Cr, which are provided by adding cadmium nitrate, lead nitrate and chromic nitrate in original soils respectively, was carried out using HS1, LS2 and OPC as stabilizer. Compressive strength of stabilized and solidified contaminated soils specimens are shown in Fig. 6. Although there is no direct correlation between the unconfined compressive strength and the leaching behavior of materials, this test was used as an indirect method to determine whether the waste has been chemically transformed into a monolith. It can be seen that unconfined compressive strength generally increased with increasing binder content and during time. Same strength trend is observed that the early age strength of HSCC and LSCC is lower than that of OPC specimens. After 28 days, strength is comparable for all specimens and LSCC is superior to HSCC and similar to cement in strength of S/S forms respectively. 28 days unconfined compressive strength ranged from 6.65 MPa to 7.45 MPa at 8% of cementitious materials content level indicates that HSCC and LSCC can meet the engineering requirement for solidification of soils. 3.9. TCLP test results The data on TCLP test are presented in Fig. 7 which shows a relationship between different stabilizers at 8% of content level and metal concentration in leachates. The stipulated standard (GB5085.3-2007) in the Chinese regulation for Pb, Cd and Total Cr is 5, 1 and 15 mg L 1 respectively. It can be seen that after S/S treatment, concentration of leached Pb(II), Cd(II) and total Cr is

Fig. 7. Test results of TCLP test for Pb, Cd and Cr in S/S matrix at 7 and 28 days of curing age.

below the standard level limitation. The samples with more stabilizers percentage had lower concentration of leached heavy metals, indicating the addition of stabilizers can facilitate the binding of heavy metals in S/S waste matrix. At the early hydration age, less hydration product such as calcium silicate hydrated (C–S–H) leads to lower strength and higher concentration of leached heavy metals in leachates. The concentration of leached heavy metals decreases from 1.83 mg L 1 to 1.54 mg L 1 for Pb, 0.62 mg L 1 to 0.11 mg L 1 for Cd and 1.15 mg L 1 to 0.08 mg L 1 for total Cr respectively. Under a condition of uncontrolled pH in leachates, final pH values ranged from 9.4 for HS1 to 12.6 for OPC. It should be concluded that the main reason of heavy metal immobilized in cement-based S/S matrix is due to the alkaline nature and buffering capacity provided by CH and C–S–H. Strength of HS1 and LS2 specimen are lower than that of OPC, it indicates that although more RHA content in matrix leads to a decrease of strength of stabilized forms, effect of S/S for Pb and total Cr is enhanced due to physical adsorption of heavy metals in matrix. Concentrations of Pb and Cr increase with an increase of pH value of leachates because Pb and total Cr are easier to be leached at high pH value solution environment than Cd. However, further investigations of selected heavy metals at different controlled pH value by using different leachant need to be discussed.

4. Conclusions

Fig. 6. Test results of unconfined compressive strength at 7 and 28 days of curing age.

According to the results in this study, a number of conclusions can be drawn. Ground BOFs and RHA can be used as the supplementary materials. Composite cementitious mortars with a partial substitution of cement at total BOFs and RHA contents between 20% and 40% exhibited compressive and flexural strength results similar to those of neat cement mortars. Besides, these fresh mortar mixtures prepared by composite cementitious showed the better workability in water demand, fluidity, setting time and soundness as well.

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For evaluation of high substitution composite cement (HSCC), water absorption rate increased with an increase of BOFs and RHA contents. A decrease of mass loss and an increase of strength showed that sulphate attack resistance performance of HSCC is superior to that of cement. An obvious reduction of dry shrinkage values indicated that HSCC has better dry shrinkage resistance ability than cement. Same trend was observed in freezing and thawing test which showed HSCC with less than 40% of BOFs contents and more than 30% RHA contents under this investigation was superior to cement in freeze–thaw resistance performance after 50 cycles. For evaluation of low substitution composite cement (LSCC), a concrete design and compressive strength was conducted. With 0.4 of water to binder ratio and 0.38 of sand to binder ratio, compressive strength at 28 days of LSCC concrete in the 47.14 MPa to 52.51 MPa range is obtained. S/S and TCLP test results indicated that both of HSCC and LSCC can be used as stabilizers as cement in fix of Pb, Cd and Cr contaminated soils. 28 days unconfined compressive strength of stabilized soils ranged from 6.65 MPa to 7.45 MPa at 8% of cementitious materials content level. Concentration of leached Pb, Cd and total Cr was below the standard level limitation, while compared to cement, effect of immobilization of Pb and total Cr was enhanced with an increase of RHA contents due to physical adsorption of heavy metals in composite cement matrix. Acknowledgment Financial and technical supports from Independent Innovation Fund provided by Wuhan University of Technology are gratefully acknowledged. References [1] China National Rice Research Institute. . [2] M.F. Rossella, N. Antonio, Effect of off-white rice husk ash on strength, porosity, conductivity and corrosion resistance of white concrete, Constr. Build. Mater. 31 (2012) 220–225. [3] R.S. Gemma, Effect of rice-husk ash on durability of cementitious materials, Cem. Concr. Compos. 32 (2010) 718–725. [4] M.F.M. Zain, M.N. Islam, F. Mahmud, M. Jamil, Production of rice husk ash for use in concrete as a supplementary cementitious material, Constr. Build. Mater. 25 (2011) 798–805. [5] P.K. Mehta, Properties of blended cements made from rice husk ash, ACI J. 74 (9) (1977) 440–442. [6] RILEM committee 73-SBC, Final report: siliceous by-products for use in concrete, Mater. Struct. 21 (1988) 69–80. [7] N. Moyad, A. Khalaf, Hana.A. Yousif, Use of rice husk ask in concrete, Int. J. Cem. Compos. Lightweight Concr. 6 (1987) 241–248. [8] M. Nehdi, J. Duquette, A. El Damatty, Performance of rice husk ash produced using a new technology as a mineral admixture in concrete, Cem. Concr. Res. 33 (2003) 1203–1210. [9] S.K. Agarwal, Pozzolanic activity of various siliceous materials, Cem. Concr. Res. 36 (9) (2006) 1735–1739. [10] E.A. Basha, R. Hashim, H.B. Mahmud, A.S. Muntohar, Stabilization of residual soil with rice husk ash and cement, Constr. Build. Mater. 19 (6) (2005) 448– 453.

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