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Accelerated calcite precipitation (ACP) method for recycled concrete aggregate (RCA)
Construction and Building Materials 125 (2016) 749–756
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Accelerated calcite precipitation (ACP) method for recycled concrete aggregate (RCA) Boo Hyun Nam a,⇑, Jinwoo An a, Heejung Youn b a b
Department of Civil, Environmental and Construction Engineering, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816, United States Department of Civil Engineering, Hongik University, Sangsu-dong 72-1, Mapo-gu, Seoul 04066, Republic of Korea
h i g h l i g h t s Evaluate short- and long-term geochemical reaction of RCA regarding calcite precipitation. Develop Accelerated Calcite Precipitation (ACP) method for RCA. Use the short-term ACP to evaluate the effect of rock type and fines. Use the long-term ACP to determine the lifetime calcite precipitation. Utilize petrographic analysis tools to check existing chemical conditions of RCA.
a r t i c l e
i n f o
Article history: Received 29 January 2016 Received in revised form 9 August 2016 Accepted 12 August 2016 Available online 29 August 2016 Keywords: Recycled concrete aggregate (RCA) Carbon dioxide Calcite precipitation Accelerated calcite precipitation Drainage French drain
a b s t r a c t Recycled concrete aggregate (RCA) is often used as a replacement of virgin aggregate in embankments, roadbed, asphalt and concrete pavements. The use of RCA in drainage systems as pipe backfill materials has not received a large attention due to clogging potential. Calcite precipitation is one of major causes of permittivity reduction in filter fabrics. The calcite formation is a long-term process; thus, a means to evaluate the potential of calcite precipitation in an accelerated manner is necessary. In this study, an accelerated calcite precipitation (ACP) procedure of RCA was developed, and its performance was evaluated. Two types of ACP methods were devised: short-term and long-term simulations. The long-term ACP procedure can be used to determine maximum lifetime calcite precipitation while the short-term ACP procedure can be used to evaluate the effect of test variables (e.g. aggregate type, fine content, etc.) on the calcite precipitation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction RCA is one of major construction demolition wastes and its beneficial reuse has received a large attention in civil engineering sectors because of economical construction, environmentally friendly materials, reduced cost of waste disposal, and conservation of natural resources [1–4]. RCA consists of aggregate, approximately 65–70% by volume, and cement paste, the remaining 30–35% by volume [2]. The high content of cement paste can significantly affect the properties of RCA, causing higher porosity and absorption and lower abrasion resistance [5–8]. Abbas et al. [9] reported the mortar content of RCA could be as high as 41% by volume. Concrete structures containing Salem and Burdette [10] also reported
⇑ Corresponding author. E-mail addresses: [email protected] (B.H. Nam), [email protected] (H. Youn). http://dx.doi.org/10.1016/j.conbuildmat.2016.08.048 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
that RCA-combined concrete results in a reduced freeze-thaw resistance due to the high absorption of the RCA. Several studies investigated the performance of RCA in those applications in pavement base/subbase layers [4,11–14]. Overall, RCA as base/subbase materials has demonstrated good stability; however, RCA also showed poor permeability performance and clogging of the drainage system [12–15]. Drainage performance can be reduced significantly due to the re-cementation potential of RCA [13]. Several studies reported the potential that the unhydrated residual binder within RCA can continue to hydrate [16,17]. The hydration products such as unhydrated cement, calcium hydroxide (CH), and dicalcium silicate (C2S) present in the adhered mortar of RCA are able to hydrate again and form rehydration products, which creates an internal structure net and may involve self-stabilization and development of compressive strength [13,17,18]. However, recent studies have demonstrated that RCA has no significant re-cementation activities, especially when RCA are stored outside for long period of time [19–24].
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In addition to the recementation, several field studies have reported clogging of a geotextile filter due to accumulation of calcium carbonate (CaCO3, or calcite) precipitation in RCA [11,25–27]. A study in Iowa Department of Transportation (DOT) [28] observed the growth of calcite deposits on the drain outlet wire mesh rodent guard and in some cases it caused complete clogging (see Fig. 1). The Minnesota DOT (MnDOT) has extensive experiences with the use of RCA in conjunction with a geotextile filter fabric [29]. It was found that the filter fabric had lost on average 50% permittivity in 4 years and 53% in 8 years. Samples from the top and bottom of the pipe exhibited less loss of permittivity, while the sidewall samples showed the greatest loss in permittivity. This is due to the phenomenon that the bottom samples would have gathered less calcite precipitation since the bottom of the pipe is submerged for longer periods of time, being exposed to less of the carbon dioxide (CO2) in the atmosphere than the top and side walls. The conclusion derived was that about 17–84% of the permittivity loss was due to calcite precipitation, while the rest of the loss was due to non-carbonate material (fines). Bruinsma et al. [30] presented a MnDOT study of the Lakeville test beds in 1989, where a series of test beds were constructed containing RCA and virgin aggregates. The test beds with the RCA exhibited slightly higher pH values than the test beds with virgin aggregates. After 3 years, test samples of the filter fabric were obtained from the top and bottom of the wrapped edges drains for permittivity testing. Samples taken from both the beds with virgin aggregate and RCA showed a loss in permittivity; however, the permittivity losses for the RCA test beds were determined to be mostly due to calcite precipitate. Approximately 70% of the loss was due to calcite buildup while 30% was from non-carbonate material. There is an urgent need of a means to evaluate the potential of calcite precipitation for the use of RCA in drainage systems, especially in a fast manner because geochemical reaction of in-field RCA may last several decades. In this study, an accelerated calcite precipitation (ACP) procedure was developed. Since French drain system is the proposed application, this study investigates RCA No. 4 gradation, but the proposed ACP method can be used for any other application such as base layers (with different gradation). This paper discusses physical and chemical assessment of RCA and then describes the detail of ACP procedure. The physical assessment includes specific gravity, absorption, unit weight, void, and abrasion resistance tests while the chemical assessment includes Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX), and X-ray diffraction (XRD) tests. The devised ACP testing method consists of short- and long-term simulation procedures. In the short-term ACP, maximum amount of Ca2+ is
extracted from RCA at a time, and unlimited CO2 is supplied to the water containing the finite amount of Ca2+. On the other hand, in the long-term ACP, Ca2+ is continuously extracted from a finite aggregate, and CO2 is unlimitedly supplied to form the calcite precipitation. The short-term procedure evaluates the effect of test variables such aggregate type, fine content, and particle distribution while the long-term ACP procedure can determine the maximum lifetime calcite precipitation under the ‘‘worst” scenario such as acid and unlimited CO2 environment. 2. A Model of calcite precipitation from the outdoor stockpiled RCA Unlike typical RCAs, the RCA obtained in this study does not contain portlandite but include significant amount of calcite (CaCO3), jennite (Ca9H2SI6O18(OH)86H2O), larnite (Ca2SIO4), and quartz (SiO2). The absence of portlandite in XRD analysis (see Fig. 2) indicates that the RCA has been outdoor stockpiled for a long period of time; thus, all portlandites may have been used for the carbonation process. Chemical interactions among water, CO2, and solid calcite result in the secondary of precipitation of calcite. An appropriate chemical model of calcite precipitation is identified as below.
CaCO3in RCA þ H2 CO3 ¼ Ca2þ þ 2HCO3
ð1Þ
CO2dissolved in water þ H2 O ¼ H2 CO3
ð2Þ
Gaseous CO2 from the air dissolves into the water and the dissolved CO2 in the water escapes into the air until the equilibrium.
CO2in the air ¼ CO2dissolved in water
ð3Þ
The chemical equilibrium is ‘‘dynamic”. If the equilibrium is disturbed by increasing the amount of CO2 in the air, the reaction Eq. (2) will adjust by ‘‘reversing” the nature of the disturbance. The amount of CO2 in the air will be reduced by dissolving some of the ‘‘excess” in water. That is, in Eq. (2) the forward direction is favored. The reaction continues until the balance between CO2 in the air and CO2 dissolved in water is that given by Eq. (3).
ð4Þ Assuming that this system is at equilibrium, any process that increases the amount of CO2 will promote the production of H2CO3. The increased H2CO3 will cause the reaction Eq. (1) to shift to the right, which means that CaCO3 will dissolve. Therefore, as
Fig. 1. Photos of clogging due to calcite precipitation and recementaiton of RCA [28].
B.H. Nam et al. / Construction and Building Materials 125 (2016) 749–756
Fig. 2. XRD Spectrum of RCA fines.
CO2 increases, calcite is dissolved. On the other hand, the reverse is also true. Any process reducing the amount of CO2 in the system will cause calcite precipitation. Generally, gases like CO2 get less soluble in warmer solutions. Therefore, if the system is at equilibrium, heating the water will lead to the reduction of the amount of the gas, then Eq. (2) will shift so as to produce more CO2 which results in a decrease in the amount of carbonic acid. Eq. (1) will shift to the left to produce more carbonic acid, resulting in calcite precipitation. Precipitation generally refers to a process which causes rapid nucleation to form small crystals. The nucleation rate is dependent on the surface energy. Either catalytic activity (CO2) of the liquid phase or change of temperature can cause the reduction of the surface energy which will lead to crystal growth. 3. Materials and methods 3.1. Physical tests RCA was obtained from a local construction and demolition waste recycling facility in Orlando Florida. Basic physical tests were conducted on both RCA and limestone (reference aggregate). The specific gravity and absorption were determined in accordance with ASTM C127 [31]. The unit weight and percent voids were measured as specified in ASTM C29 [32]. The bulk unit weight was determined by filling a rigid wall container in three layers. Each layer was rodded with 25S using a tamping rod. With the unit weight and specific gravity, the percent voids for each gradation were then calculated. The abrasion resistance of RCA was measured with the use of the HM-70A Los Angeles Abrasion Machine in accordance with ASTM C535 [33]. 3.2. Petrographic analysis RCA was ground to fine particles (passing the sieve No. 200) by using a ball mill. The powder samples were then chemically characterized by using the Energy Dispersive X-ray (EDX) and X-ray diffraction (XRD) tests according to ASTM C295 [34]. The EDX and XRD were used to characterize the chemical compositions and crystallographic structure of RCA, respectively. In the grinding process, unhydrated cement grains can be exposed if any, and those can be detected in the XRD analysis.
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maximum amount of Ca2+ is extracted from RCA at a time, and unlimited CO2 is supplied to the water containing the finite amount of Ca2+. The amount of calcite precipitation is finite but the effect of affecting parameters can be evaluated. For the material preparation, 1) obtain 5 kg of RCA, 2) place RCA in a container and 3) fill with water (either tap or distilled water) until all RCAs are submerged. The same water-to-aggregate ratio should be used. Once the material is prepared, 4) inject CO2 through the water at a rate of 10 ft3/hr over 3 h and 5) filter out the water to remove RCA fines by using the ASTM D5907 filtration equipment. To obtain calcite, 6) place the leachate (the filtered water) in an oven at 110 °C for 4 h and, 7) take the filtered water out from the oven and filter out again following ASTM D5907 to separate water and calcite precipitation. To get the maximum amount of calcite, 8) repeat the process (6 and 7) with the retained water until negligible calcite is precipitated. The detail procedure of the short-term calcite precipitation is presented in Table 3. After the first cycle with RCA and water, the filtered water is retained, thus, it can be reused, and additional cycles are performed on the filtrate only. In other words, a sufficient amount of calcium ion for the short-term method is obtained from one extraction with RCA, and then unlimited CO2 is supplied to the calcium ion in solution. This method accelerates the production of calcite by injecting CO2 directly into the solution. A portion of the gaseous CO2 is dissolved into the solution and reacts with H2O, resulting in carbonic acid (H2CO3). The increase of carbonic acid encourages the dissolution of free calcium ions and calcium hydroxide (Ca(OH)2). Therefore, any process that increases the CO2 content in the solution also increases the dissolution of calcium ions. It should be noted that Eqs. (1)–(3) are all reversible reactions, that is, the reaction can occur towards the right hand side of the equation or to the left hand side of the equation. This experiment takes advantage of this principle by dissolving the most possible calcium ions, filtering RCA fines, and then precipitating the calcite out of solution. The calcite can be precipitated by heating up the calcium-rich solution in an oven to decrease the solubility of CO2 in the water. Fig. 4a shows the solubility of CO2 in water at different temperatures. A photo of the filtration equipment (as required by ASTM D5907 [44] is shown in Fig. 4b. 3.3.2. Long-term ACP procedure The purpose of the long-term method is to determine the maximum lifetime calcite precipitation under the ‘‘worst” environment that involves acid and CO2-rich environment. The principle is that Ca2+ is continuously extracted from a finite aggregate and CO2 is unlimitedly supplied to form the calcite precipitation. The Ca2+ extraction from the aggregate will be repeated until no Ca2+ extraction is made. The long-term simulation procedure is presented in Table 4. Unlike the short-term method including a limited amount of calcium ions, the aggregate is reused as the continuous calciumion donor until no more Ca2+ is extracted from the aggregate. In the meantime, CO2 is supplied at each cycle to form the calcite precipitation. This method required many cycles of CO2 injection and filtering procedures, which is time-consuming work. The summation of calcite produced over all cycles is considered the lifetime calcite produced by RCA. 4. Results and discussion
3.3. Accelerated calcite precipitation (ACP) methods 4.1. Physical properties 3.3.1. Short-term ACP procedure The purpose of the short-term method is to evaluate the effect of test variables such as aggregate type, fine content, and gradation in the formation of calcite precipitation. The principle is that the
The measured physical properties of RCA and limestone are summarized in Table 1. As shown in the table, Specific gravity of RCA is lower than that of limestone, showing 2.16 and 2.42 respec-
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Void
(a)
(b)
(c)
(d)
Amorphous C-S-H
Calcite
Fig. 3. SEM and EDX data showing (a) the amorphous region of C-S-H in RCA (red rectangular box) (b) EDX analysis of amorphous C-S-H region (Ca/Si = 1.07), (c) the presence of calcite in RCA is evident by its rhombohedral crystal structure, (d) EDX analysis on a point on calcite.
Fig. 4. Solubility of CO2 in water at various temperatures and filtration equipment required for ASTM D5907.
tively. This can be explained by the fact that RCA contains larger void content; thus referred as lightweight aggregate. In addition, the measured absorption values of RCA were relatively high,
compared to the typical absorption value of normal aggregates used in concrete, ranging from 0.5 to 4% [16]. The higher absorption of RCA is caused by larger voids in the RCA. The L.A.
B.H. Nam et al. / Construction and Building Materials 125 (2016) 749–756 Table 1 Physical properties of RCA. Tested Value
Table 4 Long-term calcite precipitation simulation procedure for RCA. Aggregate
Specific gravity Unit weight (g/cm3) Voids (%) L.A. abrasion (%) Absorption (%)
Limestone
RCA
ASTM
2.42 n/a n/a 36.50 3.04
2.16 1.21 43.90 43.70 6.43
ASTM ASTM ASTM ASTM ASTM
C C C C C
127 [31] 29 [32] 29 [32] 535 [33] 127 [31]
Procedure
Task description
Material preparation
T1. Obtain 5 kg of RCA (with specific gradation depending on applications)1 T2. Place RCA in a container and fill with water (either tap or distilled water) until all RCAs are submerged. The same water-to-aggregate ratio should be used 2 T3. Inject CO2 into the water at a rate of 10 ft3/hr over 3 h. The pH of about 6.0 is recommended to be remained during the injection T4. Filter out the water to remove RCA fines by using the ASTM D5907 filtration equipment T5. Place the leachate (the filtered water) in an oven at 110 °C for 4 h T6. Take the filtered water out from oven and filter out again following ASTM D5907 to separate water and calcite precipitation T7. Reuse the RCA and fill with water in a container. Repeat the process from Tasks 2 through 6 until no calcite precipitation is produced3
Extraction
Filtration
abrasion value measured 43.70, which is under the FDOT limit of 45% [13]. This abrasion value indicates that RCAs are very susceptible to degradation and the generation of fines during the aggregate handling procedure, such as transporting, stockpiling, or placing. In a study performed by Kuo [1], RCA samples collected from seven Districts in the state of Florida exhibited similar results. Kahraman and Toraman [15] reported the L.A. abrasion values for virgin limestone aggregates at 28.9% mass loss. 4.2. Petrographic analysis The chemical compositions of RCA fines obtained with EDX are listed in Table 2. The major elements present within the RCA are calcium (Ca), silicon (Si), and aluminum (Al), which are the main chemical components of concrete. The XRD spectrum analysis shown in Fig. 2 indicates that the main mineral components of RCA are calcite (CaCO3) and quartz (SiO2). This observation conTable 2 Chemical compositions of RCA fines. Element
Test values (% by weight)
Oxygen (O) Carbon (C) Aluminum (Al) Silicon (Si) Calcium (Ca) Iron (Fe) Sodium (Na) LOI
35.12 20.79 2.30 7.15 24.62 1.63 0.18 8.21
Table 3 Basic Procedure of Calcite Precipitation from RCA. Procedure
Task
Material preparation
T1. Obtain 5 kg of RCA (with a selected gradation depending on applications)1 T2. Place RCA in a container and fill with water (either tap or distilled water) until all RCAs are submerged. The same water-to-aggregate ratio should be used2 T3. Inject CO2 through the water at a rate of 10 ft3/hr over 3 h. The pH of about 6.0 is recommended to be remained during the injection T4. Filter out the water to remove RCA fines by using the ASTM D5907 filtration equipment T5. Place the leachate (the filtered water) in an oven at 110C for 4 h T6. Take the filtered water out from the oven and filter out again following ASTM D5907 to separate water and calcite precipitation T7. Retain the filtered water and inject CO2 again into the retained water3; repeat the process (Tasks 5 and 6) with the retained water until negligible calcite is precipitated 4
Extraction and filtration
Calcite precipitation Calcite obtain
Repeat
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1 RCA can be used as base course or pipe backfill materials. For each application, the aggregate gradation specified in the standards should be tested. 2 The water-to-aggregate ratio of 1:3 in volume is recommended. 3 The reduced CO2 injection time of 10 min. is recommended because the pH reaches a minimum within the first 10 min. 4 Typically, total four cycles are recommended.
Calcite formation Filtration
Repeat the process
1 RCA can be used as base course or pipe backfill materials. For each application, the aggregate gradation specified in the standards should be tested. 2 The water-to-aggregate ratio of 1:3 in volume is recommended. 3 The RCA will be repeatedly re-used to extract calcium ions till no extraction.
firms that RCA is composed mainly of hydrated cement, sand, and limestone, which is consistent with the EDX results. However, no evidence of the existence of portlandite (Ca(OH)2), the byproduct of cement hydration, was found. This observation supports not only the carbonation processing mechanism [11,35,36] that portlandite gradually transforms into calcite over time but also the leaching-out of portlandite because of its relatively high solubility [36,37], especially when RCA is stored in open stockpiles in Florida at high temperatures and high relative humidity for a long time. The absence of portlandite in RCA was also found in other research [11,17]. Loss on ignition (LOI) was measured at 1000 °C to determine the total volatile content of RCA including the combined water with silicate crystalline lattices. The result of LOI is higher than natural aggregate (LOI: around 2 [46]) which indicates that the amount of water either bound or trapped in cement paste is much higher than natural aggregate. The microscopic morphology and corresponding chemical compositions of the RCA were investigated by using SEM and EDX (see Fig. 3). The EDX data presented in Fig. 3b were obtained from the locations selected in the SEM images (a rectangular box). A large number of pores are clearly seen in all SEM images; these may lead to intrinsic characteristics of high water absorption and low density of the RCA paste. The major elements present within RCA are Ca, Si, and Al, but shows different ratios. Ca–Si ratios based on chemical compositions were also calculated. The Ca–Si ratio is directly related to the CaO concentration [38]. The Ca–Si ratio corresponds to the physical and chemical properties of C-S-H. C-S-H is formed by the hydration of belite (C2S) and alite (C3S) [35,36], and the Ca–Si ratio of C-S-H generally falls in the range of 1.2–2.3 [39], which decreases with time [40]. CaCO3 was detected (see Fig. 3c) and EDX data indicate that two Ca–Si ratios of 1.1 and 2.0 were observed from the microscopic analyses. From the morphology, corresponding chemical compositions, and calculated Ca–Si ratio, Fig. 3a is considered to be the old C-S-H structure, which suggests that the RCA paste has been fully hydrated. The SEM image and EDX analysis shown in Fig. 3b appear to be old jennite (Ca9H2SI6O18(OH)86H2O) as one analogue of C-S-H, and its Ca–Si ratio of 2.0 agrees with other research studies on jennite [39,41]. Calcite with its distinctive rhombohedral structure was detected from the SEM image, as shown in Fig. 3c. The corresponding EDX data with a large amount of Ca and high Ca–Si ratio demonstrates the presence of an environment rich in Ca and also indicates that the
B.H. Nam et al. / Construction and Building Materials 125 (2016) 749–756
4.3. ACP methods 4.3.1. Short-term ACP The result from the short-term calcite precipitation test with RCA (with No. 4 gradation) is shown in Fig. 5. The RCA produced about 1.6 g and 0.2 g of calcite precipitation in the first and second cycles, respectively. After the first cycle, each cycle produces a decreasing amount of calcite for the same amount of CO2 provided. Since almost zero calcite precipitation after the 4th cycle, further CO2-insertion cycle was stopped at the fourth. 4.3.1.1. Effect of aggregate type. Calcite precipitations of RCA and limestone having the same gradation were determined. Before testing, the aggregate samples were washed off to remove dust or fines attached around the aggregates. The same procedure listed in Table 1 was followed, but only three cycles were repeated in order to expedite the process. To reduce testing variations, testing was repeated twice for each aggregate type, and the averages of total calcite precipitation are presented in Fig. 6. The RCA and limestone produced 1.46 g and 0.74 g of calcite precipitation, respectively, which is almost twice the calcite from RCA than limestone. The RCA includes limestone aggregate component as
1.6 1.4
Calcite (g)
1.2 1 0.8 0.6 0.4 0.2 0
1.46
Limestone 1.26
RCA
1 0.74 0.56
0.5 0.18
0.10
0.09 0.01
0
1
2
3
Cycle
Total
Fig. 6. Short-term simulated calcite precipitation from limestone and RCA.
well as hydrated cement paste. As a result, the RCA contains more calcium-ion donors and result in higher amount of calcite than limestone. 4.3.1.2. Effect of fines. The effect of fine content on the calcite precipitation was also evaluated. In order to control the fine content, RCA was washed off, and then 0, 2, 4, and 6% of fines (passing sieve No. 200) were added to the washed RCA. Four cycles of the CO2 injection were conducted. The same procedure presented in Table 3 was followed and the test results are shown in Fig. 7. The results illustrate that a higher percentage of fines produces higher amount of calcite precipitation. While the washed RCA can still produce calcite, the addition of 6% fines can double the calcite produced from the washed RCA with 0% fines. 2.5 2 1.5 1 0.5 0 0
2
% Fines
4
6
Fig. 7. Effect of fines in calcite precipitation.
Sum of 6 consecutive cycles (g)
1.8
1.5
Calcite (g)
hydration reaction of this RCA paste has been completed and the hydrated cement minerals have been carbonated over time. Similar results were found in other research [42]. It is important to note that calcite precipitation is affected by such parameters including storage condition and time, raw materials (difference types of cement and aggregate) and mix design (mix proportioning) in concrete. Particularly, the storage condition can significantly affect chemical properties of RCA because geochemical reaction continues as far as RCAs are exposed to water and CO2. For example, fresh concrete contains C–S–H and portlandite, but the portlandite reacts with CO2 and causes carbonation. The level of portlandite also determines the level of pH. One the other hand, the long-term outdoor stored RCAs may not contain portlandite and unhydrated cement grains. Therefore, effective assessment tools to better understand the material variation issues are the XRD and EDX because they can quantify the chemical components in RCAs. Cement and aggregate types, specially cement type, also can’t be overlooked since the percentages of compound compositions are different depending on cement types. ASTM C150 [50] classifies cement into five types (Type I–V) based on chemical composition, physical properties and special usage. For instance, Type I has 45– 55% of Tri-calcium silicate (C3S) which is the major chemical compound of cement. On the other hand, Type IV has 25–35% of C3S [51]. This difference can affect not only the degree of cement hydration but also the level of carbonation of portlandite.
Calcite (g)
754
2.5
5 Cycle Sum
2 1.5 1 y = -0.3746x + 2.6603 R² = 0.9809
0.5 0 1-6
1
2
3
4
Cycle # Fig. 5. Calcite produced from initial 5 h of CO2 injection.
7-12
13-18
19-24
25-30
31-36
37-42
43-54
Cycle #
Fig. 8. Decreasing trend of consecutive calcite precipitation cycles and predicted calcite reduction (1 Set = 5 calcite simulation cycles).
B.H. Nam et al. / Construction and Building Materials 125 (2016) 749–756
755
9
y = -0.0051x2 + 0.4077x R² = 0.9984
Cumulative Calcite (g)
8 7 6 5 4 3 2 1 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
Cycle # Fig. 9. Cumulative calcite precipitation.
RCA is being produced with a series of processes including crushing, stockpiling, transporting, and placing before being used in construction. During these processes, RCA fines are produced. The deposition of RCA fines on filter fabrics may accelerate physical clogging and reduce the filter fabric’s permittivity, creating a likelihood of calcite precipitation reactivity. As seen in Fig. 7, smaller particles have larger surface area and lead to more active chemical reaction, result in higher calcite precipitation. 4.3.2. Long-term ACP The test results were grouped by 6 consecutive cycles (1 set = 6 cycles of calcite simulation) and the calcite precipitations produced during each set are shown in Fig. 8. Grouping 6 cycles at a time shows the trend of decreasing calcite. The cumulative calcite precipitations over total 36 cycles are shown in Fig. 9. The trendline presented in the figure shows a nonlinear shape with two-order polynomial equation. As expected, there is a decreasing trend in the amount of calcite precipitation over the 36 consecutive cycles. When this process is continued the RCA will reach a point in which the majority of the calcium has been extracted and reacted with CO2 to form the maximum possible calcite from the 5-kg RCA sample. This information will be useful for estimating the total possible calcite potential from a known quantity of RCA. Over time it can be expected that the total calcite production can be up to 8.8–9.0 g. The trendline shown in Fig. 9 is meaningful to drainage engineers because total estimated calcite precipitation can be determined prior to the construction. With 5-kg RCA (No. 4 gradation), total calcite precipitation is about 9.0 g which is less than 1% of material by weight. Practically, less than 1% is negligible considering accumulation of fines surrounding soils in the field. In other words, calcite precipitation may not cause filter clogging but transported fines from surrounding soils. Likewise, the long-term ACP method can be used to check RCA for any other drainage application. 5. Summary and conclusions One of RCA’s beneficial utilizations is its use in drainage systems as pipe backfill materials; however, many studies have raised a clogging concern due to accumulation of calcite precipitation, reducing the permittivity of filer fabric. Clear understanding of the physical and chemical properties of RCA is essential to promote the beneficial use of RCA; more importantly engineers need an effective and accelerated tool to evaluate long-term clogging potential due to calcite precipitation. This paper presents the development of accelerated calcite precipitation (ACP) procedure and also utilization of petrographic
analysis tools (SEM, EDX, and XRD) for understanding existing chemical conditions of RCA. The authors developed two types of the ACP methods that are short-term and long-term ACP procedures. The short-term ACP is aimed at evaluating the effect of testing variables such as, but not limited to, aggregate type and fine content while the long-term ACP is aimed to determine maximum life-cycle calcite precipitation (under acid and unlimited CO2 conditions). A series of proof-of-concept tests were conducted. According to the short-term ACP tests, limestone exhibited lower calcite precipitation than RCA and increase of fine content (smaller than 75 lm) increase the calcite precipitation. The long-term ACP test (after 36 cycles) produces a total calcite precipitation of 8.1 g out of 5 kg of RCA, which is about 0.16% by weight. This amount may be negligible; however; it is possible to have significant calcite precipitation because RCA’s chemical property may vary depending on its source and storage condition. If significant calcite precipitation is expected, engineers may think of aggregate washing prior to the use of RCA in construction. This study also illustrates that the combined use of SEM, EDX, and XRD can indicate potential source of Ca2+ donors that are the main source of calcite precipitation. With the RCA used in this study, SEM and XRD show significant amount of calcite and no existence of portlandite. Thus, one can infer the stored environment of RCA; for example, outdoor storage conditions under weathering (e.g. rainfall) for a long period of time. Acknowledgement This study was carried out with the support of ‘‘Enhancement of the natural hazard response system in rural communities by benchmarking the U.S.A. hazard response systems (PJ010891)”, National Institute of Agricultural Sciences, Rural Development Administration, Republic of Korea. In addition, the author also appreciates technical laboratory supports of Mr. Zachary Behring. References [1] K.Y. Ann, H.Y. Moon, Y.B. Kim, J. Ryou, Durability of recycled aggregate concrete using pozzolanic materials, Waste Manage. 28 (6) (2008) 993–999. [2] C.-S. Poon, Z.H. Shui, L. Lam, Effect of microstructure of ITZ on compressive strength of concrete prepared with recycled aggregates, Constr. Build. Mater. 18 (6) (2004) 461–468. [3] R. Zaharieva, F. Buyle-Bodin, F. Skoczylas, E. Wirquin, Assessment of the surface permeation properties of recycled aggregate concrete, Cement Concr. Compos. 25 (2) (2003) 223–232. [4] C.F. Hendriks, H.S. Pietersen, Sustainable Raw Materials: Construction and Demolition Waste, RILEM Publications, Cachan, France, 2000. [5] M.S. de Juan, P.A. Gutiérrez, Study on the influence of attached mortar content on the properties of recycled concrete aggregate, Constr. Build. Mater. 23 (2) (2009) 872–877.
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[6] A. Shayan, A. Xu, Performance and properties of structural concrete made with recycled concrete aggregate, Am. Concr. J. Mater. J. 100 (5) (2003) 371. [7] S. Nagataki, A. Gokce, T. Saeki, Effects of recycled aggregate characteristics on performance parameters of recycled aggregate concrete, Am. Concr. Inst. Spec. Publ. 192 (2000) 51–72. [8] M.E. Vancura, D. Tompkins, L. Khazanovich, PCC Mix Designs Using Recycled Concrete Pavements, Presented at 21st Annual Transportation Research Conference, St. Paul, MN, 2010. [9] A. Abbas, G. Fathifazl, I.O. Burkan, A.G. Razaqpur, B. Fournier, S. Foo, Proposed method for determining the residual mortar content of recycled concrete aggregates, J. ASTM Int. 5 (1) (2008). [10] R.M. Salem, E.G. Burdette, Role of chemical and mineral admixtures on physical properties and frost-resistance of recycled aggregate concrete, ACI Mater. J. 95 (1998) 558–563. [11] Y. Song, P.S.K. Ooi, E. Hellebrand, D.W. Muenow, Potential for Tufa precipitation from crushed concrete containing coarse basaltic and fine coralline sand aggregates, Environ. Eng. Geosci. 17 (1) (2011) 53–66. [12] T. Bennert, A. Maher, The use of recycled concrete aggregate in a dense graded aggregate base course, Department of Civil and Environmental Engineering, Center for Advanced Infrastructure and Transportation, Rutgers University, Piscataway, N.J., 2008. [13] P. Paige-Green, Preliminary evaluation of the reuse of cementitious materials, Proc. 29th Annual Southern African Transport Conference, Pretoria, South Africa, 2010, pp. 520–529. [14] M. Chen, J. Lin, S. Wu, Potential of recycled fine aggregates powder as filler in asphalt mixture, Constr. Build. Mater. 25 (10) (2011) 3909–3914. [15] R.G. Hicks, NCHRP Synthesis of Highway Practice 175: Moisture Damage in Asphalt Concrete, TRB, National Research Council, Washington, D.C., 1991. [16] L. Chai, C.L. Monismith, J.T. Harvey, Re-Cementation of Crushed Material in Pavement Bases, Institute of Transportation Studies, University of California, Davis, 2009. [17] C.-S. Poon, X.C. Qiao, D. Chan, The cause and influence of self-cementing properties of fine recycled concrete aggregates on the properties of unbound sub-base, Waste Manage. 26 (10) (2006) 1166–1172. [18] Z. Shui, D. Xuan, H. Wan, B. Cao, Rehydration reactivity of recycled mortar from concrete waste experienced to thermal treatment, Constr. Build. Mater. 22 (8) (2008) 1723–1729. [19] J.-Y. Kim, B.H. Nam, Z. Behring, B.A. Al Muhit, Evaluation of re-cementation reactivity of recycled concrete aggregate fines, Transp. Res. Rec. 2401 (2014) 44–51. [20] Z. Behring, J. Kim, B. Nam, M. Chopra, J. Shoucair, Drainage performance evaluation of reclaimed concrete aggregate, in: Geotechnical Special Publication 245: Innovative and Sustainable Use of Geomaterials and Geosystems, 2014, pp. 33–40. [21] B.H. Nam, Z. Behring, J.Y. Kim, M. Chopra, J. Shoucair, C.-S. Park, Evaluating the use of recycled concrete aggregate in a French drain applications, ASTM J. Test. Eval. 43 (2) (2014) 1–11. [22] B. Nam, Z. Behring, M. Chopra, J. Shoucair, S. Nam, Investigation on clogging potential of recycled concrete aggregate in French drain systems, IFCEE (2015) 2787–2796. [23] Z. Behring, B. Nam, J. Kim, M. Chopra, J. Shoucair, Clogging potential of recycled concrete in road drainage, in: T&DI Congress 2014, 2014, pp. 153–162. [24] B.H. Nam, Z. Behring, J. Kim, M. Chopra, Evaluate the Use of Reclaimed Concrete Aggregate in French Drain Applications, Report No. BDK78TWO97712, Florida Department of Transportation, Tallahassee, FL, 2014. [25] M. Snyder, Use of Crushed Concrete Products in Minnesota Pavement Foundations. Report MN/RN-96/12 to Minnesota Department of Transportation, 1995. [26] R.W. Muethel, Calcium Carbonate Precipitate from Crushed Concrete. Research Report No.R-1297, Michigan Department of Transportation, Materials and Technology Division, Lansing, MI, 1989. [27] R. Tamirisa, Study of Highway Base/Subbase Aggregates that Cause Depositions of Calcareous ‘‘Tufa” in Drains Master’s Thesis, University of Toledo, Department of Civil Engineering, Toledo, OH, 1993.
[28] R. Steffes, Laboratory study of the Leachate from crushed Portland cement concrete base material, Report No. MLR-96-4, Iowa DOT, September 1999, 1999. [29] M. Snyder, J. Bruinsma, A review of studies concerning the effects of unbound concrete bases on PCC pavements drainage, Transp. Res. Rec. 1519 (1996) 51–58. [30] J.E. Bruinsma, K.R. Peterson, M.B. Snyder, Chemical approach to formation of calcite precipitate from recycled concrete aggregate base layers, Transp. Res. Rec. 1577 (1997) 10–17. [31] ASTM C127, Standard Test Method for Density, Relative Density (Specific Gravity), and Absorption of Coarse Aggregate, American Society for Testing and Materials, West Conshohocken, PA, 2012. [32] ASTM C29, C29M, Standard Test Method for Bulk Density (‘‘Unit Weight”) and Voids in Aggregate, American Society for Testing and Materials, West Conshohocken, PA, 2010. [33] ASTM C535, Standard Test Method for Resistance to Degradation of Large-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine, American Society for Testing and Materials, West Conshohocken, PA, 2000. [34] ASTM C295, C295M, Standard Guide for Petrographic Examination of Aggregates for Concrete, American Society for Testing and Materials, West Conshohocken, PA, 2012. [35] S. Mindess, J.F. Young, D. Darwin, Concrete, Prentice Hall, Upper Saddle River, N.J., 2003. [36] P.K. Mehta, P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials, McGraw-Hill, New York, 2006. [37] P.C. Hewlett, Lea’s Chemistry of Cement and Concrete, Elsevier ButterworthHeinemann, Oxford, United Kingdom, 2004. [38] F. Bergaya, B.K. Theng, G. Lagaly, Handbook of Clay Science, Elsevier, Oxford, United Kingdom, 2011. [39] I.G. Richardson, G.W. Groves, The structure of the calcium silicate hydrate phases present in hardened pastes of white Portland cement/blast-furnace slag blends, J. Mater. Sci. 32 (18) (1997) 4793–4802. [40] P. Barnes, J. Bensted, Structure and Performance of Cements, Taylor & Francis, London, 2002. [41] H.F.W. Taylor, Proposed structure for calcium silicate hydrate gel, J. Am. Ceram. Soc. 69 (6) (1986) 464–467. [42] V.J. Subramani, Atomistic Simulations for the Prediction of Physical Properties of Cement-Based Materials, ProQuest, Mich, Ann Arbor, 2008. [44] ASTM D5907, Standard Test Methods for Filterable Matter (Total Dissolved Solids) and Nonfilterable Matter (Total Suspended Solids) in Water, West Conshohocken, PA, 2013. [46] Derick Wade Immelman, The influence of percentage replacement on the aggregate and concrete properties from commercially produced coarse recycled concrete aggregate, 2013 (Thesis). [50] ASTM, Standard Specification for Portland Cement. C150/C150M-16e1, American Society for Testing and Materials, West Conshohocken, PA, 2016. [51] S.H. Kosmatka, B. Kerkhoff, W.C. Panarese, Design and Control of Concrete Mixtures, 14th ed., Portland Cement Association, Skokie, IL, 2002.
Further reading [43] S.N. Ghosh, Advances in Cement Technology, Pergamon Press, Elmsford, N.Y., 1983. [45] Fred. Goldberg, Rate of increasing concentrations of atmospheric carbon dioxide controlled by natural temperature variations, Energy Environ. 19 (7) (2008) 995–1011. [47] J.A. Gard, H.F.W. Taylor, G. Cliff, G.W. Lorimer, A reexamination of Jennite, Am. Mineral. 62 (1977) 365–368. [48] I. Kusachi, C. Henmi, K. Henmi, Afwillite and Jennite from Fuka, Okayama Prefecture, Japan, Mineral. J. 14 (1989) 279–292. [49] I.G. Richardson, G.W. Groves, The structure of the calciumsilicate hydrate phases present in hardened pastes of white Portland cement/blast-furnace slag blends, J. Mater. Sci. 32 (1997) 4793–4802.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Accelerated calcite precipitation (ACP) method for recycled concrete aggregate (RCA) Boo Hyun Nam a,⇑, Jinwoo An a, Heejung Youn b a b
Department of Civil, Environmental and Construction Engineering, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816, United States Department of Civil Engineering, Hongik University, Sangsu-dong 72-1, Mapo-gu, Seoul 04066, Republic of Korea
h i g h l i g h t s Evaluate short- and long-term geochemical reaction of RCA regarding calcite precipitation. Develop Accelerated Calcite Precipitation (ACP) method for RCA. Use the short-term ACP to evaluate the effect of rock type and fines. Use the long-term ACP to determine the lifetime calcite precipitation. Utilize petrographic analysis tools to check existing chemical conditions of RCA.
a r t i c l e
i n f o
Article history: Received 29 January 2016 Received in revised form 9 August 2016 Accepted 12 August 2016 Available online 29 August 2016 Keywords: Recycled concrete aggregate (RCA) Carbon dioxide Calcite precipitation Accelerated calcite precipitation Drainage French drain
a b s t r a c t Recycled concrete aggregate (RCA) is often used as a replacement of virgin aggregate in embankments, roadbed, asphalt and concrete pavements. The use of RCA in drainage systems as pipe backfill materials has not received a large attention due to clogging potential. Calcite precipitation is one of major causes of permittivity reduction in filter fabrics. The calcite formation is a long-term process; thus, a means to evaluate the potential of calcite precipitation in an accelerated manner is necessary. In this study, an accelerated calcite precipitation (ACP) procedure of RCA was developed, and its performance was evaluated. Two types of ACP methods were devised: short-term and long-term simulations. The long-term ACP procedure can be used to determine maximum lifetime calcite precipitation while the short-term ACP procedure can be used to evaluate the effect of test variables (e.g. aggregate type, fine content, etc.) on the calcite precipitation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction RCA is one of major construction demolition wastes and its beneficial reuse has received a large attention in civil engineering sectors because of economical construction, environmentally friendly materials, reduced cost of waste disposal, and conservation of natural resources [1–4]. RCA consists of aggregate, approximately 65–70% by volume, and cement paste, the remaining 30–35% by volume [2]. The high content of cement paste can significantly affect the properties of RCA, causing higher porosity and absorption and lower abrasion resistance [5–8]. Abbas et al. [9] reported the mortar content of RCA could be as high as 41% by volume. Concrete structures containing Salem and Burdette [10] also reported
⇑ Corresponding author. E-mail addresses: [email protected] (B.H. Nam), [email protected] (H. Youn). http://dx.doi.org/10.1016/j.conbuildmat.2016.08.048 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
that RCA-combined concrete results in a reduced freeze-thaw resistance due to the high absorption of the RCA. Several studies investigated the performance of RCA in those applications in pavement base/subbase layers [4,11–14]. Overall, RCA as base/subbase materials has demonstrated good stability; however, RCA also showed poor permeability performance and clogging of the drainage system [12–15]. Drainage performance can be reduced significantly due to the re-cementation potential of RCA [13]. Several studies reported the potential that the unhydrated residual binder within RCA can continue to hydrate [16,17]. The hydration products such as unhydrated cement, calcium hydroxide (CH), and dicalcium silicate (C2S) present in the adhered mortar of RCA are able to hydrate again and form rehydration products, which creates an internal structure net and may involve self-stabilization and development of compressive strength [13,17,18]. However, recent studies have demonstrated that RCA has no significant re-cementation activities, especially when RCA are stored outside for long period of time [19–24].
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In addition to the recementation, several field studies have reported clogging of a geotextile filter due to accumulation of calcium carbonate (CaCO3, or calcite) precipitation in RCA [11,25–27]. A study in Iowa Department of Transportation (DOT) [28] observed the growth of calcite deposits on the drain outlet wire mesh rodent guard and in some cases it caused complete clogging (see Fig. 1). The Minnesota DOT (MnDOT) has extensive experiences with the use of RCA in conjunction with a geotextile filter fabric [29]. It was found that the filter fabric had lost on average 50% permittivity in 4 years and 53% in 8 years. Samples from the top and bottom of the pipe exhibited less loss of permittivity, while the sidewall samples showed the greatest loss in permittivity. This is due to the phenomenon that the bottom samples would have gathered less calcite precipitation since the bottom of the pipe is submerged for longer periods of time, being exposed to less of the carbon dioxide (CO2) in the atmosphere than the top and side walls. The conclusion derived was that about 17–84% of the permittivity loss was due to calcite precipitation, while the rest of the loss was due to non-carbonate material (fines). Bruinsma et al. [30] presented a MnDOT study of the Lakeville test beds in 1989, where a series of test beds were constructed containing RCA and virgin aggregates. The test beds with the RCA exhibited slightly higher pH values than the test beds with virgin aggregates. After 3 years, test samples of the filter fabric were obtained from the top and bottom of the wrapped edges drains for permittivity testing. Samples taken from both the beds with virgin aggregate and RCA showed a loss in permittivity; however, the permittivity losses for the RCA test beds were determined to be mostly due to calcite precipitate. Approximately 70% of the loss was due to calcite buildup while 30% was from non-carbonate material. There is an urgent need of a means to evaluate the potential of calcite precipitation for the use of RCA in drainage systems, especially in a fast manner because geochemical reaction of in-field RCA may last several decades. In this study, an accelerated calcite precipitation (ACP) procedure was developed. Since French drain system is the proposed application, this study investigates RCA No. 4 gradation, but the proposed ACP method can be used for any other application such as base layers (with different gradation). This paper discusses physical and chemical assessment of RCA and then describes the detail of ACP procedure. The physical assessment includes specific gravity, absorption, unit weight, void, and abrasion resistance tests while the chemical assessment includes Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX), and X-ray diffraction (XRD) tests. The devised ACP testing method consists of short- and long-term simulation procedures. In the short-term ACP, maximum amount of Ca2+ is
extracted from RCA at a time, and unlimited CO2 is supplied to the water containing the finite amount of Ca2+. On the other hand, in the long-term ACP, Ca2+ is continuously extracted from a finite aggregate, and CO2 is unlimitedly supplied to form the calcite precipitation. The short-term procedure evaluates the effect of test variables such aggregate type, fine content, and particle distribution while the long-term ACP procedure can determine the maximum lifetime calcite precipitation under the ‘‘worst” scenario such as acid and unlimited CO2 environment. 2. A Model of calcite precipitation from the outdoor stockpiled RCA Unlike typical RCAs, the RCA obtained in this study does not contain portlandite but include significant amount of calcite (CaCO3), jennite (Ca9H2SI6O18(OH)86H2O), larnite (Ca2SIO4), and quartz (SiO2). The absence of portlandite in XRD analysis (see Fig. 2) indicates that the RCA has been outdoor stockpiled for a long period of time; thus, all portlandites may have been used for the carbonation process. Chemical interactions among water, CO2, and solid calcite result in the secondary of precipitation of calcite. An appropriate chemical model of calcite precipitation is identified as below.
CaCO3in RCA þ H2 CO3 ¼ Ca2þ þ 2HCO3
ð1Þ
CO2dissolved in water þ H2 O ¼ H2 CO3
ð2Þ
Gaseous CO2 from the air dissolves into the water and the dissolved CO2 in the water escapes into the air until the equilibrium.
CO2in the air ¼ CO2dissolved in water
ð3Þ
The chemical equilibrium is ‘‘dynamic”. If the equilibrium is disturbed by increasing the amount of CO2 in the air, the reaction Eq. (2) will adjust by ‘‘reversing” the nature of the disturbance. The amount of CO2 in the air will be reduced by dissolving some of the ‘‘excess” in water. That is, in Eq. (2) the forward direction is favored. The reaction continues until the balance between CO2 in the air and CO2 dissolved in water is that given by Eq. (3).
ð4Þ Assuming that this system is at equilibrium, any process that increases the amount of CO2 will promote the production of H2CO3. The increased H2CO3 will cause the reaction Eq. (1) to shift to the right, which means that CaCO3 will dissolve. Therefore, as
Fig. 1. Photos of clogging due to calcite precipitation and recementaiton of RCA [28].
B.H. Nam et al. / Construction and Building Materials 125 (2016) 749–756
Fig. 2. XRD Spectrum of RCA fines.
CO2 increases, calcite is dissolved. On the other hand, the reverse is also true. Any process reducing the amount of CO2 in the system will cause calcite precipitation. Generally, gases like CO2 get less soluble in warmer solutions. Therefore, if the system is at equilibrium, heating the water will lead to the reduction of the amount of the gas, then Eq. (2) will shift so as to produce more CO2 which results in a decrease in the amount of carbonic acid. Eq. (1) will shift to the left to produce more carbonic acid, resulting in calcite precipitation. Precipitation generally refers to a process which causes rapid nucleation to form small crystals. The nucleation rate is dependent on the surface energy. Either catalytic activity (CO2) of the liquid phase or change of temperature can cause the reduction of the surface energy which will lead to crystal growth. 3. Materials and methods 3.1. Physical tests RCA was obtained from a local construction and demolition waste recycling facility in Orlando Florida. Basic physical tests were conducted on both RCA and limestone (reference aggregate). The specific gravity and absorption were determined in accordance with ASTM C127 [31]. The unit weight and percent voids were measured as specified in ASTM C29 [32]. The bulk unit weight was determined by filling a rigid wall container in three layers. Each layer was rodded with 25S using a tamping rod. With the unit weight and specific gravity, the percent voids for each gradation were then calculated. The abrasion resistance of RCA was measured with the use of the HM-70A Los Angeles Abrasion Machine in accordance with ASTM C535 [33]. 3.2. Petrographic analysis RCA was ground to fine particles (passing the sieve No. 200) by using a ball mill. The powder samples were then chemically characterized by using the Energy Dispersive X-ray (EDX) and X-ray diffraction (XRD) tests according to ASTM C295 [34]. The EDX and XRD were used to characterize the chemical compositions and crystallographic structure of RCA, respectively. In the grinding process, unhydrated cement grains can be exposed if any, and those can be detected in the XRD analysis.
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maximum amount of Ca2+ is extracted from RCA at a time, and unlimited CO2 is supplied to the water containing the finite amount of Ca2+. The amount of calcite precipitation is finite but the effect of affecting parameters can be evaluated. For the material preparation, 1) obtain 5 kg of RCA, 2) place RCA in a container and 3) fill with water (either tap or distilled water) until all RCAs are submerged. The same water-to-aggregate ratio should be used. Once the material is prepared, 4) inject CO2 through the water at a rate of 10 ft3/hr over 3 h and 5) filter out the water to remove RCA fines by using the ASTM D5907 filtration equipment. To obtain calcite, 6) place the leachate (the filtered water) in an oven at 110 °C for 4 h and, 7) take the filtered water out from the oven and filter out again following ASTM D5907 to separate water and calcite precipitation. To get the maximum amount of calcite, 8) repeat the process (6 and 7) with the retained water until negligible calcite is precipitated. The detail procedure of the short-term calcite precipitation is presented in Table 3. After the first cycle with RCA and water, the filtered water is retained, thus, it can be reused, and additional cycles are performed on the filtrate only. In other words, a sufficient amount of calcium ion for the short-term method is obtained from one extraction with RCA, and then unlimited CO2 is supplied to the calcium ion in solution. This method accelerates the production of calcite by injecting CO2 directly into the solution. A portion of the gaseous CO2 is dissolved into the solution and reacts with H2O, resulting in carbonic acid (H2CO3). The increase of carbonic acid encourages the dissolution of free calcium ions and calcium hydroxide (Ca(OH)2). Therefore, any process that increases the CO2 content in the solution also increases the dissolution of calcium ions. It should be noted that Eqs. (1)–(3) are all reversible reactions, that is, the reaction can occur towards the right hand side of the equation or to the left hand side of the equation. This experiment takes advantage of this principle by dissolving the most possible calcium ions, filtering RCA fines, and then precipitating the calcite out of solution. The calcite can be precipitated by heating up the calcium-rich solution in an oven to decrease the solubility of CO2 in the water. Fig. 4a shows the solubility of CO2 in water at different temperatures. A photo of the filtration equipment (as required by ASTM D5907 [44] is shown in Fig. 4b. 3.3.2. Long-term ACP procedure The purpose of the long-term method is to determine the maximum lifetime calcite precipitation under the ‘‘worst” environment that involves acid and CO2-rich environment. The principle is that Ca2+ is continuously extracted from a finite aggregate and CO2 is unlimitedly supplied to form the calcite precipitation. The Ca2+ extraction from the aggregate will be repeated until no Ca2+ extraction is made. The long-term simulation procedure is presented in Table 4. Unlike the short-term method including a limited amount of calcium ions, the aggregate is reused as the continuous calciumion donor until no more Ca2+ is extracted from the aggregate. In the meantime, CO2 is supplied at each cycle to form the calcite precipitation. This method required many cycles of CO2 injection and filtering procedures, which is time-consuming work. The summation of calcite produced over all cycles is considered the lifetime calcite produced by RCA. 4. Results and discussion
3.3. Accelerated calcite precipitation (ACP) methods 4.1. Physical properties 3.3.1. Short-term ACP procedure The purpose of the short-term method is to evaluate the effect of test variables such as aggregate type, fine content, and gradation in the formation of calcite precipitation. The principle is that the
The measured physical properties of RCA and limestone are summarized in Table 1. As shown in the table, Specific gravity of RCA is lower than that of limestone, showing 2.16 and 2.42 respec-
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Void
(a)
(b)
(c)
(d)
Amorphous C-S-H
Calcite
Fig. 3. SEM and EDX data showing (a) the amorphous region of C-S-H in RCA (red rectangular box) (b) EDX analysis of amorphous C-S-H region (Ca/Si = 1.07), (c) the presence of calcite in RCA is evident by its rhombohedral crystal structure, (d) EDX analysis on a point on calcite.
Fig. 4. Solubility of CO2 in water at various temperatures and filtration equipment required for ASTM D5907.
tively. This can be explained by the fact that RCA contains larger void content; thus referred as lightweight aggregate. In addition, the measured absorption values of RCA were relatively high,
compared to the typical absorption value of normal aggregates used in concrete, ranging from 0.5 to 4% [16]. The higher absorption of RCA is caused by larger voids in the RCA. The L.A.
B.H. Nam et al. / Construction and Building Materials 125 (2016) 749–756 Table 1 Physical properties of RCA. Tested Value
Table 4 Long-term calcite precipitation simulation procedure for RCA. Aggregate
Specific gravity Unit weight (g/cm3) Voids (%) L.A. abrasion (%) Absorption (%)
Limestone
RCA
ASTM
2.42 n/a n/a 36.50 3.04
2.16 1.21 43.90 43.70 6.43
ASTM ASTM ASTM ASTM ASTM
C C C C C
127 [31] 29 [32] 29 [32] 535 [33] 127 [31]
Procedure
Task description
Material preparation
T1. Obtain 5 kg of RCA (with specific gradation depending on applications)1 T2. Place RCA in a container and fill with water (either tap or distilled water) until all RCAs are submerged. The same water-to-aggregate ratio should be used 2 T3. Inject CO2 into the water at a rate of 10 ft3/hr over 3 h. The pH of about 6.0 is recommended to be remained during the injection T4. Filter out the water to remove RCA fines by using the ASTM D5907 filtration equipment T5. Place the leachate (the filtered water) in an oven at 110 °C for 4 h T6. Take the filtered water out from oven and filter out again following ASTM D5907 to separate water and calcite precipitation T7. Reuse the RCA and fill with water in a container. Repeat the process from Tasks 2 through 6 until no calcite precipitation is produced3
Extraction
Filtration
abrasion value measured 43.70, which is under the FDOT limit of 45% [13]. This abrasion value indicates that RCAs are very susceptible to degradation and the generation of fines during the aggregate handling procedure, such as transporting, stockpiling, or placing. In a study performed by Kuo [1], RCA samples collected from seven Districts in the state of Florida exhibited similar results. Kahraman and Toraman [15] reported the L.A. abrasion values for virgin limestone aggregates at 28.9% mass loss. 4.2. Petrographic analysis The chemical compositions of RCA fines obtained with EDX are listed in Table 2. The major elements present within the RCA are calcium (Ca), silicon (Si), and aluminum (Al), which are the main chemical components of concrete. The XRD spectrum analysis shown in Fig. 2 indicates that the main mineral components of RCA are calcite (CaCO3) and quartz (SiO2). This observation conTable 2 Chemical compositions of RCA fines. Element
Test values (% by weight)
Oxygen (O) Carbon (C) Aluminum (Al) Silicon (Si) Calcium (Ca) Iron (Fe) Sodium (Na) LOI
35.12 20.79 2.30 7.15 24.62 1.63 0.18 8.21
Table 3 Basic Procedure of Calcite Precipitation from RCA. Procedure
Task
Material preparation
T1. Obtain 5 kg of RCA (with a selected gradation depending on applications)1 T2. Place RCA in a container and fill with water (either tap or distilled water) until all RCAs are submerged. The same water-to-aggregate ratio should be used2 T3. Inject CO2 through the water at a rate of 10 ft3/hr over 3 h. The pH of about 6.0 is recommended to be remained during the injection T4. Filter out the water to remove RCA fines by using the ASTM D5907 filtration equipment T5. Place the leachate (the filtered water) in an oven at 110C for 4 h T6. Take the filtered water out from the oven and filter out again following ASTM D5907 to separate water and calcite precipitation T7. Retain the filtered water and inject CO2 again into the retained water3; repeat the process (Tasks 5 and 6) with the retained water until negligible calcite is precipitated 4
Extraction and filtration
Calcite precipitation Calcite obtain
Repeat
753
1 RCA can be used as base course or pipe backfill materials. For each application, the aggregate gradation specified in the standards should be tested. 2 The water-to-aggregate ratio of 1:3 in volume is recommended. 3 The reduced CO2 injection time of 10 min. is recommended because the pH reaches a minimum within the first 10 min. 4 Typically, total four cycles are recommended.
Calcite formation Filtration
Repeat the process
1 RCA can be used as base course or pipe backfill materials. For each application, the aggregate gradation specified in the standards should be tested. 2 The water-to-aggregate ratio of 1:3 in volume is recommended. 3 The RCA will be repeatedly re-used to extract calcium ions till no extraction.
firms that RCA is composed mainly of hydrated cement, sand, and limestone, which is consistent with the EDX results. However, no evidence of the existence of portlandite (Ca(OH)2), the byproduct of cement hydration, was found. This observation supports not only the carbonation processing mechanism [11,35,36] that portlandite gradually transforms into calcite over time but also the leaching-out of portlandite because of its relatively high solubility [36,37], especially when RCA is stored in open stockpiles in Florida at high temperatures and high relative humidity for a long time. The absence of portlandite in RCA was also found in other research [11,17]. Loss on ignition (LOI) was measured at 1000 °C to determine the total volatile content of RCA including the combined water with silicate crystalline lattices. The result of LOI is higher than natural aggregate (LOI: around 2 [46]) which indicates that the amount of water either bound or trapped in cement paste is much higher than natural aggregate. The microscopic morphology and corresponding chemical compositions of the RCA were investigated by using SEM and EDX (see Fig. 3). The EDX data presented in Fig. 3b were obtained from the locations selected in the SEM images (a rectangular box). A large number of pores are clearly seen in all SEM images; these may lead to intrinsic characteristics of high water absorption and low density of the RCA paste. The major elements present within RCA are Ca, Si, and Al, but shows different ratios. Ca–Si ratios based on chemical compositions were also calculated. The Ca–Si ratio is directly related to the CaO concentration [38]. The Ca–Si ratio corresponds to the physical and chemical properties of C-S-H. C-S-H is formed by the hydration of belite (C2S) and alite (C3S) [35,36], and the Ca–Si ratio of C-S-H generally falls in the range of 1.2–2.3 [39], which decreases with time [40]. CaCO3 was detected (see Fig. 3c) and EDX data indicate that two Ca–Si ratios of 1.1 and 2.0 were observed from the microscopic analyses. From the morphology, corresponding chemical compositions, and calculated Ca–Si ratio, Fig. 3a is considered to be the old C-S-H structure, which suggests that the RCA paste has been fully hydrated. The SEM image and EDX analysis shown in Fig. 3b appear to be old jennite (Ca9H2SI6O18(OH)86H2O) as one analogue of C-S-H, and its Ca–Si ratio of 2.0 agrees with other research studies on jennite [39,41]. Calcite with its distinctive rhombohedral structure was detected from the SEM image, as shown in Fig. 3c. The corresponding EDX data with a large amount of Ca and high Ca–Si ratio demonstrates the presence of an environment rich in Ca and also indicates that the
B.H. Nam et al. / Construction and Building Materials 125 (2016) 749–756
4.3. ACP methods 4.3.1. Short-term ACP The result from the short-term calcite precipitation test with RCA (with No. 4 gradation) is shown in Fig. 5. The RCA produced about 1.6 g and 0.2 g of calcite precipitation in the first and second cycles, respectively. After the first cycle, each cycle produces a decreasing amount of calcite for the same amount of CO2 provided. Since almost zero calcite precipitation after the 4th cycle, further CO2-insertion cycle was stopped at the fourth. 4.3.1.1. Effect of aggregate type. Calcite precipitations of RCA and limestone having the same gradation were determined. Before testing, the aggregate samples were washed off to remove dust or fines attached around the aggregates. The same procedure listed in Table 1 was followed, but only three cycles were repeated in order to expedite the process. To reduce testing variations, testing was repeated twice for each aggregate type, and the averages of total calcite precipitation are presented in Fig. 6. The RCA and limestone produced 1.46 g and 0.74 g of calcite precipitation, respectively, which is almost twice the calcite from RCA than limestone. The RCA includes limestone aggregate component as
1.6 1.4
Calcite (g)
1.2 1 0.8 0.6 0.4 0.2 0
1.46
Limestone 1.26
RCA
1 0.74 0.56
0.5 0.18
0.10
0.09 0.01
0
1
2
3
Cycle
Total
Fig. 6. Short-term simulated calcite precipitation from limestone and RCA.
well as hydrated cement paste. As a result, the RCA contains more calcium-ion donors and result in higher amount of calcite than limestone. 4.3.1.2. Effect of fines. The effect of fine content on the calcite precipitation was also evaluated. In order to control the fine content, RCA was washed off, and then 0, 2, 4, and 6% of fines (passing sieve No. 200) were added to the washed RCA. Four cycles of the CO2 injection were conducted. The same procedure presented in Table 3 was followed and the test results are shown in Fig. 7. The results illustrate that a higher percentage of fines produces higher amount of calcite precipitation. While the washed RCA can still produce calcite, the addition of 6% fines can double the calcite produced from the washed RCA with 0% fines. 2.5 2 1.5 1 0.5 0 0
2
% Fines
4
6
Fig. 7. Effect of fines in calcite precipitation.
Sum of 6 consecutive cycles (g)
1.8
1.5
Calcite (g)
hydration reaction of this RCA paste has been completed and the hydrated cement minerals have been carbonated over time. Similar results were found in other research [42]. It is important to note that calcite precipitation is affected by such parameters including storage condition and time, raw materials (difference types of cement and aggregate) and mix design (mix proportioning) in concrete. Particularly, the storage condition can significantly affect chemical properties of RCA because geochemical reaction continues as far as RCAs are exposed to water and CO2. For example, fresh concrete contains C–S–H and portlandite, but the portlandite reacts with CO2 and causes carbonation. The level of portlandite also determines the level of pH. One the other hand, the long-term outdoor stored RCAs may not contain portlandite and unhydrated cement grains. Therefore, effective assessment tools to better understand the material variation issues are the XRD and EDX because they can quantify the chemical components in RCAs. Cement and aggregate types, specially cement type, also can’t be overlooked since the percentages of compound compositions are different depending on cement types. ASTM C150 [50] classifies cement into five types (Type I–V) based on chemical composition, physical properties and special usage. For instance, Type I has 45– 55% of Tri-calcium silicate (C3S) which is the major chemical compound of cement. On the other hand, Type IV has 25–35% of C3S [51]. This difference can affect not only the degree of cement hydration but also the level of carbonation of portlandite.
Calcite (g)
754
2.5
5 Cycle Sum
2 1.5 1 y = -0.3746x + 2.6603 R² = 0.9809
0.5 0 1-6
1
2
3
4
Cycle # Fig. 5. Calcite produced from initial 5 h of CO2 injection.
7-12
13-18
19-24
25-30
31-36
37-42
43-54
Cycle #
Fig. 8. Decreasing trend of consecutive calcite precipitation cycles and predicted calcite reduction (1 Set = 5 calcite simulation cycles).
B.H. Nam et al. / Construction and Building Materials 125 (2016) 749–756
755
9
y = -0.0051x2 + 0.4077x R² = 0.9984
Cumulative Calcite (g)
8 7 6 5 4 3 2 1 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
Cycle # Fig. 9. Cumulative calcite precipitation.
RCA is being produced with a series of processes including crushing, stockpiling, transporting, and placing before being used in construction. During these processes, RCA fines are produced. The deposition of RCA fines on filter fabrics may accelerate physical clogging and reduce the filter fabric’s permittivity, creating a likelihood of calcite precipitation reactivity. As seen in Fig. 7, smaller particles have larger surface area and lead to more active chemical reaction, result in higher calcite precipitation. 4.3.2. Long-term ACP The test results were grouped by 6 consecutive cycles (1 set = 6 cycles of calcite simulation) and the calcite precipitations produced during each set are shown in Fig. 8. Grouping 6 cycles at a time shows the trend of decreasing calcite. The cumulative calcite precipitations over total 36 cycles are shown in Fig. 9. The trendline presented in the figure shows a nonlinear shape with two-order polynomial equation. As expected, there is a decreasing trend in the amount of calcite precipitation over the 36 consecutive cycles. When this process is continued the RCA will reach a point in which the majority of the calcium has been extracted and reacted with CO2 to form the maximum possible calcite from the 5-kg RCA sample. This information will be useful for estimating the total possible calcite potential from a known quantity of RCA. Over time it can be expected that the total calcite production can be up to 8.8–9.0 g. The trendline shown in Fig. 9 is meaningful to drainage engineers because total estimated calcite precipitation can be determined prior to the construction. With 5-kg RCA (No. 4 gradation), total calcite precipitation is about 9.0 g which is less than 1% of material by weight. Practically, less than 1% is negligible considering accumulation of fines surrounding soils in the field. In other words, calcite precipitation may not cause filter clogging but transported fines from surrounding soils. Likewise, the long-term ACP method can be used to check RCA for any other drainage application. 5. Summary and conclusions One of RCA’s beneficial utilizations is its use in drainage systems as pipe backfill materials; however, many studies have raised a clogging concern due to accumulation of calcite precipitation, reducing the permittivity of filer fabric. Clear understanding of the physical and chemical properties of RCA is essential to promote the beneficial use of RCA; more importantly engineers need an effective and accelerated tool to evaluate long-term clogging potential due to calcite precipitation. This paper presents the development of accelerated calcite precipitation (ACP) procedure and also utilization of petrographic
analysis tools (SEM, EDX, and XRD) for understanding existing chemical conditions of RCA. The authors developed two types of the ACP methods that are short-term and long-term ACP procedures. The short-term ACP is aimed at evaluating the effect of testing variables such as, but not limited to, aggregate type and fine content while the long-term ACP is aimed to determine maximum life-cycle calcite precipitation (under acid and unlimited CO2 conditions). A series of proof-of-concept tests were conducted. According to the short-term ACP tests, limestone exhibited lower calcite precipitation than RCA and increase of fine content (smaller than 75 lm) increase the calcite precipitation. The long-term ACP test (after 36 cycles) produces a total calcite precipitation of 8.1 g out of 5 kg of RCA, which is about 0.16% by weight. This amount may be negligible; however; it is possible to have significant calcite precipitation because RCA’s chemical property may vary depending on its source and storage condition. If significant calcite precipitation is expected, engineers may think of aggregate washing prior to the use of RCA in construction. This study also illustrates that the combined use of SEM, EDX, and XRD can indicate potential source of Ca2+ donors that are the main source of calcite precipitation. With the RCA used in this study, SEM and XRD show significant amount of calcite and no existence of portlandite. Thus, one can infer the stored environment of RCA; for example, outdoor storage conditions under weathering (e.g. rainfall) for a long period of time. Acknowledgement This study was carried out with the support of ‘‘Enhancement of the natural hazard response system in rural communities by benchmarking the U.S.A. hazard response systems (PJ010891)”, National Institute of Agricultural Sciences, Rural Development Administration, Republic of Korea. In addition, the author also appreciates technical laboratory supports of Mr. Zachary Behring. References [1] K.Y. Ann, H.Y. Moon, Y.B. Kim, J. Ryou, Durability of recycled aggregate concrete using pozzolanic materials, Waste Manage. 28 (6) (2008) 993–999. [2] C.-S. Poon, Z.H. Shui, L. Lam, Effect of microstructure of ITZ on compressive strength of concrete prepared with recycled aggregates, Constr. Build. Mater. 18 (6) (2004) 461–468. [3] R. Zaharieva, F. Buyle-Bodin, F. Skoczylas, E. Wirquin, Assessment of the surface permeation properties of recycled aggregate concrete, Cement Concr. Compos. 25 (2) (2003) 223–232. [4] C.F. Hendriks, H.S. Pietersen, Sustainable Raw Materials: Construction and Demolition Waste, RILEM Publications, Cachan, France, 2000. [5] M.S. de Juan, P.A. Gutiérrez, Study on the influence of attached mortar content on the properties of recycled concrete aggregate, Constr. Build. Mater. 23 (2) (2009) 872–877.
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