Scholarly article on topic 'Utilization of black liquor as concrete admixture and set retarder aid'

Utilization of black liquor as concrete admixture and set retarder aid Academic research paper on "Civil engineering"

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{"Rice straw black liquor" / Workability / Concrete / "Set retarder" / Admixture}

Abstract of research paper on Civil engineering, author of scientific article — Samar A. El-Mekkawi, Ibrahim M. Ismail, Mohammed M. El-Attar, Alaa A. Fahmy, Samia S. Mohammed

Abstract The utilization of black liquor, produced by the pulp and paper industry in Egypt, as a workability aid and set retarder admixture has been investigated. This approach may help eliminate the environmentally polluting black liquor waste. It also provides a low cost by-product, which can be widely used in the construction industry. The properties of black liquor and its performance on concrete at two different ratios of water to cement have been studied. The results revealed that black liquor from rice straw pulp increases concrete workability, improves compaction, and reduces honeycombing. Moreover, it retards the initial and final set time and enhances uniform compaction. The effect of incorporating small portions of silica fume has been investigated. The ageing effect of this material over a period of one year, to determine its safe storage period, has been studied. Finally, this admixture was found to comply with the relevant Egyptian standards.

Academic research paper on topic "Utilization of black liquor as concrete admixture and set retarder aid"

Journal of Advanced Research (2011) 2, 163-169

ORIGINAL ARTICLE

Utilization of black liquor as concrete admixture and set retarder aid

Samar A. El-Mekkawi a *, Ibrahim M. Ismail a, Mohammed M. El-Attar b, Alaa A. Fahmy a, Samia S. Mohammed a

a Chemical Engineering Department, Cairo University, Giza, Egypt

b Material Research Laboratory, Structural Engineering Department, Cairo University, Giza, Egypt

Received 17 July 2010; revised 26 December 2010; accepted 10 January 2011 Available online 17 February 2011

KEYWORDS

Rice straw black liquor;

Workability;

Concrete;

Set retarder;

Admixture

Abstract The utilization of black liquor, produced by the pulp and paper industry in Egypt, as a workability aid and set retarder admixture has been investigated. This approach may help eliminate the environmentally polluting black liquor waste. It also provides a low cost by-product, which can be widely used in the construction industry. The properties of black liquor and its performance on concrete at two different ratios of water to cement have been studied. The results revealed that black liquor from rice straw pulp increases concrete workability, improves compaction, and reduces honeycombing. Moreover, it retards the initial and final set time and enhances uniform compaction. The effect of incorporating small portions of silica fume has been investigated. The ageing effect of this material over a period of one year, to determine its safe storage period, has been studied. Finally, this admixture was found to comply with the relevant Egyptian standards.

© 2011 Cairo University. Production and hosting by Elsevier B.V. All rights reserved.

Introduction

As Egypt suffers from a lack of natural forests, agricultural residues represent the main source of lignocellulosic materials

* Corresponding author. Tel.: +20 12 3173841; fax: +20 2 37236556. E-mail address: samarelmekkawi@hotmail.com (S.A. El-Mekkawi).

2090-1232 © 2011 Cairo University. Production and hosting by Elsevier B.V. All rights reserved.

Peer review under responsibility of Cairo University. doi:10.1016/j.jare.2011.01.005

for pulp and paper manufacturing. In 1996 Egypt produced, as agro by-products, 2.5 million tons of rice straw and one million tons of sugarcane bagasse [1]. By 2006 production had risen to 10 million tons of rice straw and 3.5 million tons of bagasse [2]. For a long time, these two materials were used as feed stock for the Egyptian pulp and paper industry and produced huge amount of black liquor waste. The black liquors of the pulp industry, Egypt's only potential source of lignin materials, are still not used efficiently [3,4]. The utilization of black liquors in other fields or the recovery of useful chemicals from them may have significant added value.

Lignin refers to a group of phenolic polymers that confer rigidity to the woody cell wall of plants. Its chemical and physical properties differ depending on the plant type and the extraction method [5-7]. Lignin molecules are very reactive due to their many functional groups and chemical bonds, so

lignin can serve a lot of purposes as binder, dispersant, and emulsifier [8-10]. Lignosulfonates obtained in the acidic pulping process have been used as a workability aid for cement [11,12], while alkali lignin may be extracted from black liquor and sulfonated for the same purpose [13,14]. In this regards, Chang et al. treated black liquor by filtration, evaporation, sul-fonation, and drying in a spray dryer to get the finished product as solid material [13]. Using this product as concrete admixture increases the strength of concrete by 0.3% and decreases the water content by about 10.2%. However this method is more expensive due to the cost of the sulfonation process. Besides, it is well known that the separation of lignin from black liquor is, in many ways, a very difficult process. In many cases, it was considered more difficult than the pulping process itself [15-21]. Kumar et al. performed a study on paper mill effluent as a workability aid for cement mortars [22]. Various dosages (5-100%) of effluent based on water requirement extended the setting time of cement and increased the workability of cement sand mortar.

Both the Edfu pulp and paper mill, located at Edfu, the site of the Ptolemaic Temple of Horus near the remains of ancient pyramids, and the Quena pulp and paper mill, located at Qous city, produce black liquor with an average rate of 1500 tons/ day each, as a by-product of the pulping process of bagasse. These black liquors, similar to those of most of the paper mills worldwide, are used in waste heat boilers to generate steam. The Rakta pulp and paper mill, located at Abu Quer bay 35 km to the east of Alexandria, used to produce black liquor at an average rate 1.8 million m3/day, as a by-product of the soda pulping process, which uses rice straw as a raw material. This amount was mainly discharged to sea causing severe environmental problems. The difficulties of rice straw black liquor recovery, in the Rakta case, are mainly due to its high silica content [19,20] and low heat value that make silica removal and the burning of lignin in a waste heat boiler, as at Edfu and Quena, an uneconomic process [19].

Some reports describe the use of alkaline black liquor as a workability aid for mortar and concrete, and show that alkali black liquor does not have any negative effect on concrete durability or steel corrosion [23-25]. Although other researchers succeeded in using microbial community to treat alkaline black liquor [26-28], there is extra economic added value in the utilization of black liquor as a concrete admixture. Actually, for all sorts of pulping raw material, the creation of useful products from the waste liquors represents an increased income for the industry and a solution to the pollution problem. Therefore, this research studies the possibility of utilizing black liquor, as received from local paper mills, as a concrete admixture, which represents a simple, economically preferred solution to the black liquor environmental problem.

Experimental

Black liquor samples

The total solid content of the black liquor produced from rice straw pulping by the Rakta pulp mill is usually 1%, which Rakta can concentrate a small portion of it thermally, by using an existing multiple effect evaporator pilot, to 9-12 weight% total solids. The black liquor from bagasse pulping in the Quena pulp mill is concentrated to 27 weight% total solids,

and that of Edfu is concentrated to 40%. Samples from the three pulping companies were obtained. The black liquor from the Edfu pulp mill was diluted using distilled water to reach 27% total solid content to be comparable with Quena black liquor, while that of Rakta was taken from the effluent of the concentrating pilot plant with 10% total solid content.

Characteristics of black liquor

Characterization of black liquor was performed by determining the relevant parameters using devices and apparatuses available at the Chemical Engineering Laboratory, Faculty of Engineering, Cairo University. pH was measured by a Ino-lab pH meter, while specific gravity was measured by a Ertco hydrometer for heavy liquids. Total solids content was determined by drying a weighted sample at 110 0C in a dryer till constant weight was achieved. Chloride content was measured by an Orbeco 975 spectrophotometer, test no. 34, using chloride readymade vials. Sulfate content was measured by the Orbeco 975 spectrophotometer test no. 13, using the ready-made vials. Chemical oxygen demand (COD) was measured by the Orbeco 975 spectrophotometer test no. 64, and biological oxygen demand (BOD) was measured by the OxiTop IS 12 BOD measuring device that is based on pressure measurement via electronic pressure sensors. Sugar content was determined by extracting 0.5 g of the sample via boiling in 80% aqueous ethanol for 6 h. The extract was filtered, and the ethanol was removed by vacuum distillation. The aqueous sugars were extracted using 5% phenol solution and sulfuric acid 98%. Then the total sugars content were determined by measuring the absorbance of the yellow orange color at 490 nm. A standard curve was prepared using pure glucose [29]. Carbohydrate content was determined by digesting the sample using 1N sulfuric acid in a sealed tube placed overnight in an oven at 100 0C. The solution was then filtered and the total hydrolysable carbohydrate content was determined using the Phenol-Sulfuric acid method [29]. Ash content was determined by burning the material in an oven in a porcelain crucible at 450 0C for 30 min and then at 850 0C for 45 min; the residue was then gravimetrically estimated [30]. Lignin content was determined by treating the oven dried ground sample with 72% sulfuric acid with 20:1 liquid to solid ratio for four hours at room temperature, 25-30 0C, then diluted to 3% sulfuric acid and boiled for four hours under reflux. The lignin was filtered on a weighed ashless filter paper and washed with hot distilled water till neutrality; then ash free lignin was gravimetrically estimated [31].

Concrete and cement testing

Several tests had to be carried out on concrete containing black liquor in order to ascertain the reliability of the product and the conformity of the concrete to Egyptian Standards, as explained below. To achieve the objectives of this research work, two concrete mixes were selected: one contained a water cement ratio equal to 0.5 to represent commonly used concrete in Egypt; the other contained a water cement ratio equal to 0.4, which corresponds to concrete with higher strength. Moreover, tests were conducted on cement paste produced by mixing cement with water and black liquor to verify the initial setting time and final setting time of the cement.

Table 1 Mixing proportion for concrete mixes.

Cement content (kg) 400

Fine aggregate/coarse aggregate (wt. ratio) 0.5

Black liquor/water (vol.%) 0, 5, 15, 25, 35

Water/cement (wt. ratio) 0.4, 0.5

Concrete materials

The fine aggregates used were local natural sand graded to satisfy the requirements of the Egyptian Code of Practice, ECP 2006, [32]. Relative density and fineness modulus were calculated according to the ECP. The coarse aggregates were round gravel with a maximum nominal size of 25 mm diameter and were sieved to remove particles smaller than 2.38 mm diameter as specified by the ECP. Relative density, bulk density, and water absorption were defined according to the ECP. Cement used was OPC, CEM I, according to the ES 4756-1/2006 standard. The physical and mechanical properties of cement such as fineness, setting time, and compressive strength were defined according to the ECP. Table 1 shows the mixing proportions of the concrete mixes, which follow Egyptian Standards.

Slump test

The slump test was carried out as a measure of the workability of the fresh concrete. The fresh test sample was taken from the pan mixer immediately after the mixing procedure was completed and poured into a metal slump cone with a bottom diameter of 200 mm and a top diameter of 100 mm and a height of 300 mm. The procedures were applied according to Egyptian Standards (1658/1989) as required by the ECP [32].

Concrete compressive strength

Compressive strength was measured on hardened concrete with a calibrated hydraulic press according to the ECP [32], using concrete cubic specimens of 150 mm side length.

The specimen compressive strength fcc is defined by the formula:

fcc = F/Ac

where F is the maximum load before collapse of the specimen in kg and Ac is the cross section area of the specimen in cm2.

Splitting tensile strength

The tensile strength was calculated using the indirect tensile strength with a calibrated hydraulic press according to the ECP [32], using a concrete cylinder specimen of 150 mm diameter and 300 mm height, and applying the following formula:

Splitting tensile strength = 2F/ndL

where F is the breaking load in kg, L is the cylinder length in cm, and d is the cylinder diameter in cm.

Sulfate content

Aggressive chemicals influence building materials because they affect the safety and durability of structures. The sulfate content should not exceed 4% of the cement weight used, according to the ECP [32]. The aim of this test is to precipitate sulfate in the form of barium sulfate using barium chloride. The percentage of sulfate in concrete is calculated as follows:

SO3% = (W/Wl) x 0.343 x 100 S = (SO3/M)x 100

where W is the precipitate's weight, W1 is sample's weight, 0.343 = molecular weight of SO3/molecular weight of BaSO4, M is the percentage of cement content in concrete, and S is the percentage of sulfate of cement content.

Chloride content

The chlorides that exist in water, cement, gravel, and black liquor are calculated by reference to the ECP [32] as a percentage of the cement content. This test is based on extracted chloride salts then titrate with silver nitrate 0.1 N using potassium chromate as a detector. The percentage of chloride is calculated using the following equation:

Cl% = V x 0.1 x 1/W x 1/M x 35.5 x 100

where V is the volume of silver nitrate (0.1 N) in ml, W is the weight of the sample in g, M is the percentage of cement content in concrete and 35.5 = molecular weight of chlorine.

Results and discussion

Characteristics of black liquor

Usually, the chemical composition of rice straws and bagasse differs according to plant type and origin. Hence, the composition of the black liquor produced during the pulping process in different plants may not be the same, even if the same pulping process is used. However, black liquor consists generally of lignin, hemicelluloses, cellulose, and silica. Minor constituents such as fats, wax, resins, mucilage, and gums [33] exist in small portions. Table 2 shows the average chemical composition of rice straw and bagasse in Egypt [20]. The aim of the pulping process is to produce cellulosic pulp by breaking lignin that is cementing cellulose fibers together and removing it with the rest of the undesirable compounds, which will all together form the black liquor. Basically rice straw has a lower lignin content than bagasse straw and a higher ash content, which comprises the silica content as shown in Table 2. In fact black liquor has the same characteristics.

Table 2 The chemical composition of rice straw and bagasse in Egypt.

Hemicellulose (%) Cellulose (%) Lignin (%) Ash (%)

Rice straw 19.3 45 18.9 14.7

Bagasse 23.6 48.4 22.7 1.3

Table 3 Analysis of black liquor produced from the Rakta,

Edfu and Quena pulp mills.

Property Rakta Edfu Quena

Specific gravity at 15 °C 1.04 1.128 1.128

pH 7.22 12.7 12.3

Total solid content (g/l) 93.7 278.1 271

Chloride content (mg/l) 426 3500 3550

Sulfate content (mg/l) 1516 6952 6950

Sugar content (g/l) 28.38 68 59.5

Hydrolysable carbohydrates (g/l) 32.8 85 61.3

COD (mg/l) 151,200 200,400 199,500

BOD (mg/l) 39,900 43,900 43,900

The pulping process in the Rakta pulp mill is based on digesting rice straw under 170 0C and pressure 7 atm. After the digesting process, all content is transferred to a blow tank where the pressure is reduced to 1 atm. Then the digested pulp and the black liquor are passed to rotary filters, where the pulp is also washed several times so that the final effluent collected is a highly diluted black liquor with low pH. In this study, rice straw black liquor contains 58.97 g/l lignin and 1.89 g/l ash of 9% total solid, while that of bagasse straw contains 190.22 g/l lignin and 2.88 g/l ash content of 27% total solid content. The physical and chemical characteristics of black liquor from the Rakta, Edfu, and Quena pulp mills were determined by the analytical methods explained above and the results are shown in Table 3.

Both Edfu and Quena black liquor have high pH values, while Rakta black liquor is almost neutral, which was not expected. This low pH may be due to the washing water that is added to the black liquor in the plant; and to the fermentation process that takes place during storage at the evaporation plant. It was not possible to get fresh concentrated black liquor. Formaldehyde was added to the received samples to stop further fermentation. This is supported by the slight decrease in the pH value of Rakta black liquor with the increase in aging time as is shown in Table 4. Rakta black liquor has the lowest chloride content since it has the lowest total solid content and is produced from different agricultural residue raw materials. Black liquor from bagasse has a higher sugar content than black liquor from rice straw, which was expected. Edfu black liquor has the highest sugar content and hydrolysa-ble carbohydrates. All black liquor streams have high values of BOD and COD, due to the high content of sugars, carbohydrates, and lignin, which reinforces the importance of the previously discussed environmental problem.

Rice straw black liquor was examined over one year. It is clear that the micro-organisms' nutrients, carbohydrates, are

consumed gradually, which leads to a decrease in the organic contents as shown in Table 4.

Effect of rice straw black liquor on concrete performance

Black liquor was added as a partial replacement for mixing water, so that the total liquid, water and black liquor, and cement amount are invariant and similar to the control mix of w/c = 0.4 and w/c = 0.5. The minimum limit of acceptable slump to ensure proper compaction of concrete on site is assumed to be equal to 70 mm. Fig. 1 shows the effect of the addition of the rice straw black liquor on concrete slump. It is worth noting that the maximum expected error of the shown resulting values is 3%.

Fig. 1 shows that the slump result for the control mix carried out at w/c = 0.4 was almost zero, which means that this ratio is not appropriate for the materials used in this mix. The reason is that the workability of the concrete run at w/c = 0.4 (water content equals 160 l/m3 of concrete) is very small and does not allow practically for the proper mixing and compaction of concrete, which in turn reduces both the slump and the compression strength. By adding black liquor at the same w/c ratio, the slump value increased till it reached a maximum value of 15% black liquor replacement percentage, after which slump is slightly decreased. The black liquor acts as a dispersing agent by neutralizing the electrostatic charges of the concrete mixture, especially the cement. This neutralization minimizes agglomeration of the solid particles allowing them to mix better with water. This will increase the slump, reduce honeycombing, and increase the compression strength, as can be seen in Fig. 2, which represents the effect of using rice straw black liquor on the compressive strength of concrete.

Increasing the black liquor amount above a certain limit may reduce concrete compressive strength due to the extra amount of the charged ions that may recharge the solids again reducing their ability to mix with water. Hence, both the slump and the compression strength will start to decrease with the extra increase in the black liquor amount above a maximum value, which was found to be 15% water replacement by black liquor. A similar trend was found at w/c ratio = 0.5, where the slump value shows a maximum at 15% black liquor replacement.

Moreover, Figs. 1 and 2 show the effect of aging of black liquor on concrete slump and compressive strength respectively. At 15% black liquor replacement, slump value decreases from 160 to 100 mm when using black liquor stored for one year at w/c ratio = 0.5. Meanwhile, at same black liquor replacement percentage, compressive strength decreases from 27.5 to 25 MPa when using black liquor stored for one

Table 4 Analysis of black liquor produced from Rakta at three different ages.

Property Recent product 6 months age 12 months age

Specific gravity at temp. 15 0C 1.04 1.038 1.03

pH 7.22 6.64 6.5

Total solid content (g/l) 93.7 90.8 73

Sugar content (g/l) 28.38 16.75 10.98

Hydrolysable carbohydrate (g/l) 32.8 29.92 12.2

COD (mg/l) 151,200 122,000 101,600

BOD (mg/l) 39,900 29,900 26,900

Fig. 1 Effect of rice straw black liquor on concrete slump.

Fig. 2 Effect of ageing of rice straw black liquor on concrete compressive strength.

Fig. 3 Effect of rice straw black liquor on concrete splitting tensile strength.

year at w/c ratio = 0.5. It may be concluded that aging for up to six months is safe enough to ensure acceptable values for slump and compressive strength without large reductions.

Fig. 3 shows the effect of rice straw black liquor on concrete splitting tensile strength. It has the same general trend similar to the compression strength by showing maximum values of splitting tensile strength at 15% black liquor replacement.

Analysis of the previous results indicates that the most suitable replacement percentage of black liquor is 15% of the water volume required. The increase in compressive strength compared to the control mix will be referenced to be a ''gain in compressive strength''. The gain in compressive strength for the optimum dose, 15% black liquor replacement, at w/c = 0.4, was calculated to be 85% and 78% after seven days and 28 days curing respectively for recent product; while in the case of w/c = 0.5, it achieves gains in compressive strength of 58% and 10.2% after seven days and 28 days curing, respectively. These improvements can be enhanced by adding certain amounts of fumed silica.

Chemical analysis of hardened concrete

Hardened concretes were analyzed to determine the concentration of chlorine and sulfate ions, which are soluble in water. The chlorine ion causes corrosion to reinforcing bars while the sulfate ion is harmful to concrete and causes cracking, so the two ions have upper limits in the ECP. The maximum allowable limit for chlorine ions in concrete according to the ECP is 0.15% for concrete that may be in contact with a chlorides-contained environment and 0.3% for concrete that will be protected from a chlorides-contained environment. On the other hand, the upper limit for sulfate ions is 4%. It is worth confirming that adding black liquor as an admixture to concrete will not increase the values of these ions above the ECP limits. The results shown in Table 5 are all within the ECP limits. These results prove that using black liquor as a replacement for mixing water will not inversely affect concrete durability.

Effect of silica fume on the selected mixes

The effect of adding silica fume to concrete mixes from Rakta with a replacement percentage of black liquor equal to 15% was studied. The silica fume was added as a replacement percentage of the cement weight in small portions, less than 5% of cement weight, to increase strength without increasing water content. According to the results that are displayed in Table 6, silica fume decreases slump to an unacceptable value for mixes at w/c = 0.4, while adding silica fume up to 5% to mixes at w/c = 0.5 improves strength with acceptable slump.

Setting time

Black liquor produced from rice straw pulping contains sugars, so it tends to be a retarder. The initial setting time of cement paste produced by mixing cement with water and black liquor with various percentages was greater than the minimum limits (75min) required by Egyptian standards (ES 4756-1/2006) [32]; also the final setting time was less than the maximum limits as shown in Table 7.

The effect of bagasse black liquor on concrete performance

Bagasse black liquors produced from the Edfu pulp mill and the Quena pulp mill have a high sugar content. They were

Table 5 Chlorine and sulfate ions in 28 days hardened concrete.

Concrete mixes Chlorine ions % of cement weight Sulfate as SO3 % of cement weight

w/c = 0.4, 15% replacement 0.131 2.655

w/c = 0.5, 5% replacement 0.119 2.138

w/c = 0.5, 15% replacement 0.149 3.266

w/c = 0.5, 25% replacement 0.159 3.3

Table 6 Effect of silica fume added to mixes using 15% black liquor.

Silica fume replacement 0% 0.5% 5%

Slump (mm) w/c = 0.4 85 40 30

w/c = 0.5 130 90 80

Compressive strength 28 days (MPa) w/c = 0.4 27.5 34.3 29.4

w/c = 0.5 26.7 33.8 26.7

Table 7 Effect of black liquor from Rakta on cement paste setting time.

Dose (%) Initial setting time Final setting time

h min h min

0 1 18 2 37

10 1 49 3 55

20 2 27 53

30 2 54 6 46

Fig. 4 Effect of bagasse black liquor on concrete compressive strength after 28 days of curing.

examined at water to cement ratios of 0.4 and 0.5 with replacement percentages of 5% and 15%. Fig. 4 shows the effect of these black liquors on concrete compressive strength. It is obvious that both kinds of bagasse black liquors mixed with concrete resulted in compressive strength less than the minimum acceptable value of the ECP, 250 kg/cm2 (24.5 MPa), and hence cannot be practically used in reinforced concrete applications. For these considerations black liquor from bagasse is not acceptable as a concrete admixture and there is no need to investigate it further.

Conclusions

The use of black liquor produced by the pulp and paper industry in Egypt is investigated. Black liquor is considered as a low cost admixture to increase the workability and retard setting of concrete. The results of this research show that black liquor produced from rice straw noticeably increases the workability of concrete with maximum performance at 15% water replacement by black liquor. It helps to improve compaction and to reduce honeycombing. It is also acts as a retarder due to its high sugar content. At a water to cement ratio of 0.4% and 15% black liquor replacement of water, the compressive strength of the concrete increased by 85% and 78%, as compared with control mix, after curing for seven days and 28 days respectively. In addition, the gain in strength for mixes using 15% black liquor replacement at water to cement ratio of 0.5 was 58% and 10.2% after curing for seven days and 28 days, respectively. This product is safe to be used for reinforced concrete based on the results of chemical analysis of hardened concrete, according to the Egyptian Code of Practice, over a maximum storage period of six months. On the other hand, overdose above 15% causes decreases in concrete workability and compressive strength. Silica fume can be added up to 5% cement replacement for mixes that uses 15% black liquor at w/c = 0.5, which improves strength with acceptable slump. Bagasse black liquors have negative effects on concrete performance as they reduce the compres-sive strength and should not be utilized for concrete applications.

It is finally concluded that the use of black liquor produced from the Rakta pulp and paper mill in Egypt as a partial replacement for mixing water improved workability, compres-sive strength, and setting time without harmful effects on concrete durability. On the other hand, the use of bagasse black liquors produced from the Edfu and the Quena pulp mills is not acceptable due to reductions in concrete compressive strength.

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