Ind. Eng. Chem. Res. 2007, 46, 5463-5467
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RESEARCH NOTES Crystallization Technology for Reducing Water Permeability into Concrete Awni Al-Otoom* Department of Chemical Engineering, Jordan UniVersity of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan
Abdelaziz Al-Khlaifa and Ahmed Shawaqfeh Department of Chemical Engineering, Mutah UniVersity, P.O. Box 7, Mutah, Karak 61710, Jordan
A new water-based crystallization technology to minimize the water-related problems of concrete is presented in this study. This technology is dependent on the formation of sodium acetate crystals inside the pores of the concrete after concrete spray treatment with its aqueous solution. Results have indicated a significant reduction in water permeability, as a result of concrete treatment with this solution after full curing. Results have also indicated that an optimum solution of 20 wt % sodium acetate provides the best minimization of water penetration into concrete without altering the physical and performance properties of the concrete. 1. Introduction Concrete is the most used structural material in the world, because of its unique mechanical properties. However, concrete is considered to be a porous medium; therefore, the penetration of undesired substances can cause progressive damage in this medium. Although water is very important to concrete during the hardening stages, it is considered to be an undesirable substance that could cause severe damage to the concrete. Moisture-associated problems in concrete can be summarized as follows: Freeze-thaw damage. The change in the volume of water inside the pores of concrete causes stresses on particles adjacent to these pores. The stresses may lead to severe damage in the structure of the concrete. The behavior of concrete during freeze-thaw cycles is dependent on internal factors such as water content, pore structure, and distribution. It also is dependent on external factors such as hydraulic pressure that develops when water in the saturated pores freezes, osmotic pressure that is caused by the movement of water from the smaller pores to the larger pores, different thermal contraction of the constituents, the temperature gradient, and the chemical action of factors that affect freeze-thaw cycles.1,2 Alkali silica reaction (ASR). Water is considered to be a catalyst for the alkali silica reaction (ASR), which forms a gellike substance that can contribute to stress formation on the structure of concrete. The amount of available moisture, the nature of the reactive silica, the amount of reactive silica, and the particle size of the reactive material are among the most important factors that affect the ASR. All these factors, and mitigating measures, were comprehensively studied in related literature.3-6 Acidic attack. Water can be considered to be acidic, when compared to concrete; therefore, the penetration of water can cause an acid-base reaction that can damage the concrete. * To whom correspondence should be addressed. Fax: 962 65155058. E-mail address:
[email protected].
Chloride ion diffusion. Water can carry destructive ions, such as the Cl- ion, which can severely corrode the reinforcement of the concrete. Much research that has been related to concrete deterioration and steel rebar corrosion are widely made in a sulfate solution or chloride solution.7-9 A review on the effect of Cl- ion diffusion on concrete pore structure is presented by Suryavanshi and Swamy.10 Sulfate attack. Sulfate attack also can lead to the deterioration of concrete substrates. The attack of sulfates on concrete is due to two principal reactions: (i) the reaction of Na2SO4 and Ca(OH)2 to form gypsum, and (ii) the reaction of the formed gypsum with calcium aluminate hydrates to form ettringite. In addition, it is noticed that MgSO4 reacts with all cement compounds, including C-S-H, thus decomposing cement, and subsequently forming gypsum and ettringite.11 A comprehensive study on the sulfate attack is produced by Grammond.12 A summary of the aforementioned problems has also been presented by Bin-Yair.13 Various systems are currently used to prevent or minimize water penetration into concrete, to avoid the previously mentioned water-associated problems of the concrete. Barrier systems are the most-used system to prevent water penetration. These systems are mainly polymeric systems (epoxy resin, acrylic resins, bitumen systems, etc.). They can be installed on the positive or negative side of the water pressures. A comprehensive study on the effectiveness of such systems has been presented by Barbucci et al.14 Barrier systems can be very successful in preventing water penetration. However, other factors can jeopardize the selection of these barrier systems. The resistance of these systems to acidic attacks or sulfate attacks, the adhesion of these systems to different concrete substrates, service life, and economic factors are among the selection criteria that barrier systems may fail. A major drawback on barrier systems is the one-sided protection. Penetration of water from different sides of the concrete and through joints may cause these systems to fail to protect concrete substrates.
10.1021/ie070527l CCC: $37.00 © 2007 American Chemical Society Published on Web 06/28/2007
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Silicate solution (mainly, sodium silicate) has been used extensively in the market for the purpose of waterproofing. It is well-known now that the greatest problem associated with the use of silicate solutions lies in the actual chemical reaction process that creates the gel structure within the concrete. This gel will absorb internal moisture from the concrete and begin to swell. This swelling will continue whenever moisture becomes available. The swelling would produce extreme internal pressures and stresses, even to a point where the concrete’s integrity can and will be damaged quite severely. After the gel reaction begins to swell from excess water of convenience, the resulting failure of the top layer of concrete will cause delamination of the surface of the concrete. Silicate solution works in a similar way to the damaging phenomena of ASR,15 which can be devastating to concrete. Other disadvantage of using silicate solution is that the pH of the concrete must be at certain levels to work as a waterproofing sealer. This pH level is determined by the amount of free calcium hydroxide that reacts with the silicate. Silane/siloxane products are also considered one of the most used waterproofing materials for large concrete structures, such as airports, bridges, and marine applications. It is dependent on its moisture-repellent nature to minimize the water penetration into concrete. Most of these products are solvent-based materials, and the silane itself has raised many questions regarding the environmental impact.16 The increasing environmental concerns has forced contractors to find alternative products that have less environmental impact, because of the use of solvent-based products.16 In addition, the application of silane to the concrete requires that the moisture content of the concrete be low enough to apply this water-repelling product. The objective of this study was to find a new technology for reducing water permeability into concrete substrates. Another of its objectives is to find a material that (i) can be affordable to end users, (ii) can be environmentally friendly, and (iii) have a long service life. 2. The New Technology The core of this technology is based on the formation of a salt solution of sodium acetate, which can be produced via many methods but mainly through the reaction of acetic acid and sodium carbonate (soda ash), according to the following reaction:
Na2CO3(s) + 2CH3COOH(l) f 2CH3COONa(s) + H2O(l) + CO2(g) Different salts were considered before this salt was selected. Among them were sodium tartarate, sodium citrate, and sodium carbonate. The salt of sodium acetate is used, because the rate of crystal growth is high, compared to other salt that has been studied. Sodium acetate penetrates inside the pores of the concrete. After the introduction of water, crystals start to grow, which leads to blockage of concrete pores and, therefore, reduced water permeability. The crystal of sodium acetate is a hygroscopic crystal. This would help in absorbing excess moisture in the surroundings of the concrete pores. Sodium acetate can still hold a considerable amount of water, depending on the surrounding humidity.6 Small quantities (1%) of isopropanol were added to the solutions used in this study, to reduce the surface tension of this solution and help penetration into the concrete substrate.
Figure 1. Concrete samples used for the permeability test.
3. Experimental Procedure 3.1. Materials. Analytical-grade 99% sodium carbonate powder (Na2CO3), 80% acetic acid (CH3COOH), 99.8% isopropanol (CH3CHOHCH3), and distillated water were used to make up the required solution of waterproofing. Five different solutions were prepared by the reaction of sodium carbonate and acetic acid with varying ratios of sodium acetate (from 5 wt % to 50 wt %). 3.1.1. Concrete Proportioning. The concrete consisted of (A) coarse aggregate (gravel), ranging in size from 3/8 in. to 0.5 in.; (B) fine aggregate (sand), ranging in size from 0.005 in. to 0.25 in.; (C) water (water that is suitable for drinking is used to form the concrete mixture; the water is free of organic matter and certain chemicals, such as alkaline and sulfate salts); and (D) ordinary portland cement. The mixing ratio was based on the ideal ratio of water to cement (by weight) (0.65:1), aggregates to cement (by weight) (4.5:1), and sand to gravel (by weight) ) (1:2). 3.2. Concrete Samples for Permeability Tests. Six concrete samples were prepared to study the effect of different concentrations of sodium acetate on the permeability of water. These samples were identical (40 cm × 40 cm × 15 cm). All of these samples were made from the same concrete mix. Figure 1 shows a picture of the concrete sample. The sample contains an inset cylinder at its center, with a diameter of 15 cm and height of 4 cm. The small aspect ratio is used to simulate the diffusion of water in one direction (axial direction). A plastic ring was placed during the molding process to eliminate diffusion in the radial direction. 3.3. Application of the Sodium Acetate Solution. The concrete was allowed to cure for 28 days, according to the ASTM C39 standards, and then the water penetration test was performed on these substrates. The hollow cylinders for five samples then were treated with five different concentrations of the sodium acetate solution using a typical paint brush. The application rate was ∼0.1 L/m2. One sample was left untreated, for the purpose of comparison (control). The permeability test was then performed for these treated samples. 3.4. Permeability Test. After full curing, the cylinders were filled with distilled water to a certain mark. The cylinders are then covered with polyethylene sheets, to minimize vaporization. The cylinders are then refilled to the same mark, at known intervals. The accumulated volume of water added is recorded with time. The amount of water added reflects the permeability of the water, especially when compared to the permeability of the untreated sample.
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Figure 2. Comparison between water permeability for treated and untreated samples with different concentrations of sodium acetate solution. V represents the volume of distilled water at time t, and V0 is the initial volume added to the samples.
Five of these samples were treated with different concentrations of sodium acetate solutions, as explained previously. One week after treatment, the same procedure was followed to measure the amount of water that penetrated the sample. The accumulated volume of water added is recorded with time. One sample was left untreated, for comparison purposes. 3.5. Freeze-Thaw Cycle Test. Six concrete cylinders were made from the same concrete mix used for the permeability tests. These cylinders have diameters of 4 cm and heights of 8 cm. After curing, five samples were treated with different sodium acetate solutions and one cylinder left untreated, for comparison purposes. These cylinders were exposed to cold temperatures (-4 °C) for 2 h, followed by heating in a water bath at 90 °C for 2 h. These two steps are considered as one cycle. Approximately 100 cycles were completed for these cylinders. 3.6. Compressive Strength. Nine concrete cylinders were made from the same concrete mix used to form the samples.
Six cylinders were treated with the optimum sodium acetate solution after full curing. Three samples were left untreated, for comparison purposes. Compressive strength measurements were performed for the nine cylinders, according to ASTM Standard C-39. 3.7. pH Test. Samples of the untreated and treated concrete samples were obtained, crushed, and milled. The powder obtained was then dissolved in distilled water. A standard pH meter was used to perform this test. Measurements of the pH of the treated and untreated samples were performed to understand the effect of the treatment material on the pH of the concrete. 4. Results and Discussion The crystal growth of sodium acetate was relatively fast, and the permeability of water in the samples was significantly reduced only one week after treatment with different concentra-
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Figure 3. Relationship between the concentration of sodium acetate and the magnitude of permeability reduction.
Figure 4. Results of freeze-thaw experiments for treated and untreated samples. W is the weight at certain cycles, W0 is the initial weight of the samples treated.
tions of the solution. Figure 2 shows a comparison between the volumes of water that penetrated the samples before and after treatment. The reduction in permeability increased when the concentration of the sodium acetate in the treatment solution increased from 10% to 20%. However, the magnitude of the reduction decreases when the concentration increases beyond 20%. An optimum for this reduction was observed near a sodium acetate concentration of 20%, with approximately half the amount of water penetrated after treatment, as appears in Figure 3. The reduction beyond this point is believed to be reduced as a result of the increasing viscosity of the solution and, hence, the reduction in the penetration of the treatment solution inside the concrete substrate. During wet conditions, the crystals, being hygroscopic, swell in the presence of moisture; therefore, they block the intrusion of moisture. However, during dry conditions, the crystals allow the evaporation of moisture, and the crystals shrink back to their original mature size. Samples were repeatedly merged in water, and show the same waterproofing effect, showing that the crystals are strongly bonded to the structure of the concrete.
Figure 5. Samples after freeze-thaw cycle experiments.
The crystals did not show any signs of effervescence at the optimum solution concentration used in this study. However, at higher concentrations of the solution, some degree of effervescence was observed, because of the low penetration levels of the solution, as a result of the increasing solution viscosity. The freeze-thaw cycle experiments have confirmed the reduction in water permeability, as a result of treatment by sodium acetate solution. As shown in Figure 4, the weight loss
Ind. Eng. Chem. Res., Vol. 46, No. 16, 2007 5467 Table 1. Compressive Strength of Treated and Untreated Concrete sample untreated sample samples treated with 20% sodium acetate samples treated with 30% sodium acetate
maximum load (× 103 N)
compressive strength (× 106 N/m2)
945 995
42 44.2
950
42.2
Table 2. Results of pH Tests for Treated and Untreated Samples concentration of sodium acetate
pH
control 10% 20% 30% 40% 50%
11.79 11.98 12.08 12.11 12.13 12.16
of treated samples with different concentrations of that solution is not significant. On the other hand, a complete failure of the untreated samples was encountered after ∼40 cycles. This can be explained by the reduction of water penetration inside the pores for treated samples; therefore, the expansion and contraction of water molecules is not as significant as that for the untreated samples. Figure 5 presents a photograph for samples after the freeze-thaw cycles. The untreated samples (control) clearly experienced complete failure. Once again, the optimum concentration of the treatment solution (20%) had shown the lowest decrease in the sample weight, which again verifies the optimum concentration that should be used for the treatment of concrete substrates. The compressive strength tests for treated samples were performed according to ASTM Standard C-39 after two weeks of treatment with different solutions of sodium acetate. Results, as shown in Table 1, have confirmed that treatment with sodium acetate solutions, including the optimum solution (20%), did not have any impact on the compressive strength of the concrete substrates. The pH test has shown a slight increase in the pH, as a result of treatment with different concentrations of sodium acetate, as shown in Table 2. This indicates better protection for concrete reinforcement. A cross section of a concrete was taken 1 h after treatment, to visualize the penetration depth of the proposed treatment
solution into the concrete. Figure 6 clearly shows that the treatment solution penetrates to ∼0.5 in. However, it is expected that crystals should grow even deeper than 0.5 in. 5. Conclusions This study presents a new technology for minimizing waterrelated problems. This technology is based on crystallization of the salt solution of sodium acetate, which can be produced via the reaction of acetic acid and sodium carbonate. It is believed that the crystals of this salt grows relatively fast and, therefore, minimize pore volumes inside the concrete. An optimum concentration of the solution is ∼20 wt %. This technology is believed to be cost-effective, when compared with short-service-life barrier systems. It also becomes an integral part of the concrete substrate and can be expected to increase the service life of the concrete. Literature Cited (1) Cordon, W. A. Freezing and Thawing of Concrete; Mechanisms and Control; ACI Monograph 3; American Concrete Institute; Detroit, 1966. (2) Pigeon, M.; Prevost, J.; Semard, J. M. Freeze-thaw durability versus freezing rate. J. Am. Concr. Inst. 1985, (September-October), 684. (3) Mather, B. How to make concrete that will not suffer deleterious alkali-silica reaction. Cem. Concr. Res. 1999, 29 (8), 1277. (4) Shehata, M. H.; Thomas, M. Use of ternary blends containing silica fume and fly ash to suppress expansion due to alkali-silica reaction in concrete. Cem. Concr. Res. 2002, 32 (3), 341. (5) Aquino, W.; Lange, D. A.; Olek, J. The influence of metakaolin and silica fume on the chemistry of alkali-silica reaction products. Cem. Concr. Compos. 2001, 23 (6), 485. (6) Shehata, M. H.; Thomas, M. The effect of fly ash composition on the expansion of concrete due to alkali-silica reaction. Cem. Concr. Res. 2000, 30 (7), 1063. (7) Santhanam, M.; Cohen, M. D.; Olek, J. Mechanism of sulfate attack: a fresh look. Part 1. Summary of experimental results. Cem. Concr. Res. 2002, 32, 915. (8) Santhanam, M.; Cohen, M. D.; Olek, J. Mechanism of sulfate attack: a fresh look. Part 2. Proposed mechanisms. Cem. Concr. Res. 2003, 33, 341. (9) Shamsad, A. Reinforcement corrosion in concrete structures, its monitoring and service life predictionsA review. Cem. Concr. Compos. 2003, 25, 459. (10) Suryavanshi, A. K.; Swamy, R. N. Influence of Penetrating Chlorides on the Pore Structure of Structural Concrete. Cem. Concr. Aggregates 1998, 20, 169. (11) Hekala, E. E.; Kishar, E.; Mostafa, H. Magnesium sulfate attack on hardened blended cement pastes under different circumstances. Cem. Concr. Res. 2002, 32, 1421. (12) Crammond, N. The occurrence of thaumasite in modern constructions a review. Cem. Concr. Compos. 2002, 24, 393. (13) Bin-Yair, M. The durability of cement and concrete in sea water. Desalination 1967, 3, 147. (14) Barbucci, A.; Delucchi, M.; Cerisola, G. Organic coating for concrete protection. Liquid water and water vapor permeabilities. Prog. Org. Coat. 1997, 30, 293. (15) Alkali-Silica Reaction: Minimising the risk of damage to concrete, Report No. TR30, Third Edition, UK Concrete Society, London, 1999. (16) Chamberland, D. Approved alternative to silane. Concrete (London) 2005, 39, 36.
Figure 6. Cross section of the treated concrete, showing the penetration depth of the proposed treatment solution.
ReceiVed for reView April 16, 2007 ReVised manuscript receiVed June 13, 2007 Accepted June 19, 2007 IE070527L
