versión On-line ISSN 0718-5073
Rev. ing. constr. v.23 n.1 Santiago abr. 2008
Revista Ingeniería de Construcción Vol.23 N°1, Abril de 2008 www.ing.puc.cl/ric PAG.18-25
Improvement of physical and chemical properties of concrete with brazilian silica rice husk (SRH)
La mejora de propiedades físicas y químicas de hormigón con silice de cáscara de arroz (SRH) brasileña
Fernanda Giannotti da Silva* 1, Jefferson B. L. Liborio**, Paulo Helene*
* University of Sao Paulo, Sao Paulo SP, BRAZIL
** University of Sao Paulo, Sao Carlos-SP, BRAZIL
The use of industrial and agricultural waste substitutions for Portland cement has greatly contributed to sustainable development practices. The increase in cement demand will be met by the use of supplementary cementing materials, in order to minimize the clinker consumption. In this study, the effects of silica extracted from rice husk (SRH) as a partial replacement of cement in concretes with low water/binder are reported. The properties investigated include compressive strength, splitting tensile strength, water absorption by immersion, water absorption by capillarity and resistance to chloride ion penetration. The research indicates that SRH is an effective mineral addition for designing durable concrete presenting an optimal performance when the replacement ratio of Portland cement is around 10%,
Keywords: Rice husk silica, concrete, compressive strength, choride ion penetration
El uso de desechos industriales y agrícolas como segmento en la producción de cemento Portland ha contribuido de manera importante en la práctica de desarrollo sustentable. El aumento en la demanda de cemento coincide con el uso de materiales aglomerantes suplementarios para minimizar el consumo de clinker. En este estudio, se informan los efectos de la sustitución parcial de cemento Portland por sílice extraído de la cascara de arroz (SRH) en hormigones con baja relación agua/materiales cementicios. Las propiedades investigadas incluyen resistencia a la compresión, resistencia a la tracción, absorción de agua por inmersión, absorción de agua por capilaridad y resistencia a la penetración de iones de cloruro. La investigación indica que SRH es una adición mineral eficaz para el diseño de hormigones durables que presenta una actuación óptima cuando la proporción del reemplazo de cemento de Portland está alrededor de 10%,
Palabras Clave: Sílice de cascara de arroz, hormigón (concreto), resistencia a la compresión, penetración de iones de cloruro
Ordinary Portland cement is acknowledged as the major construction material throughout the world. The production rate is approximately 2.1 billion tons/year, and is expected to grow exponentially to about 3.5 billion tons/year by 2015 (Coutinho, 2003). In Brazil, 45 million tons were produced in 2006. Most of the increase in cement demand will be met by the use of supplementary cementing materials, in order to reduce the green gas emission (Metha, 1994; Bentur, 2002).
Industrial wastes, such as blast furnace slag, fly ash and silica fume are being used as supplementary cement replacement materials and, recently, agricultural waste are also being used as pozzolanic materials concretes, such as silica from rice husk and rice husk ash (Liborio, 2000; Sensale, 2006). When pozzolanic materials are incorporated to concrete, the silica present in these materials reacts with the calcium hydroxide released during the hydration of cement and forms additional calcium silicate hydrate (C-S-H), which improves durability and the mechanical properties of concrete (Malhotra et al., 1996; Igarashi et al., 2005)
In Brazil, the rice husk volume produced in the paddy milling process is high, and represents 51% of South America production. Part of this husk is used as a source of energy (fuel) in the paddy milling process itself, generating a huge volume of ash. The ash, which has no useful application, is usually dumped directly in the environment and causes pollution and air contamination. Thus, the use of by-products is an environmental-friendly method of disposal of large quantities of materials that would otherwise pollute land, water and air.
The aim of the present investigation is to evaluate SRH as supplementary cementitious materials with reference to mechanical and durability properties of hardened concretes. In the first phase, the chemical composition, physical properties and characterization of SRH were carried out. In the second phase, studies on concrete specimens were conducted, such as compressive strength, splitting tensile strength, water absorption by immersion, water absorption by capillarity and resistance to chloride ion penetration.
2. Experimental program
2.1 Material used
For the production of concretes, the following materials were used: high early-strength Portland cement (CP V ARI, similar to ASTM C150); natural quartz sand with 2.4mm maximum particle size and 2.03 fineness modulus; coarse aggregate of basaltic origin with 9.5mm maximum particle size and 5.29 fineness modulus and silica from rice husk (SRH), added as partial cement replacement material (5% and 10%).
The SRH was produced in laboratory. The rice husk is immersed into an acid solution, followed by washing in water until the pH of the solution becomes neutral. After being dried, the husk is burnt, ground to required fineness and burnt again at a temperature of 600°C, which makes the product white and amorphous (Figure 1). It is important to emphasize that the silica properties depend on burning time, temperature, cooling time and grinding conditions.
Figure 1. Phases of silica from rice husk production: rice husk(a), rice husk ash before (b) and after (c) of grinding and silica from rice husk (d)
2.2 Physical and chemical analysis of cement and SRH
Mineralogical analysis of SRH was carried out by diffraction analysis. Specific surface area of cement and SRH were obtained using BET's air permeability apparatus and physical properties such as specific gravity and specific surface were determined following ASTM standards. The chemical analysis of cement and SRH also were carried out. The cement pastes reference and with SRH were used to determine the initial and final setting in accordance with ASTM C191.
2.3 Mix proportions of concrete specimens
The cement/dried total aggregates ratio in this work was 1:3.5. The concrete composition used in this research is presented in Table 1.
Table 1. Mixture proportions and fresh concrete properties
The concrete was mixed in a laboratory drum mixer for a total of three minutes. After mixing, 100mm diameter x 50mm thick cylindrical specimens were cast from each mix for compressive strength, splitting tensile strength and water absorption, and 100mm diameter x 50mm thick cylindrical specimens were cast for chloride penetration test. After casting, all specimens were left covered in the casting room for 24h and then demolded and transferred to a moist curing room (23±2°C and RH > 95%) until the time of the test.
The compressive strength of concretes was determined after 1, 3, 7, 28, 63 and 91days moist curing, as specified by ASTM C39. The splitting tensile strength test was conducted on concrete cylinders after 28 days of moist curing according ASTM C449.
Percentage of water absorption is a measure of the pores volume or porosity in hardened concrete, which is occupied by water in saturated conditions. Water absorption by capillarity values of concrete specimens after 7 and 28 days of moist curing were measured. Coefficient of water absorption is suggested as a measure of permeability of water. This is measured by rate of absorbed water by dry concrete in a period of 3, 6, 24, 48 and 72 hours (ASTM C 1585).
The resistance to chloride ion penetration in terms of charge passed values through concrete specimens after 7 and 28 days moist curing were measured as per ASTM C 1202. The positive reservoir of the cell was filled with 0.3M NaOH solution, while the negative reservoir was filled with 3% NaCl solution. A DC potential of 60V was applied across the specimens faces, and the current was recorded at every 30min interval, covering a total period of 6h. By knowing the current and time history, the total charge passed through the specimen was computed by the following formula:
where Q is the total passed (coulombs), 10 is the current immediately after voltage is applied (amperes), I, is the current at time after voltage is applied (amperes). Compatible with the charge passed, the chloride ion penetration risk of concretes is established (Table 2).
Table 2. Chloride ion penetration risk following ASTM
3. Results and discussion
3.1 Physical and chemical analysis of cement and SRH
Mineralogical analysis of SRH carried out by X-ray diffraction is shown in Fig. 2. The material consists essentially of an amorphous silica structure.
The morphology of SRH was verified by scanning electronic microscopy and compared with the microscopy of silica fume (SF). It was observed that SRH is extremely finer and their particles are lengthier than SF, needing larger amount of water to maintain the same workability of concretes with SF (Figure 3).
The physical and chemical characteristics of cement and SRH are compared in Table 3.
The specific surface area of SRH is found to be higher than that of Portland cement and the specific gravity is found to be lesser than that of Portland cement. The chemical analysis data indicate that SRH has four times higher silica content than Portland cement.
Figure2. X-Ray diffraction of SRH
Figure 3. SEM micrograph: a) Silica from rice husk and b) Silica fume
Table 3. Physical and chemical characteristics of cement and SRH
3.2 Setting time of blended cement
The percentage of cement replacement content versus initial and final setting time chart (Figure 4) shows that even increasing the SRH content decreases the initial and final setting time.
Figure4. Initial and final setting times of pastes with and without SRH
The initial setting time measured for 0% and 10% SRH is found to be 450min and 400min, recpectively. The final setting time measured for 0% and 10% SRH is found to be 860mon and 800min, respectively. This behavior can be similar to an accelerated effect, but ib fact, cloud be explained as a filter effect, closing the grains and indicating initial and final setting reducing time.
3.3 Compressive strength
The compressive strength values of concrete with and without additions are shown in Table4 and compressive strength evolution in Figure 5. The reference specimen was prepared with 100% Portland cement and was used for comparison with the concretes with SRH (5 and 10%).
Table 4. Compressive stregnth of concretes with and without SRH
Figure 5. Compressive strength evolution of concretes
Comparison of the data for days of curing time shows that the compressive strength increases when the SRH is added to concrete, especially as a 10% replacement for cement. This increase in strength may partially be dueto the pozzolanic reaction, filler effect and to a better dispersion of cement perticles caused os superplastizer addition, as reported by erarly reseraches (Uchikawa et al... 1997; Bui et al., 2005). The high specific area of SRH led to a number of nucleation sites for additional hydration products. It can be seen that the relative increase in compressive strength attains 80% of the amximun value obtained (91 days) at 7-day curing. Although SRH has been added as a replacement for Portland cement, even at early ages, the compressive stregnth values increase as compared to reference concrete values. The reasons for early compressive strength development of concretes with SRH are due to fineness, amorphous phase, specific area and degree of reactivity of SRH (99.7%).
3.4 Splitting tensile strength
The splitting tensile strength values of concretes with and without SRH after 28 days of curing are shown in Figure 6
Figure 6. Splitting tensile strength values of studied concretes
As observed in the compressive strength, the splitting tensile strength values of concretes with SRH were higher than concrete control. It can be clearly seen that the increase is not proportional, since the filler effect is more significant in the compressive strength (Bhanjaa et al., 2005). The increase in splitting tensile strength of concrete with 10% SRH was 12% in relation to control concrete.
3.5 Water absorption
Water absorption by immersion results are shown in Table 5.
Table 5. Water absorption by immersion of concretes with and without SRH
The effect of SRH is a little significant in concretes at 7 days of curing and, at 28 days, the water absorption values of control concrete was lower as compared to concrete with addition. The water absorption was expected to be lower in concretes with SRH, given that the addition decreases the porosity of materials. Thus, some tests of mercury intrusion porosimetry were carried out in pastes in the same proportion of concretes and the results are shown in Figure 7. The total porosity obtained in pastes with SRH was 2.31% and 3.33% in the control concrete.
Figure 7. Mercury intrusion porosimetry of pastes with and without SRH
It can be clearly seen that the material porosity decrease when the addition is incorporated to the concrete; however, the pores of materials are not disconnected, allowing water penetration. According to CEB 192 classification, all the concretes are considered of medium absorption.
3.6 Water absorption by capillarity
The results of water absorption by capillarity are shown in Figure 8.
Figure 8. Water absorption by capillarity of concretes with and without SRH: a) at 7 days and b) at 28 days
According to Figure 7, the results previously presented were proved. The benefits of SRH in concrete were lower than in mechanical properties, since the coefficient of water absorption values progressively increase with the increase in SRH content, especially after 28 days of age. After 7 days of curing, the concrete with 10% SRH presented a little decrease in coefficient of water absorption values; also, the values of concrete with 5% SRH and the control concrete were practically equal. It is observed, from the water absorption data, that 10% SRH concrete specimen indicate 12% reduction after 72 hours of test (67.5 min1/2).
Other researches with additions extracted from rice husk, as rice husk ash, reported that highly reactive pozzolana are able to reduce the size of voids in hydrated cement paste, thus making them impermeable even at early ages (Cook, 1986). In this study, there is the pore refinement in SRH concretes compared to control concrete, however, the reduction in permeability was negligible. Similar results were observed in another research (Liborio, 2000), which can be due to the fact that with rice husk ash, the filler effect of the smaller particles (28.8 m2/g) in the mixture is higher than the pozzolanic effect. In this study, this fact may have occurred also due to flocculation of SRH particles. As the particles are extremely finer than other additions incorporated into the concrete, there are the agglomerations of the grains, which are not dispersed at the moment of mixture, reducing the surface area to react.
3.7 Chloride ion penetration
The results for the test of resistance to penetration of chloride ions into concrete, measured by means of electric charge passed through the specimens (coulomb) at 7 and 28 days, are given in Figure 9. At 7 and 28 days, the control mixture showed the highest value.
Figure 9. Charge passed of concretes with and without SRH
The substitution of SRH for cement yields a significant reduction in the total charge passed at 7 and 28 days. This reduction amounts to 19% and 40% for 5% SRH and 10% SRH, respectively, at 7 days. At 28 days, the same content substitution showed reductions of 24% and 50%. Thus, although the water penetrates in pores of concrete with addition (according to water absorption test), the chloride ion do not penetrate, because of the small pores diameter.
Following ASTM C 1202 classification (Table 2), the control concrete and 5% SRH present moderate risk of chloride ion penetration, while the concrete with 10% SRH presents a low chloride ion penetration risk.
A study carried out by Bentz et al. (2000) showed that, considering the reduced capillary porosity present at equal degrees of cement hydration in the systems containing silica fume, along with the change in the nanostructure and diffusivity of the C-S-H, after equal hydration times, a 10% addition of silica fume could result in a chloride ion diffusivity more than 15 times smaller than that in a comparable concrete made without silica fume. Although the chloride ion diffusivity has not been verified for the charge passed, it was observed that the values could be even smaller, compared with other researches (Sensale, 2006; Uchicawa et al., 1997). Thus, the efficiency of the addition will be even higher if there is a better dispersion of the particles in mixtures.
Even so, the use of SRH reduced the chloride ions ingress, verified by charge passed, indicating that there is an increase in the initiation period in steel corrosion and, consequently, a substantial increase in service life of concrete structures subjected to aggressive environments, such as marine atmosphere.
For the concrete mixture tested in this study, it was found that:
■ The specimens with silica extracted from rice husk showed higher compressive strength values when compared with their equivalent mixture without addition, already at the early ages. The reasons for early compressive strength development of concretes with SRH are due to fineness, amorphous phase, specific area and degree of reactivity of SRH.
• The concretes with silica extracted from rice husk presented a refinement of pores structure; however, the performance in the impermeability properties (water absorption and water absorption by capillarity) was of little significance. It is possible that there was particle flocculation of SRH, reducing the surface area to react.
■ In chloride ion penetration, concretes with SRH showed a sharp drop in total charge passed values when compared with the control concrete, reaching reductions of 50% in 10% SRH concrete at 28 days of curing.
In general, the results obtained in this study clearly indicate that the addition of SRH as cement replacement materials provides additional improvements in compressive and splitting tensile strength and resistance to chloride ion penetration. Thus, SRH may be utilized as effective mineral addition for designing durable concrete structures. The best content of silica recommended to be added in a volumetric substitution to the Portland cement is 10%.
The authors would like to thank State of Sao Paulo Research Foundation for the financial support.
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Fecha de recepción: 20/ 01/ 2008 Fecha de aceptación: 20/ 03/ 2008 PAG. 18-25