A high unburned carbon fly ash concrete´s performance characteristics
Claudia Patricia Valderrama 1 Janneth Torres Agredo2, Ruby Mejía de Gutiérrez3
1 B.Sc., Civil Engineering. Student M.Sc., in Structures, Universidad Nacional de Colombia, Bogota. cpvalderramap@unal.edu.co.
2 Ph.D., in Engineering, Unviersidad del Valle, Colombia. Associate professor, Universidad Nacional de Colombia, Palmira. Materials and Environment Research Group. GIMMA
3 Ph.D., in Chemical Sciences, Universidad Complutense de Madrid, Spain. Permanent professor, Universidad del Valle, Colombia. Composite Materials Research Group (CENM)
ABSTRACT
Engineering today requires that structures are strong and durable; the latter concept is a decisive factor in their design and construction. The scientific community continues developing new cementitious materials and improving traditional concrete´s properties, specifically reducing permeability by incorporating materials such as pozzolans. This paper analyses the effect of fly ash (FA) added to concrete on mechanical strength regarding compression, capillary absorption and chloride permeability and their behaviour compared to concrete containing silica fume (SF). An optimum 10% mechanical strength was found for fly ash; however, this increased with addition, resulting in positive effects on durability. Fly ash had lower performance for all properties evaluated when compared to silica fume.
Keywords: Fly ash, silica fume, concrete, durability, permeability, compressive strength.
Received: september 11th 2009. Accepted: January 30th 2011
Introduction
A pozzolan is defined as being "a siliceous material or silicaalumina which, once powdered, is able to react chemically with calcium hydroxide in the presence of water at room temperature to form compounds possessing hydraulic properties" (NTC, 2004). Silica fume (SF) is a concrete additive having pozzolanic activity, being mainly composed of amorphous silica from the ferrosilicon industry and fly ash (FA), which is a contaminant residue obtained during coal-fired utility and industrial boiler combustion.
FA´s chemical characteristics depend on the type of coal from which it came and combustion variables, such as temperature, coal burning time (Ahmaruzzaman, 2010). It is a highly heterogeneous product, mainly consisting of silica (SiO2), alumina (Al2O3) and ferric oxide (Fe2O3) and, to a lesser extent, magnesium oxide (MgO), calcium (CaO), sulphate (SO3) and alkalis (Na2O and K2O) (Canals, 2007). It should be stressed that FA pozzolanic activity mainly depends on glassy phase type and amount, mineral component fineness, particle morphology and the level of unburned carbon or organic matter expressed as its loss of ignition (LOI) (Mehta, 1989). According to the above, adding FA to cement affects fresh and hardened concrete properties and may modify their durability. its advantages include increased workability, reduced permeability (Lorenzo, 1993), reduced hydration heat, increased long-term mechanical strength (Bouzoubaa 1993, Raghu 2008, Naik 1998) and good performance in aggressive environments (Mehta 1989, 1996 Gopalan, Sahmaran 2009, Alhozaimy 1996, Amapadu, 2002, Ahmaruzzaman, 2010).
This article gives the results of evaluating the mechanical and durable performance of blended concretes with FA from a power plant located in the Boyaca Department, Colombia. This was compared to that obtained using a commercial pozzolan, such as silica fume.
Materials And Experimental Procedure
The fly ash (FA) used for the study came from the "Termopaipa IV" power plant in the town of Paipa. Table 1 shows the chemical and physical properties of the cement (type III Portland) and fly ash (FA) used. FA chemical analysis results, and taking the ASTM C618 standard or its equivalent (NTC 3493) into account, showed that it was FA class F. It should be stated that the high unburned carbon content present in the FA (10.68%) exceeded the value specified in the standard specification for coal FA (6%). The same regulation stated that the use of Class F pozzolan containing up to 12.0% loss on ignition may be approved by the user if either acceptable performance records or laboratory test results were to be made available. Varying results have been reported regarding the effect of unburned carbon on FA concretes’ physical, chemical and mechanical properties (Ha 2005, Pedersen 2008, Ahmaruzzaman, 2010) and it has been estimated that this value must have a maximum of 8%.The other material used in this study was silica fume (SF), which was supplied by a commercial provider.
Blended concretes were cast using FA (10%, 20% and 30%) and SF (10%) as cement replacement for evaluating mechanical strength and durability. Crushed gravel and river sand were used as coarse and fine aggregates when mixing the concrete. The coarse aggregate had 12.7 mm maximum nominal size, 2,624 kg/m3 specific gravity, 1,438 kg/m³ unit weight and 3.1% absorption.
The sand’s specific gravity, absorption, unit weight and fineness modulus were 2,560 kg/m3, 1.8%, 1,593 kg/m3 and 2.62, respectively. The water/cementitious material ratio remained constant (0.5); this value was based on durability requirements specified in NSR-98, item C.4.2, meaning that a superplasticizer additive had to be incorporated. The specimens were cured in Ca(OH)2-saturated water at room temperature for periods of 28, 70, 100 and 130 days. Table 2 shows the proportions used in the mixtures.
Concrete samples were tested to determine their compressive strength, following ASTM C39 standard procedure. Durability tests such as initial surface absorption (ISAT), water absorption rate and rapid chloride permeability were determined in accordance with BS 1881, ASTM C1585, ASTM C1202 standard procedures, respectively.
Mechanical treatment of fly ash
FA reactivity can be increased by mechanical activation, but its effect depends on the milling equipment used (Molina, 2008). This allows incorporating higher percentages of FA for greater concrete mixture performance or adding small proportions with similar results to that obtained with highly reactive pozzolans, such as silica fume (SF) (Ahmaruzzaman, 2010). FA was subject to milling in a ball mill for 30 and 45 min in this study, resulting in reduced average particle size. Optimum particle size (19.8μm) was determined on the compressive strength of cement mortars having 10% FA; results are given in Table 3.
Figure 1 shows the particle size distribution of the FA selected for mortar preparation determined by laser particle size technique using Mastersizer 2000 equipment.
Results and Discussion
Compressive strength
The compressive strength test was conducted according to ASTM C39, using 10X20cm cylinders; the results are presented in Figure 2. It was observed that strength increased with curing age for the specimens, regardless of addition type or proportion (Bouzoubaa 2002, Lopez 2003, Santaella 2004). FA specimens showed resistance less than that for the control sample (0% addition) at an early age (28 days), indicating low reactivity. The 10% FA sample had 12.3% increase in strength compared to reference sample with 130 day curing time, such result being consistent with that reported by Molina (2008) and Santaella (2004). Increased FA percentage (adding up to 30% FA) produced a significant loss of strength (Yilmaz and Olgun, 2008). This behaviour was directly related to the quality and reactivity of the additive and progress in hydration product development (Papadakis 1999, Sahmaran 2009, López García 2003). On the contrary, the specimen with SF had 6% higher compressive strength than the reference mortar, matching the behaviour described by Lopez Garcia (2003).
Durability tests
Initial surface absorption test (ISAT)
The ISAT test was carried out in line with BS3 British Standard 1881 part 208; the results are presented in Figure 3. According to the graph, concretes having added FA showed behaviour related to replacement ratio. The 30% FA concrete was more permeable (Rukzon and Chindaprasirt, 2008); this result was directly related to its lower mechanical strength. However, increasing curing age and times measured in the test led to a decreased difference in initial FA concrete surface absorption compared to control sample, especially for 60 minute and 100 day curing times, the difference being about 20% and 33% for specimens at 10% and 20% FA, respectively. This also showed that there was a marked increase in FA surface absorption (20% to 30%), a situation which did not occur in specimens containing 10% and 20% FA.
The specimen without addition (control) had the lowest initial surface absorption for all curing ages, thereby agreeing with previous studies (Arango 2003, Nokken 2004). The difference in absorption between SF and 10% FA was low for the same age and testing time (around 6.6%), reflecting that FA helped reduce surface absorption later on. These results were consistent with 10% FA compressive strength for long curing times. According to Ramachandran (2001), it is considered that concrete has high permeability when surface absorption is greater than 0.20 (ml/ m2s), medium in the 0.15 to 0.07 (ml/m2s) range and low if less than 0.07 (ml/m2s) for 60 minutes´ testing. Based on the average absorption values reported for each sample, all concretes had good performance, the 30% FA specimen had medium permeability and the remaining mixtures low permeability.
Water absorption rate (sorptivity)
This test was conducted in line with ASTM C1585, using 7.5 cm diameter and 15 cm height cylindrical concrete specimens after 28 and 100 days curing. The capillary absorption coefficient (K) was calculated from data obtained in the test; the results are presented in Figure 4. The best performance was presented in 30% additive concrete which could be attributed to increased clogging of capillary pores in the material as a result of higher density (Ahmaruzzaman, 2010); however, the results differed by no more than 3% compared to concrete without addition. It was seen that increasing curing time decreased capillary permeability for mixtures incorporating FA, such pattern matching that reported by other researchers (Nokken 2004, Schwarz 2008, Gopalan 1996). The specimens with SF had the lowest capillary absorption coefficient, being attributed to high silica fume fineness and its early reactivity, leading to accelerated pozzolanic reaction and decreased capillary pore size in the concrete (ACI 2000). Overall, the results obtained from measuring this property demonstrated the positive effect of the two additions being evaluated.
Resistance to chloride ion penetration
This technique was carried out according to ASTM C1202, using 10 cm diameter and 5 cm height cylindrical specimens after 28, 70 and 100 days curing. Figure 5 shows that concrete specimens having 10% added SF and 30% FA had greater resistance to chloride ion penetration for the curing ages tested here. It was seen that by 100 days all specimens classified as having moderate behaviour, according to ASTM C1202 (but results recorded for samples having 10% SF and 30% FA were closer to 2,000 coulombs) had limited values between the low and moderate range in standard specification. It should be noted that the value reported for the samples with 30% added FA was 18% higher than specimens with SF, demonstrating the positive effect of increased FA addition percentage. This effect could be attributed (in addition to the pozzolanic reaction) to higher chloride binding ability (Dhir 1999, Cheewaket 2010). These results were consistent with those of Molina (2008) and Sahmaran (2009).
Conclusions
The results led to concluding that:
The FA used in the study was classified as class F, but there was still greater than 10% unburned content, explaining the poor performance observed in this study in some of the properties being evaluated;
Optimum FA addition rate was 10% from a mechanical point of view, although there was increased positive effect percentage regarding capillary absorption and chloride permeability;
Compared to silica fume, FA had lower performance for all properties tested, except chloride penetration resistance; comparable results of 10% SF to 30% FA were obtained in this test; and
Future studies should evaluate the mechanical strength and durability of concrete mixtures having additive percentages above 30% FA, so as to analyse both the benefits and drawbacks of using this by-product. Test methods for reducing the level of unburned carbon, such as flotation, should also be considered to improve FA quality.
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