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Revista ingeniería de construcción

On-line version ISSN 0718-5073

Rev. ing. constr. vol.29 no.2 Santiago Aug. 2014

http://dx.doi.org/10.4067/S0718-50732014000200005 

 

Performance of mortars produced with the incorporation of sugar cane bagasse ash

 

Pamela Camargo Macedo*, Adriana Maria Pereira*, Jorge Luis Akasaki*, Cesar Fabiano Fioriti1*, Jordi Payá**, José Luiz Pinheiro Melges*

* Universidade Estadual Paulista (UNESP). BRAZIL

** Universitat Politécnica de València (UPV). SPAIN

Dirección de Correspondencia


ABSTRACT

This paper presents the technical feasibility of using ash in mortars, from burning sugarcane bagasse, to generate power as a mineral admixture. The sugarcane bagasse ash (SCBA) underwent chemical characterization and x-ray diffraction tests. The milling of the material was studied, together with the composition of the mortars containing the SCBA at 3%, 5%, 8% and 10% ratios by weight, as partial replacement of fine aggregates. The mechanical properties were studied by compressive and diametral tensile strength tests. Some properties related to the durability of the material were also studied, such as drying shrinkage, capillary absorption and accelerated reactivity. The work was performed using low pozzolanic activity SCBA. However, the results indicate that it is possible to use SCBA as partial replacement of fine aggregates in the production of mortars.

Keywords: Sugarcane bagasse ash, mineral admixture, mortar


1. Introduction

Data point to Brazil as the world's largest sugarcane producer and also its derivatives (sugar and ethanol). This scenario portrays a continued growth trend, mainly due to the incentives for biofuel production as an alternative energy.

For the 2013/2014 sugarcane crop, according to experts, if the climate remains stable - with rain and sun incidence - the next season could reach approximately 580 million tons of sugarcane. The last 2011/2012 season processed approximately 532 million tons of sugarcane (TAGUCHI, 2012).

The sugarcane juice extraction process generates large quantities of bagasse (about 30% of ground sugarcane), biomass that is very important as an energy source. Nearly 95% of all sugarcane bagasse produced in Brazil is burned in boilers to generate steam, which generates bagasse ash residues, which in most cases does not meet adequate practices, hence becoming a serious environmental problem. Composed primarily of silica, SiO, the sugarcane bagasse ash (SCBA) has the potential to be used as a mineral admixture in mortars and concretes (Cordeiro et al., 2008). Estimates show that for every ton of sugarcane processed approximately six kg of SCBA are generated (SALES and LIMA, 2010).

A problem managers of processing plants currently face regards the disposal of these ashes. In most cases, the SCBAs are utilized in the field as organic fertilizer for sugarcane crops (even though it contains few nutrients) or are discarded in nature without efficient management (SOUZA et al., 2007). Other authors have reported the presence of several heavy metals in SCBA samples, indicating that its use as fertilizer should be restricted (SALES and LIMA, 2010).

The construction industry has a high potential for incorporating this type of residue as raw material, contributing to technological, environmental and economic regional development (ROCHA and CHERIAF, 2003).

Some researchers have been studying this alternative as mineral admixture in cementitious materials such as mortars and concrete.

Ganesan et al. (2007) found that in the mortars they produced, the initial and final setting times increased by increasing the SCBA content as cement replacement.

Souza and collaborators (2007) investigated the partial replacement of cement by SCBA in mortars and found that up to 20%, the addition of ash residues is technically possible without any loss in mechanical properties.

Cordeiro et al. (2008) also studied the replacement of SCBA by cement in mortars, pointing out that the pozzolanic effect of the residue greatly depends on the particle size and fineness. These properties can be achieved by specific burning procedures and the milling of the material.

Other studies have also shown the feasibility of using SCBA as replacement for fine aggregates. This alternative offers an advantage in the preparation stage of the residue, as it can be used as inert material in natura, which does not require lengthy milling times and firing at high temperatures. According to Lima et al. (2009) they observed an increase in compressive strength at 28 days in mortars with the replacement of up to 50% of natural sand.

Ganesan et al. (2007) used seven traces of mortar, one as a reference trace and the other six with 5%, 10%, 15%, 20%, 25% and 30% of cement replaced by SCBA. The application of up to 20% of SCBA was considered optimal from the performance perspective - high early strength, reduction of water permeability and resistance to chloride ion penetration, hence improving the durability of the material.

Cordeiro, G. C. et al. (2010) used high-carbon content SCBA in the manufacture of mortars. The compressive strength, compactness and drying shrinkage were evaluated. Due to the low ash reactivity, the results indicated loss of strength and increased shrinkage in relation to the ash cement replacement, which only improved its compactness.

Singh et al. (2000) studied the chemical deterioration caused by sulfuric acid (H2SO4, N/60), measured by prism expansion tests. They found that the expansions in the cement pastes containing 10% of SCBA were significantly smaller than the expansions in the reference pastes. This acid resistance improvement was attributed to the pozzolanic reaction with calcium hydroxide and the reduced permeability due to the presence of SCBA.

The test results of accelerated chloride ion penetration into the cement pastes performed by Ganesan et al. (2007) showed that the total charge passing, in Coulombs, decreases with the increasing SCBA ratio replacing the cement, proving that the addition of ash provides good resistance to chloride ion penetration.

As mineral additions usually reduce exudation, which prevents water rising up to the surface, there is a cracking potential due to drying shrinkage, particularly when these additions are used at higher levels, and when there is a more intense water evaporation rate (SILVA, 2007).

Moreover, SCBA can also be applied to concrete. Researchers have shown that the addition of ash as fine aggregate did not change the mechanical properties of concrete produced with three types of cement (SALES and LIMA, 2010).

However, the study of applying alternative materials to concrete and mortars requires special attention in terms of its durability. The incorporation of mineral admixtures to cement composites have a beneficial effect on some of its properties, which can be explained by the decrease in pore size, increased channel tortuosity and reduced pore interconnectivity (HELENE, 1995).

Thus, the purpose of this work is to verify the performance of mortars produced with different SCBA ratios, as partial replacement of fine aggregates. Therefore, the mechanical properties and durability of the material were analyzed.

2. Materials and methods

The entire experimental program was conducted at the Civil Engineering Laboratory of CESP - Companhia Energetica de Sao Paulo and the Civil Engineering Laboratory of UNESP -Universidade Estadual Paulista, both located in the city of Ilha Solteira, Brazil.

2.1 Cement

As this study investigated the use of a mineral addition, by substituting sand for SCBA, cements with no additions were chosen to better qualify the reactivity of the material. Thus, the cement used was CPV-ARI (Portland high-early strength cement), according to NBR 5733 (ABNT, 1991), which despite its low addition ratio in its composition, it can be found on the market more easily than CPI (Portland cement with no additions).

2.2. Fine aggregates

Natural sand was used, characterized by granulometric tests NBRNM 248 (ABNT, 2003). Table 1 shows their characterization.

Table 1. Characterization of fine aggregates

2.3 Superplasticizer

A superplasticizer was used in order to keep a consistent flow in all traces without changing the water to cement ratio. The water reducing additive chosen is a normal 'third generation' setting liquid, with a recommended dosage by the manufacturer of 0.20% to 0.90% of cement weight.

2.4 Sugarcane bagasse ash

The SCBA sample was collected in a processing plant in the northwestern region of São Paulo, from a decantation pond, and where the sugarcane bagasse is burned in boilers, generating the SCBA, which mixes with the water in the gas scrubber. This water, along with the sugarcane washing water, is taken to the decantation pond, where the liquid that is separated from the solid parts of the ash is reused by the plant.

Next, Table 2 shows the chemical composition of SCBA, according to NBR 12653 (ABNT, 2012), where the SCBA used is in accordance with the chemical requirements for pozzolanic material, which contains high SiO2+Al2O3+Fe2O3 ratio, reduced firing loss and low alkali content available.

Table 2. Chemical analysis of SCBA sample

The ashes were homogenized and remained in a kiln to dry for 24 hours at a temperature of 110°C and then 24 hours more exposed to sunlight. Next, the ash underwent milling in a ball mill composed of steel cylinders, used to produce cements. 5.0 kg of powdered ash were ground at different milling times of 20, 30, 40 and 50 minutes. Table 3 presents the fineness values, according to NBR 12826 (ABNT, 1993). Thus, the milling process was deemed satisfactory for providing ash grain size reduction and for providing better material uniformity.

Table 3. SCBA milling results

In the study of particle size variation according to the SCBA milling times, the laser particle size analysis technique was used. The importance of this test is associated with the reduction control and distribution of ash particle size and the optimization of the milling process. Table 4 shows the particle sizes, which are 10% and 90% of the material mass, D10 and D90, respectively, and also the average size.

Table 4. SCBA Diameter obtained by laser particle size (μη).

According to the results of specific surface, fineness index and particle size distribution, 20 minutes for ash milling was used as the ideal time for applying the mortar, as it improved grain uniformity, reduced the average size of the particles and for being the shortest time studied, since the process consumes energy, becoming an additional cost. Figure 1 shows the appearance of the milled SCBA for 20 minutes.

Figure 1. Aspect of SCBA used in mortars

A study of the pozzolanic activity with cement was also conducted due to the milling times. Table 5 presents the study oi pozzolanic activity with cement, based on NBR 5752 (ABNT, 2012). All ashes showed pozzolanic activity index (PAI) below the minimum value of 75% set by NBR 12653 (ABNT, 2012).

Thus, according to the chemical requirements (chemical composition and pozzolanic activity) required by this standard, this SCBA cannot be considered pozzolanic material, as it has low chemical reactivity with cement.

Table 5. Study of SCBA pozzolanic activity

Finally, x-ray diffraction was carried out on the SCBA used in the mortars (Figure 2), exhibiting intense peaks of silicon alpha oxide and alpha quartz, which demonstrates the existence of its crystalline phases. However, a small shift in the baseline angle (2Θ) 20° was observed, suggesting a small amount of amorphous phase. This test confirmed the low reactivity of the material, considering it as non pozzolanic due to the predominant crystalline phases found. Thus, it was incorporated as a replacement for part of the fine aggregates.

Figure 2. Aspect of SCBA used in mortars

2.5 Compositions of mortars

The chemical analysis and x-ray diffraction results showed a less effective pozzolan material in the cement replacement. Thus, it was decided to incorporate the SCBA, replacing part of the fine aggregate mass.

An investigation was conducted to define the water to cement ratio and the consistency that would be suitable for all traces with the partial replacement of fine aggregates by SCBA, requiring the use of a superplasticizer. The water to cement ratio 0.48 was adopted and based on the 1:3 ratio, the replacement of fine aggregates by SCBA was performed in percentages of 0%, 3%, 5%, 8% and 10%. Table 6 shows the composition of the traces used.

Table 6. Study of SCBA pozzolanic activity

It should be mentioned that it was necessary to introduce a minimal amount of superplasticizer in the incorporations of 5%, 8% and 10%, which is adequate to maintain the w/c ratio constant, promoting good grain dispersion and increasing the fluidity of the paste.

It was also observed that for the 10% incorporation, the consistency was reduced due to using the maximum amount of additive specified by the manufacturer. For this reason, it was not possible to introduce higher sand replacement ratios for SCBA. The mortar for the 10% trace was quite dry and very difficult to work with. Moreover, a gradual darkening in the color of the specimens was noted due to the increased SCBA content in the mortars.

2.6 Compressive strength

Five cubic specimens were molded (40 x 40 x 40 mm) per trace, for each age, with rupture at 7, 28 and 56 days, taking the arithmetic average between them as the ultimate tensile strength. The specimens were molded and cured following the procedures of NBR 13279 (ABNT, 2005).

2.7 Tensile strength by diametral compression

This test followed the recommendations of NBR 7222 (ABNT, 2011). Five specimens were molded (50 x100 mm) per trace, for each age, with rupture at 7, 28 and 56 days, taking the ultimate tensile strength as the arithmetic average.

2.8. Drying shrinkage

The determination of drying shrinkage variation of the mortar bars, due to the use of pozzolanic materials, was performed in accordance with NBR 12650 (ABNT, 2012). For this test only two traces were used, the Control trace (no addition) and the trace with 10% of SCBA. Three prismatic specimens were molded (25x25x285 mm) for each trace, taking its arithmetic average as the end result.

2.9. Accelerated reactivity

The potential reactivity test of the aggregates was performed according to NBR 15577-5 (ABNT, 2008, which assessed the alkali-aggregate reaction by monitoring the expansion of the mortar bars. This test was performed in order to check the possible effects of SCBA on reducing the expandability of the bars. Three prismatic specimens (25x25x285 mm) were prepared for each trace, taking its arithmetic average as the end result. This test used the Control traces of 5% and 10% of SCBA.

2.10 Capillary absorption

The test to determine capillary absorption was performed by NBR 9779 (ABNT, 2012) on five cylindrical specimens (50 χ 100 mm) per trace, which followed the molding according to NBR 13279 (ABNT, 2005). The specimens were cured until the ages of 7 and 28 days and then placed in a drying oven to constant mass. Next, they were removed from the oven and kept outdoors to cool in order to avoid high initial rates of water absorption. In addition, waterproofing additive was applied in two layers on the side surface of the specimens, leaving the free base for the penetration of water and for measuring the height of the water layer.

3. Results and discussion

3.1 Compressive strength

From the results in Figure 3, it was found that after 7 days the compressive strength of the traces with 3%, 5% and 8% of SCBA are very similar and lower than the compressive strength of the Control trace, but they evolve at the age of 28 and 56 days, indicating that the compressive strength evolution is slower.

Figure 3. Compressive strength

However, at 56 days all traces with SCBA had higher compressive strength than the Control trace, which can be attributed to the physical effect of filling the voids with fine grains of SCBA. There was a 23.3% increase of compressive strength at 56 days for the Control trace with 10% of SCBA.

The presence of the superplasticizer in the compressive strength increase in the traces with 5%, 8% and 10% of SCBA is insignificant, since there was a compressive strength increase in the trace with 3% (which has no additive) in relation to the Control trace.

3.2 Tensile strength by diametral compression

According to Figure 4, there was a tensile strength increase by diametral compression in the trace with 3% ot SCBA in relation to the Control trace, for all ages. However, after the trace with 5% of SCBA, the tensile strengths remained practically the same and lower than the tensile strengths of the trace with 3%. Therefore, for this test, the trace with 3% of SCBA was considered as optimal replacement content.

Figure 4. Tensile strength by diametral compression

3.3 Drying shrinkage

Table 7 shows the values obtained in the drying shrinkage determination of the mortars, due to the use ot pozzolanic material. The retraction of the Control trace gained a value of 0.052%, and in contrast, the trace with 10% of SCBA resulted in 0.078% of shrinkage.

Table 7. Drying shrinkage

In short, there was a 0.026% increase in drying shrinkage due to the use of SCBA, although this increase was less than the maximum of 0.05% set by NBR 12653 (ABNT, 2012), which classifies pozzolanic materials.

3.4. Accelerated reactivity

Figure 5 shows the test results of potential accelerated reactivity. The dashed line indicates the expansion threshold, which below that the material can be considered potentially innocuous at 30 days, as stated by NBR15577-5 (ABNT, 2008). The three traces (Control, 5% and 10% of SCBA) are considered potentially reactive.

Figure 5. Potential accelerated reactivity

However, at 30 days the two traces containing SCBA (5% and 10%) had similar expansions and below the expansion of the Control trace, with a decrease of approximately 15%. This reduction in expansion indicates that incorporating SCBA in mortars has some pozzolanic activity.

3.5 Capillary absorption

Figures 6 and 7 show capillary absorption at 7 and 28 days, respectively. For both ages, there is a capillary absorption reduction when any SCBA contents are added. The addition of these fines to the mortars is directly related to physical effects, since it promoted filling the voids in the paste, and this hampered the movement of water through the capillaries.

Figure 6. Capillary absorption at 7 days

Figure 7. Capillary absorption at 28 days

There was a very marked decrease, 27% and 38%, at 7 and 28 days, respectively, of capillary absorption at the end of the test (72 hours) for the Control trace, compared to the trace with 3% of SCBA. However, with this substitution ratio the decrease was lower, indicating that the trace with 3% has greater efficiency to reduce absorption.

4. Conclusions

According to the characterization results of SCBA, the work was performed with low pozzolanic activity materials. However, it was confirmed that SCBA can be used as partial replacement of fine aggregates for the production of mortars.

The milling process was an important step, with 20 minutes as the ideal milling time as it provides grain size uniformity and also its reduction, confirmed by specific surface area and laser particle size.

The chemical composition tests and pozzolanic activity showed that SCBA cannot be classified as pozzolanic material, according to the criteria of NBR 12653 (ABNT, 2012).

Although for the drying shrinkage test this incorporation resulted in a value that is below the limit specified by the aforementioned standard.

The diffractogram of SCBA revealed crystalline structure peaks and also indicated a deviation from the baseline angle (2θ) 20°, suggesting the existence of a small amount of amorphous material. It is necessary to perform other tests such as thermal analysis to further verify the pozzolanic activity of the material.

It should be taken into consideration that the evaluation methods for pozzolanic material are controversial. Gava and Prudencio Jr. (2007), for example, dispute the use of chemical and physical characteristics and the degree of pozzolan amorphism for its classification and selection.

The compressive strength results showed increased levels in all incorporation ratios of SCBA, assigned to the physical effect of filling the voids by the fine grains.

There was a tensile strength increase by diametral compression in the trace containing 3% of SCBA, compared with the Control trace, which was considered the optimal replacement ratio.

The addition of SCBA fines for all replacement ratios led to the reduction in capillary absorption, since it promoted the filling of voids, preventing the entry of aggressive agents in the mortar.

Given the above, it is believed that the introduction of SCBA can be considered an alternative material for the production of mortars and possibly concretes, as it improves its various properties, and particularly as it reduces the environmental impact caused by its inadequate disposal.

5. Notes

1Civil Engineer, Teacher at the Department of Planning, Urbanismand Environment, School of Science and Technology, University Estadual Paulista (UNESP)

6. References

 

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E-mail: fioriti@fct.unesp.br

Fecha de Recepción: 22/01/2014 Fecha de Aceptación: 17/06/2014