Due to their pozzolanic activity, ashes from agro-industrial waste are used for the partial replacement of cement as well as to partially replace fine aggregates used in the production of mortars1. Ash composition and mineralogy and its effect on mortar properties is mainly affected by burning conditions. Studies carried out by Ribeiro and Morelli2 show that at 500°C formation of calcium silicates and aluminates has not yet begun; at 600°C the highest levels are recorded at 700°C a slight decrease of the acidic oxides responsible for the pozzolanic activity of the ashes is noted. Cordeiro et al.3 worked also with sugarcane bagasse at temperatures in the range of 400-800°C, with increments of 100°C and a burning time of 3 h. The results obtained by Cordeiro et al.3 show at 400°C a completely amorphous material while indicating that crystallization initiates at 500°C.
Arum et al.4 studied the effect of ash from rice husk (RHA) and coconut husk, in the range of zero to 15% and showed a maximum increase on the compressive strength at 10% RHA, while the bark ash of coconut showed a continuous growth up to 15% of ash content in the tested samples. Bie et al.5 produced RHA at 600-700°C and burning times of 1-2 h. The results of Bie et al.5 show some crystallization in samples prepared at 700°C and some deterioration of their compressive strength compared to values obtained with ash prepared at 600°C. The best results were obtained with 10% additive, burning for 1 or 2 h. The values obtained are comparable or even slightly higher than those of conventional mortar.
Jamil et al.6 start by the calculation of the ideal amount of ashes, based on the stoichiometry of the pozzolanic reaction between amorphous silica and Ca(OH)2, an approach that do not consider the filler effect (physical component of pozzolanic activity) and lead therefore to an optimal ash content different from the value obtained experimentally.
Non conventional mortars show low values of compressive strength for the lower curing times. These results can be explained by a slower development of the mechanical resistance of mortars with addition of pozzolanic materials, when compared to reference mortars7. According to Shatat8, in lower curing ages, materials show a greater filler effect, whereas for higher curing ages, greater pozzolanic activity of these materials is registered.
Although there is a general evidence of this trend, the results obtained by Moraes et al.9 with sugarcane bagasse ash (SCBA), show, even for curing ages of 3 and 7 days, comparable values of the compressive strength of reference mortars and mortars with ash addition. This may be evidence that results obtained depend from other variables like the ratio of the different components, viscosity, consistency and decanting time of mortars10, but also the areas where a certain product was cultivated, as evidenced by X-ray diffraction results obtained with RHA from different regions11.
In the present study, a comparative study of the effect of the ashes from rice husk, sugarcane bagasse and corn cob was carried out. The water/binder ratio was kept constant, with all the limitations that this entails in the work ability of some compositions, which in some cases limited the maximum content of ash added.
MATERIALS AND METHODS
Present research encompassed the following phases: Ash production from the 3 samples and the determination of the amount of amorphous phase in each ash sample, the production of mortars; and the performance of the compressive and flexural strength after 2, 7 and 28 days curing age.
Study area: This study was conducted in Maputo Mozambique as part of the research project at the University Eduardo Mondlane, from February, 2016 to December, 2017. The agricultural material was obtained in local villages from peasants, for corn cob, rice ask was obtained from a agricultural farm in the north of Maputo (Inácio de Sousa, Lda.). Sugarcane bagasse was obtained from a local sugar mill company, namely Companhia Açucareira de Xinavane.
Ashes production: The ashes were prepared by burning the material (corn cob, rice husk and sugarcane bagasse) on a kitchen stove using aluminium pots. The samples were grinded for 30 min using a porcelain ball mill, to facilitate calcination. The pulverized material was transferred into a nickel crucible and placed in a muffle at preset temperature.
The optimization process for the three selected materials followed the experience described in the literature3,5,12. Selected temperatures ranged from 550-800°C with 50°C intervals and burning times ranging from 1-4 h with 1 h intervals. The test specimens were prepared using ash produced under optimal conditions.
Mineralogical analysis as well as the amorphous content determination in the ashes were done by XRD using a panalytical Empyrean diffractometer.
||Amounts of different components of the mortars used in the preparation of test specimens
Preparation of the mortar specimens: The preparation of the test specimens and the performance of the compressive and bending stress tests after 2, 7 and 28 days of cure, followed the methodology described in Macie et al.13, which is based on ASTM standard14 91977, norms NM15 NP 197-1: 2000 and NBR16 9778. For this purpose, mortar compositions were formulated, keeping the contents of the components constant, varying only the percentage of cement, which was replaced by varying amounts of ash of the 3 materials.
For mortars preparation a local Portland cement 42.5 N was used, produced by Cimentos de Moçambique and sand from Corrumana, a dam located in the District of Moamba-Maputo Province. The sand was manually washed to reduce dust and other impurities, dried, milled and sieved for the production of 6 classes of fine aggregate according to standardized process17.
Preparation of the mortars for test specimens followed norm18 NP EN 196-1:2006 the amount of sand (1350 g) and water (225 mL = 225 g) was kept constant, while cement was partially replaced by quantities of ash in different percentages as shown in Table 1. The ratio of cement (plus ash): Sand was kept constant(1:3) corresponding to 25% cement to 75% sand in each mortar and with a ratio of 225/450 g (water/cement), making a trace of 1:3:0.5 (cement:sand:water), according to norm18 NP EN 196-1:2006. Bending and compressive stress tests were conducted in a Controls press.
Optimization of the ash production process: Figure 1a-c show the results of the amorphous content in the samples of RHA, SCBA and corn cob ash (CCA), calcined at different temperatures (550, 600, 650, 700 and 750°C) and different calcinations time (2-4 h).
A uniform behaviour, with amorphous content in the order of 99% for the different calcination times tested and in the temperature range of 550-750°C, is observed in Fig. 1a (RHA). Similar behaviour is observed with SCBA for 2 and 3 h, while at 4 h a different behaviour is observed (Fig. 1b).
Amorphous content in (a) Rice husk (b) Sugarcane bagasse and (c) Corn cob ashes as function firing temperature and time
A slight reduction in amorphous content is observed at 650°C. The amorphous contents were below 50%. The CCA curves showed no regular behaviour (Fig. 1c), especially at 2 and 3 h burning.
The overall X-ray diffraction tests for the different temperatures the results are presented in Fig. 2-4. Heat treatment did not produce significant changes in the mineralogy of RHA, mainly composed by quartz and cristobalite, neither the heating time have produced (Fig. 2). The SCBA XRD patterns show the presence of quartz (SiO2), albite (NaAlSi3O8) and microcline (KAlSi3O8). Quantitative results (Fig. 1) show a slight increase in crystalline content as the heating temperature increases. Increasing the heating time favours crystalline content. The CCA patterns show the presence of cloritoide ((Fe, Mg, Mn)2, Al4Si2O10(OH)4), jasmundite (Ca11(SiO4)4SO2) and quartz (SiO2).
XRD patterns of rice husk ashes (RHA) (a) Effect of temperature and (b) Effect of firing tim
Q: Quartz (SiO2)
XRD patterns of sugarcane bagasse ashes (SCBA) (a) Effect of temperature, (b) Effect of calcination time
Q: Quartz (SiO2), A: Albite (NaAlSi3O8), Mcr: Microcline (KAlSi3O8)
XRD patterns of corn cob ashes (a) Effect of temperature and (b) Effect of calcination time
Ar: Arcanite (K2SO4), Chl: Cloritoide ((Fe, Mg, Mn)2Al4Si2O10(OH)4), Q: Quartz (SiO2), K: Kalicinite (KHCO3), J: Jasmundite (Ca11(SiO4)4SO2), D: Dolomite (CaMg(CO3)2))
Compressive strength of mortars: Figure 5a and b present results of the compressive strength of mortars with various ash contents. To facilitate comparison, the value of the compressive strength of the reference mortar (mortar with 100% cement) and the minimum value of the compressive strength were introduced in Fig. 5a and b. With RHA and CCA mixtures with 20 and 25% of ash content were not performed do to difficulties in work ability of the mixtures.
Results indicate that after 2 days of curing all samples performed above the threshold value of compressive strength, 10 MPa (Fig. 5a).
At 28 days of curing time only mortars with 5, 10 and 15% of RHA showed values of compressive strength higher than 42.5 MPa, the threshold value indicated by the norm NM NP EN 197-1:2005 (Fig. 5b).
Flexural strength of mortars: Although it is not a decision-making value for assessing the feasibility of using a given material, good performance in this parameter may contribute to the acceptance of the material. These results are graphically presented in Fig. 6a and b.
The results of the flexural strength after 2 days curing age show, only for the CCA and only for the 5% additive, flexural strength values higher than the conventional mortar.
Compressive strength of cement mortars with different ash contents at (a) 2 days and (b) 28 days curing age
RHA: Rice husk ash, CCA: Corn cob ash, SCBA: Sugarcane bagasse ash
Flexural strength of cement mortars with different ash contents at (a) 2 days and (b) 28 days of curing age
RHA: Rice ask ash, CCA: Corn cob ash, SCBA: Sugarcane bagasse ash
From 2-7 days curing age, there is a significant increase in the flexural strength of RHA and SCBA, with all the 3 types of ashes showing flexural strength values higher than those of conventional mortar at 5% additive, with RHA exhibiting the highest values. The RHA and CCA show higher values for 5 and 10% additive, while SCBA show values higher than reference mortar only for 5% additive.
At 28 days the RHA showed values higher than the reference mortar at 5 and 10%, SCBA showed a value comparable to that of conventional mortar at 5%, while that of CCA showed a lower value in the entire additive contents tested. At 15% the RHA and SCBA show a value that amounts to 96% of the value of the conventional mortar.
The results show a remarkable development of the pozzolanic activity of CCA in early curing ages, as evidenced by the results of the flexural strength (Fig. 6a, b), but the development of the pozzolanic activity of these ashes is then supplanted by the ashes of RHA and SCBA, in later stages of curing (Fig. 6b).
The evaluation of the content of amorphous material of ashes prepared from the 3 samples shows for RHA a content of amorphous material of 99%, for temperatures up to 700°C, results consistent with those obtained by Bahurudeen and Santhanam19 and Bié et al.5. Ashes from sugarcane bagasse and corncob show lower values, in the order of 40-45 and 55-60%, respectively. Although the amorphous material content is used as an indication of the pozzolanicity of the material, its use must be done with some care, because the fraction of the only acidic oxides (SiO2 and Al2O3) are responsible for the pozzolanic reaction with Ca(OH)2. The amorphous material in these ashes includes other components, which do not necessarily contribute for the pozzolanic activity.
Comparison of the X-ray diffraction diagrams basically shows the following results:
|| The RHA does not show major changes with the heat treatment. They have few crystalline phases (quartz and cristobalite) and high levels of amorphous material (<1% of crystalline material)
||The SCBA also had little variation of the composition, with 3 crystalline phases (albite, quartz and microcline) and amorphous material content in the order of 40%
||The CCA show a significant increase in the content of crystalline phases with temperature and a more complex composition (greater number of crystalline phases), with a content of amorphous between 50 and 70%
Chemical composition of rice husk, sugarcane bagasse and corn cob
Source: Rezende et al
, Pointner et al
, Ummah et al
According to Ribeiro and Moreli2, Benassi et al.11 and Safiuddin et al.20, RHA should give ashes free from crystalline material. The presence of crystalline material may be caused by the way these wastes were processed prior to their burning. According to Ribeiro and Moreli2, adequate washing of the residue of bagasse, prior to its burning, should lead to the production of ashes free from crystallines phases.
A look at the approximate chemical composition of rice husk, sugarcane bagasse and corn cob (Table 2) shows.
||Presence of silica in rice husks only
||Significant ash content in sugarcane and corn cob
|| Considerable contents of lignin, cellulose and hemicellulose in all the 3 samples
Results in Table 2 show that the amorphous material in studied samples is not necessarily attributed to the presence of silica, but to other components, for instance lignin which is present in form of an amorphous three-dimensional macromolecule, which can form amorphous phases but do not show the pozzolanic reaction with Ca(OH)2. Use of more objective methods is required; those better quantify and separate the chemical and physical components of the pozzolanic activity of ashes.
The RHA shows the best performance, with values of compressive strength comparable to that of conventional mortars, at 5% of additive and values of compressive strength equal to or above the minimum value prescribed in the standards, up to 15% of the additive and for the cure time of 28 days.
The pozzolanic activity index, assessed from compressive strength results, show a different behaviour for the three samples. The RHA show a slow development of pozzolanic activity at early curing ages (2 and 7 days) but increased at 28 days, results in agreement with Antiohos et al.1 and Khan et al.24. The SCBA show an increase of pozzolanic activity with increasing curing ages in the interval 15-25%, results which are in agreement with Pereira et al.25. Corn cob presents a significant early development at curing age 2 days but decreasing at 7 and 28 days curing ages.
Ashes from rice husk, sugarcane bagasse and corn cob show promising results for partial replacement of cement in mortars. Sugarcane bagasse and rice husk ashes showed the most promising results with additions up to 15% of ashes replacing cement. Corn cob ashes showed a good earlier development of the properties at the curing age of 2 and 7 days and not showing a significant further improvement after this. The poor performance of corn cob ashes can be explained by the fact of the optimization parameters for ashes production was only concerned to the temperature. Another reason could be the lower content of silica in these ashes.
This study discovers the use of corn cob residue in the production of ashes with potential to be used as cement additives could help reducing environmental burden of the cement industry, as well lowering construction costs. This study will help the researchers to uncover the mortar properties of corn cub which many researchers were not able to explore.
The authors wish to express their gratitude to the Fundo Italiano de Ajuda Multisectorial (FIAM) for the financial support and to the Laboratório Engenharia de Moçambique (LEM) for the technical contribution and support.