Early-age effect of corn cob ash as a partial replacement for Portland cement in concrete

. One of the effects of supplementary cementitious materials (SCMs) in concrete is the dilution effect which is as a result of partially replacing Portland cement with SCMs. The dilution effect of corn cob ash (CCA) with respect to strength gain and transport properties of concrete is the focus of this study. 100 mm concrete cubes were prepared with corn cob ash partially replacing Portland cement at 15% and 30% using w/b ratio of 0.4 and 0.6. The effect of CCA replacement level on the compressive strength development between 3 and 28 days, and transport properties at 28 days of curing was done. This was compared to the effect of fly ash at the same replacement level and 100% Portland cement content. The results showed that compressive strength decreases with increasing ash content and increases with increasing curing age. The dilution effect of CCA was more pronounced at the two replacement levels than fly ash with a marginal gain or strength loss compared to w/c ratio corresponding to each w/b ratio. The oxygen permeability index obtained for CCA concrete were lower compared to that of fly ash concrete at the same replacement levels and PC concrete at the same w/b ratio. Also, the water sorptivity index and chloride conductivity index for CCA concrete were higher than that of fly ash and PC concrete at the same w/b ratio.


Introduction
Currently, there are various streams of materials that have been investigated as supplementary cementitious materials (SCM) ranging from industrial products and by-products (e.g.metakaolin, slag and fly ash), to agro-based materials (sugarcane bagasse ash and rice husk ash) and natural materials (pumice).Often these materials are used to partially replace Portland cement (PC) in cementitious systems and influence the properties of the system by different mechanisms to modify and improve the properties of the system in which they are present.The behaviour exhibited by SCMs is often a function of their material properties combined with the properties of PC, which is the main hydrating material in the system.As SCMs are often less reactive than PC, they have been reported to create extra nucleation sites for the precipitation of the hydration product of Portland cement [1].The creation of nucleation sites influences the pore structure of the system and subsequently affects the microstructure of the system, which contributes to, and significantly influences the mechanical and durability properties of cementitious system.* Corresponding author: Damilola4fa4@gmail.comA known effect of SCMs in cementitious systems is their dilution effect which has a relation with the quantity of the PC in the system.Dilution effect of SCMs has two phases, one is the reduction in the quantity of PC, and the other is the increase in the w/c ratio of the system [2].Both effects have an influence on the properties of cementitious system especially compressive strength and the pore structure.The hydration of PC with time densifies the microstructure of cementitious systems but with increase in w/c, the interparticle spaces increases thereby increasing the pore network of the system.Dilution effect is compensated for by increasing the fineness of SCMs and subsequent pozzolanic reaction of reactive SCMs.The fineness of SCMs, as fine particles has been associated with increased degree of hydration of PC [3] while the pozzolanic reaction further contributes to the formation and growth of hydration products.The behaviour of SCMs in cementitious systems will therefore depends on their characteristic properties and interaction with PC.
The need to seek alternative to fossil fuels has seen research into the use of biomass which is considered MATEC Web of Conferences 364, 02011 (2022) https://doi.org/10.1051/matecconf/202236402011ICCRRR 2022 renewable in nature with comparable efficiency to some conventional fuel sources [4].With the increased use of biomass as fuel sources, ashes from biomasses will become available, and likewise the need to dispose the ashes.This has led to the investigation of ashes from agricultural wastes as SCMs in cementitious systems.Ashes from some agricultural wastes (e.g.rice husk ash, sugarcane bagasse ash, palm oil fuel ash) have been investigated as potential SCMs and found to have good material properties [5][6] [7] .Corn cob ash (CCA) has also been investigated to a certain extent for its potential as a partial substitute for PC in cementitious systems.Study has shown that CCA affects the early age compressive strength of concrete, yielding a strength lower than plain PC concrete up to 28 days and for replacement level of 10% and above [8].The study further recommended 8% CCA content as the optimum for structural application and a minimum curing period of 120 days.The curing period recommended by the study is rather too long for onsite application, hence there is a need to improve the early age strength.
In this study, the early-age up to 28 days compressive strength and transport properties of concrete containing CCA at levels higher than 10% were investigated in order to determine the effect of using CCA as a major (above 10% content) component in cement production.Also, it is crucial to understand the performance of CCA in concrete to help in the proper classification and subsequent utilisation of the material.Also, other studies [9][10] have shown that the inclusion of CCA reduces the compressive strength development of concrete, it is important to know why this is so as it will help to improve the further development of the material.

Experimental set-up
The materials used in this investigation include a CEM 1 52.5RPortland cement, crusher sand as the fine aggregate and 22 mm granite stone as the coarse aggregate.Due to availability of limited equipment, the corn cob ash was obtained by burning corn cobs in a drum furnace continuously until the ash was obtained.The obtained corn cob ash was further ground in small portions to improve the fineness of the particles in a laboratory disk mill.To obtain as homogenous a blend of the ash as possible, the grounded ash was placed in a closed container charged with steel balls and mixed in a rotating concrete drum mixer for about 45 minutes.The corn cob ash and fly ash were used to partially substitute PC at 15% and 30% levels, while water to total cementitious material ratio, w/cm ratio of 0.4 and 0.6 were used to cast test specimens.Five mix proportions, 100% PC, 85/15 FA (85% PC and 15% FA), 85/15 CCA (85% PC and 15% CCA), 70/30 FA (70% PC and 30% FA) and 70/30 CCA (70% PC and 30% CCA) were studied.Also, to study the dilution effect of the CCA, another set of concrete samples were prepared with plain PC only as the binder at the w/pc ratio corresponding to each w/cm ratio (i.e.0.4 and 0.6) studied and the effect compared.Potable water was used in the preparation of the concrete.Fly ash was used in this study based on the somewhat similar chemical composition with CCA and availability.Durability index (DI) tests were carried out on the concrete at the age of 28 days using concrete discs samples.The concrete discs specimen were of 70 ± 2 mm diameter and thickness of 30 ± 2 mm.At the testing age , 100 mm cubes were removed from water, cored and cut using a diamond saw, the concrete discs were then oven dried at 50°C for seven days prior to the commencement of the DI tests.Three DI tests namely oxygen permeability, water sorptivity and chloride conductivity tests were performed on the concrete.The oxygen permeability testing of the concrete was done according to the SANS 3001-CO3-2.The same samples used for the oxygen permeability were used for the water sorptivity test.The essence of the test is to measure the transport of moisture through the concrete pores under the influence of capillary action.The chloride conductivity test was done according to SANS 3001-CO3-3 to assess the diffusion of chloride ions in the specimen.

Physical and chemical composition of the materials
Table 1 shows the result of the XRF analysis of the oxide composition carried out on the fly ash and corn cob ash as well as some physical properties of both materials.The analysis showed that the CCA contained significant amount of silica, although the combined silica, iron oxide and alumina was lesser than that recommended by ASTM for pozzolans.The silica is often seen as a contamination from the soil on which the plant grows since corn cobs is a lignocellulosic material.Compared to CCA, the fly ash has significant amount of alumina and met the combined oxide requirement of ASTM.Also, the CCA has a high amount of K2O and LOI which is evident in the dark grey colour of the ash, showing it to contain some unburnt materials.The particle size distribution of the materials was determined using the liquid procedure of Anton Paar particle size analyser which uses laser diffraction method.The result of the size distribution showed that the fly ash was finer than the corn cob ash while the surface area of CCA is larger than that of fly ash although the difference between the two is little.

Compressive strength
The results of the compressive strength test is presented in Figure 2. The results showed that compressive strength varies with both w/cm ratio and curing ages and this is consistent with the strength characteristics of conventional concrete.Also, up to the 28 days investigated, the strength values of both fly ash and CCA were lower compared to plain PC and the strength values for CCA were lower compared to fly ash.This is due to the fact that pozzolanic supplementary cementitious materials rely on the hydration product of PC (especially the portlandite) for their reaction and subsequent strength development.The major controlling mechanism can be said to be the hydration reaction of PC and the filler effect of the SCMs.The hydration of PC results in the formation of strength enhancing product, calcium silicate hydrate (C-S-H) which grows with time on the surface of the PC and SCMs and also fills interparticle spaces in the cementitious system.The partial substitution of PC by pozzolanic SCMs on one hand reduces the quantity of PC relative to plain PC system at the same w/cm ratio, hence the lower strength values than plain PC at early age up to 28 days.On the other hand, this is compensated for by the particle size of the SCMs leading to the filler effect of the material, hence their strength development is slow at early ages up to 28 days of curing.Further, as fly ash particles are finer than that of CCA, the filler effect compensation becomes significant for PC/fly ash concrete leading to higher strength than PC/CCA concrete.Comparing the two w/b ratios, the strength at w/cm ratio of 0.4 was higher than at w/cm ratio of 0.6 and this is also consistent with the behaviour of conventional concrete.At higher w/pc ratio than 0.4, there is a dilution effect as there is more water than the theoretical (w/pc ratio of 0.4) necessary for the complete hydration of plain PC particles [11].The rate of gain of strength by PC/fly ash concrete increases with increasing curing age while it decreases for PC/CCA concrete (Table 2).A likely reason for this is the fact that fly ash has more amorphous content than CCA hence it is more reactive than the CCA studied.At 28 days it is believed that significant degree of hydration of PC has occurred and pozzolanic reaction of amorphous fly ash has commenced hence the increasing rate of strength gain.Also, since CCA has limited amorphous content pozzolanic reaction is limited, thus, with increasing CCA content, there is lesser hydration products in CCA systems than in fly ash system.Also, the rate of gain of strength at 15% PC replacement level was higher than at 30%, this may be linked to the differences in the quantity of CCA replacing PC.Also, for all replacement levels investigated, the strength increases with increasing curing age while it decreases with increasing ash content in the mix.The hydration of cement and proper curing of concrete are responsible for strength development with time [11], thus, all the mix studied were hydrating, though at different rates.water to total cementitious material ratio per mix When compared to 0.4 w/cm ratio, the rate of gain of strength at both replacement levels investigated were higher at 0.6 w/cm ratio.This effect is linked to the workability of the concrete mix as the at 0.6 w/cm was more workable than at 0.4 w/cm ratio.Also, the strength values obtained at 0.6 w/cm ratio were lower than at 0.4 w/cm ratio, this is expected as the cement content used was lower than the cement content at 0.4 w/cm.Compressive strength is known to reduce with increasing w/cm ratio and reducing cement content [11].
Compared to other studies, [8]- [10], [12] there is an improvement in the early age compressive strength, although the rate of gain of strength at 3 and 7 days seems to diminish at 28 days.A clear reason for this could not be ascertained whether the PC used or the CCA material characteristics.A likely reason maybe the rapid hardening effect of the PC with impact felt at early ages, while CCA act as a filler at these ages.
Comparison of compressive strength development in concrete containing CCA and concrete containing fly ash, Figs.3(a)-(d), showed that the fly ash had higher strength than CCA except at w/cm ratio of 0.6 where at early ages the strength of both SCMs are comparable.This is expected given the fact that fly ash has more amorphous content than CCA, although the latter has a higher surface area.This implied that for the materials studied, reactivity of the materials is more important to strength development than surface area.

Dilution effect of the SCMs
The dilution effect of SCMs at a given w/cm ratio has two phases, increased water to Portland cement, w/pc ratio and reduced Portland cement content due to partial substitution of the PC by SCMs.In PC/SCM system, the w/c ratio corresponds to the ratio of the mix water to the PC content only, while the w/cm ratio corresponds to the ratio of the mix water to the PC/SCM content.Thus at a given w/cm ratio, there is a higher w/pc ratio which corresponds to the amount of SCM used compared to plain PC system.Dilution effect affects the compressive strength of concrete resulting in lower strength prior to the commencement of significant pozzolanic reaction of the SCM [13].Dilution effect of SCMs is the resulting effect of replacing Portland cement partially with a SCM with a consequential effect on the compressive strength development of the concrete.The dilution effect is taken to be the reduction or gain in compressive strength by concrete containing SCM when compared to concrete made with plain Portland cement.Thus at the same cement content, for w/cm ratio of 0.4 and at 15% replacement level, the corresponding w/pc ratio is 0.47 while it is 0.57 for 30% replacement level.
The dilution effect of fly ash and CCA based on the effect of increased w/pc ratio is presented in Figs.4(a)-(d) For both materials, their dilution effect increases with increasing ash content in concrete while that of CCA leads to a reduction in strength when compared to PC at higher curing age.Compared to fly ash, the dilution effect of CCA is more pronounced at both replacement levels, and this can be associated to the differences in their reactivity and possibly fineness.At 0.4 w/cm ratio, where fly ash was consistently gaining strength with age compared to its increased w/c ratio effect at both replacement level, CCA had a strength loss or marginal strength gain.At 0.6 w/cm ratio, the dilution effect of fly ash concrete increased with increasing ash content with strength gain compared to the increased w/pc ratio.For CCA, there is a marginal gain in strength and strength loss at 15% replacement level compared to the increased w/pc ratio but at 30% however, there is a gain in strength compared to the increased w/pc ratio.A likely explanation for this could be that at the increased w/pc ratio, there is more free water in the PC system than is necessary for hydration, creating porous pore network in the concrete whereas the SCMs tends to reduce the amount of this water.

Durability indices of concrete
The results of the durability index tests (oxygen permeability, water sorptivity and chloride conductivity) are presented in Figs.5-7.The increase in OPI between 0.4 and 0.6 w/cm ratio for most of the mixes can be linked to the capillary porosity, size and interconnectivity of the pores, which increases with increasing w/b ratio [14].This had influenced the micro and macro pores present in the concrete resulting in reduced OPI value at 0.4 w/b.At the same w/b ratio and w/c ratio, fly ash had higher OPI values than CCA showing that concrete containing CCA is more permeable and porous while the microstructure is less dense compared to plain PC and fly ash.This is so given that PC is the sole hydrating material and the CCA is more crystalline than amorphous thereby increasing the interparticle spaces and influencing the pore network and distribution.Also, the reduction in OPI values with increasing w/b ratio for fly ash at 30% content implied a reduction in permeability with increasing fly ash content, indicative of better packing of the particles and denser microstructure than obtained in CCA concrete.CCA concrete showed lower values compared to the increased w/pc ratio while fly ash showed higher values and this is also evident in the lower strength values obtained for the CCA concrete.Also, the OPI values for CCA concrete reduces with increasing content while those of fly ash increases with increasing content.This further showed that the dilution effect of CCA was not adequately compensated for by its reduced particle size.The OPI values for CCA can be classified as poor while those of fly ash can be classified as excellent [15].
The water sorptivity index increases with increasing w/b ratio for plain PC and fly ash concrete between 0.4 and 0.6 w/cm ratio and this can be linked to the reduction in the PC content leading to an increase in the interparticle spaces and capillary porosity as increase in w/c ratio increase the permeability of concrete [11].However, for CCA, sorptivity index reduces with increasing w/b ratio which can be linked to the workability of the mix as the mix at w/cm ratio of 0.6 was more workable than at 0.4.At w/cm ratio of 0.4, there is an increase in the sorptivity of both fly ash and CCA compared to plain PC and this can be due to the slow developemnt of pozzolanic reaction and its effect on microstrcuture at early ages especially for fly ash.A similar trend was recorded for slag in the study of Otieno et al. ( 2020) and was attributed to the slow reaction of slag at early ages resulting in slow rate of microstructural refinement.Also, compared to the increased w/c ratio, the reduced sorptivity at both 15% and 30% fly ash and CCA content for w/cm ratio of 0.6 showed that the increased interparticle spaces was compensated for to certain extent by the inclusion of both SCMs.
The chloride conductivity index (CCI) increases with increase in w/b ratio for all the mixes showing the sensitivity of the test to differences in w/b ratio.This can also be linked to the increase in pores sizes and their interconnectivity as well as capillary porosity with increase in w/b ratio [14].The dilution effect of using fly ash and CCA results in higher CCI values compared to the increased w/pc ratio at w/cm ratio of 0.4 except for 30% fly ash which has a lower value than the increased w/c ratio.The fly ash behaviour is expected given that it is finer than CCA thus resulting in a better pore refinement and better microstructure than CCA.At 0.6 w/cm ratio, both fly ash and CCA had lower values of CCI when compared to the increased w/c ratio.This showed that the dilution effect of using both SCMs was compensated for and there is pore refinement to certain extent due to the inclusion of and fineness of the materials.

Conclusion
The effect of using CCA as a partial replacement for PC on the early age properties of concrete compared to fly ash has been studied, there is an improvement in the early age compressive strength especially at 3 and 7 days with a marginal gain in strength at 28 days.The dilution effect of CCA in concrete is significant compared to fly ash at the same w/cm and w/pc ratio resulting in marginal gain in strength or strength loss compared to increased w/c ratio.Also, CCA influenced the transport properties of concrete and thus affects the microstructure of concrete showing susceptibility to oxygen gas and water ingress but a good resistance to chloride ion ingress.

Figure 1 :
Figure 1: Drum furnace for the calcination of the corn cob 100 mm concrete cubes were cast and cured in water at a temperature of about 23°C until the testing date.Compressive strength of each concrete cube, was determined by placing the sample centrally in the compression machine and compression load applied continuously at a uniform rate of 150 kN/min until the cube fails.The compressive strength was calculated by dividing the failure compression load by the area of the cubes' surface in contact with the compression load.

Figure 2 :
Figure 2: Compressive strength development in concrete

Figure 5 :
Figure 5: Oxygen permeability index of concrete at 28 days

Table 1 :
Properties of the CCA and fly ash

Table 2 :
Variation in compressive strength development compared to plain PC