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Assessment of Embodied Energy and Carbon IV Oxide Emission of Concrete Containing Corncob Ash

: The purpose of this study is to assess the Embodied Energy (EE) and Carbon IV Oxide (CO 2 ) emissions saving potentials of Corn Cob Ash (CCA) as partial replacement of Ordinary Portland Cement (OPC) in concrete. Cement manufacture is energy intensive and contributes considerable amount of CO 2 emissions into the atmosphere. Globally, Concrete is the most consumed man-made material and about 95% of CO 2 emissions from a cubic meter of concrete are from cement manufacturing. In this study, the experimental plan was designed to carry out compressive strength, flexural strength, density and water absorption tests on the concrete using 0, 5, 10, 15 and 20% CCA contents to replace OPC. Inventory method of analysis was used to determine the EE and CO 2 emission for all the concrete mixes. The results indicated that the water absorption, density, compressive and flexural strength decreased with increase in CCA content and increased with curing period. The optimum blend was obtained at 10% CCA and 90% OPC contents. The EE and CO 2 emission decreased with increase in CCA contents. At 20% CCA content the EE was 2382 MJ/m 3 which is 12.04% less than that of control samples. Also, 16.37% embodied CO 2 emission saving was obtained for samples containing 20% CCA. The regression equations generated gave standard deviation, S, < 1.0, P-value < 0.05, T-statistics > T24, 0.05 and F-statistics > F1, 23, 0.05. All these indicated that there is good relationship between the predictors and the responses.


Introduction
The worldwide increasing demand for energy is one of the major causes for the unsustainable development of our Planet. Organization for Economic Co-operation and Development (OECD) reported that from 2007 to 2030 energy demand is projected to increase by about 40% which is about 15.24 billion tonnes of petroleum equivalent [1]. Concrete is considered as the world's most consumed man-made material. Every one tonne of concrete leads to CO 2 emission which varies between 0.05 to 0.13 tonnes. About 95% of all CO 2 emissions from a cubic meter of concrete are from cement manufacturing. The annual global production of concrete is about 4.54 billion tonnes. If the production of this building material remains at this frightening level, it is expected that about 3.5 billion tonnes of cement would be produced by the end of 2050 which amounts to doubling the CO 2 emissions [2]. Energy used in concrete production includes EE in cement, energy used to extract and process aggregates, transportation energy, and energy used in the concrete plant. The EE of cement production contributes a large proportion of the total EE of concrete. The EE of structural components makes up about a quarter of the EE of all elements installed at the time of construction [3]. Also, process of cement production emits large amount of CO 2 into the atmosphere, every tonne of cement produced releases approximately one tonne of CO 2 into the atmosphere which is a major contributor for greenhouse effect and the global warming [4]. The calcination process is responsible for 50% of CO 2 emissions, 45% due to burning fuel and 5% due to mining and transportation [5].
Olivier, Janssens-Maenhout, and Peters [6] reported that in 2015, CO 2 emissions generated by carbonate oxidation in the cement clinker production process, the main constituent of cement and the largest of non-combustion sources of CO 2 from industrial manufacturing, contributed about 4.0% of the total global CO 2 emissions while fuel combustion emissions of CO 2 related to cement production are of approximately the same level, so, in total, cement production accounts for roughly 8% of the total global CO 2 emissions. González and Navarro [7] estimated that the selection of building materials with low impacts can reduce CO 2 emissions by up to 30%. Therefore, partial replacement of OPC with supplementary cementitious materials such as CCA is expected to yield a significant reduction in EE and CO2 emissions.
According to the United Nation Centre for Human Settlement (UNCHS) [8], the main step that can be adopted to mitigate the consumption of energy and CO 2 emission associated with construction materials is to reduce their total primary energy consumption. Increasing the efficiency of energy use in construction materials production is important for three reasons, apart from the obvious advantage of energy saving: it can help to make durable construction materials available at bearable costs; it will help to reduce the environmental degradation caused by the excessive use of biomass fuels; and it will also help to reduce the need for imported construction materials or production processes. The aim of energy analysis is to determine the total quantity of energy that should be taken from primary energy resources in order to produce a given material. The analysis also has to include the amount of energy expended in obtaining the raw materials and the energy used in transporting them to the factory. It should also include the energy used to produce and maintain the machinery used in the production process. The total energy calculated in this way is the energy embodied in that material [8].

Corncob Ash
Corncob is an agricultural by-product obtained from maize. It is the hard thick approximately cylindrical central core of maize (on which are borne the grains). The ash resulting from burning the corncob at elevated temperature (450°C-650°C) is known as corn cob ash (CCA) and it has been classified as pozzolana [9].
CCA has pozzolanic properties and has been shown to chemically react with the calcium hydroxide released during the hydration process of cement to form cement compounds. Active pozzolanas gain their binding properties when they react with calcium hydroxide in cement in the presence of water. Studies showed that CCA improves the properties of cement materials, based on its pozzolanic properties [10,11].

Embodied Energy
EE is the amount of energy consumed in all processes associated with the production of a material, from mining and processing of natural resources to manufacturing, transporting and then the delivery of the product [12]. It includes the direct and indirect energies consumed in processing the product. Direct energy is consumed in various on-site and off-site operations like construction, prefabrication, transportation and administration. Indirect energy is the energy used during the manufacturing of building materials, in the main process, upstream processes and downstream processes and during renovation, refurbishment, and demolition. This includes initial embodied energy, recurrent embodied energy and demolition energy [13].
Various approaches to measure EE employ different system boundaries and collect data from different sources, which could result in significantly different values of EE for the same product. System boundaries may range from a restrictive analysis of direct energy required for a particular process, to an expansive analysis including energy used by entire industrial input chains and society as a whole. Analyses may consider only purchased fossil fuel energy inputs, or may include renewable sources or combustible process by-products. Data may be direct measurements of energy used by a particular machine or factory, or may be aggregated for an entire industrial sector [14].
Basically there are four methods of measuring EE: process analysis, input-output analysis, hybrid analysis and inventory analysis. This research work uses the inventory method of analysis developed by Hammond and Jones at University of Bath [15,16]. They published an Inventory of Carbon and Energy (ICE), which is a database containing EE and CO 2 values per functional unit of material (which could be in form of area (m 2 ), mass (kg) or volume (m 3 ) for different common building materials. The data for this inventory was extracted from peer reviewed literature on the basis of a defined methodology and criteria. It is considered that the strict criteria used in the selection of source material for the creation of the ICE serve to significantly increase its accuracy and relevancy.
According to Berndt [17], there are several essentials which can reduce the environmental impact factor and CO 2 intensity of concrete used for construction, which include maximizing the concrete durability, conservation of materials, use of waste and supplementing cementing materials and recycling of concrete. Partial replacement of cement with waste supplementary cementitious materials such as fly ash, ground granulated blast furnace slag, silica fume, rice husk ash and metakaolin not only improves the concrete durability and reduce the risk of thermal cracking in mass concrete but also emits less CO 2 than cement.
In view of the above, this study is aimed at investigating the EE and CO 2 emission saving potentials of CCA when used as partial replacement of cement in concrete.

Materials
Ordinary Portland cement (OPC) was used throughout the research. It has a consistency of 36%, initial setting time of The oxides composition of the OPC is presented in Table 1 The fine aggregate (sand) used was obtained from a stream at Bayara village along Bauchi-Dass road with maximum size of 4.75 mm and specific gravity of 2.63. The coarse aggregate used was crushed normal weight igneous rock aggregate with maximum size of 20 mm obtained from a quarry site in Bauchi. It has specific gravity of 2.65, aggregate crushing value of 14.5% and aggregate impact value of 7.64%. The materials were tested in accordance with BS EN 1097: 6 [20], BS 812: 2 [21], BS 812: 110 [22] and BS EN 933: 1 [23] specifications. The corncobs used were collected from white floury maize variety and stored in polythene bags for about three months before burning. The CCA was produced using controlled burning process in a kerosene powered kiln at elevated temperature of about 500°C. The resulting CCA was ground using mortar and pestle before sieving with 212 µ m sieve size. Figure 1 shows the flow diagram for the production of CCA while oxides composition of the CCA is presented in Table 1. The physical properties of the CCA determined were the specific gravity (2.27) and loose bulk density (304 kg/m 3 ). The water used was portable drinking water obtained from tap source, within the laboratory.   [24,25]. Concrete cubes of size 100mm x100mm x 100mm and beams of size 100mm x 100mm x 500mm were cast using the designed mix and cured for 90 days respectively. The cubes and beams were tested for density, water absorption, compressive and flexural strengths in accordance with BS 1881 [26], BS EN 12390: 7 [27], BS EN 12390: 3 [28] and BS EN 12390: 5 [29] specifications respectively.

Determination of EE and CO 2 Emission
The EE and CO 2 emission of the concrete samples containing CCA were determined using inventory method of analysis. The main advantage of this method is that it has greater accuracy and flexibility than the other methods and tedious procedures that involve chemical equations are avoided by using emission factors. The EE and CO 2 emission values for various mixes were calculated using the model equations presented in equations (1, 2, 3, 4, 5) and (6). The boundary for the EE and CO2 analysis was 'Cradle to Site'.

Cradle to Site EE Cradle to Gate EE Transportation EE
(1) Cradle to Gate EE Quantity of material EE coefficien Therefore, The CO 2 Emission was calculated using: Where m is the wastage factor (%), C, S, A, W and R (in Kg) of cement, sand, aggregate, water, and cement replacement respectively. T is the transportation energy calculated using equation (11). The parameters x C , x S , x A , x W , and x R are the EE coefficients/CO 2 emission per kilogram for the materials mentioned above (Hammond and Jones, 2008).
Generally, waste materials are assumed to have zero EE. Therefore, corn cob has zero EE and CO 2 emission at the point of collection in the farm. The CCA is produced using controlled burning in a kiln at temperature of about 450 0 C. The process of burning the corncob to produce CCA consumed energy and emitted CO 2 . Therefore, the EE and CO 2 emission of the CCA was calculated from the EE of fuel (Kerosene) used to power the kiln as follows: 10 liters of kerosene was used to power the kiln. Density of kerosene is 780 kg/m 3 .  (7) shows the relationship between the EE and embodied carbon (EC) equivalent developed by Hammond and Jones [30].
Equation (9) was obtained by substituting equation (8) into equation (7) and CO 2 was made the subject of the relation.  Table 2 showed the EE coefficients and CO 2 emission factors for the materials. These factors were obtained from the database of Inventory of Carbon and Energy (ICE) developed at the University of Bath.

Transportation EE and CO 2 Emission
Transportation of construction materials consume a lot of fuel energies and emit CO 2 into the atmosphere. These energies and CO 2 are also embodied into the materials. According to Bribian, Capilla & Uson (2011) EE and CO 2 emissions due to transportation of all the materials were calculated using the linear correlation shown in equation (11). Table 3 shows the values to be applied in order to evaluate the impact of transporting one tonne by various means of transportation, where d i is the distance travelled by the i-th mode of the transport (in km) and m i represents the coefficients applied to the i-th form of transport.
EFGHIJKFL MNJGOL = N = × P = + N ( × P ( + N . × P . (11) Table 4 showed the locations and average haulage distances from point of manufacture or collection of each material to the laboratory.  Tables 5 and 6 showed the computations of EE and CO 2 emissions respectively from excel spreadsheets using equations (3,6) and (11) for concrete containing 5% CCA as OPC replacement.

Mechanical Properties
The results of mechanical properties of concrete cured at 90 days is presented in Table 7. Generally the densities decreased with increase in CCA content. The density decreased from 2473 kg/m 3 at 0% CCA content to 2397 kg/m 3 at 20% CCA content. These lie within the range of 2200 to 2600 kg/m 3 specified as the density of normal weight concrete [32]. Also, the water absorptions decreased with increase in CCA contents. The low range of water absorption obtained is as a result of a less permeable calcium silicate hydrate (C-S-H) produced when SiO 2 from CCA reacted with Ca(OH) 2 from cement [33]. The lower the water absorption of concrete the more durable is the concrete. Durability of concrete is improved by decreasing its porosity and transport properties [34]. The results also show decrease in compressive and flexural strengths with increase in CCA contents. The optimum blend was obtained at 10% CCA and 90% OPC contents with compressive strength value of 30.17 N/mm 2 . This agrees with the findings of other researchers [35,10] and [36]. This strength gain can be attributed to the cementitious products formed as a result of hydration of cement and those formed when lime reacts with the pozzolana incorporated [37]

Embodied Energy
The results of EE of concrete containing CCA were presented in Table 8. The table showed the quantities of materials in kilogram required to produce the batch of fifteen sample cubes, their EE in Mega Joule, the EE per cubic meter of concrete and the percentage of EE saving of the CCA. The results indicated that the EE decreases with increase in CCA contents. Figure 2 shows the plot of the variation of EE with CCA contents. The EE decreased from 40.6211 MJ (2708 MJ/m 3 ) at 0% CCA content to 35.7296 MJ (2382 MJ/m 3 ) at 20% CCA content. At 10% CCA content the percentage decease in EE of the concrete is 6.02% while at 20% CCA content is 12.04%. The high percentage of EE saving obtained indicates that OPC is the major contributor of the concrete EE. The regression model for the relationship between EE and CCA contents is presented in equation (12). Table 9 shows the results of regression analysis of EE. The standard deviation, S=7.4388 x10 -15 . The T-statistics are 1.5764x10 16 and 1.1625x10 15 for constant and CCA terms respectively. These values are greater than the T-critical (T 24 , 0.05=1.711) at 5% level of significance. The probability of the t-statistic, P=0.0000 for the constant and CCA terms which is less than the P-value (0.05). All these show that there is good relationship between the CCA content and the EE and the model equation is a good predictor of the response (EEc). Table 10 shows the results of ANOVA for the EE of the concrete. The results show the effect of CCA on EE of the concrete at 5% level of significance The F-statistic is 1.4916 x10 06 for both the constant and CCA terms. The value is greater than the F-critical (F1, 23, 0.05=4.28). The probability of the F-statistic, P=0.0000 for the constant and CCA terms which is less than the P-value (0.05). These show that the variation in EE of the concrete is associated with the variation in CCA contents.

Embodied Carbon IV Oxide Emission
The results of CO 2 emission of concrete containing CCA were presented in Table 11. The table presented the quantities of the materials in kilogram required to produce the batch of fifteen sample cubes, their CO 2 emission in kilogram, the CO 2 emission per cubic meter of concrete and the percentage of CO 2 emission saving of the CCA. The results indicated that the CO 2 emission decreases with increase in CCA contents. Figure 3 shows the graph of  (13).
The regression results for the relationship between CO 2 emission and CCA content are presented in Table 12. The standard deviation of error in the model, S=7.861 x 10 -09 . The T-statistics are 2.155x10 09 and 2.418x10 08 for constant and CCA terms respectively. These values are greater than the Tcritical (T24, 0.05=1.711) at 5% level of significance. The probability of the T-statistic, P=0.000 for the constant and CCA terms which is less than the P-value (0.05). All these show that there is an excellent linear relationship between the CO 2 emission and CCA content, and therefore CCA is a useful predictor of the regression model. Table 13 shows the results of ANOVA for the CO 2 emission of the concrete. The results show the effect of CCA on the CO 2 emission of the concrete at 5% level of significance. The F-statistic is 5.846 x10 16 which is greater than the F-critical (F1, 23, 0.05=4.28). The probability of the F-statistic, P=0.00 which is less than the P-value (0.05). These show that the CCA content has significant effect in variation of CO 2 emission of the concrete.

Summary
The EE and CO 2 emission analysis of concrete containing CCA was explored. The CCA was proven to reduce the EE and CO 2 emission of the concrete. Empirical relations that relate EE, CO 2 emission and CCA contents were developed using Minitab Statistical Software. The performance of the models was tested using ANOVA.

Conclusion
Based on the results of this research work the following conclusions can be drawn: 1. The use of CCA to replace OPC in concrete has the potential to reduce its EE. The energy savings increased with increase in CCA contents. Concrete containing 20% CCA have 12.04% EE saving. 2. The CCA also has the potential to reduce the CO 2 emission of the concrete. At 20% CCA content the CO 2 emission savings was 16.37%. 3. The compressive and flexural strength decreased with increase in CCA contents. The optimum blend was obtained at 10% CCA and 90% OPC contents. 4. The CCA increased the durability of concrete by decreasing its water absorption.

CCA (%) Carbon IV oxide emission (kg)
CO2c = 5.868 -0.05376 CCA 0.05=1.711) at 5% level of significance. 7. The calculated F-statistics exceeded the F-critical (F1, 23, 0.05=4.28) at 5% level of significance. These indicate that the CCA content has significant effect in the reduction of the EE and CO2 emission of the concrete.

Recommendation
Based on the results obtained from this study, CCA is recommended for use as cement replacement in mitigating EE and CO 2 emission of concrete. With regards to mechanical properties, not more than 10% CCA content is recommended as cement replacement in concrete.