Miloudi M*, Merbouh M and Glaoui B
FIMAS Laboratory, Tahri Mohammed University, Bechar, Algeria
Received Date: July 29, 2017; Accepted Date: August 12, 2017; Published Date: August 17, 2017
Citation: Miloudi M, Merbouh M, Glaoui B (2017) Microstructure and Properties of Concrete Using Coal Waste (Heap) as Partial Replacement of Fine Aggregates in Hot Weather. Innov Ener Res 6:167.
Copyright: © 2017 Miloudi M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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This research work is to examine the partial replacement of fine aggregate in concrete mistreatment coal waste. It involved the study of strength properties of the concrete with totally different proportions of coal waste as partial replacement in fine aggregate. The major problem sweet-faced by the globe nowadays is that the environmental pollution. The study consists of comparing the properties of a reference concrete with concrete incorporating aggregate of coal waste (heap) at seven levels of heap as content, fine aggregate is partially substituted with coal waste. Different ratios of partial replacement is done like 0%, 1%, 2%, 3%, 4%, 5%, and 6%, and two levels of temperature (25 and 50ºC). The simulation of the hot climate in the laboratory is subjected to temperature of 50ºC with relative humidity of about 10%, wind speed at 10 km/h and E/C ratio constant between all mixtures.
The results revealed that coal waste (heap) inclusion was more effective and can be used for enhancement of concrete properties. The optimum heap varied between 1-4%. However, the concrete cured in hot temperatures, the resistances was positively affected by the inclusion of coal waste and suggest that beyond 28 d age, the difference in strength properties between them is significant.
Hot-weather concreting; Coal waste (heap); Environment; Compressive strength; X-ray diffraction
Recent developments in concrete technology take into account the inclusions of bottom ash, coal waste, fuel ash, fly ash, etc… in the preparation of concrete. The use of this waste as a partial or total fine aggregate replacement for fine aggregate is reported in the literature [1- 7]. So this totally wastes produced by thermoelectric power plants have the ability to be used in concrete and mortar. At present in America, coal waste is mainly used for the following applications: Aggregate for concrete, Road base and sub-base, Structural fill, Backfill, asphalt and masonry, manufactured soil products.
The aim of the present investigation is to study and specify the effect of hot weather on the strength of concrete containing different percentage of granulated coal waste (heap) for partial replacement of the fine aggregate.
The results in high temperatures and low humidity as found in hot weather conditions demonstrate an improper curing of concrete, a substantial reduction in strength and an increase in permeability and cracks. A limited number of investigations have been conducted so far regarding concrete performance in hot weather conditions.
The negative effects of hot weather on the strength properties of concrete have thus been well recognized [8-10]. This is due to high air temperatures, low relative humidity, wind speed, and the intensity of solar radiation. In addition, there are different ways of treatment that can be used to improve these negative effects. The undesirable effects of hot weather on the properties of fresh concrete according to many literatures include increasing;
• Water demand for required consistency;
• Rate of slump loss;
• Velocity of setting and hardening;
• Plastic shrinkage.
The American Concrete Institute takes the view that it is impractical to recommend a maximum limiting temperature because circumstances vary widely. A satisfactory limit in one case would be highly unsatisfactory in others. The materials for concreting in hot weather should be kept as cool as possible and the measures are recommended. These measures apply to work carried out in countries with hot climates .
Bogue noticed that for the normal range of Portland cements, one half the heat of hydration is released between 1 and 3 days after casting, about three-quarters of it is released during the first 7 days after casting and about 83% to 91% is released during the first 6 months after casting. Indeed, the heat of hydration released by cement depends on its chemical composition and is approximately equal to the sum of the hydration heats of the individual pure compounds when their respective proportions by mass are hydrated separately . The compounds of tricalcium aluminate (C3A) and tricalcium silicate (C3S) are known to contribute most of the heat of hydration with C3A contributing the bulk of the heat of early age of hydration. It follows that, by reducing the proportion of C3A and C3S in Portland cement, its heat of hydration can be reduced.
As mentioned by Soroka and Ravina , the stiffening of the fresh concrete, and the associated drop loss, are mainly caused by the hydration of the cement. Some evaporation of the mixing water and, in some cases, also the absorption of water by dry aggregates may be additional causes. All of these effects reduce the amount of free water in the fresh concrete mix. As a result, the flow of the mixture is decreased, i.e. the stiffening takes place. The raise in the hydration temperature from 20ºC to, for example, 40ºC, increases, the hydration rate by a factor of 2.41 in the first hours. That is, the accelerating effect of temperature on the hydration rate of Portland cement is very important. This accelerating effect of temperature is obviously well recognized and is supported by a considerable body of experimental data. As a result, a higher temperature, due to its effect on the rate of hydration, will result in shorter set times and a higher rate of loss of sag.
Different viewpoints exist with respect to the effect of high curing temperature on the development of medium-term compression strength of concrete. Mustafa and Yusof have suggested that the medium-term compression strength of a concrete cured at high-temperature will not be adversely affected, provided that the ambient relative humidity during curing is high enough.
Ordinary concrete cured at high temperatures (above 30ºC) has been shown to possess properties of resistance lower early and medium term strength properties than ordinary concrete cured at 20ºC. This reduction is probably due to cracking caused by thermal stresses. Mustafa nevertheless suggested that because of the reduction in the macro porosity of the concrete mass caused by maturity, a concrete subject to both a high curing temperature and ambient humidity should not be adversely affected on the medium term strength properties.
It is well established that the strength of concrete a few days old is increased as the curing temperature is increased. However, the ultimate strength of concrete mixed and cured at high temperatures (i.e. above about 25ºC) is generally not as great as that of the same concrete mix produced at lower temperatures.
A 15% reduction in the 28 days strength was found during an investigation into the manufacture of high-strength concrete in a tropical climate, when the concrete was mixed at a high temperature, but cured at about 23ºC.
Environmental problems caused by the existence of heaps in the vicinity of urban areas, these heaps of coal continue to make life difficult for thousands of inhabitants. At the same time that the warnings about global warming were becoming more audible, this coal waste, which is a major emitter of greenhouse gases, becomes the living environment for citizens who no longer know which saint to devote to. Given the many nuisances of this fossil fuel: In addition to greenhouse gas emissions, this coal devastates the plantations, threatens biodiversity, silicosis the lungs, pollutes the atmosphere of cities, and disfigures the extraction landscapes.
Recently, there is an increasing demand for strength concrete in the construction industry. This investigation was an action to evaluate the addition of waste heap of coal on the strength of concrete under hot weather conditions with low relative humidity and as well wind. The data obtained in this investigation would be useful in determining the performance of waste heap concrete structures in hot weather.
Climatic data of the studied areas
The region of Béchar is located in the south west of Algeria in the Saharan desert, the climate is of continental desert type which is characterized by a very hot summer which can reach (+43ºC in the shade), which can exceed 45 or 50ºC on the ground, and a very cold winter (1ºC to 2ºC). Knowing that the hot period during the days of the year is too long (8/12 months approximately).
Cement: The cement used was CEM II/B 42.5, according to the Algerian standard NA 442 (similar to type EN197-1/ NF P15-301). Table 1 shows the chemical and physical characteristics of the cement used.
|Content (%)||Cement||Fine aggregates||Coarse aggregates||Coal heaps|
|Silicon dioxide SiO2||17.5||41.15||3.46||22.14|
|Iron (III) oxide Fe2O3||2.88||1.8||0.67||8.3|
|Calcium oxide CaO||59.98||28.64||52.29||3.44|
|Magnesium oxide MgO||1.76||1.58||0.46||2.29|
|Tricalcium Aluminate C3A||5||-||-||-|
|Tricalcium Silicate C3S||56||-||-||-|
|Initial setting time (min)||145||-||-||-|
|Final setting time (min)||250||-||-||-|
|Specific gravity (g/cm3)||3.1||2.5||2.75||2|
Table 1: Chemical and physical properties of the used materials and coal waste (heap).
Fine aggregate: The specific gravity was found to be 2.50 and fineness modulus of 2.20, classified within the range 0/3 (d/D), according to the Algerian standard NA 255 (similar to type EN1097-6/ NF P18-650-6).
Coarse aggregates: A two gravel were used, classified within the range 3/8 and 8/15 (d/D), according to the Algerian standard NA 255 (similar to type EN1097-6/ NF P18-650-6), with a specific gravity 2.50 and 2.73 respectively.
Coal waste (heap): Use in current study, so that has been collected from the settling of the Kenadsa olden Coal mine, in Bechar, southern-west Alger. The chemical and physical characteristics are given in Table 1. The W/C ratio for all mixtures considered in this study was 0.51. The details of mixing with and without of coal (heap) are shown in Table 2.
|Concrete||Mix proportion (kg/m3)|
|Cement||15 mm Coarse aggregate||8 mm Coarse aggregate||Fine aggregate||Coal waste (heap)||Water|
Table 2: Mix details of coal waste (heap) concrete.
The chemical composition of the coal waste was analyzed by X-ray Energy Dispersive Spectrometry (EDS). The calcium content is very low 03.44% relative to the fine aggregate and the sum of SiO2+Al2O3+Fe2O3 reaches 43.62%. It was found to contain 2.29% MgO. The total content of CaO+MgO is very low 05.63%.
Casting and curing
To simulate a setting and curing in hot weather in the laboratory, a climatic chamber of 1800 × 1000 × 800 mm3 was used. This enclosure is equipped with electric heaters, an electric fan and a thermo-hygrometer to regulate and monitor indoor environmental conditions. The temperature was maintained at 50ºC for 24 hrs with Relative Humidity (RH) of about 10% and wind speed at 10 km/h.
An extensive program of tests has been prepared to study the effect of these concreting conditions in hot weather on the behavior of the various concretes, the testimony of ordinary concrete and concretes with coal waste (Specimens of 100 × 100 × 100 mm3).
All batches of concrete were cast inside the laboratory. Batch 0 was prepared using the mix proportions shown in Table 2 and contained ordinary concrete without additions of coal waste (heap); Batch 1 to 6 was prepared using the same mix proportions except that used to substitute the volume of fine aggregates by the coal waste. Each sample was cast in 3 layers in a steel mold, each layer being compacted by external vibration.
Batch 0 was used to cast 36 concrete cubes that were tested for compressive and the same number for each batch from 1 to 6. The number of specimens was doubled for the 7 batches, with the aim of comparing the severe hot climate 50ºC with the normal climate 25ºC. After demolding, the specimens were subjected at 20 ± 2ºC under water to enable us to compare the compressive strength at 2, 7, 14, 28, 90 and 365 days.
Figure 1 shows the results of compressive strengths obtained for concrete wastes of coal and test concretes tested at 2, 7, 14, 28, 90 and 365 days.
All the concrete cubes of the coal waste concretes and reference concrete tested at 2 days and which had been cured at 50ºC had a higher initial compressive strength than those cured during the same period of time at the standard curing temperature of 25ºC; which is justified by the acceleration of hydration reactions by raising the temperature of the middle environment, the concrete gains more hydrates which contribute to the improvement of its compactness and its compressive strength.
Concrete (0- 6%) of hot climate, gains between 44% and 46% of their strength at 2 days. It also makes clear in curing temperature at 50ºC, the concretes a gain of resistance to 7 days compared to concrete with a standard curing temperature of 25ºC, this gain is equal to between 3% and 8%. Consequently, the results obtained indicate that, in the influence plus of high temperature curing, the compressive strength of the cubes of the waste coal concrete high than that of the reference concrete.
It was observed that the mixes with replaced fine aggregates had elevated difference from the reference concrete as age increased to 365 days; the difference between reference concrete and coal waste mixes left between 1% and 6% for all mixes (except 5% and 6% coal waste mix which has showed higher decrease at all ages, but that also has decreased with increase in age).
Thus, not much difference in strength was observed from 0% to 4% replacement of fine aggregate with equal percentages of coal waste. Also, the strength observed at 4% was highest as compared to the other coal waste mixes, but more than that of reference concrete mix. The maximum strength was obtained at replacement of 4% coal waste in the replaced mixes, which can be adjudged as optimum mix.
Compressive strength of all coal waste mixes increased with age. At 2 days, all coal waste mixes also showed the strength superior than reference concrete mix but as the age increases to 365 days gradually attained strengths marginally high than the reference concrete in curing temperature at 25ºC and 50ºC as shown in Figure 1.
The difference in the strengths of various mixes from reference concrete mix was observed to increase for various coal waste mixes as given in Figure 1 with increase in age from 2 to 365 days. The reference concrete mix gained increase in strength from 2 to 7 days of 80.45%, from 7 to 14 days of 37.08% from 14 to 28 days of 43.73%, from 28 to 90 days of 13.23%, and from 90 to 365 days of 9.50% which is characteristic of normal concrete. The coal waste mixes attained a relatively constant strength of 71–73% from 2 to 7 days, 38–41% from 7 to 14 days, 46– 65% from 14 to 28 days, 11–12% from 28 to 90 days, and 10-13% from 90 to 365 days.
The rate of gain of the early compression strength of the coal waste was always higher than that of the corresponding reference concrete cubes. These results indicate that in the case of high temperature curing the partial replacement of fin aggregate with coal waste generally improve the initial compressive strength and its rate of gain of the resulting concrete.
When cured at temperatures of 50ºC, coal waste concretes and reference concrete did not reach the 28 day compressive strength at the standard curing temperature of 25ºC which gives a lower of equal to 28 days between 10% and 20%. According to these results, a high temperature curing that causes an increase in water evaporation, this water evaporated before the completion of the cement hydration, resulting in a decrease in strength.
The influence of concrete mix temperature on the concrete was studied by Abbasi et al.  Test specimens were prepared and cured in hot weather at various concrete mix temperatures. They concluded that when the concrete mix temperature reached 45ºC, even with proper precautions for hot weather effects, the strength of the specimens could be reduced by as much as 25%.
From the observations, it is inferred that 1%, 2%, 3% and 4% coal waste (heap) specimens have attained the positive effect and given the values strength to the reference concrete. The addition of coal waste (heap) to more than 4% reduces the compressive strength value since the waste of coal (heap) imparts brittle to the concrete characteristics.
Alsayed and Amjad said on the strength of normal concrete support the idea that in severe hot climates, intermittent water spraying with and without burlap cover is an acceptable way for concrete to harden. The compressive strength of the cured specimens by other means has been reduced by more than 14%, and therefore may not be recommended for hardening of concrete in a hot and dry climate. Reduction of the compressive strength of cured specimens in the field can be attributed to the deleterious effects of adverse climatic conditions .
X-ray Diffraction (XRD)
XRD technique was conducted to analyze the components of concrete mixes and the results are shown in Figures 2-5. The X-ray diffraction pattern and analysis of the concrete mixes i.e. reference concrete mix, and 4% coal waste (heap) mixes was carried out at age of 28 days.
These diffractograms are presented in the form of more or less pronounced peaks spread on the goniometric positions (0º to 70º) of the pulse counter X in a geometrical configuration (θ, 2θ) according to the Bragg diffraction conditions. In all the mixes, CaCO3, SiO2, CaMg(CO3)2 and Ca(OH2) peaks are visible indicating that they may be totally consumed or overlapping of the peaks of unhydrated cement by that of SiO2 may have occurred as all analyzed mixes were concrete specimens with large number of aggregate particles containing quartz which resulted in intensive SiO2 peaks. Hence, as shown, SiO2 peak indicating free silica, in reference concrete was observed at superior 10000 with a pronounced peak of higher intensity located at 2θ=26.6º in cured at temperatures of 50ºC and at the standard curing temperature of 25ºC.
The X-ray diffraction pattern observed in coal waste mix was similar to reference concrete mix as the overall replacement of the fin aggregate was only 4%. The 4% coal waste (heap) mix showed SiO2 peak between 4000 and 4500 in cured at temperatures of 50ºC and at the standard curing temperature of 25ºC.
For these specimens cured at temperatures of 50ºC; and similarly for concretes curing at temperatures of 25ºC., the disappearance of larnite and hartuite in the concrete of 4% coal waste is due to the response of the reaction of SO3 with high sulfur content by chemical analysis of the coal waste (heap).
As our concretes contain a large fraction of aggregates (68%) by mass relative to the cement fraction which is equal to 13%, we mainly observe the peaks of the calcite resulting from the aggregates.
The following conclusions could be arrived at from the study about performance of the concretes under hot weather conditions: In general, the coal waste (heap) replacement was more effective in the concrete with high strength than the reference concrete.
The maximum 28 day compressive strength was obtained for concrete with optimum coal waste of 4% at temperature of 25ºC and 50ºC, also coal waste (heap) concrete exhibited optimum 90 and 365 day strength at 4% coal waste level.
Generally, for hot weather concreting, the optimum coal waste replacement for concrete appears to vary in the range of 1-4% was acceptable compared of reference concrete.
The mechanical behavior of the concrete with coal waste showed strengths comparable to that of reference concrete except for 5 and 6% mix, at the age of 365 days. Furthermore, it was observed that the greatest increase in compressive was achieved by substituting 4% of the fine aggregate with coal waste in replaced mixes at curing temperature of 25ºC. Also, the maximum replacement could be taken as 4% at curing temperature of 50ºC.
The inclusion of coal waste as fine aggregate does not affect the strength properties negatively as the strength remains within limits. The concrete was endowed with comparable mechanical properties and greater resistance to aggressive agents (chemical, physical and environmental).
The possibility of substituting natural fine aggregate with industrial by-product aggregate such as coal waste offers technical, economic and environmental advantages which are of great importance in the present context of sustainability in the construction sector.