ACHIEVEMENT THE CLASS OF CONCRETE C 50/60 , REPLACING THE CEMENT CONTENT USING THE WASTE MATERIALS FROM TEC
1Kabashi, N.,1Krasniqi.C., 1Dautaj,A.
1Department of Civil Engineering, University of Prishtina, str. Bregu I Diellit, nn, 10000 Pristina, Kosova
ABSTRACT
According the development of technology in recent years a variety of “clean” technologies for coal combustion have been developed. In this study we analyze the possibilities of using the waste materials produced during a power plant process in Kosova, in particular such a mineralogical admixture for cement and for concrete. ‘During the last few decades, these “waste” materials have seen a transformation to the status of “by-products” and more recently “products” that are sought for construction and other applications. The study includes basic chemical analyses of materials ranging from fillers to fly ashes. It also compares the properties of performance of concrete in function of replacement of cement with fly ash. The results are based on experiment that aimed at reducing the amount of waste materials, and evaluation the properties of various concretes, including their fresh and hardening properties according to the EN 206-1 standard. The environmental impact of fly ash progressively worsening as it require since reasing field storage. In experiments, depend of replacement of cement with waste materials and we try to find the optimum of waste materials to obtain the improvement concrete properties in civil engineering works.
Keywords: Chemical analyses, fly ash, mineral admixture, properties of concrete
1. INTRODUCTION
Coal is a complex, heterogeneous material, in widespread use as an energy source throughout the world. When pulverized coal is burnet to generate electrical power, extremely large quantities of fly ash and bottom ash are produced. Burning of coal takes place at high temperature whereat waste material is obtained. The coal is first milled to obtain fine powder, increasing thus the surface area and allowing it to burn more quickly. In these pulverized coal combustion (PCC) systems, the powdered coal is blown into combustion chamber of a boiler where it is burnt a temperature 1200 - 1700 0C.
Coal ash is widely available in Europe, because according to Eurostat data for year 2009 around 30% of all the power generated in the EU27 is still coal-based which results in gross inland consumption of 292 000000 tones of coal. Utilization of this material has several environmental and technical benefits. It has developed by the years and is based on requirements of standards or other specifications. Still the European Standards EN 450-1 and EN 450-2 for fly ash in concrete are under revision; as a result it is necessary to have more research works in this field for different types of fly ash and in different possibilities of utilization of it. Coals compose products are: fly ash, bottom ash, boiler slag, FGD Gypsum, SDA Product, FBC ash.
The use of coal ashes must be conducted smoothly with an environmentally friendly technology. Using a fly ash it is important to consider its influence on the environment. A simple, economical, and beneficial method of using fly ash is also needed to have less environmental impact. Until now, based on experience of industrial development countries the main aim of using fly ash is cement industry such a cementius material and in this research is using such an additional material in production of concrete.
In European Union only from 15 members during 2004 the production was about 64 million tons of different ash, presented in percentage, in fig.1
Fig 1. Production of coal fly ash in EU[9]
The utilization of fly ash in construction industry from 15 members of European Union, during 2004, in total utilization was 22 million tones, for different types of works in percent, is presented in fig.2
Fig 2.Utilization of coal fly ash in EU
2. MATERIALS
In this case fly ashes used in the experiment were discharged from power plant in Obiliq, near to Prishtina. Fly ash was taken right after technological process of combustion of coal in power plant (not from the deposited field).
Chemical and physical analyses of coal fly ash are shown on table 1 and table 2 below. Analyses are done by licensed Institute IMS AD based on European and local Standards and test methods: EN 196 – 2; EN 196 – 5; EN 196 – 6
2.2. Chemical analysis
Table 1.Chemical analyses of coal fly ash components
Loss on ignition % / 2.30SiO2 % / 19.28
Al2O3 % / 5.67
Fe2O3 % / 4.85
CaO % / 42.92
MgO % / 4.31
SO3 % / 19.41
S % / 0.00
Na2O % / 0.58
K2O % / 0.36
MnO % / 0.09
Cl % / 0.00
Moisture 45C % / 0.29
Water 45C % / 0.00
CO2 % / 0.70
Insoluble residue in HCl/Na2CO3 % / 11.05
Insoluble residue in HCl/KOH % / 6.58
Total: / 99.75
Reactive CaO % / 28.44
Reactive SiO2 % / 15.69
2.3. Physical – Mechanical analysis
Table 2- Physical mechanical properties of fly ash
Fineness:*0.200 / 0.1
*0.090 / 4.9
*0.063 / 8.1
*0.043 / 12.5
Density [gr/cm3] / 2.83
Specific Surface [cm2/gr] / 6600
Puzzolanity : MPa / 0.09
Flexural strength / 4.5±0.3
Compressive Strength / 21.1±0.7
Strength after 28 days [MPa] / 0.00
Flexural strength / 3.7±0.2
Compressive Strength / 15.0±0.7
· Table 3.Chemical requirements for Fly Ash according the EN 450-1
Loss on ignition / chloride / Sulfuric anhydride / Free calcium oxide / Reactive calcium oxide / Silicon dioxideSiO2;Al2O3;Fe2O3
(2-7)%
Cat B / ≤0.10% / ≤3.0% / ≤2.5% / ≤10.0% / ≥70%(sum)
Cat C
ü / ü / − / ü / − / −
· Table 4.Physical requirements of Fly Ash
Fineness EN 452-1 / Activity index / Soundness / Initial setting time35 %≤ 40 % / 78%≥75 % / ≤ 10 mm / (W/C)*=0.30
(W/C)=0.35 / 3h 15 min
4h 25 min
ü / ü / ü / ü
· Table 5. Compressive strength (2 days &28 days) N/mm2
CEM II 42.5 / CEM II 42.5 +Fly ash17.5/51.5 / 13.25/40.37 / 78.4 %
ü
Based on the chemical and physical-mechanical properties of samples from Kosovo fly ash and chemical requirements of fly ash for using in concrete according to European Standards EN 450 – 1 the conclusion is that the first step is completed in more requested parameters, which serves as a proof to continue further for next phase, to complete the data.
2.4 Fly ash in concrete
Fly ash is pozzolan - pozzolan is a siliceous or alumino siliceous material that, in finely divided form and in the presence of moisture, chemically reacts with the calcium hydroxide released by the hydration of Portland Cement to form additional calcium silicate hydrate and other cementitious compounds. In light of this fact, this study is intended to develop using industrial waste material to produce concrete.
The hydration reactions are similar to the reactions occurring during the hydration of Portland Cement. Thus, concrete containing Fly Ash pozzolan becomes denser, stronger and generally more durable in long term as compared to straight Portland cement concrete mixtures. Quality of concrete will be in focus, without any compromise about it.
Adding fly ash to concrete is focused on following aspects:
o Reduces costs
o Conserves resources and reduces waste
o Reduces greenhouse gas emissions
o Reduces the drying shrinkage of concrete
o Promotes concrete workability
2.4.1Fresh Concrete
Based on lot of research papers, using coal fly ash to produce the concrete, either by replacing a part of cement or by replacing a part of fine aggregate, the proportion of concrete ingredients will change, specially the changes has been in water / cement ration (w/c), which is related by expression 1:
W / (C+F) = W/C (1 – F / C + F) (1)
This means that when the water to binder ratio is the same, the concrete with more fly ash has higher water to cement ratio. Thus leads we at the fact that as much as percentage of fly ash is increased as concrete component as much water percentage should increase too. Fly ash particles provide nucleation points for the formation of hydration products and hydration process, and in this case to create the lower hydration of heat.
Despite this, during preparing this experiment it was not necessary to increase of water content, even when percentage of fly ash in mixing mass was increased the water content was same.
2.4.2. Hardened Concrete
The reactivity of fly ash develops slowly due the large particle size and the stability of glass particles. In this case of research the strength of concrete even after 7 days is generally higher, which fact was not expected, due to the replacement of cement with fly ash.
The chemical composition of fly ash may provide some differences in this aspect; example dissolution of alumina and sulfate containing compounds from the ash may lead to formation of ettringnite at early ages.
The benefit of using the fly ash is the control of the heat hydration of concrete. Also from point of view of environmental conditions the using of fly ash is representing in durability of concrete through the main parameters of durability. The durability of concrete is often associated with the porosity and transport properties of concrete.
3. EXPERIMENTAL WORK
3.1. Optimization of using the fly ash in class of concrete C 50/60
Using of fly ash in concrete in this case means analyzing 8 different approved mixing designs by cement replacement with fly ash in different percent of mass.
3.2. Preparing works
Concrete with eight different contents of fly ash and with same type of cement were prepared in concrete laboratory.
There have been prepared 120 samples. Eight series of 15 cubic samples with ribs 15x15x15cm length. In eight series there have been applied the different percentage of fly ash, starting from 10% until 50%, included also the control mix to serve for comparing the compressive strength in different ages of concrete samples after 3, 7, 21, 28 and 56 days.
In table 5 are presented the set of Mix design using in this paper:
Table5. Eight different mix design for producing concrete.
The concrete for the compressive strength tests was prepared and tested according the EN 206. The plastic and metal molds are used during the preparation of the concrete samples. A small concrete mixer was used during the preparation of concrete samples, and after that the concrete samples were cured in moist environment with 99 % humidity, and tested in different ages.
The ratio between the cement and Fly ash in different Mix Designs is presented in following fig 3.
|Fig. 3- proportion between the Cement and Fly Ash
From fig 3 it is shown that in linear way has change the proportion of fly ash and cement in all different mixing designs, research was carried on until this two lines are touched, it means until replacement of cement with fly ash achieve 50% of cement content.
In lab during the time of producing samples there have been done some tests for examination of fresh concrete properties. Slump test, one of most important examination to know in advance some properties of different mix designed of concrete. In fig 4, below, it is shown measurement of slump for control and 7 different mix designed concrete.
Fig 4. Measurement of slump for control and 7 different mix designs of concrete.
Air entrainment is a necessary component of concrete mixtures, especially when concrete is exposed to freezing and thawing environments. Due to changing materials, conditions of mixing, and methods of placing concrete, achieving target air contents requires attention at the design, specification, and construction stages. So, as next important test for fresh concrete in the lab have been done the measurement of quantity of air absorbed, and the results are shown in figure 5.
Fig 5. Percentage of air absorption in fresh concrete for control and 7 different mix designs.
Laboratory investigations is planned to achieve a high strength of concrete of C 50/60 by part replacement of supplementary cementitious materials like Fly ash and additive by varying percentage with cement, and to estimate the optimal percentage of contents combination of supplementary cementitious materials. Experiment was carried out to achieve high strength and durable concrete by minimizing the quantity of cement.
4. RESULTS AND DISCUSSION
During the analyses of fresh concrete, increase the replacement of cement with fly ash even with 50%, results were with good workability, and the concrete was with good workability cohesion forces between the particles, so the mass was sticky and flow. By increasing the quantity of fly ash in content, workability of concrete will improve. The compressive strength of concrete is one of the most important parameters required for concrete. This series of test determines the strength attained by concrete whose cement quantities have been replaced with fly ash from 0% to 50 %. The behavior of the different concrete samples in different ages are tested and presented in following figure6.
Fig 6. Compressive strength against Fly Ash Percentage
It is easy finding out the optimal percentage of coal fly ash in concrete based on this laboratory research by adding until 50% of fly ash in concrete and then to analyze the optimal results.