EFFECTS OF THREE TYPES OF IRON AND STEEL SLAG ON FRESH AND HARDENED PROPERTIES OF ORDINARY PORTLAND CEMENT
Seyed Vahid Hosseini1, Shahnavaz Eilbeigi2, Mohammad Reza Nilforoushan1
1Faculty of Materials Science and Engineering, Shahrekord University
Rahbar Boulevard, Shahrekord, Chaharmahal Bakhtiari Province, Post Code: 8815915655, Iran
2MSc. Mechanical Engineering, University of Texas at Arlington (UTA) 720 N cascades, Fort Worth, TX, Post Code: 76137, USA Keywords: BOF-slag, EAF-slag, GBF-slag, Cement, Slurry, Paste, Mortar
Abstract
Slag is a by-product of different metal extraction and refining processes. This paper reports the results of an experimental study on replacing part of ordinary Portland cement (OPC) with iron and steel slags. In Iranian iron and steel plants, slags are generated at three different stages of processing; Basic Oxygen Furnace (BOF), Electrical Arc Furnace (EAF) and Granulated Blast Furnace (GBF) are the slags used in this study. In this respect, some mixtures were made with 10%, 20% and 30 wt% of each slag replaced in ordinary Portland cement. For this study, the effects of slag additions in cement was determined by measuring of electrical conductivity and pH of slurry, setting time of paste and mechanical strength of mortar at various ages. According to the results, 10% EAF or BOF slag can safely replace part of OPC in mortars.
Introduction
Recently, utilization of waste materials in other industries has been attracted by researchers.
However, most industrial slags are being used without taking full advantage of their properties or disposed rather than used. In Iranian iron and steel plants, slags are generated at three different stages of processing and in view of that categorized as:
i. Granulated blast-furnace slag (GBFS) is a glassy granular material of the manufacture of pig iron from iron ore, limestone and coke. The liquid slag is rapidly cooled by quenching to get an almost completely amorphous material. Up to now, the use of GBFS in cement and concrete technology has been extensively discussed [1-3].
ii. Electric arc furnace slag (EAFS) is essentially based on addition of burned lime, dolomite [Ca.Mg(CO3)2] and etc. together with steel scraps in electrical arc furnace under high voltages. After separating from molten metal, the liquid slag cooled by a combination of spraying water and air. The water quenched electric arc furnace slag has a physical appearance which is partly granular and partly flaky. In the type of air cooled, high Fe-oxide content coupled with the highly crystalline nature of the slag are proposed to be the reasons for its chemical inactivity during the process of hydration in the presence of clinker or lime. Hence the EAFS is used mainly as aggregates for landfills and roads [4-6].
iii. Basic oxygen furnace slag (BOFS) produced in the process of conversion pig iron to steel in a basic oxygen furnace. The principle of the basic oxygen furnace is to blow oxygen and neutral gas into the furnace to decrease carbon contents. Moreover,
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
during the conversion of pig iron, lime and dolomite are added in the converter. Lime is used to fix the silicon and phosphorous contained in pig iron and dolomite is added to protect the refractory brick. At the end of the conversion, BOF slag is separated from steel because of their different specific gravities and then cooled slowly in the air with water spraying [7-9].
There are some reasons for trying to apply the utilization of slags in the manufacturing of cement or, alternatively, as cement replacement materials in concrete. One good reason is the decrease of a significant amount of slag being sent to landfill each year. Other reasons are the potential for reducing energy consumption and carbon dioxide emissions within the cement industry, and to save natural resources [1, 8-9].
In addition to the above reasons, replacement of clinker by slags had another advantages such as low heat of hydration, high sulfate and acid resistance, better workability, and good ultimate strength and durability [10]. There are two different approaches for the incorporation of iron and steel slags in cement production. The first one involves the use of slag, mixed with limestone and clay, as raw material feed to the cement kiln [2]. This may be a solution to the disposal problem but there is not any energy benefit, (because of the slag must be clinkered) or economic benefit (one low-cost material is substituted for another). A more attractive approach is the utilization of iron and steel slags in the production of special cements [8, 11-15].
In this study, we introduce experimental results: mechanical properties of hardened cement pastes and morphology of product obtained by X-ray diffraction, Scanning electron microscopy under different conditions. In order to have a clear description of the amount of slag that can safely replaces OPC in mortar by improving its properties.
Materials and methods
Raw materials
The slags are produced by Esfahan Steel Company. One kilogram of each batch was taken for experiments randomly and mixed completely. The mixed Batches were dried, crashed and milled by various mills such as ball and fast mills, in order to make a powder which pass the #200 sieve ( <75 μm). The chemical composition of slags and OPC used in this study was determined by X- ray fluorescence and the results are listed in Table 1. X-ray diffraction patterns of raw materials are shown in Fig. 1.
Characterization and testing procedures
The mineralogical structure of materials was analyzed by X-Ray diffraction. The apparatus was Bruker D8 Advance with nickel-filtered CuKα1 radiation (λ=1.5406 Å). The effects of slag additions on the rheological properties of slurries were investigated by the measurement of pouring time of slurry through a 5 mm orifice in the standard funnel. The values of electrical conductivity and pH of Slurries were measured in situ using Consort C933 multi-parameters portable.
The setting times of the pastes were determined by the Vicat apparatus. A needle of a known weight and area was used in this method [16, 17]. Two distinct stages of setting were recorded in the laboratory for pastes: the initial set (time of commencement of the setting) and the end, or final, set. The mechanical strength measurements of mortars were performed on a Baldwin
machine in a load control regime with a loading rate of 2 MPa/min. Three specimens were made and tested for each data point. The specimens were tested at 3, 7, 28 and 90 days after casting.
Scanning electron microscopy (SEM) observation were done by Leo 435-VP, operating at EHT=20 kV.
The slags were replaced by part of cement based on the design shown in table 2. For each blended of slag and cement, dry powder was mixed in the ball mill for 10 minutes to get a uniform composition.
Table I. Chemical composition of raw materials
Mineral OPC BOFS EAFS GBFS
SiO2 21.7 10.2 13.9 36.4
CaO 63.5 56.1 49.1 38.0
Al2O3 5.9 2.1 2.3 8.3
MgO 1.8 1.5 7.7 9.5
Fe (total) 3.1 20.5 17.1 1.9
MnO - 2.6 6.3 1.0
V2O5 - 2.2 2.1 0.1
P2O5 - 1.2 0.5 -
TiO2 0.6 2.1 - 4.3
K2O 0.7 0.4 - 0.3
L.O.I 2.7 1.1 1.0 0.2
Specific gravity (g/cm3) 3.15 3.47 3.28 2.74
Figure 1. X-ray diffraction patterns of materials
Table II. Mix proportions of blends (wt %)
Mixture name OPC
(%)
BOFS (%)
EAFS (%)
GBFS (%)
Ref 100 - - -
BOF10 90 10 - -
BOF20 80 20 - -
BOF30 70 30 - -
EAF10 90 - 10 -
EAF20 80 - 20 -
EAF30 70 - 30 -
GBF10 90 - - 10
GBF20 80 - - 20
GBF30 70 - - 30
Results and Discussion
Slurry
Rheological properties
The batch of slurry was considered 50 grams powder together with water to cementitious materials ratio (w/cm) equal to 0.5. The blend was mixed in a rotary mixer for 5 minutes in order to get uniform slurry. Based on these results (Table 3), by increasing amounts of BOFS, the pouring time increased while GBFS and EAFS acted in the reverse manner. This may be due to the higher basic composition of BOFS which release Ca ions right after mixing with water, compared to the other slags. It has been mentioned as follows by other researchers, when cement mixed with water, due to hydration reactions, calcium (Ca) and hydroxyl (OH) ions go into solution within the first 10 min. The degree of hydration describes the process of hydration, and directly is related to the fraction of the hydration products or porous structure in a hydration system in cement based materials [18-20].
pH
The pH of the slurry plays an important role in the hydration process and in determining the nature of C-S-H. It was reported that C-S-H does not form in a solution with a pH below 9.5 [16]. In order to have a rough estimation of slag hydration, the pH of slurries made with different slag additions were measured at 10 and 60 minutes of hydration, respectively. The results are shown in table 3. According to the results, the alkalinity of all slags is closely the same.
Electrical-Conductivity
Electrical conductivity process occurs mainly due to ion transport through the pore solution in a cement-based system and it is an important parameter to study the hydration process of cement pastes at early stages. The variation of conductivity as a function of time can indeed reflect on internal changes of the pore solution of cement paste with time. On the other hand, electrical conductivity of cement slurries decreased with time [21]. The conductivity of various mixtures of slag and OPC were measured. Based on the results, by replacement of slag as part of OPC, the electrical conductivity of slurries decreased, this may be the case for increase in the setting time
of the pastes. Results of electrical conductivity of slurries are shown in table 3. Due to specimen of Ref at time of 60 min had been set, their values of pH and EC could not be measurable.
Table III. Values of measured pouring time (Sec), pH and electrical conductivity (mS/c) of slurries
Paste and mortar
Setting behavior of the pastes
The initial and final setting time of the pastes was evaluated and the results are graphically presented in figures (2, 3 & 4). Based on the results, the initial and final setting time of blended cement paste both increased which was depended on the amount and kind of slag used in the blended cement. This may initially be due to the reduction in the amount of C3A but finally due to the reduction in the Ca ion releasing from the cement part of the pastes which effects on the crystals growth in the during final setting of the pastes.
Figure 2. Setting time of blended cement pastes with BOFS
Figure 3. Setting time of blended cement pastes with EAFS
Mixture Name
Pouring time (Sec)
pH EC*
10 min 60 min 10 min 60 min
Ref 7.15 12.97 - 10.02 -
BOF10 8.29 12.71 12.90 9.06 9.54
BOF20 9.03 12.20 12.92 8.21 9.01
BOF30 11.67 12.28 12.96 7.65 8.60
EAF10 5.82 12.79 12.80 9.96 10.02
EAF20 5.07 12.77 12.75 9.33 9.32
EAF30 4.86 12.78 12.64 8.90 8.93
GBF10 6.78 12.74 12.85 9.29 9.72
GBF20 6.33 12.72 12.80 8.56 8.86
GBF30 5.97 12.68 12.76 7.93 7.80
*Electro-conductivity (mS/c)
Figure 4. Setting time of blended cement pastes with GBFS
Mechanical properties of mortars
Both flexural and compressive strength of mortars was measured as per Iranian standard ISIRI393 [22]. Mortars prisms (Dimensions 40×40×160 mm) and cubes (Dimensions 50×50×50 mm) were made with cement, sand and water (1:3:0.5) for flexural and compressive strength, respectively, which were first cured in accordance with the standard until ages 3, 7, 28 and 90 days. According to the results, additions of slag at early ages have negative effects on the mechanical properties of blended cements but it recovers at longer times. Based on these results EAFS and BOFS may safely replaces up to 15% of OPC but GBFS decreases mechanical properties of cement at early ages and it does not recovers even up to 90 days of hydration.
Results are shown in Table 4.
Table IV. Mechanical strengths of mortars made containing slags at different ages of curing
Sample Name
Mechanical Strength (MPa)
3 days 7 days 28 days 90 days
Flexural Compressive Flexural Compressive Flexural Compressive Flexural Compressive
Ref 2.7 21.28 2.9 36.91 4.0 43.29 5.4 51.20
BOF10 2.5 18.03 2.8 30.22 3.5 40.70 4.9 47.56
BOF20 2.4 14.92 2.7 21.54 3.2 37.91 4.7 44.95
BOF30 2.1 11.63 2.4 18.00 3.0 34.32 3.9 40.37
EAF10 2.4 19.54 2.7 31.97 3.8 41.22 5.2 50.21
EAF20 1.9 16.93 2.4 28.03 3.4 39.75 4.4 48.91
EAF30 2.1 12.73 2.3 22.80 3.1 36.44 3.7 46.29
GBF10 2.4 20.01 2.9 34.46 3.6 42.82 4.9 51.32
GBF20 2.2 17.22 2.6 30.50 3.2 39.63 3.9 49.08
GBF30 1.8 14.98 2.2 20.19 2.6 34.22 2.9 41.76
Microstructural investigations of the hardened cement pastes
Microstructure of cement pastes after 7 and 90 days of hydration was investigated by scanning electron microscopy (SEM) to have a clear understanding of the effect of various slags on the hydration products of hardened cement pastes. The electron micrographs are shown in figures 5- 8. Microstructure of OPC shows a dense texture composed of tobermorite (CSH) and plates of Ca(OH)2 . By replacement of 30% BOFS with cement (Fig. 6), the paste becomes porous due to the formation of lumps of Ca(OH)2 after 7 days which turns to plates after 90 days and needle like crystals of ettringite [Ca6Al2(SO4)3(OH)12·26H2O]. Fig. 6 compared to Fig. 7, reveals due to the presence of less lime in the composition of EAFS, the needle like crystals of ettringite and platy crystals of calcium hydroxide are well formed after 7 days but at 90 days, there is not much difference between hydration products of BOFS and EAFS. By looking at Fig. 8 from the hardened paste of GBFS, it seems that needle crystals of ettringite has formed after 7 day of hydration but, after 90 days, the crystals in the paste becomes lumpy covered with ill platy crystal of tobermorite [Ca5Si6O16(OH)2]. When the results of micro-structural studies are compared with mechanical strength in table 4, the pore mechanical strength of GBFS is interpretable. Due to the micro cracks and bigger crystal size that leads to less mechanical strength.
Figure 5. SEM Images of reference sample at: a) 7 & b) 90 (days)
Figure 6. SEM Images of BOF30 sample at: a) 7 & b) 90 (days)
Figure 7. SEM Images of EAF30 sample at: a) 7 & b) 90 (days)
Figure 8. SEM Images of GBF30 sample at: a) 7 & b) 90 (days)
Hydration products
For study of the hydration products, pieces of pastes with ages of 3, 7, 28 and 90 days were left in acetone and ether solution in order to stop their hydration. In order to have a clear understanding of the hydrated phases which formed during hydration of cement and slag the X- Ray diffraction pattern of the pastes after 28 days was taken. It seems from the pattern that shown in fig. 9, the pastes with 30% of EAFS or BOFS has stronger peaks of Ca(OH)2 and tobermorite compared to GBFS. These phases are the most important phases that contribute to the mechanical properties of the paste. Besides, Ca6Fe2(SO4)3·32H2O, FeO and FeO(OH) and Ca6Fe2(SO4)3·32H2O are also identified in samples.
Figure 9. X-ray diffraction of reference and blended cements, hydrated at 28 days Conclusions
In this study, the effects of additions various types of iron and steel slags on fresh and hardened properties of OPC were investigated by measuring rheological properties, pH, electrical conductivity of slurries and Setting behavior, Mechanical properties of the pastes and mortars.
The following conclusions may be drawn from the obtained experimental data:
X-ray diffraction showed that the main minerals present in BOF slag mostly like the one in OPC. The activity of BOF slag in cement-based mortars was evaluated and the results show that BOF slag has a poor hydraulic activity at younger ages, but it becomes
activated after 7 days of hydration. The compressive strength of the blended pastes was similar to OPC at 90 days. This means that it has a good hydraulic activity and can safely replace part of OPC in blended concretes.
EAFS has similar behavior to BOFS due to its similarity to their chemical composition but additions of GBFS does not improve the properties of mortars which may be due to its lower amounts of lime in its chemical composition.
The pH of the mixing solution is expected to have a significant effect on the nature of C-S-H by affecting the chemistry of the pore solutions. A high pH in the pore fluids may help the formation of C-S-H of a low C/S ratio and/or a different physical distribution because of the solubility of silica.
As it is a well-known fact, the hydration process in cement paste results in the formation of C–S–H, calcium CH, ettringite and other compounds. During hydration, the capillary pores in hardening cement paste are gradually filled up with hydration products and the solid phases form a rigid microstructure with increasing strength.
SEM observations showed microstructure of pastes become more compact with passing time from 7 to 90 days for all of samples. Moreover, in reference sample we saw there are some crystalline phases greater than cements containing slag.
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