J. Cent. South Univ. Technol. (2009) 16: 0768-0773
DOI: 10.1007/s11771-009-0128-x
Recycling of industrial waste and performance of
steel slag green concrete
LI Yun-feng(李云峰)1, 2, YAO Yan(姚 燕)2, WANG Ling(王 玲)2
(1. College of Civil Engineering and Architecture, Shandong University of Science and Technology,
Qingdao 266510, China;
2. China Building Materials Academy, Beijing 100024, China)
Abstract: Workability and mechanical properties of steel slag green concrete with different types of steel slag and different dosages of admixtures were investigated. The effectiveness of steel slag powder on suppressing alkali aggregate reaction (AAR) expansion was assessed using the method of ASTM C441 and accelerated test method. Experimental results show that mechanical properties can be improved further due to the synergistic effect and mutual activation when compound mineral admixtures with steel slag powder and blast-furnace slag powder are mixed into concrete. In addition, about 50% decrease in expansion rate of mortar bars with mineral admixtures can be achieved in AAR tests. Mineral admixtures with steel slag powder as partial replacement for Portland cement in concrete is an effective means for controlling expansion due to AAR.
Key words: concrete; steel slag; mechanical properties; durability; alkali aggregate reaction
1 Introduction
Steel slag is a solid waste discharged in large quantities by the iron and steel industry in China. The recycling of steel slag will inevitably become an important measure for the environment protection and therefore can lead to great social significance [1]. The manufacturing of Portland cement is a highly energy-intensive process. One of the most significant activities stressed by the engineers and scientists related to concrete industry aims at a high percentage of replacement of clinker in cement with secondary raw materials, with the possibility of improvement of cement characteristics and durability of concrete. By means of these approached, the consumption of natural raw materials, thermal and electric energy, together with the reduction of CO2 emissions can be decreased. The less energy intensive materials being sought are industrial by-products that are easily available, require little or no pyro-processing and have inherent or latent cementitious properties. Such industrial by-products are commonly called as supplementary cementitious materials or mineral additives. Mineral additives are usually available in large quantities and can be used to replace Portland cement in green concrete [2-3], which include fly ash [4], blast-furnace slag [5-7], steel slag [8], silica fume, natural pozzolans, rice-husk ash, and metakaolin [9-10]. These mineral additives are added in the cement as extra binder materials, and the benefits of using these materials in terms of workability are well established. The studies have been performed to determine mechanical properties, thermal properties, transport mechanisms and the influence of mineral additions on the durability of blended concrete.
Steel slag includes a certain scale mineral such as C2S and C3S, and can be applied to cement and concrete as mineral admixtures [11]. When super fine steel slag powder is mixed with concrete as an active admixture, concrete properties will be improved greatly so as to produce high performance concrete. The hydration heat can be effectively decreased. Resistance to abrasion and durability, workability and mechanical properties of the concrete can be enhanced. When compound mineral admixtures with steel slag powder and blast-furnace slag powder are mixed with concrete, the performance of concrete can be improved further due to the synergistic effect and activation of each other [12-15]. Therefore, the mineral admixture will be a perfect component of green concrete and its utilization will be a valuable resource for recycling. Mechanical properties of steel slag green concrete and the effectiveness of using steel slag powder for suppressing alkali aggregate reaction (AAR) were studied in this work.
2 Experimental
2.1 Materials
Four kinds of steel slag powders were selected to prepare concrete specimens. Chemical compositions and properties of steel slag powders and blast furnace slag are listed in Table 1. The 42.5R Portland cement was used in concrete, and the blast furnace slag (S95) was also used as a partially replaced material in the cement and concrete.
2.2 Mixing proportions of concrete
The concrete specimens were prepared with two types of mineral admixtures, steel slag powder and blast furnace slag powder. Only steel slag powders were mixed into concrete with dosages of 10%, 20%, 30% and 40% (mass fraction, relative to cement), respectively. The total admixture content was 30% by mass of cement when compound mineral admixture was used. The compound mineral admixtures were prepared with mass ratios of steel slag powder to blast furnace slag being 0.5, 1.0 and 2.0, respectively. The concrete specimens prepared with mineral admixtures are summarized in Table 2.
2.3 Casting of concrete specimens
Cubic concrete specimens with dimensions of 100 mm×100 mm×100 mm were prepared to determine the compressive strength of steel slag concrete. Square prism specimens with dimensions of 100 mm×100 mm×400 mm were prepared to determine the flexural strength of steel slag concrete using simple beam three-point loading. After casting, the specimens were covered with wet burlap and left in the casting room at a temperature of (20±5) ℃ for a period of 24 h. The specimens were then demoulded and cured in saturated calcium hydroxide solution for 2, 6 and 27 d, respectively. The mechanical properties of concrete were then measured.
3 Properties of steel slag green concrete
3.1 Workability
Workability of steel slag green concrete can be usually improved due to the mixing of mineral admixture. Test results show that bleeding and viscosity of fresh steel slag concrete are better than those of plain concrete without admixture. The measured slumps of concrete are summarized in Table 3.
The slump of concrete will increase when steel slag powder is used, while the slump of concrete will decrease after the concrete is mixed into blast furnace slag. The slump of concrete with compound mineral admixture increases with the increase of content of steel slag powder. In summary, it is clear that the hydration process of steel slag powder has different mechanisms
Table 1 Chemical compositions and properties of steel slag powder and blast furnace slag
Table 2 Mixing proportions of steel slag concrete
Table 3 Initial slumps of concrete with mineral admixture (Unite: mm)
with Portland cement due to its different compositions. The hydration velocity of steel slag powder is slower than that of Portland cement. However, steel slag powder changes the size and distribution of hole in the cement, and the cement mortar has better microstructure so as to increase the fluidity of cement with steel slag powder.
3.2 Compressive strength
The compressive strength of concrete specimens was determined after curing for 3, 7 and 28 d. The variation of compressive strength of steel slag concrete is shown in Fig. 1. As expected, the compressive strength increases with age in all the concrete specimens. The compressive strength of concrete with only steel slag
Fig.1 Compressive strength of concrete with mineral admixture vs curing time: (a) Steel slag A; (b) Steel slag B; (c) Steel slag C; (d) Steel slag D; (e) Steel slag D and blast furnace slag
decreases with the mixed content; the compressive strength is lower than that with blast furnace slag concrete. However, an increase in compressive strength of concrete with steel slag is higher than that with blast furnace slag after curing for 7 and 28 d, respectively. The compressive strength of concrete with compound mineral admixture (mixing code 6) is the highest.
3.3 Flexural strength
The flexural strength of concrete specimens with four kinds of steel slag and blast furnace slag was also determined after curing for 3, 7 and 28 d. The variation of flexural strength of steel slag concrete is shown in Fig.2. Test results show that flexural strength of steel slag concrete tends to increase fast compared with compressive strength at later age. When steel slag content is 10% of the total cement, the flexural strength is higher than that of the plain concrete after curing for 28 d. The flexural strength of concrete with compound mineral admixture shows the improvement due to the synergistic effect. The highest value is measured in concrete with steel slag powder to blast furnace slag ratio of 2.0.
According to the test results, compressive and flexural strengths of concrete with steel slag powder tend to decrease while the content of steel slag increases.
Fig.2 Flexural strength of concrete with mineral admixture vs curing time: (a) Steel slag A; (b) Steel slag B; (c) Steel slag C; (d) Steel slag D; (e) Steel slag D and blast furnace slag
Properties of concrete will be greatly enhanced when compound mineral admixture with steel slag powder and blast furnace slag is mixed into concrete. Steel slag and blast furnace slag show the synergistic effect and mutual activation in concrete. On the other hand, steel slag concrete has lower strength at early age. Therefore, steel slag should be used to control hydration process so that concrete with high strength and good performance can be obtained.
4 Effectiveness of steel slag powder on suppressing AAR
4.1 ASTM C441 tests
ASTM C 441 uses Pyrex glass aggregate in mortars to accelerate the testing and provides reliable results. Mixing proportions of AAR tests with Pyrex aggregate are listed in Table 4.
Table 4 Mixing proportions of AAR test with Pyrex aggregate
The inhibition effect of steel slag powder and blast furnace slag on the AAR expansion rate is shown in Fig.3.
It is seen from Fig.3 that the AAR expansion may be effectively inhibited by the addition of mineral powders, and the best result is achieved when steel slag powder and blast furnace slag are added (Test No.4). The expansion rate of the mortar bar after 56 d decreases to about 50% of that of the controlled specimens.
Fig.3 AAR expansion rates from ASTM C441 tests after curing for different times
4.2 Accelerated tests
When actual aggregate is tested, a suitable method could be used to assess the effectiveness of steel slag powder on suppressing AAR expansion under condition of high temperature and more concentrated alkali solution. The specimens were cured in the solution of 1 mol/L NaOH at 80 ℃. Mixing proportions of AAR tests with actual fine aggregate are listed in Table 5.
Table 5 Mixing proportions of AAR test with fine aggregate
The inhibition effect of steel slag powder and blast furnace slag on the AAR expansion rate is shown in Fig.4.
It is seen from Fig. 4 that the AAR expansion with fine aggregate might be effectively inhibited by the addition of mineral powders, and the best result was achieved when blast furnace slag was added (Test No.3). The expansion rate of the mortar bar after 28 d decreases to about 40% of that of the controlled specimens.
Fig.4 AAR expansion rates from accelerated tests after curing for different times
5 Conclusions
(1) Concrete with good performance can be produced using mineral admixtures consisting of steel slag powder and blast furnace slag.
(2) Compressive and flexural strengths of concrete with steel slag powder tend to decrease as the content of steel slag increases. Properties of concrete will be greatly enhanced when compound mineral admixture of steel slag powder and blast furnace slag is mixed into concrete. Steel slag and blast furnace slag show synergistic effect and mutual activation in concrete. On the other hand, steel slag concrete has lower strength at early ages. Therefore, steel slag can be used to control the hydration process so that concrete with high strength and good performance can be obtained.
(3) The effectiveness of steel slag powder on suppressing AAR expansion is assessed using the method of ASTM C441 and the accelerated test method. The results show that mineral admixture with steel slag powder as partial replacement for Portland cement in concrete is an effective means for controlling expansion due to AAR. At most, a 50% decrease in expansion rate of mortar bars can be achieved. According to the research, properties and durability of concrete can be greatly improved using mineral admixtures such as steel slag powder.
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(Edited by CHEN Wei-ping)
Foundation item: Project(2006BAF02A00) supported by the National Science and Technology Pillar Program during the 11th Five-Year Plan Period of China; Project(08-2-1-18-nsh) supported by the Science and Technology Program of Qingdao City, China
Received date: 2009-03-05; Accepted date: 2009-06-28
Corresponding author: LI Yun-feng, PhD; Tel: +86-13061429101; E-mail: hhliyunfeng@126.com
