轉爐石（basic oxygen furnace slag）具有高比重、高硬度、低磨損率及粗糙度大等特性適合作為工程用之級配粒料，但因為轉爐煉鋼製程所需加入大量石灰，造成轉爐石常含有大量未反應的游離氧化鈣（f-CaO）與高鹼特性，與水氣及二氧化碳接觸後會產生水化作用及碳酸化作用發生膨脹反應造成在工程上運用之不便。加速碳酸化轉爐石能達到相同於自然風化的效果降低膨脹特性及溶出特性。
本研究發現加速碳酸化對於轉爐石此類活性材料的安定化方式，不僅能縮短熟化時間達到降低膨脹的效果，對材料工程性質的提升也有幫助。透過不同粒徑（3.5-7 mm、7-15 mm及15-25 mm）轉爐石經加速碳酸化處理後，物理及力學性質產生不同程度之變化。由試驗結果顯示，（1）加速碳酸化後轉爐石比重增加及吸水率下降，得知其表面生成分子量大且緻密之碳酸鈣層，亦使得碳酸化後轉爐石在抗磨損與抗凍蝕表現有所提升。（2）在力學性質試驗方面，加速碳酸化後轉爐石的加州載重比值、最大剪應力與筒壓強度也有提升之情況，並且以筒壓試驗之抗壓強度最為顯著。（3）由膨脹試驗發現轉爐石粒徑愈小膨脹率愈大，經加速碳酸化後轉爐石膨脹反應有明顯趨緩的現象。
轉爐石作為鋪面級配材料的替代量上限，經本研究設計之試驗結果顯示，依據ASTM D1241級配粒料設計規範第一型之A級配粗粒料之替代量上限為31%；B級配粗粒料之替代量上限為29%；C級配粗粒料之替代量上限為27.5%；D級配粗粒料之替代量上限為44%。經加速碳酸化後轉爐石粗粒料替代量約可提升7.5%至10.5%。以不超過本研究之替代量上限為原則，作為轉爐石添加鋪面級配方法，可符合級配粒料之工程品質規範。

SUMMARY

Basic oxygen furnace (BOF) slag is characterized by high hardness, low abrasion, and high roughness. It is suitable for use as a graded aggregate in engineering applications. However, the conversion process generates a substantial amount of unreacted free CaO, resulting in expansion and, thus, causing the use of slag in engineering applications to be inconvenient. Accelerated carbonation of BOF slag is equivalent to natural weathering and can effectively shorten the weathering time. This study revealed that accelerated carbonation not only reduces expansion behaviors but also strengthens physical and mechanical properties. According to test results, the abrasion resistance, soundness, California bearing ratio, shear strength, and compressive strength of BOF slag increased after carbonation.

INTRODUCTION

The integrated steel-making process produces a large amount of BOF slag. BOF slag contains a large amount of unreacted free CaO. When free CaO comes in contact with moisture and CO2, hydration and carbonation occur, causing swelling in earthwork materials. Accelerated carbonation leads to the formation of a calcium carbonate layer on the surface of calcium-based substances. This surface layer reduces swelling and dissolution, and may be responsible for changes in the engineering properties. Many studies have compared the BOF slag and natural aggregate in engineering materials, but few have analyzed the effect of the changes in the material properties before and after the carbonation of BOF slag on the engineering properties. Therefore, this study analyzes BOF slag with various particle sizes before and after accelerated carbonation and discusses the changes in engineering and mechanical properties. Experimental results confirm the effect of the formation of calcium carbonate on the material properties of BOF slag. A mechanical behavior model is constructed, which may be helpful for applying BOF slag to civil engineering.

MATERIALS AND METHODS

BOF slag produced by an integrated steel plant in Kaohsiung, Taiwan, was used in this study. The particle sizes were 3.5-7, 7-15, and 15-25 mm. Test samples including the six kinds of fresh and carbonated in different particle sizes. The accelerated carbonation device was a large rotary kiln, curing 15 to 20 kg per batch, so as to reduce the errors between samples. The optimal control conditions obtained in previous research as the base for BOF slag carbonation. The optimal control conditions were 24h curing in a reaction tank with a CO2 concentration of 40%, water vapor content of 60%, and a temperature of 200 °C. This study carried out a series of physical and mechanical tests on the BOF slag before and after carbonation to determine changes in the engineering properties. The tests included specific gravity and water absorption tests (ASTM C127-12), the Los Angeles abrasion test (ASTM C535-03), the soundness test (ASTM C88-05), the swelling test (ASTM D4792-00), the California bearing ratio (CBR) test (ASTM D1883–07), the direct shear test (ASTM D5321-08), and the lightweight aggregate cylinder compressive strength test (CNS 14779).

RESULTS AND DISCUSSION

Effect of accelerated carbonation on engineering properties

The basic physical test data show that the carbonated BOF slag changed. Besides the changes in specific gravity and water absorption, the hardness and erosion resistance of the material improved. The results of mechanical properties tests are shown the CBR and direct shear test value of BOF slag after carbonation increased slightly. In addition, the cylinder compressive strength test data show that the strength for particle sizes of 3.5-7, 7-15, and 15-25 mm increased by about 8.63, 6.72, and 7.09 MPa after carbonation, respectively. Since the cured calcium carbonate layer covers the material surface, the total surface area of the samples with the smallest particle size was largest, and thus the amount of carbonation was largest. The cylinder compressive strength test results confirm that carbonation improves the engineering properties of BOF slag.

Effect of accelerated carbonation on expansion ratio

The expansion of the carbonated BOF slag becomes slow around the fifth day, whereas the fresh BOF slag keeps swelling. The expansion rate of carbonated BOF slag is lower than that of fresh BOF slag after day 7, and decreased by 0.19% ± 0.03% on day 8.

Evaluation of the BOF slag used as graded aggregate material

The results showed that uncarbonated BOF slag used as graded aggregate material (ASTM D1241) could replacement 27.5 to 44%. In addition, the carbonated BOF slag used as graded aggregate material could replacement 35 to 52%.

CONCLUSION

The main research findings are as follows: (1) Under optimal control conditions of 24-h carbonation time, 200 °C, 40% CO2, and 60% H2O, the high alkalinity of BOF slag was reduced. Carbonation causes a reaction on the surface of BOF slag. This calcium carbonate layer fills up the surface pores and covers the alkaline matter inside the BOF slag, thus decreasing the pH value and free CaO content and changing the physical and engineering properties. (2) According to the physical properties test results, the specific gravity and water absorption were affected by carbonation. The Los Angeles abrasion test and soundness test show that the hardness and erosion resistance of BOF slag increased. (3) The mechanical properties test shows that the engineering properties were improved after carbonation. The smaller the particle size is, the more apparent is the improvement, indicating that the improvement is related to the degree of carbonation. The fine aggregate samples had the largest area of the calcium carbonate layer, and thus their test values increased most significantly. (4) According to the analysis of mechanical behavior of the carbonated BOF slag, the stress bearing capacity of BOF slag increases significantly The increase in the material strength represents assistance to the engineering aggregates.

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