Application of ferronickel slag for structural concrete as a binder

Abstract

페로니켈 슬래그는 니켈강의 제련 시 발생하는 산업부산물로, 건설재료로써는 잔골재를 대체하여 사용되어 왔다. 그러나 잔골재로 사용 시 원재료의 높은 실리카 함량으로 인하여 알칼리-실리카 반응성의 위험이 나타나 잘 사용되지 않다가 최근들어 시멘트 대체재로의 활용 방안이 주목받고 있는 재료이다. 페로니켈 슬래그의 연간 발생량은 전 세계적으로 매우 많으며 특히 철강산업이 발전한 국가들을 중심으로 발생량이 높고 대부분 매립으로 인한 문제가 생기고 있다. 이를 시멘트 대체재로 사용할 경우에는 부가가치를 더할 수 있다는 장점이 있기 때문에, 본 연구를 수행하는 배경이 되었다. 해당 재료에 대한 기초 강성 평가나 원재료에 대한 연구는 수행되어왔지만, 내구성의 경우에는 부식에 대한 영향성 평가가 거의 이루어지지 않아 본 연구에서 이에 대한 평가를 수행하였다. 페로니켈 슬래그를 혼입한 콘크리트 평가에 앞서 원재료에 대한 기초물성을 평가하여 미분말 페로니켈 슬래그의 화학적 조성, 입도, 입형 등에 대한 기초 분석을 수행하였다. 콘크리트 품질 평가를 위해 페로니켈 슬래그의 결합재 함량을 3 단계로 나누어 시험편 배합을 준비하였으며, ‘초기 수화거동 및 굳은 콘크리트의 강성, 페로니켈 슬래그 시멘트 매트릭스의 수화특성, 굳은 콘크리트의 부식 영향성 및 알칼리-실리카 반응성 평가’에 대한 실험적 평가가 수행되었다. 또한 본 연구에서 얻은 내구성 시험 결과를 통해 페로니켈 슬래그 콘크리트의 내구수명을 산정하고, 적용 배합에 대한 전과정 평가(Life cycle assessment)를 수행하여 환경부하와 현장 적용성에 대한 검증을 수행하였다. 페로니켈 슬래그 콘크리트의 초기 수화는 보통포틀랜드시멘트를 사용했을 때보다 더디며 이에 따라 초기에 압축강도 발현도 낮게 나타나는 경향을 보였다. 그러나 90일 압축강도가 28일보다 10 MPa 이상 증가하는 것을 보았을 때 장기적으로 계속해서 반응이 일어날 수 있을 것으로 보이며, 이는 원재료 내의 실리카 함량으로 인한 포졸란 반응이 일어나는 것의 증거로 판단된다. 치환량이 증가할수록 포졸란 반응성이 높아지고 기존 수화물의 수화도는 낮아지며, 28일 이후 압축강도의 발현을 고려하면 낮은 수화도에 의한 압축강도 감소가 포졸란 반응으로 회복되는 것을 확인할 수 있다. X선 회절분석, 주사현미경분석, 열중량분석을 통해 수화특성을 파악했을 때 해당 포졸란 반응의 가능성 및 페로니켈 슬래그 미분말의 반응 산물인 마그네슘계열 물질의 존재가 확인되었다. 페로니켈 슬래그 미분말의 높은 마그네슘 함량은 콘크리트 부식 내구성에 대해서는 요소에 따라 장, 단점이 나타났다. 염소이온 확산과 고정화 측면에서는 보통포틀랜드시멘트보다 표면염화물량이 1/3 수준으로 낮고 고정화 능력도 다소 높은 것으로 나타나 내구성이 높을 것으로 예측되었으나, 치환율이 높아질수록 공극량이 증가하여 탄산화에 대한 저항성이 매우 낮게 나타났다. 또한 마그네슘계열 수화물의 페이스트 내 산 저항성이 시멘트의 칼슘계열 수화물 보다 현저히 낮게 나타나는 것을 확인하였다. 이를 내구수명에 반영하였을 때는, 각 요소들의 영향성이 상쇄되는 효과로 인하여 내구수명이 소폭 증가하거나 오히려 떨어지는 경향이 나타났다. 따라서 종합적 평가를 근거로 페로니켈 슬래그 콘크리트의 품질과 내구성 확보를 위한 타당한 혼입량이 제시되었으며, 필요환경에 따라(40 MPa 이하 낮은 강도요구수준 등) 현장 적용성이 충분히 가능할 것으로 판단된다.|Ferronickel slag (FN) is a by-product obtained from nickel-steel alloy, and it was used as aggregates in construction material, however, it was not used in concrete due to alkali-silica reactivity (ASR). Recently, the utilization of FN is emerging in research, due to the high output amount among the countries having a steel industry. The disposal problems are continuously arising in those countries. This research has the background of utilizing the FN to a cementitious binder because of the add-on value instead of paying for the disposal. So far, studies have been done to the evaluation of strengths or basic study of the raw material characteristics, but durability assessment, especially about corrosion, was not dealt at all. And this study intensively focused on the monitoring of corrosion in the FN cementitious system. Prior to the evaluation of FN concrete quality, raw material characteristics pulverized FN including chemical composition, particle distribution, morphology, and etc. were analyzed. The replacement of FN in a binder was varied in 3 levels to evaluate concrete quality by experiments regarding ‘early-stage hydration behavior with the strength development, hydration characteristics of FN blended cement matrix, and durability assessment on corrosion and ASR’. Also, applicability in-field use was verified by calculating the service life of the concrete from the obtained durability test results and conducting life cycle assessment (LCA) for the mixes with different replacement ratio. The low strength development at the initial hardening of concrete was found out in FN concrete compared to the ordinary Portland cement (OPC) concrete because of late early hydration. However, the compressive strength of FN concrete at 90 days was about 10 MPa increased from 28 days which possibly shows a potential pozzolanic reaction that is expected from silicates in the raw material. The presence of Mg-based reaction products were identified in X-ray diffraction, scanning electron microscopy, and thermogravimetry analysis showing the potential pozzolanic reactivity. The high magnesium content in FN showed the pros and cons in the corrosion resistivity. In terms of chloride transport, the surface chloride content of FN was 1/3 of OPC and the binding capacity of chloride ion was slightly higher in FN than OPC. On the other hand, the coarser pore structure of largely replaced FN resulted in resistivity against carbonation. And the Mg-based hydration products in FN concrete showed critically lowered pH in the cement system than Ca-based ones. Concerning the influencing factors of corrosion, there is an offset effect on the service life prediction showing little increase or decrease of relative service life as the FN replacement ratio increased. To conclude the assessment results, an adequate replacement ratio of FN in the total binder was suggested to secure concrete quality and durability within the required environment (such as low compressive strength design under 40 MPa), then it is expected to satisfy a certain field application.; Ferronickel slag (FN) is a by-product obtained from nickel-steel alloy, and it was used as aggregates in construction material, however, it was not used in concrete due to alkali-silica reactivity (ASR). Recently, the utilization of FN is emerging in research, due to the high output amount among the countries having a steel industry. The disposal problems are continuously arising in those countries. This research has the background of utilizing the FN to a cementitious binder because of the add-on value instead of paying for the disposal. So far, studies have been done to the evaluation of strengths or basic study of the raw material characteristics, but durability assessment, especially about corrosion, was not dealt at all. And this study intensively focused on the monitoring of corrosion in the FN cementitious system. Prior to the evaluation of FN concrete quality, raw material characteristics pulverized FN including chemical composition, particle distribution, morphology, and etc. were analyzed. The replacement of FN in a binder was varied in 3 levels to evaluate concrete quality by experiments regarding ‘early-stage hydration behavior with the strength development, hydration characteristics of FN blended cement matrix, and durability assessment on corrosion and ASR’. Also, applicability in-field use was verified by calculating the service life of the concrete from the obtained durability test results and conducting life cycle assessment (LCA) for the mixes with different replacement ratio. The low strength development at the initial hardening of concrete was found out in FN concrete compared to the ordinary Portland cement (OPC) concrete because of late early hydration. However, the compressive strength of FN concrete at 90 days was about 10 MPa increased from 28 days which possibly shows a potential pozzolanic reaction that is expected from silicates in the raw material. The presence of Mg-based reaction products were identified in X-ray diffraction, scanning electron microscopy, and thermogravimetry analysis showing the potential pozzolanic reactivity. The high magnesium content in FN showed the pros and cons in the corrosion resistivity. In terms of chloride transport, the surface chloride content of FN was 1/3 of OPC and the binding capacity of chloride ion was slightly higher in FN than OPC. On the other hand, the coarser pore structure of largely replaced FN resulted in resistivity against carbonation. And the Mg-based hydration products in FN concrete showed critically lowered pH in the cement system than Ca-based ones. Concerning the influencing factors of corrosion, there is an offset effect on the service life prediction showing little increase or decrease of relative service life as the FN replacement ratio increased. To conclude the assessment results, an adequate replacement ratio of FN in the total binder was suggested to secure concrete quality and durability within the required environment (such as low compressive strength design under 40 MPa), then it is expected to satisfy a certain field application.