游離氧化鈣(f-CaO)及游離氧化鎂(f-MgO)是指在鋼碴中未反應的氧化鈣及氧化鎂，遇水生成氫氧化鈣(Ca(OH)2)及氫氧化鎂(Mg(OH)2)，有產生體積膨脹之虞，而使鋼碴在工程材料應用上有所限制。目前，測定游離氧化鈣的方法為ASTM C114-30，游離氧化鎂尚無標準測定方法，而這些方法被用來判斷鋼碴f-CaO及f-MgO含量以決定鋼碴摻配量之依據。可是，不同的煉鋼製程產生不同鍛燒溫度之氧化鈣及氧化鎂，其物化特性互異，因此有需要進一步釐清CaO及MgO之反應性及其在工程材料應用上之影響。本研究以不同鍛燒溫度之氧化鈣及氧化鎂，探討f-CaO及f-MgO的含量及其對水泥漿體膨脹特性之影響，並藉此針對氧化鈣及氧化鎂之反應性建立創新的篩選法以評估電孤爐還原碴之膨脹潛勢。

不同鍛燒溫度之氧化鈣及氧化鎂分為輕燒(700-1000°C)、重燒(1000-1400°C)及死燒(1500°C)，以模擬煉鋼過程中可能出現之氧化鈣及氧化鎂形態。研究結果發現提升煅燒溫度之氧化鈣及氧化鎂有較大之顆粒及晶體尺寸，並有較少之比表面積。在檸檬酸及水化反應測試中，愈高煅燒溫度之氧化鈣及氧化鎂有較低的活性，而死燒氧化鎂更為明顯。在f-CaO及f-MgO測試中發現，輕燒和重燒氧化鈣及氧化鎂比死燒有較高f-CaO及f-MgO 含量，依文獻說明其在水泥漿體中有較大之膨脹潛勢。但由熱壓膨脹試驗發現只有死燒氧化鈣及氧化鎂會使水泥漿體膨脹，證明了f-CaO及f-MgO含量的測定無法準確判斷水泥漿體之膨脹特性。本研究提出「水化反應測試法」，得到不同鍛燒溫度氧化鈣及氧化鎂之水化反應速率，可以作為氧化鈣及氧化鎂添加於水泥漿體中體積穩定的判斷依據。

研究發現含有死燒氧化鈣及氧化鎂水泥漿體的膨脹機制是因為死燒氧化鈣及氧化鎂延遲水化反應，生成氫氧化鈣、氫氧化鎂及鈣礬石類之晶體，破壞水泥漿體之內部結構。此外，透過所建立的水化反應測試法快速測定發現還原碴中氧化鎂活性與死燒氧化鎂相似，顯示還原碴有很高的膨脹潛勢。而由熱壓膨脹試驗得知摻配20 wt.% 還原碴之水泥漿體健性並不穩定，其膨脹機制與含死燒氧化鎂之水泥漿體相同，顯示水化反應測試法是作為篩選鋼碴膨脹潛勢之可行方法。

Free-CaO (f-CaO) or free-MgO (f-MgO) refer to the unreacted CaO or MgO in steel slag. The application of steel slag as an engineering material has been limited due to the presence of f-CaO and f-MgO, which will form Ca(OH)2 and Mg(OH)2 when hydrated, and may cause volumetric expansion. At present, the determination of f-CaO content is the ASTM C114-30 method, while there is no standard method for f-MgO content determination. These methods are used to determine the f-CaO and f-MgO content in steel slag, and are the basis for judging the amount of steel slag that can be blended in cement. However, CaO or MgO varied in terms of basic characteristics depending on the calcining temperature during the process of steel refining. It is necessary to further clarify the reactivity of CaO and MgO, and its influence when applied as an engineering material. This study investigated the effect of different levels of calcination in CaO and MgO on the expansion of cement paste as well as its influence on the test results for f-CaO and f-MgO content. Furthermore, an innovative screening method is developed to evaluate the ladle slag expansion potential through the identification of CaO and MgO reactivity.

The calcinated CaO and MgO were classified into the lightly (700-1000°C), hard (1000-1400°C) and dead-burnt (1500°C) states in order to simulate their possible states in ladle slag formed in an electric arc furnace. Increases in the calcination temperature of either CaO or MgO have been found to result in a larger particle size, larger crystal grain size and a smaller specific surface area. The citric acid reactivity test and hydration test conducted in this study indicated a decrease in the reactivity of CaO and MgO under higher calcinating temperatures. Dead-burnt MgO was almost unreacted in the hydration test. The test results of f-CaO and f-MgO content measured with calcinated CaO or MgO showed that the lightly and hard-burnt CaO or MgO had a higher potential to cause cement paste unsoundness. However, in the autoclave expansion test, only dead-burnt CaO and MgO were found to lead to the volumetric expansion of cement paste. This indicated that the f-CaO and f-MgO content test cannot precisely predict the expansion property of cement paste. This study presents the “Hydration Test” as a basis of judgement for soundness of cement paste added with calcinated CaO or MgO, based on the testing of calcinated CaO and MgO reaction rate.

The mechanism of dead-burnt CaO and MgO in cement paste that leads to unsoundness was believed to have been due to delayed hydration to Ca(OH)2, Mg(OH)2 and ettringite-like crystals that will damage the internal strength of cement paste. The MgO reactivity of the ladle slag was screened by “Hydration Test”. It was found to be similar to that of dead-burnt MgO, thus showing ladle slag to have a high expansion potential. By conducting an autoclave expansion test, cement paste with 20 wt.% ladle slag was expanded, and the expansion mechanism was found to be the same as that for dead-burnt MgO in cement paste, showing the “Hydration Test” to be a reliable screening method for the expansion potential of steel slag.

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