The Influence of CaO and MgO on the Mechanical Properties of Alkali-Activated Blast Furnace Slag Powder

CaO and MgO are both reported as effective activators for blast furnace slag. However, the synergistic effect of these two components on the mechanical properties of alkali-activated blast furnace slag remains unclear. In this study, the flexural and compressive strengths of alkali-activated blast furnace slag powder with MgO and CaO range from 0% to 30% by the mass ratio of alkali-activated blast furnace slag powder are investigated. Moreover, the dry shrinkage rate of alkali-activated blast furnace slag powder is measured. One percent refractory fibers by volume of binder materials are added in the alkali-activated blast furnace slag. Some refractory fibers are treated with water flushing, meanwhile, some refractory fibers are directly used without any treatment. Finally, the scanning electron microscope, the thermogravimetric analysis curves and the XRD diffraction spectrums are obtained to reflect the inner mechanism of the alkali-activated blast furnace slag powder’s mechanical properties. The water-binder ratios of the alkali-activated blast furnace slag powder are 0.35 and 0.42. The curing ages are 3 d, 7 d and 28 d. The measuring temperature for the specimens ranges from 20 °C to 800 °C. Results show that the flexural and compressive strengths increase with the increased curing age, the decreased water-binder ratio and the addition of refractory fibers. The water-treated refractory fibers can improve the mechanical strengths. The mechanical strengths increase in the form of a quadratic function with the mass ratio of MgO and CaO, when the curing age is 3 d, the increasing effect is the most obvious. A higher water-binder ratio leads to an increasing the drying shrinkage rate. The activated blast furnace slag powder with CaO shows a higher drying shrinkage rate. The mechanical strengths decrease with the increasing testing temperature.


Introduction
The expansion of MgO through hydration reaction has a certain compensation effect on the shrinkage of concrete. Meanwhile, the shrinkage reduction effect is more economical and effective than that of traditional additives. Therefore, MgO is commonly used in practical civil engineering [1,2]. Due to the large volume of hydration products (Mg(OH) 2 ), the shrinkage of the materials can be reduced [3][4][5]. Moreover, the MgO can improve the dense of the materials. Additionally, the magnesium oxide can be used to improve the long-term volume stability of slag cement in slag system [6][7][8].
The active MgO was firstly used in ordinary Portland cement in 2001 by Harrison [9]. As reported in some journals, the MgO has been proved to be an effective slag activator [10][11][12]. MgO with high specific surface area and high reactivity can improve the compactness and strength of the cement by reducing porosity. Yi et al. [13]. found that the addition of MgO is effective to activate the mechanical property of blast furnace slag powder. In their study, the compressive strength of blast furnace slag powder with MgO cured in the standard curing

Specimens' Preparation
Firstly, the potash water glass and sodium hydroxide are mixed in the NJ-160A cement paste mixer (Wuxi Jianyi instrument factory, Wuxi, China) and stirred with the speed of 140 r/min for 1 min. After this mixing is finished, the blast furnace slag powder is added to the cement paste mixer and 1 min of stirring with the speed of 140 r/min is provided. Then, the water and refractory fiber are poured into the mixer and stirred at the speed of 140 r/min for 2 min, and then, another 2 min stirring with the speed of 285 r/min is provided for the mixture. After the stirring, all fresh mixture is poured to form specimens with size 40 × 40 × 160 mm 3 . When the pouring is finished, the specimens are used for vibration and compaction by a vibrating, then the specimens are cured in the environment of 65% relative humidity and 20 • C for 1 day and then demolded. Finally, all specimens are moved to the standard curing room with relative humidity and temperature of 98.1% and 20 • C, respectively, until the required curing age.

Measurement of Mechanical Strength and Dry Shrinkage Rate
HYE-300B cement flexural and compressive constant stress testing machine produced by Beijing Sanyu Weiye, Beijing, China, is used for the measurement of mechanical strengths. The loading speeds for the compressive and flexural strengths are 2.4 kN/s and 0.05 kN/s respectively. Meanwhile, the specimens with this size are applied in the measurement of drying shrinkage. The testing methods can be found in ASTM C109/C109M-2011a ASTM International standard [33]. Automatic universal testing machine equipped with a temperature controllable box shown in Figure 1 is provided to determine the mechanical strength with different temperatures. The testing temperatures of specimens with waterbinder ratio of 0.35, 10% MgO and 20% CaO by mass ratios of the total binder materials are 20 • C, 200 • C, 400 • C, 600 • C and 800 • C. Specimens are kept in these temperatures for 4 h before testing. The experimental details can be found in Feng's research [34]. mens with size 40 × 40 × 160 mm 3 . When the pouring is finished, the specimens a for vibration and compaction by a vibrating, then the specimens are cured in the e ment of 65% relative humidity and 20 °C for 1 day and then demolded. Finally, a mens are moved to the standard curing room with relative humidity and temper 98.1% and 20 °C, respectively, until the required curing age.

Measurement of Mechanical Strength and Dry Shrinkage Rate
HYE-300B cement flexural and compressive constant stress testing machi duced by Beijing Sanyu Weiye, Beijing, China, is used for the measurement of mec strengths. The loading speeds for the compressive and flexural strengths are 2.4 k 0.05 kN/s respectively. Meanwhile, the specimens with this size are applied in th urement of drying shrinkage. The testing methods can be found in ASTM C109/ 2011a ASTM International standard [33]. Automatic universal testing machine eq with a temperature controllable box shown in Figure 1 is provided to determine chanical strength with different temperatures. The testing temperatures of specime water-binder ratio of 0.35, 10% MgO and 20% CaO by mass ratios of the total bin terials are 20 °C, 200 °C, 400 °C, 600 °C and 800 °C. Specimens are kept in these te tures for 4 h before testing. The experimental details can be found in Feng's resea The dry shrinkage rate is carried out by the following steps. The BC-160 type length comparator produced by Cangzhou Bowei, Cangzh China is placed on a flat desktop. The top of the dial indicator is aligned with "0 the specimens are put into the length gauge and observe the dial indicator to r length. The testing methods can be found in JGJT70-2009 Chinese standard [35].
The dry shrinkage rate is calculated by Equation (1). The dry shrinkage rate is carried out by the following steps. The BC-160 type length comparator produced by Cangzhou Bowei, Cangzhou City, China is placed on a flat desktop. The top of the dial indicator is aligned with "0". Then, the specimens are put into the length gauge and observe the dial indicator to read the length. The testing methods can be found in JGJT70-2009 Chinese standard [35].
The dry shrinkage rate is calculated by Equation (1).
where ε is the dry shrinkage rate. L 1 and L t are the lengths of specimen cured for 1 day and 28 days, respectively. The diagrammatic sketch of the dry shrinkage rate is illustrated in Figure 2.
where ε is the dry shrinkage rate. L1 and Lt are the lengths of specimen cured for 1 d and 28 days, respectively. The diagrammatic sketch of the dry shrinkage rate is illustra in Figure 2.

Steps of Micro-Performance
Central crushing part of specimen is selected for the measurements of scanning e tron microscope and the thermogravimetric analysis. Samples with mung bean size vacuum sprayed with gold. After that, the samples are used for obtaining the scann electron microscope photos by the thermal field emission scanning electron microsc SU5000 produced by Hitachi Hi Tech Co., Ltd., Toko, Japan. Meanwhile, the remain samples are ground into a powder by a ball mill and used for obtaining the thermogra metric curves. TGADSC3+ thermogravimetric and synchronous thermal analyzer p duced by mettletoledo, Zurich, Switzerland is used for obtaining the thermogravime curves. The temperature for thermogravimetric test ranges from 20 °C to 1000 °C. So powdery sample is used for the measurement of X-rays diffraction spectrum with Bro X-ray diffractometer D2PHASER, provided by Germany Brooke company, Berlin, G many. The measuring details can be found in Wang's research [36]. Figure 3 is the flexural and compressive strengths of blast furnace slag activated water glass and refractory fibers. It can be seen from Figure 3; the flexural and comp sive strengths gradually increase with the increasing curing age. The mechanical streng increase rapidly when the curing age ranges from 1 d to 7 d. Meanwhile, when the cur age increases from 7 d to 28 d, the flexural and compressive strengths increase slow This is attributed to the fact that the reactivity of the alkali activated blast furnace sla high at early curing age and then is low at later curing age [37]. Therefore, the mechan strengths increase rapidly at early curing age and increase slowly at later curing age [ Moreover, as illustrated in Figure 3, the mechanical strengths are decreased by hig water-binder ratio and increased by the addition of refractory fibers. The refractory fib treated by water can effectively improve the flexural strength of alkali activated s However, the water treated refractory fibers demonstrate the negative effect on the al activated blast furnace slag powder. Compared with the blast furnace slag activated only CaO or MgO, the flexural strength of this activated blast furnace slag is increased 11.7~32.8%, and the compressive strength is increased by 15.6~37.2% [38].

Steps of Micro-Performance
Central crushing part of specimen is selected for the measurements of scanning electron microscope and the thermogravimetric analysis. Samples with mung bean size are vacuum sprayed with gold. After that, the samples are used for obtaining the scanning electron microscope photos by the thermal field emission scanning electron microscope SU5000 produced by Hitachi Hi Tech Co., Ltd., Toko, Japan. Meanwhile, the remaining samples are ground into a powder by a ball mill and used for obtaining the thermogravimetric curves. TGADSC3+ thermogravimetric and synchronous thermal analyzer produced by mettletoledo, Zurich, Switzerland is used for obtaining the thermogravimetric curves. The temperature for thermogravimetric test ranges from 20 • C to 1000 • C. Some powdery sample is used for the measurement of X-rays diffraction spectrum with Brooke X-ray diffractometer D2PHASER, provided by Germany Brooke company, Berlin, Germany. The measuring details can be found in Wang's research [36]. Figure 3 is the flexural and compressive strengths of blast furnace slag activated by water glass and refractory fibers. It can be seen from Figure 3; the flexural and compressive strengths gradually increase with the increasing curing age. The mechanical strengths increase rapidly when the curing age ranges from 1 d to 7 d. Meanwhile, when the curing age increases from 7 d to 28 d, the flexural and compressive strengths increase slowly. This is attributed to the fact that the reactivity of the alkali activated blast furnace slag is high at early curing age and then is low at later curing age [37]. Therefore, the mechanical strengths increase rapidly at early curing age and increase slowly at later curing age [37]. Moreover, as illustrated in Figure 3, the mechanical strengths are decreased by higher water-binder ratio and increased by the addition of refractory fibers. The refractory fibers treated by water can effectively improve the flexural strength of alkali activated slag. However, the water treated refractory fibers demonstrate the negative effect on the alkali activated blast furnace slag powder. Compared with the blast furnace slag activated by only CaO or MgO, the flexural strength of this activated blast furnace slag is increased by 11.7~32.8%, and the compressive strength is increased by 15.6~37.2% [38].  Figure 4 shows mechanical strengths of activated blast furnace slag powder with different dosages of MgO. As illustrated in Figure 4, the mechanical strengths of activated blast furnace slag powder increase with the increasing dosage of MgO. The enhancement effect on mechanical strengths of activated blast furnace slag powder is more obvious when the curing ages are 1 d and 3 d. This is attributed to the hydration products (hydrotalcite and hydrated magnesium silicate gel) formed by MgO and water. However, when the curing age is higher than 3 d, the mechanical strengths increase slowly. This is ascribed to the fact that the ion concentration will decrease with the increase of hydration process and the increase of gel. The increased gel may absorb on the surface of the hydration thus delaying the increase of mechanical strength [39]. Compressive strength/MPa  Figure 4 shows mechanical strengths of activated blast furnace slag powder with different dosages of MgO. As illustrated in Figure 4, the mechanical strengths of activated blast furnace slag powder increase with the increasing dosage of MgO. The enhancement effect on mechanical strengths of activated blast furnace slag powder is more obvious when the curing ages are 1 d and 3 d. This is attributed to the hydration products (hydrotalcite and hydrated magnesium silicate gel) formed by MgO and water. However, when the curing age is higher than 3 d, the mechanical strengths increase slowly. This is ascribed to the fact that the ion concentration will decrease with the increase of hydration process and the increase of gel. The increased gel may absorb on the surface of the hydration thus delaying the increase of mechanical strength [39].  Figure 4 shows mechanical strengths of activated blast furnace slag powder with different dosages of MgO. As illustrated in Figure 4, the mechanical strengths of activated blast furnace slag powder increase with the increasing dosage of MgO. The enhancement effect on mechanical strengths of activated blast furnace slag powder is more obvious when the curing ages are 1 d and 3 d. This is attributed to the hydration products (hydrotalcite and hydrated magnesium silicate gel) formed by MgO and water. However, when the curing age is higher than 3 d, the mechanical strengths increase slowly. This is ascribed to the fact that the ion concentration will decrease with the increase of hydration process and the increase of gel. The increased gel may absorb on the surface of the hydration thus delaying the increase of mechanical strength [39].  The compressive and flexural strengths of activated blast furnace slag powder with swelling agent which is mainly composed by CaO is shown in Figure 5. It is observed in  The compressive and flexural strengths of activated blast furnace slag powder with swelling agent which is mainly composed by CaO is shown in Figure 5. It is observed in Figure 5, the compressive and flexural strengths firstly increase with the dosage of CaO ranging from 0% to 10%. However, when the addition of CaO is higher than 10%, the compressive and flexural strengths decrease with the increasing dosage of CaO. Therefore, 10% by mass of the total binder materials is the threshold value of the strength. Moreover, the increasing curing age and lower water-binder ratio led to higher mechanical strength. The hydrolysis rate of CaO is faster than that of MgO, the addition of CaO will be rapidly hydrolyzed to form Ca(OH) 2 , which will continuously ionize Ca 2+ and OH − into the solution. High concentration of OH − is activated to the dissolution of blast furnace slag powder. Small CaO content results in lower Ca/Si molar ratio inner solution. Therefore, the pH value in the early stage is increased by the addition of CaO. Consequently, the large amount of free SiO 4 4− reacts with Mg 2+ and the amount of hydrated calcium silicate increases, thus the mechanical strengths are improved. However, when the CaO content is higher than 10%, the Ca/Si molar ratio increases, and SiO 4 4− preferentially forms hydrated calcium silicate with Ca 2+ , thus inhibiting the reaction between SiO 4 4− and Mg 2+ , hindering the formation of hydrotalcite and hydrated magnesium silicate [40].  Figure 5, the compressive and flexural strengths firstly increase with the dosage of C ranging from 0% to 10%. However, when the addition of CaO is higher than 10%, compressive and flexural strengths decrease with the increasing dosage of CaO. The fore, 10% by mass of the total binder materials is the threshold value of the strength. Mo over, the increasing curing age and lower water-binder ratio led to higher mechan strength. The hydrolysis rate of CaO is faster than that of MgO, the addition of CaO w be rapidly hydrolyzed to form Ca(OH)2, which will continuously ionize Ca 2+ and OH − i the solution. High concentration of OH − is activated to the dissolution of blast furnace s powder. Small CaO content results in lower Ca/Si molar ratio inner solution. Therefo the pH value in the early stage is increased by the addition of CaO. Consequently, large amount of free SiO4 4− reacts with Mg 2+ and the amount of hydrated calcium silic increases, thus the mechanical strengths are improved. However, when the CaO cont is higher than 10%, the Ca/Si molar ratio increases, and SiO4 4− preferentially forms drated calcium silicate with Ca 2+ , thus inhibiting the reaction between SiO4 4− and M hindering the formation of hydrotalcite and hydrated magnesium silicate [40].

Influence of MgO and CaO on Mechanical Strengths
(a) (b)  Figure 6 shows the drying shrinkage rate of activated blast furnace slag powder. shown in Figure 6, higher water-binder ratio leads to increasing the drying shrinkage r of the activated blast furnace slag powder. This is attributed to the fact that the free wa inner activated blast furnace slag powder decreases obviously due to the improvemen hydration degree by the increased curing age and higher water-binder [41]. The decrea free water leads to increasing the drying shrinkage rate. Moreover, activated blast furn slag powder with CaO shows higher drying shrinkage rate. This is ascribed to the fact t the expanding effect of CaO is lower than that of MgO; therefore, the drying shrink rate of blast furnace slag powder with CaO is lower.   Figure 6 shows the drying shrinkage rate of activated blast furnace slag powder. As shown in Figure 6, higher water-binder ratio leads to increasing the drying shrinkage rate of the activated blast furnace slag powder. This is attributed to the fact that the free water inner activated blast furnace slag powder decreases obviously due to the improvement of hydration degree by the increased curing age and higher water-binder [41]. The decreased free water leads to increasing the drying shrinkage rate. Moreover, activated blast furnace slag powder with CaO shows higher drying shrinkage rate. This is ascribed to the fact that the expanding effect of CaO is lower than that of MgO; therefore, the drying shrinkage rate of blast furnace slag powder with CaO is lower.  Figure 7 shows the scanning electron microscope photos of alkali-activated blast furnace slag powder with 10% MgO and 30% MgO, respectively. The compactness of the alkali-activated blast furnace slag powder is improved when more dosage of MgO is added. This is attributed to the fact that the hydrated magnesium silicate gel formed by MgO and water can cover on the surface of the hydrated calcium silicate gel and fill the pore structure of the hydration products [42,43]. Moreover, as discovered in Figure 7, the hydration products are more compact when the curing age is higher. This is due to the fact that the with the increase of hydration degree, the pores in the cement stone will be filled gradually, and the compactness will be improved.  Figure 7 shows the scanning electron microscope photos of alkali-activated blast furnace slag powder with 10% MgO and 30% MgO, respectively. The compactness of the alkali-activated blast furnace slag powder is improved when more dosage of MgO is added. This is attributed to the fact that the hydrated magnesium silicate gel formed by MgO and water can cover on the surface of the hydrated calcium silicate gel and fill the pore structure of the hydration products [42,43]. Moreover, as discovered in Figure 7, the hydration products are more compact when the curing age is higher. This is due to the fact that the with the increase of hydration degree, the pores in the cement stone will be filled gradually, and the compactness will be improved.  Figure 7 shows the scanning electron microscope photos of alkali-activated blast furnace slag powder with 10% MgO and 30% MgO, respectively. The compactness of the alkali-activated blast furnace slag powder is improved when more dosage of MgO is added. This is attributed to the fact that the hydrated magnesium silicate gel formed by MgO and water can cover on the surface of the hydrated calcium silicate gel and fill the pore structure of the hydration products [42,43]. Moreover, as discovered in Figure 7, the hydration products are more compact when the curing age is higher. This is due to the fact that the with the increase of hydration degree, the pores in the cement stone will be filled gradually, and the compactness will be improved.  Figure 8 shows the results of thermogravimetric analysis of alkali-activated blast furnace slag powder with 10% MgO and 30% MgO cured for 28 d. It can be observed from Figure 8, the thermogravimetry of alkali-activated blast furnace slag powder decreases with the increasing temperature. The DSC curves are shown in Figure 9. As can be observed in Figure 9, the corresponding temperatures of the endothermic peak of all the DSC  Figure 8 shows the results of thermogravimetric analysis of alkali-activated blast furnace slag powder with 10% MgO and 30% MgO cured for 28 d. It can be observed from Figure 8, the thermogravimetry of alkali-activated blast furnace slag powder decreases with the increasing temperature. The DSC curves are shown in Figure 9. As can be observed in Figure 9, the corresponding temperatures of the endothermic peak of all the DSC curves are 119 • C, 285 • C and 947 • C. This is attributed to the fact that when the temperature ranged from 0 • C to 119 • C, the thermogravimetry decreases due to the evaporation of free water in the pore solution. Meanwhile, when the temperature increases from 119 • C to 285 • C, the thermogravimetry decreases [44]. This is ascribed to the decomposition of hydrotalcite. Moreover, when the temperature increases from 285 • C to 947 • C, the thermogravimetry decreased, this is because of the decomposition of hydrated calcium silicate [45].

Microscopic Analysis
(c) (d)  Figure 8 shows the results of thermogravimetric analysis of alkali-activated blast furnace slag powder with 10% MgO and 30% MgO cured for 28 d. It can be observed from Figure 8, the thermogravimetry of alkali-activated blast furnace slag powder decreases with the increasing temperature. The DSC curves are shown in Figure 9. As can be observed in Figure 9, the corresponding temperatures of the endothermic peak of all the DSC curves are 119 °C, 285 °C and 947 °C. This is attributed to the fact that when the temperature ranged from 0 °C to 119 °C, the thermogravimetry decreases due to the evaporation of free water in the pore solution. Meanwhile, when the temperature increases from 119 °C to 285 °C, the thermogravimetry decreases [44]. This is ascribed to the decomposition of hydrotalcite. Moreover, when the temperature increases from 285 °C to 947 °C, the thermogravimetry decreased, this is because of the decomposition of hydrated calcium silicate [45].  The X-ray diffraction (XRD) spectra curves of specimens with 10% M MgO cured for 28 d are shown in Figure 9. It can be observed from the XRD s The X-ray diffraction (XRD) spectra curves of specimens with 10% MgO and 30% MgO cured for 28 d are shown in Figure 9. It can be observed from the XRD spectra curves, the hydrated calcium silicate, unhydrated magnesium oxide, hydrotalcite and hydrated magnesium silicate exist in Figure 9. As illustrated in Figure 9, the hydrated magnesium silicate's intensity of specimens with 30% MgO is higher than that of specimens with 10% MgO, which further confirms that MgO has a higher ability to promote the mechanical strengths of alkali activated blast furnace slag powder. Figure 10 shows the mechanical strength of alkali-activated blast furnace slag powder with water-binder ratio of 0.35, 10% MgO and 20% CaO by mass ratios of the total binder materials. It can be observed from Figure 10, the flexural and compressive strengths decrease with the increasing temperature. This is attributed to the fact that the free water in the specimen evaporates, resulting in holes and cracks. Therefore, the stress concentration forms during the loading process, so the strength decreases significantly. Moreover, higher temperature leads to the hydrated calcium silicate gel inside the specimen decomposes. Then, the bound water in it removes, which reduces the bonding of alkali-activated blast furnace slag powder and reducing the mechanical strengths. Compared with Aydin's research, the flexural strength and compressive strength of alkali-activated blast furnace slag powder measured at the temperature of 800 • C with assembly unit of CaO and MgO are increased by 24.1% and 13.2% [24].  Figure 11 shows the scanning electron microscope photos of alkali-a furnace slag powder measured at high temperatures of 200 °C, 400 °C, 600 ° As illustrated in Figure 11, the fibrous hydration products can be found in electron microscope photos. When the temperature is higher than 400 °C, m observed. The SEM photos further exhibit that the mechanical strength of al blast furnace slag powder is decreased by higher measuring temperature.  Figure 11 shows the scanning electron microscope photos of alkali-activated blast furnace slag powder measured at high temperatures of 200 • C, 400 • C, 600 • C and 800 • C. As illustrated in Figure 11, the fibrous hydration products can be found in the scanning electron microscope photos. When the temperature is higher than 400 • C, more cracks are observed. The SEM photos further exhibit that the mechanical strength of alkali-activated blast furnace slag powder is decreased by higher measuring temperature. Figure 11 shows the scanning electron microscope photos of alkali-activated blast furnace slag powder measured at high temperatures of 200 °C, 400 °C, 600 °C and 800 °C. As illustrated in Figure 11, the fibrous hydration products can be found in the scanning electron microscope photos. When the temperature is higher than 400 °C, more cracks are observed. The SEM photos further exhibit that the mechanical strength of alkali-activated blast furnace slag powder is decreased by higher measuring temperature.

Conclusions
The mechanical strength and drying shrinkage rate of alkali-activated blast furnace slag powder are researched. The conclusions are acquired as follows.
The water-binder ratio of the alkali-activated blast furnace slag powder demonstrated negative effect on the following flexural and compressive strengths. Specifically, the mechanical strengths increase with an increasing curing age and a lower water-binder ratio. Meanwhile, the mechanical strengths decrease with the increasing temperature.
The addition of MgO and CaO leads to improve the mechanical strengths of alkaliactivated blast furnace slag powder. While the hydrolysis rate of CaO is faster than that of MgO.
A higher water-binder ratio leads to increasing the drying shrinkage rate. The activated blast furnace slag powder with CaO shows a higher drying shrinkage rate. The measuring temperature can lead to decreasing the mechanical strengths of activated blast furnace slag powder.

Data Availability Statement:
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest:
The authors declare no conflict of interest.