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Article

Mutual Activation Mechanism of Cement–GGBS–Steel Slag Ternary System Excited by Sodium Sulfate

by
Jiuwen Zhu
1,
Hongzhi Cui
1,
Lingzhi Cui
2,
Shuqing Yang
1,
Chaohui Zhang
1,2,
Wei Liu
1 and
Dapeng Zheng
1,*
1
Key Laboratory for Resilient Infrastructures of Coastal Cities (MOE), College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, China
2
Shenzhen Metro Group Co., Ltd., Shenzhen 518026, China
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(3), 631; https://doi.org/10.3390/buildings14030631
Submission received: 11 January 2024 / Revised: 1 February 2024 / Accepted: 6 February 2024 / Published: 28 February 2024
(This article belongs to the Section Building Materials, and Repair & Renovation)
Browse Figures
Versions Notes

Abstract

:
To promote the large-scale recycling of solid waste, the hydration characteristics of blended cement with different amounts of GGBS (ground granulated blast-furnace slag) and SS (steel slag) were investigated. The optimum blending amounts of GGBS and SS in cement were 40% and 10% by mass, and the optimum dosage of Na2SO4 in the C50-S40-SS10 (50 wt.% cement–40 wt.% slag–10 wt.% steel slag) system was 2 wt.%. The flexural and compressive strengths of the C50-S40-SS10 system after adding 2 wt.% Na2SO4 are 57.95% and 9.28% higher than that of pure cement at 28 d. XRD, FT-IR and Ca(OH)2 content analysis were chosen to investigate the hydration products of pure cement and blended cement. The results show that GGBS enhanced the hydration of both cement and SS. And GGBS contributed to the generation of calcium silicoaluminate hydrate (C–A–S–H) in the blended cement system. The addition of Na2SO4 promoted the hydration reaction and contributed to the generation of ettringite (AFt) in the ternary system. The hydration heat evolution results showed that GGBS and SS can reduce the hydration heat of cement. Na2SO4 had similar effects and delayed the time of AFt conversion to monosulfide calcium sulphoaluminate (AFm). A mutual activation mechanism of cement–GGBS–SS ternary system mixed with Na2SO4 was proposed in this study.

1. Introduction

Ground granulated blast-furnace slag (GGBS), which is one kind of by-product of steel manufacturing, has been used as a kind of supplementary cementitious material (SCMs) for more than one hundred years [1]. On the one hand, replacing part of the cement with GGBS can reduce the carbon emission in the cement production process. On the other hand, the activated SiO2 and Al2O3 in GGBS will decompose under alkaline conditions and then react with Ca(OH)2 to form cementitious material. The pozzolanic reactions of GGBS can promote the hydration of cement and generate additional calcium silicate hydrate (C–S–H) and aluminum substituted calcium silicate hydrate, i.e., calcium silicoaluminate hydrate (C–A–S–H) gel, leading to an improvement in gelation and the structural density of cement [2,3,4,5,6,7]. Previous studies have achieved better excitation by combining GGBS with other materials or activators [4,8,9,10,11,12]. Recently, it has been studied that the synergistic effect of alumina-enriched mineral admixtures (such as GGBS) and limestone has been shown to maximize the contribution of admixtures to cement gelling properties [13].
As a kind of by-product of the steel industry, most of the SS is used as building materials or subgrade materials [14,15,16]. Similar to cement, SS contains calcium silicate (i.e., dicalcium silicate (C2S) and tricalcium silicate (C3S)), tetracalcium aluminoferrite (C4AF) and other active mineral components [17]. Compared with cement, the reactive activity of SS is relatively lower because of its slow cooling process and high content of inert components [18]. In addition, it is important that the chemical composition of SS varies greatly with the changes in raw materials and processes, which is unfavorable for the actual promotion and application of SS [17,19,20]. This also adds difficulty to the study of GGBS–SS composite cementitious materials.
At present, some researchers have explored the use of mechanical grinding and the chemical excitation method to improve the slag activity in order to achieve a more efficient use of SS [18,19,21,22]. As shown in Figure 1, the current study is limited to the exploration of the composite effect of GGBS–SS [22,23,24], cement–GGBS [5,6,7,9,10,11] or cement–SS [14,15,16,19,20,25]. The pozzolanic reactions of active silicon aluminum oxide in GGBS can promote the hydration of C3A, C3S, C2S and C4AF in cement and SS, so that SS and GGBS can interact with each other to enhance the hydration rate and increase the amount of C–S–H precipitation. Ultra-fine SS can improve the density of cement and has little influence on its mechanical properties when the replaced content is low [18]. In this way, GGBS and SS can be used as cement admixtures for large-scale recycling. Studies have shown that GGBS can make up for the deficiency of SS and a suitable GGBS/SS ratio is conducive to the development of concrete strength and helpful to control the hydration heat release to a lower range [23,24]. However, the reaction mechanism of the synergistic effect of SS and GGBS is still unclear, and the excitation mechanism of the chemical activator also needs further exploration when GGBS and SS are used in concrete.
Typically, activators are important influencing factors on the performance of alkali-activated composite materials [26,27]. Traditional activators, such as NaOH [28] and Na2SiO3 [29], have been widely used for slag-containing blended cement. However, due to the high hidden energy and carbon content brought by the production of NaOH and Na2SiO3 [30], and the high cost and high corrosiveness [31], their widespread application is limited. As a naturally occurring industrial by-product with an uncomplicated production process, Na2SO4 is safe, cheap and highly sustainable. Therefore, understanding the activation effect and mechanism of Na2SO4 is crucial for promoting its research and application progress. In this research, GGBS and a kind of hot stew SS are selected to explore the mutual activation mechanism of the components in the cement–GGBS–SS ternary system. Na2SO4 is used as an activator in the system to explore its excitation effect and mechanism.

2. Experimental Details

2.1. Materials

According to the provisions of GB8076-2008 [32] and ASTM C150-2012 standards [33], ordinary Portland cement (OPC), which had a specific surface area of 330.5 m2/kg, was used in this study. The GGBS used in this work was supplied by Dunshi new building materials Co., Ltd. (Tangshan, China). and had a specific surface area of 537.5 m2/kg. The SS supplied by Shanghai Metallurgical Environmental Engineering Co., Ltd. (Shanghai, China). was a type of hot stew SS, which had a specific surface area of 675.3 m2/kg. To obtain the chemical compositions of cement, GGBS and SS, an X-ray fluorescence spectroscopy (XRF, S4-Explorer, Bruker, Saarbrucken, Germany) was employed. The results are summarized in Table 1.
According to Table 1, the main components of GGBS are SiO2, Al2O3 and calcium magnesium oxide, and the basicity index of GGBS can be calculated:
M = (CaO + MgO)/(SiO2 + Al2O3) = 0.83 < 1.0
where M is the basicity index. The basicity index represents the activity of the GGBS [34]. Equation (1) demonstrates that the GGBS used in this work is acidic slag. In addition, the chemical composition in Table 1 shows that SS contains much higher Fe2O3 and much lower CaO than cement.
X-ray diffractograms (XRD) and SEM images of cement, slag and SS are summarized in Figure 2. Figure 2 shows that the GGBS composition is relatively simple, mainly containing SiO2, Al2O3 and other oxides, which is consistent with the conclusion of XRF, and the particles have an irregular shape, a dense structure and a smooth surface. The main mineral components of cement are C3S, C2S, C3A, C4AF and CaSO4, and cement has a surface topography similar to GGBS. In contrast, SS particles have a slightly loose structure and a rough surface, and the mineral composition of SS is more complex. The most abundant active mineral composition in SS is C2S, along with a small amount of C3S and C4AF. In addition, a large number of inert components, such as the solid solution of the RO phase (continuous solid solution formed by MgO, FeO and MnO), is present in SS, which is an inert component and the main reason for restricting the large-scale application of SS. In this experiment, some inert or negligible trace components were excluded, and the experimental focus was placed on these active mineral components that account for a large proportion and are involved in the mutual activation process in the ternary system. Add the Na2SO4 activator to the system to study the mutual activation mechanism between the activator and components.

2.2. Cement Pastes

In order to obtain the best proportion of GGBS and SS in cement, the blended cement pastes with different levels of replacement by weight are shown in Table 2. Eleven mixtures were prepared by varying the GGBS and SS contents with the water–solid ratio (w/b) maintained at 0.4. The preparation of the composite cement paste was based on the DBJ/T13-196-2014 standard [35]. The mechanical properties of the samples are tested after curing to different ages under the standard condition.

2.3. Test Methods

The mineral components of the hydration product were explored by X-ray diffraction (XRD, D8 Advance, Bruker) with Cu Kα radiation (λ = 1.5405 Å). The XRD uses a D max/RB diffractometer. The scanning range varied between 10° and 70° with a scanning rate of 0.1°/s.
A Nicolet 6700 Fourier Transform Infrared (FT-IR) Spectrometer was employed to analyze the chemical bond changes in the hydration products in different systems. The scanning range was from 400 to 4000 cm−1 by 32 times per measurement.
A Quanta TM 250 FEG Environmental Scanning Electron microscope (ESEM) and Energy Dispersive Spectrometer (EDS) were used to observe the changes in the surface morphology and mineral composition of hydration products.
The isothermal calorimetry of different systems was conducted on 5.0 g of paste prepared with a 0.4 w/b ratio. The heat of the hydration reaction was measured continuously for 72 h at 25 °C by an isothermal calorimeter (TAM Air 8-channel, TA Instruments, Newcastle, WA, USA).
The mechanical properties of hardened cement paste were in accordance with GB/T 17671-1999 (ISO method) [36].
The hydration degree of cement with different admixtures was tested by the TG/DTA6300 thermogravimetric analyzer (TG STA 409PC, NETZSCH-Gerätebau GmbH, Selb, Germany). The 30–1000 °C temperature range was used for this test and the heating rate was set at 10 °C/min.

3. Results and Discussion

3.1. Mechanical Properties of Composite Cement

Figure 3 and Figure 4 show the mechanical properties of composite cement. The flexural strength shown in Figure 3 has different degrees of decline after mixing with different contents of GGBS or SS in cement. However, when the substituted amount of GGBS and SS are 40 wt.% and 10 wt.%, respectively, the flexural strength of the composite cement reaches the maximum at any hydration age. The compressive strength shown in Figure 4 also has different degrees of decrease after mixing with different contents of GGBS or SS in cement at an early hydration age (3d and 7d). However, in the later stage of hydration, the compressive strength shows a slight increase when the GGBS content is 40 wt.% and the SS content is 10 wt.%. It can be indicated that the optimum content of GGBS and SS in cement are 40 wt.% and 10 wt.% in this study.
With the aim of obtaining the optimum amount of Na2SO4 in the C50-S40-SS10 system, different amounts of Na2SO4 were added to this system (as shown in Table 3), and the mechanical properties were tested after curing to a certain age under standard conditions (Figure 5 and Figure 6). Figure 5 shows that the flexural strength of the C50-S40-SS10 system is obviously improved after the addition of Na2SO4. The flexural strength of the blank group is 8.8 MPa, and when the dosage of Na2SO4 is 2%, it increases by 57.95% to 13.9 MPa. In addition, when the dosage is higher than 2%, the growth rate of the flexural strength becomes slower. Figure 6 shows that with the increase in the amount of Na2SO4, the compressive strength of the C50-S40-SS10 system showed a rising trend at the early age. When the Na2SO4 content is 2%, the compressive strength of the system reaches its highest at 28 d (69.5 MPa), which is 9.28% higher than that of pure cement. Thus, 2% is chosen to be the optimum amount of Na2SO4 in the C50-S40-SS10 system.
The strength of pure cement, composite cement and composite cement with different contents of Na2SO4 was analyzed synthetically. In order to clarify the mutual activation mechanism of each component in the C50-S40-SS10-N2 system with the best mechanical performance in the ternary system and explore the excitation effect and mechanism of Na2SO4, a comparative analysis of microscopic experiments was conducted. The following five groups of samples were selected: pure cement, C50-S50, C50-SS50, C50-S40-SS10 and C50-S40-SS10-N2.

3.2. XRD Results of Hydration Products

The order of the hydration reaction rates of the four main mineral components in cement is C3A > C3S > C4AF > C2S. Therefore, after the mixing of cement and water, the hydration reactions of the four mineral components are shown in Equations (2)–(5) [37]. Figure 7 shows the XRD patterns of different system hydration products curing for 3 d and 28 days in the range of 6–14°, which shows the impact of the different contents of GFBS and SS on the evolution of AFt and AFm phases. Figure 7a shows that AFt is contained in the hydrated product of pure cement at 3 d, which is due to the presence of about 5 wt.% CaSO4 in pure cement. As shown in Equation (6), the C3AH6 produced by Equation (2) will react with CaSO4 to produce ettringite (AFt) [38].
3CaO·Al2O3 + nH2O → C4AH13 + C2AH8 → C3AH6
C3S + nH2O → xCa2+ + H2SiO42− + 2(3 − x)OH− → xCaO·SiO2·yH2O + (3 − x)Ca(OH)2
4CaO·Al2O3·Fe2O3 + 7H2O → 3CaO·Al2O3·6H2O + CaO·Fe2O3·H2O
C2S + nH2O → xCa2+ + H2SiO42− + 2(2 − x)OH− → xCaO·SiO2·yH2O + (2 − x)Ca(OH)2
3C3AH6 + 3CaSO4 +26H2O → 3CaO·Al2O3·3CaSO4·32H2O (AFt)
Equation (1) indicates that the GGBS used in the experiment is a kind of acidic slag, and its activity is more likely to be excited under alkaline conditions. Studies have shown that, under the effect of OH−, Si–O and Al–O bonds can break down, then react with Ca2+ to produce C–S–H gel and C–A–S–H gel [9,39]. The corrosion of active SiO2 by Ca(OH)2 is shown in Equations (7) and (8) [12]. Equation (7) is the neutralization reaction of silicon hydroxyl groups of GGBS to produce C–S–H, and Equation (8) indicates that the internal Si–O bond is gradually destroyed and the [SiO4]4− structure is disintegrated.
≡Si-OH + Ca2+ + H2O → xCaO·SiO2·yH2O
≡Si-O-Si≡ + OH− → 2≡Si-OH
Ca(OH)2 generated by Equations (3) and (5) in the system continuously dissolves Ca2+ and OH−. OH− is retained in the pore fluid to maintain a higher pH, and Ca2+ reacts with ≡Si-OH to generate C–S–H, so that the reaction is carried out continuously.
The aluminum oxide contained in the GGBS can react with Ca2+ in the solution to produce C3AH6 under alkaline conditions (as shown in Equations (9) and (10)) [14]. In addition, the silicon aluminum oxide in GGBS can dissolve in the alkaline environment in cement to generate the C–A–S–H gel (as shown in Equation (11)).
Al2O3 + OH− → H3AlO42−
H3AlO42− + Ca2+ + OH− → C3AH6
Si-O-Al + Ca2+ + OH− → C-A-S-H
When cement is replaced by 50 wt.% of the GGBS, the content of CaSO4 in the system is reduced and the amount of C3AH6 is relatively increased. As shown in Equation (12), the AFt will react with the C3AH6 produced by reaction (2), (4) and (9) to generate AFm [38]. The XRD patterns of 3 d and 28 d paste samples clearly showed reflection peaks for AFm at about 10.7° (as shown in Figure 7).
3CaO·Al2O3·3CaSO4·32H2O (AFt) + C3AH6 → 3CaO·Al2O3·CaSO4·12H2O (AFm)
Wang et al. found that SS showed a higher reaction rate in the later stage of hydration, which can effectively accelerate the later hydration reaction in the blended cement system [23]. The hydration of SS is similar to pure cement (as shown in Equations (3)–(5)). Therefore, CaSO4 content decreased when the cement is replaced by 50 wt.% SS, and due to the low hydration rate of SS, the reaction of C50-SS50 is mainly based on the hydration of cement at the early stage. Figure 7 shows that more AFt in the hydration products was found in the C50-SS50 system at 3 d, while the amount of AFt was slightly decreased at 28 d. This may be due to the fact that the system does not have enough CaSO4, which leads to the conversion of AFt to AFm in the late stage of hydration.
Compared with C50-S50, the dosage of GGBS is slightly lower in the ternary system. In addition, Figure 7a shows that the hydration product of the ternary system (without Na2SO4) at 3 d has a weak reflection peak standing for AFt at 9.1° and a strong reflection peak of AFm at 10.7°, which proves that there was still a certain amount of AFt that was not converted to AFm at 3 d. When the hydration age reached 28 d, the AFt reflection peak disappeared, indicating that all AFt had transformed into AFm (Figure 7b).
Na2SO4 will precipitate with Ca(OH)2 to generate CaSO4 and NaOH (Equation (13)) after it was added in the ternary system, which improves the alkalinity of the system and contributes to the depolymerization of the [SiO4]4− and the [AlO4]4− [40].
Na2SO4 + Ca(OH)2 → CaSO4 + NaOH
The CaSO4 produced by the reaction of Equation (13) has better dispersibility than the original CaSO4 in cement, and is easier to react with C3AH6 to generate AFt. When Na2SO4 is added into the ternary system, at the early stage of hydration, the SO42− and Ca2+ in the composite cement pore solution can react with the aluminum oxide to form AFt [14]. And this chemical reaction is shown as Equation (14), which also contributes to the increase in the amount of AFt in the ternary system (Figure 7b).
Al2O3 + Ca2+ + OH− + SO42− → 3CaO·Al2O3 ·3CaSO4 ·32H2O (AFt)
≡Si-OH + NaOH → 2≡Si-O-Na + H2O
≡Si-O-Si≡ + 2NaOH → 2≡Si-O-Na
The erosion of [SiO4]4− by NaOH is shown in Equations (15) and (16) [41]. In the ternary system, the Ca(OH)2 generated by Equations (3) and (5) is continuously eluted Ca2+ and OH−. ≡Si-O-Na is soluble in the void solution, so Na+ can be replaced by Ca2+ to generate the C–S–H precipitate, while OH− is retained in the pore fluid to maintain a higher pH, so that the above reaction is carried out continuously.
As the hydration reaction of SS is slow, the GGBS in the ternary system will still consume the Ca(OH)2, which was produced by SS hydration, to promote the hydration of the ternary system. Because of the existence of cement, for the generated C–S–H, a good gelation can be presented. In addition, the C–A–S–H generated by the pozzolanic reactions of GGBS and the constant formation of AFt due to the presence of Na2SO4 can effectively fill the gap between the gel and the unhydrated particles [42]. At the later stage of hydration, this is also the main cause to enhance the mechanical properties of the ternary system.

3.3. FT-IR Analysis of Hydration Products

The FT-IR spectra of different samples curing for 3 d and 28 d are presented in Figure 8. The broad peak at about 3430 cm−1 as well as the narrow peak at about 1634 cm−1 for all samples are related to the tensile vibrations of the O–H bonds in chemical-bound H2O and the H–O–H bending vibrations in the interlayer H2O [38,43]. It can be seen from Figure 8 that all the samples exhibit an obvious and wide peak at 973 cm−1, which stands for the asymmetric stretching vibrations of Si–O bonds or Al–O bonds in C–(A)–S–H [8,44]. Compared with the pure cement and binary system samples, the vibration band at 973 cm−1 for the ternary system samples was slightly lower at 3 d, and then increased to a higher wavenumber at 28 d of curing. This indicates that the ternary system, especially after the addition of Na2SO4, increases the dosage of Al in C–(A)–S–H and reduces the Ca/Si ratio during the 3 days of hydration. During the hydration period from 3 d to 28 d, the polymerization of C–(A)–S–H was improved, and the tetrahedral material was condensed as a result of the improved aluminum content in the gel [45,46]. This indicates that Na2SO4 has a significant effect on the decomposition of activated alumina in GGBS and the generation of C–(A)–S–H gel. The weak vibration band at 465 cm−1 and 789 cm−1 of C50-S50 and the ternary system samples also show the presence of Al elements in the network of cemented materials [47]. The sharp peaks at about 3637 cm−1 for the cement and C50-SS50 system pastes at 3 d and 28 d are related to the stretching vibration of the O–H bond in Ca(OH)2 [48]. In contrast, the reflection peaks at 3637 cm−1 for the three systems mixed with GGBS are relatively weak, indicating that the GGBS has significant consumption on the Ca(OH)2 in system (Figure 8a). The peak at 613 cm−1 for the C50-S40-SS10-N2 system is due to the bending vibration of the S–O bonds in SO42−, and it disappeared at the hydration time of 28 d, indicating that the activation of Na2SO4 on the system is mainly concentrated in the early stages of hydration. The shoulder-like vibrating band at about 1487 cm−1 (asymmetric stretching of the C–O bond) and the small vibrating band at 870 cm−1 (the out-of-plane bending of the C–O bond) indicate the presence of carbonate in the system (Figure 8a) [4].

3.4. Hydration Heat Evolution

The early hydration heat release characteristics of pure cement and blended cement with or without Na2SO4 over the first 72 h of hydration reaction at 25 °C are shown in Figure 9. As reported, the hydration process of pure cement is roughly divided into five stages: (1) hydration induction period; (2) dormant period; (3) hydration acceleration period; (4) hydration deceleration period; (5) hydration stabilization period [18]. A few minutes after the mixture of blended cement and water, the first exothermic peak appeared due to the initial hydration of C3A and C3S as well as the formation of AFt [23]. Then the hydration of blended cement enters the dormant period. The silicate mineral in the cement hydrate reacts to generate C–S–H. A saturation state of Ca(OH)2 concentration can be reached gradually. The concentration of active silica–alumina in the slurry decreases and the rate of hydration and heat release is slow [23]. It can be seen from Figure 9a, compared with pure cement, that the influence of adding 50 wt.% GGBS in cement on the dormant period is limited. The reason for this can be attributed to that the GGBS used in this work is acidic slag (Equation (1)). When the Ca2+ in the system reaches a certain concentration, the GGBS will quickly carry out a volcanic ash reaction, which makes the dormancy period shorter. However, the dormant period of C50-SS50 and the ternary system are significantly longer. It was reported that mixing SS in cement increases the setting time due to the fact that SS extends the dormant period [16,20]. The cause of this phenomenon can be attributed to the low activity of SS [25]. The ternary system with or without Na2SO4 has a longer dormancy period due to the early formation of AFt, which covers the surface of the cementitious material particles and delays the hydration reaction. And a longer setting time facilitates the transportation and pouring of concrete in many cases.
Figure 9a shows that the second exothermic peak of all composite cement material samples is much lower than pure cement. Although the second exothermic peak of SS appeared at the earliest time (at about 4.5 h), the exothermic peak was the lowest (about 0.6 mW/g). The second exothermic peak of the ternary system increased after the addition of Na2SO4, which proved that the accelerator could increase the hydration rate of C2S and C3S in the system. However, the second exothermic peak of the ternary system is lower than that of the C50-S50 system, indicating that the formation of AFt in the hydration induction period has an important influence on decreasing the hydration heat release rate of the binder. The third exothermic peak shown in Figure 9a represents the transformation of AFt to AFm phases in the system [5]. This is clearly shown in Figure 9a, as the transformation of AFt to AFm phases occurred earlier in C50-S50 and C50-S40-SS10 than that of C50-S40-SS10-N2. This proves that Na2SO4 contributes to the formation of AFt in the system and delays its transformation to AFm, which is consistent with the phenomenon obtained in Section 3.2.
Compared with pure cement, Figure 9b shows a reduction in the cumulative hydration heat for all blend cement material samples. The cumulative hydration heat of pure cement in 72 h is 289 J/g. The C50-SS50 system has the lowest cumulative hydration heat, which is about half that of pure cement [16]. This phenomenon is mainly due to the lower hydration activity of SS. In addition, the cumulative hydration heat of the ternary system mixed with Na2SO4 in 72 h is 239 J/g, which is significantly lower than pure cement. It is proved that adding mineral admixture in the cement, even if the chemical activator is added, will help to decrease the hydration heat of the cement, which can reduce the temperature shrinkage of the concrete structure, thereby preventing its cracking [23,49]. The excellent property of decreasing the hydration heat of GGBS and SS will contribute to its widespread use in concrete [49,50].

3.5. SEM and TG Analysis of Hydration Products

Figure 10 shows SEM images of the hydration products of different samples after 28 days of curing. It can be seen that the flaky Ca(OH)2, the needle-shaped AFt and dense C–S–H gel are cross-linked to each other. It is obvious that pure cement contains more closely packed flake Ca(OH)2. And compared with the sample containing GGBS, Figure 10c shows that the hardened paste of C50-SS50 has a looser structure. In addition, compared to Figure 10d, Figure 10e shows that the hardened paste is more compact and uniform after the addition of NaSO4. In fact, the SEM results can only be used to generally determine the composition and structure of the hardened paste. Therefore, the TG experiment will compare the hydration degree of each group by quantifying the content of Ca(OH)2 in the hardened paste.
As shown in Figure 11, during the various periods of hydration, the Ca(OH)2 content in pure cement was the highest in five groups, followed by the C50-SS50 system. And the development of the Ca(OH)2 content in these two groups increases with the growth of the hydration age; however, the rate of increase is gradually flattened. When 50 wt.% GGBS is added in cement, the Ca(OH)2 dosage decreases with the growth of the age and reaches the lowest content among the five groups after 21 days of hydration. This proves that the pozzolanic reactions of GGBS can consume Ca(OH)2 with a higher rate than the Ca(OH)2 formation by cement hydration.
Figure 11 shows that the content of Ca(OH)2 in the C50-S40-SS10 system showed a tendency to decrease first and then increase with the growth of the hydration age, which proves that the Ca(OH)2 consumption rate is higher than its formation rate in the early age of the ternary system. At a later stage of hydration, the pozzolanic reactions of GGBS slowed down and the hydration reaction of SS accelerated, which improved the Ca(OH)2 dosage in the ternary system. After adding Na2SO4 in the ternary system, the Ca(OH)2 dosage also showed a trend of decreasing first and then increasing with the growth of the hydration age. However, the content of Ca(OH)2 is lower than that of the ternary system without Na2SO4 in all hydration ages. It is proved that Na2SO4 promotes the hydration reaction of GGBS in the ternary system at all hydration ages.
For pure cement and the C50-SS50 system, the fitting curve of the Ca(OH)2 content changing with the hydration age is presented in Figure 12. It can be found that, for pure cement and the C50-SS50 system, the Ca(OH)2 dosages have a good power function relationship with the hydration age. The relationship between Ca(OH)2 content and the hydration age in pure cement is:
Y = 13.22296X0.08132 (R2 = 0.98746)
And the relationship between Ca(OH)2 content and the hydration age in C50-SS50 system paste is:
Y = 8.08059X0.17708 (R2 = 0.98159)
Derived on both sides of Functions (17) and (18), it can be seen that the growth velocity of Ca(OH)2 dosage in pure cement is dy/dx = 1.08x−0.91868. In addition, the Ca(OH)2 growth rate in the C50-SS50 system paste is dy/dx = 1.43x−0.82292. It can be proved that the Ca(OH)2 growth rate in pure cement is higher than that of the C50-SS50 system at the early stage of hydration, but the growth rate of the Ca(OH)2 content of C50-SS50 exceeds that of pure cement after a certain time. This also proves that SS has a catalytic influence on the hydration of the ternary system at the late stage.

3.6. Hydration Mechanism

The mutual activation mechanism of the cement–GGBS–SS ternary system mixed with Na2SO4 is summarized in Figure 13. Figure 13 shows that C–S–H gel and C–F–H can be generated by the hydration reactions of C3S and C2S in cement and SS. At the same time, Ca(OH)2 can be provided for pozzolanic reactions of GGBS. In turn, the continuous consumption of Ca(OH)2 can promote the cement hydration reaction, especially SS hydration in the later stages. In addition, the decomposition and recombination reactions of the active silica–alumina oxide in GGBS provide additional C–S–H and C–(A)–S–H gel, making the structure of the ternary system more compact. The C3A in cement is rapidly hydrated to produce C–A–H after mixing with water, which then reacts with calcium sulfate to produce AFt. The C–A–H produced by the reaction of Al2O3 in the GGBS with Ca(OH)2 also reacts with calcium sulfate to produce AFt; however, the time of the reaction is relatively late. Moreover, AFt will be converted into AFm in the later stages of hydration if the content of calcium sulfate in the ternary system is not enough. The addition of Na2SO4 provides Na+ and SO42− with high dispersibility into the ternary system. The presence of Na+ also increases the pH of the system, and the SO42− can react with the Ca2+ in the system to produce CaSO4, which is more dispersible, making it easier to react with C–A–H to generate AFt.

4. Conclusions

The mutual activation mechanism of the cement–GGBS–SS ternary system mixed with Na2SO4 was experimentally studied and summarized in this research. The main conclusions can be summarized as follows:
The optimum blending amount of GGBS and SS in cement is 40 wt.% and 10 wt.%, respectively. The compressive strength of the composite cement is slightly higher than pure cement. The optimum blending amount of Na2SO4 in the C50-S40-SS10 system is 2%. The flexural strength of the C50-S40-SS10 system increases by 57.95% to 13.9 MPa, and the compressive strength is 9.28% higher than that of pure cement at 28 d.
Based on the analysis of AFt, AFm and C–A–S–H gel in products of the hydration reactions by XRD and FT-IR, the hydration process of C3A in cement, the pozzolanic reactions of activated alumina in GGBS and the promoting effect of Na2SO4 on the formation of AFt in the ternary system were confirmed.
The comparative analysis of the hydration heat evolution and Ca(OH)2 dosage of pure cement and blended cement confirms the effect of GGBS on Ca(OH)2 consumption and the improvement of the hydration reaction of cement and SS. The enhancement effect of Na2SO4 on pozzolanic reactions and its influence on the hydration heat evolution rate of the ternary system were also confirmed.

Author Contributions

Data curation, J.Z., L.C. and S.Y.; Formal analysis, S.Y., C.Z. and D.Z.; Funding acquisition, H.C. and D.Z.; Methodology, H.C., W.L. and D.Z.; Supervision, H.C. and W.L.; Writing—original draft, J.Z.; Writing—review and editing, H.C., L.C., C.Z., W.L. and D.Z. All authors have read and agreed to the published version of the manuscript.

Funding

We sincerely appreciate the funding support from the National Natural Science Foundation of China (No.: 52370145), Shenzhen Key Project of Basic Research (No.: JCYJ20200109114203853) and Shenzhen Metro Group Co., Ltd. (No.: SZDT-JSZX-ZC-2020-0022).

Data Availability Statement

The data presented in this study are available in the article.

Conflicts of Interest

Authors Lingzhi Cui and Chaohui Zhang were employed by the company Shenzhen Metro Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. The scope of the previous studies.
Figure 1. The scope of the previous studies.
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Figure 2. X-ray diffractograms and SEM images of cement, GGBS and steel slag.
Figure 2. X-ray diffractograms and SEM images of cement, GGBS and steel slag.
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Figure 3. Flexural strength of pure cement and cement with different contents of GGBS and steel slag.
Figure 3. Flexural strength of pure cement and cement with different contents of GGBS and steel slag.
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Figure 4. Compressive strength of pure cement and cement with different contents of GGBS and steel slag.
Figure 4. Compressive strength of pure cement and cement with different contents of GGBS and steel slag.
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Figure 5. Flexural strength of pure cement and C50-S40-SS10 paste with different contents of Na2SO4.
Figure 5. Flexural strength of pure cement and C50-S40-SS10 paste with different contents of Na2SO4.
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Figure 6. Compressive strength of pure cement and C50-S40-SS10 paste with different contents of Na2SO4.
Figure 6. Compressive strength of pure cement and C50-S40-SS10 paste with different contents of Na2SO4.
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Figure 7. XRD patterns of samples: (a) hydration for 3 d; (b) hydration for 28 d.
Figure 7. XRD patterns of samples: (a) hydration for 3 d; (b) hydration for 28 d.
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Figure 8. FT–IR spectra of samples: (a) hydration for 3 d; (b) hydration for 28 d.
Figure 8. FT–IR spectra of samples: (a) hydration for 3 d; (b) hydration for 28 d.
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Figure 9. Hydration heat evolution of pure cement and blend cement: (a) hydration heat evolution rate; (b) cumulative hydration heat.
Figure 9. Hydration heat evolution of pure cement and blend cement: (a) hydration heat evolution rate; (b) cumulative hydration heat.
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Figure 10. SEM of hydration products of different samples after 28 days of curing: (a) pure cement, (b) C50-S50, (c) C50-SS50, (d) C50-S40-SS10, (e) C50-S40-SS10-N2.
Figure 10. SEM of hydration products of different samples after 28 days of curing: (a) pure cement, (b) C50-S50, (c) C50-SS50, (d) C50-S40-SS10, (e) C50-S40-SS10-N2.
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Figure 11. Changes in Ca(OH)2 content with hydration age.
Figure 11. Changes in Ca(OH)2 content with hydration age.
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Figure 12. Fitting curve of Ca(OH)2 content in pure cement and C50-SS50 system.
Figure 12. Fitting curve of Ca(OH)2 content in pure cement and C50-SS50 system.
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Figure 13. Hydration reaction mechanism of cement-slag-steel slag system activated by Na2SO4.
Figure 13. Hydration reaction mechanism of cement-slag-steel slag system activated by Na2SO4.
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Table 1. Chemical composition of materials.
Table 1. Chemical composition of materials.
MaterialsSiO2Al2O3Fe2O3CaOMgOK2OMnOP2O5TiO2SO3
OPC18.74.223.3366.331.970.854——0.110.272.77
GGBS35.814.40.6934.96.571.010.24——0.661.86
Steel slag10.782.3230.2243.66.550.0242.011.760.50.25
Table 2. Blended cement pastes with different content of GGBS and steel slag.
Table 2. Blended cement pastes with different content of GGBS and steel slag.
Cement (%)Slag (%)Steel Slag (%)
C10010000
C50-S5050500
C50-S45-SS550455
C50-S40-SS10504010
C50-S35-SS15503515
C50-S30-SS20503020
C50-S25-SS25502525
C50-S20-SS30502030
C50-S15-SS35501535
C50-S10-SS40501040
C50-S5-SS4550545
C50-SS5050050
Table 3. Pure cement and C50-S40-SS10 system with different contents of Na2SO4.
Table 3. Pure cement and C50-S40-SS10 system with different contents of Na2SO4.
Cement (%)GGBS (%)Steel Slag (%)Na2SO4 (%)
C100100000
C50-S40-SS105040100
C50-S40-SS10-N0.55040100.5
C50-S40-SS10-N15040101
C50-S40-SS10-N1.55040101.5
C50-S40-SS10-N25040102
C50-S40-SS10-N2.55040102.5
C50-S40-SS10-N35040103
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Zhu, J.; Cui, H.; Cui, L.; Yang, S.; Zhang, C.; Liu, W.; Zheng, D. Mutual Activation Mechanism of Cement–GGBS–Steel Slag Ternary System Excited by Sodium Sulfate. Buildings 2024, 14, 631. https://doi.org/10.3390/buildings14030631

AMA Style

Zhu J, Cui H, Cui L, Yang S, Zhang C, Liu W, Zheng D. Mutual Activation Mechanism of Cement–GGBS–Steel Slag Ternary System Excited by Sodium Sulfate. Buildings. 2024; 14(3):631. https://doi.org/10.3390/buildings14030631

Chicago/Turabian Style

Zhu, Jiuwen, Hongzhi Cui, Lingzhi Cui, Shuqing Yang, Chaohui Zhang, Wei Liu, and Dapeng Zheng. 2024. "Mutual Activation Mechanism of Cement–GGBS–Steel Slag Ternary System Excited by Sodium Sulfate" Buildings 14, no. 3: 631. https://doi.org/10.3390/buildings14030631

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