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Article

The Early Age Hydration Products and Mechanical Properties of Cement Paste with Steel Slag Powder as Additive under Steam Curing Conditions

by
Baoliang Li
1,*,
Xue Lu
1,
Binbin Huo
2,
Yuheng Du
1,
Yuyi Liu
1,
Yongzhen Cheng
1 and
Zejun Liu
1
1
Faculty of Architecture and Civil Engineering, Huaiyin Institute of Technology, Huai’an 223001, China
2
State Key Laboratory of Coal Resources and Safe Mining, School of Mines, China University of Mining and Technology, Xuzhou 221116, China
*
Author to whom correspondence should be addressed.
Buildings 2023, 13(9), 2192; https://doi.org/10.3390/buildings13092192
Submission received: 10 August 2023 / Revised: 25 August 2023 / Accepted: 26 August 2023 / Published: 28 August 2023
(This article belongs to the Section Building Materials, and Repair & Renovation)
Browse Figures
Versions Notes

Abstract

:
To explore the feasibility of the application of steel slag powder (SSP) in steam-cured precast concrete, 0% and 20% SSP were used to replace cement and prepare cement paste, and the early age performance of steam-cured (80 °C for 7 h and 7 d) SSP-blended cement paste, including different types and amounts of hydrates, the microstructure and mechanical properties were investigated and compared with those of 28 d standard-cured SSP sample. The results show that SSP addition promotes the generation of laminar C-S-H gels and granular C-S-H gels after an initial 7 h steam curing. Further extending the lasting time of 80 °C steam curing to 7 days favors the production of hydrogarnet and crystalline C-S-H, of which the amount of formation of hydrogarnet in SSP composite cement paste is less and the particle size is smaller than those in the control sample. However, steam curing increases the gap between the number of hydrates formed in SSP-blended cement paste and the control paste. The delayed hydration effect of SSP on cement offsets the promoting effect of steam curing on the hydration of cement; in consequence, the incorporation of SSP seems to be detrimental to the hydration of steam-cured cement paste.

1. Introduction

Steel slag is a kind of industrial metallurgical slag coming from the steel-making industry, which makes up 12–20% of steel production [1]. The annual emission of steel slag reaches a hundred million tons in China [2], while its utilization rate is less than 30% [3]. Abundant unprocessed steel slags not only occupy farmland but also cause environmental pollution [4].
Steel slag with a high angularity and rough surface texture has excellent mechanical properties and is rich in calcium, silicon and iron, which make the steel slag very suitable for use as a substitute to aggregate and supplement cementitious material (SCM) in concrete [5]. The existence of substantial dense oxides (e.g., CaO, SiO2, Fe2O3, FeO) endow steel slag with a high density [5,6]; therefore, concrete with steel slag aggregate (SSA) has high strength, while the existence of conductive Ferrite in the form of FeO/Fe3O4/Fe makes SSA concrete have a high conductivity [7,8]. Li et al. [9] found that SSA concrete exposed to high temperatures (400~600 °C) shows a relatively high residual strength due to the inherent stability of SSA. In addition, the rough surface of SSA helps to improve the bonding ability in the interfacial transition zone (ITZ) [5,8,10]. It has been reported that SSA can also improve the shielding effect [11], chloride ion resistance [12], freeze-thaw resistance [13] and sulfate resistance of concrete [14]. However, the use of untreated steel slag to replace all or part of the aggregate is still limited strictly in China due to its poor volume stability related to dead-burnt free-CaO (f-CaO) and free-MgO (f-MgO) and alkaline leachates [15], which is unfavorable for use on site.
Now, the major application of steel slag in concrete is as a mineral admixture. Steel slag has a potential hydraulic property due to the presence of cement clinker phase (e.g., C3S and C2S). However, the hydration activity of steel slag is much poorer than cement clinker [16,17], owing to its much slower cooling rate.
Generally, the steel slag powder (SSP) addition (less than 30%) is beneficial to the improvement of workability. Further increasing the amount of SSP does not significantly improve the fluidity, but it will significantly reduce the slump loss of concrete due to the low reactivity of SSP. Pan et al. [18] found that self-compacting concrete with 10% SSP shows excellent mechanical properties, resistance to chloride penetration and carbonation. Guo et al. [19] identified that concrete containing SSP can reduce the drying shrinkage and improve the resistance to abrasion.
However, the use of SSP as SCM will reduce the mechanical properties of concrete. To enhance the hydration reactivity of SSP, autoclave curing [20], mechanical grinding [21], carbonation [22], alkali activation [23,24] and acid activation [25,26] are the commonly used treatment methods in the laboratory. Autoclave curing [20] could accelerate the hydration of dead-burnt f-CaO and f-MgO, whose slow reactions are adverse to the volume stability of concrete. Mechanical grinding is more effective in increasing the hydration reactivity of SCM, but the presence of many Fe phases in steel slag undoubtedly increases the energy consumption of grinding; in addition, toxic substances such as Cr, Ni and V could possibly leach during grinding [21]. After carbonation curing, CaCO3 with a dense shell is produced on the surface of the binding material, which can provide more crystal nuclei and hasten the hydration of C3S [22]. Alkaline activation can speed up the hydration of SSP by increasing the nucleation points and the pH value of the liquid phase [23,24]. Modifying the surface of SSP by weak acid (formic acid, phosphoric acid) has been popular among steel slag treatments in recent years, which can increase the surface roughness, thereby improving the specific surface area and hydraulic reactivity of SSP [25,26]. However, the above measures not only increase the disposal costs of SSP but also increase the complexity of the preparation process of concrete, which are not conducive to the popularization and application on site.
Steam curing is not only one of the ways to improve the reactivity of SCM but also a common curing measure to prepare a prefabricated concrete element [27]. In recent years, prefabricated components have attracted more attention for their advantages, e.g., low energy consumption, short construction time, improved construction quality as well as reusability and recyclability of their components [27,28]. Although steam curing improves the demoulding strength of prefabricated components, it also brings internal damage to the concrete, which is detrimental to durability [29,30,31]. To migrate the adverse effects of steam curing, the main measures currently adopted include improving the steam curing system and replacing part of the cement with SCM [27], of which GBFS is the most commonly used SCM for preparing prefabricated concrete components. Under the severe shortage of high-quality SCM, it is not clear whether SSP can replace GBFS as an SCM for preparing precast concrete. Additionally, there is relatively little research regarding the activation effect of steam curing on SSP.
Elevated temperature undoubtedly facilitates the hydration of ordinary cement and SSP, and the high alkali environment generated by rapid cement hydration under steam curing also favors the hydration of steel slag. However, there is little literature regarding the influence of SSP on cement hydration under steam curing, particularly the early age hydrate assemblages, microstructure and mechanical properties. Since concrete is generally transported to the construction site for use immediately after demoulding, the demoulding strength is an important evaluation index for precast concrete. Therefore, the influence of SSP on the early performance of prefabricated components determines whether SSP can be used in the preparation of prefabricated concrete components under steam curing.
For this reason, the early age performance of steam-cured (80 °C for 7 h and 7 d) SSP-blended cement paste, including the types and amounts of hydrates, microstructure and mechanical properties, were investigated and compared with that of a 28 d standard-cured SSP sample. In addition, to better understand the properties of steam-cured SSP-blended cement paste, the performance of the steam-cured GBFS-blended sample from [27] was also compared in this investigation. This investigation revealed that steam curing is more conducive to the hydration of pure cement than that of SSP-blended cement. Further extending the lasting time of steam curing favors the production of hydrogarnet and crystalline C-S-H. These results are helpful for guiding the application of SSP and understanding the action mechanism of SSP in steam-cured concrete.

2. Materials and Methods

2.1. Materials

Portland cement (PC) PII 52.5 and steel slag powder (SSP) from Jiangsu Rongda New Materials Co., Ltd. (Nantong, China) were used in this investigation. The specific surface area of PC and SSP were 356 m2/kg and 442 m2/kg, respectively. Table 1 displays the chemical composition of PC and SSP measured by XRF. It should be noted that compared with the chemical composition of PC, the content of Fe2O3, MgO and Al2O3 in SSP is higher, while the content of CaO is relatively lower.
Figure 1 indicates the cumulative distribution and differential distribution of particle size in PC and SSP, which were obtained by a laser particle-size analyzer. Compared with SSP, the particle size distribution (PSD) range of PC is much wider, which is very consistent with the specific surface area results. However, two peaks appeared in the PSD curves of both PC and SSP; for PC, the peak values are 7 μm and 20 μm, while for SSP, the peak values are 5.5 μm and 20 μm, respectively.
Figure 2 gives the XRD result of SSP. The phases in SSP primarily include C2S, C2F, RO phase (CaO-FeO-MnO-MgO solid solution), Ca(OH)2, CaCO3, f-CaO and f-MgO, besides the amorphous phase. The existence of Ca(OH)2 and CaCO3 is the main reason for the high loss-on-ignition (LOI) of SSP in Table 1.
In addition, ISO standard sand with a fineness modulus of 2.75 and tap water were used.

2.2. Sample Preparation

In order to keep consistent with the experimental conditions and mix proportion of ref. [27], 20% SSP was used to replace cement to prepare cement paste and mortar under three curing conditions including two steam curing conditions and one standard curing condition, of which 80 °C steam curing for 7 days was used in this investigation to activate the reactivity of SSP and keep the similar maturity to standard curing for 28 days (20 °C × 28 days = 560 °C·days) [27,32]. The corresponding mix ratio and curing condition can be seen in Table 2 and Figure 3.
To detect the mechanical properties of specimens, 40 mm × 40 mm × 160 mm mortar prisms with the mix proportion 450 g binder: 1350 g standard sand: 225 g water were prepared according to GB/T 17671-1999 [33] and the same curing condition in Table 2.

2.3. Test Methods

In order to explore the phase composition and microstructure of the sample under different curing conditions, the hydration of the Ref and S20 samples after demoulding was stopped by anhydrous ethanol and then dried in a vacuum-drying oven at 60 °C for 24 h [27]; subsequently, the phase composition and microstructure of paste specimens were characterized by XRD, SEM-EDS and TG-DTG.
A D8-Discover X-ray diffractometer was used to check the phase composition in SSP and paste samples. The corresponding scanning speed and scanning region range were 3°/min and 5~65°, respectively.
A scanning electron microscope (SEM, FEI Sirion, FEI, Eindhoven, The Netherlands) and its accompanying energy dispersive X-ray spectra (EDS) were used to determine the morphology and composition of the phases in hardened pastes.
A TG-DTG test was characterized from 60~950 °C with a speed of 10 °C/min under the condition of N2 atmosphere using STA 449 F3 Jupiter.
Water demand, soundness and setting time were performed based on the standard GB/T 1346-2011 [34] and GB/T 750-1992 [35]. It should be noted that due to the presence of f-CaO and f-MgO (Figure 2) in SSP, two methods of soundness testing, including the Le-Chatelier test and the autoclave expansion test, were considered in this investigation.
The fluidity of fresh mortars was tested based on the standard GB/T 2419-2005 [36], which was characterized by the traditional drop table flow test.
The flexural and compressive strength of mortars were measured based on GB/T 17671-1999 [33], of which the flexural strength was measured with specimens of dimension 40 × 40 × 160 mm, under the loading rate of 50 ± 10 N/s, while the compressive strength was carried out at the loading rate of 2400 ± 200 N/s.

3. Results

3.1. Water Requirement, Soundness and Setting Time

Table 3 shows the impact of SSP on the basic properties of cement slurry, including water requirement, soundness, setting time and fluidity. Although the specific surface area of SSP is much smaller than that of PC, an SSP addition has little influence on the water requirement and the fluidity when the content of SSP is small.
The soundness is one of the problems that must be paid attention to when SSP is used as cementitious material. As shown in Table 3, the Le-Chatelier expansion value of cement paste with SSP is less than 5 mm, which is specified as the limiting value in GB/T 1346-2011 [34]; however, the expansion of SSP composite paste is much larger than that of Ref due to the presence of f-CaO in the SSP. It has been reported that long-term weathering or aging is an effective technique to alleviating the volume stability problem of SSP caused by f-CaO [1]. Additionally, due to the presence of f-MgO (Figure 2) in the SSP, autoclave expansion of SSP-blended cement paste was also considered in this investigation; however, owing to small amounts of MgO (Table 1) in the SSP, the autoclave expansion rate is also much lower than the limiting value (0.8%) of GB/T 750-1992 [35].
In addition, the setting time of S20 is also very noteworthy, which is much longer than that of Ref, whether it is the initial or final setting time. Some of the literature [37,38,39] attributed the reason why steel slag extended the setting time of cement paste to the substantial MgO, MnO and P2O5 distributed in the clinker phase (C2S and C3S), which reduced the hydration reactivity of SSP. In addition, the lower reactivity of SSP may also be related to the dense structure and larger grain size of C3S and β-C2S crystal formed at high temperature, as well as the transformation of β-C2S into non-hydraulic γ-C2S during slow cooling [1]. Zhuang et al. experimentally demonstrated that the presence of C12A7 in SSP (Figure 2) retards the hydration of C3S significantly [40]. C12A7 releases abundant Al3+ that may be adsorbed by the surface of C3S, accordingly, hindering the hydration of the cement [40]. The initial setting time is a key criterion determining the pre-curing time of steam curing [41]. The longer the initial setting time of concrete, the longer the required pre-curing time. Under the same pre-curing time conditions, it can be predicted that steel slag powder may not be conducive to cement hydration under steam curing.

3.2. XRD Analysis of Hydrates

The XRD results of the crystal phases in Ref and S20 are given in Figure 4. The most common crystal hydrate assemblages of hardened cement paste, Ca(OH)2 (CH) and ettringite (AFt), appear in the 28 d standard-cured pure cement paste, apart from the un-reacted cement clinker phase (C3S, C2S and C4AF). Moreover, as limestone powder is a common filler in cement, hemicarbonate (Hc: Ca4Al2O6(CO3)0.5(OH)·11.5H2O) is also formed. When the initial curing temperature of the cement paste increases to 80 °C, the AFt becomes unstable and gradually transforms into monosulfoaluminate (AFm) [27]. Further extending the lasting time of steam curing, hydrogarnet (C3ASH4) begins to form.
Generally, C3AH6 is the most common chemical formula of hydrogarnet, which is formed by the transformation of metastable phases such as CAH10, C2AH8 and C4AH13 [42]. This transformation leads to a reduction in solid-phase volume and an increment in porosity, thus causing a decrease in strength [42]. However, due to the application of Si-rich mineral admixtures, hydrogarnet often exists in the form of C3ASH4 in concrete. Jappy et al. found that Al in hydrogarnet can be partially or completely replaced by Fe, and Si can be completely replaced by 4H; therefore, the phases in the C3A(Fe)H6~C3A(Fe)S3 series are collectively referred to as hydrogarnet [43,44]. Consequently, according to the above theory, it can improve the content of Si-rich SCMs with high reactivity in the cementitious material, and/or it can increase the Si/Al ratio of cementitious material, making it a preferentially generate C2ASH8 phase instead of a C4AH13 phase under steam curing, thereby avoiding the transition from the C4AH13 phase to the hydrogarnet phase [45].
In addition to the formation of monocarbonate (Mc: Ca4Al2O6CO3·11H2O) in the 28 days standard-cured cement paste mixed with SSP, the SSP addition has a relatively small impact on the types of hydrates formed under various curing conditions (Figure 4b). It is worth noting that although f-MgO is found in the SSP, no Mg containing hydrates appeared in S20 cured under various conditions.
The proportion of MgO in SSP is only 6.60% (Table 1), and MgO may exist in the following three forms (Figure 2): the RO phase, f-MgO and amorphous phase. The RO phase is an inert ingredient in SSP that has almost no hydration activity at room temperature; under autoclave curing conditions (215 °C and 2 MPa for 3 h), the hydration reactivity of the RO phase decreases exponentially with the growth of its FeO/MgO molar ratio (X) [46]. When X is greater than 1.5, the RO phase hardly reacts, while when X is smaller than 0.5, the hydration of the RO phase becomes relatively easy, and it will also undergo hydrated expansion [46]. Furthermore, the RO phase also has a negative effect on the hydration of SSP. The RO phase is mainly distributed around C2S, which not only affects the grinding efficiency of SSP but also hinders the hydration of C2S [47]. At the same time, the smooth surface of the RO phase also affects its adhesion with C-S-H gels, thereby reducing the mechanical properties of SSP [48].
f-MgO subjected to high-temperature dead burning has a perfect crystal structure with coarse grains and few crystal defects, resulting in very low hydration reactivity [49]. It has been reported that the higher the sintering temperature, the lower the reactivity of MgO and the longer the hydration process lasts (up to 6–8 years) [50,51]. However, the delayed expansion caused by the formation of Mg(OH)2 derived from f-MgO hydration brings great risks to the volume stability of concrete; accordingly, autoclave curing is often used to evaluate the volume stability caused by f-MgO [52]. Although high-temperature (80 °C) curing can speed up the reaction of lightly burnt MgO [53,54], no Mg-bearing hydrated phases have been found in this investigation, which is related to both the low content of f-MgO and the short steam curing time.
MgO presented in the amorphous phases is similar to MgO presented in GBFS and can react and form hydrotalcite in high alkali environments, thereby helping to improve the performance of concrete [55], but this part of MgO will not undergo hydrated expansion. Similarly, due to the small amounts of amorphous phase MgO in SSP, the formation of hydrotalcite is not detected.

3.3. Phases and Microstructure under SEM-EDS

3.3.1. Phases and Microstructure under S7h

Figure 5 and Figure 6 demonstrate the phases and microstructure of S7h Ref and S20, respectively. The phases in S7h Ref principally include fibriform C-S-H gels and flaky CH (Figure 5a,c). After steam curing for 7 h, the arrangement of fibrous C-S-H is relatively loose. The fibers are long (1–2 μm) and the Ca/Si ratio in C-S-H is large (2.48) (Figure 5b), which reflect that the hydration degree of cement under the S7h condition is low. Additionally, the barbaric growth and uneven distribution of hydrates at high temperatures result in many pores (Figure 5a) that appeared on the sample cross-section; however, the interweaving between the various phases in the cement paste is still relatively dense (Figure 5d), and only a small number of scattered particles adhering to the surface of the sample appear.
As observed in Figure 6, the hydrates in S7h S20 are still primarily fibriform C-S-H (Figure 6a), but compared to that of Ref, its fibers are finer and the interweaving between the fibrous C-S-H is less. In addition, the composition of fibrous C-S-H is also affected by elements such as Al, Fe, K and S in the SSP (Figure 6b). Furthermore, it is worth noting that the SSP incorporation affects the morphology of the hydration products: laminar and granular C-S-H gels (Figure 6c) also appear in S7h S20. By comparing the composition of the fibrous C-S-H (Figure 6b), it is evident that the formation of laminar and granular C-S-H is not only related to the influence of elements such as Al, Fe, K and S but also to its lower Ca/Si ratio (3.02 to 2.20 seen in Figure 6b,d).
By comparing the composition (Figure 5b) of C-S-H in Ref, it is obvious that the uptake of Al2O3 by C-S-H (Figure 6b,d) is easier in S20, which is mainly correlated with the higher content of Al2O3 in SSP (Table 1), and the Al2O3 in the SSP mainly exists in the C12A7 phase (Figure 2) with relatively higher reactivity [56,57]. The same phenomenon is also found in GBFS composite cement paste [27].
However, it is surprising that there are many scattered particles and pores in the cross-section of S20 (Figure 6e), and there is no effective connection between the various phases (Figure 6e), reflecting the lower strength of S7h S20.

3.3.2. Phases and Microstructure under S7d

Figure 7 and Figure 8 present the phases and microstructure of S7d Ref and S20, separately. Extending the duration of steam curing does not change the phase composition of hardened cement paste, as they are still mainly fibriform C-S-H gels (Figure 7a,d) and flaky CH (Figure 7a,c). It is interesting to note that many 1~2 μm spherical crystalline particles (Figure 7a,c) appear near the surface of the sample. Based on its morphology [58] and the aforementioned XRD analysis (Figure 4a), it can be concluded that they are hydrogarnet. After steam curing for 7 d, abundant hydration products appear on the cross-section of the sample, indicating a further improvement in the strength.
As identified in Figure 8, the C-S-H morphology (Figure 8a) in S7d S20 is similar to that in S7h S20, but compared to that in S7h, the fiber in the S7d C-S-H gels is shorter and thicker (Figure 8a) and has a lower Ca/Si ratio (1.88 seen in Figure 8b). In addition, a rod-shaped hydration product (Figure 8a) with a length of around 5 μm is also found, and its composition (Figure 8c) indicates that it is a crystalline C-S-H. Undoubtedly, the formation of crystalline C-S-H is related to the high curing temperatures. During long-term elevated temperature curing, amorphous C-S-H gel is no longer stable, and it gradually changes to a more stable crystalline C-S-H through decomposition and reproduction, thus reducing the cementitious performance of C-S-H and increasing the permeability, which is unfavorable for the durability of concrete [59].
Moreover, many granular hydrates with sizes of around 1 μm (Figure 8d) appear, whose composition are close to C3ASH4. Combined with the above XRD analysis (Figure 4), it can be inferred that they are hydrogarnet. However, it is worth noting that the size of hydrogarnet in S20 is tinier than that in Ref, which is possibly associated with the abundant reactive Fe in the steel slag. It is reported that Al in hydrogarnet can be partly or wholly replaced by Fe (Figure 8e); this substitution will affect the structure of hydrogarnet and may affect its particle size [43,44]. Furthermore, high-temperature curing not only facilitates the generation of C-(A)-S-H gels containing Al but also facilitates the production of hydrogarnet and AFm (Figure 4). The above-hydrated products require the participation of Al in the formation process; in consequence, there is competition for Al among various phases during the formation process. Nevertheless, the Al/Si ratio of hydrogarnet in the SSP samples (0.90) is still higher than that (0.69) in the GBFS sample [27], but the particle size of hydrogarnet in the SSP samples is much smaller. Therefore, the level of Al content may not be the determining factor of the particle size of hydrogarnet in the SSP sample. Also, it is worth noting that Mg occupies a higher proportion in the composition of hydrogarnet (4.29% in Figure 8e) in the SSP sample than the GBFS sample (0.87%) [27], but whether Mg can participate in the formation of hydrogarnet and affect its performance needs further study.
It is interesting to note that many cubic (Figure 8f) and spherical (Figure 8g) CaCO3 were produced on the surface of S20, which may be related to the carbonation of the sample during the preparation process. Generally, CaCO3 possesses three crystal forms (e.g., calcite, vaterite and aragonite) and a variety of morphologies. The formation of the various morphologies of CaCO3 is mainly influenced by external pH values, impurity ions, additives and the supersaturation degree of the solution [60].
Despite the fact that many hydrates are generated in S7d S20, there are also many pores (Figure 8f), which is attributed to both the long-term (7 days) 80 °C steam curing and the lower reactivity of the steel slag.

3.3.3. Phases and Microstructure under N28d

Figure 9 and Figure 10 reveal the phases and microstructure of N28d Ref and S20, respectively. For Ref, the dense reticulate C-S-H gels are closely intertwined with the layered CH (Figure 9a,b), forming a dense structure (Figure 9d). At 28 d, the Ca/Si ratio of C-S-H in Ref is roughly 2.70, and its precipitation is also affected by Al, S, K and Fe (Figure 9c).
Different from Ref, reticular and granular C-S-H (Figure 10a,c) appear in N28d S20. After 28 days of curing, the microstructure of the reticular C-S-H in S20 is also very dense. In addition to the small Ca/Si ratio (1.57), its precipitation is also affected by Mg, Al, S, K and Fe (Figure 10b). Undoubtedly, these are also the reasons for the formation of granular C-S-H (Figure 10c,d). However, unlike the denser structure of Ref, the cross-section in N28d S20 contains more pores (Figure 10e), which will affect the mechanical properties of S20.

3.4. Hydrates Measured by TG/DTG

To investigate the effect of SSP on the formation of hydrates, the DTG/TG curves of Ref and S20 cured under different conditions were measured in Figure 11 and Figure 12, separately. It is evident that the DTG curve in the range of 300~390 °C under S7d is significantly different from the other two curing conditions, mainly due to the decomposition of hydrogarnet [27], which closely matches the analysis of XRD and SEM mentioned above. However, the formation amount of hydrogarnet in S20 is less than that in Ref (Figure 11).
The Ca(OH)2 (CH) content is an important index to evaluate the reaction degree between cement and SCM [61,62]; therefore, to quantitatively analyze the influence of SSP on the hydrates, the mass loss in the ranges of 60–950 °C and 60–600 °C, as well as the CH content, were calculated and tabulated in Table 4 based on the TG results in Figure 12 [27]. It has been reported that the mass loss in the temperature range from 60 °C to 600 °C can directly affect the mechanical properties and reflect the non-evaporative water content of samples [63]; therefore, the mass loss in the ranges of 60–600 °C is defined as the non-evaporative water of paste samples in this investigation.
The non-evaporative water content in S7h and S7d Ref reaches 94.9% and 100% of N28d Ref, while these values in S20 are 90.3% and 95.5%, indicating that early age steam curing promotes the hydration of Ref, while this effect decreases after SSP addition. Moreover, under the above three kinds of conditions, the non-evaporable water content of S20 reaches 90.4%, 90.7% and 95.0% of that of Ref, respectively. This ratio under steam curing is much lower, which also means that steam curing is more conducive to the hydration of Ref than that of S20.
The CH content of S20 is 88.6%, 79.7% and 94.6% of that of Ref under S7h, S7d and N28d, respectively. This ratio in steam curing is also much lower, suggesting that steam curing is more beneficial to the formation of CH in Ref rather than in S20. Wang and Suraneni concluded that, unlike the hydration of GBFS, the hydration of SSP neither consumes nor produces CH [56]. From this perspective, it can be seen that the CH in S20 mainly comes from the hydration of PC, and the SSP incorporation is more helpful to the hydration of PC under standard curing conditions rather than under steam curing conditions.
It is worth noting that the carbonation of samples under S7h is more serious than that under S7d and N28d (Figure 12 and Table 4). Generally, the preparation and TG/DTG test of samples under different curing conditions were conducted on the same time; thus, the sample under S7h underwent a longer storage time and a more severe carbonation.

3.5. Compressive Strength and Flexural Strength

The mechanical properties of Ref and S20 after steam curing and standard curing are concluded in Figure 13. It is obvious that the compressive strength and flexural strength of S20 are both smaller than those of Ref, whether under steam curing or standard curing conditions. This is not only related to the fewer hydrate assemblages formed in the S20 mortar (Table 4) but also to the fact that the S20 mortar has more pores (Figure 6, Figure 8 and Figure 10) under various curing conditions.
The compressive strength activity index of S20 under S7h, S7d and N28d conditions is 71.3%, 78.4% and 81.0%, respectively, while this ratio for the flexural strength is 78.1%, 93.1% and 83.0%, separately, of which the law of compressive strength is in accordance with the results of the non-evaporative water content in Table 4.
In addition, the flexural strength activity index is slightly higher than the compressive activity index under the above three conditions, which is primarily correlated with the reduction in the formation amount (Table 4) of CH in cement paste mixed with SSP. Wang et al. [64] found that fewer CH contents and a smaller CH size can improve the bonding strength between aggregates and pastes, thereby improving the flexural strength.

4. Discussion

A comparison of the properties of SSP composite cementitious material and GBFS composite cementitious material under steam curing:

4.1. Phases and Microstructure

Compared to cement, SSP is rich in Al2O3, MgO and Fe2O3 (Table 1), and contains reactive minerals such as C2S, C2F and C12A7 (Figure 2); thus, it can take part in early age cement hydration. Therefore, SSP addition changes both the composition and the morphology of hydrates in the cement paste; in consequence, laminar and granular C-S-H gels form under early age steam curing conditions and 28 days standard curing conditions.
Although the MgO content of SSP (6.60%) is similar to that (6.72%) of GBFS [27], there is no hydrotalcite found in the SSP composite cement paste. This is because GBFS is mainly composed of the amorphous phase, and there is no doubt that MgO in GBFS mainly exists in the amorphous phase [27], while MgO in SSP primarily exists in the RO phase and f-MgO rather than the amorphous phase, of which the reactivity of MgO in the RO phase and f-MgO is very low, and it hardly reacts or reacts slowly under conventional conditions [46,49,50,51].
The Al2O3 content in GBFS (14.87%) is higher than that in SSP (7.12%); however, the Al/Si ratio (0.90) in hydrogarnet in the SSP sample is higher than that (0.69) in the GBFS sample [27]. Al2O3 in GBFS primarily exists in the glass phase and processes relatively high reactivity, which can participate in the precipitation process of hydrate assemblages, especially under steam curing conditions [27]. Al2O3 in steel slag is mainly found in C12A7 and the amorphous phase in this investigation, among which C12A7 is a component of rapid-set cement that can react quickly with water in the absence of gypsum. It is obvious that the reactivity of Al2O3 presented in C12A7 is not lower than that presented in the amorphous phase in GBFS. Therefore, despite the fact that the content of Al2O3 in SSP is less than that in GBFS, the hydrogarnet in SSP composite cement paste has a higher Al/Si ratio (0.90 to 0.69).
However, the particle size and amount of hydrogarnet in the SSP sample are smaller than that in the GBFS sample [27]. Comparing the composition of hydrogarnet in these two binders, it is found that hydrogarnet in the SSP sample has higher Fe2O3 and MgO, of which Fe2O3 in SSP is presented not only in the non-active RO phase but also in active C2F (Figure 2). Fe2O3 presented in the latter can replace Al2O3 to participate in the precipitation of hydrogarnet, thereby affecting its crystal structure and particle size [43,44]. Whether MgO can replace CaO in the formation of hydrogarnet and affect its particle size needs further study.
The non-evaporative water content in S7h and S7d Ref reaches 94.9% and 100% of N28d Ref, while these values in S20 are 90.3% and 95.5%, and in G20 (GBFS blended cement paste) they are 100.2% and 116.1% [27], respectively. Therefore, GBFS is very effective in promoting the hydration of steam-cured cement; on the contrary, steel slag powder seems to have a negative effect on the hydration of steam-cured cement.
Additionally, it is worth mentioning that long-term 80 °C steam curing facilitates the generation of crystalline C-S-H without cementitious performance (Figure 8) [59], which is detrimental to further improvement of concrete strength. Therefore, long-term steam curing or sustained high-temperature environments should be avoided in the concrete preparation process on site.

4.2. Mechanical Property

From Table 2, it can be observed that SSP seriously delays cement hydration; under this condition, the promoting effect of steam curing and the retarding effect of SSP on cement hydration cancel each other; therefore, many pores and scattered particles appeared in S20 under steam curing (Figure 6). Furthermore, compared to standard curing, steam curing increases the gap between the number of hydrates that form in S20 and Ref (Table 4). A similar phenomenon can also be found from the compressive strength activity index of SSP composite cement mortar (Figure 13). Therefore, SSP addition is more helpful to cement hydration under standard curing than steam curing.
Although GBFS can also delay cement hydration, it is very limited, and due to its high reactivity, it has little negative impact on the strength of early age steam-cured cement mortar [27].

5. Conclusions

To explore the effect of steel slag powder (SSP) on the properties of precast concrete under steam curing, the early age performance of steam-cured (80 °C for 7 h and 7 d) SSP-blended (20% substitution) cement paste, including the types and amounts of hydrates, microstructure and mechanical properties were investigated and compared with those of a 28 d standard-cured SSP sample. Based on the above analysis, the main conclusions are as follows:
(1) Under the condition of 80 °C steam curing for 7 h, SSP can take part in the hydration of cement and promotes the formation of laminar and granular C-S-H gels; however, after steam curing, the microstructure of SSP-blended cement paste is relatively loose.
(2) 80 °C steam curing for 7 days is favorable to the generation of hydrogarnet and crystalline C-S-H; however, the formation amount and particle size of hydrogarnet in SSP-blended cement paste are smaller than those in pure cement paste.
(3) Under steam curing for 7 h, 7 d and standard curing for 28 d, the non-evaporable water content of S20 is only 90.4%, 90.7% and 95.0% of that of Ref, respectively. Similarly, the compressive strength of S20 is only 71.3%, 78.4% and 81.0% of Ref, separately. Therefore, compared to standard curing, SSP addition seems to be unfavorable for the formation of hydrates and the improvement of the mechanical properties of steam-cured cement paste.

Author Contributions

Conceptualization, B.L. and B.H.; methodology, B.L., B.H. and Y.L.; investigation, B.L., X.L. and Y.D.; resources, Y.L. and B.L.; data curation, B.L., B.H. and Z.L.; writing—original draft preparation, B.L., X.L., Y.D. and Y.C.; writing—review and editing, B.L., B.H., Y.L., Y.C. and Z.L. All authors have read and agreed to the published version of the manuscript.

Funding

This investigation was sponsored by the Huai’an Key Research and Development Program (Frontier Foundation) (No. HAQ202202), Jiangsu Province Industry-University-Research Project (No. BY2022370) and Jiangsu Province Construction System Science and Technology Project (No. 2020ZD85, 2021ZD74, 2022ZD100).

Data Availability Statement

All data generated or used during the study appear in the submitted article.

Acknowledgments

We would like to thank Baizhan Ding and Dawei Wang from Huai’an Construction Science Research Institute Co., Ltd., Tao Sun form Huai’an Guhe Concrete Co., Ltd. as well as Yuechang Tan from Huashi Group for their assistance in providing experimental environments and materials.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Cumulative distribution and differential distribution of particle size in PC and SSP.
Figure 1. Cumulative distribution and differential distribution of particle size in PC and SSP.
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Figure 2. XRD pattern of SSP.
Figure 2. XRD pattern of SSP.
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Figure 3. The schematic diagram for steam curing.
Figure 3. The schematic diagram for steam curing.
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Figure 4. XRD patterns of Ref (a) and S20 (b) under different curing conditions.
Figure 4. XRD patterns of Ref (a) and S20 (b) under different curing conditions.
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Figure 5. SEM of S7h Ref: (a) fibriform C-S-H gels; (b) composition of C-S-H in area 1; (c) flaky CH; (d) cross-sectional structure.
Figure 5. SEM of S7h Ref: (a) fibriform C-S-H gels; (b) composition of C-S-H in area 1; (c) flaky CH; (d) cross-sectional structure.
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Figure 6. SEM of S7h S20: (a) fibriform C-S-H gels; (b) composition of C-S-H in area 2; (c) laminar and granular C-S-H gels; (d) composition of C-S-H area 3; (e) loose microstructure.
Figure 6. SEM of S7h S20: (a) fibriform C-S-H gels; (b) composition of C-S-H in area 2; (c) laminar and granular C-S-H gels; (d) composition of C-S-H area 3; (e) loose microstructure.
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Figure 7. SEM of S7d Ref: (a) fibriform C-S-H gels; (b) composition of C-S-H in area 4; (c) granular hydrogarnet; (d) abundant hydrates.
Figure 7. SEM of S7d Ref: (a) fibriform C-S-H gels; (b) composition of C-S-H in area 4; (c) granular hydrogarnet; (d) abundant hydrates.
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Figure 8. SEM of S7d S20: (a) fibriform C-S-H gels and crystalline C-S-H phase; (b) composition of C-S-H in area 5; (c) composition of area 6; (d) spherical hydrogarnet; (e) composition of hydrogarnet in area 7; (f) cubic CaCO3; (g) spherical CaCO3; (h) pores.
Figure 8. SEM of S7d S20: (a) fibriform C-S-H gels and crystalline C-S-H phase; (b) composition of C-S-H in area 5; (c) composition of area 6; (d) spherical hydrogarnet; (e) composition of hydrogarnet in area 7; (f) cubic CaCO3; (g) spherical CaCO3; (h) pores.
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Figure 9. SEM of N28d Ref: (a) tightly packed hydrates; (b) reticulate C-S-H gels; (c) composition of C-S-H in area 8; (d) dense microstructure.
Figure 9. SEM of N28d Ref: (a) tightly packed hydrates; (b) reticulate C-S-H gels; (c) composition of C-S-H in area 8; (d) dense microstructure.
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Figure 10. SEM of N28d S20: (a) dense C-S-H gels; (b) composition of C-S-H in area 9; (c) granular C-S-H gels; (d) composition of C-S-H in area 10; (e) pores.
Figure 10. SEM of N28d S20: (a) dense C-S-H gels; (b) composition of C-S-H in area 9; (c) granular C-S-H gels; (d) composition of C-S-H in area 10; (e) pores.
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Figure 11. DTG results of samples prepared under steam curing and standard curing conditions: (a) Ref; (b) S20.
Figure 11. DTG results of samples prepared under steam curing and standard curing conditions: (a) Ref; (b) S20.
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Figure 12. TG results of samples prepared under steam curing and standard curing conditions: (a) Ref; (b) S20.
Figure 12. TG results of samples prepared under steam curing and standard curing conditions: (a) Ref; (b) S20.
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Figure 13. Compressive strength (a) and flexural strength (b) of Ref and S20 mortars.
Figure 13. Compressive strength (a) and flexural strength (b) of Ref and S20 mortars.
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Table 1. Chemical analysis of PC and SSP, wt.%.
Table 1. Chemical analysis of PC and SSP, wt.%.
CompositionCaOSiO2Al2O3Fe2O3SO3MgOK2OMnOP2O5Na2OMnOLOI
PC64.4720.874.873.592.522.130.650.090.290.110.092.4
SSP38.6218.467.1222.51.086.600.152.111.250.212.116.2
Note: LOI represents loss-on-ignition.
Table 2. The mix ratio, curing condition and the corresponding labels.
Table 2. The mix ratio, curing condition and the corresponding labels.
SamplePCSSPWaterCuring Condition
Ref100030S7h: cured at 80 °C steam for 7 h
100030S7d: cured at 80 °C steam for 7 days
100030N28d: cured at 20 ± 2 °C, R.H. ≥ 95% for 28 days
S20802030S7h
802030S7d
802030N28d
Table 3. Basic performance of Ref and S20 samples.
Table 3. Basic performance of Ref and S20 samples.
GroupWater Requirement
(wt. %)		SoundnessSetting Time (min)Fluidity (mm)
Le-Chatelier Expansion (mm)Autoclave Expansion (%)InitialFinal
Ref27.40.50.08172237210
S2027.22.50.15230327220
Table 4. The non-evaporative water and CH content of samples based on TG results in Figure 12, wt.%.
Table 4. The non-evaporative water and CH content of samples based on TG results in Figure 12, wt.%.
SpecimensCuring Condition60–950 °C Mass Loss60–600 °C Mass LossCH Content
RefS7h18.3314.8314.71
S7d18.1515.6318.61
N28d17.8015.6315.21
S20S7h16.8013.4113.03
S7d15.8514.1814.84
N28d16.8014.8514.39
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MDPI and ACS Style

Li, B.; Lu, X.; Huo, B.; Du, Y.; Liu, Y.; Cheng, Y.; Liu, Z. The Early Age Hydration Products and Mechanical Properties of Cement Paste with Steel Slag Powder as Additive under Steam Curing Conditions. Buildings 2023, 13, 2192. https://doi.org/10.3390/buildings13092192

AMA Style

Li B, Lu X, Huo B, Du Y, Liu Y, Cheng Y, Liu Z. The Early Age Hydration Products and Mechanical Properties of Cement Paste with Steel Slag Powder as Additive under Steam Curing Conditions. Buildings. 2023; 13(9):2192. https://doi.org/10.3390/buildings13092192

Chicago/Turabian Style

Li, Baoliang, Xue Lu, Binbin Huo, Yuheng Du, Yuyi Liu, Yongzhen Cheng, and Zejun Liu. 2023. "The Early Age Hydration Products and Mechanical Properties of Cement Paste with Steel Slag Powder as Additive under Steam Curing Conditions" Buildings 13, no. 9: 2192. https://doi.org/10.3390/buildings13092192

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