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Article

Effects of Sodium Silicate Alkali Sludge on the Rheological and Mechanical Properties of an Alkali-Activated Slag System

by
Liyan Gao
1,
Lijie Ren
2,
Xiaomei Wan
2,3,*,
Zuquan Jin
2,3 and
Hong Wang
2
1
School of Science, Qingdao University of Technology, Qingdao 266520, China
2
School of Civil Engineering, Qingdao University of Technology, Qingdao 266520, China
3
Cooperative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone, Qingdao 266520, China
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(1), 90; https://doi.org/10.3390/su16010090
Submission received: 21 November 2023 / Revised: 16 December 2023 / Accepted: 19 December 2023 / Published: 21 December 2023
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Abstract

:
The application of alkali sludge (derived from sodium silicate production) as a supplementary material for the cementitious system of alkali-activated slag was investigated through tests of mechanical strength, rheology, heat of hydration, and microscopic analysis. The enhanced alkali sludge dosage and alkali equivalent content increased the yield stress and plastic viscosity of the alkali-activated slag while decreasing the fluidity due to the better water absorption of the alkali sludge, which increased the flocculation of the particles. The compressive strength reached the maximum, with 30% alkali sludge and 7% alkali equivalent content. The addition of the alkali sludge postponed the formation of the hydrated calcium silicate (C-S-H) gel and, therefore, delayed the peak heat of hydration, but ultimately increased the total hydration heat release. The major hydration product of calcium silicate hydrate was found in the alkali-activated slag with a 10–30% blend of alkali sludge. This work provides a reference for the utilization of alkaline solid waste from the chemical industry as an alkali activator in cementitious construction materials.

1. Introduction

Sodium silicate is extensively used in various industrial fields; however, during its manufacturing, a large quantity of alkali sludge is generated. A preliminary investigation indicated that sodium silicate production in China generates approximately 4 to 5 million tons of sodium silicate alkali sludge annually, and most of it is transported to storage yards. Consequently, the alkali in the sludge seeps into the ground, resulting in soil and water pollution and posing a serious threat to environmental security [1,2,3].
Currently, there are rare but effective methods for the recovery and utilization of sodium silicate alkali sludge. However, some other chemical alkaline wastes have been investigated for use as alkali activators in cementitious materials. Bayat et al. [4] investigated red mud (10–40%) as an activator for alkali-activated slag (AAS) cement; the addition of red mud reduced the fluidity of the cement and improved the compressive strength at an appropriate dosage. Specifically, the compressive strength first increased with the increase in red mud dosage until reaching a maximum at a blend of 20% red mud. A microstructure analysis showed that the major hydration product was aluminum-rich calcium aluminosilicate hydrate (C-A-S-H) gel. Sreelekshmi et al. [5] investigated AAS containing 0–75% red mud, and similarly found that an increase in the red mud dosage reduced the fluidity. They also observed the increasing-then-decreasing trend for compressive strength, but only under water-curing conditions; under standard curing conditions, the compressive strength decreased with the increase in red mud dosage. Li et al. [6] used soda residue derived from the manufacturing of sodium carbonate as an activator for the activation of fly ash, and found that the compressive and flexural strength first increased and then decreased as the soda residue/fly ash ratio increases. Zhao et al. [7] found that the compressive and flexural strengths of AAS increased with increases in soda residue dosage, and the hydration of soda residue produced a mixed structure of C-S-H, C-A-S-H, and sodium aluminosilicate hydrate (N-A-S-H) or calcium/sodium aluminosilicate hydrate ((C, N)-A-S-H). Liu et al. [8] reported that the maximum strength of soda residue/slag mortars with a binder-to-sand ratio of 3:1 and a soda residue/slag mass ratio of 1:4 was reached on day 28. Lin et al. [9] investigated soda-residue-activated slag systems and found that the mechanical strength first increased and then decreased with the soda residue dosage, while the fluidity showed a downward trend.
In this study, the alkali sludge derived from the manufacturing of sodium silicate was used to prepare alkali-activated cementitious materials for the first time. The effects of alkali sludge dosages and alkali equivalent contents on the rheological properties, mechanical properties, micro-morphology, hydration process, and hydration products of the AAM systems were detected. Also, the sustainability and economics of industrial alkaline solid waste are overviewed herein. The results will provide more valuable references regarding the utilization of industrial alkaline solid waste as an alkali activator in AAM binding materials.

2. Materials and Methods

2.1. Raw Materials

The cementitious materials used in the experiments included a S95 slag acquired from Qingdao Qingjian New Material Group Co., Ltd., Qingdao, China, with a water requirement ratio of 108% and a sodium silicate alkali sludge from Qingdao Hiwinsolvay Chemicals Co., Ltd., Qingdao, China, with a water requirement ratio of 120% after drying at 80 °C for 24 h and grinding in a ball mill. The chemical compositions of the slag and alkali sludge were measured by XRF and are given in Table 1, and their particle size distributions, analyzed by a Malvern 2000/3000 Laser LSP (Malvern Panalytical Company, Malvern, UK) and micromorphology characterized by a JED JSM 7500F scanning electron microscope (JEOL Ltd., Tokyo, Japan), are shown in Figure 1 and Figure 2. Sodium hydroxide solutions prepared by an analytical pure sodium hydroxide from Shanghai Eppie Chemical Reagent Co., Ltd., Shanghai, China, were used as the activators, and a standard sand with a particle size of 0.08–2 mm was used.

2.2. Specimen Preparation and Test Method

Mortar mixtures and specimens with sizes of 40 mm × 40 mm × 160 mm were prepared according to the mix proportions in Table 2. The fluidity of the mortar mixtures was tested by the jump table method, as detailed in the GB/T 2419-2005 [10] “Test Method for Fluidity of Cement Mortar”. The rheological properties of cementitious pastes were analyzed using a rheometer (Discovery Hybrid Rheometer-2, TA Instruments Inc., New Castle, DE, USA). The rheological testing and analysis process consisted of two stages, namely, the pre-shear stage and the data collection stage. In the pre-shear stage, the shear rate was increased from 0 to 200 s−1 in 20 s, then maintained at 200 s−1 for 60 s, and finally decreased from 200 s−1 to 0 in 20 s. In the data collection stage, the shear rate was increased from 0 to 200 s−1 in 100 s and then decreased from 200 s−1 to 0 in 100 s. Prismatic mortar specimens (40 mm× 40 mm× 160 mm) were used for the tests according to GB/T 17671-2021 [11] “Test Method of Cement Mortar Strength”.
The samples were analyzed by SEM-EDS with energy-dispersive X-ray spectroscopy (JEOL JSM-7500F). The samples needed to be dried, fixed with conductive adhesive, and sprayed with gold coating. The samples were determined to fall into the wavelength range of 400 cm−1–4000 cm−1 using a FTIR spectrometer (Thermo Scientific, Waltham, MA, USA, Nicolet iS10), and KBr was used as a matrix. The heat of hydration of the samples was analyzed by an isothermal calorimeter (TAM Air, TA Instruments Inc.) for 72 h.

3. Results and Discussion

3.1. Fluidity

The fluidity of the mortars with different alkali sludge dosages and alkali equivalent contents ranged from 143 to 228 mm (Figure 3). The fluidity decreased with the increase in alkali sludge dosage and alkali equivalent content. This is primarily because the alkali sludge had a loose structure with high porosity and high surface energy, which increased the water absorption of the system. SEM images of the alkali sludge and slag (Figure 2) show that the alkali sludge particles had relatively rough surfaces, while the slag particles had smoother surfaces. This means that the alkali sludge had a greater water requirement than slag. With the increase in alkali equivalent content, the pH of the activator increased, thereby increasing the amounts of OH− and [SiO4]4− ions in the system and expediting the depolymerization of the slag and the formation of hydration products. Furthermore, the viscosity of the solution increased with the increase in alkali equivalent content and led to a decrease in fluidity. This also remained consistent with the results obtained from rheology, where plastic viscosity and yield stress increased with the increasing alkali equivalent content when the amount of alkali sludge was kept constant.
Figure 4 shows the rheological curve of the AAS without alkali sludge. The shear stress increased with the increase in shear rate, with dramatic variation in the initial stage, and decreased with the decline in the shear rate in the descending stage. Similarly, Li et al. [12] found a dramatic variation in the shear stress of water glass-activated slag during the ascending phase, which may have occurred as a result of the formation and subsequent rupture of primary C-S-H gels owing to the interaction between silicate ions in the activator and calcium ions released during the partial dissolution of the slag [3,13,14]. Therefore, the curve of the descending stage was used for rheological analysis. The ascending and descending curves did not overlap and formed a hysteresis loop, suggesting that the microstructure of the paste was destroyed [15,16,17].
It is evident in the rheological curves in Figure 5 that the shear stress and viscosity increased with the alkali sludge dosage when the shear rate was constant. This can be attributed to the fact that structural flocculation was promoted by the alkali sludge, resulting in relatively high shear stress. With the increase in shear rate, the viscosity decreased, indicating that AAS–alkali sludge is a shear-thinning material. Brownian motion played a dominant role when the shear rate was relatively low, resulting in high resistance and viscosity; with increase in shear rate, the viscosity decreased as particles moved together to achieve an overall flow. The inter-particle structures changed, and the randomness of the stationary state distribution could not be restored by Brownian forces; the particles moved more freely than those at a low shear rate, and, hence, the viscosity decreased [18,19,20].
The variation in yield stress with the alkali sludge dosage and alkali equivalent content are shown in Figure 6a. The yield stress tended to increase with the increasing alkali sludge dosage. This is because alkali sludge increases the water requirement of the paste and expedites particle flocculation. Similarly, the plastic viscosity also increased with alkali sludge dosage (Figure 6b) because the high water requirement of the alkali sludge resulted in a reduction in the free water content and inter-particle spacing, thereby increasing the particles’ resistance to movement. Furthermore, owing to the irregular surface of the alkali sludge particles, the fluidity of the paste was reduced, resulting in an increasing plastic viscosity. Dai et al. [21] investigated the effects of the water-to-binder ratio on AAS–fly ash systems and found that the yield stress, plastic viscosity, and thixotropy index values of the paste increased as the water-to-binder ratio decreased.
Model fitting of alkali-activated slag with different dosages of alkali sludge revealed that NaOH-activated slag–alkali sludge complied with Bingham fluid. The yield stress of the paste acquired from the intercept of the fitted rheological regression curves increased with the alkali sludge dosage and alkali equivalent content generally (Figure 6a). Increasing the alkali equivalent content accelerated the dissolution of the slag, resulting in the formation of more reaction product and then an increased yield stress [22,23,24]. Zhang et al. [25] investigated how the NaOH solution concentration affected the rheological properties of geopolymer paste and found that the yield stress increased rapidly at a low alkaline content, and then decreased with an increasing alkali concentration. Additionally, they reported that the yield stress and plastic viscosity were enhanced by flocculation resulting from interactions among the solution and colloid particles, such as van der Waals and electrostatic forces. Furthermore, silicon and aluminum particles precipitated from the particle surface at high NaOH concentrations.
The plastic viscosity acquired from the slope of the fitted rheological regression curves also increased with the alkali sludge dosage and the alkali equivalent content (Figure 6b). This trend occurred because the viscosity of the solution and the resistance to particle movement increased with the increasing alkali sludge dosage and alkali equivalent content. Vance et al. [26] reported that the activator concentration and viscosity affected the plastic viscosity of alkali-activated fly ash paste significantly.

3.2. Mechanical Properties

The results of mechanical properties of the AAS–alkali sludge are summarized in Figure 7. With the increasing alkali sludge dosage, the compressive strength increased until it reached a maximum of 48 MPa at 30% of alkali sludge dosage, and then decreased as the alkali sludge dosage was further increased, although it was still greater than that of the specimen without alkali sludge. This was mainly because to the alkali sludge absorbed more water than the slag. Large amounts of SiO2 and Al2O3 in the alkali sludge react with alkaline substances in humid environments to produce more cementitious substances (e.g., C-S-H and C-A-S-H).
Although alkali sludge improved the compressive strength to a certain extent, when the solution-to-binder ratio was too low, the mortar fluidity was poor and bubbles were difficult to eliminate. Therefore, the setting and hardening times were reduced; thus, dense casting was more difficult and the compressive strength decreased. This is consistent with the results of liquidity and rheological tests. Figure 7 also shows that the flexural strength first increased and then decreased with the alkali sludge dosage. The maximum flexural strength of 9.1 MPa was attained with an alkali sludge dosage of 10%.
Figure 7 also shows that the compressive and flexural strengths increased with the increasing alkali equivalent content. This is because the amount of OH− ions in the solution increased with the equivalent alkali content, expediting the dissolution of the slag. Fang et al. [27] reported that an increase in alkali equivalent content accelerated the hydration and formation of C-S-H gel. Moreover, the gels had denser microstructures and higher compressive strength. In this work, when the alkali equivalent content was 7% or 5%, the maximum compressive strength was reached with a 30% alkali sludge dosage; when the alkali equivalent content was 4%, the maximum compressive strength was reached with a 20% alkali sludge dosage. This occurred because the low alkali equivalent content and pH of the solution meant that the alkali was insufficient to promote the dissolution of elements such as silicon, calcium, and aluminum from the cementitious material [28], causing the maximum compressive strength to be reached at a lower alkali sludge dosage.

3.3. Heat of Hydration

The heat of hydration was evaluated at an alkali equivalent content of 7% and alkali sludge dosages of 0%, 20%, and 30%. Additionally, a pure cement paste (A0, P.O 42.5 R) with a water-to-binder ratio of 0.49 was used as a control. The heat of hydration curves of various samples are shown in Figure 8 and Table 3. It is notable that the heat of hydration processes of the AAS–alkali sludge and cement paste were similar, with stages for the initial reaction, induction, acceleration, and retardation [29,30,31]. However, the heat of the hydration curve of the pure cement paste exhibited three peaks, while those of the AAS-alkali sludge samples only had two peaks. The third peak of the cement curve is associated with the secondary aluminum phase reaction of C3A in cement with gypsum to form ettringite (AFt). The first peak of the AAS–alkali sludge samples, which appeared rapidly after mixing with water, corresponded to the dissolution of the slag and alkali sludge in an alkaline environment. The second peak was associated with the formation of C-S-H caused by the reaction between silicon and calcium ions produced by slag dissolution during the acceleration stage.
The first exothermic peak of the AAS–alkali sludge samples appeared significantly earlier than that of ordinary Portland cement. However, with the increase in the alkali sludge dosage, the first heat of hydration peak appeared later in the hydration process. The change in peak time was related to the introduction of silicon ions by the alkali sludge into the cementitious system, which altered the alkalinity of the solution and reduced the ion dissolution rate. As the alkali sludge dosage increased, the second peak appeared at a significantly later time, suggesting that the addition of alkali sludge hinders the reaction progress and prolongs the formation time of C-S-H gels. Generally, the addition of alkali sludge increases the release of the total heat of hydration, which is consistent with the compressive strength results.

3.4. FTIR Analysis

FTIR spectroscopy was conducted on the AAS–alkali sludge samples, and the results are shown in Figure 9. The spectra show that an asymmetric O-H stretching vibration occurred at 3400 cm−1 due to the presence of weaker adsorbed water on the surface or within the geopolymer. The bending vibration of H-O-H at 1640 cm−1 occurred when the alkali sludge dosages were 10%, 30%, and 40%, which may have been caused by interlayer water adsorption in the unhydrated alkali sludge [32,33]. The absorption peak at 1420 cm−1 corresponds to symmetric stretching vibrations of C-O. The absorption peak at 950 cm−1 corresponds to the asymmetric stretching vibrations of Si–O and Al–O bonds formed by hydration in the cementitious products. This vibration indicates the occurrence of the reaction between Al3+ and [SiO4]4− in the structure, resulting in the formation of Si-O-Al vibration and an increase in silicate with non-bridging oxygen. With the increase in alkali sludge content, the absorption peak at 950 cm−1 gradually increased and became stronger, indicating the number of cementitious phases in the hydration product C-A-S-H. This is because the increase in alkali sludge increased the concentration of [SiO4]4- in the solution, which can quickly react with Ca2+ and Al3+ when dissociated from slag, increasing the quantity of hydration products. Additionally, the enhanced and sharpened peak indicates an increase in the volume of the C-S-H gel phase. Absorption peaks at 440–690 cm−1 correspond to the bending vibrations of T–O–Si groups and the stretching vibrations of [AlO4]4− tetrahedra. These peaks were observed at 645 cm−1 for the current samples. Duxson et al. [34] noted that the hydration product was converted from an aluminum-rich phase gel to a silicon-rich phase gel when the wavenumber of the peak of T–O–Si increased. The absorption peak at 430 cm−1 correlates to the in-plane bending vibrations of Si–O bonds [35].

3.5. SEM Micromorphology

The SEM images of the samples with various alkali sludge dosages (alkali equivalent contents of 7%) and with various alkali equivalent contents (alkali sludge dosages of 30%) are shown in Figure 10. Numerous flaky and reticular substances are present in Figure 10. As judged by the FTIR results, C-S-H and C-A-S-H were the main hydration products. Compared with the sample without alkali sludge, the sample with 10% alkali sludge exhibited some polymerized hydration product; with the alkali sludge dosage increasing to 20%, the hydration product formed clusters; and with the alkali sludge dosage increasing to 30%, the polymerized hydration product became patchy with a denser morphology. Further increases in the alkali sludge dosage decreased the degree of polymerization, corresponding to the reduction in compressive strength. The EDS results in Table 4 show that the calcium-to-silicon ratio first decreased and then increased with the increasing dosage of alkali sludge. It was reported that the degree of C-S-H polymerization and mean chain length decreased with the increasing calcium-to-silicon ratio [36,37,38]. The decreasing calcium-to-silicon ratio means that more and more silicon was introduced into the system with the blend of the alkali sludge. However, with more alkali mud being blended, the degree of polymerization of C-S-H began to decrease, and the calcium-to-silicon ratio also rose. On the other hand, with the increase in the alkali equivalent content, the micro-structure became denser. This is primarily because the hydration rate of slag increased with the increasing alkali concentration. The EDS results also demonstrate that the calcium-to-silicon ratio decreased with the increase in the alkali equivalent content.

3.6. Sustainable Development and Circular Economy Potentials

Alkali-activated materials (AAM) represent a highly efficient and sustainable green alternative to traditional ordinary Portland cement (OPC), capable of achieving higher activator efficiency and sustainability by utilizing local solid waste precursors, resulting in an approximately 80% reduction in CO2 emissions [39]. The United Nations announced 17 Sustainable Development Goals (SDGs) at the end of 2015 as a continuous plan to transform the planet (people, prosperity, and planet) by 2030, wherein the construction and building materials industry can effectively contribute to the realization of the UN’s SDGs [40]. China also places great importance on solid waste management and has established the “People’s Republic of China Solid Waste Pollution Prevention and Control Law” as the core of its legal system for solid waste management. This includes laws such as the “Cleaner Production Promotion Law” and the “Circular Economy Promotion Law”, along with environmental protection planning, comprehensive resource utilization planning, recycling economy planning, and other strategies as the main driving forces in the system. It incorporates principles such as comprehensive resource utilization, “three at the same time” (simultaneous development of economic construction, ecological environment, and social progress), the polluter pays principle framework, producer responsibility extension, clean production, a comprehensive resource utilization evaluation system, and a tax incentives system. Additionally, various policy tools with broad coverage and flexible applications are in place.
The application of industrial effervescent alkali sludge, a solid waste product, in the field of building materials not only holds significant importance for sustainable development, but also brings about economic benefits. Firstly, the use of alkali sludge in alkali-activated materials instead of cement helps to reduce the erosion of natural resources caused by cement production and mitigates the negative impact of solid waste on the environment.
On one hand, the application of alkali sludge promotes the sustainable development of the construction industry. Cement production is an extremely energy-intensive process, and if not properly disposed of, alkali sludge as a waste product can become an environmental burden. By applying it to alkali-activated materials, the demand for traditional cement can be reduced, thus slowing down the cement industry’s over-exploitation of raw materials such as limestone and helping to maintain ecological balance. According to statistics, the cement consumed annually by the global construction industry accounts for about 8% of the total CO2 generated by human activities [41], and the rational use of alkali sludge will help to reduce this figure and lessen the negative impact of the construction industry on climate change.
On the other hand, the use of alkali sludge also creates economic value for the enterprise. The traditional cement production process consumes a large amount of energy and raw materials, while alkaline sludge, as an alternative material for cementitious systems, not only reduces the consumption of energy in the production process, but also achieves the purpose of resource regeneration through the utilization of waste. This not only helps to reduce the production costs of enterprises, but also meets the social demand for sustainable development. Compared with traditional cement, alkali-activated materials are more environmentally friendly and have preferable durability. Furthermore, the addition of solid waste adds a new value to its life cycle, which achieves not only a cycle of resources, but also an economic cycle [40].

4. Conclusions

The utilization potential of alkali-activated materials of alkali sludge derived from the manufacturing of sodium silicate was explored, and we have provided a more comprehensive discussion of its properties in this study.
With increasing alkali sludge dosage and alkali equivalent content, the fluidity of the alkali-activated slag–alkali sludge decreases, while the yield stress and plastic viscosity increase.
Alkali sludge has a significant effect on improving the strength of the system in alkali-activated slag. With alkali equivalent contents of 7% or 5%, the maximum compressive strength could be reached at an alkali sludge dosage of 30%, while with an alkali equivalent content of 4%, the maximum compressive strength was reached at an alkali sludge dosage of 20%.
Heat of hydration analyses revealed that the addition of alkali sludge delays the appearance of the exothermic peak, but ultimately increases the amount of heat released. With an increasing alkali sludge dosage, the calcium-to-silicon ratio in the cementitious system first decreased and then increased. With an increasing alkali equivalent content, the micro-structure of the cementitious system became denser.
The use of alkali mud as a cementitious material in the field of building materials can help to guide the construction industry toward sustainable development. This green, economic development model will play an increasingly important role in the construction materials industry in the future.

Author Contributions

Conceptualization, X.W.; methodology, X.W.; investigation, L.G. and L.R.; writing—original draft preparation, L.G., L.R. and H.W.; supervision, X.W.; writing—reviewing, X.W. and Z.J.; writing—editing, H.W.; funding acquisition, X.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China, grant number 51878365 and Natural Science Foundation of Shandong Province, grant number ZR2023ME030.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data, models, or codes that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest. The funder had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Particle size distributions of (a) slag and (b) alkali sludge.
Figure 1. Particle size distributions of (a) slag and (b) alkali sludge.
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Figure 2. Particle size distributions of (a) slag and (b) alkali sludge (mag = 500×).
Figure 2. Particle size distributions of (a) slag and (b) alkali sludge (mag = 500×).
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Figure 3. Effect of alkali sludge dosage and alkali equivalent content on fluidity.
Figure 3. Effect of alkali sludge dosage and alkali equivalent content on fluidity.
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Figure 4. Rheological curve of AAS without alkali sludge.
Figure 4. Rheological curve of AAS without alkali sludge.
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Figure 5. Effect of alkali sludge dosage on rheological properties of AAS pastes. (a,c,e) Stress and (b,d,f) viscosity as a function of the shear rate of pastes with alkali equivalent contents of (a,b) 7%, (c,d) 5%, and (e,f) 4%.
Figure 5. Effect of alkali sludge dosage on rheological properties of AAS pastes. (a,c,e) Stress and (b,d,f) viscosity as a function of the shear rate of pastes with alkali equivalent contents of (a,b) 7%, (c,d) 5%, and (e,f) 4%.
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Figure 6. Effects of alkali sludge dosage and alkali equivalent content on (a) yield stress and (b) plastic viscosity.
Figure 6. Effects of alkali sludge dosage and alkali equivalent content on (a) yield stress and (b) plastic viscosity.
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Figure 7. Effect of alkali sludge dosage on mechanical strength of mortars. (a,c,e) Compressive strength and (b,d,f) flexural strength of mortars with alkali equivalent contents of (a,b) 4%, (c,d) 5%, and (e,f) 7%.
Figure 7. Effect of alkali sludge dosage on mechanical strength of mortars. (a,c,e) Compressive strength and (b,d,f) flexural strength of mortars with alkali equivalent contents of (a,b) 4%, (c,d) 5%, and (e,f) 7%.
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Figure 8. Heat of hydration at different alkali sludge dosages. (a) Heat of hydration curve and (b) cumulative heat release rate.
Figure 8. Heat of hydration at different alkali sludge dosages. (a) Heat of hydration curve and (b) cumulative heat release rate.
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Figure 9. FTIR spectra of pastes with (a) different alkali sludge dosages and (b) different alkali equivalent contents.
Figure 9. FTIR spectra of pastes with (a) different alkali sludge dosages and (b) different alkali equivalent contents.
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Figure 10. SEM images of AAS–alkali sludge samples. (a) N7–0 (Mag = 500×, 5000×). (b) N7–10% (mag = 500×, 5000×). (c) N7–20% (mag = 500×, 5000×). (d) N7–30% (mag = 500×, 5000×). (e) N7–40% (mag = 500×, 5000×). (f) N4–30% (mag = 500×). (g) N5–30% (mag = 500×). (h) N7–30% (mag = 500×). (The figure in the upper right corner in (a–e) represents a 5000× magnification of the red square area with arrows).
Figure 10. SEM images of AAS–alkali sludge samples. (a) N7–0 (Mag = 500×, 5000×). (b) N7–10% (mag = 500×, 5000×). (c) N7–20% (mag = 500×, 5000×). (d) N7–30% (mag = 500×, 5000×). (e) N7–40% (mag = 500×, 5000×). (f) N4–30% (mag = 500×). (g) N5–30% (mag = 500×). (h) N7–30% (mag = 500×). (The figure in the upper right corner in (a–e) represents a 5000× magnification of the red square area with arrows).
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Table 1. Chemical compositions of slag and alkali sludge, mass %.
Table 1. Chemical compositions of slag and alkali sludge, mass %.
CaOSiO2Al2O3MgOSO3Fe2O3K2ONa2OTiO2MnOOthers
Slag57.1222.5310.993.371.780.920.360.211.870.470.38
Alkali sludge3.8074.565.620.7360.1510.4871.5412.740.0960.0320.238
Table 2. Mix proportions of specimens.
Table 2. Mix proportions of specimens.
Alkali Equivalent
Content		Slag, kg/m3Alkali Sludge, kg/m3NaOH, kg/m3Sand, kg/m3
N4–04%3500171.5744
N4–10%31535171.5744
N4–20%28070171.5744
N4–30%245105171.5744
N4–40%210140171.5744
N5–05%3500171.5744
N5–10%31535171.5744
N5–20%28070171.5744
N5–30%245105171.5744
N5–40%210140171.5744
N7–07%3500171.5744
N7–10%31535171.5744
N7–20%28070171.5744
N7–30%245105171.5744
N7–40%210140171.5744
Table 3. Characteristic parameters of heat of hydration at various alkali sludge dosages.
Table 3. Characteristic parameters of heat of hydration at various alkali sludge dosages.
Heat at 72 h1st Peak2nd Peak3rd Peak
Integral
(J/g)		TimePeak
(mW/g)		Integral
(J/g)		TimePeak
(mW/g)		Integral
(J/g)		TimePeak
(mW/g)		Integral
(J/g)
A0305.9817 min89.395.579 h 52 min3.1887.4917 h 11 min1031168.47
N7–0174.574 min84.165.195 h 19 min2.5254.94———
N7–20%193.827 min94.417.6219 h 23 min3.1190.57———
N7–30%199.6810 min88.317.88121 h 43 min2.5286.30———
Table 4. EDS results of AAS–alkali sludge sample.
Table 4. EDS results of AAS–alkali sludge sample.
ElementN7–0N7–10%N7–20%N7–30%N7–40%N4–30%N5–30%
O43.8344.7328.1744.2941.8547.4139.54
Na6.086.648.4210.4812.086.9710.59
Mg3.784.060.180.383.920.435.56
Al7.197.31.823.795.072.556.58
Si15.5217.9539.4424.8425.2720.7619.71
Ca23.6119.0820.8111.8611.1513.8711.81
Ca/Si1.060.740.530.330.0.470.42
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Gao, L.; Ren, L.; Wan, X.; Jin, Z.; Wang, H. Effects of Sodium Silicate Alkali Sludge on the Rheological and Mechanical Properties of an Alkali-Activated Slag System. Sustainability 2024, 16, 90. https://doi.org/10.3390/su16010090

AMA Style

Gao L, Ren L, Wan X, Jin Z, Wang H. Effects of Sodium Silicate Alkali Sludge on the Rheological and Mechanical Properties of an Alkali-Activated Slag System. Sustainability. 2024; 16(1):90. https://doi.org/10.3390/su16010090

Chicago/Turabian Style

Gao, Liyan, Lijie Ren, Xiaomei Wan, Zuquan Jin, and Hong Wang. 2024. "Effects of Sodium Silicate Alkali Sludge on the Rheological and Mechanical Properties of an Alkali-Activated Slag System" Sustainability 16, no. 1: 90. https://doi.org/10.3390/su16010090

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