
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39251860
72083
10.1038/s41598-024-72083-x
Article
Mechanical properties and microscopic mechanism of solid waste based binder solidified stone waste
Wang Zimou 12
Yang Junjie 12
Wu Yalei wuyalei@ouc.edu.cn

12
1 https://ror.org/04rdtx186 grid.4422.0 0000 0001 2152 3263 College of Environmental Science and Engineering, Ocean University of China, Qingdao, 266100 China
2 https://ror.org/04rdtx186 grid.4422.0 0000 0001 2152 3263 The Key Laboratory of Marine Environment and Ecology of the Ministry of Education, Ocean University of China, Qingdao, 266100 China
9 9 2024
9 9 2024
2024
14 2096016 5 2024
3 9 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
The stone waste generated by stone industry occupy land resources, cause safety hazards and need to be efficiently resourcefully utilized. In this study, the CGF solid waste based binder (abbreviated as CGF) with calcium carbide residue (CCR), ground granulated blast furnace slag (GGBS), and fly ash (FA) as components was developed to solidify the stone waste. Through “treating waste with waste”, the resource utilization of solid waste was realized. The mechanical properties and reaction mechanism of CGF solidified stone waste were investigated through unconfined compressive strength (UCS), XRD, and SEM–EDS tests. The results show that CGF has the better solidify effect on stone waste, and its strength meets the requirements of the road base material standards. Compared to cement, the CGF solidified stone waste existed higher UCS at both 7 and 28 d of curing. The UCS of CGF solidified stone waste reaches 2.93 and 4.42 MPa under curing of 7 and 28 d at 5% binder content, which is 1.61 and 1.37 times higher that of P.O. 42.5 cement. Furthermore, the primary mineral-based stone wastes will not react with the binder, and the CGF generates gelling products such as C-S–H C-A-H, and C-A-S–H through alkali-activated reactions between the components of CGF. These gelling products enhance the UCS of solidified stone wastes through cementing and filling effects. The findings provide a feasible approach with low-carbon emission and low-cost for resourceful utilization of stone wastes.

Keywords

Stone waste
Solid waste based binder
Mechanical properties
Microscopic mechanism
Subject terms

Environmental impact
Civil engineering
National Natural Science Foundation of China52378380, 52078474 and 51779235 issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

With the acceleration of urbanization and the continuous increase in demand for high-quality architectural decoration, the market demand for stone products shows a continuous growth trend1,2. However, stone waste is the inevitable byproduct of stone production3. Stone waste is produced during mining, during stone processing, and in the installation of the finished product, and is characterized by a particle size less than 4.75 mm. The engineering properties of stone waste are poor, as it cannot be directly used, and often, piles of waste are abandoned, leading to the occupation of land resources and environmental pollution, as well as posing risks of land-slides and other safety hazards4,5. For instance, China’s annual stone processing process produces about 859 × 104 t of stone waste6, accounting for 31% of the global total. The granite stone waste generated in granite mining and stone processing is more than 100 × 104 t per year. Wulian County is the largest stone industry base in China, with more than 200 mining sites and an annual output of more than 200 × 104 m3 of barren material and more than 3,000 × 104 m2 of processed plates. At present, the stone waste pile covers an area of about 130 × 104 m2, with a total pile volume of about 3 × 108 t. Therefore, its resource utilization is the key to solving the problem of stone waste accumulation.

Currently, the resource utilization of stone waste is mainly found in industries such as cement and brick production7,8, the substitution of a certain amount of sand in concrete9–13, and other fields. Classification and resource utilization of stone waste according to particle size, as shown in Table 1. In recent years, concerns have been raised regarding the carbon footprint of the cement industry14 due to the high energy consumption, level of emissions, and pollution in the cement production process15–17. On the other hand, the massive accumulation of solid waste occupies land resources; causes pollution of the surrounding soil, water, and air; and threatens the safety of the human environment14. Therefore, the development of a solid-waste-based binder for the total replacement of cement has become a research hotspot. It has been shown that it is feasible to use a binder whose components consist entirely of solid waste to reinforce soft soil. The use of calcium carbide residue (CCR), etc., as an alkali activator, and ground granulated blast furnace slag (GGBS), fly ash (FA), etc., as pozzolanic ash material for reinforcing soft soils has been reported18–22. In addition, solid waste based binder has been applied to solidify soft soils14,21–27, solidify/stabilize heavy-metal-contaminated soils28, semi-solidified marine soft soils29, fiber-reinforced solidified soils30, and protecting foundations for offshore wind turbines31.Table 1 Classification and resource utilization of stone waste according to particle size.

Stone waste	grain size /mm	Resource utilization approach	
stone chip	0.075 ~ 4.75	Concrete fine aggregates, filling materials	
stone powder	 < 0.075	Cementitious materials, ceramics, glass, etc	

The existing research mainly highlights the following aspects of the problem: the resourceful use of stone waste mainly involves combining it with cement to form cement mortar, concrete, etc., but does not fundamentally solve the environmental problems; in terms of the use of stone waste in solidified soil, the research on the mechanical properties of solid-waste-based binders for solidified stone waste is relatively limited, and the action mechanism is not clear.

In summary, this paper uses calcium carbide residue (CCR), ground granulated blast furnace slag (GGBS), and fly ash (FA) compound to form an solid waste based alkali-activated binder (CGF). A comparative analysis of the mechanical properties of the solidified stone waste formed using cement as a binder was conducted to study the strength of different binder content and mixing amounts and the different curing time of the CGF-solidified soil. With the help of XRD and SEM–EDS, the mechanism of the CGF-solidified soil was examined, and the effect of the CGF binder was analyzed. EDS was used to analyze the mechanism of CGF-solidified stone waste. Therefore, solidified stone waste by using solid waste not only reduces the amount of cement used, thus mitigating carbon emissions, but also realizes the problem of resource utilization of stone waste and solid waste in general. This study reveals the curing mechanism of CGF-stone waste, which can seek other ways of resource utilization of stone waste, which is the key to completely solve the problem of stone waste accumulation. Due to China’s requirements for “carbon emissions” and “carbon neutrality” issues to be addressed, the research results have certain practical and theoretical significance for the utilization of solid waste resources and the realization of China's “double carbon” goal.

Materials and methods

Test raw material

The raw material of stone waste was obtained from Wulian County, Shandong Province, China. The physical properties of the raw stone waste and all levels of particles were determined, and the specific gravity, water content, liquid limit, and plastic limit test tests were implemented in accordance with the Standard for Geotechnical Test Methods (GB/T50123-2019)32. Particle size grading was carried out on the stone waste, and different particle sizes of stone waste were obtained through sieving: stone chips and stone powder (Fig. 1A) . In the stone waste, stone chips ac-counted for 26.2% of the content, and stone powder accounted for 73.8%. Particles with sizes less than 0.075 mm exceeded 70% of the full weight, the inhomogeneity coefficient was 85, and the coefficient of curvature was 0.42. The physical properties of the stone waste and the particles of all grades of mixture were determined. The stone waste had no plasticity; the liquid limit and plastic limit of the stone waste particles measuring less than 0.005 were 39.5 and 18.9%, respectively; and the plasticity index was 20.6, as shown in Table 2.Fig. 1 Apparent state of stone waste and cementitious materials. (A) Apparent condition of stone waste (B) Figure of cementitious material.

Table 2 Basic physical properties of stone waste.

Particle density (GS)	Plastic limit (%)	Fine stone powder (< 0.005 mm)	
Liquid limit (%)	Plastic limit (%)	Plasticity index (Ip)	Liquidity index (IL)	
2.68	15.6	39.5	18.9	20.6	1.1	

The all-solid-waste binder used in the test belongs to the alkali-activated binder (Fig. 1B), and the action mechanism is based on alkali activation of reactive silica-aluminium materials, and reorganization of the network structure in an alkaline environment, the condensation reaction on the surface of the vitreous mineral phase, and the generation of alkali-excited products with gelling properties. The CGF used in the test belongs to the all-solid-waste alkali-activated binder, which consists of alkali activator and pozzolanic materials. CCR is a by-product of the hydrolysis of calcium carbide to produce ethylene, with a pH value of more than 12.0, which can provide an alkaline environment and serve as an alkali activator; GGBS is the waste slag produced in the process of iron-smelting, which belongs to the high-reactivity pozzolanic materials; FA is the fly ash collected by baghouse dust collectors of coal-fired power plants, which belongs to the low-reactivity pozzolanic materials. The cumulative curve of particle size gradation is shown in Fig. 2.Fig. 2 Particle size distribution curves of each component of binder and test soil.

Stone wastes (Stone Waste A and Stone Waste B) were taken from different areas of Wulian County, and the two samples were subjected to XRD tests and compared. Import the raw XRD spectra of the waste sample into Jade 6.0 software. After searching and matching, it was determined that the main crystal phase of the sample was quartz (SiO2; PDF#85-0796), with some sodium feldspar (Na(AlSi3O8); PDF#83-1658), calcium feldspar (Ca(Al2Si2O8); PDF#73-1435), and microplagioclase feldspar (KAlSi3O8; PDF#19-0932). From this, it was determined that the mineralogical composition of Wulian stone waste is mainly quartz and feldspar (Fig. 3).Fig. 3 XRD patterns and chemical composition of stone waste.

The XRD patterns of each component of the all-solid-waste binder measured by XRD test are shown in Fig. 4. The main mineral phases of CCR are Ca(OH)2 (PDF#76-0571) and CaCO3 (PDF#88-1807); the main mineral phases of GGBS and FA are amorphous vitreous phases, and there are almost no crystalline mineral phases except for calcium-aluminium feldspar in GGBS, and there are no crystalline mineral phases in FA.Fig. 4 XRD patterns of the components of CGF.

Ratio design

Previous study has showed that the strength of CGF solidified pure clay-particles soil, pure silt-particles soil, and pure sand-particles soil was higher than that of P.O. 42.5 cement in solidifying these soils13. Therefore, CGF442, CGF451, and CGF541 were used to solidify stone waste, and the P.O. 42.5 cement was used as a control. The test scheme of solidified stone waste was shown in Table 3.Table 3 Test scheme of solidified stone waste.

Type of binder	Binder content (%)	Water–binder ratio	Curing time (days)	Tests	
CGF442*	5, 10, 15, 20	1.0	7, 28	UCST

XRD

SEM–EDS

	
CGF451**	
CGF541***	
P.O. 42.5	
*CGF442 (mass ratio of CCR:GGBS:FA is 4:4:2),**CGF451 (mass ratio of CCR:GGBS:FA is 4:5:1), and ***CGF541 (mass ratio of CCR:GGBS:FA is 5:4:1) were taken as binders in this paper, respectively.

Specimen preparation process

The unconfined compressive strength test (UCST) was conducted after curing for 7 d and 28 d, which was strain-controlled and the rate of vertical displacement was fixed at 1%/min. After the UCST, the specimen was sampled from the hand broken sample and dried at 50 ℃ until constant weight; X-ray fluorescence (XRF, ARL OPTIM′X, USA) was conducted to analysis the content of CGF binders’ component and cement. The information collection time: 3 min, the incident light energy: 16.4 keV, the energy resolution: < 2 × 104. X-ray diffractometer (XRD, X′ PERT POWDER, Netherlands) was performed to test the composition of mineral phase. X-ray generator: 40 kV voltage, 40 mA phototube current, Cu rotating anode target; scanning method: 2θ goniometer, accuracy 0.002◦; scanning angle:5–75°. Main technical parameters of SEM (Japan, HITACHI S-4800), used to observe CGF stone waste road base material under different conditions and to analyze the microstructure in a pointwise manner. Secondary electron imaging resolution: 1.0 nm@15 kV, magnification: 20 × -800000 × , accelerating voltage: 0.1–30 kV, sample stage: three-axis motor stage, maximum sample size: Ф100 mm.

The tests were implemented in accordance with the Standard for Geotechnical Test Methods (GB/T50123-2019)32. Prepare CGF442, CGF451, and CGF541 binders according to the mass ratio, and mix well. Dry the raw stone waste in an oven at 105 ℃ for 24 h, and then mix it with the binder according to the ratio to form a mixture of stone waste and binder, Weigh the required amount of water according to the water-binder ratio of 1.0, add the mixture, and stir evenly for 5 min. After adding water, pour the slurry into the test model in three layers, pounding each layer and chiseling the surface layer of slurry after each round of pounding. Put the specimens into the test model and then into the standard curing box (95% relative humidity, 20 ℃) and cure for 24 h. Remove the model after 24 h of curing and continue curing until the set curing time. After curing, the specimens were subjected to unconfined compression strength, and the stress–strain curve of each specimen was recorded at the same time. After compression, the specimens were put into an oven and baked at 60 ℃ for 24 h, and then one part of the natural cross-section of the flake was subjected to SEM and EDS testing. The other part was ground and sieved through a sieve of 0.075 mm mesh for the XRD test; the schematic flow chart of the test was shown in Fig. 5.Fig. 5 Test flow diagram.

Results and discussion

Influence of binder content on strength

Figure 6(A) showed the damage morphology of the solidified stone waste specimens with different binder content of CGF and P.O. 42.5 cement at 28 d. It can be clearly seen that with the increase in the binder content, the crack damage of the solidified stone waste material gradually increased.Fig. 6 Failure patterns and maximum crack widths of solidified stone waste. (A) Damage morphology of the solidified stone waste (B) The maximum width of the cracks of the CGF solidified stone waste (C) the maximum width of the cracks of P.O. 42.5 cement solidified stone waste.

The damage degree of the cement solidified stone waste is obviously larger than that of CGF solidified stone waste, and with the increase in the binder content, the surface damage of the material gradually increases. There is a wider fissure, and the surface damage runs through the whole specimen. The statistics regarding the maximum width of each specimen are shown in Fig. 6(B) and Fig. 6(C). It can be seen that, under the same binder content, the maximum width of the cracks of the CGF solidified stone waste is 60% ~ 84% of the maximum width of the cracks of P.O. 42.5 cement solidified stone waste. With the increase in the binder content, the growth rate of the maximum crack width of the specimens gradually decreases.

The stress–strain curves of each group of specimens are shown in Fig. 7. From the figure, it can be seen that the stress–strain curves of different binders and ratios of solidified stone waste are basically similar, and all of them have obvious peak stresses. The stress–strain curves of the specimens are all of softening type, and the curves can be roughly divided into three stages. The first stage is the compaction stage: under the action of the external force, the corresponding stress increases, and the stress–strain relationship is close to linear before it reaches the yield stress. At this point, the specimen is in the stage of elastic deformation, and there are no obvious cracks yet. In the second stage, with the increase in the loading, tiny cracks begin to appear in the specimen, which is now in the plastic yield stage. The third stage is the post-destruction stage: at this point, the stress decreases and the slope of the curve is negative.Fig. 7 Stress–strain curves of different solidified stone waste with different proportions. (a) Binder content: 5% (b) Binder content: 10% (c) Binder content: 15% (d) Binder content: 20%

The unconfined compressive strength (UCS) of solidified stone waste was taken the peak stress of stress–strain curves in Fig. 7. The UCS results of stone waste specimens prepared with different binders are shown in Figs. 8, 9. Independent of the binder type, the UCS of the specimens increased with the increase in curing time, and the strength growth rate was faster at the beginning of curing before gradually slowing down and leveling off. Under the conditions of the same curing time and the same binder content, the strength of the CGF451, CGF442, CGF541, and P.O. 42.5 cement stone waste specimens increased gradually with the in-crease in the curing time. The UCS of the CGF451 stone waste specimen was the highest, followed by CGF442 and CGF541, respectively. This is due to the fact that the mineral powder is a high-activity pozzolanic material, and the content of mineral powder influences the strength of solidified soil12,13. Conversely, fly ash is a low-activity pozzolanic material with an optimum content of 8% ~ 10%33. Except for the CGF541 group, the strength of the two CGF group specimens was greater than that of the cement specimen group. The UCS of the CGF451 specimens was higher than that of the cement specimens at 7 d, but the trend of UCS increase between 7 and 28 d was not significant compared to the cement specimens.Fig. 8 7 d UCS of different cementing materials and proportions of stone waste materials.

Fig. 9 28 d UCS of different cementing materials and proportions of stone waste materials.

The UCS of each group of specimens gradually in-creased with the increase in the proportion of binder, and the gelling products produced by the alkali-activated reaction of the binder had a wrapping effect on the stone waste particles. The binder content showed a positive correlation with the unconfined com-pressive strength of the specimens, and this UCS increased more quickly during the increase in the binder content from 5 to 15%, and increased more slowly during the increase from 15 to 20%. In the solidified stone waste specimens, internally generated gelling products bond the stone waste particles and fill the cracks between them, thus enhancing the strength of the specimen.

Through the results of the strength test, we can find that comparing the two ratios of CGF451 and CGF442, the strength of the solidified stone waste material specimens grows with the increase of GGBS content; comparing CGF442 and CGF541, the GGBS content remains unchanged, and the strength of the solidified stone waste material specimens increases with the increase of FA content.

Influence of curing time on strength

The relationship between the UCS of the CGF451, CGF442, CGF541, and P.O. 42.5 cement solidified stone waste specimens and the binder content under different aging conditions is shown in Fig. 10. Independent of the binder type, the strength of the solidified stone waste increased with the curing time. The strength of the solidified stone waste increased more quickly at the beginning of curing, and then gradually slowed down and leveled off; the strength of the CGF solidified stone waste was higher than that of cement solidified stone waste of the same curing time, among which CGF451 had the best effect of solidification. The growth rate of in-tensity decreases with increasing degree of response. Through the strength-age relationship, we can find that the initial strength of cementite waste material is lowest at 5% dosing and CGF541 strength is lowest at 10%-20% mixing ratio. The strength addition rate of cement is the highest at different dosing ratio conditions. The cement growth rate was 79.66% at 10% mixing ratio; the strength growth rate of CGF stone waste material was close to the cement growth rate with the increase of doping, and at 20% mixing ratio, the strength growth rate of cement stone waste material was 30.14%, and the strength of CGF451 stone waste material was 27.35%.Fig. 10 UCS of solidified stone waste material at different curing times.

XRD testing and analysis

XRD tests were carried out on the CGF and cement stone waste specimens after curing for 7 and 28 days. The XRD patterns of the two specimens are shown in Fig. 11. The horizontal coordinates indicate the scanning intervals of the XRD tests, and the vertical coordinates indicate the height of the peaks. From the XRD patterns of the CGF451 stone waste specimens at 28 d, it can be seen that the compositions of the physical phases at different binder were the same, all of which were C-S-H, C-A-H, C-A-S-H, etc. The Ca(OH)2 mainly originated from the calcium carbide slag in CGF451, but the diffraction peaks of Ca(OH)2 were not detected in the pattern at 28 d. This indicated that, with the increase in the curing time, the Ca(OH)2 was continuously consumed as an alkali activator, which promoted the hydration reaction and generated more alkali-excited cementitious products, thus enhancing the strength of the specimen.Fig. 11 XRD pattern of 28 d solidified stone waste material samples.

Cement reaction mechanism

It can be seen that the phase composition of the cement stone waste specimen contains characteristic physical phases of hydration products such as C-S–H (hydrated calcium silicate), C-A-H (hydrated calcium aluminates), C-A-S–H (hydrated calcium silica-alumina), and AFt (calcium alumina). These hydration-generated products are the main binder substances that stabilize the waste particles of the stone, which is the main substance that stabilizes the waste particles.

CGF reaction mechanism

The main mineral phases of CGF451 stone waste mainly include quartz, feldspar, C-S-H, C-A-H, and C-A-S-H. It can be observed that stone wastes dominated by primary mineral phases do not chemically react with the binder, and the solidification mechanism depends mainly on the interactions between the components of binder. Namely, the dissolution of CCR yields Ca2+ and OH-, which undergo polymerization with reactive SiO2 and Al2O3 in GGBS and FA to generate reactive [SiO4]4- tetrahedra and [AlO4]5- tetrahedra. The reaction of [SiO4]4- and [AlO4]5- with Ca2+ and OH- forms C-A-H and C-S–H, as shown in Eqs. 1 and 2. Furthermore, excessive [AlO4]5- reacts with C-S-H to generate C-A-S-H, as shown in Eq. 3.1 xCa2++y[SiO(OH)3]-+(z-x-y)H2O + (2x-y)OH-→Cx- Sy- Hz(C - S - H)

2 4Ca2++2[Al(OH)4]-+6H2O +6OH-→C4- A - H13(C - A - H)

3 C - S - H + [Al(OH)4]-→C - A - S - H

SEM testing and analysis

From the XRD analysis above, it was found that, when CGF451 and P.O. 42.5 cement were added to the stone waste, a series of cementitious products was generated by the hydration reaction. In order to compare and analyze the influence law of curing time on the CGF451 and cement stone waste specimens, the two specimens with a curing time of 7 d and 28 d were subjected to SEM testing, respectively.

Micro-morphological analysis of cement stone waste specimens at 7 d and 28 d

Wulian stone waste is mainly dominated by a primary mineral monoclinic structure, the mass proportion of which reaches more than 90%. The morphology of the waste is lumps and flakes, mainly flaky agglomerates with an aggregated structure. The particles exhibit significant point-surface contact, and there are a large number of pores between the particles. Figure 12 shows the microscopic morphology for cement stone waste specimens at 7 d and 28 d. Stone waste particles were generated on the surface of flocculent C-S-H, but at this point, there was very little C-S-H and it did not completely cover the stone waste particles. Large cracks appeared in the middle of the cementitious product, and the size of the cracks and their distribution were not uniform. In the cement stone waste specimens cured for 28 d, significant changes were observed in terms of the microscopic morphology. In addition to generating more flocculent C-S-H, a large number of needle-and-rod-like crystals were generated in the stone waste particles and were irregularly arranged, like weeds. The crystals’ diameter was less than 1 μm, and they had lengths of 5 ~ 100 μm. From the previous EDS analysis, it can be seen that, for the needle-and-rod-like crystals in the AFt, compared to 7 d, the cleavage was significantly reduced. The cleavage between the stone waste particles is generated by hydration. The cracks between the stone waste particles were filled and cemented by the generated hydration products, and the large and small stone waste particles and cementation products were closely linked together to form a three-dimensional skeleton structure. The number and size of the cracks were significantly reduced, and the compactness of the soil was significantly improved, which demonstrates the superiority of the 28 d samples compared with the 7 d samples.Fig. 12 Microstructure of cement stone waste samples.

Micro-morphological analysis of CGF stone waste specimens at 7 d and 28 d

Figure 13 shows the microscopic morphology of CGF451 stone waste specimens at 7 d and 28 d. At 7 d, it was found that the flocculated C-S-H generated on the surface of the stone waste particles inside the CGF451 specimens was greater and had a more complete structural development compared with that on the cement specimens. Moreover, it was found that the flocculated C-S-H bonded the stone waste particles, and there were fewer internal cracks than in the cement specimens. At 28 d of curing, according to the previous EDS analysis, the C-S-H generated under the CGF system had a larger Ca/Si atomic ratio and a more stable structure than that generated by the cement system. The types of cementitious products did not increase, but more flocculated C-S-H was generated, which gradually formed a three-dimensional skeleton structure. Compared with the flocculated C-S-H, its cementing, filling, and wrapping ability was stronger. The whole SEM interface was almost entirely covered by the three-dimensional mesh structure of the C-S-H, which completely wrapped the stone waste particles with a very dense structure, and almost no cracks occurred, which is significantly different from the microscopic morphology of the cement stone waste specimens.Fig. 13 Microstructure of CGF451 solidified stone waste.

EDS test and analysis

In order to analyze the properties of the gel products and the atomic percentage of each element in the specimens, EDS spectroscopy was carried out on the cement stone waste and CGF451 stone waste specimens at 28 d. The EDS and SEM tests were carried out in parallel with each other, and the test positions were associated with the morphology of the typical gelling products.

The microscopic morphology and EDS elements of the gel materials generated from the cement and CGF stone waste specimens at the curing time of 28 d are shown in Fig. 14. Typical hydration products such as flocculated products (test point 1), needle-and-rod products (test point 2), and flocculated products (test point 3) in the cement system and the CGF system were tested, respectively. It can be seen that the elements formed under the cement and CGF451 stone waste systems mainly include O, Na, Al, Ca, and Si. The Ca/Si atomic ratio in the hydration product C-S-H is generally located between 1.2 and 2.3, and the smaller the Ca/Si atomic ratio, the more it indicates that the crystal structure is unstable, and with a larger Ca/Si atomic ratio, the crystal structure is more stable. The Ca/Si atomic ratio of test point 1 is 0.13, of point 3 is 2.51and test point 4 is 1.15, so it is presumed that the C-S-H gelling content generated under the CGF451 stone waste system is higher. The elemental composition of test point 2 is consistent with the crystalline elements of AFt (molecular formula 3CaO-Al2O3-3CaSO4-32H2O), in which the Si element is the C-S-H attached to the surface of AFt. The early strength (7 d) is related to the dissolution speed of Si and Al in the pozzolanic material, the dissolution speed of Si is slow and the dissolution speed of Al is fast, therefore, the strength growth of the specimen shows a positive correlation with the Ca/Si ratio and a negative correlation with the Si/Al ratio; the late strength (28 d) is only related to the dissolution speed of Si in the pozzolanic material. Therefore, the strength growth of the specimens is negatively correlated with the Ca/Si ratio and positively correlated with the Si/Al ratio.Fig. 14 EDS analysis of CGF451 and cement solidified stone waste.

Conclusions

In this study, CGF451, CGF442, CGF541, and P.O. 42.5 cement were used to solidifying the stone waste, respectively. The deformation characteristics and strength characteristics of solidified stone waste were analyzed. In addition, X-ray diffraction, scanning electron microscopy, and energy spectroscopy analyses were conducted to reveal the different action mechanisms of CGF and P.O. 42.5 cement on the stone waste. The main conclusions are as follows:The stress–strain curves and damage morphology of CGF solidified stone wastes showed similar changing patterns, i.e., all of them were typical brittle damage forms, which became more significant with the curing time. Moreover, the cracks developed in the solidified stone waste specimens from the top to the bottom, followed by the gradual peeling of the outer layer.

At the curing time of 7 and 28 d, the UCS of solidified stone waste specimens increased with curing time. Under the conditions of the same binder and curing time, the binder content showed a positive correlation with the UCS of the specimens. The UCS of CGF451 was the highest, followed by CGF442, CGF541, and cement, respectively. The UCS of CGF451 solidified stone waste reaches 2.93 and 4.42 MPa under curing of 7 and 28 d at 5% binder content, which is 1.61 and 1.37 times higher that of P.O. 42.5 cement. Compared to cement, the UCS of CGF specimens increased rapidly in the early curing time (7d) but slowed down in the later curing time (28 d).

The XRD test showed that the alkali-activated gelling products of the CGF stone waste specimen mainly include C-S-H, C-A-H, and C-A-S-H, and the hydration products of the cement stone waste specimen mainly include C-S-H, C-A-H, C-A-S-H, and AFt. Ca(OH)2 mainly originates from CCR, but diffraction peaks for Ca(OH)2 were not found in the analysis, which indicated that Ca(OH)2 was continuously consumed as an alkali activator to promote the hydration reaction.

Compared to soils containing secondary minerals, the primary mineral-dominated stone wastes were not involved in the hydration reaction of binder. The gelling production cemented the stone waste particles and filled the voids in stone waste, thus reducing the cracks inside the specimen. Therefore, the overall structure was stabilized, and the strength of the specimen was further enhanced. The aforementioned results were confirmed by EDS analysis.

Acknowledgements

The authors express our deepest gratitude to all the volunteers who participated in this study. This study is supported by the National Natural Science Foundation of China (Grant Nos. 52378380, 52078474 and 51779235). Data Availability Statement: The data presented in this study are available on request from the corresponding author.

Author contributions

Wang: Conceptualization, Data curation, Formal analysis, Writing – original draft & ed-iting, Visualization, Investigation. Yang: Funding acquisition, Writing – review & editing. Data curation, Investigation. Wu: Writing – review & editing. Data curation, Investigation.

Data availability

The data presented in this study areavailable on request from the corresponding author.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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