Research on the Mechanical Properties and Microstructure of Modified Silt Sediment Geopolymer Materials

The treatment of silted sediment in the river is a global problem. The accumulation of waste sediment will lead to an adverse impact on the environment. In this paper, the silted sediment was reused to produce geopolymer composite materials via alkali-activated gelling modification. The effects of the modifiers of sodium silicate solution, quicklime, and Na2SO4 admixture, and the dosage of the slag, fly ash, and silica fume admixture, and curing conditions and age, on the compressive strength and microstructure of the geopolymer-modified sediment materials were studied. The crystalline phase and hydration products of the modified sediment geopolymer composites were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), respectively. A compressive strength test was conducted to evaluate the mechanical properties of the composites. The results showed that the type and dosage of modifier, amount of mineral admixture additive, cure conditions, and cure age had significant effects on the mechanical properties of the composites. The effect of the addition of mineral admixture on the compressive strength of the modified sediment specimens was more noticeable than that of the modifier. The compressive strength of the geopolymer-modified specimens was greatly increased by the addition of mineral dopants. When 10 wt.% silica fume is added, the compressive strength reaches a maximum value of 33.25 MPa at 60 days. The SEM-EDS results show that the C-S-H gels and C-A-S-H gels were the main hydration products. The results indicate that river siltation sediment is an excellent raw material for geopolymer-modified materials. It is feasible to produce reliable and sustainable hydraulic engineering materials by using river sediment geopolymer-modified materials.


Introduction
In recent years, with the development of the economy and society, environmental protection has become the main concept of development in most countries in the world. High-energy consumption and high-emission materials are gradually being replaced by new green materials, and the shortage of traditional building materials is becoming more and more serious. The production of conventional Portland cement-based building materials accounts for approximately 6% of total global CO 2 emissions [1,2].
Geopolymers are considered a new green building material with different applications in various fields such as coatings and adhesives [3], fiber composite production [4], decorative stone products [5], thermal insulation [6], construction materials, low energy tiles [7], The above-mentioned studies show that sediment materials can be modified into geopolymer materials by alkali-activation to replace some cement production. However, current geopolymer modification studies do not consider comprehensively enough impact conditions, and it is difficult to refer to them in subsequent studies. If we can consider the impact of modifier type and admixture, external admixture type and admixture and various other conditions comprehensively, we can bring into play the great potential of geopolymer-modified sediment materials.
The purpose of this paper was to investigate the variation in compressive strength of modified silt-sediment geopolymers with the type and dosage of modifier and admixture, and the effect of curing conditions and curing age on the compressive strength, microstructure, hydration processes, and types of products produced. This leads to a better explanation of the mechanism related to modifying silt-sediment geopolymer materials. The mole ratio of SiO 2 to Na 2 O in the sodium silicate solution determines the alkalinity of the activator solution, which, in turn, affects the properties of the sample. Therefore, by choosing the mole ratios of SiO 2 to Na 2 O in the sodium silicate solution as 3.0, 2.5, 2.0, and 1.5, their effects on the compressive strength of the samples were investigated. The compressive strength of the samples is shown in Figure 1. to gel-modify Yellow River sediment. The compressive strength of the Yellow River sediment-modified composites increased as the dosage of Ca(OH)2 increased, and the addition of NaOH resulted in a rapid increase in early strength. However, too high a dosage can lead to deterioration in compressive strength.

Results and Discussion
The above-mentioned studies show that sediment materials can be modified into geopolymer materials by alkali-activation to replace some cement production. However, current geopolymer modification studies do not consider comprehensively enough impact conditions, and it is difficult to refer to them in subsequent studies. If we can consider the impact of modifier type and admixture, external admixture type and admixture and various other conditions comprehensively, we can bring into play the great potential of geopolymer-modified sediment materials.
The purpose of this paper was to investigate the variation in compressive strength of modified silt-sediment geopolymers with the type and dosage of modifier and admixture, and the effect of curing conditions and curing age on the compressive strength, microstructure, hydration processes, and types of products produced. This leads to a better explanation of the mechanism related to modifying silt-sediment geopolymer materials.

The Effect of the Mole Ratio of SiO2 to Na2O of the Sodium Silicate Solution on the Compressive Strength
The mole ratio of SiO2 to Na2O in the sodium silicate solution determines the alkalinity of the activator solution, which, in turn, affects the properties of the sample. Therefore, by choosing the mole ratios of SiO2 to Na2O in the sodium silicate solution as 3.0, 2.5, 2.0, and 1.5, their effects on the compressive strength of the samples were investigated. The compressive strength of the samples is shown in Figure 1. From Figure 1 it can be seen that the compressive strength of the samples increases with the decrease in the mole ratio of SiO2 to Na2O in the sodium silicate solution, and the compressive strength of the samples increases with curing age. The Ca(OH)2 generated by the reaction of quicklime promotes the activation of the sediment and reacts with the reactive Si and Al components in the sediment to generate gelling substances. The hydration products C-S-H gel and C-A-S-H gel fill the gaps between the particles, making the sediment particles bond more tightly and improving the integrity of the samples; thus, enhancing the compressive strength of the samples. Samples of group A4 reached the highest strength of 5.19 MPa at the curing age of 90 days. However, this does not meet the From Figure 1 it can be seen that the compressive strength of the samples increases with the decrease in the mole ratio of SiO 2 to Na 2 O in the sodium silicate solution, and the compressive strength of the samples increases with curing age. The Ca(OH) 2 generated by the reaction of quicklime promotes the activation of the sediment and reacts with the reactive Si and Al components in the sediment to generate gelling substances. The hydration products C-S-H gel and C-A-S-H gel fill the gaps between the particles, making the sediment particles bond more tightly and improving the integrity of the samples; thus, enhancing the compressive strength of the samples. Samples of group A4 reached the highest strength of 5.19 MPa at the curing age of 90 days. However, this does not meet the strength requirements of flood control stone. If the strength of the samples were improved by only using NaOH to reduce the mole ratio of SiO 2 to Na 2 O in the sodium silicate solution, the strength requirements may still not be achieved and it is not economical and satisfactory enough. Therefore, it is necessary to consider changing other conditions to improve the performance of the samples.

The Effect of Quicklime Dosage on Compressive Strength
The above tests found that it was not satisfactory to improve the compressive strength of the samples only by changing the mole ratio of SiO 2 to Na 2 O in the sodium silicate solution. In addition, during the curing of the samples, it was found that the samples showed different degrees of surface alkali precipitation when the mole ratio of SiO 2 to Na 2 O in the sodium silicate solution was 1.5. Therefore, subsequent tests were carried out with sodium silicate solution with a mole ratio of SiO 2 to Na 2 O of 2.0, and the influence on the performance of the samples was investigated by changing the dosage of quicklime. The compressive strength of the samples is shown in Figure 2. strength requirements of flood control stone. If the strength of the samples were improved by only using NaOH to reduce the mole ratio of SiO2 to Na2O in the sodium silicate solution, the strength requirements may still not be achieved and it is not economical and satisfactory enough. Therefore, it is necessary to consider changing other conditions to improve the performance of the samples.

The Effect of Quicklime Dosage on Compressive Strength
The above tests found that it was not satisfactory to improve the compressive strength of the samples only by changing the mole ratio of SiO2 to Na2O in the sodium silicate solution. In addition, during the curing of the samples, it was found that the samples showed different degrees of surface alkali precipitation when the mole ratio of SiO2 to Na2O in the sodium silicate solution was 1.5. Therefore, subsequent tests were carried out with sodium silicate solution with a mole ratio of SiO2 to Na2O of 2.0, and the influence on the performance of the samples was investigated by changing the dosage of quicklime. The compressive strength of the samples is shown in Figure 2. From Figure 2 it can be seen that the compressive strength of the samples increases with the increase in the dosage of quicklime. The compressive strength of samples in groups B1, B2, and B3 increased with increasing curing age, while the compressive strength of samples in group B4 with 10% quicklime admixture increased sequentially at curing ages of 7 days, 28 days, and 60 days, but decreased once the curing age reached 60 days. This is because the un-reacted Ca(OH)2 on the surface of the samples absorbed the CO2 in the air to generate CaCO3 due to the over-dosage of quicklime, which caused the volume of the samples to expand and small cracks to appear. Therefore, the dosage of admixture in the sediment geopolymer composites should not be too much, and excessive free OH − residue in the samples will weaken the gel structure and damage the integrity of the material [37][38][39][40]. The compressive strength of the modified specimens reached a maximum of 6.48 MPa at the curing age of 60 d when the lime dosing was 10%, which was 24.86% higher than the maximum strength of 5.19 MPa in the above test of the effect of sodium silicate solution on the compressive strength of the modified specimens. Through the test of the effect of quicklime dosage, it was found that the addition of quicklime could effectively improve the early mechanical properties of the samples, and the optimal dosage was 5-9%, but the compressive strength at 90 days of curing age was less than 10 MPa, and it was necessary to consider changing other conditions to further improve the properties of the samples. From Figure 2 it can be seen that the compressive strength of the samples increases with the increase in the dosage of quicklime. The compressive strength of samples in groups B1, B2, and B3 increased with increasing curing age, while the compressive strength of samples in group B4 with 10% quicklime admixture increased sequentially at curing ages of 7 days, 28 days, and 60 days, but decreased once the curing age reached 60 days. This is because the un-reacted Ca(OH) 2 on the surface of the samples absorbed the CO 2 in the air to generate CaCO 3 due to the over-dosage of quicklime, which caused the volume of the samples to expand and small cracks to appear. Therefore, the dosage of admixture in the sediment geopolymer composites should not be too much, and excessive free OH − residue in the samples will weaken the gel structure and damage the integrity of the material [37][38][39][40]. The compressive strength of the modified specimens reached a maximum of 6.48 MPa at the curing age of 60 d when the lime dosing was 10%, which was 24.86% higher than the maximum strength of 5.19 MPa in the above test of the effect of sodium silicate solution on the compressive strength of the modified specimens. Through the test of the effect of quicklime dosage, it was found that the addition of quicklime could effectively improve the early mechanical properties of the samples, and the optimal dosage was 5-9%, but the compressive strength at 90 days of curing age was less than 10 MPa, and it was necessary to consider changing other conditions to further improve the properties of the samples.

The Effect of Na 2 SO 4 Dosage on Compressive Strength
Based on the above test, Na 2 SO 4 was added to investigate the influence of its content on the compressive strength law of the test samples. The compressive strength of the samples is shown in Figure 3.

The Effect of Na2SO4 Dosage on Compressive Strength
Based on the above test, Na2SO4 was added to investigate the influence of its content on the compressive strength law of the test samples. The compressive strength of the samples is shown in Figure 3. From Figure 3 it can be seen that the compressive strength of the samples of group A3 without the admixture of Na2SO4 increases with increasing curing age. When Na2SO4 was added, initially, the compressive strength of the samples increased, and the compressive strength increased as the Na2SO4 dose increased. However, with the increase in the curing age, the strength decreased substantially, and some of the samples even appeared to disintegrate as the curing time was extended. The addition of SO4 2− caused this phenomenon by reacting with the Ca(OH)2 in the samples to generate CaSO4, which then reacted with hydrated calcium aluminate to produce calcium alumina (tri-sulfur-type hydrated calcium sulfur aluminate). The samples expanded in volume, causing them to crack at a later stage, and the strength first to increase and then decrease. Therefore, sulfate-based reagents are not suitable for use as gel-modifiers for sediment, and it was necessary to consider changing other conditions to further improve the properties of the samples. Mineral admixture is widely used in cement-based materials as an easily produced admixture. Therefore, mineral admixture was added in subsequent tests to effectively improve the compressive strength of the samples.

The Effect of Mineral Admixtures on Compressive Strength
Additions of 10% slag, fly ash, and silica fume, respectively, were made to improve the properties of the samples containing 7% sodium silicate solution and 6% quicklime and study the effect of mineral admixture type on compressive strength. The compressive strength of the samples is shown in Figure 4. From Figure 3 it can be seen that the compressive strength of the samples of group A3 without the admixture of Na 2 SO 4 increases with increasing curing age. When Na 2 SO 4 was added, initially, the compressive strength of the samples increased, and the compressive strength increased as the Na 2 SO 4 dose increased. However, with the increase in the curing age, the strength decreased substantially, and some of the samples even appeared to disintegrate as the curing time was extended. The addition of SO 4 2− caused this phenomenon by reacting with the Ca(OH) 2 in the samples to generate CaSO 4 , which then reacted with hydrated calcium aluminate to produce calcium alumina (tri-sulfur-type hydrated calcium sulfur aluminate). The samples expanded in volume, causing them to crack at a later stage, and the strength first to increase and then decrease. Therefore, sulfate-based reagents are not suitable for use as gel-modifiers for sediment, and it was necessary to consider changing other conditions to further improve the properties of the samples. Mineral admixture is widely used in cement-based materials as an easily produced admixture. Therefore, mineral admixture was added in subsequent tests to effectively improve the compressive strength of the samples.

The Effect of Mineral Admixtures on Compressive Strength
Additions of 10% slag, fly ash, and silica fume, respectively, were made to improve the properties of the samples containing 7% sodium silicate solution and 6% quicklime and study the effect of mineral admixture type on compressive strength. The compressive strength of the samples is shown in Figure 4. From Figure 4 it can be seen that the three mineral admixtures of slag, fly ash, and silica fume can effectively improve the strength of the composite samples, and the modification effect of silica fume is better than that of slag and fly ash. The highest strength was achieved for the silica fume-modified samples of group D3, which reached 33.25 MPa  From Figure 4 it can be seen that the three mineral admixtures of slag, fly ash, and silica fume can effectively improve the strength of the composite samples, and the modification effect of silica fume is better than that of slag and fly ash. The highest strength was achieved for the silica fume-modified samples of group D3, which reached 33.25 MPa at the age of 60 days after curing. Compared with the highest compressive strength of 6.48 MPa of the specimens in the earlier test without admixture modification, the compressive strength improvement effect increased significantly by 413.12%.The compressive strength of D1 and D2 group samples increased with the increase in curing age, and the increase was faster in the early stage and slower in the later stage, while the compressive strength of D3 group samples mixed with silica fume increased with the increase in curing age before 60 days and slightly decreased after 60 days. The compressive strength of silica fume-modified samples increased and then decreased with the increase in curing age because of the high activity of silica fume and its small particle size, the fast reaction of gel-modification, the rapid consumption of water in the samples, and the dry shrinkage in the dry air at the later stage of curing, which caused cracks in the samples and led to the decrease in compressive strength. Significantly, the 90-day curing age strength of the materials after the addition of mineral admixture is greater than 10 MPa, which meets the requirements of engineering applications.

The Effect of Curing Conditions on Compressive Strength
From the above tests, it can be seen that the compressive strength of the mineral admixture-modified samples is better than that of the samples without admixture. For gelling materials, curing conditions also play a crucial role in their performance [41]. After studying the effect of the modifier type, the B2 group of samples was selected to study the effect of the curing conditions on the compressive strength of the samples. The curing conditions are shown in Table 1, and the compressive strength of the samples is shown in Figure 5. Natural curing is curing at room temperature, while standard curing for reference concrete is maintaining 20 ± 2 • C and humidity not less than 95% in a standard curing room.   From Figure 5 it can be seen that the curing conditions have an effect on the compressive strength of the samples. Compared to the compressive strength of the samples at the age of 28 days in natural curing, the compressive strength of the samples at the age of 28 days in standard curing and curing after immersion for 7 days increased, but the increase was smaller, while the compressive strength at the age of 28 days in immersion curing From Figure 5 it can be seen that the curing conditions have an effect on the compressive strength of the samples. Compared to the compressive strength of the samples at the age of 28 days in natural curing, the compressive strength of the samples at the age of 28 days in standard curing and curing after immersion for 7 days increased, but the increase was smaller, while the compressive strength at the age of 28 days in immersion curing decreased. When comparing the compressive strength of E5 and E6, the compressive strength of natural curing for 28 days after high-temperature curing at 90 • C for 12 h is 3.35 MPa, which is 26.8% higher than the 1.25 MPa before natural curing, so the hightemperature drying curing method can significantly improve the compressive strength of the samples.

The Effect of Immersion Curing on Compressive Strength
The above test demonstrated that immersion curing has an effect on the compressive strength of the samples. In order to further investigate the effect of immersion curing on the compressive strength of long-age samples and short-age samples, the samples of groups A and B with 7 days and 90 days curing were taken as the objects of study, and each group of samples was immersed for 24 h and dried for 3 days, and the compressive strength was tested after the samples were dried and compared with the compressive strength before immersion to determine the relationship, and the strength changes are shown in Figures 6 and 7.  The change in compressive strength of the samples in Figures 6 and 7 shows that the strength of the samples in groups A and B at 7 days curing has increased through immersion. The largest increase in compressive strength of the samples was in the A2 group, from 1.01 MPa to 3.34 MPa, an increase of 203.69%. Conversely, the compressive strengths of the samples at 90 days curing were all reduced by immersion, and the largest reduction was from 2.87 MPa to 1.64 MPa in group A1, a reduction of 44.25%. It can be seen that early immersion can improve the compressive strength of the sample, bur prolonged immersion decreases the compressive strength. The reason for this phenomenon may be due to the fact that immersion in water in the early stage can accelerate the dissolution of active The change in compressive strength of the samples in Figures 6 and 7 shows that the strength of the samples in groups A and B at 7 days curing has increased through immersion. The largest increase in compressive strength of the samples was in the A2 group, from 1.01 MPa to 3.34 MPa, an increase of 203.69%. Conversely, the compressive strengths of the samples at 90 days curing were all reduced by immersion, and the largest reduction was from 2.87 MPa to 1.64 MPa in group A1, a reduction of 44.25%. It can be seen that early immersion can improve the compressive strength of the sample, bur prolonged immersion decreases the compressive strength. The reason for this phenomenon may be due to the fact that immersion in water in the early stage can accelerate the dissolution of active Si and Al components, accelerate their hydration reaction, and make the compressive strength of the sample increase rapidly; conversely, when the immersion is carried out in the later stage of curing, the hydration reaction is completed, and water will enter the microcracks inside the sample, which will then expand, weaken the bonding of the material inside the sample, and destroy its integrity, leading to the reduction in compressive strength. It is also possible that, when immersed in water for curing, there is too much water around the hydration products, which hinders their cohesion reaction and leads to a decrease in strength [42].  The change in compressive strength of the samples in Figures 6 and 7 shows that the strength of the samples in groups A and B at 7 days curing has increased through immersion. The largest increase in compressive strength of the samples was in the A2 group, from 1.01 MPa to 3.34 MPa, an increase of 203.69%. Conversely, the compressive strengths of the samples at 90 days curing were all reduced by immersion, and the largest reduction was from 2.87 MPa to 1.64 MPa in group A1, a reduction of 44.25%. It can be seen that early immersion can improve the compressive strength of the sample, bur prolonged immersion decreases the compressive strength. The reason for this phenomenon may be due to the fact that immersion in water in the early stage can accelerate the dissolution of active Si and Al components, accelerate their hydration reaction, and make the compressive strength of the sample increase rapidly; conversely, when the immersion is carried out in the later stage of curing, the hydration reaction is completed, and water will enter the microcracks inside the sample, which will then expand, weaken the bonding of the material inside the sample, and destroy its integrity, leading to the reduction in compressive strength. It is also possible that, when immersed in water for curing, there is too much water around the hydration products, which hinders their cohesion reaction and leads to a decrease in strength [42].  -0466) appears at around 27.9 • 2θ. The initial reaction of the modifier and mineral admixture-activated sediment seems to lead to the formation of C-S-H gels with carbonation in the C-S-H gels and the formation of CaCO 3 at later maintenance ages [43]. From the analysis of the literature [44], the diffraction peaks at 20.9 • 2θ and 50.1 • 2θ are the characteristic diffraction peaks of the calcium silicate hydrate (Ca 1.5 Si 3.5 ·nH 2 O). However, it was difficult to determine the diffraction characteristic peaks of C-S-H because they overlap with calcite peaks and quartz peaks. The presence of C-S-H gel needs to be further investigated using SEM-EDS. After geopolymer-modification, the microcline (KAlSi 3 O 8 , PDF#19-0932) peak at 27.5 • 2θ and the albite (NaAlSi 3 O 8 , PDF#09-0466) peak at 27.9 • 2θ became lower. The reduction of albite (NaAlSi 3 O 8 , PDF#09-0466) peak compared to microcline (KAlSi 3 O 8 , PDF#19-0932) peak in the quicklime-and slag-modified specimens is more obvious. Thus, it can be inferred that the sediments of the Xiaoluan River have potentially volcanic ash-active Si and Al.  Figure 8 shows the XRD of the original and modified sediment samples. Some diffraction peaks have changed. Sharp peaks are observed at around 20.9° 2θ and 26.6° 2θ for the original sediment sample, indicating the presence of quartz (SiO2, PDF#46-1045), and albite (NaAlSi3O8, PDF#09-0466) appears at around 27.9° 2θ. The initial reaction of the modifier and mineral admixture-activated sediment seems to lead to the formation of C-S-H gels with carbonation in the C-S-H gels and the formation of CaCO3 at later maintenance ages [43]. From the analysis of the literature [44], the diffraction peaks at 20.9° 2θ and 50.1° 2θ are the characteristic diffraction peaks of the calcium silicate hydrate (Ca1.5Si3.5·nH2O). However, it was difficult to determine the diffraction characteristic peaks of C-S-H because they overlap with calcite peaks and quartz peaks. The presence of C-S-H gel needs to be further investigated using SEM-EDS. After geopolymer-modification, the microcline (KAlSi3O8, PDF#19-0932) peak at 27.5° 2θ and the albite (NaAlSi3O8, PDF#09-0466) peak at 27.9° 2θ became lower. The reduction of albite (NaAlSi3O8, PDF#09-0466) peak compared to microcline (KAlSi3O8, PDF#19-0932) peak in the quicklime-and slag-modified specimens is more obvious. Thus, it can be inferred that the sediments of the Xiaoluan River have potentially volcanic ash-active Si and Al.  Figure 9 shows the scanning electron microscopy images of the composites after the modification of the mineral admixture. In order to determine the mineral composition of the new materials, we focused on the modified sediment particles observed in the SEM images of the materials, and acquired EDS energy spectrum to analyze the composition at   Figure 9 shows the scanning electron microscopy images of the composites after the modification of the mineral admixture. In order to determine the mineral composition of the new materials, we focused on the modified sediment particles observed in the SEM images of the materials, and acquired EDS energy spectrum to analyze the composition at the positions shown as "A," "B," and "C" in Figure 9. The results of the energy spectrum analysis are shown in Table 2. The scanned parts contain mainly carbon, oxygen, magnesium, aluminum, silicon, calcium, iron, etc. The weight percentages of Si and Ca elements confirm that C-S-H gel is one of the products in the composite. Inspection of the SEM images shows that, first, the gel-modified products are mainly glassy and amorphous particles, which contain a small number of impurities. Second, Figure 9(1) shows that the voids of the sediment particles are filled with some white material, which is clearly shown in location A. The white material fills the gaps between the particles and densely wraps the sediment particles. This gelatinous substance combines the stability of the sediment particles and the solid mass, helping to reduce the microcracks in the sample and thus improve its strength. Based on the above analysis, the white material may be the C-S-H gel, mainly produced by the mineral admixture under alkaline conditions. Although there were some cracks and holes in the internal structure, the samples were denser than before. The gel produced during alkali-activation plays an important role in filling the holes and improving the strength of the composite. The results show that the amount of C-S-H gel is small, which may be due to the low amount of mineral admixture and the fact that CO 2 in the air can carbonize the C-S-H gel.  Table 2. The scanned parts contain mainly carbon, oxygen, magnesium, aluminum, silicon, calcium, iron, etc. The weight percentages of Si and Ca elements confirm that C-S-H gel is one of the products in the composite. Inspection of the SEM images shows that, first, the gel-modified products are mainly glassy and amorphous particles, which contain a small number of impurities. Second, Figure 9(1) shows that the voids of the sediment particles are filled with some white material, which is clearly shown in location A. The white material fills the gaps between the particles and densely wraps the sediment particles. This gelatinous substance combines the stability of the sediment particles and the solid mass, helping to reduce the microcracks in the sample and thus improve its strength. Based on the above analysis, the white material may be the C-S-H gel, mainly produced by the mineral admixture under alkaline conditions. Although there were some cracks and holes in the internal structure, the samples were denser than before. The gel produced during alkali-activation plays an important role in filling the holes and improving the strength of the composite. The results show that the amount of C-S-H gel is small, which may be due to the low amount of mineral admixture and the fact that CO2 in the air can carbonize the C-S-H gel.

Conclusions
(1) River and lake sediment can be made into new geopolymer materials using the alkaliactivated gel modification method. The sediment geopolymer materials produced have a maximum strength of 33.25 MPa, and the sediment geopolymer materials made according to this method have the potential to be used in water conservation projects such as rock preparation along rivers and lakes. (2) Alkali dosage, mineral admixture type, and dosage, as well as curing condition and curing age, are significant factors affecting the mechanical strength of the composites. The strength of the samples will increase with increasing alkali dosage, but an excessive amount of alkali will have a negative impact on the compressive strength. Sulfate is not suitable as an alkali activator for geopolymer modification, and it will greatly reduce the durability of the modified material. Mineral dosage can significantly improve the early strength of the composites. (3) High-temperature curing can accelerate the hydration reaction process and improve the reaction efficiency. Immersion curing can promote the growth of compressive strength of modified specimens in the early stage, while for long-age specimens immersion curing will weaken their compressive strength. (4) The compressive strength of the samples was significantly enhanced after mixing with slag, fly ash, and silica fume. The XRD and SEM-EDS test results showed that the alkali-activated products were mainly flocculent or honeycomb geopolymer gels. The geopolymer gels were mainly amorphous hydrated calcium silicate (C-S-H) gels  Table 2. Average molar ratio of basic elements at sample points A, B, and C in Figure 9.

Conclusions
(1) River and lake sediment can be made into new geopolymer materials using the alkaliactivated gel modification method. The sediment geopolymer materials produced have a maximum strength of 33.25 MPa, and the sediment geopolymer materials made according to this method have the potential to be used in water conservation projects such as rock preparation along rivers and lakes. (2) Alkali dosage, mineral admixture type, and dosage, as well as curing condition and curing age, are significant factors affecting the mechanical strength of the composites. The strength of the samples will increase with increasing alkali dosage, but an excessive amount of alkali will have a negative impact on the compressive strength. Sulfate is not suitable as an alkali activator for geopolymer modification, and it will greatly reduce the durability of the modified material. Mineral dosage can significantly improve the early strength of the composites. (3) High-temperature curing can accelerate the hydration reaction process and improve the reaction efficiency. Immersion curing can promote the growth of compressive strength of modified specimens in the early stage, while for long-age specimens immersion curing will weaken their compressive strength. (4) The compressive strength of the samples was significantly enhanced after mixing with slag, fly ash, and silica fume. The XRD and SEM-EDS test results showed that the alkali-activated products were mainly flocculent or honeycomb geopolymer gels. The geopolymer gels were mainly amorphous hydrated calcium silicate (C-S-H) gels and hydrated calcium aluminate (C-A-S-H) gels, which then reacted with carbon dioxide to produce calcium carbonate, and the crystalline phase of the calcium carbonate in the composites was determined by XRD.

Raw Material
The raw material for this experiment was taken from the Xiaoluan River in the western part of Weichang County, Hebei Province, China. Its particle morphology was observed using a SteRE0 Discovery V8 microscope from Zeiss, Germany, as shown in Figure 10. Referring to the subsequent XRD analysis, it can be seen that, at 300× magnification, the raw-material particles were mainly composed of crystalline particles with different grain sizes. At 1200× magnification, it was observed that the crystalline particles were irregular and translucent. and hydrated calcium aluminate (C-A-S-H) gels, which then reacted with carbon dioxide to produce calcium carbonate, and the crystalline phase of the calcium carbonate in the composites was determined by XRD.

Raw Material
The raw material for this experiment was taken from the Xiaoluan River in the western part of Weichang County, Hebei Province, China. Its particle morphology was observed using a SteRE0 Discovery V8 microscope from Zeiss, Germany, as shown in Figure  10. Referring to the subsequent XRD analysis, it can be seen that, at 300× magnification, the raw-material particles were mainly composed of crystalline particles with different grain sizes. At 1200× magnification, it was observed that the crystalline particles were irregular and translucent. The raw materials were mixed and then oven dried at 105 °C to a constant weight. After being cooled to room temperature, the particle size distribution (PSD) of the raw materials was measured with an NKL62000 type laser particle distribution meter from Jinan Nextech Analytical Instruments Co., Zhengzhou, China, and the fineness modulus of the Xiaoluan River sediment was also determined ( Figure 11). As shown in Figure 11, it was found that the PDS of the raw materials was mainly between 150 and 500 μm, and was relatively concentrated and narrowly distributed. The chemical composition of the Xiaoluan River sediment was analyzed using an X-ray fluorescence spectrometer (XRF-1800) with Cu and Kα emission targets from Shimadzu Corporation, Japan (Table 3). From Table 3, it can be seen that the main components of the Xiaoluan River sediment are SiO2, Al2O3, CaO, Fe2O3, Na2O, and K2O. The content of these six oxides is approximately 95%. The raw materials were mixed and then oven dried at 105 • C to a constant weight. After being cooled to room temperature, the particle size distribution (PSD) of the raw materials was measured with an NKL62000 type laser particle distribution meter from Jinan Nextech Analytical Instruments Co., Zhengzhou, China, and the fineness modulus of the Xiaoluan River sediment was also determined ( Figure 11). As shown in Figure 11, it was found that the PDS of the raw materials was mainly between 150 and 500 µm, and was relatively concentrated and narrowly distributed. The chemical composition of the Xiaoluan River sediment was analyzed using an X-ray fluorescence spectrometer (XRF-1800) with Cu and Kα emission targets from Shimadzu Corporation, Japan (Table 3). From Table 3, it can be seen that the main components of the Xiaoluan River sediment are SiO 2 , Al 2 O 3 , CaO, Fe 2 O 3 , Na 2 O, and K 2 O. The content of these six oxides is approximately 95%.   Powdered samples of sediment were prepared using a mortar and pestle to ensure that the powdered samples were dry and could pass through a sieve of less than 20 μm. These samples were then used to fill a slide with a groove volume of 50 mm 3 . Then XRD data of the Xiaoluan River sediment with different grain sizes were recorded using a Bruker D8 Advance diffractometer (Cu target, k = 0.154 nm). The X-ray tube was operated at 40 kV and 40 mA. All scans were measured over an angular range of 5 to 70° 2θ at a rate of 2°/min and a step size of 0.02°. Figure 12 shows the XRD of the Xiaoluan River sediment. It can be seen that the main mineral components are quartz (SiO2, PDF#46-1045), anorthite (CaAl2Si2O8, PDF#41-1481), anorthite sodian ((Ca, Na)(Al, Si)2Si2O8), albite (NaAlSi3O8, PDF#09-0466), microcline (KAlSi3O8, PDF#19-0932), and hematite (Fe2O3, PDF#33-0664). The crystallinity of the quartz content is high, so its chemical composition is stable. The anorthite peaks are more significant in the fine-grained sediment, indicating that anorthite is mainly present in the fine-grained Xiaoluan River sediment. Powdered samples of sediment were prepared using a mortar and pestle to ensure that the powdered samples were dry and could pass through a sieve of less than 20 µm. These samples were then used to fill a slide with a groove volume of 50 mm 3 . Then XRD data of the Xiaoluan River sediment with different grain sizes were recorded using a Bruker D8 Advance diffractometer (Cu target, k = 0.154 nm). The X-ray tube was operated at 40 kV and 40 mA. All scans were measured over an angular range of 5 to 70 • 2θ at a rate of 2 • /min and a step size of 0.02 • . Figure 12 shows the XRD of the Xiaoluan River sediment. The crystallinity of the quartz content is high, so its chemical composition is stable. The anorthite peaks are more significant in the fine-grained sediment, indicating that anorthite is mainly present in the fine-grained Xiaoluan River sediment.

Admixture
The modifiers used in the test were sodium silicate solution (the mole ratio of SiO2 to Na2O was 3), 99.9% pure analytical NaOH reagent, 99% pure analytical Na2SO4 reagent, and quicklime produced by Tianjin Comio Reagent Co., Tianjin, China. NaOH was used to adjust the mole ratio of SiO2 to Na2O of the sodium silicate solution. Since sodium silicate solution and quicklime were inexpensive and easily available materials and had relatively little environmental pollution, the modified new composite material could be ap-

Admixture
The modifiers used in the test were sodium silicate solution (the mole ratio of SiO 2 to Na 2 O was 3), 99.9% pure analytical NaOH reagent, 99% pure analytical Na 2 SO 4 reagent, and quicklime produced by Tianjin Comio Reagent Co., Tianjin, China. NaOH was used to adjust the mole ratio of SiO 2 to Na 2 O of the sodium silicate solution. Since sodium silicate solution and quicklime were inexpensive and easily available materials and had relatively little environmental pollution, the modified new composite material could be applied to the manufacture of river basin flood-protection stones.
The S95 slag and silica fume used as mineral admixtures in this study were produced by Henan Borun Foundry Materials Co., Ltd., Zhengzhou, China, and fly ash was produced by Henan Gongyi Yuanheng Water Purification Materials Plant, Zhengzhou, China. If these industrial by-products, which are not easily disposed of, are used industrially for secondary use, they will not only bring economic and environmental benefits but will also solve the disposal problem of large amounts of waste [45,46]. Slag is made by reprocessing the floating molten slag on the surface of the iron from the ironmaking furnace [47]. Fly ash is a by-product of the combustion of pulverized coal in thermal power plants, and the growing industrialization in recent years had produced a large amount of fly ash [48]. Previous studies have shown that more than 750 million tons of fly ash are produced annually, but only about 17-20% was re-used [49]; silica fume belongs to the category of industrial waste utilization technologies, and results from the rapid oxidation and condensation precipitation of a large amount of SiO 2 and silica fumes from mineral-heated electric furnaces during the smelting of ferrosilicon and industrial silicon. Silica fume is less dense and has to be processed using specialized encryption equipment [50,51]. The chemical composition of the mineral admixtures is shown in Table 4, and the XRD of the mineral admixture is shown in Figure 13. It can be seen that the main mineral component of fly ash is mullite (Al 6 Si 2 O 13 , PDF#15-0776) and quartz (SiO 2 , PDF#46-1045), the main mineral component of slag is calcium silicate (Ca 3 SiO 5 , PDF#16-0407), and the main mineral component of silica fume is silicon dioxide (SiO 2 , PDF#12-0708).

Casting, Curing and Testing
The sample was a cylindrical body D × H = 50 mm × 50 mm. The steel film pressureforming method was used. The mold was a hollow cylindrical cylinder with an inner diameter of 50 mm, an outer diameter of 60 mm, and a height of 130 mm. The pressure bar was a solid cylinder with a diameter of 49.8 mm and height of 150 mm. The spacer was a solid cylinder with a diameter of 49.8 mm, height of 10 mm, and a cylinder with an outer

Casting, Curing and Testing
The sample was a cylindrical body D × H = 50 mm × 50 mm. The steel film pressureforming method was used. The mold was a hollow cylindrical cylinder with an inner diameter of 50 mm, an outer diameter of 60 mm, and a height of 130 mm. The pressure bar was a solid cylinder with a diameter of 49.8 mm and height of 150 mm. The spacer was a solid cylinder with a diameter of 49.8 mm, height of 10 mm, and a cylinder with an outer diameter of 80 mm, an inner diameter of 55 mm, and height of 55 mm was used for sample demolding.
To account for the pressure formed by the samples, the density was set at 2100 kg/m 3 , and the mass of each raw material was calculated and weighed according to the material ratio. The volume of the sample was 98.125 cm 3 , so the mass of each sample was 206.06 g. After weighing and mixing, the material was mixed with a cement slurry mixer and then filled into the mold by drawing a horizontal line on the pressure bar 70 mm from the bottom, thus maintaining the height of the sample at 50 mm. After 5 min, the sample was demolded. After demolding, the samples were numbered and kept under the same conditions, sealed in plastic sealing bags, and placed in an oven at 90 • C for 12 h before curing, so that the compressive strength of the samples developed rapidly and reached a certain degree of compressive strength in a short period of time. The samples were kept in air for 3 days, 7 days, 28 days, and 90 days, respectively. The compressive strength of the samples was tested using an electronic universal testing machine with a range of 100 kN, and the displacement rate was 0.05 mm/min. The average value of three samples was taken as the compressive strength. The mineral content of the modified samples was determined using an X-ray diffractometer, and the microstructure of the modified samples was observed by scanning electron microscopy (SEM).

Mix Proportion Design
The mixed proportion of the samples is shown in the Table 5 The effect of various factors on the compressive strength of the samples was investigated by varying the composition of the mix and the admixture. Based on a preliminary test, the best solid-liquid ratio of 88:12 was selected considering the formation of the samples and preventing the mixed water from being squeezed.