3.1 Water absorption
Figure 3 displays the test results of the water swelling ratio of the SAP with different particle sizes in solid waste pore solutions with sodium silicate moduli of one and two. As the SAP particle size decreases, the water SAP swelling ratio in solid waste pore solutions with sodium silicate moduli of one and two both increases, indicating that the SAP with smaller particle sizes has better water absorption capacity. This may be because, under the same mass, the SAP with smaller particle sizes has a larger surface area and greater contact area with the solid waste pore solution[33], allowing the water absorption capacity of the SAP to be utilized more effectively, resulting in a higher water swelling ratio.
Furthermore, compared with the SAP in Modulus 1, the water swelling ratio of the SAP with all three different particle sizes in Modulus 2 decreased. This may be because Modulus 2 has a higher pH value compared to Modulus 1. The higher pH value environment encourages the precipitation of metal ions in fly ash and slag, so that the ion concentration in the external solution of the SAP increased. When the ion concentration in the external solution of the SAP increases, water flows from the low-concentration area to the high-concentration area, causing a certain amount of water to be released from the SAP[34].
3.2 Fluidity and setting time
3.2.1 Influence of SAP content on fluidity and setting time
For solid waste filling materials, higher fluidity is beneficial for pumped grouting filling[35–37]. Water absorption increases with the SAP quantity, and the content of free water influences the fluidity of the paste. In addition, a shorter setting time allows rapid use of solid waste filling materials in construction. The SAP reduces the content of free water in the solid waste paste, which directly influences the initial and final setting times of the paste.
To investigate the influence of the SAP content on fluidity and setting time, fly ash and slag with the same proportion were selected. Sodium silicate with a modulus of one was utilized as the alkali activator. The water–binder ratio was set to 0.45. The SAP was incorporated at contents of 0, 0.1, 0.2, 0.3, and 0.4% with a particle size larger than 100 mesh, respectively. The test results are shown in Fig. 4.
The fluidity of solid waste paste continuously decreased with the increase in SAP content, from 13.8 cm at an SAP content of 0% to 9.5 cm at an SAP content of 0.4%. The fluidity decreased by 2.5 cm at an SAP content of 0.1%. With the increase in SAP content, the fluidity kept decreasing, but the downward trend slowed. The fluidity decreased by 0.5 cm on average for every 0.1% increase in the SAP content. This may be because the SAP absorbed free water, but the content of free water in solid waste paste was limited. When free water was absorbed by the SAP to saturation, the increase in SAP content did not cause significant fluctuations in the fluidity of the paste[31].
The initial setting time of the paste continuously decreased with the increase in SAP content, from 31 min at an SAP content of 0% to 14 min at an SAP content of 0.4%. The final setting time of the paste also continuously decreased with the increase in SAP content, from 62 min at an SAP content of 0% to 31 min at an SAP content of 0.4%. This may be because SAPs have excellent water absorption capacities; the more the SAP incorporated, the lower the amount of free water in the paste, leading to a shorter initial setting time. In addition, the increase in SAP content enhanced the SAP spatial distribution proportion in the paste. The free water absorbed by the SAP was released during hydration[33, 38]. The SAP, with a higher distribution proportion released more water, which accelerated the hardening process of the paste, resulting in a shortened final setting time.
3.2.2 Influence of SAP particle size on fluidity and setting time
The SAP expansion ratio test indicates that SAPs with different particle sizes possess varying water absorption capacities, with the water absorption capacity increasing as the particle size decreases. Consequently, particle sizes of the SAP may influence the fluidity and setting time of the paste.
To investigate the effect of SAP particle size on fluidity and setting time, fly ash and slag with the same proportion were selected. Sodium silicate with a modulus of one was utilized as the alkali activator, and the water–binder ratio was set to 0.45. An SAP with particle sizes of 30–50 mesh, 50–100 mesh, and larger than 100 mesh was incorporated at a content of 0.4%. The test results are shown in Fig. 5.
The SAP particle sizes of 50–100 mesh and larger than 100 mesh exhibited a similar impact on the fluidity of the paste, maintaining a range of 9.25 cm to 9.75 cm. However, the fluidity with an SAP particle size of 30–50 mesh was 10.5 cm. This could be attributed to the different spatial distribution proportions of SAPs with varying particle sizes. Given the same mass, smaller SAP particles had a more uniform spatial distribution and greater water absorption. Furthermore, smaller SAP particles had a larger specific surface area and increased contact with water in the solid waste paste. The enhanced contact area facilitated water transmission through capillary action between fly ash and slag particles, weakening the steric effect of these particles [39, 40]. Consequently, water absorption by the SAP increased, and free water in the external solution of the SAP decreased, leading to lower fluidity of the paste.
SAP particles with the same mass but different sizes had minimal impact on the initial setting time of the paste, remaining within the range of 14 to 15 min. The final setting time, however, increased with decreasing SAP particle size, ranging from 23 min for 30–50 mesh to 31 min for larger than 100 mesh particles. The minimal change in initial setting time could be attributed to SAPs of different particle sizes but the same mass absorbing free water to saturation. This ensured that the amount of free water needed for the hydration and agglomeration of the paste was similar, resulting in nearly identical initial setting times. However, with the same content, smaller SAP particles had a higher spatial distribution proportion and a larger specific surface area. Consequently, the paste released more water during the hydration process[33, 38], requiring a longer time to coagulate and achieve a certain strength, thus extending the final setting time.
3.2.3 Influence of sodium silicate modulus on fluidity and setting time
The ion environment and pH value of the solid waste paste can be adjusted by using different moduli of sodium silicate, which influence the water absorption capacity of SAPs in turn. To examine the impact of sodium silicate modulus on fluidity and setting time, fly ash and slag in equal proportions were chosen as the primary solid waste system. An SAP was incorporated at a content of 0.4%, and the water–binder ratio was set to 0.45. Sodium silicate with moduli of one and two was employed as an alkali activator, respectively. The test results are presented in Tables 2 and 3.
As the sodium silicate modulus increased, fluidity increased from 9.5 cm to 17.85 cm. According to the analysis in Section 3.1, increasing the sodium silicate modulus raised the pH value and ion concentration of the solid waste paste, thereby causing the free water absorbed by the SAP was released to areas of higher concentration. Consequently, the free water content and fluidity of the paste increased.
An increase in sodium silicate modulus significantly extended both the initial setting time and final setting time of the paste. The initial setting time rose from 14 to 56 min, whereas the final setting time increased from 31 to 65 min. This could be attributed to the fact that sodium silicate with a higher modulus elevated the pH value of the paste. Metal ions in fly ash and slag, such as Ca2+ and Al3+, are more likely to precipitate in a high pH environment. The rise in ion concentration and pH value hindered the water absorption capacity of the SAP, and a certain amount of water was released from the SAP due to changes in osmotic pressure[41, 42]. The increase in free water content relatively raised the water content required for paste hydration, thus prolonging the initial and final setting times.
Table 2
Fluidity with different sodium silicate moduli
Sodium silicate modulus
|
1
|
2
|
Fluidity/(cm)
|
9.50
|
17.85
|
Table 3
Setting time with different sodium silicate moduli
Sodium silicate modulus
|
1
|
2
|
Initial setting time/(min)
|
14
|
56
|
Final setting time/(min)
|
31
|
65
|
3.3 Compressive strength
3.3.1 Influence of SAP content on compressive strength
A higher compressive strength of solid waste filling materials ensures the safety and stability of construction. To investigate the impact of SAP content on the compressive strength of solid waste blocks, fly ash and slag in equal proportions were selected. Sodium silicate with a modulus of one was utilized as the alkali activator, and the water–binder ratio was set to 0.45. An SAP was incorporated at contents of 0, 0.1, 0.2, 0.3, and 0.4% with a particle size larger than 100 mesh. The test results are shown in Fig. 6.
As the SAP content increased, the 3 day compressive strength of solid waste blocks declined from 27.79 to 19.01 MPa. This could be attributed to larger amounts of the SAP absorbing excessive free water, which hindered the early hydration of the solid waste blocks. With the rise in SAP content, the long-term compressive strength of the solid waste blocks first increased and then decreased. When the SAP incorporated less than 0.2%, the 7 day compressive strength increased from 30.65 to 37.6 MPa, and the 28 day compressive strength increased from 34.45 to 40.41 MPa. However, the 7 and 28 day compressive strengths dropped to 23.43 and 29.88 MPa, respectively, as the content of the SAP increased from 0.2 to 0.4%. This could be attributed to the SAP continuously releasing water throughout the hydration process. An appropriate amount of water release would contribute to sufficient hydration of the solid waste blocks and improved compressive strength[43]. Conversely, excessive SAP contents would create large pores in the solid waste blocks during hydration, significantly compromising the compressive strength[44].
3.3.2 Influence of SAP particle size on compressive strength
The particle sizes of the SAP influence its water absorption capacity, which affect the content of free water released by the SAP during hydration in turn. The longer the water-release duration, the more sufficient the hydration of solid waste blocks, leading to higher compressive strengths. To investigate the impact of SAP particle size on the compressive strength of solid waste blocks, fly ash and slag in equal proportions were selected. Sodium silicate with a modulus of one was utilized as the alkali activator, and the water–binder ratio was set to 0.45. The SAP was incorporated with particle sizes of 30–50 mesh, 50–100 mesh, and larger than 100 mesh at a content of 0.4%. The test results are displayed in Fig. 7.
As the SAP particle size decreased, both early and later strengths improved. The 3 day compressive strength increased from 14.26 to 19.01 MPa, and the 28 day compressive strength rose from 24.27 to 29.88 MPa. This could be attributed to smaller SAP particles having higher water absorption capacities at the same mass. The free water absorbed by the SAP can be continuously released into the solid waste filling body throughout the hydration process, contributing to sufficient hydration and consolidation of the solid waste blocks. Moreover, at the same mass, smaller SAP particles have a broader spatial distribution within the solid waste concretion, allowing for more uniform absorption and release of free water, which also enhances the compressive strength of the blocks [39, 40].
3.3.3 Influence of sodium silicate modulus on compressive strength
Sodium silicate with different moduli is known to influence the pH value and ion environment of solid waste paste, which influences the SAP water absorption capacity in turn. Consequently, the compressive strength of solid waste blocks can also be impacted by the sodium silicate modulus. To examine the effect of sodium silicate modulus on compressive strength, fly ash and slag in equal proportions were chosen as the matrix material. The SAP was incorporated at 0.4% of cementitious materials, and the water–binder ratio was set to 0.45. Sodium silicate with moduli of one and two was employed as the alkali activator. The test results are presented in Fig. 8.
As the sodium silicate modulus increased, both early and later strengths improved. The 3 day compressive strength rose from 19.01 to 22.7 MPa, while the 28 day compressive strength increased from 29.88 to 35.43 MPa. This could be attributed to the higher modulus sodium silicate raising the pH value of the paste, promoting the precipitation of active ions in fly ash and slag particles. These ions increased the ion concentration of the paste, prompting the SAP to release a certain amount of water [42]. More water was provided for early and later hydration, resulting in higher early and later strengths for solid waste blocks with increased sodium silicate moduli.
3.4 Volume shrinkage
3.4.1 Influence of SAP content on volume shrinkage
Lower volume shrinkage of solid waste cementitious materials is essential for maintaining volume stability in related applications, ensuring construction safety and preventing potential disasters. The exceptional water absorption and retention capacities of SAPs allow for the water absorbed in the solid waste paste to be released until the paste solidifies to a certain strength, significantly extending the water-release duration and reducing the volume shrinkage of the solid waste paste [33]. To examine the influence of SAP content on volume shrinkage, fly ash and slag with the same proportion were selected. Sodium silicate with a modulus of one was employed as the alkali activator. The water–binder ratio was set to 0.45. The SAP was incorporated at contents of 0, 0.1, 0.2, 0.3, and 0.4% with a particle size larger than 100 mesh, respectively. The test results are presented in Fig. 9.
The drying shrinkage rate of the solid waste paste decreased with increasing SAP content throughout the curing period. The final drying shrinkage rate dropped from 0.56% at 0% SAP content to 0.19% at 0.4% SAP content, reducing to one-third of the rate without the SAP. This could be due to the SAP capturing part of the free water while continuously releasing the absorbed water as the curing age increased. The SAP with a higher content had a broader spatial distribution and superior water absorption capacity, resulting in sufficient hydration and a longer hydration duration for the solid waste paste. The water bleeding rate of the paste decreased, preventing water evaporation waste and contributing to later hydration. The water released by the SAP was utilized by the solid waste paste for subsequent hydration, effectively alleviating the paste’s shrinkage and significantly reducing its micro-strain[45].
3.4.2 Influence of SAP particle size on volume shrinkage
The SAP water absorption capacity is affected by SAP particle size, and the free water content in the solid waste filling body influences the volume shrinkage of the paste. To examine the impact of SAP particle size on the volume shrinkage of solid waste paste, fly ash and slag with the same proportion were selected. Sodium silicate with a modulus of one was employed as the alkali activator, and the water–binder ratio was set to 0.45. The SAP was incorporated with particle sizes of 30–50 mesh, 50–100 mesh, and larger than 100 mesh at a content of 0.4%.
The drying shrinkage of the solid waste paste decreased with the reduction in SAP particle size throughout the curing period. The final shrinkage of the solid waste paste diminished from 0.25% with an SAP particle size of 30–50 mesh to 0.19% with an SAP particle size larger than 100 mesh. The SAP which had a smaller particle size possessed a better inhibitory effect on the volume shrinkage of solid waste paste. This may be because, under the same quality, the SAP which had a smaller particle size exhibited a higher water absorption capacity, effectively reducing the water bleeding rate of the paste[33, 38]. Water was continuously released by the SAP during later hydration, contributing to sufficient hydration of solid waste paste and reducing the drying shrinkage of the solid waste paste. The test results are shown in Fig. 10.
3.4.3 Influence of sodium silicate modulus on volume shrinkage
The change in sodium silicate modulus impacts the pH value and ion concentration of the solid waste paste, which influences the SAP water absorption capacity in turn. To examine the influence of different sodium silicate moduli on volume shrinkage, fly ash and slag with the same proportion were selected as the main solid waste system. The SAP was incorporated at a content of 0.4%, and the water–binder ratio was set to 0.45. Sodium silicate with moduli of one and two was utilized as an alkali activator. The test results are shown in Fig. 11.
The drying shrinkage of solid waste paste increased with the rise in sodium silicate modulus. This may be because a higher sodium silicate modulus raised the pH value of the solid waste paste, causing more metal ions in the solid waste cementitious component to dissolve into the paste. The increase in pH value and ion concentration impaired the SAP water absorption capacity, which exacerbated the water bleeding rate of the paste and reduced the water content available for later hydration, resulting in an increase in drying shrinkage[41, 42].
3.5 Pore size distribution
3.5.1 Pore generation
The generation of pores with varying sizes can be attributed to the desorption of the SAP. Based on the intensity of different factors, the SAP desorption process can be divided into three stages, as illustrated in Fig. 12. The primary factor in each stage differs. In the first stage, the desorption of the SAP is mainly determined by osmotic pressure, whereas in the third stage, it is predominantly influenced by the humidity gradient. The second stage involves a relatively weaker combination of these two factors.
3.5.2 Influence of SAP content on pore size distribution
An appropriate pore size distribution is essential for ensuring excellent workability in filling materials. Researchers have found that a material’s pore structure is related to its macroscopic properties, such as compressive strength. The greater the pore size distribution, the lower the compressive strength[46–48]. During the hydration process, a certain amount of macro pores is left in the solid waste filling body due to the water released by the SAP. To examine the impact of SAP content on the pore size distribution of solid waste paste, fly ash and slag with the same proportion were selected. Sodium silicate with a modulus of one was employed as the alkali activator, and the water–binder ratio was set to 0.45. For control observation, an SAP with a particle size larger than 100 mesh was incorporated in the solid waste paste at contents of 0, 0.1, 0.2, 0.3, and 0.4%, respectively. An NMR test was performed every hour after pouring the paste and continued for three days. The test results are shown in Fig. 13.
As the SAP content increased, the pore size content of solid waste filling materials at 1 µm continuously increased, from 0 to 0.06%. The SAP particle size after water absorption was larger than the general pore diameter, which caused its position on the pore size distribution figure to differ from the general pore size distribution position. the distribution of macro pores increased as the SAP content increased, forming a second peak on the pore size distribution figure[49].
3.5.3 Variation of pore size distribution with curing age
In the previous section, it was demonstrated that the second peak of the pore size distribution resulted from the SAP after absorbing water in the solid waste paste. In other words, the position of the second peak on the pore size distribution figure reflected the distribution of the SAP in the solid waste paste. Based on this relationship, the pore size distribution of samples with the same SAP content at different hydration durations can be used to further investigate the changes and mechanisms of SAPs in solid waste filling materials and their influence on the pore size distribution.
Samples with SAP particle sizes larger than 100 mesh and a content of 0.4% were selected for analysis. As the pore size distribution curves of solid waste filling materials in the first few hours were nearly identical, hydration durations of 2 h, 7 h, and 3 d were chosen for representative analysis. The pore size distribution is shown in Fig. 14.
With the increase in hydration duration, the second peak in the pore size distribution figure shifted noticeably to the left. This demonstrated that, as hydration continued, the general volume of the SAP gradually decreased due to water release, and the decline in the first peak position indicated that small pores in the solid waste filling body were continuously being filled.
Upon incorporation with a solid waste paste, a portion of the SAP’s water absorption capacity was limited due to the high pH value and high ion concentration. However, the SAP possessed strong water absorption capacity, and a significant amount of water was still captured by the SAP even in unfavorable hydration environments[50]. As hydration proceeded, water retained in the pores became available for further hydration of the solid waste paste. Fly ash and slag were further dissolved, releasing more metal ions such as Ca2+, Al3+, and Mg2+. The release of metal ions worsened the water absorption environment of the SAP. The chelating reaction between Ca2+ and SAP carboxyl groups forced the SAP into a water-release state, significantly inhibiting the SAP’s water absorption capacity. Owing to the deterioration of the ionic environment, the SAP began to release water, and its volume continuously decreased. The pore size of the solid waste paste grew smaller, causing the second peak of the pore size distribution figure of the solid waste paste to shift entirely to the left. The decrease in the first peak location may be attributed to the filling of some pores by the hydration products generated during further hydration of the solid waste paste[51].
3.6 Microscopic mechanism
To further investigate the impact of the SAP on the internal hydration and cementation of solid waste filling materials, we selected fly ash and slag in the same proportion. We utilized sodium silicate with a modulus of one as the alkali activator and set the water–binder ratio to 0.45. Additionally, an SAP with a particle size larger than 100 mesh was incorporated at content levels of 0, 0.1, 0.2, and 0.3%. After 28 days of curing, the samples were analyzed using SEM, and the results are presented in Fig. 15.
As the SAP content increased, the cementation body content in the solid waste paste also increased. In Fig. 15A, with 0% SAP content, insufficient hydration and incomplete cementitious reaction occurred, and the cementitious components were not completely dissolved. Consequently, the fly ash (bright spheres in the image) was isolated with less cementation body around it. In Fig. 15B, with 0.1% SAP content, intact fly ash and slag particles were hardly observed, indicating a more thorough dissolution process of the solid waste cementitious components. In Fig. 15C, with 0.2% SAP content, hydration products increased significantly and formed an interlocking network structure. In Fig. 15D, with 0.3% SAP content, dissolved fly ash and slag particles were enveloped by the swelled SAP, filling the internal structure and contributing to a denser structure. Thus, incorporating the SAP facilitated further hydration and cementation of solid waste paste.
To investigate the water absorption and release mechanism of the SAP and the factors affecting changes in its water absorption capacity, we selected dried SAP, sodden SAP after water absorption, dried SAP after absorbing a solid waste leaching solution, and sodden SAP after absorbing a solid waste leaching solution for Fourier transform infrared spectroscopy. The results are presented in Fig. 15E.
SAP1 represents the SAP in a dry state, while SAP2 represents the SAP in a sodden state after water absorption. The curve shapes of SAP1 and SAP2 were nearly identical, indicating that the functional groups of SAP did not change before and after water absorption, and no chemical reaction occurred at this stage. Furthermore, the C = O stretching vibrational peak in carboxyl–COOH was observed at 1714 cm− 1 in the spectrum [52]. However, the vibrational peak at 1714 cm− 1 disappeared in both the sodden and dried SAP after absorbing the solid waste leaching solution. The carboxyl group can bind to a variety of metal ions, and the disappearance of its vibration peak was mainly due to the chelation reaction between Ca2+ and the carboxyl groups [53]. Consequently, the electrostatic repulsion between the anions in the internal network of SAP decreased, thereby reducing the water absorption capacity of the SAP and causing it to release water.
Under the influence of sodium silicate, slag and fly ash quickly dissolve Ca2+, Al3+, and Si4+ ions. Furthermore, the chelation reaction between Ca2+ and SAP decreased the SAP water absorption capacity. Consequently, all three particle sizes of the SAP exhibited a low water absorption rate in the solid waste pore solution with a sodium silicate modulus of two.
Moreover, the experimental energy spectrum results further confirmed the SAP water absorption and release mechanism. Figure 15G displays the energy spectrum of the solid waste cementation body, corresponding to 1# in Fig. 15F, while Fig. 15H displays the energy spectrum of SAP, corresponding to 2# in Fig. 15F. Apart from the elements inherent in SAP, such as O, Na, Si, and C, the solid waste filling materials contained other elements like Ca and Al. Ca2+ and Al3+ were released by the dissolution of fly ash and slag, had high contents, and penetrated into the SAP with the help of free water. However, as the SAP contained a large number of carboxyl groups, Ca2+ and Al3+ combined with carboxylic acid in the form of ionic bonds, decreasing the SAP water absorption capacity [41]. Furthermore, the selected slag contained a certain amount of Mg element, and as such, the final cementation product included Mg. Thus, through an EDS analysis, it can be inferred that a six-element system of Na2O-Al2O3-SiO2-CaO-MgO-H2O was formed.