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Article

Optimization of the Injection and Physical Properties of Sulfoaluminate Cement via the In Situ Polymerization of Acrylamide

by
Haibo Zhang
*,
Xiaotian Zhang
,
Zhiying Guo
and
Hucheng Chai
Henan Key Laboratory of Materials on Deep-Earth Engineering, School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China
*
Author to whom correspondence should be addressed.
Buildings 2022, 12(12), 2237; https://doi.org/10.3390/buildings12122237
Submission received: 1 November 2022 / Revised: 4 December 2022 / Accepted: 13 December 2022 / Published: 15 December 2022
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

:
Deep coal mining requires grouting to reinforce the coal rock walls. The conventional polymer-modified cement-based grouting materials have improved toughness but poor fluidity. The purpose of this paper was to improve the injectability of grout. The preconditions were that the physical properties of the stones are not obviously damaged and the mechanical properties are optimized. The acrylamide monomer was incorporated into the sulfoaluminate cement-based grouting material system. The effects of the in situ polymerization modification of the acrylamide on the paste properties and stone body strength were investigated. The results showed that the acrylamide mixed with the sulfoaluminate cement could increase the fluidity of the grouting material, prolong the paste setting time, and improve the flexural strength of the stone body. Compared with the blank group, the 500 mL efflux time of the 1006 mud viscometer in the 35% AM (acrylamide) dosing group decreased from 51 s to 41 s, the initial setting time increased from 15 min to 98 min, and the flexural strength at three days increased by 202.8%. The compressive stress–strain of the stone body gradually changed from brittle to ductile damage as the acrylamide dosing increased. Based on the microscopic analysis, a composite structure model of the inorganic skeleton of the flexible polyacrylamide network of toughened AFt (ettringite) was built in this paper. A mechanical study of the compressive stress–strain characteristics of the stone body was performed. It was concluded that the acrylamide in situ polymerization-modified sulfoaluminate cement-based grouting material is suitable for coal rock wall grouting reinforcement.

1. Introduction

China’s shallow coal rock mining industry is nearing its end, and the change to deep coal mining is imperative [1]. However, the geology and operating conditions in kilometer-deep wells are harsh. Based on the experience accumulated from the problems encountered in the roadway excavation of 47 kilometer-deep wells in China, deep mining has to face the problems of high ground stress in the tunnels and increased permeability pressure in the rock walls of the roads. Due to the soft rock characteristics of the roadway walls [2,3], high impacts, high-temperature work, strong stress disturbances during the excavation of the roadway group, and strong stress disturbances during coal seam mining are formed. This leads to several safety issues that pose significant safety threats to miners.
Grouting methods are widely used to reinforce fractured surrounding rocks in coal mines [4,5]. Organic grout is expensive and easily causes pollution. Cement grout is difficult to inject into tiny pores, and stone bodies are brittle. They will be damaged when the surrounding rock deforms and weakens the reinforcement effect. Therefore, a lot of research [6,7,8,9] has been done on toughening and modifying stone bodies by adding polymer into cement-based materials. Pang et al. [10] studied the effects of lotion epoxy resin (EEP) and non-lotion epoxy resin (NEP) on cement hydration. The results showed that the strength and toughness of the material increase with the increase in the epoxy content. Tang et al. [11] mixed isocyanate and a curing agent into a cement paste and found that the PU (polyurethane) produced by the polymerization of the two could effectively improve the early toughness of the cement-based materials. The amount of PU added was 0.5%, which was 38%, 26.61%, and 36.40% higher than the flexural strengths of the blank group at 1 d, 3 d, and 7 d, respectively. Sun et al. [12] found that they could fill the pores of cement stone bodies to a certain extent by adding SBR into the cement, making the stone body structures more compact. Chen et al. [13] showed that there are two functions between the polyacrylate and Portland cement. One is that the carboxyl group in the polyacrylate can provide the site for forming hydrogen bonds with water molecules in the Portland cement paste and hydrogen atoms in the protonated silicate tetrahedrons. The other is that the sodium ions and calcium ions form O-Na-O and O-Ca-O salt bridges to connect the oxygen atoms in the polyacrylate and silicate. Thus, the toughness of the composite materials is improved. Geng et al. [14] used isocyanate (TDI) containing highly active groups (-NCO) as an intermediate to form a “bond bridge” connection between the epoxy resin and organosiloxane, partially replacing the role of hydrogen bonding in modify cement-based materials. They found that the impact strength of the stone body was 1.9 times that of the blank group, and the elongation at break was 4.2 times that of the empty group. Therefore, the polymer modification is significant in improving the toughness of the grouting materials.
The addition of most polymers improves the toughness of cement-based materials well. However, this leads to an increase in paste viscosity in the early stage and a decrease in paste injectivity. However, one advantage of acrylamide is that it can be polymerized in situ. This advantage causes the viscosity of the paste to significantly decrease in the early stage. This advantage has attracted many scholars [15,16,17,18,19,20,21]. However, at present, the existing research mainly focuses on the effects of acrylamide on silicate systems with low water–binder ratios (<0.5) [22,23,24]. There are few reports on the role of sulfoaluminate in cement-based grouting material systems with high water–cement ratios (>0.5).
The effects of the acrylamide content on the fluidity, setting time, and strength of the sulfoaluminate cement grouting materials was systematically studied in this paper. The aim was to optimize the sulphoaluminate cement. The purpose was to improve the injectability of the paste without apparent damage and even to optimize the physical properties of the stone. A double network structure model was constructed, and the toughening mechanism analysis was carried out. The model showed that the flexible polyacrylamide network toughens the inorganic crystal skeleton. The results showed that the in situ acrylamide polymerization of modified sulfoaluminate cement-based materials could effectively avoid a certain problem. The problem was the poor flowability that occurred in the conventional polymer-modified cement paste. However, it also retained the advantage of traditional polymer-modified cementitious materials in that it could significantly improve the toughness of the stone body. In the future, the polymer–monomer in situ polymerization of modified cementitious materials has broad development prospects for grouting.

2. Experiment

2.1. Experimental Materials

The sulfoaluminate cement clinker (SAC), hard gypsum (anhydrite, AN), and lime (lime, LI) were purchased from Jiaozuo Huayan Industrial Co., Ltd. The particle size distributions are shown in Figure 1 and the chemical compositions are shown in Table 1. The mineral composition of the sulfoaluminate cement clinker is shown in Table 2. The acrylamide, ammonium persulphate (AP) initiator, and N, N-methylene-bis-acrylamide (MBA) crosslinked agent were purchased from Zhengzhou Xin Dong Co.

2.2. Sample Preparation

2.2.1. Experiment on the Optimization of the Yellow/White Material Ratio

In practical grouting applications, the SAC is usually used as one component and the AN and LI mixture as the other component for dual-liquid grouting. Therefore, the SAC was used as a grouting material component in this experiment, and it was called the yellow material. The AN and LI were mixed at a mass ratio of 7:1 as the grouting material component, which was called the white material. The grouting materials were prepared with a water/cement ratio of 0.6. The balance is shown in Table 3. The paste was injected into a 40 mm × 40 mm × 160 mm mold (Figure 2) and covered with plastic cling film. Then the molds were dismantled after 24 h of maintenance in the standard maintenance room. After demolding, the specimen was wrapped in plastic cling film and normal care was continued until the specified age and test strength were reached.

2.2.2. Acrylamide Dosing Experiment

According to the results of an experiment on the optimization of the yellow/white material ratio, the optimal balance was selected based on the physical strength of the stones. The experimental results showed that the best effect was achieved when the yellow/white pigment ratio was 5:3 (Section 3.1.3). Fixing the best ratio of yellow and white materials, the AM was dosed at 0%, 15%, 25%, and 35% of the total mass of yellow and white fabrics, respectively. For the sake of discussion, they were named AM-0, AM-15, AM-25, and AM-35. The initiator AP was dosed at 1% of the AM mass, the cross-linker MBA was doped at 2% of AM mass, and the formulation is shown in Table 4. The paste was injected into 40 mm × 40 mm × 160 mm and ϕ 50 mm × 100 mm molds (Figure 2) and covered with plastic cling film. The molds were remolded after 24 h in the standard maintenance room. After remolding, the specimens were wrapped with plastic cling film and continued to be maintained under normal maintenance conditions until reaching the specified age for the performance testing.

2.3. Test Methods

2.3.1. Fluidity

The fluidity was expressed in seconds by the time it took for 500 mL of paste to flow out of a standard model 1006 viscometer. As shown in Figure 3, the model 1006 paste viscometer was a funnel-shaped vessel. The supporting equipment included cylindrical measuring cups and sieves for paste filtration. This was done by flushing the paste viscometer with water and blocking its outlet. Then, the well-mixed cement paste was injected into the funnel through the screen. The measurement was made by placing a 500 mL measuring cup under the outlet and releasing the outlet. At the same time, the stopwatch was pressed to record the time taken for the cement paste to fill the measuring cup, indicating the liquidity of the paste, expressed in seconds.

2.3.2. Setting Time

Before the measurements, the round mold was placed on the glass plate and the instrument was adjusted so that when the initial setting needle contacted the glass plate, the pointer was aligned with the zero point of the ruler. The pastes were prepared according to the ratio in Table 3 and Table 4. The mixed pastes were installed into the round mold at once and scraped after several rounds of vibrations When performing the measurements, the round mold was placed under the test needle, the test needle contacted the paste surface, and the screw tightened. Then, the screw was loosened suddenly, the test needle sunk into the paste freely, and the pointer readings were observed. When the test needle sunk 4–6 mm from the base plate, the time recorded was the initial setting time. After the initial setting time measurement was completed, the test mold and the paste were removed from the glass plate immediately using the translation method. They turned 180 degrees, the large end of the diameter was facing upward, and the small end was facing downward on the glass plate. The Vicat instrument was taken off the initial setting needle and replaced with the final setting needle. The test needle was sunk into the paste freely and the pointer readings were observed. If the test needle plunged into the test body less than 0.5 mm, i.e., when the annular attachment could not leave traces on the test body, the time recorded was the final setting time. The details are shown in Figure 4.

2.3.3. Compressive Strength and Flexural Strength

The flexural and compressive strengths at 3 d and 28 d were tested following the “Test Method for Cementitious Sand Strength (ISO method)” GB/T 0506-2005. The colorful specimen size was 40 mm × 40 mm × 160 mm.

2.3.4. Axial Compression Strength

The axial compression tests were performed on ϕ 50 mm × 100 mm cylindrical specimens. The test instrument was a WDW−300 kN electronic universal testing machine (Figure 5). Both load and displacement controls were used; the load was nonlinear and the displacement was linear, while the loading rate was 0.2 mm/s. The loading stopped when the deformation reached 50% and the load–deformation curve was measured. Finally, the stress and strain were calculated. The points where the load was within the range of 0 ± 0.01 kN were rounded off in the test data to avoid the effect of specimen surface bumping in the experimental results.

2.3.5. XRD

The XRD analysis was performed using a D8 Advanced X-ray diffractometer (Cu target) from Bruker, Germany. In this case, the scanning range was from 5° to 80° and the scanning speed was 5 °/min.

2.3.6. SEM

A Carl Zeiss Merlin Compact scanning electron microscope was used to observe the microscopic morphology of the specimens.

3. Results and Discussion

3.1. Effect of the Yellow/White Material Quality Ratio

3.1.1. Fluidity

The fluidity variation curves of the grouting materials with different yellow and white material ratios are shown in Figure 6. It can be seen that as the proportion of white material increases, the paste flow time gradually decreases and the paste fluidity improves. This phenomenon corresponds to a series of physical and chemical changes. The early viscosity of the cement paste mainly comes from the adhesion and friction between the cement particles. AFt crystals are formed during the hydration and hardening of sulfoaluminate cement. During the crystallization process, particles will preferentially deposit at the positions with more bonding and more significant heat release, which will lead to a decrease in the Gibbs free energy of the system. Therefore, the AFt will produce heterogeneous nucleation and crystallization on the surfaces of the cement particles [13,25]. The AFt crystal formed on the surfaces of the cement particles will extend into the solution to increase the surface roughness of the cement particles. The longer the AFt crystal column is generated, the more the surface roughness of the cement particles will increase and the more outstanding the contribution to the adhesion and friction between the cement particles. The size of the AFt crystal column depends on the concentration and supersaturation degree of the AFt in part of the paste solution. The AFt was formed via the hydration reaction of the yellow sulfoaluminate cement pigment. By consulting the relevant literature [26], the solubility product constant of calcium sulfate was determined to be 7.1 × 10−5, and CaO reacted with water to generate Ca(OH)2. The solubility product constant of the calcium hydroxide was 4.6 × 10−6. According to the solubility product constant, the calcium ion concentration of the saturated calcium sulfate solution was 8.426 × 10−3 mol/L, and that of the saturated calcium hydroxide solution was 1.048 × 10−2 mol/L. The calcium ion concentration of the mixed solution of the two would be higher. The reaction of CaO with water would release a lot of heat, accelerating the hydration of calcium sulfoaluminate and the calcium oxide reaction. Therefore, the calcium ion concentration of part of the paste solution would be very high in the early hydration stage. The sulfoaluminate cement clinker in the yellow pigment mix would be rapidly hydrated when encountering water. The solubility product constant of the sulfoaluminate was not affected by the ion concentration. A high concentration of calcium ions would strongly inhibit the dissolution of sulfoaluminate; the abolition of aluminate was slow and the diffusion rate was low, so the aluminate concentration was the main control factor for AFt formation [27]. Therefore, the concentration of AFt in the paste solution was low and there were few resulting AFt crystals.
In summary, the calcium ions generated from the hydration of white sulfoaluminate cement materials inhibit the hydration of yellow sulfoaluminate cement materials, which reduces the size of the AFt crystals generated on the surfaces of the cement particles. This phenomenon weakens the adhesion and friction between the cement particles. Therefore, with the increase in the white material proportion of the sulfoaluminate cement, the fluidity of the paste improves.

3.1.2. Setting Time

The setting time variation curves for the grouting materials with different yellow and white material ratios are shown in Figure 7. It can be seen that as the proportion of white material increases, the paste setting time decreases first and then increases. This is because when the sulfoaluminate cement (white material) is present in a small amount and the yellow fabric is present in a sufficient amount, the reaction rate when generating AFt depends on the white material admixture; the higher the white material content, the higher the reaction rate. On the contrary, when the white material is sufficient and the yellow material is present in a small amount, the rate of AFt generation depends on the amount of yellow fabric; the lower the amount of white fabric, the higher the yellow material and the faster the reaction rate. The most rapid rates of hydration and hardening occur when the ratio of yellow to white materials is close to the reaction equation ratio [28], making the setting time the shortest. The experiment proved that when the balance of both reached 5:3, the initial and final setting times were the quickest. The total reaction between the yellow material and the white material in the paste occurred, as shown in Equation (1), and a large amount of AFt crystals were produced to induce the paste to set [19]. In this study, when the white material reached 60% of the yellow fabric, the reaction rate was the largest and the setting time was the shortest.
3 C 3 A · C a S O 4 + 8 C a S O 4 + 6 C a O + 96 H 2 O = 3 ( 3 C a O · A l 2 O 3 · 3 C a S O 4 · 32 H 2 O )

3.1.3. Flexural and Compressive Strengths

Figure 8 and Figure 9 show the flexural and compressive strengths of the grouting material nodules at 3 d and 28 d with different yellow and white material ratios. It can be seen that both the flexural and compressive strengths increase and then decrease as the proportion of white material increases. The compressive strength reached the maximum when the mass ratio of yellow and white materials was 5:3. This was a phenomenon that was essentially the same as the setting time of the system, in that the closer the proportioning was to the reaction equation proportioning, the faster the hydration rate. For cement paste, the quicker the hydration rate, the higher the partial supersaturation of the paste solution, which leads to faster heterogeneous nucleation and crystallization of AFt on the surfaces of cement particles [29]. The faster the crystallization rate, the larger the diameter of the AFt crystal column and the more AFt that is generated [30], which leads to higher strength of the three-dimensional crystal network of the stone body [31]. The strength of the 28 d age specimen was less improved than the 3 d specimen. This was caused by the hybrid cementitious system of calcium sulfoaluminate, gypsum, and lime. This system underwent a rapid hydration reaction to produce AFt and for fast strength development.

3.2. Effect of Acrylamide Dosing

3.2.1. Fluidity

Figure 10 shows the variation curve of the paste fluidity with different acrylamide dosing rates. It can be seen that the paste flow time decreases with the increase in the acrylamide admixture. This was because the amide group in the acrylamide can form a complex with calcium ions [32,33,34] and adsorb on the surfaces of cement particles to hinder hydration [35]. This phenomenon reduces the ion concentration in the paste solution fraction, resulting in lower rates of heterogeneous nucleation and crystallization of the AFt from the surfaces of unhydrated and incompletely hydrated cement particles [36]. This, in turn, weakens the bond strength between the cement particles [37]. When the AM dosing reached 35%, the efflux time was reduced by 19.6% compared to the blank group. It was demonstrated that the incorporation of acrylamide was beneficial in improving the paste flow.

3.2.2. Setting Time

Figure 11 shows the change curve for the paste setting times with different acrylamide dosing rates. It can be seen that the initial and final setting times for the paste first increase and then decrease with the increase in acrylamide dosing. However, the difference between the initial and final setting times keeps falling. This is because the paste strength is determined by the three-dimensional crystal network generated by the mutual lap of the AFt when the amount of acrylamide is low. If the paste was mixed with acrylamide, the acrylamide would adsorb on the surfaces of the cement particles to hinder hydration. This phenomenon would result in a lower rate of AFt generation in the system. At the same time, the higher the amount of acrylamide, the stronger the hydration hinderance effect and the greater the paste-setting rate. When the acrylamide dosing was high, the paste hardening consisted of two parts [38]. One was the three-dimensional AFt crystal network generated by the hydration of sulfoaluminate cement. The second was the polyacrylamide network generated by the polymerization of acrylamide. The generation speed of the polyacrylamide network was quicker than that of the AFt network. Therefore, when the acrylamide dose was higher, the setting time of the system decreased with the increase in the acrylamide dose. The difference between the initial and final setting times decreased with the increasing acrylamide dosing. This was because acrylamide could be complexed with the calcium ions generated via hydration [39,40]. When the concentration of calcium ions within the paste solution fraction was high, the acrylamide absorbed excess calcium ions by complexing them with calcium ions. When the concentration of the calcium ions in the paste solution fraction was low, the acrylamide released the complexed calcium ions, equivalent to the excess calcium ions generated by hydration being “stored” by acrylamide. The higher the dosing of acrylamide, the more calcium ions were “stored” at the beginning of hydration, which resulted in more calcium ions being released at the later stage. Therefore, the higher the amount of acrylamide, the smaller the difference between the initial and final setting times. The experiment proved that when the dosing amount reached 25%, the initial and final setting times were the longest, which increased by nearly 5.9 times and 3.3 times, respectively, compared with the blank group. The phenomenon whereby polymers can delay cement hydration is also common in other polymer cement systems [41,42,43].

3.2.3. Flexural Strength

Figure 12 shows the flexural strengths of the paste nodules with different acrylamide dosing rates. It can be seen that the flexural strength of the stone body increased significantly with the increase in acrylamide dosing. This was because there was a large number of entangled structures inside the polyacrylamide, which meant that the polyacrylamide was ductile. The entangled networks would open and stress relaxation would occur macroscopically to release the stress if the polyacrylamide material was stressed to a specific strength [44,45]. Compared with the blank group, the flexural strengths at 3 d for the specimens dosed at 15%, 25%, and 35% increased by 154.6%, 144.4%, and 202.8%, respectively, while the flexural strengths at 28 d increased by 185.0%, 134.6%, and 121.8%, respectively.
Many studies have shown that the flexural strength of polymer-modified cement-based materials increases with the polymer content within a specific range [46,47,48]. Some studies have also shown that the flexural strength of cement-based materials will reach a maximum at a certain polymer content [49,50,51,52]. However, for sulfoaluminate cement, the crystal product, Aft, is needle-shaped [53]. The crystal column of AFt is an ionic crystal with ionic bonds between atoms. PAM is a molecule with covalent bonds between atoms. Overall, the strength of the ionic bond is higher than that of the covalent bond, and the entangled structure between the PAM molecular chains is mainly maintained by the van der Waals force. Therefore, hidden dangers will be formed if the location of the AFt and PAM is appropriate. This hidden dangers will result in the PAM being pressed or even punctured by the AFt crystal column, causing damage to the entangled structure when the stone body is subjected to the external force. Theoretically, this puncture effect is related to the diameter of the AFt crystal column. If the diameter of the AFt crystal column is too large, the AFt crystal column will not be “sharp” enough. If the AFt crystal column is too thin, the crystal column will be fragile during the force process and the puncture effect will not be apparent. Only when the diameter of the AFt crystal column is moderate will the puncture effect reach the maximum.
AM will adsorb on the surfaces of the cement particles and hinder their hydration. Calcium ions generated from hydration will also be complexed with amide groups to prevent AFt crystallization. Therefore, with the increase in AM dosing, the diameter of the early AFt crystal column will decrease. Thus, the puncture effect of the AFt crystal column on the PAM will reach the peak when the AM reaches a specific dosing. In this paper, the flexural strength of the sample at 3 d in the 25% AM dosing group was lower than that in the 15% and 35% AM dosing groups simultaneously. This phenomenon indicated that the AFt crystal column had the most apparent puncture effect on the PAM at 3 d in the 25% AM dosing group.
The flexural strength at 28 d decreased with the increase in AM dosing, which was related to DEF (delayed ettringite formation). DEF has been mentioned in many studies in the literature. The focus of attention is on the damage caused by DEF [54,55,56,57,58] and the conditions under which DEF occurs. The damage caused by DEF means that sufficient Aft is generated at the crack of the stone body, and the AFt grows to form stress, causing crack growth [59]. The generation of DEF is related to the environment. Some researchers believe that a high temperature of 70 °C is required to delay the occurrence of AFt [60,61]. Mehta [62] found that the condition under which AFm can coexist with AFt is that the sulfate concentration in the solution is lower than the concentration required for the stable existence of AFt. The necessary conditions for the regular presence of monosulfide sulfate in an aqueous solution are a high calcium content (CaO), low sulfate content, and high temperature. Lawrence [63] studied the expansion behavior of 55 kinds of Portland cement at 65–100 °C. His conclusion showed that the vital temperature range for DEF is 65–70 °C. He also pointed out that significant expansion during high-temperature curing can significantly reduce the later growth. Some researchers [64,65] also believed that DEF could be formed without reaching such a high temperature. From a chemical point of view, all reactions were in dynamic equilibrium, but the stability constantly changed with the temperature. The occurrence of DEF is controlled by Equation (2). This theory is consistent with Mehta’s research results. When the concentration of calcium sulfate is low enough, the chemical balance will shift to the right and AFt will start to decompose and transform into AFm (monosulfoaluminate), calcium sulfate, and water. The reaction is not violent at room temperature, but the calcium sulfate generated is soluble in water, which will cause the reaction balance to continue to shift to the right when transported with water. Calcium sulfate is transported with water and enriched in other places in the stone body, which will cause Equation (2) to shift to the left and generate DEF. The local calcium sulfate concentration determines the reaction balance, and the gypsum concentration mainly depends on the gypsum consumption by the hydration reaction.
In Equation (2), the balanced right shift corresponds to the reduced AFt crystal column diameter, which results in the strength of the stone body. In Equation (2), the balanced left shift corresponds to the generation of DEF and filling of the pores. It is generally recognized that the time point for DEF to cause damage is several months or even years later. Hence, the DEF generated after 28 days is not enough to cause apparent crack expansion in the stone body, so insufficient DEF in the early stage is beneficial. Therefore, in theory, a considerable number of sulfoaluminate cement stones will experience large consumption of gypsum, the AFt decomposition process, and the process of gypsum transport with water to generate DEF. The former process corresponds to a decrease in the strength of the stone body. In contrast, the latter process theoretically will cause a strength increase, at least when the amount of DEF generated is insufficient.
The adsorption of AM on the surface of cement particles will hinder the hydration of cement particles. The complexation of the amide group and calcium ion will impede the formation of AFt. Therefore, the hydration rate of cement particles will slow with the increase in AM content. The excessive consumption of gypsum will cause the time point of the AFt decomposition to move backward, and the time point of the recrystallization of the reaction products to generate DEF will naturally move backward with water transportation. Moreover, the complexation of the amide group to calcium ions will cause the recrystallization to be delayed with the increase in AM content.
By comparing the flexural strengths of the three groups at the ages of 3 d and 28 d, we concluded that AFt decomposes into AFm and calcium sulfate, and the decomposition products are transported with water and recrystallized in other places of the stone body to generate DEF. The three dosing groups occur at different stages. The 15% AM dosing group generates a lot of DEF, the 25% dosing group develops part of the DEF, and for the 35% dosing group it cannot be determined whether the DEF generation process has just started or the AFt decomposition process has not ended.
C3A·3CaSO4·32H2O = C3A·CaSO4·12H2O + 2CaSO4·2H2O + 16H2O

3.2.4. Uniaxial Compression Stress–Strain Curve

Figure 13 shows the compressive stress–strain curves of the 3 d specimens of the paste nodules with different acrylamide dosing rates. It can be seen that the blank specimen has a breaking strain of 3.2%, and the sample crumbles and loses strength completely after reaching the peak strength. The samples showed the characteristics of brittle damage. When the acrylamide dosing amount reached 15%, the stress increased rapidly with strain and then increased slowly and the yielding phenomenon appeared. At this time, the stress is called the yield stress and the strain is called the yield strain. After the sample was compressed and yielded, the stress decreased and the sample was damaged but did not disintegrate when the strain reached 21.3%. There was still a significant residual stress. When the acrylamide dosing increased to 25% and then 35%, the stress–strain curve showed a ductile deformation. However, the yield stress decreased, the stress continued to grow, and the specimens no longer broke down. The yield stress of the stone body decreased with the increase in acrylamide dosing. This was because the incorporation of acrylamide impeded the hydration of sulfoaluminate cement particles. The decrease in hydration rate finally led to the decreased density of the AFt’s three-dimensional crystal network. Furthermore, stress relaxation occurs under high stress.
Figure 14 shows the compressive stress–strain curves of the 28 d specimens of the paste nodules with different acrylamide dosing rates. It can be seen that the stress–strain curves of the other acrylamide specimens have the same variation characteristics as the 3 d specimens. however, the stresses corresponding to the same strain are significantly higher. This is because the amide group in acrylamide can form hydrogen bonds with water molecules [66]. Thus, it has good water retention ability [67]. The higher the acrylamide dose, the more water is preserved inside the stone body of the system, the better the hydration environment provided for the unhydrated cement particles, the more AFt that is generated in the middle and later stages, and the denser the three-dimensional AFt crystal network of the stone body. Therefore, the strength of the stone body is significantly increased in the later stages.

3.3. XRD Phase Analysis

Figure 15 shows the XRD patterns of the 3 d specimens of the paste nodules with different acrylamide admixtures. It can be seen that the addition of acrylamide did not change the type of mineral crystals produced via paste hydration. The main products were ettringite (AFt) and residual gypsum (CaSO4). However, the AFt diffraction peak gradually decreased and the CaSO4 diffraction peak gradually increased with the increase in AM dosing. This indicated that AM is indeed adsorbed on the surface of cement particles to hinder hydration, which causes a decrease in the hydration rate of the sulfoaluminate cement. The amount of cement consumed during hydration decreases, the remaining CaSO4 from the hydration reaction increases, and the partial supersaturation of the paste solution decreases, which in turn causes a reduction in the generated AFt.

3.4. SEM Morphology Observations

Figure 16 shows the microscopic morphology of the 3 d specimens of the paste nodules with different acrylamide dosing rates. It can be seen that a large number of columnar AFt crystals lap each other in the structure of the blank specimen, and the aluminum adhesive (AH) bonds the AFt crystals and fills the pores (Figure 16a). The diameter and length of the columnar AFt crystals were reduced by the incorporation of acrylamide. The surface was covered with polyacrylamide (PAM). With the increase in the acrylamide dosing, the diameter and length of the columnar AFt crystals gradually decreased, while the PAM gradually increased and formed a continuous matrix phase (Figure 16b,c). This microscopically proves that the incorporation of acrylamide hinders the hydration of sulfoaluminate cement particles. This reduces the supersaturation of some ions in the paste solution during hydration, leading to a decreased diameter, length, and total amount of AFt crystals generated. However, polyacrylamide adsorbs on AFt surfaces by complexing with calcium ions, which leads to the formation of mechanical structure [68]. It was demonstrated that when the dosing amount reached 35%, the PAM formed a dense and continuous matrix phase, and the AFt crystals became a dispersed and enhanced stage (Figure 16d).
Figure 17 shows the microscopic morphology of the 28 d specimens of the paste nodules with different acrylamide dosing rates. As with the three day microscopic morphology, the density of the AFt crystal column at 28 d also decreased with the increase in AM content. The AFt PAM double network structure was also most intuitive in the 25% AM content group. When the AM content was 35%, the AFt was also covered mainly by PAM, which corresponded to the macro change rule of the stone body in the axial compression test from brittle to ductile fractures. Compared with the 3 d results, the diameter of the AFt crystal column in the 28 d white groups at 15%, and 25% AM dosing rates group decreased significantly, which proved that AFt would be decomposed into AFm and CaSO4 because of a reaction imbalance with the hydration reaction from a microscopic perspective. When the AM content was 35%, it could still be seen that the column diameter and density of the AFt decreased, which corresponded to the fact that the overall flexural strength at 28 d was lower than the strength at 3 d.

3.5. Discussion

In the present study paste system, there were sulfoaluminate cement clinker particles, gypsum particles, lime particles, acrylamide monomers, cross-linker monomers, and initiators (Figure 18). Calcium sulfoaluminate, gypsum, and lime undergo hydration reactions to form AFt and aluminous gum [69]. The AFt crystals lap each other to create an inorganic rigid skeleton structure, as shown in Figure 19a. The acrylamide monomer and cross-linker can polymerize in situ under the action of the initiator to form a flexible polyacrylamide network structure [26], as shown in Figure 19b. The inorganic skeletal network and the organic loose network are interspersed together to form a loose polyacrylamide network and toughened inorganic skeletal composite structure, as shown in Figure 19c.
At the beginning of the paste mixing, acrylamide can adsorb on the surfaces of inorganic particles to play a water-reducing role [19]. Therefore, the acrylamide admixture increases and the system fluidity is improved (Figure 10). The hydration reaction is delayed because of the adsorption of acrylamide. This results in a longer setting time for the system (Figure 11). As the reaction proceeds, the flexible polyacrylamide network and toughened inorganic rigid skeletal structure within the paste nodules are gradually formed. During the uniaxial compression stress–strain test, the wooden inorganic skeleton was the main stressed structure initially, and the stress increased rapidly. When the inorganic frame was destroyed, the yield point appeared in the curve and the flexible network still kept the structure intact, with the stress starting to increase slowly (Figure 13 and Figure 14). For the blank specimens with only the rigid inorganic skeleton structure, the samples showed brittle damage characteristics. The samples exhibited ductile damage characteristics with the gradual increase in the flexible polyacrylamide network. Due to the encapsulation of polyacrylamide (Figure 16 and Figure 17), the generation of AFt and aluminum gum was inhibited. The inorganic skeletal structure content of the specimens decreased and the yield stress decreased with the increase in acrylamide dosing.

4. Conclusions

The effects of acrylamide regarding the in situ polymerization–modification of the sulfoaluminate cement and the modification mechanism were analyzed in this paper. The significant conclusions are stated below:
(1)
By adjusting the proportions of yellow and white materials in the sulfoaluminate cement, it was found that the overall structural strength was the highest after setting and hardening when the balance of yellow and white materials was 5:3;
(2)
With the increase in acrylamide content, the early viscosity of the slurry decreased significantly and the setting time increased significantly. This was in line with the purpose of improving the injectability of grout;
(3)
The uniaxial compressive stress–strain curve of the blank group specimens showed a brittle fracture characteristic. The acrylamide-doped group exhibited ductile fracture behavior, and the yield stress decreased with the increasing acrylamide. This meant that the requirements for optimizing the physical properties of the stones were met;
(4)
In this paper, the injectability and physical properties of AM regarding the in situ polymerization–modification of sulfoaluminate paste were analyzed, but the surface properties of the paste were not investigated. For coal and rock walls, the wettability of the paste directly affects the reinforcement effect. Therefore, the subsequent research should study the wetting effect of the paste on coal and rock walls from the perspective of the intermolecular force.

Author Contributions

Conceptualization, H.Z. and X.Z.; methodology, Z.G. and H.C.; software, X.Z.; validation, H.Z., X.Z. and H.C.; formal analysis, X.Z.; investigation, X.Z.; resources, H.Z.; data curation, Z.G.; writing—original draft preparation, H.C.; writing—review and editing, H.C.; visualization, Z.G.; supervision, Z.G.; project administration, H.Z.; funding acquisition, H.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The support from the National Natural Science Foundation of China (U1905216) and the National Key R&D Program of China (2017YFC0603004) at Henan Polytechnic University is appreciated.

Institutional Review Board Statement

This study don’t require ethical approval.

Informed Consent Statement

This study don’t involve humans.

Data Availability Statement

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

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Particle size distributions of (a) sulfoaluminate cement clinker, (b) anhydrite, and (c) lime.
Figure 1. Particle size distributions of (a) sulfoaluminate cement clinker, (b) anhydrite, and (c) lime.
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Figure 2. The test molds. (a) Prism stone mold (b) Cylinder stone mold.
Figure 2. The test molds. (a) Prism stone mold (b) Cylinder stone mold.
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Figure 3. The 1006 mud viscometer.
Figure 3. The 1006 mud viscometer.
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Figure 4. The Vicat apparatus.
Figure 4. The Vicat apparatus.
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Figure 5. The uniaxial compression test setup: (a) test program interface; (b) test equipment.
Figure 5. The uniaxial compression test setup: (a) test program interface; (b) test equipment.
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Figure 6. Effect of the ratio of A to B on the fluidity of the paste.
Figure 6. Effect of the ratio of A to B on the fluidity of the paste.
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Figure 7. Effect of the ratio of A to B on the setting time of the paste.
Figure 7. Effect of the ratio of A to B on the setting time of the paste.
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Figure 8. Effect of the ratio of A to B on the flexural strengths of the cement stone at the ages of 3 d and 28 d.
Figure 8. Effect of the ratio of A to B on the flexural strengths of the cement stone at the ages of 3 d and 28 d.
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Figure 9. Effect of the ratio of A to B on the compressive strengths of the cement stone at the ages of 3 d and 28 d.
Figure 9. Effect of the ratio of A to B on the compressive strengths of the cement stone at the ages of 3 d and 28 d.
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Figure 10. Effect of the AM content on the fluidity of the paste.
Figure 10. Effect of the AM content on the fluidity of the paste.
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Figure 11. The effect of the AM content on the setting times of the paste.
Figure 11. The effect of the AM content on the setting times of the paste.
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Figure 12. Effect of the AM content on the flexural strengths of cement stone samples at the ages of 3 d and 28 d.
Figure 12. Effect of the AM content on the flexural strengths of cement stone samples at the ages of 3 d and 28 d.
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Figure 13. Uniaxial compressive stress–strain curves of the 3 d specimen.
Figure 13. Uniaxial compressive stress–strain curves of the 3 d specimen.
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Figure 14. Uniaxial compressive stress–strain curves of the 28 d specimen.
Figure 14. Uniaxial compressive stress–strain curves of the 28 d specimen.
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Figure 15. XRD pattern of 3 d specimens.
Figure 15. XRD pattern of 3 d specimens.
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Figure 16. SEM photos of 3 d specimens: (a) 0% PAM; (b) 15% PAM; (c) 25% PAM; (d) 35%PAM.
Figure 16. SEM photos of 3 d specimens: (a) 0% PAM; (b) 15% PAM; (c) 25% PAM; (d) 35%PAM.
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Figure 17. SEM photos of 28 d specimens: (a) 0% PAM; (b) 15% PAM; (c) 25% PAM; (d) 35%PAM.
Figure 17. SEM photos of 28 d specimens: (a) 0% PAM; (b) 15% PAM; (c) 25% PAM; (d) 35%PAM.
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Figure 18. Schematic diagram of AM in situ polymerization of modified sulfoaluminate paste.
Figure 18. Schematic diagram of AM in situ polymerization of modified sulfoaluminate paste.
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Figure 19. Microstructure and schematic diagram of the cement stone: (a) inorganic rigid skeleton; (b) organic, flexible network; (c) organic–inorganic interpenetrating network.
Figure 19. Microstructure and schematic diagram of the cement stone: (a) inorganic rigid skeleton; (b) organic, flexible network; (c) organic–inorganic interpenetrating network.
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Table
Table 1. A chemical analysis of the sulfoaluminate cement clinker, anhydrite, and lime (wt%).
Table 1. A chemical analysis of the sulfoaluminate cement clinker, anhydrite, and lime (wt%).
ComponentsSiO2Al2O3Fe2O3CaOMgOSO3TiO2
SAC8.5929.973.8645.211.039.441.90
AN2.930.430.2339.51.3654.660.89
LI8.462.671.0780.77.1--
Table
Table 2. The mineral composition of the sulfoaluminate cement clinker (wt%).
Table 2. The mineral composition of the sulfoaluminate cement clinker (wt%).
C4A3Sβ-C2S(-)C4AFf-SO3CaO-TiO2
60.6830.165.782.341.04
Table
Table 3. The mix proportions for the cementitious materials.
Table 3. The mix proportions for the cementitious materials.
A/gB/gWater/gA:B
200040014405:1
200080016805:2
2000120019205:3
2000160021605:4
2000200024001:1
Table
Table 4. The mix proportions for the organic components.
Table 4. The mix proportions for the organic components.
DosageA+B/gAM/gMBA/gAP/gWater/g
0%25600001536
15%25603843.847.681536
25%25606406.4012.81536
35%25608968.9617.921536
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Zhang, H.; Zhang, X.; Guo, Z.; Chai, H. Optimization of the Injection and Physical Properties of Sulfoaluminate Cement via the In Situ Polymerization of Acrylamide. Buildings 2022, 12, 2237. https://doi.org/10.3390/buildings12122237

AMA Style

Zhang H, Zhang X, Guo Z, Chai H. Optimization of the Injection and Physical Properties of Sulfoaluminate Cement via the In Situ Polymerization of Acrylamide. Buildings. 2022; 12(12):2237. https://doi.org/10.3390/buildings12122237

Chicago/Turabian Style

Zhang, Haibo, Xiaotian Zhang, Zhiying Guo, and Hucheng Chai. 2022. "Optimization of the Injection and Physical Properties of Sulfoaluminate Cement via the In Situ Polymerization of Acrylamide" Buildings 12, no. 12: 2237. https://doi.org/10.3390/buildings12122237

APA Style

Zhang, H., Zhang, X., Guo, Z., & Chai, H. (2022). Optimization of the Injection and Physical Properties of Sulfoaluminate Cement via the In Situ Polymerization of Acrylamide. Buildings, 12(12), 2237. https://doi.org/10.3390/buildings12122237

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