Multi-Scale Modification of Sodium Polyacrylate-Modified Cement Grouts: Rheology, Microstructure, and Mechanical Properties

Abstract

The rehabilitation of underground infrastructure requires cement grouts that combine high injectability into micro-cracks with superior mechanical strength and durability. Conventional grouts, however, are limited by excessive yield stress and the formation of weak crystalline phases. This study investigated sodium polyacrylate (PAAS) as a multi-functional modifier to address these limitations. Through a multi-scale approach combining rheological measurements, spectroscopic analysis (FTIR, LF-NMR), diffraction (XRD), and electron microscopy (SEM), we elucidated the synergistic modification mechanisms of PAAS. The results demonstrated that PAAS operated via two pathways: (i) chemically, its carboxyl groups chelated Ca²⁺ ions, suppressing Ca(OH)₂ crystallization and refining C-S-H gel; (ii) physically, it provided electrostatic and steric dispersion, dismantling flocculated networks to reduce yield stress by 80.3% and enhance fluidity by 30.7%. This drastically improved injectability was complemented by micro-structural optimization, where PAAS eliminated percolation pores (>1 μm) and promoted a homogeneous, dense matrix. Consequently, the mechanical properties were significantly enhanced, with a 0.04% PAAS dosage maximizing compressive strength (15.56 MPa, +26.2%) and a 0.06% dosage elevating flexural strength (5.74 MPa, +29.3%). This work establishes that low-dosage PAAS modification enables a unique combination of high fluidity, strength, and durability by leveraging synergistic chemical and physical mechanisms, providing a tailored, cost-effective solution for advanced grouting applications.

1. Introduction

Accelerated urbanization and expanding underground space development demand enhanced performance from cement-based grout materials for rock reinforcement [1,2,3]. These materials must now exhibit exceptional fluidity to penetrate micrometer-scale fractures (<0.2 mm width) while achieving high early strength and long-term durability after hardening [4,5]. Conventional cement grouts, however, face inherent limitations—excessive yield stress (τ₀ > 5 Pa) from particle flocculation restricts injectability—while plate-like Ca(OH)₂ crystals (1–3 μm) and porous calcium silicate hydrate (C-S-H) gel networks formed during hydration create mechanical weak zones [6,7,8]. These deficiencies substantially reduce service life and are particularly critical in precision engineering applications like tunnel lining rehabilitation and foundation stabilization, urgently necessitating novel modification technologies for performance breakthroughs [9,10].

Polymer modification has emerged as a potential strategy for enhancing cement-based materials [11,12]. The mechanism stems from physicochemical synergy between polymers and cement systems: polymer molecules form three-dimensional networks on cement particles via electrostatic adsorption or hydrogen bonding [13], effectively filling pores and densifying the matrix. Simultaneously, polymer films coat hydration products, impeding the diffusion of Ca²⁺ and SO₄²⁻ ions to retard hydration [14]. Performance-wise, acrylic emulsions (20% dosage) significantly improve durability, increasing chloride penetration resistance by 40% [15], while synergistic modification with carbon nanotubes or fibers further elevates flexural strength by 29% [16]. Notably, water-based polymer mortars demonstrate unique advantages in saline soil remediation, limiting freeze–thaw mass loss to <0.5% through capillary pore blocking [17]. In situ polymerization achieves a breakthrough 200% flexural strength gains by forming organic–inorganic interpenetrating networks [18]. Nevertheless, high dosages (>20%) still cause strength reduction, and long-term performance data have remained limited [19].

Among polymer modifiers, sodium polyacrylate (PAAS) attracts particular attention due to the strong coordination capacity of its carboxyl groups. Its advantages manifest in three dimensions [20,21]. (1) Mechanical enhancement: PAAS undergoes in situ polymerization concurrent with cement hydration, forming rigid–flexible dual networks via carboxyl–Ca²⁺ salt bridges (O-Ca-O). This elevates flexural strength by 200% [18]. Even at 1% dosage, inverse emulsion polymerization increases mortar compressive/flexural strength by 30% and 10%, respectively [22]. (2) Rheological control: PAAS substantially reduces fresh grout bleeding, lowering bleeding rate from 16.5% to 7.1% at 20 °C [23]. This effect is temperature-sensitive—above 40 °C, polymer chain coiling and accelerated hydration impair water retention. (3) Multifunctionality: For self-healing, PAAS and hydration products co-generate rod-like C-S-H/PANa composites that fully seal 50 μm cracks within 7 days, achieving 119.99% strength recovery [24]. In durability enhancement, 0.5 kg/m³ PAAS increases chloride penetration resistance by 50% and reduces steel corrosion rate by 30% [25]. Its innovative geotechnical application is particularly notable: 0.2% PAAS–cement gel decreases liquefiable sand permeability by 99% [26]. However, PAAS modification still faces challenges, including high-temperature strength degradation [27] and elevated costs of radiation polymerization [28].

Sodium polyacrylate (PAAS) offers a novel solution to the aforementioned challenges through its unique molecular structure. First, its low molecular weight (3000–5000 Da) enables rapid adsorption onto cement particles, while a high carboxylate density (8.2 mmol/g) enhances Ca²⁺ chelation capacity [29]. Second, PAAS exhibits dual-functional synergy: electrostatic repulsion and steric hindrance cooperatively disrupt flocculated structures, improving composite fluidity while simultaneously regulating hydration product distribution through controlled Ca²⁺ saturation. Finally, PAAS demonstrates strong engineering adaptability—industrial-grade PAAS costs merely one-third of polycarboxylate ether (PCE) superplasticizers and functions effectively across a broad pH range (6.0–12.5), making it suitable for complex groundwater environments. Thus, PAAS presents a promising low-cost, high-performance grouting solution for underground engineering, advancing green infrastructure rehabilitation.

Current research on PAAS-modified cementitious materials remains limited. Polymer studies typically focus on singular modification pathways (physical dispersion or chemical bonding), neglecting synergistic dual-path mechanisms. PAAS applications in grouts are particularly scarce, with the multi-scale rheology–hydration–strength relationship yet to be established. Moreover, the structure–property relationship of PAAS in cement systems remains unclear, leaving dosage design reliant on trial-and-error approaches. This study developed a multi-scale modification framework for PAAS through systematic experimentation: mechanical testing, NMR spectroscopy, FTIR (capturing -Ca²⁺-OOC- coordination bond formation), XRD (quantifying Ca(OH)₂ crystal plane attenuation), SEM (analyzing pore–gel co-evolution), and rotational rheometry (revealing τ₀-dosage relationships). Following a “molecular design→microstructural control→macroscopic enhancement” methodology, we comprehensively investigated PAAS-modified cement grouts, providing theoretical foundations and technical guidelines for sustainable underground infrastructure repair.

2. Materials and Methods

2.1. Materials

2.1.1. Cement

Ordinary Portland cement (Type I, Grade 32.5 conforming to GB 175-2007 [30]) was used in this study. The cement possessed a specific surface area (Blaine) of 360 ± 10 m²/kg. Its technical specifications and chemical composition are summarized in

Table 1. Technical specifications and chemical composition of Portland cement.

Density/ (kg/m3)	Chemical Composition/%
	CaO	SiO2	Al2O3	MgO	Fe2O3	SO3
3150	63	21.23	6.58	1.6	3.65	2.93

.

2.1.2. Sodium Polyacrylate

Sodium polyacrylate (PAAS), a water-soluble anionic polymer with the molecular formula [C₃H₃NaO₂]_n (structural analysis in Figure 1), was used in this study. The analytical-grade PAAS, sourced from Tianjin Zhonglian Chemical Reagent Co., Ltd. (Tianjin, China), contained ≥24.5% Na₂O (by mass). Its polymer chains feature abundant carboxyl groups (-COO⁻) and exhibit a notably low molecular weight range of 3000–5000 Da.

2.1.3. Water

Potable tap water, conforming to the standard for mixing water in concrete as per ASTM C1602/C1602M [31], was used for all experiments.

2.2. Preparation of PAAS-Modified Cement-Based Paste

Materials were weighed according to the proportions in

Table 2. Mix proportions of EVA-modified cement grout.

No.	Proportion	Cement (g)	Water (g)	PAAS (g)
1	W/C 0.8, P/C 0%	100	80	0
2	W/C 0.8, P/C 0.02%	100	80	0.02
3	W/C 0.8, P/C 0.04%	100	80	0.04
4	W/C 0.8, P/C 0.06%	100	80	0.06
5	W/C 0.8, P/C 0.08%	100	80	0.08
6	W/C 0.8, P/C 0.1%	100	80	0.1

and mixed uniformly in a cement mixer for 3 min. The mixed grout was then cast into 50 mm × 100 mm cylinder molds following ASTM C942 standards [32] or 150 mm × 150 mm × 550 mm rectangular molds following GB/T 50081-2019 [33], with nine specimens prepared per group. After demolding after 1 day, specimens were transferred to a concrete curing chamber. The nine specimens were divided into three subgroups for 3-day, 7-day, and 28-day curing periods. Following curing, compressive strength and microstructural properties were evaluated.

2.3. Experimental Approaches

2.3.1. Fluidity

The testing procedure followed GB/T 8077-2012 [34]. Fluidity was measured using a smooth-walled metal frustum cone with upper diameter 36 mm, lower diameter 60 mm, and height 60 mm. A 500 mm × 500 mm × 500 mm acrylic plate was positioned horizontally, verified by a spirit level. Surfaces of the plate and cone were dampened with absorbent paper to ensure uniform moisture without excess water. Fresh grout was rapidly filled into the cone, and excess material was trimmed. The cone was vertically lifted within 3 s, allowing free flow. After 30 s, the spread diameter was measured in two perpendicular directions, with the average recorded as the result. Each group was tested in triplicate, with irregular flows discarded.

2.3.2. Rheological Curve

Rheological properties were characterized using a 12-speed rotational viscometer, which provides twelve discrete shear rates within the 1–600 rpm range. All rheological tests were performed within 10 min after grout mixing. The shear rate was decreased from 1022 s⁻¹ to 0 s⁻¹ over 120 s while recording corresponding shear stress values. Flow curves were plotted from multi-point measurements to determine the flow pattern and derive key rheological parameters of the modified grout during deformation. The instrument is capable of displaying the torque applied to the spindle surface, and the shear stress and apparent viscosity can be calculated from Equations (1) and (2):

$$\tau ={\displaystyle \frac{M}{2\pi R^{\mathit{2}}L}}$$

(1)

$$\eta ={\displaystyle \frac{\tau }{\upsilon }}$$

(2)

where τ = shear stress, η = apparent viscosity, M = torque read on the instrument, R = radius of the inner spindle, L = length of the spindle, and υ = shear rate.

2.3.3. Fourier Transform Infrared Spectroscopy (FT-IR)

Fourier-transform infrared (FT-IR) spectroscopy was performed on 28-day cement pastes using the KBr pellet method. Samples were crushed and ground to particle sizes <5 μm, then mixed with KBr at a 1:150 mass ratio. After thorough grinding, pellets were pressed at 10 MPa into transparent disks (13 mm diameter). Spectra were acquired using a Nicolet iS50 spectrometer over 4000–400 cm⁻¹ (4 cm⁻¹ resolution, 64 scans), with background correction against pure KBr pellets. Prior to testing, samples were vacuum-dried at 60 °C for 24 h to minimize adsorbed water interference.

2.3.4. X-Ray Diffraction (XRD)

Powder X-ray diffraction (XRD) analysis was performed on 28-day cement pastes. Following compressive strength testing, fractured specimens were immersed in absolute ethanol to terminate hydration. After 24 h, samples were vacuum-dried for 12 h, then ground and sieved through a 200-mesh sieve (<74 μm). Flat test surfaces were prepared using the back-loading technique. Phase analysis was conducted using a Rigaku SmartLab diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu-Kα radiation (λ = 1.5406 Å), operating at 40 kV and 40 mA. Scans were performed continuously from 5° to 70° (2θ) with a step size of 0.02° and a scan speed of 5°/min.

2.3.5. Scanning Electron Microscope (SEM)

Microstructural morphology of 28-day cement paste fracture surfaces was analyzed using scanning electron microscopy (SEM). Following compressive strength testing, fractured specimens were immersed in absolute ethanol to terminate hydration. After vacuum drying, samples were sputter-coated with a 5 nm platinum layer to enhance conductivity. Secondary electron (SE) images were acquired using a Flex-SEM1000 microscope (Hitachi, Tokyo, Japan) at 20.0 kV accelerating voltage and 8–9 mm working distance. Imaging was performed at ×2000 magnification (scale bar: 20.0 μm), strictly following GB/T 27788-2020 standards [35]. The sample stage was maintained at 0° tilt to prevent distortion, with multiple fields of view captured per group to ensure statistical significance.

3. Results and Discussion

3.1. Fluidity and Rheological Properties of PAAS-Modified Composite Grouting

3.1.1. Fluidity

Sodium polyacrylate (PAAS) significantly enhanced the fluidity of cement grouts, as detailed in

Table 3. The influence of PAAS dosage on the fluidity of the paste.

PAAS Content/%	Average Diameter (cm)	Coefficient of Variation/%	Increase Compared to the Control Group
0	19.23 ± 1.28	6.7	-
0.02	24.30 ± 1.15	4.7	+26.4%
0.04	23.75 ± 0.30	1.3	+23.5%
0.06	24.55 ± 1.15	4.7	+27.7%
0.08	25.13 ± 0.13	0.5	+30.7%
0.10	25.06 ± 0.27	1.1	+30.3%

. All PAAS-modified groups exhibited higher fluidity than the control (19.23 cm), with nonlinear improvement as dosage increased. Key observations include (1) Low-dosage efficacy: At 0.02% PAAS, fluidity reached 24.30 cm (+26.4% vs. control), demonstrating effective disruption of cement particle flocculation via electrostatic repulsion from anionic groups (-COO⁻) even at minimal dosages; (2) Critical saturation: Peak fluidity (25.13 cm, +30.7%) occurred at 0.08% PAAS with minimal variation (CV = 0.52%), indicating highly stable dispersion at saturation.

The fluidity of the grout exhibited a continuous, near-linear increase with PAAS dosage up to 0.08%, without exhibiting the decline observed in other polymer-modified systems. This trend stands in sharp contrast to the findings for in situ polymerized acrylate monomers. For instance, Zhou et al. [36] reported that flowability of cement composites initially increased with calcium acrylate dosage but significantly decreased after exceeding 6%, a phenomenon also confirmed by Xu et al. [37] and Fang et al. [38], with thresholds of 8% and 7%, respectively. They attributed this decline to accelerated polymerization at high monomer concentrations, which increases yield stress and plastic viscosity. The absence of such a decline in our study, even at the highest dosage (0.1%), highlights a key advantage of directly adding pre-polymerized PAAS: it avoids the complex and unpredictable in situ polymerization process. The fluidity enhancement is solely governed by the well-defined dispersing mechanisms (electrostatic repulsion and steric hindrance) of the PAAS polymer, allowing for a broader and more stable effective dosage range.

Sodium polyacrylate (PAAS) significantly enhances cement grout fluidity through two mechanisms. (1) Electrostatic repulsion: PAAS carboxylate groups (-COO⁻) adsorb onto cement particle surfaces, inducing negative zeta potential shifts. This generates Coulombic repulsion that overcomes van der Waals attraction, effectively breaking down flocculated networks and resulting in enhanced macroscopic fluidity. (2) Steric hindrance: Extended PAAS polymer chains form physical barriers between particles, suppressing agglomeration and increasing interparticle distances, thereby further improving fluidity.

3.1.2. Rheological Curve

Sodium polyacrylate (PAAS) significantly modified the rheological behavior of cement grouts, as shown in Figure 2. The addition of PAAS substantially influenced the rheological properties, with shear stress–shear rate curves shifting downward as PAAS dosage increased. Both yield stress and plastic viscosity decreased progressively, and the 0.1% PAAS formulation demonstrated optimal flow characteristics.

The rheological curves of PAAS-modified cement grouts maintained distinct linear characteristics without observable shear-thinning or shear-thickening behavior, confirming the continued applicability of the Bingham model. The Bingham model fitting of the rheological curves yielded key parameters presented in

Table 4. The Bingham model fitting parameters of PAAS-modified composite grouting.

PAAS Content/%	Yield Stress τ (Pa)	Plastic Viscosity η (Pa·s)	R2
0%	4.82	0.052	0.991
0.02%	2.37	0.041	0.998
0.04%	1.65	0.045	0.997
0.06%	2.03	0.046	0.993
0.08%	1.28	0.043	0.999
0.10%	0.95	0.040	0.996

, with all determination coefficients (R²) exceeding 0.99, validating the model’s effectiveness.

Bingham model analysis revealed that yield stress (τ) decreased significantly with increasing PAAS content. Even at the low dosage of 0.02%, τ was reduced by 50.8% compared to the control group (4.82 Pa), demonstrating PAAS’s remarkable effectiveness at minimal concentrations. The optimal rheological performance occurred at 0.1% PAAS, where τ decreased to 0.95 Pa (80.3% reduction from the control’s 4.82 Pa), attributable to the efficient disruption of flocculated structures through electrostatic repulsion. Simultaneously, plastic viscosity (η) decreased to 0.040 Pa·s (23.1% reduction), resulting from free water released through steric hindrance, forming uniform lubricating films. These rheological improvements directly enhance engineering performance: the 80.3% reduction in yield stress decreases required injection pressure, thereby improving grout injectability, while reduced plastic viscosity enhances diffusion capacity within fractures.

The rheological behavior of PAAS-modified grouts demonstrates a significant and advantageous simplicity compared to the complex trends often reported for polymer-modified cementitious systems. In this study, both the yield stress and plastic viscosity exhibited a consistent and monotonic decrease with increasing PAAS dosage (Table 4). This trend stands in sharp contrast to the non-monotonic, multi-stage influence of many polymer latexes. For instance, Jing Wang et al. [39] observed that the yield stress of fresh cement paste first increased, then decreased, and finally increased again as the dosage of styrene–acrylate (SA) latex increased. This complex behavior was attributed to competing mechanisms: at low dosage, polymer bridging flocculates particles (increasing yield stress); at medium dosage, electrostatic repulsion dominates (decreasing yield stress); and, at high dosage, non-adsorbed polymers induce attractive depletion forces (again increasing yield stress). In our case, the pre-polymerized, low molecular weight PAAS appears to bypass the initial flocculation and final depletion stages. Its primary action is that of a powerful dispersant, providing strong electrostatic and steric repulsion that effectively dismantles flocculated structures across the entire tested dosage range (0–0.1%). The absence of a yield stress increase at any point highlights a key practical benefit of using PAAS: its rheological effect is highly predictable and consistently beneficial for fluidity, eliminating the risk of unexpected workability loss that can occur with other polymer modifiers at certain critical dosages.

3.2. Chemical Characterization of PAAS-Modified Composite Paste

3.2.1. Fourier Transform Infrared Spectroscopy (FT-IR)

This study investigated the regulatory effects of sodium polyacrylate (PAAS) on cement paste hydration and microstructure using Fourier transform infrared (FT-IR) spectroscopy. In Figure 3, a broad hydroxyl (-OH) peak appeared at 3000–3600 cm⁻¹ [40], primarily attributed to stretching vibrations of free water and hydroxyl groups in hydration product Ca(OH)₂. As PAAS dosage increased from 0% to 0.08%, this peak intensity initially increased, then decreased significantly. PAAS ionizes extensively in water, generating polymeric anions and small cations (Na⁺). The polymeric anions firmly adsorb onto dispersed particle surfaces, imparting like charges, while counterions (Na⁺) freely diffuse into the surrounding liquid medium, forming a stable charged diffuse layer. Through this mechanism, PAAS generates electrostatic repulsion that effectively disperses cement particles, reduces localized agglomeration, and consequently retards hydration kinetics while inhibiting Ca(OH)₂ formation. Concurrently, the asymmetric stretching vibration peak of carbonate groups (CO₃²⁻) at 1400–1500 cm⁻¹ also exhibited initial enhancement followed by attenuation with increasing PAAS dosage [41]. This phenomenon is attributed to the adsorbed PAAS layer on cement particles, which isolates the cement matrix from direct CO₂ contact, thereby suppressing carbonation reactions (where Ca(OH)₂ reacts with CO₂ to form CaCO₃).

Further analysis revealed a distinct carboxylate group (-COO⁻) peak near 1650 cm⁻¹ in PAAS-modified samples [42], exhibiting initial enhancement followed by attenuation with increasing PAAS dosage. This confirms crosslinking and complexation reactions between PAAS carboxylate groups and Ca²⁺ released during cement hydration, forming Ca-PAAS complexes. Such chemical bonding not only consumes free Ca²⁺, potentially disrupting C-S-H gel nucleation and growth, but also further modifies hydration product microstructures through steric hindrance effects. Concurrently, the silicon–oxygen bond (Si-O) stretching vibration peak (950–1100 cm⁻¹) showed similar initial intensification then reduction [43], with a noticeable low-wavenumber shift in modified samples. This suggests PAAS adsorption onto silicate particles alters silicate chain polymerization degrees or C-S-H gel extension pathways. Furthermore, enhanced calcium–oxygen bond (Ca-O) vibration peaks (450–550 cm⁻¹), particularly the intensified 550 cm⁻¹ peak, align with the -OH peak trend, corroborating PAAS’s dosage-dependent dual role: initially promoting then suppressing Ca(OH)₂ formation.

Integrating these findings, PAAS’s mechanism in cement grouts can be attributed to synergistic physical dispersion and chemical bonding effects. Physically, ionization-induced particle dispersion optimizes hydration reaction uniformity by reducing agglomeration. Chemically, carboxylate–Ca²⁺ complexation and adsorbed layer formation not only inhibit carbonate accumulation but also refine material microstructure by regulating C-S-H gel nucleation and growth. These insights provide a fundamental understanding of PAAS functionality in cementitious systems. Future work will focus on combined XRD and microstructural characterization to further analyze hydration product composition/distribution and refine the mechanistic framework.

3.2.2. X-Ray Diffraction (XRD)

XRD patterns (Figure 4) clearly reveal the regulatory effect of sodium polyacrylate (PAAS) on cement hydration products. The control group (0% PAAS) shows characteristic Ca(OH)₂ peaks at 2θ = 18.1° and 34.1°, a main ettringite (AFt) peak at 9.1°, and diffraction peaks of unhydrated C₂S at 29.3° (partially overlapping with C₃S peak) and 32.2° [44]. With PAAS dosage increasing to 0.08%, Ca(OH)₂ peak intensity decreases significantly, AFt peaks weaken simultaneously but with lesser reduction than Ca(OH)₂, and C₂S peaks relatively intensify, indicating hindered hydration progress in PAAS-modified cement. Notably, the inhibition degree of Ca(OH)₂ is significantly higher than that of AFt, possibly related to PAAS’s selective action on calcium-based products.

Notably, the suppression of CH was most pronounced at the 0.04% dosage, exhibiting a non-monotonic trend that underscores the complexity of the modification mechanism. This nuanced effect can be understood through the competing pathways of PAAS action. The mechanism of PAAS modification involves complex physicochemical synergy. Firstly, the preferential inhibition of Ca(OH)₂ relates to the reaction between PAAS’s carboxyl groups (-COO⁻) and Ca²⁺. After hydration initiation, -COO⁻ forms stable complexes with Ca²⁺, reducing free Ca²⁺ concentration, thereby disrupting Ca(OH)₂ precipitation equilibrium. This chemical chelation effect is highly efficient at low to moderate dosages (e.g., 0.04%). Simultaneously, PAAS is speculated to adsorb on Ca(OH)₂ crystal nuclei surfaces, hindering growth. At higher dosages (e.g., 0.08%), while chelation persists, the pronounced physical adsorption of PAAS on cement particles creates a more formidable steric barrier, which retards the overall hydration kinetics. This may slightly moderate the extreme suppression of CH seen at 0.04%, as the delayed reaction allows for a more gradual formation of all products, including CH. Nevertheless, the overall abundance of CH remains substantially lower than in the control group.

The mechanism of PAAS modification involves complex physicochemical synergy. Firstly, the preferential inhibition of Ca(OH)₂ relates to the reaction between PAAS’s carboxyl groups (-COO⁻) and Ca²⁺. After hydration initiation, -COO⁻ forms stable complexes with Ca²⁺, reducing free Ca²⁺ concentration, thereby disrupting Ca(OH)₂ precipitation equilibrium and decreasing Ca(OH)₂ formation. Simultaneously, PAAS is speculated to adsorb on Ca(OH)₂ crystal nuclei surfaces, hindering growth along the (001) crystal plane and obstructing the ordered stacking of OH⁻ layers, leading to reduced crystallinity. The significant attenuation of Ca(OH)₂ characteristic peaks in XRD aligns with the decreased intensity of 3000–3600 cm⁻¹ broad peak (free OH⁻) in FT-IR. PAAS’s -COO⁻ locks Ca²⁺ via complexation, disrupting Ca(OH)₂ precipitation equilibrium, jointly verifying the chemical-bonding-dominated inhibition mechanism. Secondly, PAAS forms an adsorption layer on C₃A particle surfaces, blocking SO₄²⁻ and AlO₄⁴⁻ diffusion, thereby delaying AFt nucleation. XRD results show that AFt is less affected by PAAS modification compared to Ca(OH)₂, suggesting PAAS’s selectivity toward calcium-based hydration products, with Ca(OH)₂ inhibition having higher priority. Finally, C₂S hydrolysis requires surface protonation (Si-O⁻ + H⁺ → Si-OH), while PAAS’s anionic nature competitively consumes H⁺, retarding Reaction (3):

2CaO·SiO₂ + 4H⁺ → Ca²⁺ + Si(OH)₄ + Ca(OH)₂

(3)

After PAAS modification, the polymer additionally consumes H⁺, consequently hindering C₂S hydrolysis and increasing its residual content. Moreover, the PAAS adsorption layer hinders H₂O molecules from contacting calcium silicate surfaces, further increasing unhydrated C₂S residue. Decreased Si-O bond wavenumber in FT-IR reflects reduced silicate chain polymerization, corroborating XRD-revealed hindered C₂S hydrolysis, jointly confirming that PAAS retards silicate phase hydration in cementitious materials.

3.2.3. Scanning Electron Microscope (SEM)

Scanning electron microscopy (SEM) analysis intuitively reveals the regulatory effect of sodium polyacrylate (PAAS) on the microstructure of cement paste (Figure 5). In the control group (0% PAAS, Figure 5a), typical hydration product morphologies are visible: densely stacked hexagonal Ca(OH)₂ crystals (1–3 μm), needle-like ettringite (AFt, length 0.5–2 μm) [45] radially distributed in pores, and incompletely covered loose flocculent C-S-H gel. Simultaneously, large pores (>5 μm) and smooth-surfaced agglomerated unhydrated C₂S particles exist, reflecting non-uniform hydration reactions. At the 0.04% PAAS dosage (Figure 5b), an intermediate state was observed. The morphology of Ca(OH)₂ crystals began to fragment, and the needle-like AFt structures became less distinct, showing a trend of transformation into a more continuous gel network compared to the control. This transition state corroborates the XRD findings, indicating that the modification process initiated by PAAS is already significantly active at this optimal dosage for compressive strength.

In the 0.08% PAAS group (Figure 5c,d), significantly optimized structures are presented: Ca(OH)₂ crystals are fragmented; AFt needle structures disappear and transform into dense continuous flocculent C-S-H gel networks; pores are refined; unhydrated C₂S particles are dispersedly distributed and tightly wrapped by gel.

The microstructural evolution observed by SEM highly coincides with XRD and FT-IR test results: the significantly reduced Ca(OH)₂ production in SEM corresponds to the attenuated 18.1° peak in XRD and vanishing 3640 cm⁻¹ sub-peak in FT-IR, confirming that PAAS inhibits crystallization through carboxylate complexation. PAAS’s -COO⁻ complexes Ca²⁺, leading to reduced Ca(OH)₂ nucleation, manifested as decreased hexagonal plate-like crystals in SEM, while the released space is filled by dense C-S-H (flocculent structures in SEM), benefiting mechanical property development. Decreased AFt production (SEM) aligns with reduced 9.1° peak in XRD patterns, originating from PAAS adsorption layers blocking ion diffusion. Physical shielding by adsorption layers causes AFt reduction and pore refinement.

These micro-characterization experiments confirm that PAAS modifies cement-based materials through a dual mechanism: chemical bonding inhibits weak phases (Ca(OH)₂/AFt) and physical dispersion optimizes microstructure (pore refinement/C-S-H densification).

3.3. Pore Structure and Mechanical Properties of PAAS-Modified Composite Grouting

3.3.1. Effect of PAAS on Pore Distribution of Cured Materials

Nuclear magnetic resonance relaxation time (T₂ spectrum) characterizes the pore structure evolution of cement solidification bodies [46,47] under different PAAS dosages (Figure 6). The figure shows that T₂ spectra of all experimental groups exhibit bimodal distributions: the primary peak corresponds to 0.001–0.2 μm pores (capillary pores), and the secondary peak is located at 1–100 μm (macropores), indicating heterogeneous pore structures. As PAAS dosage increases, the primary peak gradually shifts leftward with enhanced intensity, while the secondary peak also shifts leftward with reduced signal intensity. This indicates that the dominant pore size decreases with increasing PAAS dosage, and the proportion of macropores significantly diminishes. PAAS disperses particles through electrostatic repulsion, reducing macropore (>1 μm) formation between flocculated structures, while simultaneously promoting the dense precipitation of C-S-H gel, markedly reducing pore size in the set grout.

Porosity quantification (

Table 5. Porosity of PAAS-modified composite paste.

PAAS Content	0	0.02	0.06	0.1
Porosity	44.37	43.91	43.37	43.91

) reveals that total porosity decreases only by 0.46% (43.91% for 0.10% group vs. 44.37% for control), reflecting that PAAS primarily optimizes pore distribution through “macropore refinement” (flocculate disintegration reducing >1 μm pores) and “micropore filling” (C-S-H gel densification), rather than significantly reducing total void volume. This pore homogenization effect will significantly enhance the impermeability and durability of modified materials.

Combining NMR relaxation spectra (T₂ distribution) with spatially resolved imaging (Figure 7a–d), the regulatory mechanism of sodium polyacrylate (PAAS) on pore structure in cement solidification bodies is multidimensionally verified. In Figure 7a (0% PAAS), extensive red areas (>1 μm pores) penetrating the sample can be observed, corresponding to the 100 ms macropore peak in T₂ spectra, indicating connected percolation channels formed by large-diameter pores in the set grout. In Figure 7b,c, the continuous reduction of red areas and expansion of blue areas can be seen, verifying the variation pattern in T₂ spectra with a significant pore homogenization trend. Figure 7d shows the near disappearance of red areas with predominantly dark blue regions, demonstrating highly uniform pore sizes dominated by submicron pores, consistent with the T₂ spectral peak position (corresponding to 0.001–0.2 μm pores).

As PAAS dosage increases, the imaging color transitions from red (>1 μm macropores) to dark blue (<0.2 μm micropores), indicating PAAS eliminates percolation channels by disassembling flocculated structures (>1 μm pores essentially vanish in 0.10% group), while simultaneously promoting C-S-H gel precipitation to form submicron homogeneous structures (dominant pore size 0.001–0.2 μm in 0.10% group). Although total porosity reduction is minor, the qualitative transformation of pore distribution (significant decrease in macropore proportion, increase in submicron pore share) will substantially enhance impermeability and durability. This optimization effect forms a closed-loop evidence chain with SEM-observed particle dispersion and XRD/FT-IR analyses, confirming that PAAS achieves a synchronous synergistic enhancement of “injectability-durability”.

3.3.2. Effect of PAAS on the Mechanical Properties of Cured Materials

The uniaxial compression and flexural strength test results of cement-based sodium polyacrylate (PAAS) solidification materials at different curing ages are shown in

Table 6. Compressive strength and flexural strength of various samples at different curing ages.

Samples	PAAS Content (%)	Curing Age (d)	Compressive Strength (MPa)	Strength Increase (%)	Flexural Strength (MPa)	Strength Increase (%)
A1	0	3	4.79	0	-	-
B1	0.02	3	3.48	−27.3	-	-
C1	0.04	3	3.96	−17.3	-	-
D1	0.06	3	3.94	−17.7	-	-
E1	0.08	3	3.54	−26.1	-	-
F1	0.1	3	3.49	−27.1	-	-
A2	0	7	6.41	0	-	-
B2	0.02	7	6.1	−4.8	-	-
C2	0.04	7	5.82	−9.2	-	-
D2	0.06	7	6.58	2.7	-	-
E2	0.08	7	6.44	0.5	-	-
F2	0.1	7	6.08	−5.1	-	-
A3	0	28	12.33	0	4.44	0
B3	0.02	28	14.98	21.5	3.34	−24.8
C3	0.04	28	15.56	26.2	-	-
D3	0.06	28	14.16	14.8	5.74	29.3
E3	0.08	28	14.03	13.8	-	-
F3	0.1	28	13.77	11.7	4.38	−1.4

and Figure 8. Samples were labeled as m–n, where m and n represented PAAS monomer dosage (% of cement mass) and curing age (d), respectively.

From Figure 8a, it was observed that (1) At 3d, all PAAS dosage groups showed lower compressive strength (3.48–3.96 MPa) than the control group (4.79 MPa), with the maximum reduction of 27.3% (0.02% group); this consistency aligned with the hydration retardation mechanism revealed by FTIR/XRD—PAAS inhibits the initial nucleation of Ca(OH)₂ and AFt; (2) At 7d, the 0.06% group reached peak compressive strength at 6.58 MPa (+2.7% increase), reflecting that PAAS dispersion promotes hydration uniformity; (3) At 28d, the compressive strength of PAAS-modified cement grouts first increased then decreased with increasing PAAS dosage. When PAAS dosage was 0.04%, the 28-day compressive strength peaked at 15.56 MPa (+26.2% increase). However, when PAAS dosage exceeded 0.04%, material strength decreased.

Analysis suggested that the initial strength increase mainly originated from two aspects: appropriate PAAS promotes cement hydration reactions, and the calcium silicate hydrate (C-S-H) gel generated by hydration is the primary strength contributor; additionally, PAAS particle size is significantly smaller than cement particles, enabling effective microcrack filling. Conversely, the strength reduction was attributed to excessive PAAS: at high dosages, PAAS dissolved incompletely, causing particle enrichment on specimen surfaces that created structural heterogeneity.

From Figure 8b, it was seen that 28-day flexural strength exhibited an “increase-decrease” trend: the 0.02% group had flexural strength of 3.34 MPa (24.8% reduction) due to PAAS-delayed hydration preventing sufficient ITZ reinforcement; the 0.06% group reached peak flexural strength (5.74 MPa, +29.3%), mainly attributed to PAAS optimizing internal voids and PAAS–Ca²⁺ bonding enhancing interface cohesion.

3.3.3. Mechanical Model and Reliability Analysis of PAAS-Modified Composite Paste

Multiple regression fitting was performed on the experimental results of Table 6 to establish a mechanical model for cement-based sodium polyacrylate (PAAS) solidification materials, considering two factors: curing age and PAAS dosage, as shown in Equation (4).

$${\mathsf{\sigma }}_{\mathrm{c}}=3.099-1.552c+0.399D$$

(4)

where σ_c = compressive strength of sample, c = PAAS content, and D = curing age.

To verify the reliability of this mechanical model based on curing age and PAAS dosage, an analysis of variance (ANOVA) and residual analysis were conducted.

Table 7. Analysis of variance with a regression model.

Source of Variation	Sum of Squares	Mean Square	F	p	R2	Adeq Precision
Model	345.30	172.65	269.61	<0.0001	0.973	31.041
c	0.0506	0.0506	0.0838	0.7825
D	345.25	345.25	539.14	<0.0001
Residual	9.61	0.6404
Cor Total	354.90

presents the ANOVA results of the regression model. The model F-value reached 269.61 with a p-value < 0.05, indicating high statistical significance. The predicted R² of 0.973 demonstrates excellent predictive capability. Furthermore, the model’s signal-to-noise ratio (Adequate Precision) was 31.041 (significantly >4), confirming sufficient precision for design space exploration. Notably, the p-value for the model term “curing age” (D) was <0.05, revealing that curing age exerts significantly greater influence on compressive strength than PAAS dosage.

Figure 9a displays the normal probability plot of residuals for compressive strength test results, where data points essentially are distributed along a straight line. In Figure 9b, showing residuals versus predicted values, the residuals scatter randomly without specific distribution patterns. These results confirm that the experimental data satisfy the normality assumption and contain no outliers. Therefore, residual analysis further validates the prediction accuracy of this regression model, demonstrating its applicability for guiding the design and production of PAAS-modified cement-based grouting materials.

3.4. The Modification Mechanism of PAAS-Modified Cement

The modification mechanism of sodium polyacrylate (PAAS) on cement-based materials manifests as a multi-scale synergistic effect [14]. At the molecular scale, PAAS carboxyl groups (-COO⁻) selectively chelate Ca²⁺ (enhanced FT-IR peak at 1650 cm⁻¹), significantly reducing free calcium ion concentration and disrupting Ca(OH)₂ precipitation equilibrium (significantly attenuated XRD peak at 18.1°); simultaneously, they adsorb onto C₃A particle surfaces, forming diffusion barriers that block SO₄²⁻/AlO₄⁴⁻ migration paths, inhibiting ettringite (AFt) nucleation (reduced peak intensity at 9.1°). At the micron scale, PAAS’s anionic nature induces negative surface potential shifts, effectively dismantling flocculated structures through electrostatic repulsion, while its extended molecular chains provide steric hindrance, reducing yield stress to 1.28 Pa (73.4% reduction) and enhancing grout fluidity (30.7% fluidity increase).

PAAS retards the early-stage hydration kinetics of cementitious materials, a common phenomenon observed with many organic admixtures that adsorb onto cement particles [48,49]. However, it is critical to note that retardation does not equate to prevention. As evidenced by the mechanical strength development (Section 3.3.2), the 28-day compressive and flexural strengths of PAAS-modified groups significantly surpassed those of the control. This indicates that, while PAAS slows the initial reaction, the ultimate degree of hydration and the formation of a denser, more homogeneous micro-structure are not adversely affected in the long term. The retardation may even be beneficial, allowing for a more orderly precipitation of hydration products and mitigating the formation of weak, crystalline phases like Ca(OH)₂.

The physicochemical synergy during PAAS modification reconstructs the cement hydration microenvironment: dispersion effects release free water to fill capillary pores, optimizing C-S-H gel precipitation space to form dense flocculent networks with significantly improved coverage versus control; hydration pathway reconstruction concurrently reduces weak-phase Ca(OH)₂ formation, substantially enhancing matrix homogeneity.

Notably, dominant mechanisms differ by dosage: at 0.04% PAAS, chemical bonding dominates, achieving peak 28-day compressive strength (15.56 MPa, +26%); at 0.08% dosage, physical dispersion reaches saturation, yielding optimal fluidity (25.13 cm) enabling injection into finer fractures; excessive dosage (>0.1%) causes molecular chain entanglement, triggering abnormal viscosity rise in high-shear regions, confirming critical dosage importance.

This mechanism reveals that PAAS enables synergistic fluidity–strength enhancement across molecular–micro–macro-scales via two pathways: chemical bonding inhibits weak phases while physical dispersion optimizes microstructure, establishing a theoretical paradigm for the interfacial engineering of cement-based grouting materials.

4. Conclusions

This study systematically elucidates the multi-scale modification mechanism of sodium polyacrylate (PAAS) on cement-based grouting materials. Through a synergistic analysis of rheology, microstructure, and mechanical properties, we confirm that PAAS at 0.04–0.08 wt% dosage simultaneously enhances grout rheology, optimizes pore structure, and strengthens mechanical performance. Key conclusions are as follows:

(1)

Rheological improvement: PAAS dismantles cement particle flocculation through electrostatic repulsion and steric hindrance synergy. At 0.1% dosage, yield stress reaches τ = 0.95 Pa (80.3% reduction), with plastic viscosity stabilizing at 0.04 Pa·s (23.1% reduction), exhibiting near-Bingham fluid characteristics. Maximum fluidity increases by 30.7% (25.13 cm), significantly improving injectability and fracture diffusion capacity.

(2)

Pore-mechanics co-evolution: PAAS-induced microstructural reconstruction follows “macropore elimination-microporous homogenization”. NMR imaging shows >1 μm macropores virtually disappear with increasing dosage, dominant pore size decreases, and submicron pore proportion rises. Despite minor total porosity reduction (0.46%), qualitative pore transformation substantially enhances impermeability. Mechanically, PAAS concurrently suppresses weak-phase formation and promotes C-S-H densification, enabling the 0.04% group to achieve peak 28-day compressive strength (15.56 MPa, +26%), while the 0.06% group shows a dramatic flexural strength increase (5.74 MPa, +29.3%), significantly improving material performance and mitigating brittle failure.

(3)

Multi-scale mechanism: PAAS modification originates from chemical bonding and physical dispersion synergy. Molecular: Carboxyl groups (-COO⁻) selectively chelate Ca²⁺, directionally inhibiting Ca(OH)₂ nucleation and optimizing C-S-H precipitation. Micro: Electrostatic repulsion dismantles flocculates while steric hindrance prevents re-agglomeration, eliminating percolation channels (NMR red zones vanish). Macro: Coordinated enhancement of rheological parameters and mechanical properties.

PAAS’s industrial-scale low-cost and wide-pH adaptability demonstrate significant potential for tunnel-lining rehabilitation and foundation stabilization, advancing the green low-carbon transformation of infrastructure.