Superabsorbent Polymer Particles Research Articles

Superabsorbent Polymers for Internal Curing Concrete: An Additional Review on Characteristics, Effects, and Applications

Superabsorbent polymers (SAPs) are a promising admixture that can provide internal curing to freshly cast concrete and enhance concrete properties. Although many reviews have explored aspects of SAPs, the links among SAPs’ chemical and physical properties, internal curing behaviors, concrete performance, and their large-scale applications are often weakly elucidated. This paper provides an additional review of the chemical structures and physical dimensions of SAPs and their effects on the internal curing kinetic behavior as well as on concrete properties, such as workability, strength, and durability. In addition, different approaches to introducing SAP particles into concrete mixtures are also summarized. Case studies on the use of SAPs in the concrete industry are introduced to provide a better understanding of the greatest potential of SAPs in field applications. The results confirm that the utilization of SAPs in concrete mixtures provides multiple benefits such as improved water curing, reduced shrinkage, and enhanced workability. Selecting the appropriate SAPs is crucial and involves considering factors like absorption rate, durability, and stability. However, achieving uniform distribution of dry SAPs in concrete poses challenges. Further research is required to gain a deeper understanding of the impact of SAPs on transport properties and frost durability. Additionally, the absence of a standard makes it difficult to maintain consistent water-to-cement ratios. These findings provide a theoretical foundation for using SAPs to enhance concrete performance while also highlighting future research directions and challenges. In this article, scientists, engineers, and contractors will find a comprehensive explanation encompassing laboratory investigations, field implementation, and relevant guidance.

Acoustic emission analysis of self-healing properties in cementitious materials with superabsorbent polymers (SAPs) during different environments

This paper studies the coupling effect of superabsorbent polymers (SAPs) and fly ash on the self-healing performance of mortar. The three-point bending test was used to evaluate the mechanical property recovery effect of mortar specimens after curing in different environments, and the relationship between crack closure rate, flexural strength and flexural stiffness recovery was established. The amplitude and energy in acoustic emission were used to evaluate the characteristics of self-healing products. The results show that compared with the control specimens, the addition of SAP improves the crack closure rate and mechanical property recovery effect of mortar specimens. With the increase of SAP particle size and content, the mechanical property recovery rate and crack closure rate increase accordingly. The order of self-healing effect of specimens in different healing environments is saturated calcium hydroxide > water >97 % RH > air. After reloading, the amplitude number and cumulative energy of samples with SAP are larger than those of the control specimens. The SEM results also prove this, and the self-healing products in the crack area of the mortar specimens with SAP are denser than those in the control specimens.

Multi-scale study of the effect of superabsorbent polymers on microporosity, dimensional stability, and durability of cementitious materials

As internal curing agents, superabsorbent polymers (SAPs) help enhance the degree of hydration of cement particles. In the meantime, however, they affect the dimensional stability and durability of the mixtures that benefit from them. This critical aspect, which is influenced by the type, size, and dosage of SAPs, motivated the current study. To obtain holistic assessments and side-by-side comparisons, six high-performance mortar mixtures were developed based on a control mixture. The referenced mixtures contained two types, two sizes, and two dosages of SAPs. Initially, the performance characteristics of the SAP powders were determined, in terms of water absorption and desorption. Autogenous shrinkage tests were then performed to evaluate how SAPs contribute to controlling this type of shrinkage over time. The microstructures of the developed mixtures were then analyzed using mercury intrusion porosimetry, paired with scanning electron microscope images, providing fundamental insights into to the role of SAPs in both short and long terms. To correlate the findings to the mechanical properties of the developed mixtures, a set of compressive strength and abrasion resistance tests were performed. The results were then complemented with the data obtained from the rapid chloride migration tests. This study’s outcome showed that SAP particles can effectively reduce the development of autogenous shrinkage-induced strains. Additionally, the inclusion of SAPs was found to alter microstructural characteristics, increasing the density of the binding gel, while introducing capillary pores. This was confirmed through enhanced abrasion resistance and reduced chloride resistivity.

Modification of fresh and hardened properties of 3D-printed recycled mortar by superabsorbent polymers

The high water absorption level of recycled sand limits the applicability of 3D-printed recycled mortar. In this study, superabsorbent polymers (SAPs) were adopted to modify the properties of both fresh and hardened 3D-printed recycled mortar. The effects of various SAP dosages, particle sizes, and addition methods were carefully examined. X-ray diffraction (XRD) and computed tomography (CT) were adopted to observe the microstructure. The test results showed that the addition of SAP effectively improved the printability of the mixed mortar. Particularly, with the addition of SAP, the autogenous shrinkage was reduced by more than 50 %, indicating that the polymer had an obvious inhibitory effect on shrinkage. Furthermore, the hydration of cementitious materials was promoted, resulting in an increase in compressive strength by 27%–69 %. Microstructural analysis further confirmed that the addition of SAP optimized the pore structure and enhanced the density of the 3D-printed mortar but did not change the types of hydration products. Generally, for the modification of 3D-printed mortar with recycled sand, it would be recommended to add SAP at a doping level of 0.4 % with a particle size of 60–100 mesh.

Investigation of the Compressive Strength and Void Analysis of Cement Pastes with Superabsorbent Polymer.

This study aimed to experimentally investigate the compressive strength and air voids of cement pastes with varying dosages of Superabsorbent Polymer (SAP) and water-to-cement (w/c) ratios. Cement pastes were prepared using three different w/c ratios of 0.4, 0.5, and 0.6, along with different dosages of SAP ranging from 0.2% to 0.5% by weight of cement. Additionally, SAP was introduced in two forms: dry and wet. After casting the cubes, two distinct curing conditions were employed: curing at a temperature of 20 °C with a Relative Humidity (RH) of 60% (Curing 1), and water curing (Curing 2). The results revealed that the addition of SAP increased early strength when subjected to Curing 1, followed by a decrease in later strength. On the other hand, samples with SAP and water curing exhibited higher strength compared to those without SAP, especially with w/c ratios of 0.4 and 0.5. However, at a w/c ratio of 0.6, nearly all samples showed a reduction in strength compared to those without SAP. Furthermore, air void analysis was performed on all samples cured for 28 days using an image analysis technique. The samples containing wet SAP resulted in a higher total air content compared to the samples with dry SAP. Additionally, the incorporation of wet SAP in cement paste led to lower specific surface areas and a higher spacing factor than the samples with dry SAP. These findings suggest that the clumping of wet SAP particles during presoaking resulted in coarser air voids compared to the samples containing dry SAP.

Study on Alkali Reduction Effect of SAP on Cementitious Composites

In order to improve the high alkalinity environment inside the cementitious composites, the effect of SAP doping on the pH value and compressive strength of cement slurry was investigated by doping Super Absorbent Polymer (SAP). The test results show that SAP can reduce the pH value of cementitious composites at 28d to a certain extent; the pH value of cementitious composites at 28d is significantly reduced when SAP doping is 0.3%, and the compressive strength is improved compared with that of the control group; SAP particles will form cracks with the middle of the cement matrix after the release of water to reduce the alkaline material outflow from the cement matrix, which will make the pH value of SAP specimens reduced.

Effect of superabsorbent polymers on the properties of cement paste particle under different relative humidities

This study investigated the adsorption-desorption kinetics of polyacrylic acid sodium salt (AA) and polyacrylic acid-acrylamide copolymer (AM) under different relative humidities by the static water vapour adsorption experiment. TG and SEM-EDS were used to study the effect of Superabsorbent polymers (SAPs) on the hydration degree of cement without mixing water. The results show that the adsorption capacity of AA is higher than that of AM, and the desorption capacity of AA is worse than that of AM. At 97%RH, AA and AM can absorb 233.2 % and 210.5 % of water vapour from the environment. Compared with cement without SAPs, the addition of AA can increase the hydration degree of cement by 17.05%–53.58 % (one-element system) and 14.21%–30.29 % (binary system); the addition of AM can increase the hydration degree of cement by 8.82%–20.13 % (one-element system) and 4.79%–9.15 % (binary system). The degree of cement hydration on the surface of SAPs particles is higher than that of other parts. It was found that needle-like products exist on both the surface of SAPs particles and cement particles and the calcium content is similar to the TG results. Calcium hydroxide and calcium element content are ranked as AA particle surface cement > AM particle surface cement > AA cement added > AM cement added > pure cement.

Experimental Study of a Superabsorbent Polymer Hydrogel in an Alkali Environment and Its Effects on the Mechanical and Shrinkage Properties of Cement Mortars.

As internal curing self-healing agents in concrete repair, the basic properties of superabsorbent polymers (SAPs), such as water absorption and release properties, are generally affected by several factors, including temperature and humidity solution properties and SAP particle size, which regulate the curing effect and the durability of cementitious composites. This study aimed to investigate the water retention capacities of SAPs in an alkaline environment over extended periods by incorporating liquid sodium silicate (SS) into SAP-water mixtures and examining the influence of temperature. The influence of SAP particle size on mortar's water absorption capacity and mechanical behavior was investigated. Two mixing techniques for SAPs (dry and pre-wetting) were employed to assess the influence of SAP on cement mortars' slump, mechanical properties, and cracking resistance. Four types of SAPs (SAP-a, SAP-b, SAP-c, and SAP-d), based on the molecular chains and particle size, were mixed with SS to study their water absorption over 30 days. The results showed that SAPs exhibit rapid water absorption within the first 30 min, exceeding 85% before reaching a saturation point, and the chemical and temperature variations in the water significantly affected water absorption and desorption. The filtration results revealed that SAP-d exhibited the slowest water release rate, retaining water for considerably longer than the other three types of SAPs. The mechanical properties of SAP mortar were reduced due to the addition of an SAP and the improved cracking resistance of the cement mortars.

Multi-scale characterization of SAP impact on self-healing behavior in UHPC under varied crack widths and environments

This paper studies the effects of the content and particle size of superabsorbent polymer (SAP) on the self-healing behavior of precracked Ultra-high-performance concrete (UHPC) under different crack widths. In this experiment, bending and compression-resistant internal damage to UHPC was first created with different amounts and particle sizes of SAP added, and then the damaged specimens were placed in a water environment, a dry environment, and a water-dry cycle environment for self-healing. Finally, the self-healing effect of pre-cracked UHPC was evaluated from the changes in crack width, recovery of mechanical properties, and changes in ultrasonic speed. In addition, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to conduct microscopic analysis of the products of the self-healing reaction. The experimental results show that SAP itself does not participate in self-healing chemical reactions. It mainly seals cracks by absorbing water and expands and promotes the continued hydration of the internal gelling materials of UHPC through water storage to improve the self-healing performance. Therefore, in a dry environment, SAP has almost no effect on the self-healing performance of UHPC. In moisture-rich environments, SAP can significantly improve the healing of macroscopic cracks with a width greater than 50 μm. In addition, the experimental results also show that under the same environment, changes in SAP content and particle size have a consistent effect on the recovery of UHPC's mechanical properties and the recovery of ultrasonic speed. The results of this study can promote the application of internal curing in the self-healing of UHPC cracks and further reduce the maintenance cost of UHPC throughout its life cycle.

Durability of Concrete with Superabsorbent Polymer (SAP) Assessed Using Depth of Carbonation and NDT Ultrasonic Methods.

The paper concerns destructive and non-destructive (NDT) evaluation of the effect of the addition of superabsorbent polymer (SAP) used as a carrier of mixing water and a means of internal curing on the durability of concrete. The research concerns testing of five concretes-an ordinary reference concrete and four concretes differing in the content of mixing water introduced into the concrete mix in the form of pre-saturated SAP particles (25%, two variants of 50% and 75% of the total mixing water in the form of SAP hydrogel). The research consisted of 4 stages of tests. The subsequent stages involved the analysis of the effect of using SAP as a carrier of mixing water on the particular characteristics of concrete mix and hardened concrete, i.e., consistency and density of concrete mix (1st stage), carbonation tested using two indicators-phenolphthalein and thymol phenolphthalein (2nd stage), and finally: the homogeneity of the concretes' structure by means of ultrasonic method (determination of ultrasonic pulse velocity) 28 days after production (3rd stage) and 3 years after production (4th stage). The ultrasonic pulse (or wave) velocity was then correlated with the content of water applied in the form of SAP hydrogel. The statistical analysis of results showed that the method of introducing the mixing water into the concrete mix in the form of pre-absorbed superabsorbent polymer, although it changed the concrete mix consistency, did not significantly affect the concrete ability to resist carbonation. Meanwhile, after 3 years, the densification of the microstructure of concrete with SAP has been observed.

DEM‐CFD analysis of swelling behaviors of binary particle systems with a microscopic diffusion model

AbstractSwelling, that is, volumetric expansion, of particles is ubiquitous in many industrial systems containing both swellable particles and non‐swellable particles. However, the interactions between these two types of particles and the internal stress distribution within the binary particle systems are not yet fully understood. The dynamic behavior of binary mixtures with both swellable and non‐swellable particles was explored for the first time in this study by employing a microscopic diffusion swelling model that considers the change in microstructure of swelling particles implemented in the discrete element method coupled with computational fluid dynamics (DEM‐CFD). The expansion of particle bed consists of super absorbent polymers (SAPs) particles, that is, the swellable particles, and polyformaldehyde (POM) particles, that is, the non‐swellable particles, in a rectangle container is then analyzed experimentally and numerically. The results show that a tapered pattern of SAP particles and an arch pattern of POM particles are formed during the swelling process. The Lacey index is used to quantify the mixing degree of the binary particle system, which increases with time until the POM particle layer is fully encompassed by SAP particles. The mean stress distribution within the particle system is also examined, revealing that the swollen SAP particles migrate upward through a “low‐stress channel,” pushing the POM particles in the central area outward. Moreover, it is found that, for case with a relative low friction, SAP particles penetrate the POM particle layer along the container walls, while for the cases with relative high friction, SAP particles penetrate the POM particle layers through the central regions of the container.

Effect of superabsorbent polymers and expansive agent on the properties and microstructures of self-stressing high performance concrete

In order to overcome the self-desiccation and high autogenous of high performance concrete (HPC), the superabsorbent polymers (SAP) and expansive agent (EA) are considered to be compositely incorporated, which has great significance to the HPC structures. This paper presents an experimental study on the compounded effect of SAP and EA on the properties of HPC. The workability, internal relative humidity (IRH), compressive strength and free expansive ratio as well as self-stress of HPC were examined. The influencing mechanism of SAP and EA on HPC was discussed. The results showed that the compressive strength of HPC generally decreases with the increases of EA and SAP contents and additional water ratio. The water release of SAP particles mainly occurs at 7–10 d to maintain a high IRH. The higher EA content induces the larger free expansive ratio and self-stress, caused by the increased hydration products of Aft and Ca(OH)2 bulks. For a low SAP content of 0.1%, the shrinkage is alleviated, but it becomes even worse for a higher SAP content. The total amount of pores increases with the inclusions of EA and SAP, except for the harm pores. The hydration products caused by EA could fill and compensate the pores left behind by the SAP particles, showing synergetic improvement effect. The self-stress values of HPC in this study are at the range of 2.73–3.44 MPa, From the present study, the SAP content of 0.1% is more recommended for the HPC in real engineering practices.

A microfluidic-based In-situ investigation of swelling dynamics of superabsorbent polymer microparticles

This paper presents a novel microfluidic-based technique for in-situ time-lapse investigation of the dynamic water absorption (or swelling) of individual superabsorbent polymer (SAP) microparticles (MPs). Most prior studies were restricted to using gravimetric techniques to investigate the equilibrium state of the swelled SAP macroparticles, without considering their swelling dynamics. Time-lapse investigation of the swelling rate is essential for optimizing SAP-MPs' performance to meet the needs of different applications. Moreover, since swelling occurs through the surface of SAP particles, we hypothesize that decreasing their size to microscale (i.e., SAP-MPs) could potentially improve their performance due to the increased surface area-to-volume ratio. In this context, the proposed microfluidic device enabled hydrodynamic trapping of an array of individual SAP-MPs and in-situ investigation of their swelling behaviour under a microscope. The SAP-MPs were initially suspended in ethanol and loaded into the microfluidic device. Once they were trapped, the fluid medium was switched to water to trigger their swelling. Acrylic acid (AA)/acrylamide (Aam)-based SAP-MPs in size range of 250–300 µm were investigated to characterize the device's performance. The SAP-MPs' average trapping time and yield in the device were 9.56 s per SAP-MP and 92%, respectively, at an ethanol loading flow rate of 0.15 mL/min. A minimum water flow rate of 5 mL/min was required to achieve an average ethanol-to-water transition time of 10 s. This was needed to suppress the effect of the solution exchange on the swelling behavior of the SAP-MPs. The equilibrium volumetric swelling ratio (VSReq), swelling rate (SR), and initial SR of AA-Aam-based SAP-MPs were determined to be 5.59 ± 0.13 (m3/m3), 0.039 ± 0.002 (m3/m3.s), and 0.068 ± 0.010 (m3/m3.s), respectively. A significant effect of the neutralization degree (ND) on the swelling behavior of AA-Aam-based SAP-MPs was also observed. At 90% ND, the maximum VSReq of 79.45 ± 2.49 m3/m3 and SR of 1.11 ± 0.047 m3/m3·s were achieved. This microfluidic platform could be used to conduct parametric study and real-time characterization of various SAP-MPs in biological, chemical, medical, and agricultural applications.

Effects of superabsorbent polymers (SAP) on the rheological behavior of cement mortars: A rheological study on performance requirements for 3D printable cementitious materials

While the potential of incorporating superabsorbent polymers (SAP) as an additive in 3D printing cementitious materials has been a subject of interest recently, little research has been conducted on the effects of SAP incorporation on the rheological properties of cement mortars and the required performance of 3D printable cementitious materials when no other additives, such as superplasticizer (SP), are present or when SAP is added up to 1.0% by weight of the binder. Accordingly, the effects of SAP on the rheological behavior of cement mortar were analyzed in this study from the perspective of the performance requirements for 3D printable material with the SAP dosage (0%, 0.5%, and 1.0% by weight of cement), SAP particle size (100–180 and 355–425 μm), and superplasticizer (SP) presence (polycarboxylate-ether) as variables. The rheological properties of cement mortars with the same initial slump flow (180 mm ± 5 mm) were measured using a vane rotational rheometer, and their conformance to the performance requirements for 3D printable cementitious materials was studied. Static yield stress, dynamic yield stress, and plastic viscosity were calculated for each specimen using measured torque data obtained from the stress growth and flow curve experiments. The ratios of static to dynamic yield stress, plastic viscosity, and thixotropic areas were analyzed from the perspective of pumpability and extrudability, whereas the yield stress evolution and structural build-up rate were analyzed from the perspective of buildability. The results demonstrated that although the rheological properties of the SAP-incorporated cement mortar with a low water–cement ratio were similar to those of the SAP-incorporated cement mortar with a high water–cement ratio when SP was incorporated, the change in rheological properties with time and shear differed. SAP incorporation promoted the structuration of the mixtures without SP that have a high water–cement ratio due to the physical effects of SAP particles, but the effect was reversed in the mixtures with SP and a low water–cement ratio due to the water migration kinetics associated with SAP. Because a low water–cement ratio with SP is commonly applied for 3D printable materials, the findings of this study indicate that the water migration kinetics of SAP critically determines the printability of cementitious materials mixed with SAP. Given that observed effects are substantial, SAP arguably has potential as a rheology modifier for 3D printable cementitious materials.

On crack healing in fiber-reinforced cementitious composites incorporating mineral-based healing agent and superabsorbent polymer: Evaluation using modified permeability test method

In this study, the crack healing properties of fiber-reinforced engineered cementitious composites (ECCs) under incremental strains, various fiber contents, and incorporation of a mineral-based self-healing agent (SHA) and a superabsorbent polymer (SAP) were evaluated. A permeability test setup applicable to plain mortar samples was modified to evaluate the fiber-reinforced ECC samples under several strains and crack widths. Supplementary self-healing tests—resonant frequency (RF) recovery, stiffness recovery, and crack image inspection—were also conducted. Changes in the permeability, RF, stiffness, and crack images were monitored for 28 days after sample cracking, to evaluate self-healing. The findings showed that the permeability reduction of the ECCs was significantly influenced by the fiber content instead of by the addition of the SHA and the SAP. In addition, the SAP particles were observed to improve the self-healing properties of the ECC samples.

Microstructure refinement and shrinkage mitigation of alkali-activated slag paste by superabsorbent polymer

This paper investigates the effects of superabsorbent polymer (SAP) dosage, particle size, addition method, and absorption status on the properties and microstructure of alkali-activated slag (AAS) pastes. The results show that incorporating SAP can mitigate the autogenous shrinkage and short-term drying shrinkage of the AAS pastes, mainly depending on the SAP dosage and size. The release of activator can promote the geopolymerisation around the SAP particles, which improves the local micro-hardness of the pastes and reduces the amount of capillary pores. This consequently densifies the microstructure around the SAP particles and mitigates the shrinkage of the AAS pastes. The large SAP particles are found to release activator to a farther distance than the small ones, which is more effective in mitigating the autogenous shrinkage. The promoted conversion from mesopores to gel pores in the paste with large SAP is also beneficial to reducing the driving force of autogenous shrinkage. By contrast, the addition method and absorption status of SAP particles have marginal impacts on the autogenous shrinkage as the released activators travel a similar distance within the AAS pastes. Moreover, adding SAP particles increases the number of gel pores and macropores regardless of their size or absorption status, leading to an increased long-term drying shrinkage and a weakened mechanical property of the AAS paste. Overall, the findings of this study can guide the mix design and preparation of AAS materials with SAP.

Effect of additional water or superplasticizer on key characteristics of cement paste made with superabsorbent polymer and other shrinkage mitigating materials

The use of superabsorbent polymer (SAP) coupled with expansive agent (EA) or shrinkage-reducing admixture (SRA) can effectively reduce shrinkage of cement-based materials. However, SAP particles at high contents can adversely affect rheological and mechanical properties. Securing proper workability can be accomplished by additional water or adjusting the superplasticizer (SP) demand. There is an uncertainty about the efficiency of these approaches to workability compensation. This study evaluates the effect of additional water or SP on key properties of cement paste made with SAP and relatively high content of EA or SRA. Macro and micro mechanisms underlying the effect of combined shrinkage mitigating strategies on the performance of cement paste made with 0.4 water-cementitious materials ratio are investigated. Results indicate higher efficiency of increasing SP demand by 0.6% versus the addition of water by 30 g per gram of SAP. Up to 80% higher compressive strength is obtained for the mixtures made with additional SP, which is attributed to up to 33% reduction in capillary porosity and smaller pore-size distribution of the hydrated cement paste. In the presence of high EA content, the use of additional SP prevents excessive initial expansion in the paste with additional water.

The influence of SAP particle size and extra water on the dynamic mechanical behavior of SAP-modified cement-based composites

Superabsorbent polymers (SAP)-modified cement-based composites (abbreviated as composites) have excellent frost resistance, internal curing efficiency, self-sealing and self-healing capacity. However, the effects of SAP particle size and extra water on the dynamic mechanical behavior of composites are still unclear. In this study, the total porosity and macropore (>100 μm) porosity of composites were studied with a mercury intrusion porosimetry test and an image analysis. The influence of SAP particle size and extra water on the quasi-static and dynamic mechanical properties of composites was analyzed. The fracture behavior of composites was investigated based on the fractal dimension and crack morphology. The results reveal that adding extra water can respectively raise the total porosity and macropore porosity by 33 %–39 % and 36 %–50 %, thereby declining the compressive strength and elastic modulus of composites under quasi-static and dynamic compression. The rise of strain rate from 63 s−1 to 102 s−1 elevates the dynamic compressive strength, elastic modulus, and fracture degree of composites by 39 %–44 %, 4.9 %–6.0 %, and 5.2 %–7.4 %, respectively. Raising particle size of SAP and adding extra water can raise the dynamic increase factor of compressive strength and elastic modulus. Which originates from the change of crack path, the increase of fracture degree, and the delayed response of strain induced by the rise of total and macropore porosity.

Benefits and drawbacks of using multiple shrinkage mitigating strategies on performance of fiber-reinforced mortar

Shrinkage mitigating strategies have been successfully used to prevent concrete cracking. Numerous studies have demonstrated the efficiency of shrinkage mitigating materials (SMM), such as expansive agent (EA), shrinkage reducing admixture (SRA), and superabsorbent polymer (SAP) on reducing shrinkage and cracking; however, few studies have addressed the limitations of using these materials. In addition, the mechanism of negative effect on microstructure and mechanical properties when these materials are used at high contents or in combinations is not well defined. This study investigates the effect of CaO-based EA, SRA, and SAP on compressive strength development, fiber pull-out strength, and shrinkage of fiber-reinforced mortar (FRM) mixtures. Hydration kinetics and internal relative humidity are also determined. Advanced microstructural characterization techniques are employed to explore the mechanism underlying the effect of SMM on the mechanical properties and shrinkage of FRM mixtures. Test results indicate that FRM made with coupled systems of SMM limited to 5% EA, 1% SRA, and 0.25% SAP are preferred. The preferred SMM combination considering compressive strength, fiber pull-out strength, and shrinkage is the coupled system made of EA and SRA. High heat of hydration obtained using 10% EA results in excessive portlandite formation and high expansion, which adversely affects the mechanical properties. The use of 0.5% SAP with additional internal curing water contributes to the excessive expansion of SAP particles and creation of voids into the matrix, which reduce compressive strength and fiber-matrix bonding. The results revealed that the individual use of 7.5% EA and coupled use of 5% EA either with 0.5% SRA or 0.125% SAP can result in producing flowable, high strength, and low shrinkage FRM mixtures applicable to repair.

The Characteristics of Time-Dependent Changes of Coefficient of Permeability for Superabsorbent Polymer-Soil Mixtures.

Water uptake dynamics of superabsorbent polymers (SAP) in soil is of key importance for the optimum application of these materials in environmental engineering and agriculture, so goal of this paper is to determine time dependent values of coefficient of permeability for various SAP-soil mixtures. Retaining water in soil is a key requirement in critical zones to support plant growth. There is an urgent need for technologies that can increase soil water retention, given the increasing prevalence of droughts and scarcity of clean water as the climate changes, combined with the rising demand for food by a growing world population. SAPs are materials that can absorb significant amounts of water, and thus have tremendous potential to help increase water retention in soil. However, while some studies have characterized the equilibrium swelling behavior of SAPs in soil, how their addition influences the time-dependent flow of water through soil remains poorly understood. Here, we address this gap in knowledge by directly measuring the coefficient of permeability of SAP-soil mixtures, testing different soil grain sizes, SAP grain sizes, and different SAP-soil ratios. We find that SAP addition can dramatically hinder the flow rate of water through soil—reducing the permeability by several orders of magnitude, and in some cases causing complete blockage of water infiltration, at mass fractions as small as 1%. In this scenario coefficient of permeability of 1.23 × 10−4 m/s dropped by a factor of ~10 after 14 min, a factor of ~100 after 36 min, and by nearly a factor of ~1000 after 63 min, eventually causing complete blockage of infiltration after 67 min. Authors concluded that in this particular situation the size and quantity of SAP particles was enough to nearly completely fill the available pore space resulting in rendering the soil column almost completely impermeable. Moreover, we demonstrate that these effects are well-described by a simple hydraulic model of the mutual interactions between SAP and soil grains, providing more generally-applicable and quantitative principles to model SAP-soil permeability in applications. Ultimately, this work could help evaluate the optimal proportions and grain sizes of SAPs to use for a given soil to simultaneously achieve a desirable permeability along with increased water holding capacity in the plant root zone.