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the mixture, the result is a high-performance concrete (HPC) that forms a denser ... HPC has a very dense microstructure to ensure an optimum particle...
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Chapter 17

Using Polymer Science To Improve Concrete: Superabsorbent Polymer Hydrogels in Highly Alkaline Environments Kendra A. Erk* and Baishakhi Bose School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States *E-mail: [email protected].

When added to fresh concrete mixtures, superabsorbent polymer hydrogel particles behave as internal curing agents, capable of absorbing and subsequently releasing large amounts of water which reduces self-desiccation and volumetric shrinkage of cement and, in turn, results in hardened concrete with increased strength and durability. In this chapter, the interactions between polyelectrolyte hydrogel particles and alkaline cementitious mixtures are described with an emphasis on how swelling behavior and internal curing performance is controlled by the chemical composition and morphology of the hydrogel particles. The beneficial impacts of hydrogel addition on cement microstructure and mortar compressive strength are highlighted, including the preferential formation of specific inorganic phases within the cement that appear to be influenced by the organic chemistry of the hydrogel particles.

Introduction Concrete, the largest volume building material in use worldwide, is produced at levels twice that of steel, aluminum, and wood combined; and due to its versatility, accessibility, low price and high durability, it provides shelter for an estimated 70% of the world’s population (1, 2). Compared to other building materials, including steel, it possesses high compressive strength and excellent resistance to corrosive fluids, assuring its ubiquity in construction for long into the future. However, conventional concrete can be easily adulterated, leading to poor © 2018 American Chemical Society

quality mixtures subject to shrinkage and cracks as well as structures requiring frequent repair and replacement. Indeed, the amazing devastation following the 7.0-magnitude earthquake in Haiti in 2010 was directly linked to the poor quality concrete used for housing and other buildings (3). In addition, the frequent repair and replacement required for conventional concrete has a significant environmental impact as concrete production accounts for 5-8% of global CO2 emissions (4). To increase the overall performance of conventional concrete, a variety of chemical additives have been developed so that when incorporated into the mixture, the result is a high-performance concrete (HPC) that forms a denser microstructure upon curing, resulting in increased strength and durability. HPC has a very dense microstructure to ensure an optimum particle packing and a water-to-cement ratio so low that all the water is consumed during the hydration reaction (i.e., during cement curing) (5). The dense microstructure engenders a strong and durable structure (6) with reduced impact on the environment (7). The main technical challenge associated with HPC is the autogenous (volumetric) shrinkage that is encountered during the early stages of curing, eventually resulting in the formation of microcracks, increased porosity, and an overall reduction in strength (8). Shrinkage increases in magnitude as result of incomplete hydration of cement due to the scarcity of water to fuel curing, causing the mixture to self-desiccate (9, 10). Compressive stresses develop in the system driven by the negative capillary pressure within the aqueous fluid in the mixture (termed ‘pore fluid’), ultimately leading to collapse of the cement microstructure (9, 11). The lack of water inside the system can be offset by use of internal curing agents that effectively replenish the water reservoir from inside the mixture and aid in cement hydration. Among other methods, the use of superabsorbent polymers (SAP) is becoming increasingly popular for the internal curing of HPC. SAP hydrogel particles can absorb water and undergo voluminous expansion up to 1,000 times their original weight (12). Thus, SAP can provide a means of controlled release of an ample reservoir of water for internal curing of HPC (13, 14). Hence, the degree of autogenous shrinkage and microcracking in concrete is effectively reduced while ensuring the formation of a highly durable and strong concrete (9, 15–17). There are also additional benefits of using SAP particles as internal curing agents in concrete, some of which are active fields of research: concrete freeze/thaw resistance can be enhanced by control of size and shape of pore systems (18–27), thermal expansion can be mitigated (28) and cracks can be filled which facilitates crack healing (18, 29). The use of hydrogel particles also provides more control over the rheological properties (18, 30) of fresh concrete and a reduction in spalling due to fires (31). Use of SAP particles in HPC has its own set of challenges that needs to be carefully considered. The most important issue with hydrogel particles is that they are not chemically inert in cement mixtures although they are considered to be by many in the building materials community (32, 33). Multivalent cations, which are naturally occurring in the pore fluid of fresh concrete mixtures, have been shown to negatively impact the swelling characteristics of hydrogel particles, causing unexpected deswelling and subsequent collapse of the polymer networks (33, 34). Hence the stability of SAP particles is of significance and degradation needs to be 334

minimized for the particles to serve their internal curing purpose. Furthermore, the release of water at an ideal time is of upmost importance. A fast release of water before the final setting of the cement mixture would cause increased porosity in the mixture and corresponding reduction in strength while a slower release of water would not be able to effectively counteract self-desiccation and the resulting autogenous shrinkage. Also, nonuniform distribution of the SAP particles within the mixture and reduction of SAP particle size by coarse aggregates (rocks) during concrete mixing needs to be carefully monitored to ensure optimum benefits of using SAP in concrete.

Background Hydrogel Chemistry Hydrogel particles used as internal curing agents are composed of polyelectrolyte molecules (typically poly(acrylic acid-acrylamide) random copolymer molecules) that are covalently crosslinked to form a three-dimensional polymer network (35). As stated previously, hydrogel particles are not chemically inert within cementitious mixtures due to the presence of strongly alkaline pore fluids (pH > 12). In the case of hydrogel particles containing acrylic acid, the carboxylic acid (COOH) functional groups undergo deprotonation in alkaline conditions as the pKa of acrylic acid is approximately 4.5 (36), forming anionic COO- moieties in the polymer network and allowing significant amounts of water to be absorbed by virtue of ion-dipole interactions (33, 37–39). For these charged polymer networks, the primary mechanism of hydrogel particle swelling can be attributed to the creation of a chemical potential gradient between the particle and the surrounding fluid as the anionic network results in a higher concentration of free counterions within the hydrogel particle compared to the surrounding fluid in order to preserve electroneutrality within the system (40). This gradient in turn creates an osmotic pressure gradient which drives the diffusion of water into the hydrogel particle along with other ions in solution. The particle will continue to swell until the net osmotic pressure is reduced to zero. Cations present in the aqueous fluid (including sodium, calcium, and aluminum ions) will be electrostatically attracted to the COO- moieties and form ionic complexes that effectively act as crosslinks within the polymer network, the formation of which decreases the equilibrium absorption capacity of the hydrogel (37, 41) and can ultimately lead to collapse of the polymer network as we recently demonstrated (33). A schematic illustrating the various stages of hydrogel swelling is shown in Figure 1. In the concrete materials community, there is a growing body of research to accurately quantify the absorption and desorption behavior of hydrogel particles in cementitious pore fluids as certain hydrogel compositions (including some commercially available products) have been found to display strong sensitivity to the mono- and multivalent cations that are naturally present in pore fluids depending on mixture age (32, 33, 42–46). As the anionic nature of the hydrogel is increased (e.g., by greater concentration of acrylic acid in the network), ion-induced deswelling is enhanced due to the greater presence of anionic sites 335

in the polymer network which can complex with counterions in solution (see Figure 1c) (34). Additionally, when the pH is highly alkaline (pH > 12-13, as is typical for pore fluids), the amide groups in the acrylamide segments are partially hydrolyzed to form anionic COO- moieties as well, encouraging swelling even in majority-acrylamide copolymer particles (47).

Figure 1. Idealized schematic of a polyelectrolyte hydrogel network in (a) the dry state, (b) swollen with water, and (c) deswollen in the presence of counterions. AA and AM indicate acrylic acid and acrylamide segments.

Synthesis pathways for hydrogel particles can be broadly classified into solution polymerization (48) and suspension polymerization (49). Hydrogel particles made from solution polymerization have uneven, angular shapes while hydrogel particles made from suspension polymerization are typically spherical or ellipsoidal in shape. The major constituents of both the pathways are same, i.e., water, monomer (acrylic acid (AA) and acrylamide (AM)), and neutralization solution (NaOH); the composition of synthesized hydrogel particles may be controlled by varying the amounts of these constituents. Note that for the sake of brevity, here hydrogels will be referred to by the weight percent of their first component only and denoted with abbreviations of AA and AM. The solution polymerization procedure is hereby summarized, adapted from Zhu, et. al. (43) Water, monomer, and neutralization solutions are added to scintillation vials followed by the crosslinker solution N-N’methylenebisacrylamide (MBAM) and initiator solutions. The vials are placed in a temperature-controlled oil bath at 50-60°C until gelation is observed. The thermally controlled bath ensures that gelation occurs within 1 to 8 hours. The hydrogel particles are then soaked and washed with water to remove any unreacted monomer, cut into rough, cm-sized pieces and dried in an oven between 50-80°C for 8 hours. The pieces are then ground and sieved for further use. Grinding and sieving may be repeated as required to obtain the desired hydrogel particle size distribution (see Figure 2). 336

Figure 2. Size distribution of dry, sieved hydrogel particles: (a) spherical 17% AA, (b) spherical 83% AA, (c) angular 17% AA, and (d) angular 83% AA. Reproduced with permission from reference (34), Copyright 2018 Springer.

The inverse suspension polymerization procedure to create spherical hydrogel particles requires the formation of a dispersed aqueous phase within an organic continuous phase. The aqueous droplets are stabilized by a surfactant, and the droplet size is a function of the mixing speed (e.g., 400-1200 rpm) with greater speeds creating smaller droplets and, in turn, smaller hydrogel particles. The aqueous phase is prepared by mixing water, monomer, neutralization solution, crosslinker, and an initiator. The aqueous phase is then added to the organic phase (e.g., cyclohexane containing the surfactant) and stirred continuously as a catalyst is added to the mixture to initiate polymerization of the monomer within the dispersed droplets. The solution is heated at a specified temperature until the reaction is complete and the hydrogel particles are then filtered and rinsed. Details of this procedure can be obtained from Davis, et. al. (50) Figure 2 and 3 depict the particle size distribution and optical microscopy images, respectively, of hydrogel particles prepared by solution polymerization (angular in shape) and suspension polymerization (spherical in shape). 337

Figure 3. Optical microscopy images of dry (a) spherical and (b) angular 17% AA hydrogel particles. Reproduced with permission from reference (34), Copyright 2018 Springer.

Cement Chemistry Concrete is mainly comprised of cement, aggregates and water (51, 52). As a simple approximation, concrete is composed of two phases: hydrated cement paste and aggregates. Therefore, concrete inherits its properties from its two constituent phases and the interfaces between the phases. The interface is known as the interfacial transition zone and is usually the mechanically weakest area in concrete. The aggregates in concrete are mainly divided into two types: fine aggregates (sand) and coarse aggregates (rocks). Supplementary cementitious materials (e.g., silica fume, fly ash) may be used in HPC to impart specific properties to the hardened concrete (53). The hydration reactions that occur within a fresh concrete is due to the cement paste. Various different types of Portland cement are used in concrete mix design, depending on the type of structure to be constructed. Overall, the raw material for Portland cement contains four oxides: CaO, SiO2, Al2O3, and Fe3O3, which forms the clinker (53). Clinker together with gypsum (CaO.2H2O), forms Portland cement and their composition is adjusted to impart specific properties necessary in the concrete produced from it. The main composition of Portland cement with their chemical formulae (54) are shown below in Table 1.

Table 1. Chemical Composition of Portland Cement Cement Compound

Chemical Formula

Tricalcium silicate (C3S)

Ca3SiO5 or 3CaO·SiO2

Dicalcium silicate (C2S)

Ca2SiO4 or 2CaO·SiO2

Tricalcium aluminate (C3A)

Ca3Al2O6 or 3CaO ·Al2O3

Tetracalcium aluminoferrite (C4AF)

Ca4Al2Fe2O10 or 4CaO·Al2O3·Fe2O3

Gypsum

CaSO4·2H2O

338

The addition of water causes hydration of each of these compounds and this is referred to as ‘curing’ of concrete. The calcium silicates are mainly responsible for contribution of strength to hardened mixtures. Tricalcium silicate is responsible for early strength in concrete (first 7 days) (55) along with the setting of cement (i.e., the time taken for the transition of cement from a fluid paste to a rigid material) (54, 56). Dicalcium silicate, which reacts more slowly, imparts strength at later stages (2, 44, 48, 52, 57, 58). Tricalcium aluminate affects setting time of cement and contributes to the hardening of cement.. Significant heat is generated during hydration of C3A (2, 48) and hence a higher proportion of this constituent is avoided to prevent cracking in concrete (59) C4AF contributes to the color of the cement and acts as a flux during the production of cement (60). It does not have a significant effect in strength gain. Gypsum is added to cement to prevent ‘flash setting’, a phenomenon where a higher percentage of C3A in cement leads to immediate stiffening of cement (54). The hydration reactions relevant to strength development (58) are shown below (Table 2).

Table 2. Reactions Pertinent to Strength Development in Cement Paste Tricalcium silicate + Water → Calcium silicate hydrate + Calcium hydroxide 2 Ca3SiO5 + 7 H2O → 3 CaO·2SiO2·4H2O + 3 Ca(OH)2 + 173.6 kJ Dicalcium silicate + Water → Calcium silicate hydrate + Calcium hydroxide 2 Ca2SiO4 + 5 H2O → 3 CaO·2SiO2·4H2O + Ca(OH)2 + 58.6 kJ

The hydration reactions produce calcium silicates (commonly known as CSH) and calcium hydroxides (referred to as CH). The behavior of cement paste can be explained by the hydration mechanisms and associated chemistry involved with it. At the onset of hydration, there is a rapid liberation of CH into the pore fluid, which results in the development of shell-like formations of CSH on the cement grains (54, 61). This layer hinders further reactions which allows the cement paste to remain workable for a few hours. With time, this outer shell is disrupted due to osmotic pressures (54) and/or by further growth of CH (62). CH forms thin, plate-like crystals that are brittle in nature. The crystalline gelatinous mass (2), mainly comprised of CSH, contributes to adhesive properties (2, 57) and strength by a combination of cohesive bonds due to van der Waals’ forces and chemical bonds (54). In general, the hydration of Portland cement involves a progression of reactions between the solid cement components and an aqueous fluid. The fluid is initially water, and, as the reactions progress, the water is converted to a complex alkaline, sulfate-bearing solution soon after mixing (63). An interconnected internal pore structure results when the cement sets and is filled with the aqueous pore fluid. Only a small extent of hydration is complete before setting occurs and hence most cement hydration involves reactions with pore fluid (63). The aqueous pore fluids are highly alkaline in nature (pH greater than 12) (64) and are usually concentrated solutions of alkali hydroxides and a small fraction of other components (63). Analysis of pore solution by previous researchers have shown 339

that it consists primarily of ions such as sodium, potassium, and hydroxide, in addition to marginal amounts of multivalent calcium and sulfate ions (64).

Hydrogel-Cement Interactions When incorporated into cement, hydrogel particles are generally used in very small amounts (generally only 0.2% by weight of dry cement). The hydrogel and cement are mixed in their dry state, the required amount of water and superplasticizer are added, and the mixture is hand or vacuum mixed in the laboratory (or machine mixed in the field). Use of a vacuum mixer can yield more uniform mixing in addition to ensuring that any porosity would be due to capillary water and hydrogel particles and not due to non-uniformity in the manual-mixing process (34). The superplasticizer, generally a water reducing admixture (such as a polycarboxylate comb polymer (65, 66)), is added to ensure good workability of the cement paste as the paste for HPC has a very low water-to-cement (w/c) ratio. The addition of water in the mixture immediately causes the hydrogel particles to swell such that the hydrogel particles are most likely fully swollen by the time the cement mixture is placed (i.e., cast). Figure 4 is an illustration of the phenomena occurring in cement containing hydrogel particles from the time it is cast to the time when the cement has reasonably hardened. As the hydrogel particles are used to ensure proper hydration of the cement matrix, the interaction between the cement paste and hydrogel particles is of primal significance; the coarse aggregate (pieces of rocks) and fine aggregate (sand) have been omitted from the illustration for the sake of simplicity. Also, it is presumed that a similar process of hydration would occur in mortar and concrete samples.

Figure 4. Cross-section of cement mixture containing hydrogel particles and cement grains: (a) immediately after mixing and placement; (b) after a few minutes to hours (before final setting of cement); and (c) after a few days when a substantial amount of cement has reacted and the hydrogel particles have partially deswollen (dehydrated). Initially, the cement grains start to hydrate from the water available in the mixture causing hydration products to form an outer layer on the grains as aforementioned and illustrated in Figure 4b. This causes a retardation in 340

the process of hydration as the unhydrated core of the cement grains will have reduced access to water. As the cement curing proceeds, water is extracted from the hydrogel particles to fuel the hydration reactions, driven by osmotic pressure gradients in the system and facilitated by the formation of capillary networks in the cement matrix. Eventually the hydrogel particles deswell (as shown in Figure 4c) from a combination of two factors: the extraction of water by the cement paste and the intrusion of cations from the pore fluid into the polymer network. Even after a few days (or years, for some types of cement), hydration reactions are still incomplete, and there is always some unreacted cement grains in the cement paste (C2S takes a long time to undergo hydrolysis). It is possible that the cement grains in close vicinity to the hydrogel particles may undergo faster and greater amounts of hydration than those further away, especially if there is an inhomogeneity in the dispersion of the hydrogel particles throughout the cement paste. Over time, hydration products form on the outer surfaces of the voidspace that remains from the dehydrated hydrogel particles (described in detail later). Since the hydration reactions of C3S start immediately after water is added to the dry cement grains, it would take a few minutes to hours for the hydration products to form. Meanwhile, depending on the composition of the hydrogel particles, cement alkalinity, and the availability of water, it might take a few minutes to hours for the particles to reach maximum swelling capacity. The deswelling of the hydrogel particles creates voidspace (porosity) in the hardened cement paste (Figure 5), which can enhance the durability and freezethaw resistance of concrete (21). However, careful dosage of hydrogel particles has to be ensured so as to not significantly reduce the compressive strength of the concrete due to the increased porosity from hydrogel-related voidspace.

Figure 5. Scanning electron micrograph of a 24-hour cured mortar cross-section showing a void remaining from an angular hydrogel particle.

Hydrogel Swelling Behavior The gravimetric “tea-bag” method (46, 47, 67, 68) is a common method for evaluating the swelling capacity, Q, of a collection of hydrogel particles. The procedure is fairly simple and involves loading a known mass of dry particles into a prewetted, commercially available tea-bag and immersing the tea-bag into a 341

desired solution. At specified time intervals, the tea-bag is removed, excess water is allowed to drain, and the tea-bag is weighed to obtain the swelling capacity, given by the following equation:

where ms is the mass of the wet, swollen samples and md is the initial dry mass. Effect of Hydrogel Size, Morphology, and Crosslinking Density Figure 6 displays results obtained from swelling tests performed on two different size distributions (106–425 µm and 425–850 µm, respectively) of angular hydrogel particles containing 2 wt.% covalent crosslinking and immersed in pure water and a salt solution (calcium nitrate solution) (43). The results show that in both solutions, particle size distribution causes a significant change in swelling rate, as the smaller size distribution displays a faster swelling rate than the larger distribution considering that particles ideally start at Q = 0 (dry). Note that the values of Qeq at relatively long times (i.e., beyond 30 minutes) are independent of particle size.

Figure 6. Swelling behavior of 17% AA angular particles of two different sizes immersed in pure water and 0.025 M CaNO3 solutions. Reproduced with permission from reference (43), Copyright 2014 Springer. Hydrogel particle shape (e.g., spherical particles from suspension polymerization vs. angular particles from solution polymerization) was observed to have no prominent effect on the apparent swelling kinetics for collections of hydrogel particles measured using gravimetric tea-bag tests (34), although individual, isolated particles with greater surface area to volume ratios would be expected to swell more quickly. An increase in covalent crosslinking density within the polymer network limits the volumetric change that is possible during 342

swelling (67) and results in an overall decrease in measured absorption capacity and thus decreased values of Qeq (43).

Comparison of Hydrogel Swelling Behavior in Various Solutions A general trend can be observed when hydrogel swelling curves in different solutions are plotted (see Figure 7) (32). A higher percentage of acrylic acid (AA) in the polymer network generally increases the hydrogel’s maximum swelling capacity. However, due to the anionic nature of AA (described previously in this chapter), ionic complexes can form between the COO- moieties and the multivalent cations present in salt solutions, which collapses the network by expelling water that was initially bound to the polymer. By comparison, hydrogel particles containing a majority of acrylamide (AM) segments in their networks are largely unaffected by the presence of cations as AM has relatively few anionic sites at pH < 12. Hence the presence of cations and increasing cationic valency has a negative impact on maximum absorption capacity of hydrogel particles. Tap water, which contains a plethora of ions, also leads to reduced swelling compared to reverse osmosis (RO) water; note that in practice, most concrete mixtures are formulated using municipal tap water.

Figure 7. Swelling behavior of (a) 17% (b) 33% (c) 67% and (d) 83% AA angular hydrogel particles submerged in various types of solutions, including sodium (pH 6.7±0.2), calcium (pH 6.7±0.2), aluminum (3.8±0.2) salt solutions, RO water (pH 6.7±0.2), tap water (pH 7.3±0.2) and pore fluid (pH 12±0.2). Reproduced with permission from reference (32), Copyright 2016 Springer. 343

Further scrutiny of the swelling ratios observed in aluminum sulfate solutions indicates that swelling is suppressed as the concentration of trivalent Al3+ in solution is increased (see Figure 8) (33). Additionally, real-time observations indicate that the hydrogel particles develop a stiff outer shell with elastic modulus values proportional to the concentration of AA in the network, most likely due to localized collapse/densification of the outer regions of the polymer network due to Al3+ complexation (33). The presence of a mechanically stiff shell is evidence of different transport behavior of aluminum ions within a polymer network compared to sodium and calcium ions; and indeed, the ionic complexation of trivalent ions may be considerably stronger than that of divalent ions (33, 69).

Figure 8. Swelling behavior of four different compositions of angular hydrogel particles in (a) 0.005M and (b) 0.025M of aluminum sulfate solution. Reproduced from reference (33), Copyright 2017 MDPI.

Swelling characteristics, including maximum absorption values and the sorption kinetics, are important indications of the usefulness of the hydrogel particles as effective internal curing agents for concrete mixtures. These factors must be kept in mind when selecting the amount of water for concrete mixture design and when the concrete is intended for use in places of high salinity (e.g., coastal regions) or places with increased water hardness. It is important to note that the results from swelling tests in specific solutions are merely an indication of general trends (i.e., the relative sensitivity of a particular hydrogel composition to the presence of ions in solution) and cannot be used to accurately predict how a hydrogel will behave in a cementitious mixture. The pore fluids sometimes used in gravimetric tests (discussed in the next section) are an idealized condition of an infinite reservoirs of water and ions and is in stark contrast to the low water-to-cement ratio HPC mixtures where increased competition for water could result in reduced swelling kinetics and absorption capacities (32). 344

Hydrogel Swelling in Cementitious Pore Fluids There is a prominent difference in swelling behavior when hydrogel particles are immersed in cementitious pore fluid (see Figure 9) (32). Pore fluid naturally contains a complex mixture of mono- and multivalent ions – Na+, K+, Ca2+, Mg2+, Al3+, Si4+ among others (70) – all by-products of the ongoing hydration reaction between the Portland cement grains and water. For hydrogel particles containing a greater concentration of AM, the swelling capacity is fairly constant over time, while for particles containing a greater concentration of AA, a more parabolic swelling-deswelling behavior is observed (32). The alkaline pH of the pore fluid (12.5-13.8) (71, 72) causes the deprotonation of the COOH groups of the AA segments in the polymer network and initially leads to greater swelling. However, the exposed COO- also serves as sites for cations to bind, resulting in rapid deswelling, especially for particles containing the highest concentration of AA (i.e., 83% AA in Figure 9). This extreme sensitivity of some hydrogel compositions to the presence of cations has to be kept in mind while selecting the appropriate hydrogel compositions for internal curing of concrete as differences in cement compositions will generate pore fluids with different constituent ions and hence alkalinities.

Figure 9. Swelling behavior of different compositions of angular hydrogel particles immersed in pore fluid with pH = 12±0.2. Reproduced with permission from reference (32), Copyright 2016 Springer.

Impact of Hydrogel Particles on the Microstructure of Cement For any discussion of the use of hydrogel particles in cementitious mixtures to be complete, a close scrutiny and analysis of the hydrogel-cement microstructure is necessary. As described previously and illustrated in Figure 4, the water-swollen hydrogel particles deswell during the course of the hydration reaction, leaving 345

behind voids in the cement matrix (refer to Figure 5 and see Figure 10 and 11). Interestingly, microscopy of hydrogel-containing samples revealed the formation of specific inorganic phases within the voids including calcium hydroxide (CH) and calcium-silicate-hydrate (CSH), the latter being the ‘glue’ that binds the cement grains together and is thus primarily responsible for concrete’s exceptional compressive strength (33).

Figure 10. Scanning electron micrographs of cement paste containing (a) 17% and (b) 33% AA angular hydrogel particles. The blue areas indicate CH formation inside the remaining hydrogel voidspace; note that some of the remaining dehydrated hydrogel is visible in (a). Reproduced from reference (33), Copyright 2017 MDPI.

It was also observed that CH was more likely to develop in cement that was internally cured with AM-rich hydrogel particles. As previously mentioned, the amide groups present in AM are less sensitive to cation-induced deswelling of the polymer network; thus, these AM-rich hydrogel particles may retain water longer to facilitate a greater conversion of calcium silicates to CH and CSH (refer to Table 2). Figure 11 clearly illustrates the significant growth of CH crystals within the hydrogel voidspace for cement paste containing 100% AM (0% AA) hydrogel particles. Energy Dispersive Spectroscopy (EDS) performed on the features of interest displayed results which corresponded to composition of CH (within the range of uncertainties) and lacked any of the elements (Al, S, etc.) characteristic of ettringite formation (another product of hydration of cement). The dependence of CH formation on hydrogel particle composition is reported in Figure 12, indicating that the AM-rich hydrogel particles (17 and 33% AA) contained voids with the greatest amounts of hydrated product (33). These results demonstrate that hydrogel particles could potentially be engineered to refill voidspaces with inorganic phases, leading to further increases in compressive strength dependent entirely on the organic chemistry of the hydrogel particles (33, 34). 346

Figure 11. Scanning electron micrographs of cement paste containing 100% acrylamide spherical hydrogel particles, showing significant CH growth in the hydrogel voidspace (note that hydrogel particle is not visible).

Figure 12. Percentage of voids in 3-day cured paste samples filled with CH and any other hydrated products (CH + CSH) as a function of hydrogel composition. Reproduced from reference (33), Copyright 2017 MDPI.

Impact of Hydrogel Particles on the Mechanical Properties of Mortar Effect of Hydrogel Composition on Autogenous Shrinkage and Durability One of the primary reasons for using hydrogel particles as concrete internal curing agents is because the water released from the particles has been found to reduce autogenous shrinkage (9, 22, 42, 73). ASTM C1698 (74) is generally followed for the determination of autogenous shrinkage of mortar samples. Figure 13 shows results of autogenous shrinkage testing of mortar samples containing 347

a similar dosage (0.2% by weight of cement) but different compositions of hydrogel particles (32). The slopes of the control mortar samples remain negative throughout the test, indicating a perpetual process of self-desiccation and corresponding volumetric shrinkage, measured here as negative strain. All the samples containing hydrogel particles showed significant decrease in autogenous shrinkage compared to the controls. Thus, it is evident that hydrogel particles can effectively alleviate the shrinkage caused by self-desiccations (73, 75–79).

Figure 13. Effect of hydrogel particle compositions (17-83% AA) on the linear autogenous strain of mortar samples mixed at a w/c ratio of 0.32±0.03. Reproduced with permission from reference (32), Copyright 2016 Springer. Additionally, the incorporation of hydrogel particles to cementitious mixtures can have a positive effect in mitigating freeze-thaw effects in concrete (18–26) and thus increase the durability of pavements in cold-weather climates. Two common types of durability tests are water permeability (80) and rapid chloride permeability tests (81). Researchers have shown that the presence of hydrogel particles in concrete cause drastic decreases in water and chloride permeability when compared to hydrogel-free control samples under similar conditions (82). Hydrogel-modified concrete has also been shown to decrease the tensile creep of concrete (83). Creep is associated with the movement of water and development of microcracks over a long duration of time, and hence, any increase in creep resistance may be an additional benefit of using hydrogel-based internal curing agents in concrete. Effect of Particle Morphology on Compressive Strength The effect of hydrogel particles on the overall compressive strength of hardened cementitious composites (including mortar and concrete) is fraught with contradiction across literature. While some researchers report an increase in compressive strength following addition of hydrogel particles to the mixture, data reporting significant decreases in strength can also be found. Although addition of hydrogel particles for use as internal curing agents directly contributes to 348

increased strength of cementitious mixtures by creating a denser cement matrix and mitigating shrinkage and the subsequent development of stress cracks, it also increases the porosity within the microstructure and can increase relative humidity, both of which lead to decreased strength (9, 22, 84–86). It is also worth noting that in contrast to impact-resistant polymer composites, in which the addition of micron-sized rubber particles can increase the strength and toughness of the material (e.g., rubber-modified epoxies (87, 88)), here the dehydrated hydrogel particles do not have a strong interfacial bond with the surrounding cement matrix (refer to images in Figure 10). Thus, it is unlikely that any significant elastic energy is ever transferred between the cement matrix and the hydrogel particles. Figure 14 demonstrates the effect of hydrogel particle shape on the compressive strength of mortar (cement containing fine aggregate) (34). All mortar samples containing hydrogel particles displayed greater 28-day compressive strengths than the hydrogel-free control mixture, and there were no significant differences observed between mortar containing angular particles and mortar containing spherical particles. The mechanical performance of the internally cured mortar is even more impressive considering that the hydrogel particles will leave behind voids within the cement matrix, such that the density of these hydrogel-containing samples is expected to be less than the density of the hydrogel-free control samples. These results are promising as concrete mixture design is generally performed based on 28-day compressive strength values. It might be possible that for some hydrogel compositions, the development of compressive strength is delayed as the particles release water over different timescales. Also it is noteworthy that although the hydrogel particles are ultimately designed for use as internal curing agents in concrete, results from mortar and cement compression tests are typically used to infer the hydrogel particle’s effectiveness as an internal curing agent. This is reasonable since concrete contains coarse aggregates (rocks) which makes it less vulnerable to failure than either mortar or cement by restricting crack propagation.

Figure 14. Average compressive strength of mortar samples at various ages containing different hydrogel compositions and particle shapes but very similar size distribution (reported in Figure 2). Reproduced with permission from reference (34), Copyright 2018 Springer. 349

Conclusions and Implications High-performance concrete mixtures derive significant benefit from the inclusion of water-swollen hydrogel particles. These internally cured mixtures ultimately have increased compressive strength and service life due to the reduced volumetric shrinkage during curing that is afforded by the hydrogel particles as well as increased freeze-thaw resistance resulting from the residual voidspace. The internal curing performance of hydrogel particles is strongly dependent on the physical and chemical structure of the internal polymer network, especially the sensitivity of the polyelectrolyte molecules to the presence of multivalent cations which occur naturally in fresh cementitious mixtures. By determining these key structure-property-performance relationships through careful experimental study, the chemical composition of the polymer network can be successfully designed to create hydrogel particles that have a desired sorption behavior and can trigger the growth of specific inorganic phases within cement microstructure, leading to the development of next-generation high-performance concrete materials.

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