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Influence of Silica Fume and PCE Dispersant on Hydration Mechanisms of Cement Weina Meng, Piyush Lunkad, Aditya Kumar, and Kamal Khayat J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08121 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 19, 2016

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The Journal of Physical Chemistry

Prepared for Submission to The Journal of Physical Chemistry C (2016)

Influence of Silica Fume and PCE Dispersant on Hydration Mechanisms of Cement Weina Meng1, Piyush Lunkad1, Aditya Kumar2*, and Kamal Khayat1 1. Center for Infrastructure Engineering Studies (CIES), Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology (S&T), Rolla, MO 65409-0340. 2. Department of Materials Science and Engineering, Missouri University of Science and Technology (S&T), Rolla, MO 65409-0340.

*Corresponding author Aditya Kumar, Assistant Professor, Dept. of Materials Science & Engineering Missouri University of Science and Technology B49 McNutt Hall, 1400 N. Bishop, Rolla, MO 65409-0340 Email: [email protected] Phone: 573-341-6994

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Prepared for Submission to The Journal of Physical Chemistry C (2016) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Partial replacement of ordinary portland cement by silica fume (SF) accelerates its rate of hydration reactions. This acceleration is attributed to the enhanced heterogeneous nucleation of the main hydration product, i.e., calcium-silicate-hydrate (C-S-H), on the extra surfaces provided by SF. However, such enhancement of C-S-H nucleation is suppressed in the presence of polycarboxylate ether (PCE) dispersant, which is added to regulate the fluidity and rheological properties of fresh paste. A generalized phase boundary nucleation and growth (pBNG) model with time-dependent growth of C-S-H is used to fit the hydration rates of plain and binary (10% to 30% SF) cement pastes prepared with and without PCE. The results show that while SF accelerates cement hydration, increments in hydration rates are significantly smaller in relation to the extra surface area provided by SF. This is because of the agglomeration of SF particles which renders up to 96% of their surface area unavailable for C-S-H nucleation. Further, it is shown that the hydration of cement, in both plain and binary pastes, is suppressed in relation to the PCE dosage. This is because of: (a) adsorption of PCE molecules onto cement and SF surfaces resulting in inhibition of sites for product nucleation, and (b) interaction of PCE with CS-H, which suppresses growth of C-S-H throughout the hydration process. It is shown that the effects of nucleation site inhibition by PCE are more pronounced in SF as compared to cement. The outcomes of this study improve our understanding of the mechanisms that drive the hydration of cement in the presence of SF and PCEs.


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Introduction The reaction between ordinary portland cement and water, i.e., hydration, involves the dissolution of anhydrous phases concomitant with the precipitation of hydration products. Calcium-silicate-hydrate (C-S-H, as per cement chemistry notation: C = CaO, S = SiO2, H = H2O) is the main hydration product1 composed of SiO2 tetrahedra and layers of calcium and oxygen atoms interspersed by water within a highly disordered framework2,3. C-S-H nucleates and grows heterogeneously on the cement’s surface, wherein the strong electrostatic bonding between the nanometer-scale particles binds the paste cohesively4–7. In the presence of inorganic mineral additives (e.g., limestone, quartz, rutile), enhanced heterogeneous nucleation of C-S-H on the extra surfaces provided by the minerals results in acceleration of overall hydration kinetics. This acceleratory effect is typically termed as the filler effect 4,5,8, wherein the term filler signifies the chemically inert nature of the mineral additives.

Silica fume (SF), composed of amorphous silicon dioxide (SiO2), is typically used to prepare high-performance concrete with enhanced durability and strength. Past studies have shown that SF produces enhancement in cement hydration rates9–11 at early ages by offering preferential sites for product nucleation, via the filler effect. In addition, in such systems, SF can dissolve and react chemically with portlandite (i.e., Ca(OH)2: calcium hydroxide) formed during the hydration of cement12–14 to form pozzolanic C-S-H. The pozzolanic activity of SF, however, is kinetically slow and restricted at early ages9,13 due to its slow dissolution15,16 and sparse availability of portlandite in the system, thus limiting the formation of pozzolanic C-S-H. Nevertheless, minor quantity of pozzolanic C-S-H in the system could still produce profound effect on cement’s hydration kinetics. More specifically, Thomas et al.4 hypothesized that pozzolanic C-S-H acts as


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a energetically-favored template (i.e., a seed) for the nucleation and growth of C-S-H formed due to the hydration of C3S and C2S phases present in cement. As C-S-H continues to form and grow in an autocatalytic manner away from the cement particles, the hydration of cement is enhanced. These reactivity enhancements induced by SF at early ages often lead to poor fresh properties (e.g., rapid setting, poor workability) and, thus, require the use of chemical admixtures to suppress reaction rates as well as to enhance fluidity.

Comb-shaped polycarboxylate ether (PCE) superplasticizers are an established class of dispersants used to regulate the fluidity and rheological properties of high-performance concrete. PCE molecules adsorb onto cement and SF surfaces, and evoke electrostatic and steric repulsive forces to mitigate agglomeration of fine particles 17–22. However, PCEs suppress hydration kinetics of cement17,18,23–25. While the exact mechanisms describing the interactions between PCE and cement particles are not well understood, a limited number of studies conducted in the past decade have forwarded some hypotheses. It has been speculated that the adsorption of the negatively charged backbone of PCE molecules onto the surface of cement particles results in inhibition of surface dissolution and C-S-H nucleation sites, thus causing suppression of cement’s reactivity18,19,21. In contrast, Ridi et al.19 argued that length and chain density of the uncharged side chains (as opposed to the charged backbone) of PCE molecules are the main factors affecting the early age hydration of cement. In recent studies, Valentini et al. 25 and Artioli et al. 23 suggested that in the presence of PCE, the nucleation of C-S-H switches from heterogeneous to homogeneous. This results in a higher supersaturation requirement for C-S-H precipitation, which subsequently leads to slower hydration kinetics. However, in a prior studies19,26 it was shown that PCE additions simply result in alterations in rate constants (i.e.,


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growth rate and nucleation density) of the nucleation and growth process, while C-S-H continues to grow heterogeneously on the solid-phase boundaries.

Despite important contributions reported on the influence of SF and PCE on cement hydration, the underlying mechanisms driving these reactivity alterations, especially in the context of binary pastes which contain both SF and dispersant, are poorly understood. Knowledge of fundamental mechanisms that drive cement hydration, and how these are altered in the presence of SF and dispersants is vital to determine optimum amount of SF, and the corresponding dispersant type/dosage, for high-performance concrete that guarantees sufficient workability, and retains sufficient reactivity of cement at the same time.

This study employs a confluence of experimental methods and computer simulations to elucidate the effects of PCE and SF additions on the early age (i.e., up to 30 h) hydration kinetics of plain and binary cement pastes. Due to the complexities associated with modes of interaction between PCEs and cementitious phases (e.g., adsorption, steric hindrance, electrostatic effects, etc.) in relation to the intrinsic properties of the PCE (e.g., molecular architecture, charge density, etc.18,20,25), this study uses a single type of PCE, and places focus on describing its influence on hydration kinetics of both plain and binary pastes. The hydration rates are measured across a wide range of PCE dosages and SF replacement levels. Though in practice, high-performance concretes are proportioned using ≈10% SF27, this study attempts to generically describe the role of SF on cement hydration rates across a wider range (i.e., up to 30%) of replacement levels. A phase boundary nucleation and growth (pBNG) model with time-dependent growth rate of hydration products is employed to describe the progress of hydration in such systems. The


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outcomes provide new understanding and quantification of rate controls of cement hydration in the presence of SF and PCE.

Experimental Section A Type I ordinary portland cement with an estimated Bogue phase composition of 61% C3S, 8% C2S, 6% C3A, 9% C4AF, and 3.4% of gypsum (CaSO42H2O) was used in this study. The SF powder used is a commercially-available product composed of amorphous SiO2 at nominal purity (i.e., 96 mass%). A commercially-available PCE dispersant with a solid mass content of 23% and a density of 1050 kg. m-3 was used. The molecular architecture of the comb-shaped PCE polymer consists of a polymethacrylic backbone (14.1 nm) with grafted polyethylene oxide side chains (25.1 nm). These characterizations were performed using a combination of gel-permeation and size-exclusion chromatography techniques.

The particle size distributions (PSDs) of the solids were measured using a Beckman Coulter static light scattering (SLS) analyzer (LS13-320) using a 750 nm laser source that is incident on a dilute suspension of ultrasonicated powder particles in isopropanol (see Figure S1, Supporting Information). The median particle sizes (d50, µm) of cement and SF were determined as 9.98 µm and 0.23 µm, respectively. By factoring the bulk density (i.e., 3150 kg. m-3 and 2250 kg. m-3 for cement and SF, respectively, as measured using a pycnometer), the total specific surface area (SSA, m2.kg-1) of cement and SF were calculated from the PSDs as 379 m2. kg-1 and 18200 m2. kg-1, respectively. These results were found to be in good agreement with the BET specific surface areas provided by the suppliers of these materials.


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All pastes were prepared at a constant liquid-to-solid ratio (l/s) of 0.45, by mass. To describe the influence of SF on the reaction response of the paste, cement was partially replaced by SF at 0%, 10%, 20%, and 30%, by mass. In accordance with these increasing replacement levels, the waterto-cement ratios (mass basis) are 0.450, 0.500, 0.563, and 0.643, respectively. As reported in prior studies5,28, the kinetics of hydration of cement in a paste are essentially independent of the water content. As such, these changes in the water-to-cement ratios are expected to have negligible impact on the cement’s hydration kinetics. To investigate the influence of PCE on cement hydration rates, the admixture was added directly to mixing water at dosages of 0%, 0.6%, 1.2%, 1.8%, and 2.4% (by mass of the binder, i.e., cement + SF). It is clarified that these dosages represent the total (i.e., solid + liquid) mass of the PCE – the aforementioned dosages would amount to 0%, 0.038%, 0.276%, 0.414%, and 0.552% of the active solid component of the PCE per unit mass of the binder. The upper bound of dosage, i.e., 2.4%, was determined by saturation point test 29. The lower PCE dosages correspond to 25%, 50%, and 75% of the maximum dosage, respectively. Further details pertaining to the composition of the solids, mixing procedures, and determination of the maximum PCE dosage are provided in the supporting information.

The rate and extent of hydration were monitored up to 30 h after mixing using isothermal conduction calorimetry (Calmetrix I-CAL 8000) programmed to maintain the sample at a constant temperature of 20 °C ± 0.1 °C. The cumulative and differential heat release were normalized by the enthalpy of cement hydration (as calculated from mass fractions and enthalpy of individual phases1), 472 J. gcement-1, to determine the extent of reaction (i.e., degree of hydration, α, expressed as the fraction of cement reacted) and the rate of reaction (i.e., dα/dt, h-1),


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respectively, as functions of time. This method of derivation of α and dα/dt is based on the assumption that the measured heat release is solely on account of cement hydration, that is, discounting heat release due to the pozzolanic reaction of SF with portlandite. This assumption is reasonable because various previous studies have shown that the pozzolanic reaction of SF releases very small amounts of heat1,4, and occurs at very slow rates within the first 30 h9,14. To verify the low heat release and slow kinetics of the pozzolanic reaction of SF, the time-dependent reactivity of the SF in a saturation lime solution was monitored using isothermal microcalorimetry (see Supplementary Information). A Netzsch STA 409 PC thermal analyzer was used to identify and measure the quantities of phases present at different reaction times. The mass loss (TG) and the differential mass loss (DTG) traces were used to quantify both the extent of hydration and phases present in the system, which for these samples includes evaporable and non-evaporable water, portlandite and calcite, if any may have formed during carbonation. The values of α determined from isothermal calorimetry and DTG methods (i.e., as described in30) were found to be within ±4% across a range of samples and hydration times. To obtain information about the microstructure, secondary electron images of pastes at the age of 24 h were obtained from a FEI Quanta 200 scanning electron microscope (SEM).

Phase Boundary Nucleation and Growth Model A modified pBNG formulation is applied to describe the effects of SF and PCE additions on the hydration kinetics of cement. In ordinary portland cement, all four phases (C3S, C2S, C3A and C4AF) react with water simultaneously. C2S and C4AF are slower reacting phases and, hence, do not release substantial amounts of heat at early ages1,31,32. The aluminate phase, i.e., C3A, reacts with water and aqueous SO42- ions rapidly within the first few minutes to form ettringite33,34,


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which continues to grow subsequently for the next several hours at a very slow (and near constant) rate, thus releasing little heat. As such, in classical pBNG models applied to cement pastes4,5,32,35–37, including the model used herein, kinetics of cement hydration are assumed to be dominated by the hydration of the C3S phase. A single product of constant density is assumed to form at a given nucleation event, and its subsequent growth on solid-phase boundaries is treated as the rate-controlling mechanism that drives the kinetics during the early ages of cement hydration. Based on these criteria, at any given time t (h), the volume fraction of hydration products precipitated in the system can be described by Equation (1)35,38,39.

X(t) = 1 − exp [−4aBV . Gout (t) ∙ t (1 −

FD (Gpar (t)√π ∙ Idensity ∙ t) Gpar (t)√π ∙ Idensity . t

)]

(1)

where, X (unitless) is the volume fraction of the reactant transformed to product, Gout (µmh-1) is the outward growth rate of the product away from the substrate (i.e., surfaces of solids) on which it is assumed to nucleate, Gpar (µmh-1) is the lateral growth rate of the product assumed to be isotropic in the two dimension (2D) plane parallel to the substrate, aBV (µm-1) is the total area of solid substrate per unit volume of the paste (Equations 2a-2b), FD is the f-Dawson function (Equation 2c), and Idensity (µm-2) is the nucleation density of the product, i.e., the starting number of total supercritical nuclei produced per unit surface area of the substrate (cement and SF particles).

In the formulation shown in Equation 1, Idensity is assumed to be a constant with respect to time (i.e., site saturation conditions, wherein rate of nucleation = 0.0 µm-2h-1), thus implying that the growth of the product phase begins from nuclei that form at very early ages, and no further


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nuclei are permitted to form after this very initial nucleation burst. Digressing from the classical form of pBNG, the model used herein employs a modification involving the use of an anisotropic time-dependent growth rate of the product. It is worthy to state that this is based on an implementation highlighted by Bullard et al.39 and Oey et al.38, to capture the temporal growth of C-S-H as a function of its supersaturation in the pore solution. It is pointed out that while the system’s chemistry is not considered explicitly in the model, the evolution of the growth rates obtained by fitting the experimental reaction rates is expected to be a direct manifestation of the temporal variation of C-S-H supersaturation38. As both the outward (Gout) and the parallel (Gpar) growth rates vary with time, a constant 2:1 ratio for Gout:Gpar is assumed. This relationship between Gout and Gpar represents anisotropic growth of needle-like domains of the product32,35, and agrees with recent experimental data of the geometry of C-S-H growth at early ages40. As hydration products grow outward into the solution phase and parallel to the plane of the boundary area, they are not permitted to penetrate the cement particles, as was noted by Scherer et al.32,35. However, this does not result in voids at the product-substrate interface. This is due to random orientation of the cement and SF grains, such that as hydration progresses, the growth of the C-S-H from neighboring grains would occupy the space left by the receding grains.

aBV =

w c

( [ ρWater

SSASolid (1 − z) + + ) ρcement ρSF ] 1

SSAsolid = SSAcement + aSF SSASF 2

z (1 − z)

(2a)

(2b)

x

FD (x) = exp−x ∫ exp(y 2 ) dy

(2c)

0


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where, w/c (unitless) is the ratio of the mass of water to the mass of cement, ρ is the density (water = 1000 kg. m-3, cement = 3150 kg. m-3, and SF = 2200 kg. m-3), z (unitless) is the fraction of cement replaced by SF, and SSAsolid (m2. kg-1) is the total exposed area of the substrate per unit mass of cement. Here, the parameter aSF (unitless) acts as a free variable used in the simulations to represent the reactive area fraction of SF, i.e., fraction of area that remains unaffected by agglomeration and, thus is able to provide sites for C-S-H nucleation. The introduction of this variable to account for agglomeration of SF particles is based on past studies41–43. By combining SSAsolid and Idensity, the total number of supercritical product nuclei (Nnuclei, unit of gcement-1) produced per gram of cement can be calculated (Equation 3). Nnuclei = SSAsolid Idensity

(3)

The volume fraction of the reactant transformed to products (X), as calculated from Equation (1), and the degree of hydration (α, unitless) of cement are related by a constant B (unitless) described in Equation (4):

α(t) = BX(t)

1 = B

ρcement ρproducts

w c ∙ ρcement + 1 ( ρwater )

1 1 c+ ρ −ρ cement water +( ) 1 1 − ρproducts ρwater

(4a)

(4b)


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where, ρproducts is the bulk density of combined hydration products (assumed to be 2.070 g. cm3 2,44

)

, and the parameter c = −7.04 × 10−5 m3. g-1 represents the chemical shrinkage per kilogram

of cement that is consumed over the course of its reaction with water45.

With these assumed relationships to simulate the measured reaction rates, the variables that must be determined are: Gout (t), Idensity, and aSF. Of the three variables, Idensity and aSF are constants (with respect to time) whereas Gout(t) varies with respect to time. Therefore, to obtain their optimum values (or functional forms) for a given system, a Nelder-Mead based simplex algorithm38,46,47, that uses derivative-free and non-linear optimization principles, is employed in two steps. In the first step, the value of Gout is kept constant at 0.055 µmh-1, a value reported based on Scanning transmission electron microscopy (STEM) analyses of early age hydration of C3S40. The algorithm iteratively varies the values of Idensity and aSF within predefined bounds until the magnitude of the difference between the simulated and measured rates of reaction (dα/dt) is minimized. It is pointed out that up to this point, the model represents classical pBNG formulation37, wherein the anisotropic growth of the products, which nucleate at a virtual time τ (h), is kept constant throughout the hydration process. To account for the time-dependent variation in product growth, a second simulation step is employed. Here, at any given time t, the optimum values of Idensity and aSF yielded from the first step are used as constants, whereas Gout is allowed to vary iteratively within the bounds of 10-4 – 102 µmh-1 to minimize the deviation between the simulated and measured reaction rates. When the simplex algorithm converges, the value of Gout is taken to be the optimum value at that time. The optimum values of Gout for the entire duration of cement hydration are thus determined by implementing such an optimization process over the first 30 h of hydration using a time step of 0.1 h. The time-dependent Gout,


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obtained as such, mimics the non-monotonic and non-linear growth of the product in relation to its evolution supersaturation in the solution. In a prior publication38, a similar simulation scheme was employed, and it was shown that the simulated evolution of the product growth rate reproduced the intrinsic changes in the initial, as well as the time-dependent evolution of the chemistry of the solution, as determined from experiments. Therefore, while this scheme of deriving the product growth is indirect (i.e., from experimental measurements of reaction rate), the simulations are still representative of the physical processes occurring in the system, and able to capture the initial rapid growth of the product (i.e., when the supersaturation of the product in the solution is high), which cannot be effectively captured otherwise by classical pBNG models39.

Results and Discussion Figure 1 shows representative heat evolution profiles for plain and binary cement pastes prepared with different PCE dosages. SF produces significant enhancements in cement hydration rates, as marked by the leftward shift of the heat evolution curves and higher heat flow rates at the main hydration peak. While the extent of cement hydration enhancement scales broadly with the amount of SF, such enhancements are less pronounced at higher SF replacement levels (i.e., SF > 20%). In the presence of PCE, the hydration of cement in both plain and binary systems is suppressed increasingly with PCE dosage. In particular, PCE delays the onset of the acceleration, and this delay is carried over to later stages, resulting in delayed occurrence of the main hydration peak.


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To gain quantitative information of the hydration kinetics, characteristic calorimetric parameters, i.e., slope of the acceleration regime (mW. gcement-1 h-1), heat flow at the main hydration peak (mW. gcement-1), and inverse of the time corresponding to the main peak (h-1)5, indicative of acceleration or retardation in hydration kinetics were extracted and plotted against PCE dosage (Figure 2). As can be seen, at any given PCE dosage, calorimetric parameters are consistently higher in binary systems compared to the plain system. This suggests enhanced heterogeneous nucleation of the main hydration product, i.e., C-S-H, either on the extra surfaces provided by the fine SF particles, or on the pozzolanic C-S-H formed due to the reaction of SF with portlandite. Thomas et al.4 hypothesized that in cement + SF systems, the nucleating agent is the pozzolanic C-S-H rather than the silica surface itself. The authors argued that enhanced nucleation of C-S-H on pozzolanic C-S-H only becomes significant at the end of the induction period (i.e., the period corresponding to ≈1h-3h after mixing before the onset of rapid acceleration) when massive precipitation of portlandite crystals occurs6,31. As such, the time required to reach the main hydration peak would remain almost unchanged between plain and cement + SF pastes. However, as shown in Figure 2(a), at any given PCE dosage, the time corresponding to the main hydration peak decreases significantly, and approximately in proportion to the SF replacement level. This suggests that in cement pastes prepared with SF, the additional heterogeneous nucleation of C-S-H occurs primarily on the SF surfaces via the filler effect5, as has been reported in various previous studies9–11. Nucleation of C-S-H on the pozzolanic C-S-H, though possible, is expected to be a secondary effect at early ages mainly due to the slow dissolution of SF (i.e., amorphous SiO2) in the paste’s alkaline environment15,16, and scant availability of portlandite in the system. To corroborate this hypothesis, the temporal reactivity of SF in a saturation lime solution (i.e., 1.73 grams of Ca(OH)2 mixed in 1 liter of DI-water, to mimic the


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cement paste’s pore-solution environment) was monitored using the isothermal microcalorimetry method (Figure 3(a)). As can be seen, due to its low solubility in alkaline environments and its intrinsically low dissolution rate, the extent of reaction of SF after 24 hours is minuscule (i.e., ≈1.2%) as compared to that of cement (i.e., ≈41%) reacting in DI-water. These results showing low reactivity of SF in cementitious environments are in good agreement with those determined from 1H nuclear magnetic resonance (NMR) techniques in a prior study48. Next, to quantify the pozzolanic activity of SF, Ca(OH)2 (i.e., portlandite) and SF were mixed proportionally (i.e., 1:1 molar ratio) with water (i.e., l/s = 0.45), and the progress of the pozzolanic reaction, as the two compounds react with each other, was quantified using TGA at different ages (i.e., 1, 2, 3, and 24 h). As can be seen in Figure 3(b), the decrease in portlandite content on account of the pozzolanic reaction with SF is less than 1% (within the uncertainty of TG methods) at early ages (i.e., age ≤ 3h, when massive precipitation of C-S-H occurs). Even after 24 h of reaction, and in spite of the presence of excess portlandite, the decrease in portlandite content was found to be less than 2.5% - as expected, this is associated with formation of trace amounts of pozzolanic CS-H (see Figure 3(b)). These results, obtained from isothermal microcalorimetry and TG methods, therefore, confirm that in a blended cementitious system, the role of SF at early ages is characteristically that of a filler, i.e., devoid of chemical interactions with paste components.

A closer look at the calorimetric parameters (Figure 2) reveals that the enhancements in cement hydration rates of blended pastes are disproportionately smaller with respect to the augmentation in the surface area as provided by SF. For example, in spite of a ≈1400% increase in the solid surface area (calculated based on SSAs of cement and SF), the paste prepared with 30% SF only shows a ≈46.5% increase in the peak heat flow rate compared to the plain paste. These


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enhancements in cement hydration rates become progressively less pronounced at higher SF replacement levels (i.e., SF > 20%), as also noted above. This is hypothesized to be on account of one or both of these effects: (a) agglomeration of SF particles which reduces their exposed surface area and renders it unavailable for heterogeneous nucleation of C-S-H, and (b) nucleation site saturation, wherein the SF surface area by far exceeds the number density of C-S-H nuclei that can form at prevalent levels of cement content (i.e., dilution of cement content due to replacement by SF). Due to the fine size (d50 = 0.23 µm) of the SF particles, effects of particle agglomeration are expected to be more dominant compared to that of nucleation site saturation. Further details corroborating this hypothesis pertaining to the propensity of SF particles to agglomerate are described later.

As shown in Figure 2, for any given system, all three calorimetric parameters decrease with increasing PCE dosage, suggesting a direct correlation between PCE dosage and suppression of cement hydration, as reported in previous studies18,19,21,23,25,49–53. It is important to note that while PCE delays the occurrence of the main hydration peak, the extent of cement reacted up to the peak remains broadly similar (i.e., α = 0.15 to 0.17) across systems (Figure 4(a)). This suggests that regardless of the SF and PCE contents in the system, the main hydration peak occurs only when a threshold amount of hydration products have formed, as also shown in53. At later ages, however, the degree of cement hydration decreases linearly with increasing PCE dosage (Figure 4(b)). This is also evident in the portlandite contents of the pastes, which decline monotonically with PCE dosage (Figure 4(c)). This strongly suggests that the suppression of cement hydration induced by PCE at early ages is irreversible, and propagates to later ages.


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In light of these results, it is hypothesized that the suppression of cement hydration by PCE is primarily due to the adsorption of PCE molecules on the surfaces of cement particles, as has been reported in previous studies20,23,25,54. Such adsorption of PCE molecules inhibits the surface dissolution sites and, hence, delays the time needed to reach the critical supersaturation for portlandite precipitation6,23,25; this manifests as the prolongation of the induction period (see Figure 1). Beyond the induction period, the adsorbed PCE molecules continue to inhibit cement dissolution sites, as well as sites for C-S-H nucleation20,23, thus resulting in suppressed hydration of cement even at later ages. Since PCE molecules are able to adsorb on SF surfaces55,56, it is expected that nucleation sites on SF particles are also inhibited. However, the fraction of SF’s area unaffected by PCE adsorption is still able to offer additional sites for heterogeneous nucleation of C-S-H9–11 (as discussed later in the paper), thus counteracting the retarding effects of PCE, and producing faster hydration rates (Figure 2) and higher degrees of hydration (Figure 4(b)) compared to the plain system. These counteracting effects of SF and PCE on cement hydration are also corroborated in Figure 4(c), which shows that at any given PCE dosage, portlandite contents in binary pastes (i.e., with reduced cement contents) remain consistently similar (i.e., slightly higher for 10% and 20% SF and slightly lower for 30% SF) to the plain system. Similar portlandite contents across plain and binary pastes also support the results shown in Figure 3(b), in that they confirm negligible pozzolanic reaction of SF within the first 30 h of cement hydration.

Figure 5 shows the measured reaction rates of pastes compared again those simulated using the pBNG model. As can be seen, by evaluating the optimum values of the outward growth rate of the product (Gout(t)), the product nucleation density (Idensity), and the reactive area fraction of SF


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(aSF), the model is able to reproduce the experimental results. The variations in these simulation parameters are analyzed below to describe the alternations in the nucleation and growth process in relation to the initial composition of the mixture.

Figure 6(a) describes the influence of PCE and SF additions on product growth rates. As shown, products grow at rates that decrease nonlinearly by about one order of magnitude over the course of cement hydration in first 30 h (Figure 6(a)). This functional form of the product growth rate has been reported to mimic the evolving supersaturation of the C-S-H phase in prior studies38,39. It is noted that any given extent of cement’s reactivity, Gout decreases with increasing PCE dosage. This is better shown in Figures 6(b-c) which plot the values of Gout at the main hydration peak and at later ages when α = 0.30 against the PCE dosage. These results suggest that PCE continually suppresses the growth of C-S-H throughout the hydration process. Interaction between PCE and C-S-H resulting in suppression of the latter’s growth rate has also been reported in literature18,23,25. While the exact mechanisms for this are not understood, it is hypothesized that the excess PCE molecules present in the solution adsorb onto C-S-H20,54 and, subsequently cut off their access to the contiguous solution, thus resulting in inhibition of their growth rate20. At any given dosage of PCE, the provision of SF enhances Gout in accordance with its content, but these enhancements are smaller compared to the reductions induced by PCE (Figures 6(b-c)).

In addition to alterations in the product growth rates, the parameter optimizations show that both SF and PCE have profound effects on the product nucleation process occurring at early ages. The model predicts that large fractions, i.e., up to 96%, of the surface area of SF do not participate in


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the nucleation and growth process (Figure 7(a)). This is attributed to the enhanced agglomeration of the fine SF particles, which reduces their access to the contiguous solution. Effects of agglomeration become more pronounced as the SF content in the system increases, as also reported in previous studies41–43,57. It is expected that PCE alleviates the effects of agglomeration (to a certain extent), and improves flowability22,51 (see Supporting information). However, due to the adsorption of PCE molecules on the particles’ surfaces, improved dispersion of the SF particles does not necessarily result in an increment of the exposed surface area. This is confirmed in Figure 7(a), which shows a decline in aSF with increasing PCE dosage. Despite substantial reductions in its exposed surface area, SF still enhances product formation by offering additional nucleation sites in proportion to its content (Figure 7(b)). However, these nucleation sites on SF surfaces, as well as on cement surfaces, are inhibited progressively with increasing PCE dosage. Binary pastes, in particular, feature a sharp reduction in nucleation sites with increasing PCE dosage (Figure 7(b)). This suggests that the effects of nucleation site inhibition produced by PCE are more pronounced in SF. While the reasons for this are not clear, it is hypothesized that this is due to the preferential adsorption of PCE molecules on SF due to its comprehensively higher specific surface area compared with cement.

Next the fitting parameters aSF and Idensity are combined to calculate the total number of supercritical product nuclei (Nnuc) (Figure 7(c)). The trends observed here are in very good agreement with the hypotheses present above, as well as the trends derived directly from experiments (Figure 2). As can be seen, PCE suppresses the formation of product nuclei in proportion to its dosage, as reported previously in literature19,25,58. Partial replacement of cement by SF consistently enhances product formation compared to the plain system, but these


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enhancements become less pronounced: (a) at elevated PCE dosages due to the preferential adsorption of PCE molecules on SF surfaces, and (b) at high SF replacement levels (i.e., SF > 20%), wherein effects of SF particles agglomeration become dominant, thus negating the benefits of extra surface provided by SF. Based on these results, it can be said that replacement of cement by SF at high replacement level is beneficial in terms of reactivity improvements, but these benefits decline progressively with increasing dosage of PCE.

Overall, the results from the simulations show that SF enhances product nucleation, whereas PCE suppresses product nucleation as well as its growth. For all pastes described in this study, the model assumes products to nucleate and grow in a heterogeneous manner. SEM images of pastes obtained after 30 h of hydration (Figure 8) corroborate this assumption as they show C-SH forming heterogeneously around both cement and SF grain in spite of the presence of PCE at a high dosage (i.e., 2.4 %). Heterogeneous nucleation of C-S-H, albeit at suppressed rates, in paste containing PCE was also reported in a prior study19,26. Other studies23,25, however, have suggested that PCEs cause a switch in the nucleation mechanism of C-S-H from heterogeneous to homogeneous. Such drastic changes in the nucleation mechanism would require the adsorption propensity of PCE molecules to be significantly large to produce near complete inhibition of nucleation sites on the reactant’s surface. Sucrose, which is also an organic hydration retarder like PCE, suppresses nucleation sites at the C3S surface but does not enforce homogeneous nucleation of C-S-H59,60 even at higher dosages. Thomas et al.4 showed that the retarding effects of sucrose could be reversed through provision of favorable sites for heterogeneous nucleation of C-S-H (e.g., via additions of C-S-H seed). Similarly, in this study it is shown that the retarding effects of PCE can be partially counteracted through provision of additional heterogeneous


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nucleation sites on SF surfaces. These differing interpretations of the mechanisms pertaining to the retarding effects of organic chemical admixtures arise from the lack of understanding of the correlations between the intrinsic properties of the admixture (e.g., chemistry, molecular architecture and charge density of the additive molecule19,20,26,51–53) and its interaction with the cementitious phases and hydrates (e.g., adsorption capacity of the admixture’s molecules on the cement’s surface as it evolves continually during its dissolution18,20,21,54). Further research directed towards exploring these correlations would aid in refinement of the numerical models.

Conclusions A series of experiments and pBNG simulations were applied to elucidate the roles of PCE and SF on hydration kinetics of ordinary portland cement in plain and binary pastes. The results indicate that both PCE and SF affect the nucleation and growth of C-S-H right from the time of product nucleation (i.e., around the time of mixing), and throughout the hydration of cement.

SF enhances hydration of cement in relation to its content in the paste. These reactivity improvements are linked to the provision of product nucleation sites on the extra surface area provided by SF. However, due to the agglomeration of SF particles, which becomes progressively dominant with increasing SF contents, up to 96% of its surface is rendered unavailable for reaction. The results also suggest that the pozzolanic activity of SF is limited in the first 30 h of hydration and, as such, imparts negligible contribution on the nucleation and growth process, as well as hydration kinetics at early ages.


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In the presence of PCE, the hydration of cement is suppressed in proportion to the PCE dosage. This is due to the adsorption of PCE molecules on the cement’s surface, which in turn inhibits surface dissolution and product nucleation sites. This is reflected as prolongation of the induction period and delayed occurrence of the main hydration peak. In pastes containing SF, PCE molecules adsorb preferentially on SF surfaces, and act to diminish the benefits of additional nucleation sites on SF surfaces by blocking them. Furthermore, PCEs interact with the hydration products, and act to suppress their growth throughout the hydration process.

The outcomes of the work provide mechanistic insights into the origins of accelerating and retarding effects of PCE and SF respectively. While a simplified view is presented, the discussion highlights aspects which need better understanding to aid in the design of SF-based high-performance binders with optimized rheology and reactivity.

Supporting Information (a) Composition and particle size distribution of materials, (b) Description of the method used to determine the maximum dosage of PCE, (c) Description of the mixing procedure, and (d) Description of the isothermal microcalorimtery method.

Acknowledgement This research was conducted in the Advanced Construction and Materials Laboratory (ACML) of the Center for Infrastructure Engineering Studies (CIES) and Materials Research Center (MRC) at Missouri S&T. The authors gratefully acknowledge the financial support that has made these laboratories and their operations possible. This study was funded by the RE-CAST


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University Transportation Center at Missouri University of S&T under grant No. DTRT13-GUTC45.

References (1) (2) (3)

(4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

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List of Figures

(a)

(b)

(c)

Figure 1: Measured heat flow rates of pastes prepared with: (a) cement replaced by SF at different replacement levels, (b) cement with different PCE dosages, and (c) 80% cement and 20% SF with different PCE dosages. Cement, in this and the subsequent figures, is referred to as OPC (i.e., ordinary portland cement). The l/s for all pastes is 0.45. For a given system, the uncertainty in the measured heat evolution profile is ±2%.


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(a)

(b)

(c)

Figure 2: The calorimetric parameters: (a) inverse of the time to the main hydration peak, (b) heat flow at the main hydration peak, and (c) slope of the acceleration regime extracted from the calorimetry profiles. The l/s for all pastes is 0.45. For a given system, the uncertainty in each calorimetric parameter is ±2%.


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(a)

(b)

Figure 3: Isothermal microcalorimetry based determinations of the time-dependent reactivity of cement in plain paste, and SF in a saturated lime solution. (b) DTG traces showing differential mass loss traces of a SF + portlandite (1:1 molar ratio) system at 1, 2, 3, and 24 hours after mixing. For the plain paste, the degree of hydration of cement determined from isothermal calorimetry and isothermal calorimetry methods are within ±1% of each other. The highest uncertainty in phase quantifications by DTG is ±2%. For all systems, l/s = 0.450.


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(a)

(b)

(c)

Figure 4: Degree of hydration (α) of cement in plain and binary pastes: (a) as determined from the heat evolution profiles at the main hydration peak, (b) as determined from DTG analyses after 30 h of hydration. (c) shows the portlandite contents (as mass% of the binder, i.e., dry paste) in pastes after 24 h of hydration, as determined from DTG analyses. The l/s for all pastes is 0.45. The highest uncertainty in phase quantifications by DTG is ±2%.


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Figure 5: A representative set of simulated and measured reaction rates (dα/dt) for plain and binary pastes provisioned with different PCE dosages. The blue dashed line represents model output at its intermediate step, wherein the outward growth rate of the product is assumed to remain constant throughout the hydration process. The red dashed line represents the final output from the simulations, wherein the growth rate is allowed to vary with time. Simulations are deterministic and, therefore, there is no uncertainty associated with them.


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(a)

(b)

(c)

Figure 6: (a) The outward growth rate of the product (Gout) as a function of the degree of the hydration (α) of cement. The values of Gout (b) at the main hydration peak, and (c) at α = 30%. Simulation parameters are deterministic and, therefore, there is no uncertainty associated with them.

(a)

(b)

(c)

Figure 7: Parameters derived from the simulations: (a) reactive area fraction of SF (aSF), (b) nucleation density of the product (Idensity), and (c) total number of supercritical product nuclei formed per gram of cement (Nnuc) as functions of the PCE dosage. Simulations are deterministic and, therefore, there is no uncertainty associated with them.


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OPC

OPC

SF

(a)

(b)

SF

(c) Figure 8: SEM micrographs of pastes provisioned with 2.4% PCE using: (a) cement, and (b and c) 80% cement + 20% SF. Age of hydration of all pastes is 30 h. The magnifications and length scales are indicated in each image. OPC and SF represent cement and SF particles, respectively.


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Influence of Silica Fume and Polycarboxylate Ether ... - ACS Publications

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