Fractal Structure Evolution during Cement Hydration by Differential

Nov 13, 2013 - the presence of comb-shaped polycarboxylate ether superplasticizers. LT-DSC is ... generation superplasticizers to tricalcium silicate ...
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Fractal Structure Evolution during Cement Hydration by Differential Scanning Calorimetry: Effect of Organic Additives Francesca Ridi, Emiliano Fratini, and Piero Baglioni* Department of Chemistry “Ugo Schiff” and CSGI, University of Florence, via della Lastruccia 3-Sesto Fiorentino, I-50019 Florence, Italy S Supporting Information *

ABSTRACT: Low-temperature differential scanning calorimetry (LT-DSC) is used to investigate the microstructure of tricalcium silicate pastes, hydrating in pure water and in the presence of comb-shaped polycarboxylate ether superplasticizers. LT-DSC is shown to be a powerful technique, able to provide important information on the porosity and on the fractality of the porous evolving matrices by means of rapid and nondestructive measurements. In particular, LT-DSC gives a semiquantitative estimation of the evolving porosity (capillary, small gel, and large gel pores), the depercolation threshold of the capillary pores, and the fractal dimension associated with the probed porosity. The results are in good agreement with those obtained by small-angle scattering methods ensuring that this approach, based on the well-established and easily accessible DSC technique, can provide valuable information on the evolving porosity and the fractal nature of hydrating cement pastes.

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INTRODUCTION Cement hydration is a complex process that starts from a powder of heterogeneous composition and water and forms a rigid solid network with high mechanical resistance. The most important hydrated phase, acting as “glue” between the grains (unreacted phases, inert fillers, crystalline hydrated products), is the amorphous calcium silicate hydrate (briefly named C−S− H), formed upon reaction of tricalcium silicate (C3S) and, to a lesser extent, of dicalcium silicate (C2S) with water. A number of macroscopic properties (e.g., elasticity, compressive strength, resistance to degradation, transport phenomena) depend on the microstructural features of C−S−H, and their complete control would require the full understanding of both the hydrates microstructure and packing.1 For this reason, since many years, an important part of the literature is devoted to the investigation of the C−S−H microstructure2−15 and to the intrinsic relationship between the evolving microstructure and ultimate macroscopic properties.16−19 Many efforts in this field are oriented on the difficult task of providing a comprehensive picture of this complex material, whose constituent features have dimensions ranging over several orders of magnitude, from nanometers (C−S−H unit globules, interlamellar waterfilled spaces) to tens of micrometers (largest capillary pores). The increasing use of organic polymers in the industrial practice, with the aim of modifying the hydration kinetics and tuning the final properties of the material, compels to account for their possible effect on the forming C−S−H. In particular, in a recent work it was pointed out that the addition of last generation superplasticizers to tricalcium silicate pastes modifies both the structure of the C−S−H basic globules and the overall microstructural arrangement.20 © 2013 American Chemical Society

It is generally accepted that a realistic picture of disordered porous systems could be provided by a fractal description. In particular, in the case of cement pastes, the hydration consists in a “gelation” process, where the developing C−S−H phase links together the unreacted grains. This kind of process is efficiently described by the percolation theory.21,22 From a physical point of view, the term percolation refers to the formation of a long-range connectivity in stochastic systems, and its occurrence produces random fractal structures. The application of the percolation theory to an evolving porous system allows the measurement of the percolation threshold and of the fractal dimension, both directly linked to the macroscopic characteristics of the material. For example, one of the most relevant industrial challenges in the cement chemistry field is the reduction (or at least the control) of the degradation mechanisms due to the permeation of salt-rich water because of atmospheric agents. This event is strictly related to transport phenomena taking place in the specimen. According to the percolation model, the transport properties inside a paste can be rationalized in terms of two percolation thresholds: (i) the set point and (ii) the capillary porosity (de)percolation.23−25 The set point is the time when the solid grains become connected together by means of hydration products and form a rigid network. This point occurs at very low degree of hydration (α = 0.02−0.08), roughly coinciding with the end of the induction period. Even when the solid matrix is percolated, the hydration reaction inside the sample keeps on changing the microstructure of the sample. The volume of the hydrated phases Received: June 25, 2013 Revised: November 12, 2013 Published: November 13, 2013 25478

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increases until the pores (initially interconnected) depercolate. This time establishes the capillary porosity depercolation threshold. After this point, the capillary porosity becomes “closed” toward the external surface and is progressively reduced by the developing hydrate phases. This implies that the “in situ” determination of the depercolation threshold could be important to understand and control the transport properties that in most degradation mechanisms govern the rate of damage of the specimen and hence the long-term durability of the material.23,26−28 Cement pastes have been extensively studied by means of scattering techniques10,20,29−33 because the length scale of the structure (1−1000 nm) matches the dimension of the X-ray or neutron probe (i.e., the inverse of the scattering vector). The easiest information that can be achieved by the log−log plot of a scattering curve is the mass f ractal dimension. Moreover, these techniques do not require drying procedures and can be directly performed on wet samples. Hence, SAS techniques are particularly suitable to investigate the evolution of the fractal arrangement during the microstructure development.30,33 This paper aims at showing that calorimetry, a conventional and easily accessible technique, can effectively estimate the volume of the meso/macroporosity, the depercolation threshold of the capillary porosity, and the fractal dimension of hydrated cementitious samples, with results well in agreement with those obtained by other techniques like small-angle neutron or X-ray scattering, which are more sophisticated but less accessible. Furthermore, even though SANS/SAXS methods have to be considered by far the most rigorous way to attain the mass fractal dimension, the possibility to obtain this information by means of an easily accessible technique based on calorimetry is very appealing, also considering that it is possible to simultaneously couple the fractality evaluation with information like the porosity evolution and hydration kinetics. In this regard, low-temperature differential scanning calorimetry (LT-DSC) enables to follow the evolution of the microstructure throughout the hydration process and does not require special treatments of the samples. In particular, in this paper we report the results of the investigation of saturated tricalcium silicate (C3S) pastes hydrated in water and in the presence of four structurally defined organic additives that belong to the last generation of superplasticizers (polycarboxylic backbone with grafted PEO chains) used in advanced cement formulations.

Figure 1. Sketch of the PCEs molecular formula.

time) was identified through the acquisition of the hydration kinetics, as reported in a previous work.34 The td times of the pastes are 1 day for C3S/water, C3S/PCE102-2, and C3S/ PCE102-6 pastes; 3 days for C3S/PCE23-2; and 12 days for C3S/PCE23-6. At these given times, some milligrams of each paste were put in aluminum pans and studied with LT-DSC. The remaining part of each paste was put under water and stored at 20 °C. After 3, 7, 14, and 28 days some milligrams were withdrawn, externally dried with paper, and analyzed. As the induction time of the C3S/PCE23-6 paste was very long, only for this sample the sampling times were 12, 21, and 28 days. Differential scanning calorimetry measurements were performed using a DSC Q2000 (TA Instruments, Philadelphia, PA), and the obtained data were elaborated with the Q Series software, version 5.4.0. Each measurement was carried out with the following temperature program: equilibrate to 5 °C; cooling ramp from 5 to −80 °C at 0.5 °C/min; heating ramp from −80 to +10 °C at 0.5 °C/min. Scanning electron microscopy was performed on uncoated samples using a field emission scanning microscope ΣIGMA (Carl Zeiss Microscopy GmbH, Germany). The images were acquired using the in-lens secondary electron detector with an acceleration potential of 5 kV.

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RESULTS AND DISCUSSION The heating scans (from −80 °C to room temperature) of the DSC thermograms, reported in Figure 2A, show a single hump over the whole temperature range. As documented in the literature,28,35−40 this particular shape is due to the peculiar characteristics of the cement microstructure: being the distribution of the pore sizes continuous in a range spanning from nanometers to tens of micrometers, while heating, the melting of the water occurs progressively from the fractions confined in the smallest cavities to the fractions present in the largest ones. This continuous melting and release of heat produces the broad hump characteristic of the heating part of the thermogram. However, the freezing process occurs in a discontinuous way, and the cooling scans present some definite peaks, as shown in Figure 2B. This peculiar behavior has to be ascribed to the combination of the two freezing mechanisms of the water: homogeneous and heterogeneous. The homogeneous nucleation is an activated process because a free energy barrier must be surmounted to form a critical nucleus. The heterogeneous nucleation occurs at preferential sites and requires less energy than the homogeneous process. According to a very recent paper by Sanz et al.,41 at low degree of supercooling (−20 °C < T < 0 °C), due to the high free energy required for the homogeneous formation of a critical cluster, only the heterogeneous nucleation is possible. For this reason, in a saturated system, the water confined in cavities whose dimension is large enough to host critical nuclei of at least

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EXPERIMENTAL SECTION Synthetic tricalcium silicate (C3S) with BET specific surface area of 0.65 ± 0.05 m2/g and particles median radius of 4.66 μm was obtained from the CTG-Italcementi Group. PCEs were obtained from BASF. They are polymethacrylic chains partially esterified with poly(ethylene oxide) lateral chains. The molecular formula is sketched in Figure 1. Each polymer is identified by the acronym PCEp-n, where p is the number of repeating PEO units in the lateral chains and n identifies the number of free carboxylic groups per esterified group. The pastes were prepared by manually mixing 500 mg of C3S with 200 mg of water or additive solutions (1% w/w). In this way a water/cement ratio (w/c) of 0.4 and the 0.4% of PCEs by C3S mass was obtained. Each sample was maintained at 20 °C in closed containers, to avoid water loss, until the end of the acceleration period, when the microstructure due to the growing hydrated phases has formed. This time (td, diffusional 25479

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Figure 2. Heating part (A) and cooling part (B) of a typical LT-DSC thermogram recorded on cementitious saturated samples (C3S/PCE102-2).

∼8000 molecules (diameter ≈ 8 nm, that is, the size of the stable critical cluster at T ≈ −15 °C) 41 freezes via heterogeneous nucleation, even if it is isolated from the surface. Once the water in the capillary pores is frozen, the formed ice remains in contact with the liquid water present in the smaller cavities. By lowering the temperature below T < −20 °C, the size of the critical cluster sensibly decreases (at T ≈ −35 °C, diameter ≈ 3.5 nm corresponding to ∼600 molecules).41 Moreover, the nucleation free energy barrier decreases, making the homogeneous process possible to occur. In these conditions both the homogeneous and the heterogeneous nucleation mechanisms become accessible, with comparable rates. The freezing will then occur in the pores whose dimensions can host the critical cluster stable at that temperature. The appearance of peaks in the cooling curve indicates, however, that the process starts at the pore entrance, where the liquid water is in contact with the surrounding ice, which constitutes a preferential site for the nucleation. The dimensional range of these freezing pores must be comparable to the size of their entrance because, for the reasons given before, it is very unlikely that water in large pores freezes at this stage and the smaller pores cannot host the critical nucleus. In the case of cement pastes, the process of formation of the microstructure originates two classes of nanometric porosities, named small gel pores (SGP) and large gel pores (LGP), according to the Jennings’ colloidal model II. The former class (SGP) derives from the packing of individual ∼5 nm C−S−H globules: the nanometric dimension of these “building block” implies that both the pore size and the entrance size are necessarily in the range of a few nanometers (namely, 1−3 nm).11,42 This dimensional range is compatible with the peak around −40 °C in the thermogram.43,44 The latter pore class (LGP) originates from the fractal aggregation of a “superstructure” of globules, resulting in a more “open” porosity, in terms of both pore size (the literature indicates these cavities to range between 4 and 12 nm)11,39,42 and entrance size. In the cooling part of the thermogram, the freezing of the water confined in this pore class occurs in the −20/−35 °C temperature range.43,44 Analysis of Fractal Dimension. As the heating scan directly depends on the distribution of the pore sizes, it can be analyzed to obtain information on the fractality of the samples. Several studies in the literature evidence the fractal nature of C−S−H and extract the mass fractal dimension, Dm, as a function of the degree of saturation,31 the hydration time,30,33 and the degradation degree.45 As already mentioned, SAS

techniques are generally the methods of choice to investigate these properties. Previous works report the possibility to extract these quantities in the case of wet sonogels from the DSC incremental volume distribution, in very good agreement with SAXS patterns measured on the same system.46 The rationale behind the extrapolation of fractal properties from DSC relies on the fact that the melting temperature of an ice crystal confined in a pore is depressed of a quantity ΔT = T0m − Tm related to the radius R, being R = r − l, where r is the radius of the pore and l is the thickness of the nonfreezable layer of water at the solid interface. Previous estimations of l from NMR measurements report a value of 0.5 ± 0.1 nm for porous glasses.47 For this reason, no peak is observed in the DSC cooling scan for the water confined into the calcium silicate layers constituting the C−S−H primary units (“interlamellar gel pores” or IGP), the size of these pores being about 1 nm. The Gibbs−Thomson equation states that the melting temperature Tm and the radius R are inversely related as follows: ⎛ 1−2γVs ⎞ ⎟ Tm = Tm0⎜ ⎝ ΔHR ⎠

(1)

where Tm0 is the melting temperature of an ice crystal of infinite dimension, γ is the solid−liquid interfacial tension, ΔH is the specific melting heat, and Vs is the specific volume of the solid. For the water case (assuming T0m = 273.15 K, γ = 40 × 10−3 N m−1, ΔH = 334 J g−1, and Vs = 1.02 cm3 g−1), eq 1 becomes simply ΔT =

68.29 R

(2)

where R is given in nanometers. Heating from −80 °C to room temperature a porous sample with pore size distribution P(r) and saturated with water produces the melting of the liquid confined in pores of progressively increasing dimension. In other words, the registered heat flux is proportional to the incremental volume dV of the ice that melts at each temperature Tm. To provide a quantitative estimation, the detected heat flow must be independent of the heating rate. In fact, in a DSC experiment the heat flow depends on the mass of the sample, on its thermal conductivity, on the thermal contact and on the heating/ cooling rate. According to Neffati and Rault,48 a DSC thermogram registered at rates lower than 2 °C/min maintains 25480

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Figure 3. (A) DSC heating scans on the paste C3S/PCE102-2. (B) Incremental pore volume per solid mass vs temperature obtained from the scans shown in panel A. The curves are shifted for the sake of clarity.

Figure 4. Incremental pore volume per solid mass as a function of the melting depression, ΔT = T0m − Tm for the differently formulated paste investigated in this study: (A) C3S/H2O, (B) C3S/PCE102-2, (C) C3S/PCE102-6, (D) C3S/PCE23-2, and (E) C3S/PCE23-6.

DSC signals and the plots of the incremental pore volume vs the melting temperature, obtained for the paste C3S/PCE102-2. Neffati and Rault48 assumed that the heat flow, Jq, measured by DSC on porous glasses is related to ΔT by a scaling law consistent with the fractal nature of the systems. Furthermore, in some papers46,50 the mass fractal of wet gels was extracted from DSC data and compared with the results obtained from SAXS. To compare the DSC and the SAXS approaches, Vollet et al.51 used a method originally proposed to link SAXS and nitrogen adsorption data on porous systems. In their approach, the porous system was regarded as a homogeneous solid of

the equilibrium conditions, yielding real information on the accessible porosity. From the definition given above, the incremental pore volume per solid mass can be written as dV = P(r) dr, where P(r) is the pore size distribution. To obtain dV, the heating DSC signal has been normalized with the total pore volume Vp (see the table reported in the Supporting Information), obtained by integration of the whole melting peak, scaled by the bulk water density value at 0 °C (0.9998 g/cm3).49 As an example, Figures 3A and 3B respectively report the heating 25481

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density ρS where an incremental pore volume per solid unit mass dV = P(r) dr was used to account for the change of the bulk density of the porous sample, ρ(r), as a function of the pore filling steps. The resulting process can be mathematically described as r 1 1 = + P(r ) dr ρ (r ) ρS 0 (3)

around 2.4−2.6 even in the C3S/SPs cases. Similarly, a recent SANS/SAXS investigation20 on pure C−S−H phases, synthesized from C3S in excess of water or in the presence of a water solution at the same concentration of the PCEs used in this study, reported a Dm = 2.8 for the pure C−S−H phase and values around 2.6 in presence of PCEs. Dm extracted for very mature samples (6 months) by LT-DSC results in two- or three-tenths lower than what is reported by the SANS/SAXS experiment performed on the correspondent C−S−H pure phase. This small discrepancy can be attributed to the different nature of the probes and to the small differences in the samples. In the first case (i.e., different nature of the probe) while LTDSC is sensitive to the water phase, SAXS detects both the scattering contributions of the hydrated phases and the anhydrous phases, which is not always obvious to decouple. In the second case (i.e., small differences in the samples), the Dm values obtained on pure C−S−H cases are not affected by the presence of Ca(OH)2, which is always contained in the samples investigated by LT-DSC. Even a different w/c ratio could lead to small differences in the mass fractal values. All these comparisons show that the LT-DSC technique is a reliable method to access, within a certain extent, the evolution of the mass fractal dimension in cement-based materials. Evolution of Porosity. While the heating scan contains the information on the fractality of the samples, the cooling scan allows the quantification of the water confined in the porosity during each stage of the cement hydration. Figure 5 shows the cooling part of the DSC thermograms registered on C3S/water paste and on C3S/PCEs pastes during the hydration. The thermogram registered on each sample at the end of the acceleration period does not show the sharp peak between −10 and −20 °C due to the crystallization of bulk water. This means that at this time the main part of the initial water has been consumed by the formation of hydrates. Part of this water remains unreacted, constrained in the just formed microstructure. Because of this confinement, this water freezes around −40 °C. After the acceleration, the pastes have been dipped in water to ensure the whole porosity to be saturated. This procedure allows extracting quantitative information on pore volume and on its evolution during the hydration. From this point on, the DSC thermograms show three main features: an intense sharp peak in the −10/−20 °C range, due to the crystallization of bulk water contained in capillary pores; a peak in the −20/−35 °C region, corresponding to the crystallization of water in LGP pores; a peak at −40 °C due to the solidification of the water constrained in the SGP porosity. According to a procedure reported elsewhere,42 each peak was integrated, and the areas were used to calculate the amount of water involved. As the standard enthalpy of fusion varies with the temperature, to quantify the water, we used the estimation of ΔH0 by Hansen et al.53 at the mean temperature within each integrated region. Figure 4 shows the results of this calculation. The histograms display the pore volume of capillary, LGP, and SGP pores as a function of the hydration time. To estimate the pore volume, we used the density of bulk liquid water at 25 °C, 0.9998 g/cm3. This calculation does not take into account any possible change of the water density due to the confinement. Further investigations could be planned to give a more precise estimation of this assumption, which is not the purpose of the present work.49 In all the samples the capillary pore volume evolves according to a depercolation process, showing a decreasing

∫

As a matter of fact, ρ(r) will then scale with r, in the fractal range a ≤ r ≤ ξ, as ρ(r ) = ρS (r /a)Dm − 3

(4)

where a is the characteristic dimension of the smallest repeating unit and ξ is the maximum correlation length of the fractal aggregate. Combining eq 2 and eq 4, the following relation holds: dV = A(ΔT )Dm − 3

(5)

The plots dV vs ΔT on a log−log scale for the investigated samples are reported in Figure 4. Table 1 (as well as Figure SI2 in the Supporting Information) reports the Dm coefficients extracted from the fitting of the Table 1. Mass Fractal Dimension Dm (±0.1) for the Analyzed Pastes during the Hydration water PCE102-2 PCE102-6 PCE23-2 PCE23-6

3 days

7 days

14 days

2.0 2.0 2.1

2.3 2.2 2.2 1.9

2.5 2.1 2.2 2.4

21 days

28 days

6 min

2.6

2.6 2.1 2.4 2.2 2.5

2.5 2.5 2.7 2.3 2.5

log−log plots in Figure 4 from ΔTξ ≅ 1 K to ΔTa ≅ 10 K, corresponding to pores with radius between ξ ≅ 70 nm and a ≅ 7 nm, as calculated by means of eq 2. This range is directly comparable with that investigated by small-angle scattering techniques.30,52 In all the samples, immediately after the nucleation and growth period (that is after 1 day of curing for C3S/water, C3S/ PCE102-2, and C3S/PCE102-6 systems; after 3 days for C3S/ PCE23-2; and after 12 days for C3S/PCE23-6)34 the fractal dimension is very low, as expected for poorly packed systems, where the three-dimensional microstructure is still beginning to develop. As the process goes on, the microstructure of the pastes becomes increasingly packed as a consequence of the hydrated phases growth and Dm values rise. In the case of C3S/ water paste, the fractal dimension increases from 2.0 to 2.6 in the first 28 days. In a previous SAXS investigation, the mass fractal dimension in a similar C3S/water paste was shown to vary from 1.9 to 2.8 in the very same time interval, from 1 to 28 days.30 During the first month of hydration the pastes containing superplasticizers exhibit Dm values lower than C3S/water. This means that a more “open” nanoscale structure is formed in PCE-containing pastes with high w/c values (w/c = 0.4). The need of reducing the water content in real applications involving superplasticizers is well-known, as the excess water is recognized to increase the capillary porosity. The present investigation shows that high w/c values induce also the formation of a nanostructure less packed than that of the C3S/water sample, especially in the first part of the hydration process. However, after 6 months Dm reaches values 25482

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Figure 5. Low-temperature DSC thermograms recorded on (A) C3S/water, (B) C3S/PCE102-2, (C) C3S/PCE102-6, (D) C3S/PCE23-2, and (E) C3S/PCE23-6 during hydration.

Figure 6. Histograms showing the evolution of the pore volume (cm3 per g of paste) during hydration: capillary porosity (red), LGP (yellow), and SGP (blue). Error bars indicate uncertainties of ±25% for capillary porosity and of ±40% for LGP and SGP, estimated analyzing different portions of the same sample.

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Figure 7. SEM images of fracture surfaces of pastes cured 28 days: (A, B) C3S/water; (C, D) C3S/PCE102-2; (E, F) C3S/PCE102-6; (G, H) C3S/ PCE23-2; and (I, J) C3S/PCE23-6. The bar in parts A, C, E, G, and I is 2 μm. The bar in parts B, D, F, H, and J is 200 nm.

behavior during the hydration process due to the increase of the solid volume fraction, as the hydrated phases have molar volume higher than the anhydrous ones. In the C3S/water case (Figure 6A, red bars) the volume is almost constant during the first 14 days and starts to decrease at 28 days, showing a drastic drop only after 6 months. In this sample the depercolation threshold can be then estimated to occur after 28 days from the mixing. The addition of PCEs sensibly decreases the capillary volume with respect to the C3S/water sample and, in some cases, alters the percolation threshold. The sample with

PCE102-2 (having the lowest adsorption propensity, plasticizing efficiency54 and retarding power34 among these polymers) shows a behavior very similar to the paste without additives, the capillary pore space remaining percolated throughout the first 28 days of hydration, and showing a reduction only after 6 months. When C3S is hydrated in presence of PCE102-6 (Figure 6C) the depercolation of capillary porosity is anticipated (with respect to C3S/water paste) at 7 days, while PCE23-2 and PCE23-6 (Figures 6D and 6E) maintain the depercolation threshold at 28 days after mixing, despite their 25484

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low-temperature calorimetric measurements performed in equilibrium conditions could disclose the fractal dimension of the pastes, and these values are in good accordance with those obtained from small-angle scattering measurements. It was found that the mass fractal dimension, Dm, grows during the hydration from 2.0 after 3 days to about 2.6 after months, because of the packing of the matrix due to the formation of hydrated phases. For pastes prepared with w/c = 0.4, the presence of PCEs decreases the packing of the matrix during the first month of curing with respect to the C3S/water paste, as demonstrated by the lower Dm, while the fractal dimension is roughly the same in all the samples after 6 months. These results, apart the significant information on the effect of additive on the curing process of cement pastes, show that the analysis of simple DSC measurements can provide a wealth of information on the fractal properties of this important construction material.

very effective retarding action, which cause the initial dormant period to stop after 42 and 260 h, respectively (as reported previously34). The LT-DSC technique also provides the evolution of the finest microstructure in the samples. Blue and yellow bars in Figure 6 show respectively SGP and LGP pores. It is evident that the SGP volume is always higher than LGP, meaning that apart from the capillary pores (acting as internal water reservoirs), the microstructure due to the growth of C−S−H gel mainly consists of a network of SGP pores. The analysis of the LT-DSC data shows that the volume of the nanometric pores (both SGP and LGP) in all the saturated samples does not change much, maintaining almost constant values throughout the hydration process. Scanning Electron Microscopy. The microscopic analysis performed on the samples by scanning electron microscopy evidence their multiscale porosity, ranging from the micrometers to tenths of a nanometer. The SEM micrographs reported in Figure 7 show that the superplasticizers addition influences the aggregation of the C−S−H fibers and thus the porosity. The lowest magnification evidence the effect of PCEs in reducing the mean dimension of the C3S grains: this effect is evident in all samples except C3S/PCE102-2 (Figure 7C), in line with its lower efficiency. The other PCEs produce the separation of the initial anhydrous C3S into grains with dimensions of the order of the micrometer, as shown in the Figures 7E,G,I. The best performing PCE, in terms of disaggregating the anhydrous grains, is PCE23-6 (Figure 7I), in line with its high adsorption propensity and retardation efficiency with respect to the other additives of the series.34 SEM images acquired at the highest magnification provide the visual evidence that PCEs modify the C−S−H microstructure. In all cases the formed C−S−H is fibrillar. The presence of PCEs induces the growth of fibrils thinner than those observed in the C3S/water case. Moreover, PCEs affect the spatial arrangement of these fibrils: PCE102-2 (Figure 7D) induces the formation of “suprafibrils” which are bundles of aligned primary fibrils. PCE102-6 does not generate these suprastructures, while the primary fibrils are radially oriented from the anhydrous core. PCE23-X series results in a C−S−H arrangement that is intermediate among the two previously described, alternating regions with well-separated primary fibrils to regions where the bundles are present. In these cases the bundles are less compact than those formed with PCE102-2.

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ASSOCIATED CONTENT

S Supporting Information *

Histogram of the total pore volume vs time for all the investigated samples; evolution of the fractal dimension (Dm) as a function of the hydration time for all the investigated samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone +39 055 457-3033; Fax +39 055 457-3032; e-mail piero.baglioni@unifi.it (P.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors thank CTG-Italcementi for providing the C3S pure phase and Dr. S. Becker and Dr. J. Pakusch (BASF AG, Ludwigshafen, Germany) for providing the PCE superplasticizers. Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase, CSGI, Ministero per la Istruzione, Università e Ricerca (MIUR), and CTG-Italcementi are gratefully acknowledged for partial financial support.

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CONCLUSIONS It is known that some macroscopic properties of cement (degradation phenomena, elasticity, compressive strength, etc.) are influenced by porosity. For this reason the elaboration of methods able to easily access the characteristics of the pore structure during the hydration process is a task of primary importance. In this paper we showed for the first time that a very common and accessible technique like differential scanning calorimetry can be used to extract most of the relevant information on the samples porosity. We investigated the development of C−S−H microstructure during the hydration of C3S in the presence of PCEs estimating the volume of pores (capillary, large gel, and small gel pores), monitoring it throughout the hydration. We also estimated the depercolation threshold of the capillary porosity and how this value is influenced by commonly used cement additives (polymethacrylic chains partially esterified with poly(ethylene oxide) lateral chains). Furthermore, we showed that an accurate analysis of 25485

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