Journal of Building Engineering 65 (2023) 105771
Available online 23 December 2022
2352-7102/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Studying the bulk hydrophobization of cement mortars by the combination of alkylalkoxysilane admixture and
fluoropolymer-functionalized aggregate
Rafael Zarzuela S´anchez
*, Jorge Gonz´alez-Coneo, Manuel Luna, Ana Díaz, María Jesús Mosquera
**TEP-243 Nanomaterials Group, Department of Physical-Chemistry, Faculty of Sciences, University of Cadiz, 11510, Puerto Real, Spain
A R T I C L E I N F O Keywords:
Cement mortar Hydrophobicity Admixture Silane
Aggregate modification
A B S T R A C T
In this work, hydrophobic mortars are produced by combining a silane-based admixture with the pre-treatment of the aggregates with an alkoxysilane-ended fluoropolymer. A comparative study is presented to determine the effect of the components, under different curing conditions, on the hydrophobicity, mechanical performance, composition and micro-structure. The combination of both strategies allows obtaining hydrophobic properties at different curing conditions, whereas the silane loses effectiveness at high humidity and the modified aggregate at low humidity. The silane hinders cement hydration and promotes gaps in the aggregate-matrix interfacial transition zone, decreasing mechanical resistance, whereas the modified aggregate changes the interfacial transition zone morphology without significant effects on resistance. The combination of both strategies partially compensates the negative impact of the silane admixture, especially when the mortars are cured at high humidity. Thus, this combination increases versatility of the mortars and poses as a potential route to address the limitations of silane admixtures.
1. Introduction
The efforts to preserve the aesthetical and structural integrity of architectural heritage, along with the interest to protect and increase the service life of new buildings and infrastructures from the environmental factors that accelerate the degradation of their constituent materials, has led to the development of different treatments to preserve/restore the material integrity and decrease the ingress of aggressive agents (e.g. water/moisture, chlorides, sulfates, CO2) responsible of their decay [1]. Examples of such treatments include, but are not limited to, repair mortars, resin-based coatings, nanoparticle dispersions or alkaline silicates [2]. One of the most common solutions to delay weathering-related damages is the application of hydrophobic impregnation treatments based on silanes, siloxanes or their combination, which are able to penetrate into the pore structure, creating a protective barrier on the surface and external layers of the material [2]. Such treatments are used with positive results on building elements from different materials, such as natural stone and manmade materials including concrete and Portland cement mortars. The ability of these treatments to interact with the substrate, by creation of Si-O-Si bonds [3] or reaction with the cement phases [4,5], promotes a high chemical compatibility and decreased susceptibility towards aggressive environmental conditions.
* Corresponding author.
** Corresponding author.
E-mail address: [email protected] (R. Zarzuela S´anchez).
Contents lists available at ScienceDirect
Journal of Building Engineering
journal homepage: www.elsevier.com/locate/jobe
https://doi.org/10.1016/j.jobe.2022.105771
Received 21 October 2022; Received in revised form 9 December 2022; Accepted 19 December 2022
The effectiveness and durability of such treatments depends, among other factors, on the capacity to penetrate inside the material, the strength of product-substrate interactions and the physical, chemical or mechanical properties of the reaction products (e.g. surface energy, UV resistance, abrasion resistance, drying capillary pressure) [6]. Therefore, a successful treatment should consider—aside from the nature of the substrate—the application methodology and environmental conditions which may affect the aforementioned factors. When the barrier created by the surface treatment is damaged (e.g. cracking during drying due to capillary pressure, weathering, thermal expansion) [6], preferential entry pathways for the decay agents may be created, which reduce the treatment effectiveness and, in some cases, decrease their adhesion to the substrate [7]. Extending the protective effect—beyond the superficial layers—through the materials’ bulk would contribute to increase durability of these treatments and service life of the structures.
Although this may be challenging for already existing structures—as it should require replacing entire building elements—control of the mix design and use of admixtures is a common strategy to produce weathering-resistant concrete and mortars. Thus, multiple studies have been dedicated to evaluate the effectiveness of different hydrophobic admixtures (e.g. stearates, oleates, waxes, silanes, polymeric fibers) and their interaction with cementitious materials (concrete, mortars and cement paste) [8–11]. While some of these admixtures generally show positive results in terms of reducing water ingress, chloride penetration and moisture retention, these properties can be inconsistent depending on factors such as substrate composition (i.e., chemical interactions), curing conditions or homogeneity of their distribution (especially when the admixture is not water soluble). Moreover, most admixtures show a dose-dependent effect on the physical (e.g. porosity, permeability) and mechanical properties of the materials [12–14], often involving increased porosity, a lower degree of hydration and poor aggregate-matrix cohesion. Therefore, these factors should be considered to ensure that the integrity and durability of the structures are not compromised. Different strategies to compensate these effects are reported in literature, including the addition of components with pozzolanic activity (e.g. alkoxysilanes, nano-silica) [11] or the immobilization of the hydrophobic agent on a mineral substrate to avoid interference with cement hydration reactions. Zhang et al.
[15] report the use of siloxane-functionalized mica powder which can simultaneously promote the hydration reactions (i.e. acting as a SiO2 source for pozzolanic reactions) and refine the pore structure of the cured concrete.
An alternative strategy to produce hydrophobic concrete in bulk consists on the use of hydrophobic aggregates, either by partial substitution of the sand by inherently hydrophobic materials (e.g. recycled rubber tire) [16–18] or by pretreating the aggregate with a hydrophobic agent [19]. Di Mundo [16] et al. studied the hydrophobization of cement mortars by partial or total substitution of the sand with recycled tire rubber of different granulometries, obtaining a homogeneous hydrophobic behavior through the bulk. The lower resistance of rubber and decreased interaction with the cement matrix, however, significantly reduces compressive strength of the mortar, restricting the application range of this strategy to the production of non-structural lightweight materials. As observed by other authors [17,18], pre-treatment of the rubber with coupling agents or the combination with reinforcement materials can mitigate this effect. A different approach is reported by Wang et al. [19], based on pre-treatment of the fine aggregate and cement powder with a fluorinated alkylsilane. This allowed producing mortars with all-dimensional superhydrophobicity and a high freeze-thaw resistance, although the hydration rate and compressive strength are compromised due to coating of the cement grains. Therefore, the addition of components that promote either the formation of hydration products or the interaction with the aggregate and cementitious matrix could potentially compensate the aforementioned negative effects.
The composition of concrete and cement mortars is mostly represented by the cement matrix (formed by hydration reactions of anhydrous phases) and the aggregates (fine and coarse), usually siliceous or carbonatic [20]. A current trend in the protection of cementitious materials concerns promoting the chemical interactions of the hydrophobic components with the aggregate and main phases of the cement matrix (i.e. portlandite, calcium silicate hydrate gel, ettringite), which is the main contributor to the mechanical resistances [5]. Aside from the intrinsic properties of the matrix, the porosity created at the cement-aggregate interface and at the C-S-H gel itself—both of them factors that can be altered by hydrophobic admixtures—can have a high impact on durability and mechanical performance [20,21]. In terms of chemical interactions, siloxanes, silanes and related compounds hydrolyze forming reactive silanols that can form covalent Si-O-Si or Si-O-Al bonds [3] with the siliceous aggregates and silicate/aluminosilicate phases present in the hydrated cement paste. Furthermore, different authors have demonstrated that alkoxysilanes (a common component of commercial hydrophobic and consolidant formulations) are able to interact with cementitious materials through mechanisms different from the hydrolysis-condensation reactions [4,22]. In the presence of portlandite [5,23], the alkoxysilanes behave in a similar manner to pozzolanic reactions producing C-S-H gel. García-Lodeiro et al. studied the interaction of alkoxysilanes with different hydrated cement phases [4] and observed that, under the strongly basic conditions of the cement pore solution, the Si-O units can be incor- porated into the silica chains of C-S-H and react with Al-containing phases (ettringite, katoite, monocarboaluminate) to produce semi-crystalline aluminosilicate gels. The relation between these effects and the consolidating properties of ethyl silicate (TEOS) has been addressed by various authors [23–25], although it cannot be discriminated from the effect of SiO2 auto-condensation under experimental application conditions.
The interaction of silanes or siloxanes with the cement when used as admixture, however, is more complex because of the multiple simultaneous—often competing—processes occurring in the material, including: reactions with the newly formed hydration phases, auto-condensation to form silica gel, interaction with the anhydrous phases, changes in surface tension/wettability of the cement grains. Several authors have investigated the effect of alkoxysilanes and related compounds over the hydration kinetics, reaction products and mechanical resistance of the cement paste, with conflicting results due to the high dependence of the reactions on experimental conditions. Cai et al. [26]. reported that TEOS delays the hydration of cement paste, attributing the effect to the con- sumption of water during its hydrolysis. Kong et al. [27], on the other hand, observed a similar delay when adding alkylalkoxysilanes, whereas TEOS led only to a decrease in the heat of hydration at 120 h. The higher effect of the former was attributed to their adsorption on the cement particles hindering the access to water. In line with these explanations, Feng et al. [28] reported that adding the silanes in the form of oligomers or nanoparticles diminished the delay of hydration processes, although other works suggest that
pre-hydrolysis may also lead to a higher degree of adsorption on the incipient Ca(OH)2 and C-S-H and obstruct access to water during the induction period [10]. Lodeiro et al. [29] found a similar decrease in heat of hydration and delay of the induction period after the addition of oligomeric alkoxysilanes, although they also found evidences of accelerated early hydration reactions depending on the dosage. Although this effect over hydration could delay the development of mechanical strengths, some authors observed a densifi- cation of the structure and increased flexural strength due to the reaction of the alkoxysilane with the incipient portlandite, which yields C-S-H gel [10,28]. This effect, however, depends on the composition of the alkoxysilane and may be offset by the delay of cement hydration, especially when hydrophobic akylalkoxysilanes are used [10,30].
Taking into consideration the shortcomings and possible interactions of silane-based hydrophobic admixtures with cementitious material, the present work explores a route to improve the homogeneity and effectiveness of the hydrophobic properties in bulk, by combining two strategies: (1) induction of an independent hydrophobic character on the fine aggregate by functionalization with a linear perfluoropolyether possessing reactive alkoxysilyl-groups on both ends. (2) Incorporation of a silane/siloxane admixture containing hydrophobic agents, an oligomeric ethoxysilane and nano-silica to promote the formation of hydration phases, and a surfactant to increase miscibility with water. A comprehensive characterization of the mortars chemical, physical, mechanical and hydric properties was carried out considering the effect of different curing conditions for both standalone strategies and their combination.
2. Materials and methods 2.1. Fabrication of the cement mortars
The different Portland cement mortars under study were prepared by substitution of the aggregate by a pre-treated sand (via hydrophobic surface functionalization), addition of a hydrophobic alkylsilane/siloxane sol as admixture or a combination of both.
2.1.1. Synthesis of the hydrophobic sol
The hydrophobic sol (henceforth referred as PrPS) was synthetized via an ultrasound assisted sol-gel route. The reaction mixture contains 44.90% v/v of an oligomeric tetraethoxysilane (TES40 WN, from Wacker®), 44.90% v/v propyltriethoxysilane (PTEO, Dynasylan® PTEO from Evonik), 9.98% v/v polydimethylsiloxane (PDMS, Wacker® Finish WS 62 M), 0.08% v/v n-octylamine (98%, from Sigma-Aldrich), 0.15% v/v de-ionized H2O and 20 g/l of fumed SiO2 (Aerosil® OX50, from Evonik). All the components were mixed for a total volume of 100 ml and sonicated using an ultrasound probe (Sonopuls HD3200, from Bandelin) working at 74%
amplitude for 10 min. The reaction vessel was refrigerated in a water bath during sonication.
TES 40 is a mixture of monomeric and oligomeric ethoxysilanes with an average chain length of 5 Si-O units and a 41% SiO2 content after hydrolysis. The PDMS employed is a hydroxyl terminated polymer with an average chain length of 58 and a viscosity of 60 mPa⋅s.
The fumed silica (OX50), added as a silica source, consist of 40–50 nm size SiO2 nanoparticles with a specific surface of 50 m2/g.
2.1.2. Hydrophobic functionalization of the aggregate
The hydrophobic fine aggregate (henceforth referred as FA) was prepared by surface treatment of CEN standard river sand with a triethoxysilane-terminated perfluoropolyether, whose structure is represented in Fig. 1. For this purpose, a pre-hydrolyzed polymer sol was prepared by mixing 94% w/w iso-propanol, 1% w/w fluoropolymer (Fluorolink® S10, from Solvay), 4% w/w de-ionized H2O and 1% w/w acetic acid. The components were mixed under mechanical stirring for 30 min, after which the mixture became translucent.
Afterwards, the sand was manually mixed with the fluoropolymer sol in a 640 g per 100 ml proportion for 90 s. The impregnated sand was spread on a plate and cured in a stove at 40◦C during 5 days to allow reaction of the fluoropolymer with the sand surface by condensation of the Si-OH from the hydrolyzed fluoropolymer with surface hydroxyl groups of quartz. Prior to its use in mortar fabrication, the hydrophobized sand was left to equilibrate in laboratory conditions (20◦C, 40% RH) for an additional 2 days.
2.1.3. Mortar specimens preparation
Four different mortar sets were prepared by modifying the hydrophobic components while keeping w/c and c/s ratio constant. The first set is a conventional OPC mortar used as reference (RA), the second set was prepared by substituting the conventional sand with the functionalized aggregate described in 2.1.2. (FA), the third set was prepared by incorporating the hydrophobic sol described in 2.1.1. in a 2% w/w proportion respect to the cement (PrRA), the fourth set was prepared by combining the use of the hydrophobic aggregate and addition of the hydrophobic sol (PrFA). All the mortars were prepared using type II Portland cement (CEM II/B-L32.5R) and tap water, with a w/c ratio of 0.52 and a 2.5:1 sand:cement proportion.
Fig. 1. Molecular structure of the fluoropolymer Fluorolink® S10.
For the different mortar characterization and performance tests, cylindrical and prismatic specimens were manufactured. Table 1 specifies the tests and dimensions of the specimens used.
2.2. Characterization of the mortars
2.2.1. Chemical and structural characterization
The chemical interaction of the hydrophobic components with those used for mortar fabrication, and their effect over surface properties can modify the behavior during mixing, the physical or structural properties of the mortars (e.g., color, density, porosity) and promote changes in the cement hydration kinetics and reaction products. For this reason, different tests were carried out to characterize the fresh paste, components and cured mortars.
In order to check if the admixture or surface functionalization of the aggregate changes the paste rheological properties (i.e. its workability), a qualitative assessment was carried out using the mini-slump test, adapting the methodology described by other authors [31]. This test allows a comparison of the paste workability by observing the spread radius of a fresh paste. The experimental pro- cedure was as follows: (1) the pastes were mixed as described in section 2.1.3. (2) 160 ml of the paste were poured into a truncated conical mold (Ø1 =62 mm, Ø2 =46 mm, h = 85 mm) and placed over a wet glass surface. (3) The paste was left to equilibrate for 60 s, after which the mold was lifted. (2) After 4 and 8 min, the diameter of the slump was measured.
Changes in surface tension of the cement pore solution after addition of the hydrophobic sol may affect the entrapment of air, which influences the rheology of the fresh paste [32] and porosity of the final material after curing. For the measurements, a simulated pore solution was prepared by mixing 1.72 g/L of CaSO4 ⋅ 2H2O, 6.959 g/L of Na2SO4, 4.757 g/L of K2SO4 and 7.12 g/L of KOH and the PrPS sol was added in a 3.3% proportion (corresponding to the nominal water/admixture ratio in the paste). Total surface tension was determined by the sessile drop method, based on shape analysis of the droplet applying the Young-Laplace equation, using a software-controlled OCA 15 plus apparatus (software ver. SCA – 22) from Data Physics.
Modifications to the cement hydration products in the mortars cured for 28 days were studied by X-ray diffraction (XRD), FTIR spectroscopy and surface free energy measurements of powder samples extracted from the mortar (henceforth referred as OPC powders). To minimize the interference of the quartz present in the aggregate, the samples were ground with a rubber cap and passed through a Ø = 0.1 mm sieve. In order to study chemical interactions and the composition of the mortars, the OPC powders were analyzed by attenuated total reflectance FTIR spectroscopy, using a Shimadzu IRAffinity-1S equipment equipped with a MIRacle10 ATR module from Pike Technologies. The spectra were registered, directly on the powders, in the 4000-650 cm−1 range with a res- olution of 4 cm−1. The X-ray diffraction (XRD) patterns were recorded with an X-ray powder D8 Advance diffractometer from Bruker, equipped with a secondary monochromator, Cu tube X-ray, using Cu Kα radiation. Quantitative analysis of the crystalline phases was determined using Profex software, a graphical user interface for Rietveld refinement program BGMN [33]. Surface energy of the OPC powders, which is related to the hydrophobic properties, was determined by the sessile drop method using two different probe liquids, water and CH2I2. Surface energy, along with its polar and dispersive components, were calculated from the static contact angles of 2 μl droplets deposited over the powders according to the Owens, Wendt, Rabel and Kaelble method [34,35]. In order to reduce the in- fluence of surface roughness and fast absorption of the droplets into the sample, the powders were pressed into pellets at 210 MPa [32], and the angles were registered 1 s after contact with the surface. The same methodology was used to determine surface energy of the aggregate before and after surface modification with fluoropolymer.
Porosity and density of the materials are related parameters which indicate compaction of the mortars. Bulk density was calculated from the bulk volume, measured with a caliper, and weight of the mortar specimens. Total open porosity of the mortars was calculated from their skeletal volume, measured using a helium pycnometer (Ultrapyc 1200e from Quantachrome Instruments). The helium pycnometer determines the skeletal volume by measuring the pressure change of helium in a cell with a calibrated volume. Open porosity was calculated using equation (1). Measurements were performed directly by placing the cylindrical mortar specimens in the Table 1
Dimensions of the mortar samples fabricated for the different tests.
Test Dimensions
Static contact angle φ =90 mm, H = 10 mm
Water absorption by capillarity 40 mm × 40 mm x 40 mm
Water vapor permeabilitya Moisture content Drying rate
Water accessible porosity Ultrasound Pulse Velocity
Open porosity (Helium pycnometer) φ =32 mm, H = 25 mm
Density (bulk and skeletal)
Drilling Resistance Measurement System (DRMS)
Abrasion Resistance φ =90, H = 10 mm
Impact resistance 50 mm × 50 mm x 10 mm
Hardness (Vickers) Color measurements
aThe specimens for this test were cut to 40 mm × 40 mm ×10 mm.
cell. Prior to the analysis, the specimens were dried at 60 ◦C until constant weight, equilibrated in a desiccator at 20◦C for 48 h and purged by passing a He current for 4 min trhough the sample cell.
%Porosity =Vbulk− Vskel
Vbulk
⋅ 100 Eq.1
Water accessible porosity of the mortars was determined by measuring the saturation water absorption after immersion in water.
For this purpose, the dried and equilibrated mortars were submerged in de-ionized H2O at 20◦C and their weight was monitored until variation within 24 h intervals was below 0.1%, considered as saturation. Water accessible porosity (expressed as %) is calculated as the volume of water absorbed, calculated from H2O density at 20◦C, divided by the bulk volume. Color variation of the hydrophobic mortars respect to the reference may indicate interaction between the components, and can be a limiting factor for applications where aesthetical aspect of fair-faced elements is considered. Total color difference was determined from the color coordinates in the CIE L*a*b* space (observer at 10◦, illuminant D65) [36], using a ColorFlex spectrophotometer-colorimeter (from Hunterlab). All the mortar specimens were equilibrated for 5 days at 20 ◦C and 40% RH prior to the measurement.
Morphology of the phases and structure of the mortars (i.e., compactness, cement matrix-aggregate interaction) was studied through scanning electron microscopy (SEM) of their cross-section, using a Nova NanoSem 450 equipment (from FEI) working in secondary electrons mode at an acceleration voltage of 3 kV. Cross-section samples were obtained by cleaving the mortar specimens with a chisel, and were sputtered with a 5 nm Au layer before observation.
2.2.2. Hydric properties
The wetting behavior of the mortars was determined through the sessile drop method, measuring the static contact angles (SCA) of 5 μl water droplets deposited over the mortar surface. The SCA were registered using the software-controlled video analyzer previously described. A minimum of 5 droplets per specimen were used to calculate the average SCA.
Protection against water ingress inside the mortars was evaluated via a capillary water absorption test (modification of standard ASTM C1403 – 15). Prior to the test, the mortar specimens were dried at 40 ◦C until constant weight and stored in a desiccator for 2 days at 20◦C to equilibrate. A single face of the specimens was put in contact with de-ionized water, placing a soaked filter paper in- between to avoid contact with the lateral faces. Weight increase due to water absorption was registered at regular intervals up to a total time of 48 h. Absorption coefficient (WAC) was calculated as the slope of the weight of water absorbed per area unit vs time1/2 in the linear interval. Total water absorption (TWU) was calculated as the % water absorbed respect to dry weight after 48 h.
In addition to the capillary absorption, water vapor transport properties inside the mortars are important to account for, as a low permeability decreases susceptibility to carbonation but can also lead to deterioration if moisture cannot exit the pore structure (e.g., mold growth, staining, weakening of the material). To this end, the mortars were evaluated for their vapor permeability and their drying properties. Water vapor permeability was determined through the wet cup method described in ASTM E96 standard. The average permeability in metric system units was calculated from the slope of the weight loss per area vs time (WVP in g⋅s−1⋅m−2) plot, according to equations (2) and (3).
Permeance = WVP
S ⋅ (R1− R2) Eq.2
Permeability = Permeance ⋅ h Eq.3
where S is the saturation vapor pressure at test temperature (2,986.4 Pa at 24◦C), R1 is the relative humidity inside the cup (expressed as fraction), R2 is the relative humidity in the test chamber (expressed as fraction), and h is the specimen thickness in cm.
Drying properties of the mortars were evaluated according to a modification of UNE-EN 16322 standard. The mortar specimens were immersed in de-ionized water until saturation (7 days) and left to dry in a controlled atmosphere (23◦C, 50% RH), regularly registering their weight during 10 days. The first drying coefficient (D1) was calculated as the slope of the residual water content Mi (kg/m2) vs time plot during the first 7 h. The second drying coefficient was calculated as the slope of the Mi (kg/m2) vs √t (h−1/2) plot in the linear region. Where:
Mi=mi− mf
A Eq.4
mi: specimen weight at t = i hours; mf: specimen weight at the end of the test; A: exposed area (m2) tf: final time.
In order to quantify the effect of the hydrophobic components over drying rate while accounting for the differences in initial water absorption, drying index after 48 h (corresponding to the time at which the reference mortars lost >80% of the absorbed water) was calculated from the curves through equation (5).
DI =
∫t=48
0
Midt
Mt=48⋅ 48 Eq.5
2.2.3. Mechanical properties
In order to determine whether the hydrophobic admixture or modification of the aggregate alter the mechanical properties of the cured mortars, two sets of assays were performed. The first set accounts for the surface resistance (Vickers Hardness and abrasion resistance). The second set aims to evaluate the mortar cohesiveness and internal resistance (ultrasound transmission, drilling resis- tance and impact resistance). All mechanical tests were performed after curing the mortars for 28 days, followed by drying in a stove at
40◦C until constant weight and equilibration at room conditions for 2 days.
Vickers hardness was measured on the surfaces in contact with the mold using a universal durometer Centaur RB2/200, with an indentation force of 45 kgf and a 15 s dwell time. A minimum of 10 indentations per specimen were used to calculate the average hardness from the indentation size. The abrasion test was performed according to the parameters described in ASTM C1803-20 standard using a TABER® Rotary Platform Abrasion Tester Model 5135 and H-22 abrading wheels. The specimens were tested under a 1 kg load and the weight loss was registered every 100 cycles until a total of 1,000 cycles.
The drilling resistance test measures the variations in shear force required for a drill bit, spinning at a set rpm, to penetrate through a material and is related to the cohesiveness inside the mortar structure. The test was carried out using a Drilling Resistance Mea- surement System (DRMS) from Synt Technology, equipped with Ø = 5 mm flat tip diamond coated drill bits. The test parameters were:
spin rate 900 rpm and penetration rate 5 mm min−1. A minimum of 5 holes per specimen were used to calculate the average drilling resistance profiles.
In order to evaluate the impact resistance of the mortars, 50 × 50 × 10 mm tiles were placed on a sand bed and a 0.5 kg steel spherical weight was dropped on their center at increasing height intervals (5 cm) until material failure occurred. Impact resistance (L) in Joules was defined as the energy required to produce failure of the material. The values were normalized by specimen thickness to account for variations during mortar fabrication (z = 1.00 ± 0.15 cm), according to equation (6).
L∗=m ⋅ g ⋅ h ⋅1 cm
z Eq.6
where m is the sphere weight in kg, g is the gravitational acceleration in m⋅s−2, h is the drop height in m, z is the specimen thickness in cm and 1 is a correction factor (average thickness) in cm.
Ultrasound pulse transmission speed of the mortars was measured following the direct transmission method described in UNE-EN 12504-4 standard. For each mortar specimen, the speed was measured through the three axes of the prism using an Ultrasonic Tester BP7 (UltraTest GmbH). The ultrasound measurements were performed at different curing times to monitor the process. After curing for 28 days, the specimens were dried to constant weight and their elastic modulus was estimated according to eq. (7) [37].
Ep= (1 + vp
)(1 − 2vp
) (1 − vp
) (γ / g)Vp2 Eq.7
where Ep is the dynamic elastic modulus in GPa, vp is the Poisson ratio (assumed 0.22 as typical value in mortars), γ/g is the density in kg/m3, and Vp is the ultrasound pulse velocity in km/s.
Fig. 2. FTIR spectra of the: A) unmodified aggregate and B) fluoropolymer-modified aggregate. C) Wetting behavior of the aggregates using dyed water drops. D) Surface energy of the aggregates and its components calculated using the OWRK model.
3. Results and discussion
3.1. Chemical and structural characterization 3.1.1. Characterization of the components and paste
Functionalization of the aggregate surface with fluoropolymer is evidenced by the apparition of its characteristic bands in the FTIR spectra (Fig. 2A). The bare aggregate presented the characteristic quartz bands, namely: an intense Si-O asymmetrical stretching band at 1080 cm−1, a double band at ~778-790 cm−1 associated to Si-O symmetrical stretching, the symmetrical Si-O-Si bending signal at 690 cm−1 and the asymmetrical Si-O-Si bending Si-O rocking at 457 cm-1 [38], aside from the wide O-H band at ~3600 cm−1, attributable to adsorbed water and/or terminal Si-OH. After functionalization, several signals from the fluoropolymer structure are detected, including the secondary amide C=O band 1640 cm−1, H-C-H bending vibrations at ~1440 cm-1, C-F bands at ~1380 cm-1 and an increase of the 3500 cm−1 band intensity that can be attributed to O-H from the hydrolyzed ethoxysilane moieties and/or the N-H from the secondary amide. Additionally, shoulders appear on the main Si-O band around 1110 and 1000-1020 cm−1, which likely results from overlapping with characteristic signals from the fluoropolymer (FTIR spectrum of fluoropolymer is available in supporting information, Fig. S1).
Further evidence of fluoropolymer presence on the quartz surface is found on the hydrophobic behavior of the modified aggregate and the marked decrease in surface energy. As it can be observed in Fig. 2C, the water droplets quickly wet the bare aggregate surface due to its hydrophilic nature. After functionalization with fluoropolymer, the water droplets take a spherical shape, which is indicative of a hydrophobic behavior (contact angle >90◦). This property can be explained considering the surface energy values presented in Fig. 2D, as the Young-Dupr´e equation [39] predicts that the contact angle of water with any solid will increase for lower surface energies.
cos θ =γsolid− air− γsolid− water
γwater− air Eq.8
The unmodified aggregate has a surface energy of 50 mN/m, with a high contribution (40%) of the polar component due to the presence of Si-OH groups, similar to the values reported by other authors using the sessile drop method [40]. After functionalization, surface energy sharply drops to ~15 mN/m, and the polar component practically disappears. Due to their aliphatic chains and the substitution of H for fluorine atoms, fluoropolymers are characterized by extremely low surface energies (<20 mN/m), which is the reason why they are extensively used in hydrophobic and oleophobic coatings [41]. It should be noted that the calculated surface energies may be subject to inaccuracies, as the H2O and CH2I2 contact angles used for the calculation are affected by roughness of the pressed pellet surface and the model assumes an ideal flat surface.
Workability of the freshly mixed pastes after incorporating the PrPS admixture or the fluoropolymer-modified aggregate varies respect to the reference composition (RA), as evidenced by slump diameters after 4 and 8 min (Table 2). Addition of PrPS admixture (PrRA) significantly increases the paste fluidity, leading to a larger slump diameter after 4 min. Similar behaviors have been observed in previous works [11,42] and by other authors [43,44] in concrete mixes incorporating silane and/or siloxane admixtures, which can be attributed to an increased content of air bubbles in the paste due to changes in surface energy of the mixing water (i.e., the admixture acts as a surfactant). Aside from qualitative observations (i.e., more bubbles were observed in the paste with PrPS), this effect was verified by measuring the surface tension of simulated pore solutions with and without the admixture. Specifically, surface tension of the simulated pore solution decreased from 69.6 mN/m to 58.6 mN/m after addition of PrPS admixture (3.3% v/v), which may explain the stabilization of the air-water interfaces. Another mechanism that may contribute to their higher workability is related with the coating of the cement particles by the alkylalkoxysilane present in PrPS, which favors their dispersion due to steric hindrance between the alkyl chains, in a similar vein to the mechanism of plasticizers such as polycarboxylates [45].
On the other hand, workability of the pastes with fluoropolymer-modified aggregate (FA) was similar to the reference (RA), since the hydrophobic component is immobilized over the sand and unlikely to affect the water or cement particles. It is worth mentioning that, although workability of the paste is similar, the FA paste required more time to homogenize due to the lower interaction of the aggregate with the sand. The PrFA paste, containing the admixture and modified aggregate, showed an intermediate situation, with a slightly lower slump diameter compared to PrRA.
3.1.2. Characterization of the cured mortars
After curing for 28 days, the effect of the hydrophobic components over the cement hydration products at different conditions was studied by analyzing the composition (FTIR), mineralogy (XRD) and surface energy of the cement matrix samples.
The FTIR spectra (Fig. 3) shows the typical signals of the cement phases. The C=O bands at 1400, 870 and 712 cm−1 indicate the presence of calcite, resulting from carbonation of the cement matrix. These signals are more intense for the mortars cured at low Table 2
Slump diameter of the freshly mixed pastes after 4 and 8 min.
Ø (cm)
t(min) 4 8
RA 7.9 8.7
FA 7.9 8.2
PrRA 9.0 9.1
PrFA 8.6 8.8
humidity (RH 40%), in agreement with the XRD results (Table S1), probably due to their higher porosity leading to more exposure to CO2. In all spectra, the characteristic stretching S-O signals of ettringite appear as a shoulder at 1111-1114 cm−1. As also observed by XRD, this signal is more evident for the mortars cured at high humidity or by immersion. In a similar vein, the O-H band of portlandite at 3640 cm-1 [4,5] becomes more evident for the mortars cured under these conditions, evidencing their higher degree of hydration.
Under high humidity conditions the Si-O band group is centered at 960-970 cm−1, the characteristic Si-O stretching position of the Q2 units in the linear chains of C-S-H gel [5]. For the mortars cured at low humidity, the contributions at 1050-1080 cm−1, associated to reticulate SiO2 structures (e.g., formed by carbonation of the C-S-H), becomes more evident.
The differences in the spectra due to incorporation of the hydrophobic admixture or modified aggregate are generally subtle. The area ratio between the main C=O and Si-O bands (see Table 3) shows no remarkable differences for the mortars cured at high humidity, while for the curing at 40% RH, the reference mortar seems less affected by carbonation.
Peak analysis of the region of the spectra containing the Si-O signals (Fig. 3) reveals the presence of different peaks from the cement matrix: Al-OH bands at 834, 850 cm−1 (e.g. ettringite, monocarboaluminate) [46], C=O band at 873 cm−1, 913 cm−1 from Q1 Si-O of jennite-like C-S-H structure [47,48], 965 cm−1 from Q2 Si-O (C-S-H), 1011 cm−1 from long-chain Q2 Si-O, 1050–1080 and 1162 cm−1 from reticulated SiO2 structures and 1113 cm−1 from S-O (ettringite and gypsum) [4,5]. Table 4 shows the normalized area of the different contributions (excluding the signals below 873 cm−1) at varying compositions and curing conditions. Although the individual contributions fluctuate and it is difficult to establish a trend, the contribution of the Q2 Si-O signals grows for the mortars cured by immersion or high humidity. The ratio between the Q2 (965, 1011 cm−1) and reticulated SiO2 (1050, 1080, 1162 cm−1) signals clearly shows this trend, which can be related to the hydration degree indicated by XRD (Fig. 4). Another evidence of the effect of curing conditions is the position of the Q1 band, which in the case of the mortars cured at 40% RH shifts to 920 cm−1, suggesting the contribution of Q0 Si-O signals form the anhydrous phases alite and belite (characteristic signal at 935 cm−1). For the PrRA mortars, the Q2/SiO2 ratio is lower than the reference (RA), which can be attributed to the formation of an amorphous silica xerogel by auto-condensation. The higher ratio observed for PrRA cured under immersion, along with the higher amount of C-S-H detected by XRD (Fig. 4), suggest that the reaction of the admixture with the cement matrix is more favored or the SiO2 xerogel undergoes pozzolanic reactions.
The presence of the hydrophobic aggregate seems to have a minor influence by itself, as the FA and RA mortars show similar spectra. However, when combined with the hydrophobic admixture, it seems to compensate the drop in the Q2/SiO2 area ratio. This result is in agreement with the observations by XRD of the crystalline hydrated phases and unreacted anhydrous phases (Fig. 4) and may be associated to a higher effective water/cement ratio due to the lower water adsorption of the aggregate or interactions with the admixture, though further studies would be required to determine the mechanisms.
The major crystalline phases in the cement matrix powders extracted from the mortars can be identified in the diffractograms, available in supporting information (Fig. S2). The main signals of the anhydrous phases, corresponding to alite (C3S), belite (C2S), ferrite (C4F) and tricalcium aluminate (C3A) [49], decrease respect to cement, evidencing the progress of hydration reactions. The relatively large contribution of calcite may be overestimated due to the high crystallinity of calcium carbonate in comparison with the poor crystallinity of other cementitious phases. Along with the consumption of anhydrous cement phases the mortar curing is
Fig. 3. FTIR-ATR spectra of cement matrix sampled from the mortars cured at A) RH 40%, B) RH 90%, C) immersion.
evidenced by the apparition of signals corresponding to the hydration products. The main contributions correspond to portlandite (2θ
=18◦) and ettringite (2θ = 9◦). In addition, a wide signal appears at 2θ = 29◦, which can be attributed to the semi-crystalline structure C-S-H gel. As described by other authors [50], the structure of C-S-H gel is close to a poorly crystalline tobermorite. It should be noted that the presence of quartz signals corresponds to remnants of the aggregate that could not be completely separated from the matrix, however this signal and their quantification results are overestimated in this technique due to the low crystallinity of cement phases.
The same happens with calcite, most calcium carbonate produced during cement carbonation is in form of calcite that gives signal Table 3
Area ratio between the carbonate band and the Si-O band group in the FTIR spectra.
AC=O/ASi-O
RH 40% RH 90% Immersion
RA 1.5 1.2 1.2
FA 1.9 1.1 1.1
PrRA 2.0 1.1 1.3
PrFA 2.0 1.1 1.3
Table 4
Normalized area calculated by peak analysis of the νSi-O signals on the FTIR spectra.
RA 40% RA 90% RA
I FA 40% FA 90% FA
I Pr RA 40% Pr RA 90% Pr RA
I Pr FA
40% Pr FA 90% Pr FA I
913 Si-O (Q1) 3% 6% 3% 3% 4% 7% 1% 6% 7% 4% 4% 6%
965 Si-O (Q2) 25% 34% 39% 26% 47% 33% 14% 36% 33% 20% 42% 35%
1011 Si-O (Q2) 18% 25% 15% 19% 17% 26% 13% 17% 22% 18% 17% 18%
1050 Si-O (Q3,Q4) 16% 12% 12% 14% 10% 8% 27% 15% 12% 20% 13% 15%
1082 Si-O (Q3,Q4) 9% 5% 6% 11% 6% 5% 9% 4% 5% 4% 4% 3%
1113 S-O 21% 16% 21% 19% 14% 19% 22% 20% 20% 27% 18% 20%
1162 Si-O(Q3,Q4) 8% 2% 4% 8% 2% 2% 14% 3% 2% 7% 2% 3%
AQ2/AQ3,4 1.30 3.17 2.44 1.37 3.59 3.99 0.55 2.54 2.99 1.22 3.10 2.49
Fig. 4. Quantification of the crystalline phases present in the cement matrix, obtained by Rietveld refinement. (A) Ratio between major hydrated and anhydrous phases. (B–D) major hydrated phases. (E–G) major anhydrous phases. All the values were normalized in order to discard the remnant quartz from the aggregate.
more intense than other cement phases that are mainly amorphous.
In order to better understand the effect of the hydrophobic components over cement hydration processes, which ultimately affect compaction and mechanical strength of the mortars, quantification of crystalline phases by Rietveld analysis was carried out. In all cases, the contribution of quartz from the aggregate was excluded from the calculation. Fig. 4 shows the quantification results of the main hydrated and anhydrous phases; full quantification results including minor phases are available in supporting material (Table S1). In order to compare the degree of hydration between the different mortars, the ratio between the hydrated and anhydrous phases was calculated using the following equation, obtaining values (Fig. 4A) that followed a similar trend as that observed for Q2/ Q3,4 ratio calculated by FTIR.
Qhydrated
/
Qanhydrous=%Portlandite + %Ettringite + %Tobermorite
%C3S + %C2S + %C4F
For mortars of the same composition, the general trend confirms the favorable effect of water availability. Specifically, mortars cured under high humidity or immersion show an increased proportion of hydrated phases and a lower amount of anhydrous, which is in line with the FTIR analysis of the Si-O bands and portlandite (Table 4). Portlandite is the hydrated phase where the effect is more obvious, as it is formed during hydration of alite, the major phase detected in the cement. The formation of portlandite seems to be less favored for the mortars cured under immersion, which can be attributed to partial leaching/solubilization or the predominance of different hydration reactions.
It should be noted that quantification of the C-S-H (see Table S1) signal—which is related to the tobermorite structure—may be subject to errors due to its poor crystallinity. However, a clear upward trend was observed for all the samples following the order 40%
RH < 90% RH < immersion. Therefore, the higher water availability promotes two possible effects (or a combination of them): (1) hydration of C3S and C2S to form CSH. (2) The formed CSH under these conditions may have a higher crystallinity. In the case of the anhydrous phases, the effect of curing conditions is particularly visible for alite, which is only detected when cured at low humidity.
The trend fluctuates for the C2S and C4F, which represent a smaller proportion of the anhydrous cement and their hydration is associated to slower processes [51]. Anyways, the C2S proportion follows the expected trend (i.e., higher consumption with water availability) for the reference mortar (RA). The aforementioned trends are corroborated by the “hydration degree”, which is clearly lower for all the mortars cured at low humidity. The mortars without admixture (RA and FA) showed the same values for when cured at 90% and immersion, since the lower amount of portlandite of immersion samples is compensated by their higher amount of tober- morite. The amount of calcite (Table S1 in supporting material) is higher for the mortars cured at 40% RH, which can be linked to carbonation processes due to the higher porosity (Fig. 6 and Fig. S3), as also suggested by FTIR (Fig. 3).
The influence of the hydrophobic components is relatively low when they are immobilized on the aggregate (FA), especially when water availability is higher. Specifically, the amounts of portlandite are practically unchanged respect RA mortars when cured at 90%
RH or immersion, and the differences mainly arise from the minor phases, which may be attributed to localized differences at the
Fig. 5. A) Surface energy of the cementitious matrix calculated by OWRK method and their components, B) polar and C) dispersive.
matrix-aggregate interface, as evidenced by SEM micrographs (Fig. 7, S4 and S5), or variations of the effective water/cement ratio due to the lower water adsorption of the modified aggregate. On the other hand, the FA mortars cured at 40% seem to present a lower degree of hydration respect RA. However, considering that the consumption of anhydrous phases is similar in both mortars (except for a slightly higher %C2S in FA), this may be caused by an underestimation of the hydrated phases with poor crystallinity and the experimental error associated to their lower proportion.
On the other hand, the hydrophobic admixture (PrRA mortars) leads to evident changes of the hydration degree regardless of the curing conditions, which is expected considering it can freely interact with the cement grains and pore solution. Specifically, the overall hydration degree is lower due to a decrease in portlandite and increase in remnant alite, while the effect over ettringite, C2S or C4F depends on the curing conditions. Overall, it can be confirmed that the combined wt.% of remnant anhydrous increases respect to the mortar without admixture and the effect on ettringite formation is comparatively low. The components of the admixture may affect cement hydration by different simultaneous processes: (1) after hydrolysis in basic media, the hydrophobic component (PTEO) can coat the surface of the cement grains, hindering its contact with water and delaying the solubilization stage [26,27]. A similar effect has also been observed for ethoxysilanes (a component of the PrPS admixture) by other authors, although it is mitigated for its oligomeric form [28]. Related to this, in a previous study we observed that addition of the oligomeric ethoxysilane decreases heat of hydration at 7 days on Portland cement pastes [29] and delays the induction period. (2) The hydrolysis and auto-condensation re- actions of alkoxysilanes and related compounds consume water, and may compete with the initial stages of hydration. This effect may explain the higher impact over alite hydration for the mortars cured at 40% RH, where water availability is more limited. (3) On the other hand, the oligomeric ethoxysilane (and the nano-SiO2), may be able to interact with the incipient portlandite to form C-S-H gel [4]. In a recent work [29], we observed that the addition of the ethoxysilane to a cement paste promoted the early formation of C-S-H at the expense of portlandite precipitation. This mechanism is consistent with the increase of tobermorite content for all samples con- taining the hydrophobic admixture respect RA, although the %wt. of remnant anhydrous phases suggests that this effect is not the only cause of the drop in Portlandite.
The mortars combining the hydrophobic aggregate with the admixture (PrFA) show a similar phase composition to PrRA mortars but their hydration degree is higher. This effect may be related with the lower adsorption of water compared to the unmodified aggregate, which would slightly increase the effective water/cement ratio of the mixture. Comparing the three main hydration phases, PrFA showed higher portlandite and ettringite contents than PrRA, while tobermorite proportion was lower. As observed in a prior work, the oligomeric ethoxysilane might be able to react with the incipient ettringite [4]. Thus, the higher amount of this phase and portlandite, along with the decrease in tobermorite can be associated to a lower interaction between cement phases and ethoxysilane when the hydrophobic aggregate is incorporated. Regarding the amount of remnant anhydrous phases, the combined %wt. of C3S and C2S is lower for the PrFA mortars than for PrRA, suggesting that combination with the hydrophobic aggregate partially compensates the negative effects of the admixture on cement hydration. Further studies would be necessary to determine the mechanisms involved.
Surface energy of the cementitious matrix is a factor that heavily influences capillary water absorption of the mortars, as the Laplacian pressure that governs capillary flow depends on the surface tension balance in the water-pore wall-air interface [52].
Alkylalkoxysilane hydrophobic admixtures act by decreasing the surface energy of the material, either by coating the aggregate surface, modifying the cementitious phases (e.g. C-S-H gel) by incorporation of aliphatic chains in their structure [53] or forming amorphous organically modified silica gels by auto-condensation.
In a similar vein to the effect over cement hydration reactions, the surface energy values obtained for the RA and PrRA mortars Fig. 6. SEM micrographs showing the structure of the cementitious matrix and the matrix-aggregate interface of the mortars cured under 40% RH.
(Fig. 5A) confirm that the curing conditions modify the reaction products formed by the PrPS admixture. The reference mortar (RA), regardless of curing conditions, had a similar surface energy at 67–69 mN/m, which is consistent with the presence of the hydrophilic phases observed by XRD and FTIR, especially C-S-H gel which constitutes the major hydration product. Surface energy of PrRA was clearly lower when curing at 40% RH, with a decrease mainly on its polar component. This reduction is less marked for the mortars cured at 90% humidity and is barely significant when cured under immersion.
Under the strongly basic conditions of the cement pore solution, hydrolysis of the alkyl alkoxysilane units present in the admixture is a very fast process [54], and the hydrolyzed monomers may undergo auto-condensation, yielding a highly hydrophobic organically modified silica gel (with typical surface energies in the 25–30 mN/m range), or react with hydrated phases (e.g. portlandite, C-S-H gel) to form a C-S-H with a high Si content and intercalated organic chains [4]. At low humidity conditions, hydration of the cement progresses at a slower rate and auto-condensation of the hydrolyzed alkyl alkoxysilane monomers will be favored, which explains the significant reduction of surface energy. On the other hand, curing at high humidity conditions (or immersion) promotes the early formation of cement hydration phases, which will react with the alkyl alkoxysilane incorporating the monomers into their structure.
Even though the presence of alkyl chains may decrease surface energy of these phases, the effect seems to be low under the tested conditions. These explanations are supported by the FTIR spectra (Fig. 3), which only showed an increased contribution of the bands associated to reticulated silica gels (1050-1080 cm−1) for the PrRA mortars when cured at 40% humidity.
3.1.3. Physical and structural characterization of the mortars
The effect of the hydrophobic components over color of the mortars may be a limiting factor for applications where preserving consistent aesthetics of fair faced elements is a requirement. The obtained total color differences values (Table 5) allow to conclude that, in general terms, the aesthetic impact of the hydrophobic admixture or modified aggregate is minimal regardless of the curing conditions, as indicated by the ΔE* values below 3 that confirm the color difference is near the perception threshold [55]. The only remarkable ΔE* changes are observed for the PrFA mortars cured at 90% humidity or immersion in water, where the color is lighter than the reference mortar cured under the same conditions. Anyways, these differences are small considering the inherent variability of the reference mortar. Regarding the changes in color hue, only the b* coordinate follows a consistent trend, increasing for the mortars cured at higher humidity. At equal curing conditions, the b* coordinate is lower for the mortars containing the modified aggregate, which may be attributed to its lower water adsorption making the contribution of the sand to the yellow tones less noticeable.
The different effects of the hydrophobic components on the air entrapment and workability of the fresh paste, the hydration processes of cement and the internal structure (e.g., surface-aggregate interface, cracks, shrinkage) may affect the mortar porosity, which has a marked influence on the mechanical properties, susceptibility to weathering and its density.
Cross-sectional cuts of the mortars were analyzed by SEM in order to study the morphology of the hydration products in the cementitious matrix and the cohesion between matrix and aggregate (Fig. 7, S4 and S5). In agreement with the results obtained by FTIR and XRD, the SEM micrographs of the mortars cured at 40% RH (Fig. 6) evidence the formation of different phases in the mortars with admixture, aside from an effect on their compactness and interaction with the aggregate. The reference mortar (RA-40) presents a cementitious matrix with a compact aspect and an abundance of flake-like C-S-H gel structures and ettringite rods [56,57]. A similar structure is observed in the matrix of the FA mortars in terms of compactness and phase morphology, which coincides with the similar composition of the major phases detected by XRD. The only remarkable difference is the less defined structure of the C-S-H, which is likely related to localized variations in water availability [57], as the modified aggregate has a lower tendency to adsorb water. In contrast, the mortars prepared with the hydrophobic admixture (PrRA and PrFA) both presented a less compact cementitious matrix, which may be associated to the lower cement hydration rate suggested by the XRD results (Fig. 4), characterized by the presence of laminar structures, ettringite rods and poorly defined honeycomb-like structures attributable to a Si-rich C-S-H gel distorted due to the presence of alkyl chains [58]. The relative abundance of hexagonal plates characteristic of portlandite [56], which contrasts with the crystalline phase analysis by XRD, may be explained by the formation of a portlandite with poor crystallinity (non-detectable by XRD).
Regarding the cement matrix-aggregate interaction, the SEM micrographs of the reference mortar (RA) show a cohesive interfacial transition zone where the aggregate grains are covered by structures consistent with the hydration products (i.e., fiber-like C-S-H,
Table 5
CIE L*a*b* coordinates of the mortars and total color change respect to the reference mortar at each curing condition.
Specimen L* a* b* ΔE*
RA-40 77.9 − 0.2 5.0 n/a
FA-40 79.0 − 0.1 4.0 1.5
PR RA-40 78.9 − 0.3 4.9 1.0
PR FA-40 76.0 − 0.2 3.9 2.2
RA-90 80.4 0.2 6.8 n/a
FA-90 80.9 0.1 4.9 1.9
PR RA-90 82.7 0.0 6.0 2.4
PR FA-90 84.8 0.0 3.8 5.4
RA-I 83.8 0.4 7.6 n/a
FA-I 85.0 0.1 5.8 1.4
PR RA-I 85.8 0.4 7.7 3.4
PR FA-I 86.9 0.3 6.6 3.5
ettringite). The modified aggregate present in the FA mortar also shows a good interaction with the matrix, but the interfacial tran- sition zone presents a completely different structure, characterized by the presence of poorly defined laminar morphologies on both the grain surface and the interface. The structure on the grains can be associated to the presence of the fluoropolymer, which in turn may interact/co-polymerize with the matrix components (i.e., hydrated phases) or react with Si(OH)4 formed during solubilization of the anhydrous phase through its free terminal alkoxysilane functional groups. Incorporation of the hydrophobic admixture, as in PrRA mortars, leads to a less cohesive structure with visible voids between the aggregate and the cement matrix and a poor coverage of the aggregate surface with hydration products. As previously discussed, the XRD evidenced how the admixture hinders the solubilization and subsequent hydration of anhydrous cement phases. In addition to this effect, the alkylalkoxysilane present in the admixture is able to react with the Si-OH present in the aggregate (i.e., quartz) leading to an alkyl-terminated surface, which in turn decreases the ability of the aggregate to interact with the Si-OH and/or Al-OH groups present in the hydrated cement phases. However, when the hy- drophobic admixture is added in combination with the modified aggregate (PrFA mortar), the integration of the aggregate with the matrix is improved respect to the PrRA mortar and its surface is covered by phases including C-S-H and ettringite. Unlike the alky- lalkoxysilane present in the admixture, the fluoropolymer used to modify the aggregate possesses hydrolysable Si-O-C2H5 on both ends of its chain. These groups are able to interact with the cement phases or copolymerize with the hydrophobic admixture components, leading to a reticulated silica structure that may further react with the portlandite and other cementitious phases [4,5].
The SEM micrographs of the mortars cured at 90% humidity and immersion (available in supplementary material, Figs. S4 and S5) show an overall similar structure of the cement matrix respect to those cured at 40%RH, though under these conditions, ettringite is more abundant (in agreement with XRD results) and the C-S-H has a denser aspect with needle- and fiber-like structures, which typically form in conditions with a high water availability [59]. The mortars with hydrophobic admixture also possess the laminar structures observed for the material cured at 40% RH, intercalated with hydrated phases (ettringite and needle-like C-S-H). Regarding the interaction with the aggregate, the only remarkable differences are observed for the PrRA mortars, where the coverage of the aggregate grains with hydrated cement phases is higher than the same material cured under low humidity. The most likely explanation for this observation is the higher rate of cement hydration due to water availability.
The mortars cured under immersion (Fig. S5) display a C-S-H gel with an even more compact structure formed by small needles and/or fibers (Type I) and a relatively high amount of ettringite, which is consistent with the presence of excess water [57]. For the mortars containing the admixture (PrRA and PrFA), the presence of the laminar structures is less abundant than at other curing conditions. In a similar vein, the aggregate grains appear to have a higher coverage of needle-like (type I) and poorly defined honeycomb-like (type II) C-S-H structures. As previously discussed, part of the admixture may leach outside the fresh mortar under these curing conditions, which explains the higher similitude of the mortars respect their counterparts without admixture (RA and FA).
Consistently with the structure observed by SEM (Fig. 6, S4 and S5) and the degree of hydration hinted by XRD (Fig. 4), total porosity of the mortars (Fig. 7A) increases with The incorporation of the hydrophobic admixture (PrRA mortars) regardless of the curing conditions, which may be associated to different phenomena: (1) the decrease in surface tension promoted by the admixture stabilizes the air bubbles formed during mixing of cement paste, which may in turn become trapped after setting, as observed in other studies using silane or siloxane-based admixtures [11]. (2) As determined by XRD (Fig. 4), the hydrophobic admixture hinders cement hydration reactions, eventually leading to a less compact structure of the cementitious matrix. This effect on compaction is also suggested by the SEM images. The influence of this factor would explain the higher influence of the admixture on porosity of the mortars cured at 40% RH, where the effect on hydration is more remarkable. (3) The aggregate-cement matrix interface of the mortars containing the admixture shows larger voids between the phases (see SEM micrographs, Fig. 6) and a poor coverage of the sand grains.
On the other hand, incorporation of the modified aggregate (FA mortars) increases the mortar porosity, but to a lesser extent compared to the hydrophobic admixture. This increase is likely attributed to the different structures observed in the aggregate-matrix interface (Fig. 6), as the effect of the modified aggregate over hydration reactions is relatively low and the cementitious matrix itself presents a similarly compact structure compared to the reference mortar. Porosity of the PrFA mortars is overall intermediate between the PrRA and FA mortars, which is consistent with the combination of the admixture effects over cement hydration and the higher aggregate-matrix interaction observed for these mortars in comparison with PrFA.
Regarding the effect of curing over total porosity, the differences for the reference mortar are minimal and no obvious increase is Fig. 7. A) Total porosity, measured by gas pycnometry and B) Bulk density of the mortars.