Multifunctional silane-based superhydrophobic/impregnation treatments for concrete producing C-S-H gel: Validation on mockup specimens from European heritage structures

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Construction and Building Materials 367 (2023) 130258

Available online 4 January 2023

Multifunctional silane-based superhydrophobic/impregnation treatments for concrete producing C-S-H gel: Validation on mockup specimens from European heritage structures

Rafael Zarzuela

^a^,^*

, Manuel Luna

^a

, Jorge Gonzalez Coneo

^a

, Giada Gemelli

^a

, Dia Andreouli

^b

, Vasilis Kaloidas

^b

, María J. Mosquera

^a^,^*

aTEP-243 Nanomaterials Group, Department of Physical-Chemistry, Faculty of Sciences, University of Cadiz, 11510 Puerto Real, Spain

bMaterials Industrial Research and Technology Center (MIRTEC S.A.), Thiva Branch, Athens-Lamia National Road (76th km), P.O. Box 150, GR-32009 Schimatari, Greece

A B S T R A C T

Many of the concrete structures that conform our modern cultural heritage are in need of repair and protective interventions. Silane-based impregnation treatments can be used to repair onset cracks and reinforce the surface due to their ability to produce silica and C-S-H gels, and can be modified by incorporating hydrophobic precursors to create multifunctional treatments that also protect from water ingress.

Since the effectiveness of impregnation treatments is dependent on substrate properties and chemical-physical changes it may have experienced over time, validation using standard materials may not always be representative of on-site application. In this work, the effectiveness of three innovative silane-based impregnation treatments developed by our group (two of them combining superhydrophobic properties) was evaluated on mockup specimens, which simulate the properties of the cementitious materials from six different heritage structures across Europe, artificially aged to simulate weathering by three methods:

carbonation, chloride ingress and physical damages (freeze–thaw and thermal cycles).

The characterization of the treatments showed they are compatible in terms of chemical interaction, applicability and minimal aesthetical alterations. Surface resistance and ultrasound pulse measurements have been used to assess the improvement in mechanical properties. The incorporation of hydrophobic components and fumed silica has a relatively low impact over the mechanical properties while it significantly reduces water absorption and grants water repellent properties to the surface, giving rise to a superhydrophobic performance.

1. Introduction

The use of concrete and other Portland cement-based materials has shaped the architectural trends and construction techniques of the early 19th and 20th centuries [1]. Even though the versatility and advantages of these materials are evident, exposure to environmental agents is known to cause multiple pathologies to the structures [2–4], decreasing their service life and/or requiring regular maintenance and repair interventions. This is especially true for older concrete structures, as there are several non-environmental factors which may have accelerated their decay [5]: (1) knowledge of concrete chemistry and its decay mechanisms was limited, sometimes leading to suboptimal composition and fabrication techniques. (2) The use of fair-faced concrete was a popular trend, which increased the exposure of the structures to the environmental agents. (3) A lack of awareness about the cultural significance of concrete buildings have led to severe negligence in their conservation interventions.

Several of the most common decay phenomena affecting concrete structures are associated to water ingress in the pore structure from moisture, aerosols, rainfall, capillarity, etc. Water can directly affect the material through several mechanisms [2,6–8], including but not limited to: calcium leaching from the cementitious matrix, mechanical stress by freeze–thaw cycles or solubilization by acid rain. In addition to direct action, water acts as a vehicle for other decay agents [3,4,8,9], such as soluble sulphates and chlorides that cause chemical alterations or physical damages by crystallization, or biological agents responsible of biofouling. The ingress of chloride ions is specially harmful for reinforced concrete structures [4,10], since their presence accelerates corrosion of the steel rebars that can lead to detachments and cracks by volumetric expansion or even structural failure. Another environmental factor that affects concrete, especially in urban or industrial areas, is carbonation of the phases present in the cementitious matrix by reaction with CO2 [11]. In this process, components such as portlandite (Ca (OH)2) and calcium silicate hydrate react with atmospheric CO2 forming

* Corresponding authors.

E-mail addresses: [email protected] (R. Zarzuela), [email protected] (M.J. Mosquera).

Contents lists available at ScienceDirect

Construction and Building Materials

journal homepage: www.elsevier.com/locate/conbuildmat

https://doi.org/10.1016/j.conbuildmat.2022.130258

Received 15 September 2022; Received in revised form 25 November 2022; Accepted 27 December 2022

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a compact calcite structure. Even though this process actually increases hardness of the material, the lower pH causes de-passivation of the steel, eventually leading to corrosion of the rebars.

Many effective strategies to mitigate the effect of the aforementioned processes are related to a proper structural design (e.g. drainage systems, avoiding exposure to water runoffs…) or the choice of materials with a lower susceptibility [5,12,13]. To this end, it is of paramount importance to carefully design the concrete mix (i.e. cement and aggregate types, cement/water and cement/aggregate ratios, use of admixtures) taking into consideration the environmental factors and use of the building element for each case. As a general rule, water ingress and carbonation are minimized in compact concretes fabricated with a low water/cement ratio, though this increases production costs.

In the case of already existing structures, this strategy might be un- feasible or not cost-effective, as it would require major refurbishing and reconstruction. Therefore, the preferred conservation methods involve the application of different protective treatments over the concrete surface to delay the action of decay processes [14,15]. Surface treatments for concrete, according to European standard EN 1504–2, are classified in the following categories: coatings, impregnations and hydrophobic impregnations. Coatings (e.g. cementitious, epoxy or acrylic resins, paints) act by creating a continuous physical barrier over the material that prevents the ingress of harmful species in the pore structure. Although coatings may be the preferred option for protecting highly compact concrete [15], their main drawbacks are related a high dependence of their effectiveness on the coating homogeneity, limited durability compared to other treatments and their tendency to induce aesthetical changes, which may limit their use for application on his- torical or culturally significant built elements. Impregnations (e.g.

alkoxysilanes, sodium silicate, nanolime dispersions), on the other hand, act by penetrating into the pore network and forming a compact structure that enhances surface resistance, decreases or blocks the surface porosity and hinders water absorption and transport of aggressive species [14]. Generally, impregnations display a higher durability, though their effectiveness highly depends on their penetration capacity and chemical interactions with the substrate [16–20]. Hydrophobic impregnations (e.g. siloxanes, alkylsilanes), sometimes called pore lining treatments, act by coating the inner wall of the pore network, without blocking their access, and decreasing their surface energy to prevent capillary water absorption. Unlike waterproof coatings, hydrophobic impregnations have a low impact over the vapor permeability of the substrate, which is an advantage when drying properties must be preserved.

Among the different impregnation treatments, alkoxysilanes and related compounds are commonly employed due to their potential advantages [16,21], namely: (1) their low viscosity allows application by simple methods (e.g. spraying, brushing) and facilitates penetration in the substrate. (2) Their polymerization reactions occur at ambient conditions using moisture from the atmosphere or present in the substrate. (3) The reaction products of their polymerization are chemically compatible with components from the aggregates and the cementitious matrix. More specifically, the precursors can undergo auto-condensation [21,22], producing an amorphous silica xerogel capable of bonding to siliceous phases (quartz, C-S-H, etc…), or they can react with different phases from the cement matrix [23–26], yielding products with a similar composition. In a prior work [23], we demonstrated that these reactions include formation of C-S-H gel by reaction with portlandite, lengthening of the silica chains of C-S-H gel. Formation of amorphous aluminosilicate gels by reaction with Al-containing phases (e.g. ettringite, mono- carboaluminate, katoite) was also demonstrated in later works [24].

Another advantage of alkoxysilanes is their versatility, as the incorporation of precursors with specific moieties can be used to modify the functions of the treatment (e.g. using alkylsiloxanes to create organically modified silica with hydrophobic properties) or produce multifunctional treatments that combine different effects with the mechanical reinforcement of an impregnation. In a previous work, we modified a

surfactant-assisted synthesis route designed by our group [27] by adding alkylsiloxanes and nano-silica to create a multifunctional treatment with superhydrophobic properties. This route was later tuned, by considering a numerical modelling approach, in order to increase the penetration capacity of the sol in the fine pores of cementitious materials [28].

Specifically, composition of the starting sol was modified in order to reduce its viscosity, increase surface tension and, subsequently, increase its penetration.

Although the results obtained so far are promising, it is crucial to evaluate the treatments’ performance on different substrates since their effectiveness significantly depends [16–20,29] on the structural, mechanical and compositional (including weathering-induced changes) properties of the material. These aspects are of special importance for heritage buildings, where the standards for composition and specifica- tions of the concrete have changed depending on the time period and region, and materials properties have been altered after long-term weathering in contact with their environment. For these reasons, the results obtained in a laboratory setting may not always be representative and it is recommended to perform preliminary tests either ex situ (i.e.

sampling the original material) or in using mock-up samples with similar properties.

Although there are studies which compare the effectiveness of impregnation treatments on concrete and mortars with different mix designs (water:cement:aggregates ratio, admixtures) [30–33], weath- ered materials are more complex systems and the information available in scientific literature is limited [20,34]. Therefore, it is of special interest to understand how the common weathering processes of cementitious materials affect the performance of such treatments on different types of concrete. In this work, we evaluated three different products designed in our laboratory (one impregnation and two multifunctional with superhydrophobic properties) on artificially aged mockup specimens,representing the properties and weathering processes of six different case studies across Europe, selected as part of the InnovaCon- crete project [35]. The mockup specimens were prepared according to characterization data of samples extracted in situ [35–41] and artificially aged by different processes: carbonation, freeze–thaw cycles and chloride ingress.

2. Materials and methods

2.1. Preparation of the mockup specimens

Seven types of mockup specimens, representative of the composition and structural properties of the materials (concrete, mortar) employed in construction of the seven monuments selected as case studies for the InnovaConcrete European project [35–41] were prepared. These case studies were chosen as a representation of different (i) construction periods, (ii) climate and environmental conditions (i.e. urban, rural and coastal areas), (iii) conservation state (iv) decay origin and (v) characteristics and composition of the material. Fig. 1 shows the case studies and their locations.

The results of the previous characterization tests on the case studies [35–41] were used to prepare the mockups considering their: (i) material type (concrete, mortar), (ii) type of binder/cement and aggregates, (iii) water accessible porosity, (iv) resistance and causes of decay (see table S1 in supporting material). According to the above characteristics, seven different mix designs (see Table 1) were prepared. Detailed mix proportions used for mockups fabrication is available at supporting information (Table S3).

Two types of cement were used for preparing the different mockups (CEM II/A-LL 42.5 N and CEM II/B-LL 32.5 N). While modern cements may differ from those used in older constructions, a compromise was made due to unavailability or lack of information about composition of the latter. In order to meet dimensional limitations of the specimens according to the standard methods and used equipment (specimens’

dimensions not<3 times the nominal max. aggregate particle size),

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coarse aggregates smaller than 22 mm were employed. Two different types of coarse aggregate were employed, namely, a granitic aggregate (Granite-C) and a limestone aggregate containing calcite and a small amount of dolomite (Limestone-C and Limestone-M). Smaller size frac- tions of the same aggregates were used as fine aggregates (Granite-F and Limestone-F), as well as siliceous sand (Siliceous-F). Aggregate size distribution and main properties are available in supporting material (Fig. S1, Table S2).

The mix design 1 corresponds to Centennial Hall (Wroclaw, Poland).

It is based on concrete with cement binder, and granite or basalt fine and coarse aggregates. The specimens show a low porosity. The mix design 2 represents Elogio del Horizonte sculpture (Gijon, Spain). It is based on

concrete with cement binder, and limestone fine and coarse aggregates exhibiting also low porosity. Specimens with a higher porosity (MD2N) were also prepared to simulate the areas in a more advanced state of decay. The mix design 3 corresponds to three structures, Eduardo Tor- roja Institute (Madrid, Spain), ´Sródmie´scie WKD station (Warsaw, Poland) and Kaunas Ninth Fort (Kaunas, Lithuania), which are grouped together as they exhibit similar characteristics. More specifically, the mix design 3 is based on cement binder, limestone as coarse aggregates and siliceous sand as fine aggregates, and presents a low porosity. The mix design 4 corresponds to the War Memorial Tower (Torricella Peli- gna, Italy). It is a highly porous mortar with cement binder, containing carbonate and siliceous fine aggregates. The mix design 5 corresponds to Fig. 1. Location of the case studies selected as reference to prepare the mockup specimens. (A) Centennial Hall in Wroclaw, Poland – 1910–13. (B) Elogio del Horizonte in Gijón, Spain – 1989. (C) Eduardo Torroja Institute in Madrid, Spain – 1951–53. (D) ´Sródmie´scie WKD Station in Warsaw, Poland – 1962–63. (E) Ninth Fort Memorial in Kaunas, Lithuania – 1976–84. (F) War Memorial Tower in Torricella Peligna, Italy – 1961. (G) Flaminio Stadium in Rome, Italy – 1958.

Table 1

Design characteristics of the different mockup specimens used for evaluation of the products. Properties of the aggregates are available in supporting material.

mix design MD1 MD2 MD2N MD3 MD4 MD5 MD6

Case study A B B C,D,E F G G

Constr. Material Concrete Concrete Concrete Concrete Mortar Concrete Cement

Binder CEM II/B-LL

32.5 N CEM II/B-LL

32.5 N CEM II/A-LL

42.5 N CEM II/A-LL

42.5 N Lime putty

CEM II/A-LL 42.5 N

Aggregates Granite-C

Granite-F Limestone-C

Limestone-F Limestone-C

Siliceous-F Limestone-M Limestone-F Siliceous-F

Limestone-C

Siliceous-F n/a

w/c ratio 0.53 0.45 0.58 0.51 0.83 0.68 0.30

Cem:Fine:Coarse 1:3:3 1:2:2 1:2:2 1:2:2 1:5:3 1:1.8:2 n/a

aPorosity (%) 13.1 9.4 12.9 10.8 16.4 15.8 24.1

bCompr. Str. (MPa) 42.9 63.5 40.9 50.4 27.8 40.2 70.1

aWater accessible porosity determined from ASTM 642 test.

b Determined after curing for 28 days.

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Flaminio Stadium (Rome, Italy). It is based on a porous concrete with mixed cement/air lime binder and two types of fine aggregates (siliceous and limestone). The mix design 6 corresponds to the ferrocement can- opies of Flaminio Stadium. In this case, a simplified recipe for ferrocement was decided for this mix design.

For all mix designs, prismatic 135x90x80mm specimens were prepared by casting into 135x180x80 mm³EPS molds, curing for 28 days immersed in water and cutting the blocks in half (135x90x80 mm³). In order to simulate the effects of weathering on the materials, the mockup specimens were artificially aged by different processes.

Mockups of the “carbonation” series simulate the effect of long-term exposure to atmospheric CO2. The specimens were aged in a climatic chamber containing a 25 % v/v CO2 concentration under a RH 60 ± 10

% and 21 ± 2 ^◦C for 30 days. Carbonation depth achieved on the different mockups was determined by the phenolphthalein method (images in supporting information, Fig. S2).

Mockups of the “freeze–thaw” series simulate the cracks and physical decay produced in the material by weathering processes. In order to induce crack formation, the specimens were artificially aged by freeze- –thaw cycles (as specified in ASTM C-666) and thermal cycles (see Fig. S3). In summary, the specimens were subjected to 60 freeze–thaw cycles involving immersion in water at 4 ^◦C during 2.5 h and cooling in air at-18 ^◦C during 2.5 h. After equilibration to room temperature, further cracking was induced by subjecting the specimens to 30 wet-dry cycles involving soaking in water (20 ^◦C) for 15 h and then oven-drying for 8 h at 180 ^◦C.

Mockups of the “chloride” series simulate the effect of chloride ingress in areas exposed to marine aerosol. The specimens were pre- conditioned for 4 days at 40 ^◦C and subjected to 4 cycles involving immersion in a saturated NaCl solution (357 g/l) and drying at 105 ^◦C for 24 h. Afterwards, the specimens were desalinated by immersion in de-ionized water for 14 days, changing water every 24 h. Chloride penetration on the specimens, determined by staining with AgNO3 solution, is presented in supporting information (Fig. S4). Artificial ageing by chlorides was only performed on the MD2 and MD2N specimens since they represent the materials of the case study located in a coastal area.

2.2. Synthesis of the products

Three different products were synthetized via a sol–gel process, described in prior works: (1) UCA-T: an impregnation treatment for surface reinforcement [27], (2) UCA-TPS: a modification of UCA-T containing an alkylsiloxane to act as a hydrophobic impregnation treatment and fumed silica to grant superhydrophobic properties [42], (3) UCA-PRPS: a modification of UCA-PrPS containing monomeric alkylalkoxysilane to enhance absorption in the pore structure. The general synthesis process was as follows: (1) the silica precursors (alkoxysilanes and alkylalkoxysilanes) were mixed under mechanical stirring, according to the proportions given in Table 2. (2) The SiO2

nanoparticles were added (in UCA-TPS and UCA-PrPS) and dispersed in an ultrasound bath for 5 min. (3) De-ionized water and the catalyst n- octylamine (n-8) were added and the mixture was sonicated for 10 min with a Bandelin Ultrasonic HD3200 probe working at 74 % amplitude.

A summary of the products’ physical properties (viscosity, density,

surface tension and gel time) is available at supplementary information (Table S4).

2.3. Application on the mockup specimens

Prior to the product application, all the mockup specimens were pre- conditioned according to the following protocol: (1) the surface to be treated was sanded with P 180 grit wet sandpaper. (2) The debris were removed with an air gun at 3 bars and the specimens were cleaned in an ultrasound bath with de-ionized H2O for 10 min. (3) The specimens were dried in a stove at 40 ^◦C for 3 days and, afterwards, stored at room conditions (20 ^◦C, 40 % RH) for 7 days.

The products were applied by brushing until apparent saturation in two successive layers (on one of the 135x90 mm²faces), waiting 10 min for their absorption between applications. Preliminary tests showed that additional layers may cause color alterations and delay product drying.

After the second layer, the product excess on the surface was removed with a rag soaked in isopropanol. After application, the samples were stored at room conditions (20 ^◦C, 40 % RH) and left to cure for 28 days before their evaluation. Product uptake per area unit was determined by the weight difference before and right after the treatment.

2.4. Evaluation of the treatments

Possible color alterations after the treatment were evaluated through the total color change ΔE* in the CIE L*a*b* color space [43] by means of a portable colorimeter (PCE-CSM-4 from PCE instruments), using as references illuminant D65 and observer CIE 10^◦. The measurements were performed on six specimens per treatment and mockup type (for a minimum of 24 measurement points).

Infrared (ATR-FTIR) spectra were registered in the 4000–600 cm⁻¹ range using an IRAffinity-1S spectrophotometer from Shimadzu, equipped with an attenuated total reflectance module (Miracle ATR, Pike Industries). Samples were extracted 0–1 mm from the surface, ground and passed through a Ø 0.1 mm sieve to remove the fine aggregate grains.

The effect of the treatments on the mechanical properties was assessed through non-destructive tests, which are commonly used for on-site studies. Ultrasound pulse speed was measured using an Ultra- sonic Tester BP7 model from UltraTest GmbH, through the three axes of the prismatic specimens (4 specimens per treatment and mockup type), equipped with 45 kHz standard probes. Dynamic elastic modulus and compressive strength values were estimated through equations 1 and 2 respectively [44].

Ep= (1 + vp

)(1 − 2vp

) (1 − vp

) (γ/g)V_p² (1)

where Ep is the dynamic elastic modulus in GPa, vp is the poisson ratio (assumed as 0.2 as a typical value for concrete), γ/g is the density in kg/

m³, and Vp is the ultrasound pulse velocity in km/s.

fc=c⋅V_p^3.75 (1)

where fc is the estimated compressive strength in MPa and c is a factor Table 2

Proportions used in the synthesis of the products applied to the mockup specimens.

aTES40 (%v/v) ^bPDMS (%v/v) ^cPTEO (%v/v) ^dSiO2NPs (%w/v) H2O (%v/v) n-8 (%v/v)

UCA-T 100 – – – 0.50 0.16

UCA-TPS 90 10 – 2 0.15 0.08

UCA-PrPS 45 10 45 2 0.15 0.08

aWACKER® SILICATE TES 40 WN, from Wacker Chemie. Tetraethyl orthosilicate oligomer/monomer mixture with average chain length of 5.

b WACKER® FINISH WS 62 M from Wacker Chemie. Hydroxyl terminated polydimethylsiloxane with an average chain length of 58.

cDynasylan® PTEO from Evonik. Monomeric propyltriethoxysilane.

dAerosil® OX50 from Evonik. SiO2 nanoparticles with an average size of 40 nm and a BET area of 50 m²/g.

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ranging from 0.158 to 0.231. For the calculations, the value 0.231 was assumed, based on the comparison with the preliminary compression strength measurements on the sound mockup specimens (Table 1).

Surface strength of the materials was tested using a PCE-HT-225A type N Schmidt Hammer from PCE Instruments. The rebound index was calculated as the median between a minimum of 10 measurement points per specimen (4 specimens per treatment and mockup type).

Wetting properties of the surfaces were studied by measuring the static contact angle with water. For this purpose, 30 μl droplets were deposited over the treated surface and the contact angles were determined by image analysis of photographs using the ImageJ software.

Water repellence of the superhydrophobic treatments was verified qualitatively by tilting the specimens 10^◦ (i.e. a surface is considered repellent if the tilting angle is below this value) and depositing 30 μl droplets to observe their rolling/sliding behavior.

Variations in capillary water absorption after the treatments were evaluated via the contact sponge test [44] (CTS srl, Italy). For the measurements, the sponge was loaded with 5 ml water and put in contact with the specimen surface for 5 min (4 specimens per treatment and mockup type). The amount of water absorbed per time and area unit (expressed as kg⋅m^-2h⁻¹) was determined by weight difference of the sponge before and after the test.

Humidity content of the specimens was determined after equilibration for 28 days at room conditions (20 ± 3 ^◦C, 40 ± 5 % RH) by elec- trical resistivity measurements with a MM4DE Pin-Type moisture meter from General Tools & Instruments LLC.

3. Results and discussion 3.1. Evaluation of the treatments

After application of the treatments, product uptake ranged in the 0.10–0.25 kg/m²for all the products on the different tested mockups (Fig. 2). Despite the varying porosities of the different concrete types and ageing sets, there is not a clear correlation with the uptake values, likely because the number of applications was set to two layers instead of applying until saturation. Likewise, the three products under study showed similar values on all concrete specimens. Dry matter could not be accurately determined due to variations caused by the adsorption of

ambient moisture by the concrete.

The time required for the concrete surface to absorb the treatment, however, varied with the products and the specimen ageing (Table 3). In general, absorption was faster on the freeze–thaw and chloride aged specimens, where the forces exerted by the decay agents may cause micro-cracks and a general increase of the capillary pores (1–10 μm) [20], which have a higher contribution towards penetration of the treatments [28]. The carbonated and sound specimens presented a similar absorption rate despite the lower porosity of the former, likely due to the influence of the surface energy modifications after the ageing [20]. Regarding the products, UCA-TPS showed the slowest absorption rate (roughly 2–3 min slower than UCA-T on the 1st layer), which can be attributed to its higher viscosity and the accumulation of the SiO2NPs on the surface pores. On the other hand, the UCA-PrPS product was absorbed at a significantly faster rate (<2 min for the 1st layer and < 5 min for the 2nd). On the one hand, its higher surface tension promotes a stronger Laplacian pressure, the driving force governing capillary absorption and, according to modelling studies [28], the most influential factor for the penetration of the sol into the smaller pores of cementitious materials. On the other hand, the lower viscosity of PrPS also contributes to a higher penetration.

The color variations after the treatment can be a limiting factor their application on site, especially for cultural heritage elements, where preserving the original aspect is a priority. Thus, the color of the specimens was measured 28 days after treatment, when drying of the product could be considered complete. As presented in Fig. 3, the average color variations (ΔE*) on the different concrete specimens are below 4 in practically all cases, which is generally accepted as the threshold value for most restrictive applications on Cultural Heritage.

The only specimens where the value was higher did not surpass ΔE* = 5, which is considered a limit on applications where aesthetic aspect is considered. The differences between concrete type/ageing and applied product do not follow any trend, and can be attributed to the in- homogeneity of the substrate.

3.1.1. Product-substrate interaction

The FTIR of mockups samples showed characteristic bands in the 600–1600 cm⁻¹range (see Fig. 4), CO32– bands of CaCO3 from cement carbonation and dolomite aggregates at 1430, 875 and 730 cm⁻¹and Si-

Fig. 2. Average product uptake per area unit on the different mockup specimens.

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O bands from cement components and silicate aggregates in the 1200–900 cm⁻¹range. In addition, MD1 and 2 samples showed a gypsum band at 667 cm⁻¹and MD3, 4 and 5 showed a characteristic double band of quartz from the fine aggregates at 790 cm⁻¹. The main change in the FTIR spectra of treated mockup samples took place in the Si-O bands groups, where its original position was generally shifted towards higher wavenumbers. This shift indicates a higher contribution of Q3 and Q4 Si- O bands related to reticulated silica structures, confirming the presence of the treatments that produce silica coatings on the substrate surfaces [45]. The shift was hardly visible for some mockup types due to the presence of fine quartz aggregates that also contribute to Q3 and Q4 Si-O signals. On the contrary, the MD6 sample showed a large shift from 960 to 1070 cm⁻¹confirming that the treatments transform the cement surface into a highly reticulated silica structure, as a consequence of formation of a silica xerogel layer on the substrate surface and the interaction of the treatment with CSH for producing more reticulated structures [23,24]. The contribution of Si-O band around 960 cm⁻¹was higher after treatment for MD2 and MD5 samples, this is line with the reaction of alkoxysilanes with portlandite of cement for producing C-S-H whose Si-O Q2 units have a characteristic absorption at 960 cm⁻¹ [23,24]. The interaction of treatment with cement phases can be also evidenced by the increase of gypsum intensity band in MD1 samples which is related with the reaction of the treatment with ettringite for producing gypsum and aluminosilicate structures [24]. Finally, the samples treated with UCA-TPS and PrPS exhibited small bands at 1250 and 800 cm⁻¹related to vibrations of methyl groups of PDMS chains, which confirm the presence of this hydrophobic component in the surface of treated samples.

All ageing processes produced similar changes in the FTIR spectra

(see Fig. S5-7 in supporting information) of all mockups samples that were comparable to the shift in Si-O band observed for the sound mockups after the treatments. This similarity is because the carbonation process induces the transformation of CSH into CaCO3 and SiO2, decreasing the contribution of Si-O Q2 units at the same time as the Q3 and Q4 contribution was increased, which explains the Si-O band shift.

The treatment of aged mockups produced the same changes observed for the sound mortars with the exception that the shift of Si-O bands was hardly visible due to the initial changes in this band produced by the aging.

3.1.2. Mechanical properties

The mechanical performance of the concrete specimens and reinforcing effect of the treatments was evaluated through non-destructive tests, employing techniques common for on-site evaluations. It is worth mentioning that the MD6 mockups, which have no aggregates, tend to fracture by the impact from the Schmidt hammer, thus their rebound number values are unreliable and are not presented.

The rebound number obtained through the Schmidt hammer test (Fig. 5) serves as an indirect measure of the surface resistance, closely related to surface hardness and, to some extent, correlated with the compression strength of the concrete. As a general trend, the rebound indexes increase after the treatment by a 5–35 % factor, depending on the treatment and concrete type/ageing, which indicates that the products are able to improve the surface resistance. This effect is associated to the formation of a more cohesive structure with the presence of amorphous silica, formed by auto-condensation of the ethoxysilane oligomers, and the reaction products of the ethoxysilane with the cementitious phases, which have a similar composition to the substrate.

Table 3

Time required on average (in minutes) for the products to absorb on the mockup specimens depending on the ageing.

Sound Carbonation Freeze-thaw Chloride

1st layer 2nd layer 1st layer 2nd layer 1st layer 2nd layer 1st layer 2nd layer

UCA-T 5–8 10–11 5–8 12–14 2–4 7–9 3–5 12–14

UCA-TPS 8–10 ≥15 8–10 ≥15 7–8 12–14 8–9 12–14

UCA-PrPS 1–2 4–5 1–2 4–5 1–2 4–5 1–2 4–5

Fig. 3. Color change (ΔE*) of the treated surfaces after product curing (for 28 days). Dashed lines represent the maximum threshold values for application on cultural heritage objects.

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As studied in previous works [23,24,46], in the basic media of the cement pore solution, the alkoxysilane oligomers can undergo processes that include: reaction with portlandite to form C-S-H gel, incorporation into Si-O chains of the C-S-H gel increasing its mean chain length and partial decomposition of Al-containing phases (ettringite, mono- carboaluminate, katoite) to form amorphous aluminosilicate gels with a reticulated structure. The FTIR analysis demonstrated the silica presence and partially evidenced the interaction of treatment with cement phases.

Regarding the differences between the three evaluated products, UCA-PrPS sees to have an overall lower effect, which may be associated to a lower reticulation of the silica and aluminosilicate gels, as the PTEO contained in this product has three hydrolysable alkoxy- groups, as

opposed to the four present in the TEOS units. Anyways, the performance differences are minor in most cases and do not follow a clear trend.

Influence of the substrate is harder to predict, as the effectiveness depends both on the concrete type and how they are affected by the ageing processes. As expected, ageing by carbonation clearly increased the surface resistance and the treatments had an overall higher effect than on the sound specimens, which may be attributed to a higher accumulation of the product near the surface (i.e. the specimens are more compact and product absorption is hindered). In the case of specimens aged by freeze–thaw cycles, the surface resistance decreases in all specimens except MD2, which was the most compact material.

Fig. 4. FTIR spectra of the untreated and treated sound mockups samples.

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After application of the treatments, all the materials except for MD2N increase their resistance up to values similar or higher than the sound specimens, with the higher increase measured on MD4 and MD5 mockups, which were the most porous and affected by the ageing. The chloride-aged specimens, on the other hand, did not suffer a decrease in resistance after ageing, even though salt crystallization generally

produces micro-cracks. This can be explained by the presence of remnant salts in the pore structure, which alter the Schmidt hammer measurements and lead to overestimation of the material strength. On these specimens, the increase in resistance is overall lower than on the sound specimens and those aged by other processes.

The ultrasound pulse measurements (Fig. 6), in a similar vein, Fig. 5. Schmidt hammer measurements (rebound number) of the untreated and treated mockup specimens. MD6 samples are not represented because the material fractured after impact.

Fig. 6. Ultrasound pulse transmission velocity on the untreated and treated mockup specimens.

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showed similar trends regarding the effect of ageing on the specimens and a general increase in the elastic modulus (Fig. 7) after the treatment, although this technique was less sensitive in the more compact materials due to the limited penetration (generally 0.5–2 cm for impregnation treatments) respect to the specimen size. Overall, the compressive strength of the materials predicted by the UPV measurement (Fig. S8 in supporting material) is closer to the design values (Table 1) compared to the Schmidt hammer test, which is also affected by irregularities on the surface.

For the sound specimens, the increase in pulse speed is only remarkable for the MD4 and MD5 mockups, which are the least compact according to the values measured for the untreated specimens, although discrete increases were observed for MD1 and MD2 with UCA-T or MD2N and MD3 with all products. The effect of carbonation on the untreated specimens is manifested as an increased pulse transmission speed, and the enhancement after applying the treatments is minimal, unlike the surface resistance results from the Schmidt hammer, which may be explained by a lower penetration capacity due to the more compact structure of the material. Thus, the increase in mechanical resistance only occurs closer to the surface. Similar observations can be made for the chloride-aged specimens, although UCA-T shows significant (albeit small) differences. It must be noted that the higher re- sistances observed respect to the sound specimens likely result from an overestimation due to the presence of residual chloride salts in the pore structure. In the case of the specimens aged by freeze–thaw cycles, the formation of cracks by the associated mechanical stress caused a remarkable decrease in their ultrasound pulse transmission velocity, especially on the more compact mockups. For these samples, the reinforcing effect of the treatments is more evident, as the product will presumably have a higher capacity to penetrate through these cracks, filling them and forming phases that can bond with the substrate (e.g.

silica, C-S-H and aluminosilicate gels) after polymerization. It should be noted that, unlike the results obtained for surface resistance, the mockups MD1, MD2 and MD3 do not seem to fully recover their properties after the treatments, which may be associated to (1) a lower penetration of the products in these mockups, which are more compact.

(2) The presence of larger cracks (0.3–0.5 mm wide) due to the ageing process, which cannot be efficiently repaired with an impregnation treatment alone.

3.1.3. Hydric properties

The wetting properties of the mockups’ surfaces, characterized by the static contact angles (Fig. 8), are related with the capacity to prevent water absorption of the hydrophobic treatments, as the Laplacian pressure (proportional to capillary suction forces) inside the pores decreases with higher contact angles. For the untreated mockup specimens, the contact angle could not be measured, as the water droplets are quickly absorbed due to their porosity and the hydrophilic character of their constituent phases. In the case of the materials treated with the impregnation (UCA-T), the droplets are absorbed at a slow rate (2–5 min before absorption) and the surfaces present a hydrophilic character (SCA < 90^◦), in agreement with the characteristics of the reaction products formed on the surface.

On the other hand, the surfaces treated with UCA-TPS and UCA-PrPS showed a marked hydrophobic character (SCA > 90^◦), with generally high SCA values in the 120-150^◦range. In addition, all treated surfaces displayed water repellence with sliding angles below 10^◦. Video re- cordings showing water repellent behavior of the surfaces treated with UCA-PrPS are available in supporting material (Videos S1 and S2). The hydrophobic effect was lower on sound MD4, where the sliding angle was 25^◦and SCA 90^◦. The high hydrophobicity of the surfaces results from the combination of two factors: (1) the alkyl chains present PDMS and/or PTEO units decrease the surface energy of the reaction products that are formed in the surface, which is associated to the hydrophobic character. (2) The particles incorporated to the products accumulate on the surface, creating a regular nano-roughness [42,47]. It is known that the combination of both characteristics promotes a Cassie-Baxter wetting regime, where air pockets are entrapped between the roughness valleys, minimizing the interaction of the surface with liquid water and increasing the contact angles [48,49].

Protection of the treated materials against water absorption, measured by the contact sponge test, is an important parameter to

Fig. 7. Estimated dynamic elastic modulus, calculated from ultrasound pulse transmission velocity, of the untreated and treated mockup specimens.

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evaluate the overall performance, since water is associated with multiple decay processes of concrete (chloride ingress, freeze–thaw, leaching, biofouling…).

The water absorption values (Fig. S9 in supporting material) on the untreated specimens follow an overall consistent trend with the material

porosity, with the higher absorption observed for MD5, MD4 and MD3.

The higher absorption of MD6 specimens is likely due to the more hy- groscopic nature of the cement paste (i.e. they contain no aggregates).

Physically aged (freeze–thaw) mockups absorb at a higher rate, whereas a reduction is observed for the carbonated ones. The lower absorption of Fig. 8. Average static contact angles of 30 µl water droplets on the treated specimens. Dashed line at 90^◦indicates the threshold value for defining a hydrophobic surface.

Fig. 9. Decrease (expressed as % respect to the untreated control) in water absorption of the mockup specimens after the treatments. Measured by contact sponge test.

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the chloride-aged specimens may be related to the pore blockage by crystallized salts, as evidenced by ultrasound measurements (Fig. 6).

After the treatments, absorption decreased for all materials, as presented in Fig. 9. In the case of the impregnation treatment UCA-T, there are low to moderate reductions (in most cases < 50 %) in water ab- sorption rate that can be attributed to the formation of a more compact structure near the treated surfaces, even though the reaction products are hydrophilic in nature, as manifested by the contact angles (Fig. 8). A significantly higher decrease in water absorption rate is observed for the hydrophobic impregnation treatments (UCA-TPS, UCA-PrPS), reaching reductions in the 75–95 % range except for the carbonated MD1 mockups, where calculation of this value is affected by the low water absorption of the untreated material (see Fig. S9). These results are in line with the hydrophobic character observed on the treated surfaces, which further prevents water absorption in combination with the formation of a more compact structure, as evidenced by the ultrasound pulse measurements (Fig. 6). Capillary flow inside a porous material, according to Washburn model [50], becomes slower as the contact angle of water with the pore walls increases. There were no remarkable differences between the effectiveness of the two hydrophobic impregnations under study.

Regarding the influence of weathering or mockup properties, the effectiveness of the hydrophobic impregnations seems to be barely affected, suggesting that the hydrophobic nature of the material is the main responsible for the observed reductions. In the case of UCA-T there is not a clear trend, as the reduction will depend on different factors that are difficult to isolate in the experimental conditions: (1) product penetration and its effect on the pore size distribution [20,28]. (2) The formation of different reaction products (with varying surface energies) with the cement matrix, whose composition is altered by the chlorides, carbonation and thermal cycles, as observed in previous works using cement mortars [20]. (3) Incomplete hydrolysis and condensation of the precursors present in UCA-T leads to remnant ethoxy- groups [27,51,52], which lower surface energy of the material. These reaction kinetics will depend, among other factors, on the water content and pH

[52], which vary between the substrates.

Moisture content of the mockup specimens after reaching equilibrium at room conditions (20 ± 3 ^◦C, 40 ± 5 % RH) was measured in order to determine if the treatments modify the capacity of the materials to exchange water vapor with the environment (see Fig. 10). In general terms, a high moisture retention can be associated to a susceptibility towards alteration and decay processes and should be avoided.

In most cases, the treated materials retained less moisture than their untreated counterparts, especially the ones treated with the hydrophobic impregnations, though the differences between treatments are not significant on all mockups. In the case of the impregnation treatment UCA-T, the lower moisture retention can be attributed to a decrease in porosity near the surface after the product polymerizes inside the pore network and micro-cracks. The formation of phases with different chemical properties (i.e. surface energy, hygroscopicity) may also influence moisture content, although the reaction products of the sol components with hydrated phases [24] are presumably similar to the cementitious matrix. In the case of the hydrophobic impregnations (UCA-TPS, UCA-PrPS), the effect over porosity co-exists with the formation of hydrophobic phases with a low surface energy (i.e. organically modified gels) that have a lower capacity to adsorb and retain water. It should be noted that the different composition of the treated surface may also affect the impedance readings and introduce error to their conversion as humidity values. Regarding the differences between both hydrophobic impregnations, no significant variations were observed in almost all cases.

4. Conclusions

In this work, the effectiveness of three silane-based impregnation treatments (two of them with superhydrophobic properties) was evaluated on mockup specimens, which simulate the properties and weathering/ageing of the cementitious materials from different Cultural Heritage structures across Europe. For this purpose, seven different materials were prepared and artificially aged by three methods:

Fig. 10. Moisture content of the untreated and treated specimens after reaching equilibrium at room conditions (20 ^◦C, 40 % RH).

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carbonation, chloride ingress and physical damages (freeze–thaw and thermal cycles).

Under the application conditions (two layers by brushing), product uptake was in a similar range for different materials and did not follow a clear trend. The absorption rate, however, was slower for the carbonated specimens and faster for the freeze–thaw ones, indicating an influence of the surface compactness. Color alterations by the treatment were within the acceptable limits for heritage buildings (ΔE* < 4) for all product- substrate combinations after curing for 28 days.

Polymerization of the products inside the pore structure, producing amorphous silica gel by auto-condensation and C-S-H by reaction with the cementitious matrix, leads to an increase of surface resistance and elastic modulus of the treated material, as attested by Schmidt hammer and ultrasound velocity measurements. In general terms, the reinforcing effect was less significant on the materials with a higher compactness, where penetration is expected to be lower. Composition of the aggregate did not seem to play an important role under the tested conditions.

Regarding the conditions of the material, carbonation led to a higher reinforcement effect of the treatments over surface hardness but a lower effect over the bulk properties (elastic modulus). The opposite is observed for the physically aged materials, where the cracks promote absorption of the product instead of its accumulation on the surface. The three tested products performed similarly in terms of mechanical properties, indicating that the addition of hydrophobic components (siloxane and alkyl-alkoxysilane) to produce multifunctional treatments does not compromise their interaction with the cement matrix and/or aggregates.

The impregnation treatment was able to decrease water absorption rate and moisture retention in the materials due to the more compact structure, albeit this effect is highly dependent on the substrate properties and its condition, to the point no significant reductions were observed for several cases. On the other hand, incorporation of hydrophobic components to the treatment significantly decreased capillary water absorption rate, with consistent reductions of 75–90 % across the different materials, due to their ability to reduce surface energy. Mois- ture retention, on the other hand, was similar to the materials treated with the impregnation. In addition to these effects, the addition of fumed silica to the hydrophobic impregnations produces water repellence phenomena on the treated surfaces, as a consequence of Cassie-Baxter state creation, with static contact angles > 130^◦and sliding angles <

10^◦, which are potentially useful to decrease fouling and dirt pick-up.

Overall, it was observed that the three silane treatments are compatible in terms of applicability (product absorption) and aesthetical alterations over different materials subjected to varying ageing processes, and they could potentially be used for reducing water ingress and related damages on in situ applications. Reinforcement of mechanical properties, however, is minor in some of the more compact materials and the treatment choice should be reconsidered in such cases if surface reinforcement is the main objective.

CRediT authorship contribution statement

Rafael Zarzuela: Writing – original draft, Formal analysis, Investi- gation, Visualization. Manuel Luna: Writing – original draft, Formal analysis, Visualization. Jorge Gonzalez Coneo: Writing – review &

editing, Investigation, Formal analysis. Giada Gemelli: Writing – re- view & editing, Investigation, Formal analysis. Dia Andreouli: Writing – review & editing, Resources, Investigation. Vasilis Kaloidas: Writing – review & editing, Resources, Investigation. María J. Mosquera:

Writing – review & editing, Supervision, Project administration, Fund- ing acquisition.

Declaration of Competing Interest

The authors declare the following financial interests/personal re- lationships which may be considered as potential competing interests:

Maria J. Mosquera, Rafael Zarzuela, Manuel Luna has patent pending to Universidad de C´adiz.

Data availability

Data will be made available on request.

Acknowledgements

This project has received funding from the European Union’s Hori- zon 2020 - Research and Innovation Framework Programme under grant agreement No 760858; This work has been also financed by the Spanish State Research Agency R&D program 2020 (Project reference: PID2020- 115843RB-I00); Additionally, this work has been co-financed by the European Union under the 2014-2020 ERDF Operational Programme and by the Department of Economic Transformation, Industry, Knowl- edge, and Universities of the Regional Government of Andalusia (Project reference: FEDER- UCA18-106613). M. Luna would also like to thank the Spanish Government for his Margarita Salas grant (2021-067/PN/

MS-RECUAL/CD) supported by the European Union-NextGenerationEU.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.

org/10.1016/j.conbuildmat.2022.130258.

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