Fresh and hardened state behaviour of aerial lime mortars with superplasticizer

Construction and Building Materials 225 (2019) 1127–1139

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Fresh and hardened state behaviour of aerial lime mortars with superplasticizer Bruna Silva a,⇑, Ana Paula Ferreira Pinto a, Augusto Gomes a, António Candeias b a b

CEris, Department of Civil Engineering, Architecture and Georresources, Instituto Superior Técnico, Universidade de Lisboa, Portugal HERCULES Laboratory and Chemistry Department, School of Sciences and Technology, University of Évora, Portugal

h i g h l i g h t s
Study of the influence of superplasticizers on the fresh and hardened state characteristics of lime mortars.
Superplasticizers based on polycarboxylate ether (PCE) and on polynaphthalene sulfonate (PNS) were used.
PCE was found to be more effective (higher dispersing ability) and efficient (less quantity needed) than PNS.
Both superplasticizers led to a vast strength increase and only small changes in the porous structure.
Resulting lime mortars with superplasticizer have potential to be used in restoration.

a r t i c l e

i n f o

Article history: Received 13 May 2019 Received in revised form 18 July 2019 Accepted 22 July 2019

Keywords: Lime Mortar Rheology Admixture Plasticizer Restoration

a b s t r a c t Water-reducing admixtures have the potential to minimize the disadvantages that typically discourage the use of lime in restoration works. However, the knowledge on the effect of these admixtures on the properties of lime mortars is not as consolidated as it is for cement-based materials. Therefore, this paper aims at studying the influence of two types of superplasticizer, one based on polycarboxylate ether (PCE) and the other on polynaphthalene sulfonate (PNS), on the fresh and hardened state properties of lime mortars. PCE was found to be more effective (higher dispersing ability) and efficient (less quantity needed) than PNS, leading to a lower fluidity loss over time, but to a higher tendency towards bleeding. Both admixtures led to a substantial mechanical strength increase, even at early ages, and just to small changes in the porous structure of the mortars, which suggests that lime mortars with superplasticizer have potential to be used in restoration. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Lime-based mortars have been considered the most compatible to use in restoration and conservation interventions due to their similarity in terms of properties and aesthetics to the original mortars. However, they present disadvantages that systematically discourage their application, such as long setting and hardening times that hinder and delay restoration works. Moreover, the high plasticity and water retention capacity of these mortars make them more difficult to apply, since masons nowadays are used to the more fluid cement mortars. In this context, admixtures (substances added in less than 5% of binder weight to improve the properties of a material in the fresh or hardened state [1]) may minimize the reported disadvantages, without harming the compatibility with old materials. Water⇑ Corresponding author. E-mail address: [email protected] (B. Silva). https://doi.org/10.1016/j.conbuildmat.2019.07.275 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

reducing admixtures, for instance, allow reducing the need of kneading water (if the consistency is to be maintained) or increasing the fluidity of the material (if the amount of kneading water is to be kept). These admixtures, also called plasticizing admixtures, can be useful for lime mortars since their water-reducing action can avoid excessive kneading water, which can be beneficial for the carbonation process and mechanical strength development [2,3]. Indeed, there are evidences that these admixtures can increase the mechanical strength of lime mortars [4], just like in cement-based materials, and decrease their long setting time [5]. Moreover, a lower water content can account for less drying shrinkage and a lower porosity associated with water absorption (capillary pores), which can reduce the susceptibility to degradation of lime mortars. In the fresh state, the higher fluidity that can be achieved with the use of water-reducing admixtures can improve the application of these mortars and their adherence to the substrate.

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Concisely, water-reducing admixtures consist of anionic surface-active agents or surfactants that are adsorbed onto binder particles, giving them a negative charge. This electrostatic charge leads to repulsion between the particles (electrostatic repulsion) and, thus, to their dispersion. Since the attractive forces between the particles are reduced, it is easier to overcome them for the material to flow (reduction of yield stress), the reason why these admixtures are responsible for a fluidity increase, when the kneading water is kept the same [6]. At this point, a distinction between plasticizers and superplasticizers is necessary. The former are substances that generally lead to a water reduction between 5 and 10%, while the latter allow water reductions of up to 40% [6,7]. The most commonly used water-reducing admixtures nowadays are those based on lignosulfonate (LS), polynaphthalene sulfonate (PNS), polymelamine sulfonate (PMS) and polycarboxylate ether (PCE). LS is a natural polymer that derives from wood processing. The lignin in the wood pulp is removed by a sulfite reaction and is then further processed before being used for admixtures [7]. Its chemical structure contains a carbon chain with several functional groups attached, such as sulfonic, phenolic hydroxyl and methoxyl [4,6,8]. PNS molecules are composed by sulfonic groups and naphthalene double rings connected by methylene [9]. PMS is very similar in structure to PNS except that a melamine ring replaces the naphthalene double ring and the molecular weight is higher [6]. The sulfonic groups are responsible for the negative charge of these molecules [4]. Polycarboxylate ethers constitute the 3rd and newest generation of superplasticizers, being more effective than those based on PMS or PNS, due to their molecular structure. PCE molecules have a comb like shape, with a main linear backbone bearing carboxylic groups and long chains of ether groups attached [6,10,11]. In this case, the carboxylic groups are the ones responsible for the negative charge of these molecules and, thus, by the electrostatic repulsion when attached to the positively charged surface parts of binder particles. However, the long side chains of the polymer are responsible for additional repulsive forces since they do not want to overlap (steric repulsive hindrance), the latter being the dominant dispersing mechanism in this type of superplasticizers and the reason for their higher effectiveness [6,11,12]. The available literature on the use of admixtures in aerial lime mortars is scarce, unlike in cement mortars or concrete. Hence, their influence on these materials is not as consolidated. This is particularly serious given that nowadays dry mix mortars are increasingly used in construction, including in restoration works and, as it is known, these use a variety of substances to improve the performance of mortars, water-reducing admixtures being one of them. According to Seabra et al. [13], who focused on the rheological properties, the addition of a PMS-based superplasticizer to aerial lime mortars was responsible for a strong reduction of torque values, being this reduction very dependent on the dosage. The admixture promoted the dispersion and deflocculation of binder agglomerates, and to less bonded water in their surface, leaving more free water to fluidity the mixes. In terms of yield stress and plastic viscosity, the admixture led to a strong decrease of the first, but only to a slight decrease of the second, again being this effect dependent on the dosage used. Fernández et al. [5] also reported a decrease of viscosity with the addition of a PCE-based superplasticizer. Pérez-Nicolás et al. [4] made a comparative study between the addition of a superplasticizer based on PNS and another based on LS to aerial lime mortars with nanosilica, using a fixed amount of water. The authors then concluded that the latter was more effective at increasing the fluidity of the mortars, but also hindered their carbonation process. The addition of PCE-based superplasticizers to lime-based materials with nanosilica has been studied by Fernández et al. [5] and Navarro-Blasco et al. [11]. When using the admixture to

reduce the kneading water content of the mortars, Fernández et al. [5] obtained a reduction of setting time and a noticeable increase of mechanical strength, but also a decrease of porosity owing to a strong reduction of pores in the range of 1–10 mm in diameter and drastic changes in the microstructure of the mortar. Similar but less pronounced trends were observed with superplasticizer addition while fixing the water content of the mortar. The exception was the setting time that registered an increase due to the greater water content, a result also obtained by NavarroBlasco et al. [11] when comparing the performance of two different superplasticizers based on PCE on aerial lime pastes. More recently, our research group has taken an interest in this matter, and has briefly evaluated the influence of a plasticizer and a superplasticizer, both based on PCE, on the early age properties of aerial lime mortars for restoration purposes [14]. The conclusions were promising, in the sense that, both substances, but especially the superplasticizer, led to a mechanical strength increment and to a reduction of the porosity and capillary absorption. Since these changes were moderate, the resulting aerial lime mortars could maintain their compatibility with the old materials. The results suggested that the use of superplasticizers on lime mortars should be further investigated. Based on the review of the literature carried out, PCE and PNS seemed to be the most promising superplasticizers to be used in mortars for restoration purposes, given their potential to improve aerial lime mortars, without strongly affecting their carbonation process. However, still little is known about their effect on lime mortars, especially in what concerns the rheological properties and porous structure of the resulting material, these being of critical importance for the compatibility of the final solution. In the light of these gaps, this work aims at studying the effect of the addition of these two types of superplasticizer (PCE and PNS) on the fresh and hardened state properties of aerial lime mortars and, thus, to contribute to the existing knowledge on this subject. First, a thorough rheological characterization of the lime mortars with several dosages of either PCE or PNS will be performed, followed by the characterization of these mortars in the hardened state at different ages, with emphasis on mechanical strength and porous structure. 2. Materials and methods 2.1. Materials and formulations A commercial dry hydrated lime of type CL80-S (according to EN 459-1:2010 [15]) and a siliceous sand with a 0/2 particle size (according to EN 13139:2002 [16]) were used in the production of the mortars. X-Ray Diffraction analyses were performed on these materials in order to determine their mineralogical composition. A Bruker AXS D8 Advance powder diffractometer with Cu Ka1 radiation was used, and the diffractograms were taken from 5° to 75° 2h, using a 0.05° 2h increment and a time per step of 1 s. The resulting diffraction patterns showed that the lime used was composed of portlandite and, in minor quantities, of calcite (Fig. 1) and that the aggregate used mainly consisted of quartz (Fig. 2). A binder/aggregate ratio of 1:2 (in volume) was selected for the production of the mortars. Two commercially available superplasticizers (SP) were chosen based on the review of the literature carried out: one based on polycarboxylate ether (PCE), and another based on polynaphthalene sulfonate (PNS). At least 3 dosages of each type of superplasticizer were used: the minimum, the intermediate and the maximum dosages recommended in the respective technical sheets. Accordingly, 0.3%, 0.65% and 1% of PCE and 0.8%, 1.9% and 3% of PNS were used, with respect to the weight of lime. For the PNS-based superplasticizer, the dosage recommended by the manufacturer (1% of binder weight) was also used, dosage that was not defined for the superplasticizer based on PCE. Both superplasticizers were analyzed by micro-FTIR using a Bruker Tensor 27 infrared spectrometer, coupled to a Hyperion 3000 microscope, equipped with a single point MCT detector. The samples were analyzed in transmission mode, using a 15x objective and a compression diamond microcell from S.T. Japan. The spectra, presented in Fig. 3, were acquired in the infrared region of 4000–650 cm1, at a spectral resolution of 4 cm1, and are the result of averaging 32 scans.

B. Silva et al. / Construction and Building Materials 225 (2019) 1127–1139

higher water contents led to segregation. Besides the reference mortar, two groups of lime mortars with superplasticizer were produced: one in which the w/b ratio was fixed and equal to that of the reference mortar (1.44), and the other where the w/b ratio was defined in order to obtain a fixed final consistency equal to that of the reference mortar (260 ± 5 mm). The mortars were produced according to EN 196-1:2005 [21]. When used, the superplasticizers were added to the kneading water (direct addition) as advised in the respective technical sheets, and mixed for 1 min with water, prior to the addition of lime. Nevertheless, several addition methods were tested previously in order to understand if they affected the fresh state properties of lime mortars, like it occurs for cement ones. However, it was concluded that their influence on aerial lime mortars was negligible. Immediately after production, a sample of the material was taken in order to evaluate its consistency, bulk density and air content. Another sample was taken for the rheometric test. With the remaining mortar, prismatic mortar specimens with the standard dimensions (160  40  40 mm) were moulded according to EN 196-1:2005 [21] and de-moulded 7 days later. The specimens were stored in a conditioned room (20 ± 5 °C and 60 ± 10 %RH) until testing, which was performed after 14, 28, 90 and 180 days of curing.

16000

Intensity (counts)

14000

P: portlandite C: calcite

P

12000

P

10000

C

8000 6000 4000

P

P

P P

CC

2000

P

0 5

15

25

35

45

55

65

75

Fig. 1. X-ray diffraction pattern for the aerial lime used.

180000

Q: quartz M: microcline

Q

Intensity (counts)

140000

2.2.2. Bulk density and air content Bulk density and air content were determined in order to verify if the added superplasticizers increased the air content of the mixes, as it is a common secondary effect owing to their inherent surfactant properties [6,7,14,24]. Bulk density was evaluated by the filling method (final flow values higher than 200 mm) on fresh mortars according to EN 1015-6:1999 [25]. Air content was evaluated according to the pressure method described in EN 1015-7:1998 [26] for mortars with an expected air content lower than 20%. At least two determinations were made in distinct batches per type of mortar, being the presented results of bulk density and air content the average value.

120000 100000 80000 60000 40000

M

Q

20000

Q

Q

Q

Q

0 5

15

25

35

45

55

2.2. Fresh state characterization 2.2.1. Consistency The consistency of the mortars was evaluated by flow table (according to EN 1015-3:1999 [22]) and by plunger penetration (according to EN 1015-4:1999 [23]). In the former, the mortar spread diameter (flow value) was measured after the mould was removed (initial flow value) and after jolting the flow table 15 times (final flow value). In the plunger penetration test, the vertical penetration of a plunger, which has been allowed to fall freely from a height of 100 mm onto a vessel filled with fresh mortar, was measured on a scale from 0 to 70 mm, being the measured value proportional to the fluidity of the material. For both tests, the results presented are the average of at least two determinations.

Degree (2 )

160000

1129

65

75

Degree (2 ) Fig. 2. X-ray diffraction pattern for the fine aggregate used.

The IR spectrum of the PCE superplasticizer showed the presence of alkyl groups (sharp peak at 2872 cm1 characteristic of CAH stretching vibrations) and carboxyl groups (broad band at 3501 cm1 attributed to OAH stretching vibrations and a small peak at 1722 cm1 characteristic of C@O stretching vibrations), which should constitute its backbone (Fig. 3). The sharp peak at 1108 cm1 (attributed to CAOAC stretching vibrations) is indicative of the presence of ether groups on the side chains of the polymer. In this sense, the spectrum obtained has similarities to those attained by Navarro-Blasco et al. [11], Janowska-Renkas [17], Li et al. [18] and Zhang et al. [19]. In turn, the IR spectrum of the PNS-based superplasticizer showed bands typical of the presence of aromatic rings (small peak at 3072 cm1 attributed to CAH stretching, sharp peaks at 1632 and 1595 cm1 characteristic of C@C bending vibrations and several peaks between 894 and 752 cm1 due to CAH bending vibrations) and of sulfonate groups (peaks between 1183 and 1039 cm1). Other important bands corresponded to S@O and SAO stretching vibrations, respectively at 1358 and 681 cm1 (Fig. 3). This result was comparable to those obtained by Pérez-Nicolás et al. [4] and by El-Didamony et al. [20] for similar substances. An aerial lime mortar without admixtures was taken as reference (REF mortar). The water content of this mortar was selected so that valid rheological measurements could be achieved. A preliminary study led to the selection of a 24% water content (w/b ratio = 1.44) as that which enabled a stable/homogenous rheological behaviour [2], corresponding to a consistency around 260 mm (according to the flow table test). Lower water contents led to a significant thickening of the lime suspension during the rheometric test, steeply increasing torque values, whereas

2.2.3. Rheological measurements For the rheological measurements, a rotational rheometer suitable for mortars (Viskomat NT, Schleibinger Testing Systems) was used. In this type of equipment, the material is placed in a cylindrical container that rotates around a stationary paddle at a designated rotation speed profile controlled by a computer. The torque that is created when the sample flows through the paddle is recorded throughout the test, as well as rotation speed and time. This data allows plotting torque versus time curves or torque versus rotation speed curves (flow curves). From the latter, if the typical behaviour of a Bingham fluid is verified, it is possible to determine by a linear regression the parameters g and h, which are directly proportional, respectively, to yield stress and plastic viscosity [27]. In the present case, the Bingham model allowed characterizing the fluid behaviour of the mortars with superplasticizer fairly well (correlation coefficients > 0.90), with exception of the mortars with a fixed consistency due to a pronounced thickening of the lime suspension during the test, which caused a high variability of torque values. A ‘‘dwell” speed profile was used that consisted of 2 cycles, where each cycle consisted of increasing the rotation speed from 0 to 160 rpm in 1 min and then, after 5 min, decreasing it again from 160 to 0 rpm in 1 min, leading to a total testing time of 14 min (Fig. 4). The same amount of mortar was used in all rheological measurements, and the material was not re-used for other tests. The scraper was not utilized. Since the reproducibility of these tests proved to be quite high, the results presented correspond to only one test per mortar formulation. 2.2.4. Bleeding Bleeding (a form of segregation) was observed in some mixes, particularly in those in which the admixtures were added without reducing the kneading water. In order to evaluate bleeding, a graduated cylinder with a capacity of 1 l (334 mm height and 66 mm diameter) was filled with mortar up to 900 ± 10 ml, and the initial height was registered (h0 ). The top of the cylinder was sealed with plastic wrap to avoid water evaporation. The height of the water layer above the material was then measured after 3 h and 24 h standing undisturbed (hw ). Bleeding was then calculated using the following expression:

951 893 829 792 752 681

1123 1039

1350 1296 1251

1505 1455

Absorbance Units 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

1722 1651 1594

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3501 3420

1130

3500

3000

2500

2000

1500

1000

Wavenumber (cm -1 ) Fig. 3. IR spectra of the PCE-based (in blue) and of the PNS-based (in red) superplasticizers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

180

2.3. Hardened state characterization

160

2.3.1. Mechanical strength The mechanical characterization comprised flexural and compressive strength tests according to EN 1015-11:1999 [28], on at least 3 prismatic mortar specimens per type of formulation. The freshly broken halves from the flexural test were sprayed with a phenolphthalein indicator in order to qualitatively determine the carbonation depth of the specimens.

Rotaon speed (rpm)

140 120 100

2.3.2. Porosity and pore structure The porosity accessible to water or open porosity was determined as described in RILEM Test I.1 [29] on at least 2 half specimens of each mortar formulation. Porosity and pore size were also determined by Mercury Intrusion Porosimetry (MIP) analysis using an Autopore IV 9500 porosimeter from Micromeritics. The changes induced by the admixtures in the porous structure of the mortars were analyzed by Scanning Electron Microscopy (SEM). In order to do that, small pieces were cut from the samples, embedded in epoxy resin and one of their surfaces polished. These surfaces were then observed in a Phenom ProX scanning electron microscope from Thermo Fisher Scientific, in the backscattered electron (BSE) mode.

80 60 40 20 0 0

2

4

6

8

10

12

14

3. Results and discussion

Time (min) 3.1. Fresh state properties Fig. 4. Speed profile used in this study.

Bleeding ¼

hw  100% h0

During the test, the graduated cylinders were kept in a controlled environment with a temperature of 20 ± 2 °C and a relative humidity of 60 ± 5%. 2.2.5. Particle size Water-reducing admixtures are known to disperse binder particles, as already discussed. Since lime particles are very small and have a strong tendency to agglomerate in the presence of water, these substances will supposedly avoid that agglomeration. As so, the particle size of lime suspensions with the maximum dosages of the two types of superplasticizers tested (1% PCE and 3% PNS) and without any admixture (REF) was determined by laser diffraction using a Malvern Mastersizer 2000 instrument. The suspensions were obtained from the respective mortars by removing the aggregate with a 100 mm sieve.

Several dosages of either a PCE-based or a PNS-based superplasticizer were added to aerial lime mortars with a fixed w/b ratio (1.44) or with the w/b ratio required to obtain a fixed consistency (260 ± 5 mm). The properties obtained in the fresh state for the mortars with a fixed w/b ratio are presented in Table 1 and Fig. 5. It should be noted that in Fig. 5 only the initial flow values were presented given the high fluidity of the mixes that led them to surpass the flow table diameter (300 mm) after jolting the flow table, in most cases. Nevertheless, when comparing both types of superplasticizer, their effect on the mortars’ consistency was similar, so that with increasing dosage a vast increase of initial flow values and of plunger penetration depths was registered (Fig. 5). However, this increase was steeper for the PCE-based superplasticizer, given

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B. Silva et al. / Construction and Building Materials 225 (2019) 1127–1139 Table 1 Fresh state properties of lime mortars with different dosages of either a PCE or a PNSbased superplasticizer and a fixed w/b ratio. SP dosage (%)

W/b ratio (–)

Bulk density (kg/m3)

Air content (%)

REF PCE

0 0.3 0.65 1.0 0.8 1.0 1.9 3.0

1.44 1.44

1940 1950 1950 1950 1970 1970 1960 1960

1.3 1.0 1.1 1.1 0.5 0.3 0.2 0.2

PNS

1.44

3.5

24h

3h

3.0

Bleeding (%)

Mortar

4.0

2.5 2.0 1.5

70

275

60

250

50

225

40

200 FV - PCE

30

175 FV - PNS

150

20

PP - PCE

125

10

PP - PNS

100

Plunger penetraon depth (mm)

Inial flow values (mm)

1.0 300

0 0.0

1.0

2.0

3.0

4.0

SP dosage (%) Fig. 5. Initial flow values (FV) and plunger penetration depths (PP) as function of superplasticizer dosage for a fixed w/b ratio (1.44).

the better dispersion ability of PCE molecules [6]. In spite of this, when the maximum dosage was used, PNS only led to a slightly lower initial flow value (235 mm) than PCE (246 mm). With increasing dosage of superplasticizer, both admixtures led to small variations of bulk density and even smaller variations of air content (Table 1). The mortars with superplasticizer showed higher densities than the reference mortar, but with increasing dosage bulk density values presented a tendency to decrease. These results should be owing to a balance between entrapped air (that sharply decreased following the high fluidity increase) and entrained air (that rose with increasing dosage of superplasticizer, especially in PCE). The mortars’ tendency towards bleeding was also evaluated, whose results are shown in Fig. 6. The mortars with the lowest superplasticizer dosages (0.3% PCE and 0.8 and 1% PNS) presented a bleeding percentage close or even lower than the reference mortar, whereas those with higher dosages displayed a substantial increase in the volume of bleed water, as expected. Despite presenting a higher bleeding (either for the same dosage or for the maximum dosage), the mortars with PCE presented a slower ascendance of water (greater difference between the values registered after 3 h and 24 h). From a previous paper by our research group [2], difficulties in performing rheological measurements in lime mortars were reported. It was concluded that it was the breakdown of lime agglomerates, essentially due to the ‘‘ball-milling action” of the aggregate, that led to a thickening of the lime suspension and, thus, to an accentuated increase of torque values. In this sense, and

0.5 0.0

Fig. 6. Bleeding of the mortars with a fixed w/b ratio (1.44) after 3 h and 24 h.

according to the literature, plasticizers are capable of improving the dispersion and deflocculation of these lime agglomerates. The torque vs. time curves of the mortars with superplasticizer and a fixed w/b ratio are presented in Fig. 7. As can be perceived, the behaviour of the mortars was affected differently depending on the dosage of superplasticizer used, specifically: i) For low percentages of superplasticizer (0.3% PCE and 0.8% and 1% PNS), a steeper increase of torque values over time is noticeable (Fig. 7), when compared to the reference mortar. This should be owing to the dispersive power of the admixtures on lime agglomerates that leads to a greater exposed area of binder and lower free water amount and, as consequence, to a faster thickening of the lime suspension during the test; ii) For dosages equal or higher than the intermediate one (0.65% PCE and 1.9% PNS), only small differences in the torque versus time curves can be perceived (Fig. 7). This behaviour should be related to the occurrence of segregation/bleeding that originates lower torque values in these mortars due to a gradual descent of the aggregate to the bottom of the vessel, which was confirmed at the end of the test. These results are consistent with those obtained for bleeding (Fig. 6). The parameters related to yield stress and plastic viscosity taken from the up- and down- ramps are presented in Fig. 8. Regarding the results, a substantial reduction of g values was obtained with increasing dosage of superplasticizer (both PCE and PNS), especially with intermediate to high dosages; however, no clear trend was observed for h values. Over the test duration, g values rose significantly in the mortars with the lowest superplasticizer dosages (0.3% PCE and 0.8 and 1% PNS), while presenting much lower values for the mortars with higher dosages, due to the tendency of these mixes towards segregation. No clear tendencies were observed for h values yet again. Most works on this subject report a sharp reduction of g values (since the effect of these admixtures is to reduce the attractive forces between binder particles and, consequently the yield stress) and small changes (either an increase or a decrease) of h values, with increasing admixtures content, both in lime and cement-based materials

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60

60

REF

REF 0.3% PCE

1.0% PNS

0.65% PCE

40

0.8% PNS

50

Torque (N.mm)

Torque (N.mm)

50

1% PCE

30 20 10

40

1.9% PNS 3% PNS

30 20 10

0

0 0

2

4

6

8

10

12

14

0

2

4

6

8

Time (min)

Time (min)

(a)

(b)

10

12

14

Fig. 7. Torque vs. time curves of lime mortars with different dosages of (a) PCE and (b) PNS and a fixed w/b ratio (1.44).

Fig. 8. Variation of g and h parameters throughout the rheological test for mortars with different dosages of (a) PCE and (b) PNS and a fixed w/b ratio (1.44).

[6,10,13,27]. Over time, an increase of g values, especially with longer testing times is reported by Seabra et al. [13]. In this sense, the results obtained corroborate the literature. A similar characterization was performed on mortars with a fixed consistency (260 ± 5 mm), and whose results are presented in Table 2 and Fig. 9. Regarding the type of superplasticizer, PCE

was more efficient at reducing the kneading water than PNS (Fig. 9). For instance, when the same dosage was used (1% of lime weight) the PCE-based admixture allowed a water reduction of 16%, whereas the PNS-based one allowed a water reduction of only 5%. However, when the maximum dosage was used, both substances led to equivalent water reductions (16%).

1133

B. Silva et al. / Construction and Building Materials 225 (2019) 1127–1139 Table 2 Fresh state properties of mortar LM24% with different dosages of either a PCE or a PNS-based superplasticizer and fixed consistency. Mortar

SP dosage (%)

W/b ratio (–)

Final flow (mm)

Bulk density (kg/m3)

Air content (%)

REF PCE

0 0.3 0.65 1.0 0.8 1.0 1.9 3.0

1.44 1.40 1.30 1.21 1.39 1.37 1.28 1.21

261 260 ± 5

1940 1940 1920 1910 1950 1960 1980 1990

1.3 2.0 4.7 6.2 1.5 1.7 2.0 2.1

PNS

260 ± 5

18

Water reducon (%)

16 y = 15.29x R² = 0.98

14

y = 5.41x R² = 0.99

12 10 8 6 4

PNS

2

PCE

0 0.0

1.0

2.0

3.0

4.0

SP dosage (%) Fig. 9. Water reduction as function of admixture dosage for a fixed consistency (260 ± 5 mm).

Moreover, a decrease of bulk density values was registered for mortars containing PCE, indicative of its air-entraining action (air content increased approximately 380%), which prevailed over the water reduction. By contrast, an increase of bulk density values was registered for mortars with PNS as a result of kneading water reduction, since this admixture lead only to a negligible air increase.

140

100

REF 0.8% PNS 1% PNS 1.9% PNS 3% PNS

120

Torque (N.mm)

Torque (N.mm)

140

REF 0.3% PCE 0.65% PCE 1% PCE

120

In the mortars with superplasticizer where the consistency was fixed all the torque versus time curves obtained presented higher torque values and a more pronounced thickening with time than that of the reference mortar without admixtures, as depicted in Fig. 10. These results, together with the previous ones, allow pinpointing the effectiveness of the dispersing effect that waterreducing admixtures have on lime agglomerates that originates a faster thickening of the lime suspension. As a result, the determination of g and h values was hindered in most cases owing to the low correlations to the Bingham model and, therefore, these values are not presented. In order to confirm the dispersive effect of the superplasticizers, the particle size of lime suspensions with and without admixtures was determined by laser diffraction, whose results are shown in Fig. 11. The lime suspension without admixtures (REF) showed a bimodal distribution, with a large peak around 1–20 mm and a much smaller one around 20–80 mm. The addition of 1% PCE lead to the disappearance of the small peak related to particles with higher diameters due to the breakdown of the large lime agglomerates into smaller ones (a rise in the volume of particles with dimensions below 20 mm was observed), as confirmed by the stereo microscope images. A similar result was also obtained by Fernández et al. [5]. In turn, the addition of 3% PNS proved not to be as effective. Indeed, the latter substance led to a slight reduction of the volume of particles with dimensions around 7 mm and to an increase of the volume of very small particles with dimensions between 1 and 2 mm, but apparently did not affect the larger particles. It should be mentioned, however, that a sieve of 100 mm was used to remove the aggregate of the lime suspensions. With this, eventually, some lime agglomerates with diameters above that value were also removed. As so, a possible influence of the

80 60 40 20

100 80 60 40 20

0

0 0

2

4

6

8

10

12

14

0

2

4

6

8

Time (min)

Time (min)

(a)

(b)

10

12

14

Fig. 10. Torque vs. time curves of mortar LM24% with different dosages of a (a) PCE-based or a (b) PNS-based superplasticizer and a fixed consistency (260 ± 5 mm).

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Volume (%)

10 9

REF

8

1% PCE

7

3% PNS

6

1mm

5

REF

4 3 2 1 0 0.1

1

10

100

Parcle diameter (μm)

1mm

1% PCE Fig. 11. Particle size distribution of lime suspensions taken from the reference mortar and from those with superplasticizer (left) and stereo microscope images of these lime suspensions (right).

100 90 80 70 60 50 40 30 20 10 0

REF 1% PCE 3% PNS 0

60

120

Elapsed me (min)

180

To sum up, the highest dosages of PCE and PNS led to similar consistencies (if w/b ratio is maintained) and to similar water reductions (for the same consistency). However, PCE was found to be both more efficient and effective (less quantity needed and a higher/better dispersion of lime particles achieved), while losing its dispersing ability slower than PNS. The behavioural differences observed between these two superplasticizers are related to their chemical structure and, as consequence, to their dispersing mechanisms as described in the Introduction. 3.2. Hardened state properties The influence of both types of superplasticizer (PCE and PNS) was evaluated on the mechanical and physical properties of lime mortars after 14, 28, 90 and 180 days of curing. The hardened state properties were assessed on mortars produced with a fixed consistency of 260 ± 5 mm, and whose fresh state properties were already presented in Table 2. Fig. 13 shows the evolution with time of the flexural and compressive strength values obtained for the aerial lime mortars with the PCE-based superplasticizer. As can be perceived, mechanical

Penetraon depth (%)

Flow value (%)

superplasticizers on the dispersion of these larger agglomerates was not detected. The assessment of fluidity loss over time is especially important in lime mortars since it is frequent, on site, to use the same batch of mortar throughout several hours, given their slow setting. In order to evaluate this, the consistency of the mortars with the highest superplasticizer dosage (1% PCE and 3% PNS) was assessed every hour up to 3 h after mixing. The results obtained are presented in Fig. 12. According to the literature on cement-based materials, the addition of a superplasticizer is known to lead to a higher fluidity loss with time than the same mix without any admixture [30]. Fig. 12 allows confirming that this tendency can also be observed in lime mortars, with PNS leading to the highest fluidity loss, followed by PCE, when compared to the reference mortar. These tendencies corroborate existing works on cement-based materials that highlight the low fluidity loss of mixtures with PCE and elaborate on the strong loss caused by PNS [30–32]. However, the fluidity loss in cementitious materials is greater due to the onset of hydration reactions, whereas in lime mortars it derives essentially from water evaporation and flocculation of lime particles due to loss of the dispersing ability of the superplasticizer.

100 90 80 70 60 50 40 30 20 10 0

REF 1% PCE 3% PNS 0

60

120

180

Elapsed me (min)

Fig. 12. Flow value and penetration depth decrease over time obtained for the mortars with and without superplasticizer and with a fixed consistency (260 ± 5 mm).

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B. Silva et al. / Construction and Building Materials 225 (2019) 1127–1139

4.0 28d

90d

180d

1.0 0.8 0.6 0.4 0.2

3.5 3.0

20 14d 90d Carb. depth

18

28d 180d

16 14

2.5

12

2.0

10

1.5

8 6

1.0

4

0.5

2

0.0

0.0 0

0.3

0.65

0 0

1

Carbonaon depth (mm)

Flexural strength (MPa)

1.2

14d

Compressive strength (MPa)

1.4

0.3

0.65

1

PCE dosage (%)

PCE dosage (%)

Fig. 13. Flexural and compressive strength values and carbonation depth (black dots) of lime mortars with PCE at different curing ages.

with PCE. A clear decrease of porosity can be seen, as the PCE dosage increases and as the specimens age (from 14 to 180 days), reaching its minimum value at 180 days. From Fig. 15(a) it also appears that there is no relationship between entrained air content and porosity. Indeed, while the entrained air voids tend to be spherical and isolated, during curing, these can be intersected by shrinkage cracks thus becoming accessible to water and detectable by the open porosity test. In turn, Fig. 15(b) shows what occurs for lime mortars with PNS. As can be seen, open porosity decreased as PNS percentage in the mortar rises, being this decrease more pronounced (up to 17%) than that observed in lime mortars with increasing PCE dosage (up to 11%), which can be explained fairly well by the lower amount of entrained air voids (Fig. 16). Furthermore, the higher porosity obtained for the mortars with PCE, due to air entrainment (Fig. 16), appears to be the reason why the compressive strength values presented by these mortars were lower than those presented by the mortars with PNS, as can be perceived in Fig. 17, which shows good correlations between compressive strength and open porosity. The tendencies observed for open porosity with increasing superplasticizer content should then result from a balance between a decrease of capillary pores due to a lower w/b ratio, when compared to the reference mortar, and a rise in entrained air voids, in the case of mortars with PCE; or mainly from a decrease of capillary pores, in the case of mortars with PNS.

14d

28d

90d

180d

Compressive strength (MPa)

Flexural strength (MPa)

1.2

20

4.5

1.4

1.0 0.8 0.6 0.4 0.2

4.0

14d

28d

18

3.5

90d

180d

16

3.0

14

Carb. depth

12

2.5

10

2.0

8

1.5

6

1.0

4

0.5

2

0.0

0.0 0

0.8

1

1.9

PNS dosage (%)

3

Carbonaon depth (mm)

strength steadily increases with the addition of PCE, even at early ages. For instance, when 1% PCE was used, an increase of 52% and 82%, respectively for the flexural and compressive strength values, was registered at 14 days, whereas that increase was of 79% and 84% at 28 days of age. As for the lime mortars with PNS, similar conclusions were reached: strength values rose with increasing dosage (Fig. 14). Again, for 3% PNS a strength increase of 34% and 161% was obtained, respectively, for the flexural and compressive strength at 14 days, while those values were 42% and 153% at 28 days. It is noticeable the much greater increase in compressive than in flexural strength, which can be an indication that the ductility of the specimens with PNS might be lower than those without admixtures or with PCE (higher compressive to flexural strength ratio). Moreover, when comparing the effect of the same dosage of superplasticizer (1%), it can be concluded that PCE always led to higher strength values. Notwithstanding, for the highest dosage, 1% PCE gives rise to mortars with higher flexural strength but lower compressive strength than those with 3% PNS. As so, it can be concluded that, despite requiring a higher dosage, PNS is more effective at increasing the mortars compressive strength, but tendentially gives rise to less ductile mortars, which can be a problem in old buildings. Regarding porosity, Fig. 15(a) shows the open porosity values and the air content measured in the fresh state for the lime mortars

0 0.0

0.8

1.0

1.9

3.0

PNS dosage (%)

Fig. 14. Flexural and compressive strength values and carbonation depth (black dots) of lime mortars with PNS at different curing ages.

B. Silva et al. / Construction and Building Materials 225 (2019) 1127–1139

28d

90d

180d

14d

Air

28d

90d

180d

Air

7

35

7

30

6

30

6

25

5

25

5

20

4

20

4

15

3

15

3

10

2

10

2

5

1

5

1

0

0

0 0

0.3

0.65

Open porosity (%)

35

Air content (%)

Open porosity (%)

14d

1

Air content (%)

1136

0 0

0.8

PCE content (%)

1

1.9

3

PNS dosage (%)

(a)

(b)

Fig. 15. Open porosity values of lime mortars with (a) PCE and (b) PNS.

32

32

PCE

PCE

PNS 31

Open porosity (%)

31

Open porosity (%)

PNS

30 y = -0.66x + 31.78 R² = 0.99

29 28

y = -4.57x + 36.85 R² = 0.93

27

y = -1.71x + 33.02 R² = 0.98

30 29 28

y = -1.89x + 33.43 R² = 0.96

27 26

26 0.0

2.0

4.0

6.0

8.0

Air content (%)

0.0

1.0

2.0

3.0

4.0

Compressive strength (MPa)

Fig. 16. Relationship between air content and open porosity of the studied mortars at 180 d.

Fig. 17. Relationship between compressive strength and open porosity of the studied mortars at 180 d.

In terms of carbonation depth, lower carbonation depths were noted in the mortars with higher superplasticizer content at later ages (Figs. 13(b) and 14(b)), probably a consequence of the lower porosity that hindered the access of carbon dioxide to interior non-carbonated areas of the mortar. Indeed, at 180 days of age all mortars appeared to be fully carbonated (according to the phenolphthalein indicator) with exception of those with 1% PCE and 3% PNS as can be seen in Figs. 13(b) and 14(b), respectively. Fig. 18 presents the pore size distribution curves obtained for some of the studied mortars and their porosity obtained with MIP, whereas Fig. 19 shows the percentage of intruded mercury in various pore ranges. The reference mortar presented a bimodal distribution, with two peaks of main intruded volume related to medium (0.1–1 mm) and large (10–100 mm) pore diameters, in accordance to other research works [33–35]. The presence of a superplasticizer led to a division of the peak related to large pores into two smaller peaks and, thus, to an almost trimodal distribution (Fig. 18). However, in terms of pore size (Fig. 19), only slight

changes were detected, namely in the amount of small pores (<0.1 mm) that increased and in the amount of large pores (>10 mm) that decreased, especially in the mortars with 1% PCE and 3% PNS, as a result of the lower w/b ratio used in their production. Accordingly, the average pore size ranged between ca. 0.3 mm for the mortars with 1% PCE and 3% PNS and ca. 0.4 mm for the reference aerial lime mortar and that with 1% PNS. The results obtained contrast with those presented by Fernández et al. [5], in which the addition of 0.5% PCE to lime mortars was responsible for a strong reduction of pores between 1 and 10 mm, even in the mortar with excessive mixing water (w/b ratio equal to the reference mortar, 1.54). However, no reference is made regarding the air-entraining action of the admixture (air content was not determined) and PCE dosage, water/binder ratio and aggregate type were different from those used here. Comparing the effect of both superplasticizers, Fig. 19 allows conveying that, for the same dosage (1%), PNS led to a lower amount of small pores and a greater amount of large pores than

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PCE, but the maximum dosage (1% PCE and 3% PNS) led to similar porous structures in terms of pore size and porosity, according to MIP results. Regarding the latter, it is worth mentioning that the porosity values obtained with MIP were slightly higher (up to 7%) than those obtained with the RILEM I.1 test, which can be attributed to the different measuring ranges of both tests [34]. Nevertheless, MIP results corroborate the tendencies obtained with the RILEM I.1 test. Note that, the mortar with 1% PCE and that with 3% PNS should have an identical amount of capillary pores (normally ranging from 0.1 to 100 mm, based on Thomson et al. [36]), since their w/b ratio is the same, but the mortar with 3% PNS presents a lower porosity presumably due to the lower amount of pores derived from air-entrainment (pores > 10 mm in diameter), as confirmed by MIP results (Fig. 19). The SEM images obtained in the BSE mode show almost no differences in the porous structure of the mortars with and without superplasticizer (Fig. 20), as MIP results suggested. Indeed, all mortars displayed larger rounded and thinner elongated pores typical of aerial lime mortars [34,37], and where the small air-entraining action of the PCE-based superplasticizer was not evident, neither was the lowest porosity of the mortars with admixtures. In overall, PCE and PNS led to similar results in the sense that they both led to a vast mechanical strength increase (of up to 161%) and to a porosity decrease (of up to 17%), while keeping the porous structure rather unaltered. Despite this, and according to the results, PNS might be more detrimental to lime mortars, since it led to less ductile mortars and to a stronger decrease of porosity than PCE. Based on these results, superplasticizers can be useful for improving lime mortars, since they promote an increase of mechanical strength even at early ages, but still without reaching the typical strength values of a cement mortar (20 MPa), while maintaining the porous structure close to that of a pure lime mortar. Since the reported changes are moderate, compatibility with old materials can potentially be maintained, avoiding premature damage and consequent need for repair.

Fig. 18. Pore size distribution and MIP porosity (in brackets) of aerial lime mortars with PCE and PNS at 180 days of age.

100%

4.8

7.0

5.7

5.6

90%

Intruded volume

80%

38.8

42.7

70%

43.8

37.4

60% 50%

16.9

14.1

40%

17.2 14.9

30% 30.9

30.0

20%

28.9

10% 6.1

8.6

6.6

9.4

REF

1% PCE

1% PNS

3% PNS

0.1-1µm

1-10µm

10-100µm

0% <0.1µm

4. Conclusions

30.4

In this work, the effect of the addition of two types of superplasticizer (PCE and PNS) on the fresh and hardened state properties of aerial lime mortars was studied. In the fresh state, it was concluded that the addition of either type of superplasticizer to lime mortars:

>100µm

- When the kneading water was maintained, led to a higher fluidity and, thus, to lower torque values. As consequence, g values (related to yield stress) decreased sharply with admixture

Fig. 19. Percentage of intruded volume in various pore ranges.

300 µm

300 µm

(a)

300 µm

(b)

(c )

Fig. 20. SEM-BSE images of the mortars (a) without admixtures, (b) with 1% PCE and (c) with 3% PNS at 180 days of age.

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dosage. During the rheometric test, an increase of torque values (thickening of the lime suspension) was observed, especially in the mortars with low superplasticizer dosages. For higher dosages (0.65% PCE and 1.9% PNS), thickening was not observed, but segregation signs were. As consequence, g values increased over testing time in the former mortars and remained under low values in the latter; - When the consistency was maintained, led to similar torque vs. time curves to that of the reference mortar, but with progressively higher torque values and more notorious thickening throughout the test, with increasing dosage. Regarding the type of superplasticizer, PCE proved to be more effective (it led to a higher fluidity/higher water reduction, for the same dosage) and efficient (lower dosage was needed), due to its greater dispersive effect. However, for the maximum dosage recommended, both substances led to similar results. In addition, PCE displayed a lower fluidity loss over time, but a greater tendency towards bleeding and a small air-entraining action. In the hardened state, both admixtures led to an increase of the mortars’ mechanical strength (of up to 161%). For the same dosage, PCE led to higher strength values, but for the maximum dosage recommended by the manufacturer PNS led to higher compressive strength values. Porosity values decreased (up to 17%) with superplasticizer dosage, especially when PNS was used, leading to slightly lower carbonation depths, as a result. Nonetheless, the mortars with superplasticizer maintained a porous structure similar to that of the reference mortar, with only slight changes in the volume of smaller (<0.1 mm) and larger pores (>10 mm). To conclude, both types of superplasticizer were responsible for a vast mechanical strength increase (even though the resulting mortars are still much weaker than cement-based ones), without causing important changes in the porous structure of lime mortars, so that compatibility with the old masonries can be maintained. In view of these results it can be stated that these substances have potential to be used in mortars for restoration interventions. For that, however, other relevant properties of lime mortars with superplasticizer still need to be assessed, such as their modulus of elasticity, shrinkage, behaviour in the presence of water (water absorption, vapour permeability etc.) and susceptibility to degradation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to gratefully acknowledge CERIS research centre for supporting the experimental work and the Hercules laboratory for supporting the analytical characterization. The first author would also like to acknowledge the financial support provided by Fundação para a Ciência e a Tecnologia (FCT), through the doctoral scholarship SFRH/BD/107879/2015. References [1] EN 934-2 – Admixtures for concrete, mortar and grout. Concrete admixtures. Definitions, requirements, conformity, marking and labelling. European Committee for Standardization (CEN), 2009. [2] B.A. Silva, A.P. Ferreira Pinto, A. Gomes, A. Candeias, Influence of water content and mixing conditions on the properties of lime-based materials, In press.

[3] A. Izaguirre, J. Lanas, J.I. Alvarez, Ageing of lime mortars with admixtures: durability and strength assessment, Cem. Concr. Res. 40 (7) (2010) 1081–1095, https://doi.org/10.1016/j.cemconres.2010.02.013. [4] M. Pérez-Nicolás, A. Duran, I. Navarro-Blasco, J.M. Fernández, R. Sirera, J.I. Alvarez, Study on the effectiveness of PNS and LS superplasticizers in air limebased mortars, Cem. Concr. Res. 82 (2016) 11–22, https://doi.org/10.1016/j. cemconres.2015.12.006. [5] J.M. Fernández, A. Duran, I. Navarro-Blasco, J. Lanas, R. Sirera, J.I. Alvarez, Influence of nanosilica and a polycarboxylate ether superplasticizer on the performance of lime mortars, Cem. Concr. Res. 43 (2013) 12–24, https://doi. org/10.1016/j.cemconres.2012.10.007. [6] P. Aïtcin, R.J. Flatt (Eds.), Science and Technology of Concrete Admixtures, first ed., Woodhead Publishing, Cambridge, 2015. [7] J. Newman, B.S. Choo (Eds.), Advanced Concrete Technology 1: Constituent Materials, Butterworth-Heinemann, Elsevier, Oxford, 2003. [8] X. Ouyang, X. Qiu, P. Chen, Physicochemical characterization of calcium lignosulfonate – a potentially useful water reducer, Colloids Surf., A: Physicochem. Eng. Aspects 282 (2006) 489–497, https://doi.org/10.1016/ j.colsurfa.2005.12.020. [9] D. Yang, Y. Qin, Y. Du, D. Zheng, Adsorption characteristics of naphthalene sulfonate formaldehyde condensate with different molecular weights, J. Dispersion Sci. Technol. 34 (8) (2013) 1092–1098, https://doi.org/10.1080/ 01932691.2012.737749. [10] F. Puertas, H. Santos, M. Palacios, S. Martínez-Ramírez, Polycarboxylate superplasticiser admixtures: effect on hydration, microstructure and rheological behaviour in cement pastes, Adv. Cem. Res. 17 (2) (2005) 77–89, https://doi.org/10.1680/adcr.2005.17.2.77. [11] I. Navarro-Blasco, M. Pérez-Nicolás, J.M. Fernández, A. Duran, R. Sirera, J.I. Alvarez, Assessment of the interaction of polycarboxylate superplasticizers in hydrated lime pastes modified with nanosilica or metakaolin as pozzolanic reactives, Constr. Build. Mater. 73 (2014) 1–12, https://doi.org/10.1016/ j.conbuildmat.2014.09.052. [12] N. Roussel (Ed.), Understanding the Rheology of Concrete, Elsevier, 2011. [13] M.P. Seabra, H. Paiva, J.A. Labrincha, V.M. Ferreira, Admixtures effect on fresh state properties of aerial lime based mortars, Constr. Build. Mater. 23 (2) (2009) 1147–1153. [14] A.P. Ferreira Pinto, A. Gomes, B. Silva, A. Candeias, F. Vale, Influence of water reducers on the early age properties of aerial lime mortars, in: 3rd International Conference on Protection of Historical Constructions, Lisbon, 2017, pp. 337–338. [15] EN 459-1 – Building lime. Definitions, specifications and conformity criteria. European Committee for Standardization (CEN), 2010. [16] EN 13139 – Aggregates for mortar. European Committee for Standardization (CEN), 2002. [17] E. Janowska-Renkas, The effect of superplasticizers’ chemical structure on their efficiency in cement pastes, Constr. Build. Mater. 38 (2013) 1204–1210, https://doi.org/10.1016/j.conbuildmat.2012.09.032. [18] Y. Li, C. Yang, Y. Zhang, J. Zheng, H. Guo, M. Lu, Study on dispersion, adsorption and flow retaining behaviors of cement mortars with TPEG-type polyether kind polycarboxylate superplasticizers, Constr. Build. Mater. 64 (2014) 324– 332, https://doi.org/10.1016/j.conbuildmat.2014.04.050. [19] Y.R. Zhang, X.M. Kong, Z.B. Lu, Z.C. Lu, S.S. Hou, Effects of the charge characteristics of polycarboxylate superplasticizers on the adsorption and the retardation in cement pastes, Cem. Concr. Res. 67 (2015) 184–196, https://doi. org/10.1016/j.cemconres.2014.10.004. [20] H. El-Didamony, M. Heikal, I. Aiad, S. Al-Masry, Behavior of delayed addition time of SNF superplasticizer on microsilica-sulphate resisting cements, Ceramics-Silikáty 57 (3) (2013) 232–242. [21] EN 196-1 – Methods of testing cement. Determination of strength. European Committee for Standardization (CEN), 2005. [22] EN 1015-3 – Methods of test for mortar for masonry. Determination of consistency of fresh mortar by flow table. European Committee for Standardization (CEN), 1999. [23] EN 1015-4 – Methods of test for mortar for masonry. Determination of consistence of fresh mortar (by plunger penetration). European Committee for Standardization (CEN), 1999. [24] M.R. Rixom, N.P. Mailvaganam, Chemical admixtures for concrete, third ed., E. & F.N. Spon Ltd, London, United Kingdom, 1999. [25] EN 1015-6 – Methods of test for mortar for masonry. Determination of bulk density of fresh mortar. European Committee for Standardization (CEN), 1999. [26] EN 1015-7 – Methods of test for mortar for masonry. Determination of air content of fresh mortar. European Committee for Standardization (CEN), 1998. [27] P.F.G. Banfill, Use of the ViscoCorder to study the rheology of fresh mortar, Mag. Concr. Res. 42 (153) (1990) 213–221, https://doi.org/ 10.1680/macr.1990.42.153.213. [28] EN 1015-11 – Methods of test for mortar for masonry. Determination of flexural and compressive strength of hardened mortar. European Committee for Standardization (CEN), 1999. [29] RILEM Test I.1 – Porosity accessible to water. RILEM 25-PEM – Recommandations provisoires. Essais recommandés pour mesurer l’altération des pierres et évaluer l’éfficacité des méthodes de traitement. Matériaux et Construction, 13(75), 1980. [30] M. Collepardi, Admixtures used to enhance placing characteristics of concrete, Cem. Concr. Compos. 20 (2–3) (1998) 103–112, https://doi.org/10.1016/ S0958-9465(98)00071-7.

B. Silva et al. / Construction and Building Materials 225 (2019) 1127–1139 [31] S. Chandra, J. Björnström, Influence of superplasticizer type and dosage on the slump loss of Portland cement mortars—part II, Cem. Concr. Res. 32 (10) (2002) 1613–1619, https://doi.org/10.1016/S0008-8846(02)00838-4. [32] J. Gołaszewski, J. Szwabowski, Influence of superplasticizers on rheological behaviour of fresh cement mortars, Cem. Concr. Res. 34 (2) (2004) 235–248, https://doi.org/10.1016/j.cemconres.2003.07.002. [33] J. Lanas, J.I. Alvarez-Galindo, Masonry repair lime-based mortars: factors affecting the mechanical behaviour, Cem. Concr. Res. 33 (11) (2003) 1867– 1876, https://doi.org/10.1016/S0008-8846(03)00210-2. [34] M.J. Mosquera, B. Silva, B. Prieto, E. Ruiz-Herrera, Addition of cement to limebased mortars: effect on pore structure and vapor transport, Cem. Concr. Res. 36 (9) (2006) 1635–1642, https://doi.org/10.1016/j.cemconres.2004.10.041.

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[35] B.A. Silva, A.F. Pinto, A. Gomes, Influence of natural hydraulic lime content on the properties of aerial lime-based mortars, Constr. Build. Mater. 72 (2014) 208–218, https://doi.org/10.1016/j.conbuildmat.2014.09.010. [36] M.L. Thomson, J.E. Lindqvist, J. Elsen, C.J.W.P. Groot, Porosity of historic mortars, in: D. Martens, A. Vermeltfoort (Eds.), 13th International Brick and Block Masonry Conference Amsterdam, 2004. [37] B.A. Silva, A.F. Pinto, A. Gomes, Natural hydraulic lime versus cement for blended lime mortars for restoration works, Constr. Build. Mater. 94 (2015) 346–360, https://doi.org/10.1016/j.conbuildmat.2015.06.058.