Impact of a viscosity-modifying admixture on the properties of lime mortars

Journal Pre-proof Impact of a viscosity-modifying admixture on the properties of lime mortars Bruna Silva, Ana Paula Ferreira Pinto, Augusto Gomes, António Candeias PII:

S2352-7102(19)31640-7

DOI:

https://doi.org/10.1016/j.jobe.2019.101132

Reference:

JOBE 101132

To appear in:

Journal of Building Engineering

Received Date: 20 August 2019 Revised Date:

10 December 2019

Accepted Date: 18 December 2019

Please cite this article as: B. Silva, A.P. Ferreira Pinto, A. Gomes, Antó. Candeias, Impact of a viscositymodifying admixture on the properties of lime mortars, Journal of Building Engineering (2020), doi: https://doi.org/10.1016/j.jobe.2019.101132. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Impact of a viscosity-modifying admixture on the properties of lime mortars Bruna Silva1*; Ana Paula Ferreira Pinto1; Augusto Gomes1; António Candeias2 1

CEris, Department of Civil Engineering, Architecture and Georresources, Instituto Superior Técnico, Universidade de Lisboa, Portugal *

2

Corresponding author: [email protected]

HERCULES Laboratory and Chemistry Department, School of Sciences and Technology, University of Évora, Portugal

Abstract Viscosity-modifying admixtures are frequently used to ease the application of mortars, especially in ready-mixed products, with consequences on the hardened material. However, the knowledge on the influence of these admixtures on lime mortars is still incipient. In this context, this paper intends to analyse the impact of a viscosity-modifying admixture (VMA) on the fresh and hardened state properties of aerial lime mortars. In the fresh state, the VMA led to a thickening of the lime suspension, which translated into a progressive increase in torque, plastic viscosity and yield stress (after an initial decrease) with increasing dosage. At the same time, it increased air content, which made the mortars easier to handle, and increased the bleeding rate, but the amount of bleed water was lower. In the hardened state, the VMA did not improve mechanical strength owing to a porosity increase (based on a rise in large pores). Key-words: Lime; Mortar; Viscosity-modifying admixture; Restoration;

1. Introduction In conservation and restoration interventions the best option is always to apply mortars as close as possible to the original ones, which are usually based on aerial lime, in order to avoid compatibility problems. However, the drawbacks typically associated to lime mortars, such as their long setting and hardening times and high drying shrinkage, together with the loss of

1

know-how regarding the production and application of these mortars, discourage their use. As a result, traditional mortars have been progressively replaced with modern industrial readymixed mortars that are easier to prepare and apply and designed to improve hardened state properties by including a wide variety of admixtures and other additives [1-3]. This has consequences on the compatibility with the original materials, often leading to accelerated deterioration and premature failure of the final solution. To aid in the selection of the most suitable components for these mortars, the knowledge about the action mechanism of these admixtures and their contribution to the overall behaviour of the mortar is of crucial importance. After water-reducing admixtures (plasticizers and superplasticizers) viscosity-modifying admixtures (VMAs) are perhaps the most widely used admixtures in ready-mixed mortars. However, although the influence of VMAs on cement-based materials has been the subject of numerous investigations, the study of their influence on lime-based materials is still quite incipient and therefore remains poorly understood. Viscosity-modifying admixtures are typically used to improve the workability and ease the application of mortars [4, 5]. This can be an advantage, given that most masons nowadays are not used to apply the plastic lime mortars, but the fluid cement ones. Moreover, some VMAs have the ability to retain water inside the mortar, which can be useful for restoration mortars since ancient masonries are made with highly porous materials that can absorb water from the mortar, dehydrating it and hindering carbonation [7]. However, VMAs are also known to have a set-retarding action, at least on cement-based materials, and to display a strong dosagedependent behaviour: they can have thickening or dispersing effects according to the dosage used [4-8]. These substances also have implications for the hardened mortar. For instance, the improvement of workability owing to VMAs can be attributed to the air-entraining action of 2

these substances, which reduces the friction between particles. In turn, these air voids can reduce mechanical strength, which in the already weak lime mortars can be a problem, but they can also cut the capillary network and, therefore, reduce water absorption and improve the mortars’ resistance to freeze-thaw cycles [7] and salt crystallization. Most VMAs are hydrophilic, water-soluble organic polymers, being the most widely used those based on cellulose ethers, namely hydroxypropyl methyl cellulose (HPMC) and hydroxyethyl methyl cellulose (HEMC), even though the use of starches and gums has increased in recent years [9]. Both starch and cellulose are glucose polymers, i.e., sugar molecules (polysaccharides) derived from plants. In the glucose molecule, five hydroxyl groups (–OH) are attached to a six membered ring made of five carbon and one oxygen atoms. Despite the similar composition, starch and cellulose differ in the placement of the hydroxyl group attached to the number 1 carbon. To make them soluble and stable at the high pH of alkaline materials such as lime or cement mortars, they are subjected to complex chemical reactions (etherification or esterification). Viscosity-modifying admixtures work by adsorbing and fixing water molecules in their structure, thus reducing the amount of free water in the mixture and leading to an increase in viscosity. In addition, polymer chains can suffer an intertwining process and can adsorb onto neighbouring binder particles, physically holding them together, which further increases viscosity [4, 9, 10]. The behaviour of VMAs when added to cement-based materials was assessed by several authors, including Khayat [11], who published an overview of the substances used and their effects, Lachemi et al. [10, 12], Peschard et al. [13], Leemann and Winnefeld [14], Pourchez et al. [15], Patural et al. [16], Lasheras-Zubiate et al. [17], Cappellari et al. [18] and Wyrzykowski et al. [19]. However, their behaviour in aerial lime mortars was essentially evaluated by Seabra 3

et al. [20], Izaguirre et al. [6-8] and more recently by Žižlavský et al. [21-23] and Vyšvařil et al. [24, 25]. The study performed by Seabra et al. [20] focused only on the fresh state properties of aerial lime mortars with the addition of several admixtures, including a viscosity-modifying admixture based on cellulose ether (HPMC). At a constant water/binder (w/b) ratio, the rheometric tests performed led to the conclusion that the presence of HPMC initially caused an increase in torque (flow resistance), greater for higher dosages. Nonetheless, for longer agitation times torque values tended to decrease, more noticeably for higher added amounts of HPMC. The authors justified this behaviour with a rise in entrained air with agitation time and an alignment of polymer chains that led to a reduction of the flow resistance of the mortar. Plastic viscosity (h) values initially decreased but after a certain agitation time started to increase, whereas yield stress (g) values increased with agitation time. Both parameters tended to decrease with HPMC dosage. Izaguirre et al. [7] compared the effect of two VMAs on aerial lime mortars: one based on cellulose ether (HPMC) and another based on guar gum. In the fresh state, higher water amounts were needed in order to obtain the required consistency in the mortars with these admixtures. An increase in air content and setting time was also noted, especially with guar gum, the latter due to the presence of an excessive amount of mixing water. Pastes with VMA displayed a shear-thinning behaviour, that is, a reduction in viscosity with increasing shear rate. In the hardened state both admixtures were detrimental to mechanical strength and increased open porosity and permeability to water vapour. In other works, Izaguirre et al. [6, 8] evaluated the influence of several dosages of a VMA based on potato starch. The authors concluded that this admixture behaved as a thickener for dosages up to 0.3% (of the total dried mortars weight), that is, it led to a decrease in flow values, and as a plasticizer above that value. In the thickening range, however, a decrease in setting time and an improvement of

4

mechanical strength was verified, which contrasts with what has been reported for aerial lime mortars with other types of VMAs. Moreover, open porosity, water absorption and vapour permeability only changed slightly. Žižlavský et al. [21] evaluated the effect of several types of VMAs, namely gellan gum, xanthan gum, diutan gum, sodium salt of alginic acid and carrageenan, on lime mortars with a constant w/b ratio. Most of the admixtures were responsible for a decrease in fluidity and in bulk density, an increase in water retention and a slight increase in air content. However, the admixtures had different effectiveness: diutan gum led to the greatest variations in these properties and gellan gum to almost none. All the substances lowered the mechanical strength at 7 days; but at 28 days of curing diutan gum, but especially sodium salt of alginic acid, led to comparable results to those of the reference mortar. The same authors [22-25] also analysed the influence of cellulose and chitosan ethers and of guar gum derivatives on the properties of lime mortars. Based on the results obtained the authors concluded that cellulose ethers were not as suitable as chitosan ethers or guar gum to be used as VMAs in lime mortars. From the review of the literature carried out it became evident that potato starch was possibly the most useful VMA for lime mortars: besides being able to improve their properties in the fresh state, it could increase the low mechanical strengths typical of these mortars and maintain their physical properties almost unaffected, thus minimizing compatibility problems. Moreover, the still insufficient information regarding the effect of starches on lime mortars justifies the interest of this study. For these reasons, this paper intends to analyse the impact of a viscosity-modifying admixture based on potato starch on the fresh and hardened state properties of mortars made with aerial lime.

2. Materials and methods 2.1. Materials and formulations 5

A powdered hydrated lime (CL80-S according to EN 459-1:2010 [26]) and a sand of siliceous nature (0/2 mm according to EN 13139:2002 [27]) were used for producing the mortars. The mineralogical composition and particle size distribution of these materials were already presented in previous papers [28, 29]. A commercial viscosity-modifying admixture based on soluble potato starch was used. The composition of the commercial product was compared with that of a purer form of potato starch (soluble potato starch, from Panreac), by subjecting both substances to micro-FTIR analysis using a Bruker Hyperion 3000 infrared spectrometer equipped with a single point MCT detector, in transmission mode, and a diamond compression cell from S.T. Japan. The resulting IR-spectra, presented in Fig. 1, were acquired at a spectral resolution of 4 cm-1, in the infrared region of 4000-600 cm-1, and are the result of averaging 64 scans. The commercial product proved to have a composition very similar to that of the purer product. Both spectra showed the presence of hydrogen bonded hydroxyl groups (broad band between 3600 and 3200 cm-1 attributed to O-H stretching vibrations) and of C-H bonds (peaks around 2900 cm-1 characteristic of C-H stretching vibrations). The sharp peak at 1648 cm-1 can be attributed to OH bending vibrations, but also to C=O stretching vibrations, whereas the peaks between 14801300 and 1200-900 cm-1 are due to C-H bending and C-O stretching, respectively. The spectra obtained also proved to be comparable to those obtained by Izaguirre et al. [6, 8], Rosu et al. [30] and Jivan et al. [31]. The VMA was added in several percentages, namely in 0.06, 0.3 and 1.2% of lime weight, which corresponded to the lowest, middle and highest dosage of the range recommended by the manufacturer. Besides these, two other intermediate percentages were used in the fresh state tests (0.12 and 0.9%). The VMA was introduced together with the mixing water and the lime, with no additional mixing time.

6

The mortars were produced with a volumetric binder/aggregate ratio of 1:2. The water content used in the production of the lime mortars was excessive in order to evaluate the VMAs effect on bleeding and segregation. The value chosen was 26% (water/binder ratio of 1.56), based on Silva et al. [28]. A lime mortar without admixtures was taken as reference.

Fig. 1 – Comparison between the FTIR spectra of the commercial and of the purer form of soluble potato starch

The mortars production followed the procedures of EN 196-1:2005 [32]. A sample of fresh mortar was used for the rheometric test and another to evaluate consistency, bulk density and air content. The remaining mortar was used to cast prismatic mortar specimens with the standard dimensions (160x40x40 mm). These specimens were cured in a controlled environment (20±5 ºC and 60±10 %RH) and were de-moulded after 7 days of curing. 2.2. Fresh state characterization 2.2.1. Consistency

7

Consistency was evaluated by flow table and by plunger penetration, according to EN 10153:1999 [33] and EN 1015-4:1999 [34], respectively. In the flow table test, the mortar spread diameter (flow value) after the mould was removed (initial flow value) and after jolting the flow table 15 times (final flow value) was registered. In the plunger penetration test, the penetration of a plunger into a vessel filled with fresh mortar on a scale from 0 to 70 mm was measured. Two determinations were made in distinct batches and the results presented correspond to the average values. 2.2.2. Bulk density and air content Bulk density was determined according to EN 1015-6:1999 [35] and air content according to EN 1015-7:1998 [36]. The results presented for each mortar formulation correspond to the average value of two determinations made in distinct batches. 2.2.3. Bleeding Bleeding was observed in the mixes with VMA, particularly when they were left to stand. The tendency of a mortar to bleed was evaluated by filling a 1000 ml graduated cylinder with mortar up to 900±10 ml, and measuring the initial height of mortar (ℎ ). The top of the cylinder was sealed with plastic wrap to avoid water evaporation and the cylinder was left to stand in a controlled environment (20±5 ºC and 60±10 %RH). After 3 and 24 hours the height of the water layer above the material was measured (ℎ ). Bleeding was then calculated as following:

=

ℎ ℎ

100 (%)

(1)

Eventual alterations owing to segregation were also registered. 2.2.4. Rheological measurements

8

The flow behaviour of mortars can be in many cases described with sufficient accuracy by the Bingham model, according to the following equation: =

where (Pa) is the shear stress, (

(2)

+

(Pa) is the yield stress,

(Pa.s) is the plastic viscosity and

) is the shear rate.

In Binghamian fluids a minimum stress (the yield stress) is necessary for flow to occur. Above the yield stress the shear stress vs. shear rate curve has a constant slope that is generally termed as plastic viscosity. The rheological properties of the mortars with VMA were measured using a rotational rheometer (Viskomat NT, from Schleibinger Testing Systems) suitable for mortars and pastes. In this type of equipment the mortar is placed in a cylindrical vessel that rotates around a paddle according to a predefined speed profile. During the test, the equipment registers the torque generated by the mortar flowing through the paddle, the rotation speed and the time. A step speed profile was used, similar to that defined by Seabra et al. [37], which consisted in keeping the rotation speed constant at 200 rpm for 3 minutes and then decreasing it in steps of 20 rpm every 60 seconds, as shown in Fig. 3. The total test duration was 12 minutes. From the step profile data, torque values from each step were taken which allowed plotting a torque versus speed graph. If the Bingham model is assumed, the obtained data can then be fitted using a linear regression, according to the equation: T=g+ℎ where

is the torque (N.mm),

is the rotation speed (rpm) and

(3) and ℎ are parameters

directly proportional to yield stress and plastic viscosity, respectively. In this work, the

9

Bingham model proved to be well suited to describe the fluid behaviour of the studied mortars.

250

Speed (rpm)

200 150 100 50 0 0

2

4

6

8

10

12

Time (min)

Fig. 3 – Step speed profile used in this study

The reason for using this type of profile instead of using a dwell speed profile is that the first one allows determining torque values in equilibrium conditions rather than instantaneously, which yields better correlation coefficients, a fact also highlighted by Seabra et al. [37]. 2.2.5. Particle size The particle size of lime suspensions with and without the VMA was determined by laser diffraction using a Mastersizer 2000 particle size analyser from Malvern. The suspensions were prepared in the same way as the mortars (Section 2.1), but the aggregate was removed using a sieve with a mesh size of 100 µm after mixing ended. 2.3. Hardened state characterization 2.3.1. Mechanical strength Flexural and compressive strength were determined according to EN 1015-11:1999 [38], on 3 mortar specimens per type of mortar, after 14, 28, 90 and 180 days of curing. The freshly

10

broken surfaces of the specimens resulting from the flexural test were sprayed with a phenolphthalein solution in order to qualitatively determine their carbonation depth. 2.3.2. Porosity and porous structure Open porosity was determined in accordance to RILEM Test I.1 [39] guidelines on 3 half specimens of each mortar formulation, after 14, 28, 90 and 180 days of curing. In order to complement these results, Mercury Intrusion Porosimetry (MIP) analyses were carried out on mortar samples after 180 days of curing, using a Micromeritics Autopore IV 9500 porosimeter. These analyses allowed the determination of the porosity and pore size of these mortars. Scanning Electron Microscopy (SEM) analyses were used to further detect changes in the porous structure of the mortars promoted by the viscosity-modifying admixture. These were carried out on mortar samples after 180 days of curing. The samples were obtained by cutting small pieces from the mortar specimens, embedding them in epoxy resin and polishing one of their surfaces. A Thermo Fisher Scientific Phenom ProX microscope was then used to analyse the polished surfaces of these samples in the backscattered electron mode (BSE).

3. Results and discussion 3.1. Fresh state properties As already mentioned, the impact of the viscosity-modifying admixture (VMA) was evaluated on lime mortars with a high water content in order to understand the effect of this substance on the susceptibility of the mortars to segregation and bleeding. The resulting mortars were subjected to fresh state characterization (Table 2 and Fig. 3) and to rheometric testing (Fig. 4). As can be perceived from Fig. 3, the admixture led to an expected decrease in flow values and in plunger penetration depths, being this decrease more substantial for higher dosages of

11

VMA. In turn, bulk density values decreased due to a higher air content (Table 2), which may result from a small air-entraining effect of the admixture and an increase in entrapped air due to a lower fluidity. These trends are consistent with those obtained by Izaguirre et al. [6].

Table 2 – Fresh state characterization of lime mortars with a viscosity-modifying admixture (VMA)

Initial flow values Final flow values

250 Flow values (mm)

1.56

Bulk density (kg/m3) 1940 1940 1940 1910 1880 1860

Penetration depth

70

180

60

160

50

200

40 150 30 100

20

50

10

0

0 0.0

0.2

0.4

0.6

0.8

1.0

Bleeding 3h 24h (%) 0.79 2.20 1.13 1.61 1.54 1.94 1.58 1.89 1.42 1.73 1.57 1.89

Air content (%) 0.2 0.2 0.3 1.9 3.4 4.5

1.2

VMA dosage (%)

Fig. 3 – Flow values and plunger penetration values as function of VMA dosage

REF 0.06% VMA

140 Torque (N.mm)

300

w/b ratio (-)

Penetration depth (mm)

VMA dosage (%) 0 0.06 0.12 0.30 0.60 1.20

0.12% VMA

120

0.3% VMA

100

0.6% VMA

80

1.2% VMA

60 40 20 0 0

2

4

6

8

10

12

Time (min)

Fig. 4 – Torque versus time curves of lime mortars with several dosages of VMA (step speed profile)

The variation of torque with time of the studied mortars is presented in Fig. 4. With increasing percentage of VMA, an increase in torque values at the beginning of the test was registered, consistent with the tendency of the flow values/penetration depths to decrease (Fig. 3). However, as the rheometric test continues, torque values steadily decrease. Since the higher

12

the dosage, the higher the decrease, the torque values of the mortars with different added amounts of VMA all tended towards similar values (20-25 N.mm) at the end of the test. The exception was the mortar with 1.2% VMA that continuously presented the highest torque values (Fig. 4). This behaviour was due to the working mechanism of the admixture, that is, to the fixation of water molecules and entanglement of the polymer chains that led to the initial increase in viscosity and in torque values. As shearing continues, the chains dislodge and align in the direction of the flow, causing a progressive decrease in viscosity and in torque values [9, 10, 20, 37]. The air-entraining action of the viscosity-modifying admixture used (Table 2) may have further contributed to this reduction. The rheological parameters obtained from the respective flow curves are presented in Fig. 5. As can be seen, g and h values (related with yield stress and plastic viscosity, respectively) increased as VMA dosage increased, as consequence of the thickening effect promoted by the admixture. Nevertheless, g values initially decreased for dosages up to 0.12% VMA which is in line with what has been reported for VMAs based on cellulose ether [20, 37], namely the existence of a minimum value for yield stress for low VMA dosages. According to the cited works the dosage that allows the minimum yield stress value is seen as the optimal dosage by the industry, since it allows the easiest application. Despite not presenting signs of segregation, the mortars with VMA showed a tendency to bleed (release water) while at rest, being this phenomenon much faster than that observed in the reference mortar, as can be confirmed by the small differences between the bleeding percentages after 3 and 24 hours (Table 2). However, the total volume of bleeding water was lower. In terms of application, the mortars showed to be cohesive/stiff at rest, like standard aerial lime mortars, but lighter and easier to handle when subjected to shear. In this sense, this behaviour is coherent with the rheological behaviour, i.e., a peak of torque values at the beginning of the rheometric test, followed by a sharp decrease in torque values as rotation

13

speed increases (Fig. 4). This behaviour was also observed by Izaguirre et al. [6] and by Seabra et al. [37] and is typical of thickening admixtures, such as potato starch. Another important feature of mortars with admixtures that modify their rheological properties is the maintenance of those properties over time, or the stability of the mortar. This is especially important in aerial lime mortars, since on-site application of these mortars can be extended for several hours due to their slow setting. In order to assess this, flow value and penetration depth measurements were carried out for the mortar with the highest VMA dosage over a testing period of 3 hours after mixing. The results were then compared to those obtained for the reference mortar and are shown in Fig. 6. Flow values and penetration depths decreased over time, as expected. However, the mortar with 1.2% VMA only displayed a higher fluidity loss than the reference mortar after 2 hours, which was much more evident for penetration depths than for flow values. The evaporation of bleeding water might have contributed for this result, as the mortars with VMA tended to bleed when at rest, as

25

0.18 0.16

20 g (N.mm)

0.12 15

0.10 0.08

10

0.06 5

g

0.04

h

0.02

0

h (N.mm.min)

0.14

0.00 0

0.2

0.4

0.6

0.8

1

1.2

VMA dosage (%)

Fig. 5 – Yield stress (g) and plastic viscosity (h) values as function of VMA dosage

Flow value/Penetration depth loss (%)

mentioned earlier.

100 95 90 85 REF - FV 1.2% VMA - FV

80

REF - PD 1.2% VMA - PD

75 0

60

120

180

Time from mixing (min)

Fig. 6 – Fluidity loss over time for the reference mortar and for the mortar with 1.2% VMA (FV - flow values; PD - penetration depths)

14

3.2. Hardened state properties The impact of the viscosity-modifying admixture on the hardened state properties of aerial lime mortars was also evaluated. Mechanical strength was negatively affected by the use of potato starch, as can be seen in Fig. 7, but the variation of strength loss with VMA percentage was not linear. The use of an intermediate dosage of VMA (0.3%) led always to the lowest flexural and compressive strength values, whereas the lowest and highest dosages led to similar results and lying between those registered by the reference mortar and the mortar with 0.3% VMA. For instance, at 180 days, the addition of 0.3% VMA led to reductions in flexural and compressive strength of 40% and 56%, respectively, whereas for 0.06% VMA those reductions were of 18% and 40%. At early age (14 days) only the highest dosage of VMA (1.2%) was able to yield strength values close to those of the reference mortar. This result might be due to the higher porosity of the resulting mortar (Fig. 8), that on the one hand was prejudicial to strength, but on the other hand facilitated carbonation (Fig. 9). Hence, as the admixture dosage increased, a greater amount of entrained air bubbles (due to the air-entraining action of the VMA) and of entrapped air bubbles (due to a lower flow value and thus poorer mechanical compaction) were present in the mortars and carbonation depth increased as a consequence. A great variability in the results must be noted (high standard deviation), especially in flexural strength and open porosity, that can be explained by the low strength of these mortars. This made the specimens very susceptible to damage (e.g. micro cracking, crumbling), especially while de-moulding and handling them. The pore size distributions obtained with MIP for the reference mortar and for that with the maximum dosage of VMA are depicted in Fig. 10. The addition of the VMA led to a shift of the bimodal pore size distribution of the reference mortar towards greater pore sizes and to sharper peaks, which means that it led to a more uniform pore size distribution, which is in 15

agreement with the effect of potato starch and other viscosity-modifying admixtures on lime mortars [7, 8]. MIP also confirmed the higher porosity of the mortar with 1.2% VMA (Fig. 10), being the results close to those obtained with the RILEM test (Fig. 8).

0.8

14d

28d

90d

2.0

180d

Compressive strength (MPa)

Flexural strength (MPa)

0.7 0.6 0.5 0.4 0.3 0.2 0.1

14d

1.8

28d

90d

180d

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

0.0 0

0.06

0.30

0

1.20

0.06

0.30

1.20

VMA dosage (%)

VMA dosage (%)

Fig. 7 – Mechanical strength evolution over time with increasing VMA dosage

14d

28d

90d

180d 5 4

33 32

3

31

2

30 1

29 28

0 0

0.06

0.30

1.20

VMA dosage (%)

Fig. 8 – Open porosity over time with increasing VMA dosage and correlation with the air content measured in the fresh state

Air content (%)

Open porosity (%)

34

28d

90d

180d

20

Carbonation depth (mm)

35

14d

Air

16 12 8 4 0 0

0.06

0.30

1.20

VMA dosage (%)

Fig. 9 – Carbonation depth over time with increasing VMA dosage

The abovementioned porosity increase is in line with the increasing air content measured in the fresh state (Fig. 8), because the spherical and isolated pores attributed to air-entrainment can be intersected by shrinkage cracks, thus becoming accessible to water or mercury. In fact,

16

MIP results show a greater amount of pores around 10-100 µm on the mortar with 1.2% VMA when compared to the reference mortar (Fig. 11), which can be attributed to the small air entraining action of the admixture. In turn, the higher amount of pores greater than 100 µm can be attributed to entrapped air as a result of the lower fluidity of this mix, a consequence also observed by Izaguirre et al. [8]. These larger pores can be responsible for a delay of water absorption by capillarity and can constitute chambers for the expansion of ice and salt crystals (in freezing/thawing and salt crystallization phenomena, respectively), thus, avoiding damage to the mortar [40].

60 REF (29.8%)

0.18

REF

50

0.16

Intruded volume (%)

1.2% VMA (34.1%)

0.14 0.12 0.10 3 µm

Log differential intrusion (ml/g)

0.20

0.08 0.06 0.04

1.2% VMA

40 30 20 10

0.02

0

0.00 0.01

0.1

1

10

100

1000

Pore diameter (µm)

Fig. 10 – Pore size distribution curve and total porosity (in brackets) obtained for the reference lime mortar and for that with 1.2% VMA

<0.1

0.1-3

3-100

>100

Pore diameter ranges (µm)

Fig. 11 – Intruded mercury in different pore ranges obtained for the reference mortar and for that with 1.2% VMA

SEM images allowed complementing the information regarding the porous structure of the mortars. The images obtained are presented in Fig. 12, where it can be perceived that the mortar with 1.2% VMA had a porous structure similar to that of the reference lime mortar, with irregularly shaped pores, mainly of two types: thin, elongated pores and large, rounded pores. In the mortar with VMA, the pores, especially the elongated ones, are tendentially broader than those of the reference mortar, thus corroborating the higher uniformity in pore size pointed out by MIP results. Nevertheless, the rounded large pores present in these 17

mortars seem to have diameters sometimes greater than 100 µm, but since they are accessible through the thinner, elongated ones, MIP has detected them as smaller pores (ink bottle effect)[40][42].

500 µm

300 µm

500 µm

300 µm

(a) (b) Fig. 12 – SEM-BSE images of the (a) reference lime mortar and of the (b) lime mortar with 1.2% VMA

In lime mortars, the impact of VMAs on mechanical strength is not obvious. For instance, Izaguirre et al. [7] studied mortars with a guar gum derivative and with HPMC and highlighted the detrimental effect that both admixtures had on compressive strength. Conversely, Žižlavský et al. [22, 23] reported an increase in compressive strength when studying the properties of lime mortars with VMAs based on guar gum and on chitosan ethers. The differences encountered should be related with the w/b ratio adopted in these works, since 18

Izaguirre et al. [7] used a high water amount in order to obtain the same consistency in all mortars, which delayed carbonation and originated a higher porosity, whereas Žižlavský et al. [23] used a fixed water amount. In another work, Izaguirre et al. [8] evaluated the effect of potato starch and attained a higher mechanical strength in the mortars with the admixture, which contrasts with what was obtained in the present work, despite both using a constant w/b ratio. However, when analysing the effect of potato starch on the porous structure of lime mortars, Izaguirre et al. [8] verified that for the dosage range hereby studied (up to 1.2% of lime weight, equivalent to 0.2% of total dried mortars weight) there was a strong increase in the mercury intruded in the region around 1 µm, but an overall shift of the pore size distribution curves towards smaller diameters and a decrease in larger pores (around 10 µm), which heavily contrasts with what was obtained in this paper. Nevertheless, it should be noted that a much lower w/b ratio was used in [8], which can explain the differences found in the porous structures and, thus, the distinct trends obtained for mechanical strength. From the review of the literature carried out, it became evident that the w/b ratio employed and the airentraining action of the VMA (different types have different air-entraining abilities) significantly influenced the resulting porous structure (porosity and pore size) of the mortars and, consequently, their mechanical properties, being these two factors the origin of most discrepancies found in the literature. In summary, it was found that the use of the viscosity-modifying admixture was detrimental to mechanical strength, due to an increase in porosity, namely in large pores (10-100 µm). Considering these results, it would be interesting in a future work to associate the use of a viscosity-modifying admixture with a plasticizing one, since the latter is known to improve the mechanical strength of aerial lime mortars and to reduce their porosity [29]. Moreover, their combined use is very common in practice, since the addition of high dosages of a plasticizing admixture can impart excessive segregation, being the VMA useful for counteracting this tendency and stabilizing these mixes. 19

4. Conclusions This paper intended to analyse the impact of the addition of a viscosity-modifying admixture (VMA) on the fresh and hardened state properties of aerial lime mortars. In the fresh state, and for a constant w/b ratio, the addition of the VMA originated a decrease in flow values and penetration depths, in line with its thickening effect. The mortars with VMA showed a propensity to release water (bleeding) faster than the reference mortar, but the total amount of bleed water was lower. The increase in entrained air with VMA dosage meant that the mortars became lighter and easier to handle, after an initial higher stiffness. The rheological characterization confirmed this behaviour by showing very high torque values at the beginning of the test (thickening effect) followed by a progressive decrease in torque values as shearing continued (thinning effect). Yield stress and plastic viscosity increased with VMA dosage, even though the presence of a minimum value of yield stress was noted for 0.12%. In the hardened state, the VMA did not lead to an improvement of mechanical strength, which was attributed to an increase in porosity (partly due to a rise in air content). The low strength of mortars with VMA might also have facilitated the development of cracks while de-moulding and handling the specimens, further weakening them. However, the admixture was also responsible for an increase in the amount of larger pores (>10 µm) and a decrease in those of smaller size (<1 µm), which can be potentially beneficial for the mortars’ durability towards salt crystallization and freezing/thawing. Overall, it can be concluded that the use of a viscosity-modifying admixture in lime mortars, despite improving their properties in the fresh state, is detrimental to mechanical strength. Hence, its use should be carefully evaluated, otherwise resulting in excessively weak mortars.

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Declarations of interest: None

Acknowledgements The authors gratefully acknowledge the support provided by CERIS research centre and the Hercules laboratory. The first author also acknowledges Fundação para a Ciência e a Tecnologia (FCT) for the doctoral scholarship SFRH/BD/107879/2015.

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Highlights: • • • •

The impact of a viscosity-modifying admixture (VMA) based on potato starch on lime mortars was assessed The VMA had a thickening effect on the mortars: torque, yield stress and plastic viscosity increased with dosage The air entraining action of the admixture made the handling of the mortars easier The VMA did not improve the mechanical strength of the mortars due to a rise of large pores

Declaration of interest

The authors of the manuscript “Impact of a viscosity-modifying admixture on the properties of lime mortars” wish to confirm that there are no known conflicts of interest associated with this publication and that there has been no significant financial support for this work that could have influenced its outcome.

Bruna Silva, MSc Civil Eng. Corresponding author