Hydrophobic lime based mortars with linseed oil: Characterization and durability assessment

Cement and Concrete Research 61–62 (2014) 28–39

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Hydrophobic lime based mortars with linseed oil: Characterization and durability assessment Cristiana Nunes ⁎, Zuzana Slížková Institute of Theoretical and Applied Mechanics, Prosecká 809/76, 190 00, Prague, Czech Republic

a r t i c l e

i n f o

Article history: Received 12 October 2013 Accepted 26 March 2014 Available online 4 May 2014 Keywords: C. durability C. transport properties C. metakaolin E. mortar Linseed oil

a b s t r a c t Linseed oil was added to lime and lime metakaolin mortars in order to impart hydrophobic properties and investigate its resistance to weathering agents involving water transport. Different properties of the mortars with 6 months of age were evaluated: open porosity, pore size distribution, water absorption by capillarity, mechanical strength, carbonation reactions, microstructure and durability assessed by testing the resistance to sodium chloride accelerated ageing test. Significant durability improvement of both lime and lime metakaolin mortars enriched with linseed oil was achieved: remarkable capillarity reduction and consequently higher resistance to NaCl cycles. Linseed oil had a different effect on the two studied mortars: mechanical strength was slightly reduced for lime and slightly raised for lime metakaolin. The mechanism for the durability improvement was found to be related to the modification of the chemical structure rather than on the alteration of the physical properties of the mortars. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction The rediscovery of traditional building technologies for the conservation of the architectural heritage is a promising field of research trending compatibility and sustainability of the historical construction to be repaired while seeking for its retreatability and preventive maintenance. Lime mortars were used since ancient times but some decades ago changes in construction technology replaced its use and many traditional craftsman experiences were lost. Currently, the investigation of traditional lime mortar technology is a very important research topic in the field of built heritage conservation [1–3]. However, lime mortars with similar aggregate/binder ratio to ancient mortars, and thus more suitable to ensure the functional requirements for architectural heritage restoration, have presented durability problems mainly when exposed to weathering agents involving water transport, even if only occasionally. Natural or artificially pozzolanic materials were thoroughly added in ancient times in order to ameliorate lime mortar strength [4]. Different lime based mortar formulations and special application techniques through successive layers with different properties were prepared and applied according to the specific functional requirements of the building structure [5]. In some cases, in order to improve certain characteristics (e.g. rheological, mechanical) some organic additives were used [6]. Ancient treatises (e.g. Vitruvius, Pliny, Palladio) mention the use of linseed oil as an additive or as a protective treatment for mortars

⁎ Corresponding author. Tel.: +420 774854391. E-mail addresses: [email protected] (C. Nunes), [email protected] (Z. Slížková).

http://dx.doi.org/10.1016/j.cemconres.2014.03.011 0008-8846/© 2014 Elsevier Ltd. All rights reserved.

and other building materials to impart hydrophobicity. Malinowski [7] studied the linings of the Caesarea Roman aqueduct and emphasized its similarity to the solution described by Vitruvius. The author analysed the chemical composition and properties of the materials and, although the author states that some type of oil was added as recommended by Vitruvius, no traces of oil could be found. The detection of organic additives in ancient materials can be difficult not only because they were added in small amounts but also due to its degradation over time. Recent improvement of analytical methods for the identification of organic additives in ancient mortars starts to tackle the lack of information about the ancient mortar recipes with organic compounds [8]. A recent study of the mortar layers beneath the mosaics of the vaults of Saint Peter's Basilica enabled the detection of linseed oil that was also cited in ancient recipes related to the construction of the basilica [9]. Recent laboratory studies have proven the promising effectiveness of linseed oil on improving durability of mortars by granting hydrophobic properties [10–13]. Linseed oil contains unsaturated fatty acids: linolenic (48 to 60%), oleic (14 to 24%) and linoleic (14 to 19%), and saturated fatty acids: palmitic (6 to 7%) and stearic (3 to 6%) [14]. Linseed oil chemical reactivity is conferred to the triglyceride molecules by the double bonds of the unsaturated acids, which allows them to react with the oxygen of air and with one another to form a polymeric network [15]. The triglycerides present in the linseed oil will hydrolyse to glycerol when mixed with the alkaline lime based materials with fatty acid anions consuming three hydroxyl ions in the process. The carboxyl group of the fatty acid anion will coordinate strongly with calcium. The fatty acid will thus be trapped inside the mortar and the hydrophobic part of the molecule will create water repellency [12].

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This reaction is similar to the saponification process where a strong base is used to cleave the ester bond releasing the fatty acid salt and glycerol. Data from literature report that, generally, addition of vegetable oils to lime mortars reduces their mechanical strength [10,13,16,17,19]. Some other studies indicate compressive strength increment [11,18]. Vikan and Justnes [12] determined that cement mortar strength did not show a clear dependency on the vegetable oil dosage and reported that even 0.5% oil was sufficient to slow down the strength development. In order to tackle the problem of low mechanical strength that characterizes lime mortar, the present study encompasses a mixture of lime and metakaolin. Metakaolin is a pozzolanic material that has also been used in the past to improve the mechanical strength of lime mortars [20]. The air lime metakaolin mortar properly designed may be promising as repair mortar because it can achieve much higher mechanical strength than pure air lime mortar but it is not strong enough to generate stress that might lead to failure of the original system to be repaired [21]. In the Czech Republic there is a specific type of clay shale that after burning at temperatures similar to the burning of kaolinite in metakaolin production exhibits pozzolanic properties [22]. The hereby designated metakaolin corresponds to the burnt Czech clay shale. Besides being an available material in the country, burnt Czech clay shale was selected taking into account previous studies reporting its pozzolanic effectiveness [19,22,23]. The research described in this paper had two main aims: the formulation of lime based mortars for restoration purposes with improved durability to weathering involving water transport through linseed oil addition to the mixtures and to study the influence of oil on the mortars physicochemical characteristics in order to ascertain the mechanisms of its resistance to degradation. Specimens with and without oil addition were therefore prepared and tested to evaluate and compare their features focusing on the transport phenomena properties which were found to be the ground for its performance towards the NaCl ageing test. In the present research the percentage of linseed oil added in respect to the weight of binder was 1.5%. This proportion was selected on the basis of the results reported in previous studies that used proportions of 1, 5 and 10 wt.% of boiled linseed oil [11] and 1 and 3 wt.% of raw linseed oil [10]. In both studies the lime based mortars achieved the best performance with 1% of oil addition. 2. Materials, testing equipment and experimental techniques 2.1. Sample preparation The materials used for the present research have been products commercially available in the Czech Republic market. A high calcium lime (classified as CL 90S according to the European Standard ENV 459 1) from Czech Republic supplied by Vápenka Čertovy Schody a.s. Čerták® was used as the basic reference binder for all the specimens. Pure siliceous sand with controlled granulometry (particle size distribution between 0 and 4 mm) supplied by Provodínské písky a.s. was used as aggregate. The hereby designated metakaolin corresponds to burnt Czech clay shale Mefisto L05 and was supplied by České Lupkové Závody a.s. Linseed oil extracted by the cold press method from flax seeds was supplied by GRAC s.r.o. Weight proportions were used for the mortar preparation to avoid measurement imprecision on mixing process. The binder/aggregate, amount of oil and water/binder ratio used to prepare all mortar specimens is shown in Table 1. A normal consistency and a good workability measured by the flow table test [24] were achieved by using the reported amount of water (the diameter of the fresh mortar measured on the flow table was ca. 165 and 170 mm). A slightly higher amount of kneading water was necessary for the mortars with oil to achieve the same consistency as the reference which is probably assigned to the fact that the particles of binder covered with oil prevent water absorption. Other studies have also reported the need of a higher

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Table 1 Mortar identification, composition by weight and air content of fresh mixture. Mortar code

Composition

Oil/binder ratio (%)

Water/binder ratio

Air content (%)

L LO LM

Lime:sand (1:3) Lime:sand (1:3) Lime:metakaolin:sand (0.75:0.25:3) Lime:metakaolin:sand (0.75:0.25:3)

– 1.5 –

1.04 1.08 0.96

2.6 (±0.1) 5.4 (±0.2) 2.2 (±0.1)

1.5

1.02

4.0 (±0.1)

LMO

amount of kneading water to achieve the same consistency with hydrophobic admixtures [19]. An automatic mortar mixer MATEST E093 at low speed was used to mix 2.5 kg of dry mixture at each time. Binder and aggregate were mixed for 6 min and water was then added and the mixture was blended for plus 3 min. Regarding mortars with linseed oil addition, binder and aggregate were mixed for 3 min and a bit of dry mixture (approximately 50 g) was blended manually with oil in a plastic cup for 3 min while adding more portions of dry mixture in order to homogenise the mixture effectively. The oiled mixture was then added to the dry mixture and blended for plus 3 min in the automatic mixer. Water was then added and the mixture was blended for plus 3 min in the automatic mixer. Air content of freshly mixed mortar was measured with a manual air entrainment meter TT AMM17034 [25] and the determined values, average of three measurements, are presented in Table 1. The mortars were moulded in prismatic 40 × 40 × 160 mm stainless steel casts using a jolting table to compact them and to remove air bubbles and voids. A plastic foil was put on the bottom of the casts to facilitate demoulding. Mortars were prepared by groups of 6 specimens (each group made of 2.7 kg of dry mixture). The specimens were demoulded after 1 day and during the first day inside the casts and for 6 days further the specimens were kept under high relative humidity conditions at room temperature (90 ± 10% and 20 ± 5 °C). After this period, the mortar prisms were stored for 173 days at room with controlled RH and T (60 ± 10% and 20 ± 5 °C) lain on a 40 × 160 mm face over grid lined plastic shelves. Therefore, prepared mortars were hardened for 180 days previously to carry out the different tests. 2.2. Mechanical strength and hardening reactions 2.2.1. Flexural and compressive strength Flexural and compressive strength were determined with five samples in a universal traction machine, following the classic method of performing the compressive test with the half samples obtained from the flexural test performed with the common three point bending method [26]. 2.2.2. Thermogravimetric analysis TG DTA was used in order to follow the progress of the hardening reactions of carbonation and hydration. The analysis was carried out with two samples using a TA instrument, model SDT Q600 TGA/DSC in static nitrogen atmosphere at a temperature range between 20 and 1000 °C and at a controlled heating rate of 20 °C/min. The first derivative of thermogravimetry was used in order to identify the hydrated and the carbonated phases at their characteristic decomposition temperatures. 2.3. Pore space properties 2.3.1. Open porosity Open porosity test was performed by total saturation with water of five samples under vacuum and hydrostatic weighing [27]. Water absorption by total immersion at atmospheric pressure was determined by weighting five wet specimens after 48 h of immersion [28].

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2.3.2. Pore size distribution Pore size distribution was performed with a mercury intrusion porosimeter (MIP) Quantachrome Poremaster® PM 60 13 with 3 specimens from each mortar type. Two equivalent penetrometers were used with a 5 cm3 bulb and a total intrusion capacity of 0.500 cm3. Low pressure testing ranged from 6894.7 Pa (1 Psi) to 344,737 Pa (50 Psi) and high pressure analysis from 275,790.3 (40 Psi) to 172,368,925 Pa (30,000 Psi). Equilibration times were 15 s for low pressure and 30 s for high pressure. As mercury parameters, the following were used: advancing and receding contact angle = 140°; surface tension = 0.485 N/m; and density = 13.5487 g/cm3. 2.3.3. Water absorption by capillarity Water absorption by capillarity test [29] was performed with three half prismatic samples which remained after the flexural strength test, therefore the specimens varied in height from 60 to 80 mm. The fresh broken surface of each sample was made even by sawing, washed with water to remove loose particles and dried at 60 °C until constant mass. The fresh broken surface (40 × 40 mm) was the one placed in contact with the water. The lateral and top surfaces were left unsealed in order to be able to use the specimens for further tests. Each sample 40 × 40 × 60–80 mm was immersed in ca. 2 mm of water (over glass rods) inside a covered box to maintain constant hygrothermal conditions and to limit the water evaporation from the samples. The weight of the absorbed water per unit of the exposed surface, function of the square root of time (in hours), was registered. The tests were carried out until the absorption reached an asymptotic value. The capillary water absorption coefficient was determined by the angular coefficient of the curve. 2.3.4. Drying behaviour The evaluation of the drying behaviour of the mortars was performed by determining its evaporation curves [30] that express the evolution of the residual moisture content of the material along time. The evaporation curve of the mortars was determined with three samples 40 × 40 × 80 mm of each mortar type obtained by cutting 40 × 40 × 160 mm samples. Four of the six surfaces were sealed with epoxy resin leaving two 40 × 80 mm surfaces exposed. The samples were immersed in water to the point of saturation. In order to have unidimensional evaporation the base surface was sealed with plastic paraffin film leaving available for evaporation the surface 40 × 80 mm that was exposed when the samples were prepared in the casts. The samples were placed in a room at constant temperature and relative humidity (20 °C and 40%) with no artificial ventilation. The characterization of the evaporation was performed by the determination of the drying index [31] by calculating the integral of the drying curve normalized as a function of the maximum water content in relation to the time interval necessary for the samples to dry. 2.4. Microstructure One thin section (40 × 40 mm) of each hardened mortar category was prepared with epoxy resin after drying at 60 °C. Thin polished sections were analysed under the optical microscope (OM). The microscope used was a UMSP 30 Petro OPTON ZEISS. Images of fresh fracture of each mortar were collected with scanning electron microscope (SEM)

in order to study the influence of linseed oil addition on the morphology of the mortar. After the specimens were dried at 60 °C, they were broken to have a freshly fractured surface which was then coated with gold and observed under the SEM. The scanning electron microscope used was a MIRA II LMU (Tescan, Czech Republic) equipped with back scattered electron detector. The images were collected under high voltage (15 kV) at a working distance of 15 mm and under high vacuum regime. 2.5. Resistance to sodium chloride Specimens 40 × 40 × 160 mm with 180 days of age were dried to constant mass at 60 °C before being subjected to the NaCl test. Samples were immersed in NaCl solution 3wt.% at room temperature (20 ± 5 °C) for 8 h and dried at 60 °C for 17 h. Three samples were subjected to the salt ageing cycles and a group of three mortar specimens was used as the reference for the salt exposed materials and were immersed in deionised water instead of salt solution. To assess the water and salt solution absorption by water aged and salt aged specimens, respectively, the weight after the wetting period was monitored in each five ageing cycles. Mass loss was also monitored after each 5 cycles by measuring the mass remains on the vessel used for the specimen immersion bath. After 15 ageing cycles the samples were dried at 60 °C for 24 h and tested for flexural and compressive test. The halves remaining after performing the mechanical strength tests were desalinated. Desalination was carried out by immersing the samples in water at room temperature and the salt extraction was monitored by measuring the water electrical conductivity. The water was changed periodically in shorter time periods at the beginning (1, 2 and 3 days) and gradually increasing it along the process (5 days) until the electrical conductivity achieved a constant value, within the same range of water aged specimens, which occurred after approximately 1 month. After desalination the specimens were evaluated by water absorption by capillarity and MIP. Thin sections of the aged specimens were prepared and observed under the OM in order to study the effect of salt crystallization on the structural integrity of the material. 3. Results and discussion 3.1. Properties of mortars Formation of bubbles was observed when blending water with the mortars with oil which is probably related to the saponification effect that causes entrainment of air into the mixture. This fact might be related to the high air content values of the fresh mortars with oil shown in Table 1. Air content increased within a similar range of values reported in other study with sodium oleate [32] added in 0.3 and 2.4% to lime mortar. The air entrained content can have a significant effect on the basic physical properties of both fresh mortar (e.g. consistency, plasticity, volumetric density) and hardened mortar (e.g. mechanical strength, shrinkage, volumetric density and frost resistance) [33] which may explain the better workability of the fresh mortars with oil in respect to the reference. Table 2 summarizes the properties of the hardened mortars. Although porosity values determined with water are similar for all mortars (between 32 and 35%), water saturation for 2 days at atmospheric

Table 2 Pore space properties: porosity, saturation by immersion, water absorption by capillarity coefficient and drying index; and mechanical strength of the mortars. The values correspond to the average (±standard deviation). Mortar

Open porosity (%)

Porosity with MIP (%)

Saturation at 48 h (%)

Capillarity coefficient (kg·m−2·h−1/2)

Drying index

Flexural strength (MPa)

Compressive strength (MPa)

L LO LM LMO

31.9 (±0.2) 33.5 (±0.9) 34.5 (±0.3) 33.1 (±0.7)

31.9 (±1.6) 37.0 (±2.4) 32.9 (±0.8) 36.1 (±1.3)

13.2 (±0.1) 3.0 (±0.0) 17.0 (±0.2) 9.0 (±0.3)

28.0 (±0.0) 0.5 (±0.0) 14.8 (±0.0) 2.5 (±0.2)

0.70 (±0.01) 0.63 (±0.03) 0.53 (±0.03) 0.57 (±0.02)

1.8 (±0.5) 1.5 (±0.3) 1.3 (±0.2) 1.5 (±0.2)

3.9 (±1.0) 3.3 (±0.2) 5.0 (±0.7) 5.5 (±0.3)

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pressure gives remarkable lower values for LO and LMO in respect to the reference revealing the hydrophobicity of the mortars. It must be taken into account that although porosity with water is determined by forcing the water into the pores by subjecting the materials to vacuum, the water repellence effect of the oil enriched mortars might block the entrance of water in some pores resulting in slightly lower values of total open porosity. This phenomenon may explain why the values of porosity obtained with MIP are clearly higher for LO and LMO: the higher pressure used with mercury is able to surpass the surface energy of the hydrophobic pore walls. Water absorption by capillarity coefficient presented in Table 2 was reduced in ca. 98 and 83% when oil was added to lime and lime metakaolin, respectively. Fig. 1 depicts how oil addition alters the way the mortars absorb water so strongly that the absorption no longer is a linear function of the square root of time. Similar decrease in water absorption coefficients and capillarity curves was reported with 3% linseed oil addition [10]. Vikan and Justnes [12] found comparable water absorbance rate reduction for cement mortar samples with 1.5% of different vegetable oils after 3 years of curing. The water absorption rate reduction granted by vegetable oils is comparable to that achieved with commercial water repellents [22,31,34,35]. Pore size distribution curves presented in Fig. 2 may contribute to explain some results obtained with the capillary water absorption test. Mortar L exhibits a bimodal distribution with the two pore volume maxima located at ca. 0.4 and 86 μm showing a significant percentage (36%) of pores within the region 10 100 μm and it is through these that water moves faster by capillary transport [36]. These pores can possibly be assigned to shrinkage cracks. Mortar LO shows very low percentage of pores within this range, therefore fewer shrinkage cracks are present. Fig. 3 is a representative set of OM views of the mortars studied. The measured width of the shrinkage cracks under the OM shown in the L mortar micrograph lies within the range 15 to 138 μm. Oil addition seems to prevent shrinkage crack development probably by altering the rheological properties of the fresh mixture e.g. plasticity, air content. LO and LMO mortars show the presence of spherical pores ranging in radius up to ca. 220 μm and are probably assigned to air entrainment. However, this pore size radius is not present in the MIP distribution curves. This may be explained by the fact that some pores are larger than the upper range of measurement (ca. 200 μm) or because they are isolated pores into which mercury cannot enter. The presence of oil in lime metakaolin mortar slightly shifted the major peak from ca. 0.4 to 0.6 μm and slightly raised the amount of sorption pores (pore size diameter less than 0.1 μm). A similar tendency to shift the pore size distribution to higher values was observed with the use of zinc stearate in lime metakaolin mortar [19]. Although sorption pores are mostly related to the presence of hydraulic phases such as CSH

Fig. 1. Water absorption by capillarity curves of the tested mortars.

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Fig. 2. Pore size distribution of reference mortars and mortars enriched with oil: a) L mortar shows a bimodal distribution whereas LO mortar shows a unimodal distribution and the main pore size volume was shifted towards bigger pore ranges; b) LM mortar shows a unimodal distribution whereas LMO shows a bimodal distribution and the main pore size was slightly shifted towards bigger pore ranges.

(calcium silicate hydrate) and lime only mortars do not have sorption pores [37] oil addition to lime mortar has also induced the development of these pores which may be assigned to the presence of lumps of binder and oil formed during the mixing process of the dry components. Other studies reported that lime mortars with 5% olive oil caused a reduction of approximately 48% in pore volume and pore size average in respect to the reference after 28 days of curing [18]. A slight reduction in pore size diameter was also reported for the mortars tested within this project study with 90 days of age [38] but after 180 days the majority of pore sizes of L mortar were shifted towards smaller pore size ranges while the volume of pore sizes assigned to shrinkage cracks slightly increased. Regarding LO mortar no significant changes on pore size distribution curves were found between 90 and 180 days of curing. Although oil induces the formation of higher pore sizes that fall within the range of the pores that have the highest effect on capillarity suction, the water absorption by capillarity of LO and LMO is much lower because the surfaces of the pores are coated with a layer of fatty acids that change the hydrophilic surfaces into hydrophobic ones. Within the present research project water vapour diffusion coefficient was determined with mortar specimens with 90 days of age [38]. LO mortar showed an increase of 6% whereas LMO reduced the vapour permeability coefficient by 23% in respect to the reference. The reduction of water vapour diffusion coefficient in LMO mortar is significant in respect to the reference but the value obtained (1.03 × 10−6 m− 2·s−1) may be considered high enough to allow effective vapour transport within historical masonry. It may be concluded that oil addition has the ability to repel liquid water inhibiting water movement through capillarity without blocking the pores to vapour transport. The drying index presented in Table 2 shows that mortar L is the material with the lowest evaporation rate since it presents the highest

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Fig. 3. OM photomicrographs of the mortars thin sections: L mortar shows a severely cracked matrix whereas LO shows spherical pores; LM mortar presents cracks of moderate size and LMO mortar shows few thin cracks and spherical pores.

drying index value. Mortar LO dries slightly faster probably because the average of the main pore sizes is bigger. Contrarily, in lime metakaolin mortar the addition of oil seems to have slightly reduced the evaporation rate as LMO presents higher drying index in respect to LM. Although the average of the main pore sizes were slightly shifted towards bigger pore size ranges by the addition of oil, the increase in sorption pores might have had an effect on the slight decrease of the evaporation rate. Notwithstanding, the determined difference between LM and LMO drying indexes is so small that it can be considered negligible to affect the effective drying of a lime metakaolin plaster with oil. Mechanical strength values presented in Table 2 show that addition of oil to lime mortar caused a reduction of 16 and 17% on flexural and compressive strength, respectively, whereas in the case of lime metakaolin mortar, addition of oil resulted in an increase of 15 and 9% on flexural and compressive strength, respectively. However, LMO mortar with 90 days of age had shown less 24 and 40% of flexural and compressive strength absolute value, respectively, in respect to the reference LM [38]. LM mortar mechanical strength decreased in absolute value from 90 to 180 days: 24% decrease in flexural strength and 25% decrease in compressive strength. The decrease of mechanical strength of LM during the monitored period may be explained by the carbonation reaction dominance in hydration/carbonation completion [39] causing the strength reduction. However, the addition of oil to lime metakaolin mortar seems to have a positive effect on the hydraulic reactions with time, probably by inhibiting the degeneration of the hydraulic products as confirmed by the TG DTA thermographs obtained (Fig. 4) and discussed subsequently. The mechanical values obtained are similar to those determined in other study with lime metakaolin admixture with zinc stearate added 1 and 2% after 28 days of curing [19]. The same study reported a strength decrease with increasing zinc stearate dosage. The differences in moisture content determined by drying the mortar specimens for 48 h at 60 °C were found to be negligible (lower than 1wt.%) to influence the mechanical strength test results. When comparing the values of mechanical strength with the open porosity it can be noticed that the relation between the mechanical strength of

all mortars cannot be explained only by their densities since the porosities are similar although some authors assigned decreases in mechanical strength to comparable differences in porosity [40]. Other study showed that while maintaining the same values of total porosity, mortars with 3% of linseed oil showed much bigger pores and that could explain why the mechanical resistance decreased [10]. The fact that the total porosities stay approximately constant for reference and samples with oil indicates that the reduced mechanical strength for LO mortar in respect to the reference L cannot be caused only by entrainment of air as reported for cement mortars [12]. Furthermore, when analysing the thin sections (Fig. 3) LO mortar could be expected to have higher mechanical strength than L mortar since the latter showed a severe cracked structure whereas LO mortar showed no fissuration. Hence, the slightly lower mechanical strength exhibited by LO mortar may be assigned to a lower carbonation development (Fig. 4) combined with higher porosity. Thermographs depicted in Fig. 4 show that oil addition to the mortars reduces the carbonation rate as illustrated by the higher portlandite dehydration peaks between 400 and 500 °C (around 450 °C in this case) and lower calcium carbonate decomposition peaks at the temperature range 600 to 750 °C for mortars with oil in respect to the reference. The progress of the pozzolanic reaction of lime metakaolin mortars can be followed in time with the appearance of the hydrated phases within a wide temperature range of 50 to 250 °C. LMO thermograph shows a broad peak between 100 °C and 150 °C corresponding to the decomposition of CSH and CAH. These phases are barely present in the LM graph that rather shows a peak at 230 °C corresponding to one of the hydrated aluminate phases. According to Cizer [41] this phase is assigned to the presence of stratlingite that in hydrated lime metakaolin system it possibly formed due to the interaction of the CSH with alumina in the metakaolin. Biggest mechanical strengths in metakaolin lime blended mortars correspond, apart from CSH, to stratlingite and CAH phase content [41]. It is possibly due to the higher content of CSH and CAH phases in LMO mortar that the corresponding mechanical strength is higher than LM thus surpassing the mechanical strength assigned to the presence of stratlingite in LM. Furthermore, the thin section of LM

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phenomenon is probably assigned to the formation of bubbles during the mixing process of the fresh mortar thus creating space for the crystals to grow larger. The less dense structure of LO mortar may also have contributed for the slightly lower mechanical strength in respect to L mortar. Photomicrographs of lime metakaolin mortars taken under the SEM show some of the most common morphologies of CSH as fibres, flakes and tightly packed grains [44] and LMO seems to exhibit more of these crystal habits than the reference mortar confirming the TG DTA results. The growth of fibre and flake crystals due to the presence of oil may help improve the consistency of the mortar and result in better compressive strength. Therefore, the absorption of carboxyl acid derivates on the metakaolin particles may alter the hydration both by improving it and changing the morphology of the hydration products. Encapsulation by carboxyl acid derivates may thus have changed the morphology of the lime and hydrated metakaolin phases in a way which has weakened the strength of the paste in the case of LO and improved it in the case of LMO. The morphology of later products may be more affected than early ones since the concentration of fatty acid increases as the water is consumed. 3.2. Resistance to NaCl test

Fig. 4. Thermographs of the studied mortars: L and LM reference mortars show slightly higher degree of carbonation in respect to mortars with oil but LMO mortar presents higher content of the hydraulic phases CSH and CAH in respect to the reference LM.

mortar (Fig. 3) showed slight fissuration whereas no fissuration was detected in LMO thin section. Oil addition to lime metakaolin mortar seems to have a positive effect on the mortar structure both by inhibiting development of shrinkage cracks and ameliorating the progress of the pozzolanic reactions responsible for strength development. However, it must be taken into account that CSH and CAH phases correspond to the hydration reactions that initiate first and provide the initial set of the mortars and subsequently carbonation of free lime takes place and the strength of the mortar is reduced [39]. Hence, LMO higher strength in respect to LM may be assigned to the delay of these reactions because the oil droplets may block the binder grains reducing the reactivity of the metakaolin in short curing time or by preventing its degeneration in long curing time periods. Characterization of the mortar specimens at later curing ages is expected to elucidate these results. The results on carbonation reactions obtained with TG are different from those determined by the phenolphthalein method on cement mortars where the carbonation depth seemed to increase with increasing oil dosage in some instances [12]. On the other hand, the results obtained with lime metakaolin are in accordance with the hydration reactions described for cement mortars [42], the oil or its decomposition did not seem to obstruct the degree of hydration and were therefore coordinated towards the hydration products CSH rather than the surface of unreacted metakaolin grains. Organic additives are known to change the morphology of various materials [10,18,43]. Microstructural changes were observed between reference and oil enriched mortars under the SEM (Fig. 5). Mortars with oil show bigger crystals and a less compact structure. This

3.2.1. Sample ageing monitoring The performance of the mortars towards the salt crystallization test was evaluated by monitoring the water and salt solution absorption, mass variation and surface cohesion. The mortar specimen's weight registered after the wetting ageing step with the NaCl test was monitored in each 5 ageing cycles and the results are presented in Fig. 6. Mortars enriched with oil absorb a remarkably lower amount of water and salt solution in respect to the reference. The graphics plotted in Fig. 6 also point out that LO mortar hydrophobicity is not affected during ageing whereas LMO mortar shows a gradual decrease of water repellency as illustrated by the slight increase in water and salt solution absorption along the ageing cycles. Therefore, it may be inferred that the hydrophobicity granted by the oil to lime mortar is more resistant to the ageing action than on lime metakaolin mixture. The drop off on salt solution absorption for L and LM mortar on the 5th cycle may be caused by the sample saturation with salt: the motion of the ions under a gradient of concentration is reduced as the specimens become saturated with salt. Fig. 7 presents the values of mass loss during ageing that are seen to be consistent with the values of salt solution absorption, which clearly illustrate the higher resistance of mortars with oil to the ageing action. Metakaolin improves the cohesion of the mortar structure towards the degradation process and the addition of oil both to lime and lime metakaolin mortars improves the resistance to ageing by reducing the water and salt solution ingress. 3.2.2. Properties of samples after ageing Fig. 8 shows the visual aspect of the specimens after ageing and desalination. Water aged specimens did not show any visually detectable alteration. The deterioration of the salt aged samples generally occurred by development of superficial disaggregation of the material. Mortars enriched with oil show very limited differences between water aged and salt aged specimens. The results of mortar characterization after ageing are shown in Table 3. The total mass loss is a good indicator of the higher resistance to disaggregation of oil enriched mortars to the salt ageing test whereas water ageing seems to have negligible effect on the cohesion properties of the four mortar types. All mortars show higher water absorption by capillarity for water aged and salt aged samples with the exception of LO mortar. Salt aged specimens show generally higher water absorption rates but the capillarity coefficients are close to the water aged samples. Thin sections of water aged specimens (Fig. 9) showed that, apart from L mortar, no cracks or higher percentage of pores were developed after the wetting drying cycles. The transfer of moisture in the pores of a mortar also includes the transfer of the soluble ions of the mortar

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Fig. 5. SEM photomicrographs of the studied mortars: larger crystals are formed in the mortars with oil that consequently show a less dense structure.

binder. The calcium ions are transferred and may come in contact with fluids enriched in CO2 as carbonic acid and calcite will precipitate [37]. During the wetting steps narrow cracks may heal by precipitation (autogenous healing) and cause a reduction of porosity [45]. This reaction may explain the absence of fissures in LM thin section after water ageing cycles (Fig. 9). LM salt aged specimens showed some mild fissuration of the binder matrix probably due to the alteration of the calcium aluminium hydrate phases to calcium chloroaluminates. However, the water absorption by capillarity is similar in respect to the water aged specimens. Regarding these results, it must be taken into account that the desalination process might have induced the autogenous healing of the material (the samples were immersed in water for approximately 1 month). The same effect did not occur for LMO mortar that showed a slightly higher increment of water absorption by capillarity after water and salt ageing in respect to the reference although the rate of water absorption is still far below LM not aged mortar. The very low amount of water absorbed by LO mortar leads to infer that water only penetrates the outer layer of the specimen material. Hence, the reprecipitation of dissolved calcium ions is more likely to recrystallize on the mortar surface during drying that will become more impermeable. This fact may explain why LO water aged specimens show slightly lower water absorption by capillarity after wetting and drying cycles. Salt aged specimens show fissuration of the binder matrix and disconnection of the binder from the aggregate which is severe in the case of L mortar leading to conclude that the salt crystallizes in the binder structure accumulating also at the interface with the aggregate

(Fig. 9). As a result the water absorption by capillarity of L mortar increased significantly. A remarkable reduction of mechanical strength was determined for L and LO aged specimens (Table 3). Although LO salt aged thin section exhibited moderate fissuration of the binder whereas very limited fissuration was observed for the water aged specimens the determined flexural strength values are similar in both cases. Compressive strength is slightly higher for LO salt aged samples. The slight increment of the compressive strength might be assigned to the presence of salt in the pores that produces the so called “salt cementing effect” [46]. It is worth reminding that mechanical strength tests were performed in salt loaded specimens whereas the thin sections were prepared from desalinated samples. Regarding L mortar the salt cementing effect is not observed probably due to the high degree of fissuration revealed by the thin sections. Mechanical strength values of LM and LMO after ageing do not differ much from not aged samples. Regarding LM salt aged specimens the similar results may be assigned to the salt cementing effect whereas the water aged specimen results may be related to the improvement of the hydraulic reactions in wet atmosphere which means that the wetting drying cycles do not affect the mortar strength or the autogenous effect surpasses the damage caused by it. According to Scherer [47] if the contact angle between a salt crystal and the pore wall is low, then the stress can be small but if the contact angle is higher than 90°, crystallization can even create compressive stress in the material. According to this it may be inferred that the hydrophobicity of the oil enriched mortars improves durability by

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Fig. 6. Mortars water and salt solution absorption during ageing: L and LM reference mortars show a different behaviour regarding water and salt solution absorption; mortars with oil absorb significantly lower amount of water and salt solution in respect to the references.

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preventing salt solution ingress into the structure but the small amounts of salt solution that are able to penetrate into the structure can produce higher stress in the pores. Houck and Scherer [48] tested a relatively hydrophobic material (carboxylic acid as functional group) on limestone and found that the salt that had penetrated into the pores crystallized only in the centre and not on the surface. The authors assigned this phenomenon to the fact that there was not a significant capillary pressure to draw the liquid toward the surface. Instead, it retreated into the interior and the salt accumulated there. The authors deduced that the effectiveness of the polymer used was related to its effect on contact angle, rather than on crystallization pressure, since it was found that the salt retreated to the interior of the stones coated with this polymer. Nevertheless, it must be taken into account that these results concern a surface treatment and that the water vapour permeability, which was not reported, might have contributed to this phenomenon if it was lower than the untreated material. Further research on the salt solution migration and crystallization in the mortar matrix shall be undertaken in order to clarify this topic. In order to investigate the pore sizes involved in NaCl crystallization the pore size distribution of the samples after ageing followed by desalination was measured by MIP. The measurements were performed in two specimens of each mortar type on 2 mm layers sampled from the surface of the 40 mm thick mortar specimens. No difference on pore size distribution was obtained for the mortars enriched with oil. The pore size distributions obtained for the reference mortars are shown in Fig. 10 and only one curve from the two measurements performed in two different specimens is shown since both curves were very similar: L and LM do not exhibit significant changes between reference and water aged specimens and MIP curves of salt aged specimens are similar therefore it is difficult to draw sound conclusions. L salt aged specimens showed a slight increment of pore sizes in the range between ca. 0.08 and 0.2 μm and LM salt aged specimens showed a slight increase in the percentage of pores within ca. 0.05 to 0.1 μm range. A similar increment in porosity within this pore size region was reported for a salt transporting mortar after NaCl crystallization [49] and for a lime cement mortar after NaCl crystallization followed by desalination [50]. In both studies [49,50] a decrease in total porosity due to the partial filling of

Fig. 7. Mortars mass loss during ageing: mortars with oil present significantly lower mass loss in comparison to the reference mortars.

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Fig. 8. Aspect of mortar specimens after ageing and desalination: all water aged mortars and LO salt aged specimens do not show visual alterations after ageing. L and LM mortars are the most affected by the salt ageing test (scale above the specimens shows 1 mm intervals).

the pores with salt was obtained before desalination. Observation of LM salt aged thin section (Fig. 9) enabled the detection of some fissures with widths between 36 and 50 μm which were not detected by MIP. 4. Conclusions The effect of linseed oil addition as a hydrophobic additive for lime and lime metakaolin mortars was studied and the main results can be summarized as follows: • Oil addition to lime mortar produces different physical properties than when added to lime metakaolin mortar although both materials show comparable hydrophobic properties. Linseed oil changes the hydrophilic surfaces of the capillaries into hydrophobic surfaces due

to the bulky structure of the fatty acids and to the nonpolar carbon hydrogen bonds of the oil thus preventing salt solution ingress into the porous structure of the material. • Mechanical strength increment seems to develop more slowly in oil enriched mortars. This may be due to a combination of factors such as slower progression of the hardening reactions and air entrainment during mortar preparation causing porosity increment and a less dense microstructure. • The intention of improving the mechanical behaviour of lime mortar by adding the reported amount of metakaolin was achieved and, although addition of oil promoted a slower strength development, it seems to increase gradually with time which is a positive factor because one of the drawbacks of lime metakaolin based mortars for use in repairs of historic masonry is the stress that can arise from

Table 3 Properties of the aged mortars (WA: water aged; SA: salt aged). Mortar L LO LM LMO

WA SA WA SA WA SA WA SA

Total mass loss (wt.%)

Capillarity coefficient (kg·m−2·h−1/2)

Flexural strength (MPa)

Compressive strength (MPa)

0.85 (±0.01) 10.22 (±0.11) 0.46 (±0.01) 1.67 (±0.01) 0.07 (±0.00) 5.28 (±0.02) 0.05 (±0.00) 1.65 (±0.00)

36.84 (±0.41) 39.93 (±0.10) 0.48 (±0.04) 0.79 (±0.04) 17.04 (±0.03) 16.99 (±0.13) 5.93 (±0.11) 7.25 (±0.06)

0.58 (±0.04) 0.51 (±0.02) 0.45 (±0.04) 0.49 (±0.05) 1.04 (±0.12) 1.39 (±0.50) 1.12 (±0.38) 1.12 (±0.18)

1.72 (±0.04) 1.11 (±0.14) 1.31 (±0.30) 2.12 (±0.34) 5.82 (±0.48) 5.16 (±0.28) 6.07 (±0.34) 5.91 (±0.18)

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Fig. 9. Thin sections of mortar specimens after ageing: water aged mortars do not show any detectable alteration after ageing whereas salt aged mortars show crack formation which is more intense in the case of the reference mortars.

the mechanical strength fluctuations along time. Testing mortars at later ages will contribute to unveil whether oil addition delays the normal development of the hydration reactions or contributes to its stability over the course of time. • The addition of oil neither significantly change the open porosity nor the drying rate while decreasing the water absorption by capillarity which is a positive factor for the material to be used as a plaster in historical walls because it contributes for the ventilation of the masonry while preventing water ingress.

• Oil enriched mortars have clearly restrained water transport by capillarity and are therefore more resistant to the NaCl test as expressed by the less amount of water and salt solution absorption, lower mass loss and better cohesion of the specimens after ageing. • From the literature it may be inferred that the use of the optimum amount of oil and the nature of the oil and binder is a key factor for both technical and economical aspects. If the amount of oil is low the desired effects are not achieved while the use of excessive amount leads to inferior mechanical properties and increased price. The

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Fig. 10. Pore size distribution for reference and water and salt aged specimens for L (a) and (b), and LM (c) and (d) mortars.

present study proved that 1.5% of linseed oil is a good proportion to achieve satisfactory performance on lime and lime metakaolin mortars tested when compared to the results obtained by other authors with higher or lower amounts of linseed oil addition. The improvement of mortar durability by oil addition is comparable to mortars with the addition of hydrophobic compounds produced by the chemical industry (e.g. sodium oleate, calcium stearate, zinc stearate) but has the advantage of being an environmental friendly material, cost effective, innocuous and easy to manipulate. The formulated mortars with oil can be particularly suitable for application as plasters in areas exposed to rain, water absorption by capillarity from the ground or exposed to marine spray. The hydrophobic plaster is expected to cause rainwater to be averted and the wall to become drier over the course of time. LMO mortar may perform better in situations where the plaster must harden in conditions with low air contact and where mechanical strength is an important parameter in the masonry maintenance. In situ tests in different case studies will be the final step for the assessment of the mortars compatibility and durability in natural exposure conditions. Acknowledgements The present study was supported by the Czech national project MK ČR NAKI DF11P01OVV008 entitled “High Valuable and Compatible Lime Mortars for Application in the Restoration, Repair and Preventive Maintenance of the Architectural Heritage” and by the Research Development Plan RVO 68378297 at the Centre of Excellence of Telč, built with the support from the EC, from the Czech Ministry of Education, Youth and Sports (CZ.1.05/1.1.00/02.0060). The authors are grateful to Dr. Dana Křivánková, Dr. Dita Frankeová, Dr. Krzystof Niedoba and Dr. Verónika Petřanová for helping with the preparation of mortar samples, TG DTA, MIP and SEM measurements, respectively.

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