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Influence of clay minerals addition on mechanical properties of air lime–metakaolin mortars
Construction and Building Materials 65 (2014) 132–139
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Influence of clay minerals addition on mechanical properties of air lime–metakaolin mortars S. Andrejkovicˇová a,⇑, A.L. Velosa b, E. Ferraz b, F. Rocha a a b
Geosciences Department, Geobiotec Research Unit, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal Civil Engineering Department, Geobiotec Research Unit, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
h i g h l i g h t s Two sets of lime mortars with addition of clay minerals and metakaolin were analysed. Clay minerals and metakaolins cause microstructural changes of mortars. Lime with higher bulk density is more suitable to be substituted by additives. Clay minerals substituted air lime in 1st set clearly improved mechanical strengths.
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Article history: Received 5 December 2013 Received in revised form 21 April 2014 Accepted 22 April 2014
Keywords: Lime Mortar Metakaolin Sepiolite Zeolite Palygorskite Vermiculite Mechanical properties Restoration
a b s t r a c t Two sets of mortars differing in type of lime and metakaolin, with air lime:sand volumetric ratio 1:3 were prepared with the aim to be used for restoration of historic masonries. The first set involved air lime and 20 wt.% of metakaolin with more impurities (calcite and kaolinite, respectively) and with higher bulk densities compared to materials from the second set. Clay minerals (sepiolite, zeolite A, palygorskite and vermiculite) characterised by high specific surface areas and thus able to keep water in the structure and promote pozzolanic activity of metakaolins were used as additives in air lime and air lime–metakaolin mortars and their impact was evaluated in a view of mechanical strength at 28, 90 and 180 days. Substitution of air lime from the first set by clay mineral and/or metakaolin caused improvement of mechanical strengths predominantly at latter ages, while lime mortars from second set suffer by lack of binder when other additives are supplemented and just palygorskite incorporation improves flexural strength, while vermiculite and metakaolin create mixture with improved compressive strength than lime mortar alone. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction For the adequate restoration of cultural heritage, the compatibility between old mortars and the rehabilitation materials is obligatory. The survey for this compatibility is also supported by the fact that old materials to be renovated, already had evidenced to have suitable mechanical properties and acceptable performances throughout the centuries [1,2]. Recent restoration interventions are based on utilisation of analogous chemical composition of binders, aggregates and mineral additions, as they derive from the study of historic mortars [e.g. 3,4]. The main component of old renders is usually lime, occasionally supplemented by the presence of pozzolanic or other additives. For this reason, ⇑ Corresponding author. Tel.: +351 234 370 747; fax: +351 234 370 605. E-mail address: [email protected] (S. Andrejkovicˇová). http://dx.doi.org/10.1016/j.conbuildmat.2014.04.118 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
air lime mortars and/or mixed with pozzolanic additions (natural or artificial) have been studied widely with the intention to be used as mortars for the construction of historic buildings [1,5–13]. Addition of high reactive pozzolans to lime creates mortars similar to historic ones that display an advanced durability and high values of mechanical strength. Nowadays many researchers are paying attention mainly to metakaolin which is an artificial pozzolanic additive mainly due to its capacity to react with calcium hydroxide creating typical pozzolanic products [14–23]. In addition, positive effect of natural clay minerals such as sepiolite [24–28], vermiculite [29] or synthetic zeolite A [30] on lime mortar’s characteristics has also been the object of recent research. In some rehabilitation cases, it is inevitable to provide restoration interventions in places of lack of humidity, or unfavourable conditions such as difficult access to CO2 or desiccation conditions (wind, heat). This is more so in Portugal due to its climatic
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conditions, extensive coastline and building traditions. For this reason the main scope of the present study is to develop blended mortars based on lime with additions of metakaolin and clay minerals characterised by high specific surface areas, able to adsorb water molecules and provide more humid conditions promoting pozzolanic activity of metakaolin. From natural clay minerals have been chosen sepiolite and palygorskite typical also for their fibrous structure and from commercial ones zeolite type A and expanded vermiculite. Lime/aggregate volumetric ratio selected for this study was 1:3.
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21% of water was added to AL_2, PAL, AL20MK_2 and PAL20MK; 23% to FSAL20MK_2 and 19% to VAL and VAL20MK mortars. 2.1. Conditioning Mortar prisms 40 40 160 mm of all the mortars were prepared. Air lime mortars (AL, AL_2) were cured during all tested ages 28, 90 and 180 days in a chamber with a relative humidity of 65 ± 5% and temperature 20 ± 2 °C. Specimens containing metakaolin and/or FS, CS, FZ, CZ, P, V were stored in moulds for the first 2 days in a chamber at 20 ± 2 °C with a relative humidity of 95 ± 5% and then remoulded and kept for next 5 days at the same conditions. Then the specimens were maintained at a relative humidity of 65 ± 5% and temperature 20 ± 2 °C; and cured up to ages of 28, 90 and 180 days according to the Standard EN 1015-11 [32].
2. Materials, mortar composition, conditioning 3. Methods Taking into account the principal mortar components (air lime and metakaolin), two sets of mortars were prepared: 1. With powdered commercial air lime CL 90 (AL) (Calcidrata, S.A., Portugal) and siliceous river sand and formulated with air lime:sand volumetric ratio of 1:3. Lime binder was replaced by: (a) 5 wt.% of fine (FS) and coarse (CS) commercial sepiolite (Sepiolita 15/30, Minas de Paracuellos del Jarama, Madrid, Spain) and 5 wt.% fine (FZ) and coarse (CZ) commercial synthetic zeolite A pellets Phonosorb 551 (Grace Davison, USA); (b) 20 wt.% of commercial metakaolin (MK) (EcoPozz, Portugal); (c) by both, (a) and (b). Individual specimens are in the text marked as follows (Table 1). Water added to mortars was calculated to provide an appropriate workability, accomplished by the flow table test with values around 130–140 mm according to the Standard EN 1015-2 [31]. 19% of water, considering total mortar mass, was added to air lime (AL) and air lime + metakaolin (AL20MK) mortar. Forasmuch sepiolite and zeolite pellets have high specific surface areas and consequently higher water demand, 23% and 21% of water was necessary to add to fine/coarse sepiolite and zeolite mortars, respectively. 2. Second set of mortars was prepared with powdered commercial air lime (AL) (Lusical H100, Portugal) with classification CL90 and siliceous river sand and formulated with air lime:sand volumetric ratio of 1:3. Lime binder was replaced by: (a) 5 wt.% of palygorskite (P) (Minas the Torrejon, Spain) and 5 wt.% of expanded vermiculite (V) (Aguiar & Mello, artificial product of natural vermiculite calcination between temperatures of 700 °C and 1000 °C); (b) 20 wt.% of commercial metakaolin (MK) (AGS Mineraux, France); (c) by both, (a), (b) and with 5 wt.% of fine (FS) commercial sepiolite (Sepiolita 15/30, Minas de Paracuellos del Jarama, Madrid, Spain) and 20 wt.% of commercial metakaolin (MK) (AGS Mineraux, France). Individual specimens are in the text marked as follows (Table 2). Table 1 Composition of mortars (1st set). Reference
Materials
AL FSAL CSAL FZAL CZAL AL20MK FSAL20MK CSAL20MK FZAL20MK CZAL20MK
Air lime + sand Fine sepiolite + air lime + sand Coarse sepiolite + air lime + sand Fine zeolite + air lime + sand Coarse zeolite + air lime + sand Metakaolin + air lime + sand Fine sepiolite + air lime + metakaolin + sand Coarse sepiolite + air lime + metakaolin + sand Fine zeolite + air lime + metakaolin + sand Coarse zeolite + air lime + metakaolin + sand
Table 2 Composition of mortars (2nd set). Reference
Materials
AL_2 PAL_2 VAL_2 AL20MK_2 PAL20MK_2 VAL20MK_2 FSAL20MK_2
Air lime + sand Palygorskite + air lime + sand Vermiculite + air lime + sand Metakaolin + air lime + sand Palygorskite + air lime + metakaolin + sand Vermiculite + air lime + metakaolin + sand Fine sepiolite + air lime + metakaolin + sand
The fine size materials (sepiolite and zeolite) were obtained by dry grinding of the coarse (original) materials in a Ceramic Instruments mill (S2-1000-M) with porcelain jars and alumina balls, during 15 min. Particle size distribution of fine materials was performed with X-ray grain size analyser Sedigraph 5100 from Micromeritics, following the BS 3406-2 [33]. Bulk densities of materials were determined according to Certification CSTB Cahier [34]. The mineralogical composition of the specimens was determined using a Philips X’Pert diffractometer equipped with Cu Ka radiation. The microstructural and chemical homogeneity was analysed by scanning electronic microscopy, SEM/EDS (Hitachi SU 70 coupled with EDAX Bruker AXS detector). Flexural and compressive strength tests were carried out on 3 probes of individual mortar following Standard EN 1015-11 [32] on (SHIMADZU: AG-IC 100 kN) equipment, with loads of 10 and 50 N/s for flexural and compressive strength, respectively. The dynamic modulus of elasticity was determined based on the fundamental longitudinal resonant frequency following the BS 1881-209 [35].
4. Results and discussion 4.1. Characterisation of materials used for mortars preparation 4.1.1. Particle size distribution and bulk density of materials Values of particle size distribution and bulk density of all materials used for mortar preparation are reported in Table 3. The lowest D50 is that of palygorskite (0.1 lm) with bulk density 733 kg m 3. Medium values of D50 are attributed to metakaolin (France), fine sepiolite and air lime (Calcidrata) 1.3, 2.4 and 3.2, respectively. Top D50 values has air lime (Lusical) (7.0 lm), metakaolin (Portugal) (10.0 lm) and fine zeolite (16.0 lm). The highest bulk density (1140 kg m 3) of fine materials is provided by fine zeolite. Related to particle size distribution of coarse materials, expanded vermiculite is composed of the largest particles (<4 mm) and in the same way has the lowest bulk density (121 kg m 3) of all the materials (Table 3). Distribution of sand particles ranging between 0.125 and 0.5 mm is followed by coarse sepiolite 0.3–1.2 mm and finally by coarse zeolite between 1.2 and 2.5 mm. Bulk density of sand (1560 kg m 3) is the highest from all materials.
Table 3 Particle size distribution and bulk density of materials. Material
D50 (lm)
Air lime (Calcidrata) Air lime (Lusical) Metakaolin (Portugal) Metakaolin (France) Fine sepiolite Fine zeolite Palygorskite Vermiculite Coarse sepiolite Coarse zeolite Sand
3.2 7.0 10.0 1.3 2.4 16.0 0.1
Particle size distribution (mm)
Bulk density (kg m 3)
<4 0.3–1.2 1.2–2.5 0.125–0.5
460 380 673 296 526 1140 733 121 680 1290 1560
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(Fig. 2h–i) of calcium silicates, what confirm the EDAX spectra of AL20MK and AL20MK_2 in Fig. 3a–b, respectively. It is important to note that pozzolanic reaction (distribution of calcium aluminium silicate in mortar matrix) of AL20MK_2 mortar was developed in larger scale compared to AL20MK mortar.
Fig. 1. X-ray diffraction patterns of random oriented materials used for mortar preparation; (a) air lime Calcidrata, (b) air lime Lusical, (c) metakaolin Pt, (d) metakaolin Fr, (e) sepiolite, (f) zeolite, (g) palygorskite, (h) vermiculite, and (i) sand. (A-anatase, C-calcite, D-dolomite, F-feldspar, I-illite, K-kaolinite, L-leucite, Nnepheline, P-portlandite, Q-quartz, S-sepiolite, V-vermiculite. Z-zeolite type A,).
4.1.2. Mineralogical composition The XRD patterns of materials used for mortar preparation are displayed in Fig. 1. Air lime (Calcidrata) diffractogram contains portlandite and calcite appears due to carbonation of Ca(OH)2 (Fig. 1a). Air lime (Lusical) shows predominantly XRD patterns of portlandite with negligible traces of calcite. Metakaolin (Pt) was not completely calcinated, as its pattern contains kaolinite, besides quartz and feldspar; while illite, anatase and quartz are detected in XRD pattern of metakaolin (Fr). Sepiolite diffractogram is composed of dominant mineral sepiolite, including admixtures of dolomite and quartz (Fig. 1e). The synthetic zeolite pellets contain leucite and nepheline as major mineral phases and sodium zeolite type A as minor component. Palygorskite sample shows admixtures of quartz and dolomite, while expanded vermiculite contains only diffraction peaks of vermiculite. Sand is composed predominantly from quartz with minor content of feldspar (Fig. 1i). 4.2. Characterisation of mortars 4.2.1. Morphology and microstructure Scanning electron images of representative mortars at 90 days are shown in Fig. 2. Images of mortars containing both, clay mineral and metakaolin are not included as they are combination of individual morphological changes observed in lime–clay or lime–metakaolin micrographs. Basic lime mortars without any additions (AL and AL_2) show characteristic carbonated portlandite matrixes as is demonstrated in Fig. 2a. An addition of other admixtures causes the changes in the microstructure of primary mortar (Fig. 2b–i). Both fine and coarse sepiolite (Fig. 2b–c) due to their fibrous origin, cause connection between the individual components of mortar and thus create a more compacted system. Fine zeolite proved its pozzolanic activity reacting with of Ca(OH)2 by the presence of mosaic shaped fragments of calcium aluminosilicate (Fig. 2d). Coarse zeolite pellets in AL mortar provide conditions, in which fan-shaped clusters of aragonite are formed (Fig. 2e). This phenomenon is described in the work of Andrejkovicˇová et al. [30]. Palygorskite needles show analogous behaviour as coarse and fine sepiolite in term of the ‘‘bridge’’ linking with the lime binder (Fig. 2f). Particles of expanded vermiculite with typical ‘‘accordion’’ type morphology also form connection with lime particles as sepiolite and palygorskite; however in the same time produce more open pore spaces. Pozzolanic activity of both metakaolins (Portugal and France) was revealed by formation of acicular fibres
4.2.2. Mechanical strength of mortars 4.2.2.1. Flexural strength Rf. An impact of the addition of individual clay minerals and metakaolins to air lime mortars on mechanical properties was also evaluated. Flexural strength values of mortars representing first and second set at 28, 90 and 180 days are displayed in Figs. 4a and 4b, respectively. In case of the first set, at 28 days, related to the addition of clay minerals to lime (without MK), there is an increase of 14% and 50% for FSAL and CSAL and negligible change for FZAL and CZAL mortars compared to AL (0.22 MPa) (Fig. 4a). Addition of metakaolin (AL20MK mortar) shows the same behaviour as fine sepiolite on final Rf value (0.25 MPa). Fine materials in combination with MK imposed an improvement of about 27% (FSAL20MK) and 36% (FZAL20MK) in comparison with AL, while addition of coarse sepiolite and coarse zeolite to AL–metakaolin did not affect the final value of flexural strength (Fig. 4a). At 90 days of curing, all the mortars from the 1st set show a rise in flexural strength compared to age 28 days. Rf value 0.26 MPa of AL mortar, is the lowest from all mortars, meaning that all additives provide an enhancement of flexural resistance as a result of their structure and/or pozzolanic activity. The best examples are FSAL20MK and FZAL20MK, which reach double Rf (0.50 MPa) than AL mortar (Fig. 4a). Pozzolanic activity of fine zeolite alone manifested in the highest flexural strength (0.41 MPa for FZAL) of mortars without MK. This value is higher than Rf of AL20MK mortar with metakaolin (0.35 MPa). From 90 to 180 days there is visible a slight increase of flexural strength values for all the mortars without metakaolin (AL, FSAL, CSAL, FZAL and CZAL). From those with metakaolin, just mortars containing coarse materials exhibit a rise of Rf at 180 days. It is important to note that incorporation of MK did not improve flexural resistance of CZAL20MK mortar, as the final values during all curing ages are alike for CZAL and CZAL20MK (Fig. 4a). FZAL (0.44 MPa) and FZAL20MK (0.48 MPa) mortars show the highest values of Rf at 180 days, predominantly as a result of microstructure and pozzolanic activity of fine zeolite (Figs. 2d and 4a). In general, compared to AL, all the mortars besides AL20MK show higher flexural resistances at 180 days. Fig. 4b presents flexural strength of mortars from the second set. Evolution of Rf with time of curing is different when compared to mortars from the 1st set. Air lime used in these mortars contains less calcite, and metakaolin contains less impurities and above all, no kaolinite peaks were observed what means that calcination was executed in higher level compared to metakaolin from Portugal (Fig. 1b, c and d). Nevertheless, at 28 days of curing AL_2 reaches Rf 0.24 MPa (Fig. 4b) comparable with AL mortar with Rf 0.22 MPa (Fig. 4a). From all the mortars just palygorskite addition improves flexural resistance of air lime mortar of 63% and vermiculite with metakaolin (VAL20MK_2) of 42%. VAL_2, AL20MK_2, PAL20MK_2 and FSAL20MK_2 mortars show lower resistances than AL (Fig. 4b). From 28 to 90 days, Rf of all the mortars increases, but more evidently in case of mortars without MK. Top values reach AL_2 and PAL_2 mortar (0.45 MPa). None of lime–MK mortars caused improvement of flexural strength compared to AL. At 180 days, an upward trend from 90 days in Rf values show mortars without MK. Palygorskite addition to AL mortar provides the best flexural resistance 0.52 MPa, but even AL_2 mortar alone reaches 0.49 MPa. From lime–metakaolin mortars, the highest Rf shows VAL20MK_2. In general, unexpected behaviour was observed in by all the mortars containing metakaolin. According to SEM analysis (Figs. 2i and 3b), metakaolin used in second set
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Fig. 2. Scanning electron images of (a) AL, (b) FSAL, (c) CSAL, (d) FZAL, (e) CZAL, (f) PAL_2, (g) VAL_2, (h) AL20MK, and (i) AL20MK_2 mortar.
produces pozzolanic activity, though, its presence inhibits the growth of flexural strength in opposition to metakaolin (Portugal) (Fig. 2h). Even the time of curing does not effect in a progressive way the development of Rf and all the mortars with metakaolin have comparable values of flexural strength during all testing periods (Fig. 4b). Compared to AL, AL_2 shows even better flexural
resistance at 28, 90 and 180 days (Figs. 4a and 4b). Incorporation of other additives (besides palygorskite) to AL_2 mainly at later ages does not cause any improvement (Fig. 4b). It means that AL_2 mortar already contains just ‘‘accurate’’ amount of binder and other substitutions create a mixture with lack of lime to withstand flexural forces. It is important to note that the air lime used
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Fig. 4b. Flexural strength of mortars from the second set.
Fig. 3. EDAX spectra of (a) AL20MK (Fig. 2h) and (b) AL20MK_2 mortar (Fig. 2i).
Fig. 4a. Flexural strength of mortars from the first set.
in this set (Lusical) has lower bulk density compared to air lime (Calcidrata) (Table 3) and to create volumetric ratio lime:sand 1:3, AL_2 mortar required lower quantity of lime than AL mortar.
4.2.2.2. Compressive strength Rc. Compressive strength values (Rc) of mortars from both sets at 28, 90 and 180 days are shown in Figs. 5a and 5b. Related to the first set at 28 days (Fig. 5a), incorporation of clay minerals to AL mortar caused increase in Rc of 47%, 37% and 47% for CSAL, FZAL and CZAL mortars, respectively, while FSAL (0.32 MPa) mortar shows comparable value with AL (0.30 MPa). Pozzolanic activity of metakaolin manifested in 17% rise of compressive strength when compared to AL mortar, however CSAL20MK and CZAL20MK mortars show alike value as AL20MK (0.35 MPa) meaning that coarse materials at early age of curing do not have a significant impact on Rc increase on lime–metakaolin mortar. On the other hand, combination of fine materials with metakaolin provides an increment of compressive strength (FSAL20MK 0.44 MPa and FZAL20MK 50 MPa) even compared to AL or AL20MK (Fig. 5a). From 28 to 90 days, an increase of Rc was observed in case of all the mortars, following the trend of flexural strength (Fig. 4a). The least resistant mortar under compression is AL with Rc 0.43 MPa. Similarly as flexural strength evolution, all additives provide an improvement of compressive resistance as a result of their morphology and/or pozzolanic activity predominantly at later ages than 28 days. Best endurance (0.74 MPa) was achieved by CZAL and FZAL20MK followed by 0.71 and 0.66 MPa reached in CSAL20MK and FZAL mortar, respectively. AL20MK and CZAL20MK mortars show the same Rc value (0.63 MPa) confirming that coarse zeolite addition does not affect the pozzolanic reaction of MK. CSAL mortar produces at 28 and 90 days of curing the same resistances as FSAL20MK mortar 0.44 and 0.55 MPa, respectively. The lowest impact of 19% on Rc increase has fine sepiolite in AL mortar. At 180 days upward trend in values was observed in all the mortars with exception of AL20MK (Fig. 5a). Again, AL mortar is showing the lowest compressive strength value of 0.47 MPa. Top resistances provide mortars involving fine materials and MK: 0.93 and 1.0 MPa for FSAL20MK and FZAL20MK, respectively. Slightly lower Rc results show CSAL20MK (0.83 MPa) and CZAL20MK (0.88 MPa). Related to coarse zeolite, it is noticeable that its positive effect in AL20MK is evident in late ages of curing as AL20MK mortar reaches 0.52 MPa. In opposition to AL–MK mortars, coarse materials in lime mortar provide higher values than fine ones. Both, coarse sepiolite and zeolite result in similar compressive strength (0.83 MPa), while from fine materials, fine zeolite, like in case of Rf is responsible for higher resistance than fine sepiolite. Even so, FSAL mortar at 180 days exhibits the same strength as AL20MK (0.52 MPa) meaning that needle shaped
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Fig. 5a. Compressive strength of mortars from the first set.
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Fig. 6a. Elasticity modulus of mortars from the first set.
Fig. 5b. Compressive strength of mortars from the second set.
particles of sepiolite fulfil the same role as pozzolanic products of metakaolin. In Fig. 5b is reported compressive strength of mortars from second set. At 28 days, AL_2 reaches 0.34 MPa. A drop in compressive resistance causes VAL_2 mortar (0.24 MPa), but on the other hand, combination of vermiculite and MK (VAL20MK_2) provides the highest Rc result from all the mortars studied at 28 days (0.95 MPa). VAL20MK_2 showed the best flexural resistance from AL–MK mortars, too (Fig. 4b). Addition of palygorskite to lime mortar (PAL_2) causes the same effect as addition of metakaolin (AL20MK_2) giving the value of 0.44 MPa. However, incorporation of palygorskite to AL20MK_2 manifested in increase in Rc of 19% compared to PAL_2 and AL20MK_2. Substitution of lime by fine sepiolite and MK does not appear to be effective as FSAL20MK_2 shows similar value (0.36 MPa) as AL_2 mortar. At 90 days, Rc values increase for all mortars with exception of VAL20MK_2. Nevertheless, this mortar keeps the highest resistance at 90 days from all the mortars and on the contrary, VAL_2 mortar achieves the lowest Rc (0.42 MPa) from both sets at this age of curing (Fig. 5b). The most gradual increase of more than 100% in strength is achieved by AL mortar (0.77 MPa) and none of mortars with additives, besides VAL20MK_2, exceeds this value. From 90 to 180 days, mortars without MK show similar increasing trend as in case of flexural strength, while there is a decline for AL–MK mortars (Figs. 4b
Fig. 6b. Elasticity modulus of mortars from the second set.
and 5b. VAL20MK_2 keeps the best resistance (0.90 MPa) followed by AL mortar (0.79 MPa) as it was at 90 days of curing. Fine sepiolite and palygorskite addition to AL20MK_2 seems not efficient, as PAL20MK_2 and FSAL20MK_2 mortars act during all ages as AL20MK_2 mortar.
4.2.2.3. Dynamic modulus of elasticity E. Evolution of dynamic modulus of elasticity with time of curing, for mortars from the first and second set is displayed in Figs. 6a and 6b, respectively. The measurement of the frequency of ultrasonic pulses is usually used to evaluate the homogeneity/heterogeneity of the mortar, the presence of voids, cracks or additional imperfections. Elasticity modulus usually increases with curing ages, but on the contrary with mechanical resistances, a significant growth of elasticity modulus with curing time is not preferred, because renders with a high elastic modulus have low deformation capability and are therefore unsuitable for use in conservation work. E values of mortars from the first set at 28 days do not vary significantly, between 2.3 and 2.7 GPa showing that the structure of the mortars is alike during early stage of hardening (Fig. 6a). At 90 days, the most noticeable increase show AL (3.3 GPa) and AL20MK (3.7 GPa) mortars, those with more homogeneous
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structure as the frequency of pulses of these mortars if higher compared to heterogeneous mortar matrixes. From 90 to 180 days, elastic modulus of mortars follows a similar evolution as their Rc (Fig. 5a), upward trend of all the mortars besides AL20MK. According to Veiga et al. [36], one of the requests of mortars to be used as renders of ancient buildings is low elasticity modulus with maximum value of E 2–5 GPa. Maximum elasticity modulus achieved is 3.7 GPa (AL20MK and FZAL20MK), it means that these mortars fulfil mentioned requirement. The second set of mortars provides bigger variety in the E values ranging at 28 days between 2.1 (VAL_2) and 3.4 GPa (FSAL 20MK_2), reflecting higher diversities in the core of hardening mortars (Fig. 6b). Behaviour of mortars at 90 days differs from those from first set (Fig. 6a). Raising effect of E exhibit AL_2 (38%), PAL_2 (23%) and AL20MK_2 (8%) mortars, negligible changes show VAL20MK_2 and FSAL20MK_2; while diminishing effect is observed in VAL_2 (24%) and PAL20MK_2 (75%). Nevertheless, at 180 days, elasticity modulus of all the mortars increases reaching the values of 1.9 GPa (belonging to PAL20MK_2, evidently mortar with most voids and heterogeneities) to 4 GPa (AL20MK_2). AL_2 mortar during all ages of curing achieves comparable E values as AL mortar. It is evident that incorporation of additives, mainly to lime–metakaolin_2 mortars causes structural discrepancies responsible for substandard E evolution with ageing (Fig. 6b).
5. Conclusions Two sets of mortars differing in the main mortars components as air lime and metakaolin (20 wt.%) were evaluated in terms of structural changes and mechanical strength, when various natural and commercial clay minerals were added as replacement of air lime (5 wt.%). Both mortar sets were prepared with fixed lime: sand 1:3 volumetric ratio. It was confirmed that: (i) All the additives (fine and coarse sepiolite and zeolite, palygorskite, vermiculite and metakaolins) cause microstructural changes in mortar matrixes. (ii) Both metakaolins (from Portugal and France) together with fine zeolite, exhibit pozzolanic activity. Related to mortars from the first set (air lime Calcidrata and metakaolin Portugal) it was observed that: – From 28 to 90 days, flexural and compressive strength show upward trends for all the mortars. At 180 days, only mortars without metakaolin (AL, FSAL, CSAL, FZAL and CZAL) and those with metakaolin and coarse sepiolite and zeolite provide an increase in flexural strength, while compressive strength of all the mortars shows an increment with the exception of AL20MK mortar. – Incorporation of fine and coarse sepiolite and zeolite and/or their combination with metakaolin creates blended mortars with improved mechanical strengths mainly at latter ages (90 and 180 days) compared to lime mortar without any addition. – The highest flexural strength (0.48 MPa) and compressive strength (1.04 MPa) at 180 days was attained by mortar containing 5 wt.% of fine zeolite and 20 wt.% of metakaolin, both materials exhibiting pozzolanic activity as was proven by scanning electron microscopy. – Elasticity modulus of mortars at latter curing ages show a similar evolution as their compressive strength – upward trend of all the mortars besides AL20MK. Values of all the mortars at 180 days ranging from 2.7 (FSAL) to 3.7 (FZAL20MK) GPa are appropriate for use especially as renders in conservation and restoration work.
Mortars from the second set (air lime Lusical, metakaolin France) provided the following findings: – AL_2 mortar reaches at 180 days higher mechanical resistances compared to AL from first set as AL_2 involves air lime with negligible traces of calcite. – Growing tendency of mechanical resistances from 28 to 90 and later to 180 days are displayed by mortars without metakaolin (AL_2, PAL_2 and VAL_2). Those with metakaolin have comparable values of flexural strength during all testing periods and declining trend in compressive resistance from 90 to 180 days was observed. – The best flexural resistance at 180 days (0.52 MPa) is obtained by PAL_2 mortar (containing 5 wt.% of palygorskite), while AL_2 mortar alone reaches 0.49 MPa. None of mortars containing metakaolin (France) and/or clay mineral (vermiculite, palygorskite and fine sepiolite) exceeds the value of AL_2 pointing out that combination power of pozzolanic activity of metakaolin with specific shape of clay minerals in AL_2 mortars is insignificant. – Top value of compressive strength (0.90 MPa) reaches VAL 20MK_2 (with 5 wt.% of vermiculite and 20 wt.% of metakaolin France), mortar with the highest flexural strength (0.37 MPa) of mortars with metakaolin. – Mortars provide greater variety in the elasticity modulus values (reflecting their structural heterogeneities evolved in progressed way than mortars from first set) during all ages of curing and do not show any conjoint trend with their mechanical resistances. At 180 days, elasticity modulus of all the mortars increases reaching the values of 1.9–4 GPa. For restoration purposes, compatibility between old and new renders should be attained. Present research shows that to create 1:3 (binder:sand) volumetric ratio, besides mineralogical composition also bulk density of air lime and metakaolin plays an important role. From mortars to be used as renders for restoration of cultural heritage it is important that they attain high flexural strength. In this case is it lime mortar with 5 wt.% of fine zeolite and 20 wt.% of metakaolin from the first set and lime mortar with 5 wt.% of palygorskite from the second set. Acknowledgements This research was supported by the Research Projects: PEstC/CTE/UI4035/2011, METACAL – Study of lime–metakaolin mortars for building conservation (PTDC/ECM/100431/2008) financed by the Fundação para a Ciência e a Tecnologia (FCT) and COMPETE Programme. References [1] Lanas J, Alvarez JI. Masonry repair lime-based mortars: factors affecting the mechanical behavior. Cem Concr Res 2003;33:1867–76. [2] Moropoulou A, Bakolas A, Moundoulas P, Aggelakopoulou E, Anagnostopoulou S. Strength development and lime reaction in mortars for repairing historic masonries. Cem Concr Compos 2005;27:289–94. [3] Arıoglu N, Acun SA. A research about a method for restoration of traditional lime mortars and plasters: a staging system approach. Build Environ 2006;41:1223–30. [4] Marques SF, Ribeiro RA, Silva LM, Ferreira VM, Labrincha JA. Study of rehabilitation mortars: construction of a knowledge correlation matrix. Cem Concr Res 2006;36:1894–902. [5] Stefanidou M, Papayianni I. The role of aggregates on the structure and properties of lime mortars. Cem Concr Compos 2005;27:914–9. [6] Lanas J, Sirera R, Alvarez JI. Study of the mechanical behavior of masonry repair lime-based mortars cured and exposed under different conditions. Cem Concr Res 2006;36:961–70. [7] Faria P, Henriques F, Rato V. Comparative evaluation of lime mortars for architectural conservation. J Cult Herit 2008;9:338–46.
S. Andrejkovicˇová et al. / Construction and Building Materials 65 (2014) 132–139 [8] Veiga MR, Velosa AL, Magalhães AC. Experimental applications of mortars with pozzolanic additions: characterization and performance evaluation. Constr Build Mater 2009;23:318–27. [9] Velosa AL, Rocha F, Veiga R. Influence of chemical and mineralogical composition of metakaolin on mortar characteristics. Acta Geodyn Geomater 2009;153:1–6. [10] Paiva H, Velosa A, Veiga R, Ferreira VM. Effect of maturation time on the fresh and hardened properties of an air lime mortar. Cem Concr Res 2010;40:447–51. [11] Aggelakopoulou E, Bakolas A, Moropoulou A. Properties of lime–metakolin mortars for the restoration of historic masonries. Appl Clay Sci 2011;53: 15–9. [12] Arizzi A, Viles H, Cultrone G. Experimental testing of the durability of limebased mortars used for rendering historic buildings. Constr Build Mater 2012;28:807–18. ˇ erny´ R. Mechanical, [13] Vejmelková E, Keppert M, Keršner Z, Rovnaníková P, C fracture–mechanical, hydric, thermal, and durability properties of lime–metakaolin plasters for renovation of historical buildings. Constr Build Mater 2012;31:22–8. [14] Frías M, Sánchez de Rojas MI, Cabrera J. The effect that the pozzolanic reaction of metakaolin has on the heat evolution in metakaolin–cement mortars. Cem Concr Res 2000;30:209–16. [15] Cabrera J, Rojas MF. Mechanism of hydration of the metakaolin–lime–water system. Cem Concr Res 2001;31:177–82. [16] Kakali G, Perraki T, Tsivilis S, Badogiannis E. Thermal treatment of kaolin: the effect of mineralogy on the pozzolanic activity. Appl Clay Sci 2001;20:73–80. [17] Moropoulou A, Bakolas A, Aggelakopoulou E. Evaluation of pozzolanic activity of natural and artificial pozzolans by thermal analysis. Thermochim Acta 2004;420:135–40. [18] Bakolas A, Aggelakopoulou E, Moropoulou A, Anagnostopoulou S. Evaluation of pozzolanic activity and physicomechanical characteristics in metakaolin–lime pastes. J Therm Anal Calorim 2006;84:157–63. [19] Fortes-Revilla C, Martínez-Ramírez S, Blanco-Varela MT. Modelling of slaked lime–metakaolin mortar engineering characteristics in terms of process variables. Cem Concr Compos 2006;28:458–67. [20] Frías Rojas M. Study of hydrated phases present in a MK–lime system cured at 60 °C and 60 months of reaction. Cem Concr Res 2006;36:827–31. [21] García R, Vigil de la Villa R, Rodríguez O, Frías M. Mineral phases formation on the pozzolan/lime/water system. Appl Clay Sci 2009;43:331–5.
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[22] Konan KL, Peyratout C, Smith A, Bonnet J-P, Rossignol S, Oyetola S. Comparison of surface properties between kaolin and metakaolin in concentrated lime solutions. J Colloid Interface Sci 2009;339:103–9. [23] Gameiro A, Santos Silva A, Veiga R, Velosa A. Hydration products of lime– metakaolin pastes at ambient temperature with ageing. Thermochim Acta 2012;535:36–41. [24] Martínez-Ramírez S, Puertas F, Blanco-Varela MT. Carbonation process and properties of a new lime mortar with added sepiolite. Cem Concr Res 1995;25:39–50. [25] Martínez-Ramírez S, Puertas F, Blanco-Varela MT, Thompson GE, Almendros P. Behaviour of repair lime mortars by wet deposition process. Cem Concr Res 1998;28:221–9. [26] Martínez-Ramírez S, Puertas F, Blanco-Varela MT, Thompson GE. Effect of dry deposition of pollutants on the degradation of lime mortars with sepiolite. Cem Concr Res 1998;28:125–33. [27] Andrejkovicˇová S, Ferraz E, Velosa AL, Silva AS, Rocha F. Fine sepiolite addition to air lime metakaolin mortars. Clay Miner 2011;46:621–35. [28] Andrejkovicˇová S, Velosa AL, Rocha F. Air lime–metakaolin–sepiolite mortars for earth based walls. Constr Build Mater 2013;44:133–41. [29] Silva LM, Ribeiro RA, Labrincha JA, Ferreira VM. Role of lightweight fillers on the properties of a mixed-binder mortar. Cem Concr Compos 2010;32:19–24. [30] Andrejkovicˇová S, Ferraz E, Velosa AL, Silva AS, Rocha F. Air lime mortars with incorporation of sepiolite and synthetic zeolite pellets. Acta Geodyn Geomater 2012;9:79–91. [31] EN 1015-2. Methods of test for mortar for masonry Part 2: Bulk sampling of mortars and preparation of test mortars; 1998. [32] EN 1015-11. Methods of test for mortar for masonry – Part 11: Determination of flexural and compressive strength of hardened mortar; 1999. [33] BS 3406-2. Methods for determination of particle size distribution, Recommendations for gravitational liquid sedimentation methods for powders and suspensions. London: British Standard Institute; 1986. [34] CSTB Cahier 2669-4. Certification CSTB des enduits monocouches d’imperméabilisation. Modalités d’essais; 1993. [35] BS 1881-209. Testing concrete. Recommendations for the measurement of dynamic modulus of elasticity. London: British Standard Institute; 1990. [36] Veiga MR, Aguiar J, Silva AS, Carvalho F. Methodologies for characterisation and repair of mortars of ancient buildings. In: Lourenço PB, Roca P, editors. Proceedings of the 3rd international seminar on historical constructions. Guimarães; 2001. p. 353–62.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of clay minerals addition on mechanical properties of air lime–metakaolin mortars S. Andrejkovicˇová a,⇑, A.L. Velosa b, E. Ferraz b, F. Rocha a a b
Geosciences Department, Geobiotec Research Unit, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal Civil Engineering Department, Geobiotec Research Unit, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
h i g h l i g h t s Two sets of lime mortars with addition of clay minerals and metakaolin were analysed. Clay minerals and metakaolins cause microstructural changes of mortars. Lime with higher bulk density is more suitable to be substituted by additives. Clay minerals substituted air lime in 1st set clearly improved mechanical strengths.
a r t i c l e
i n f o
Article history: Received 5 December 2013 Received in revised form 21 April 2014 Accepted 22 April 2014
Keywords: Lime Mortar Metakaolin Sepiolite Zeolite Palygorskite Vermiculite Mechanical properties Restoration
a b s t r a c t Two sets of mortars differing in type of lime and metakaolin, with air lime:sand volumetric ratio 1:3 were prepared with the aim to be used for restoration of historic masonries. The first set involved air lime and 20 wt.% of metakaolin with more impurities (calcite and kaolinite, respectively) and with higher bulk densities compared to materials from the second set. Clay minerals (sepiolite, zeolite A, palygorskite and vermiculite) characterised by high specific surface areas and thus able to keep water in the structure and promote pozzolanic activity of metakaolins were used as additives in air lime and air lime–metakaolin mortars and their impact was evaluated in a view of mechanical strength at 28, 90 and 180 days. Substitution of air lime from the first set by clay mineral and/or metakaolin caused improvement of mechanical strengths predominantly at latter ages, while lime mortars from second set suffer by lack of binder when other additives are supplemented and just palygorskite incorporation improves flexural strength, while vermiculite and metakaolin create mixture with improved compressive strength than lime mortar alone. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction For the adequate restoration of cultural heritage, the compatibility between old mortars and the rehabilitation materials is obligatory. The survey for this compatibility is also supported by the fact that old materials to be renovated, already had evidenced to have suitable mechanical properties and acceptable performances throughout the centuries [1,2]. Recent restoration interventions are based on utilisation of analogous chemical composition of binders, aggregates and mineral additions, as they derive from the study of historic mortars [e.g. 3,4]. The main component of old renders is usually lime, occasionally supplemented by the presence of pozzolanic or other additives. For this reason, ⇑ Corresponding author. Tel.: +351 234 370 747; fax: +351 234 370 605. E-mail address: [email protected] (S. Andrejkovicˇová). http://dx.doi.org/10.1016/j.conbuildmat.2014.04.118 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
air lime mortars and/or mixed with pozzolanic additions (natural or artificial) have been studied widely with the intention to be used as mortars for the construction of historic buildings [1,5–13]. Addition of high reactive pozzolans to lime creates mortars similar to historic ones that display an advanced durability and high values of mechanical strength. Nowadays many researchers are paying attention mainly to metakaolin which is an artificial pozzolanic additive mainly due to its capacity to react with calcium hydroxide creating typical pozzolanic products [14–23]. In addition, positive effect of natural clay minerals such as sepiolite [24–28], vermiculite [29] or synthetic zeolite A [30] on lime mortar’s characteristics has also been the object of recent research. In some rehabilitation cases, it is inevitable to provide restoration interventions in places of lack of humidity, or unfavourable conditions such as difficult access to CO2 or desiccation conditions (wind, heat). This is more so in Portugal due to its climatic
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conditions, extensive coastline and building traditions. For this reason the main scope of the present study is to develop blended mortars based on lime with additions of metakaolin and clay minerals characterised by high specific surface areas, able to adsorb water molecules and provide more humid conditions promoting pozzolanic activity of metakaolin. From natural clay minerals have been chosen sepiolite and palygorskite typical also for their fibrous structure and from commercial ones zeolite type A and expanded vermiculite. Lime/aggregate volumetric ratio selected for this study was 1:3.
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21% of water was added to AL_2, PAL, AL20MK_2 and PAL20MK; 23% to FSAL20MK_2 and 19% to VAL and VAL20MK mortars. 2.1. Conditioning Mortar prisms 40 40 160 mm of all the mortars were prepared. Air lime mortars (AL, AL_2) were cured during all tested ages 28, 90 and 180 days in a chamber with a relative humidity of 65 ± 5% and temperature 20 ± 2 °C. Specimens containing metakaolin and/or FS, CS, FZ, CZ, P, V were stored in moulds for the first 2 days in a chamber at 20 ± 2 °C with a relative humidity of 95 ± 5% and then remoulded and kept for next 5 days at the same conditions. Then the specimens were maintained at a relative humidity of 65 ± 5% and temperature 20 ± 2 °C; and cured up to ages of 28, 90 and 180 days according to the Standard EN 1015-11 [32].
2. Materials, mortar composition, conditioning 3. Methods Taking into account the principal mortar components (air lime and metakaolin), two sets of mortars were prepared: 1. With powdered commercial air lime CL 90 (AL) (Calcidrata, S.A., Portugal) and siliceous river sand and formulated with air lime:sand volumetric ratio of 1:3. Lime binder was replaced by: (a) 5 wt.% of fine (FS) and coarse (CS) commercial sepiolite (Sepiolita 15/30, Minas de Paracuellos del Jarama, Madrid, Spain) and 5 wt.% fine (FZ) and coarse (CZ) commercial synthetic zeolite A pellets Phonosorb 551 (Grace Davison, USA); (b) 20 wt.% of commercial metakaolin (MK) (EcoPozz, Portugal); (c) by both, (a) and (b). Individual specimens are in the text marked as follows (Table 1). Water added to mortars was calculated to provide an appropriate workability, accomplished by the flow table test with values around 130–140 mm according to the Standard EN 1015-2 [31]. 19% of water, considering total mortar mass, was added to air lime (AL) and air lime + metakaolin (AL20MK) mortar. Forasmuch sepiolite and zeolite pellets have high specific surface areas and consequently higher water demand, 23% and 21% of water was necessary to add to fine/coarse sepiolite and zeolite mortars, respectively. 2. Second set of mortars was prepared with powdered commercial air lime (AL) (Lusical H100, Portugal) with classification CL90 and siliceous river sand and formulated with air lime:sand volumetric ratio of 1:3. Lime binder was replaced by: (a) 5 wt.% of palygorskite (P) (Minas the Torrejon, Spain) and 5 wt.% of expanded vermiculite (V) (Aguiar & Mello, artificial product of natural vermiculite calcination between temperatures of 700 °C and 1000 °C); (b) 20 wt.% of commercial metakaolin (MK) (AGS Mineraux, France); (c) by both, (a), (b) and with 5 wt.% of fine (FS) commercial sepiolite (Sepiolita 15/30, Minas de Paracuellos del Jarama, Madrid, Spain) and 20 wt.% of commercial metakaolin (MK) (AGS Mineraux, France). Individual specimens are in the text marked as follows (Table 2). Table 1 Composition of mortars (1st set). Reference
Materials
AL FSAL CSAL FZAL CZAL AL20MK FSAL20MK CSAL20MK FZAL20MK CZAL20MK
Air lime + sand Fine sepiolite + air lime + sand Coarse sepiolite + air lime + sand Fine zeolite + air lime + sand Coarse zeolite + air lime + sand Metakaolin + air lime + sand Fine sepiolite + air lime + metakaolin + sand Coarse sepiolite + air lime + metakaolin + sand Fine zeolite + air lime + metakaolin + sand Coarse zeolite + air lime + metakaolin + sand
Table 2 Composition of mortars (2nd set). Reference
Materials
AL_2 PAL_2 VAL_2 AL20MK_2 PAL20MK_2 VAL20MK_2 FSAL20MK_2
Air lime + sand Palygorskite + air lime + sand Vermiculite + air lime + sand Metakaolin + air lime + sand Palygorskite + air lime + metakaolin + sand Vermiculite + air lime + metakaolin + sand Fine sepiolite + air lime + metakaolin + sand
The fine size materials (sepiolite and zeolite) were obtained by dry grinding of the coarse (original) materials in a Ceramic Instruments mill (S2-1000-M) with porcelain jars and alumina balls, during 15 min. Particle size distribution of fine materials was performed with X-ray grain size analyser Sedigraph 5100 from Micromeritics, following the BS 3406-2 [33]. Bulk densities of materials were determined according to Certification CSTB Cahier [34]. The mineralogical composition of the specimens was determined using a Philips X’Pert diffractometer equipped with Cu Ka radiation. The microstructural and chemical homogeneity was analysed by scanning electronic microscopy, SEM/EDS (Hitachi SU 70 coupled with EDAX Bruker AXS detector). Flexural and compressive strength tests were carried out on 3 probes of individual mortar following Standard EN 1015-11 [32] on (SHIMADZU: AG-IC 100 kN) equipment, with loads of 10 and 50 N/s for flexural and compressive strength, respectively. The dynamic modulus of elasticity was determined based on the fundamental longitudinal resonant frequency following the BS 1881-209 [35].
4. Results and discussion 4.1. Characterisation of materials used for mortars preparation 4.1.1. Particle size distribution and bulk density of materials Values of particle size distribution and bulk density of all materials used for mortar preparation are reported in Table 3. The lowest D50 is that of palygorskite (0.1 lm) with bulk density 733 kg m 3. Medium values of D50 are attributed to metakaolin (France), fine sepiolite and air lime (Calcidrata) 1.3, 2.4 and 3.2, respectively. Top D50 values has air lime (Lusical) (7.0 lm), metakaolin (Portugal) (10.0 lm) and fine zeolite (16.0 lm). The highest bulk density (1140 kg m 3) of fine materials is provided by fine zeolite. Related to particle size distribution of coarse materials, expanded vermiculite is composed of the largest particles (<4 mm) and in the same way has the lowest bulk density (121 kg m 3) of all the materials (Table 3). Distribution of sand particles ranging between 0.125 and 0.5 mm is followed by coarse sepiolite 0.3–1.2 mm and finally by coarse zeolite between 1.2 and 2.5 mm. Bulk density of sand (1560 kg m 3) is the highest from all materials.
Table 3 Particle size distribution and bulk density of materials. Material
D50 (lm)
Air lime (Calcidrata) Air lime (Lusical) Metakaolin (Portugal) Metakaolin (France) Fine sepiolite Fine zeolite Palygorskite Vermiculite Coarse sepiolite Coarse zeolite Sand
3.2 7.0 10.0 1.3 2.4 16.0 0.1
Particle size distribution (mm)
Bulk density (kg m 3)
<4 0.3–1.2 1.2–2.5 0.125–0.5
460 380 673 296 526 1140 733 121 680 1290 1560
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(Fig. 2h–i) of calcium silicates, what confirm the EDAX spectra of AL20MK and AL20MK_2 in Fig. 3a–b, respectively. It is important to note that pozzolanic reaction (distribution of calcium aluminium silicate in mortar matrix) of AL20MK_2 mortar was developed in larger scale compared to AL20MK mortar.
Fig. 1. X-ray diffraction patterns of random oriented materials used for mortar preparation; (a) air lime Calcidrata, (b) air lime Lusical, (c) metakaolin Pt, (d) metakaolin Fr, (e) sepiolite, (f) zeolite, (g) palygorskite, (h) vermiculite, and (i) sand. (A-anatase, C-calcite, D-dolomite, F-feldspar, I-illite, K-kaolinite, L-leucite, Nnepheline, P-portlandite, Q-quartz, S-sepiolite, V-vermiculite. Z-zeolite type A,).
4.1.2. Mineralogical composition The XRD patterns of materials used for mortar preparation are displayed in Fig. 1. Air lime (Calcidrata) diffractogram contains portlandite and calcite appears due to carbonation of Ca(OH)2 (Fig. 1a). Air lime (Lusical) shows predominantly XRD patterns of portlandite with negligible traces of calcite. Metakaolin (Pt) was not completely calcinated, as its pattern contains kaolinite, besides quartz and feldspar; while illite, anatase and quartz are detected in XRD pattern of metakaolin (Fr). Sepiolite diffractogram is composed of dominant mineral sepiolite, including admixtures of dolomite and quartz (Fig. 1e). The synthetic zeolite pellets contain leucite and nepheline as major mineral phases and sodium zeolite type A as minor component. Palygorskite sample shows admixtures of quartz and dolomite, while expanded vermiculite contains only diffraction peaks of vermiculite. Sand is composed predominantly from quartz with minor content of feldspar (Fig. 1i). 4.2. Characterisation of mortars 4.2.1. Morphology and microstructure Scanning electron images of representative mortars at 90 days are shown in Fig. 2. Images of mortars containing both, clay mineral and metakaolin are not included as they are combination of individual morphological changes observed in lime–clay or lime–metakaolin micrographs. Basic lime mortars without any additions (AL and AL_2) show characteristic carbonated portlandite matrixes as is demonstrated in Fig. 2a. An addition of other admixtures causes the changes in the microstructure of primary mortar (Fig. 2b–i). Both fine and coarse sepiolite (Fig. 2b–c) due to their fibrous origin, cause connection between the individual components of mortar and thus create a more compacted system. Fine zeolite proved its pozzolanic activity reacting with of Ca(OH)2 by the presence of mosaic shaped fragments of calcium aluminosilicate (Fig. 2d). Coarse zeolite pellets in AL mortar provide conditions, in which fan-shaped clusters of aragonite are formed (Fig. 2e). This phenomenon is described in the work of Andrejkovicˇová et al. [30]. Palygorskite needles show analogous behaviour as coarse and fine sepiolite in term of the ‘‘bridge’’ linking with the lime binder (Fig. 2f). Particles of expanded vermiculite with typical ‘‘accordion’’ type morphology also form connection with lime particles as sepiolite and palygorskite; however in the same time produce more open pore spaces. Pozzolanic activity of both metakaolins (Portugal and France) was revealed by formation of acicular fibres
4.2.2. Mechanical strength of mortars 4.2.2.1. Flexural strength Rf. An impact of the addition of individual clay minerals and metakaolins to air lime mortars on mechanical properties was also evaluated. Flexural strength values of mortars representing first and second set at 28, 90 and 180 days are displayed in Figs. 4a and 4b, respectively. In case of the first set, at 28 days, related to the addition of clay minerals to lime (without MK), there is an increase of 14% and 50% for FSAL and CSAL and negligible change for FZAL and CZAL mortars compared to AL (0.22 MPa) (Fig. 4a). Addition of metakaolin (AL20MK mortar) shows the same behaviour as fine sepiolite on final Rf value (0.25 MPa). Fine materials in combination with MK imposed an improvement of about 27% (FSAL20MK) and 36% (FZAL20MK) in comparison with AL, while addition of coarse sepiolite and coarse zeolite to AL–metakaolin did not affect the final value of flexural strength (Fig. 4a). At 90 days of curing, all the mortars from the 1st set show a rise in flexural strength compared to age 28 days. Rf value 0.26 MPa of AL mortar, is the lowest from all mortars, meaning that all additives provide an enhancement of flexural resistance as a result of their structure and/or pozzolanic activity. The best examples are FSAL20MK and FZAL20MK, which reach double Rf (0.50 MPa) than AL mortar (Fig. 4a). Pozzolanic activity of fine zeolite alone manifested in the highest flexural strength (0.41 MPa for FZAL) of mortars without MK. This value is higher than Rf of AL20MK mortar with metakaolin (0.35 MPa). From 90 to 180 days there is visible a slight increase of flexural strength values for all the mortars without metakaolin (AL, FSAL, CSAL, FZAL and CZAL). From those with metakaolin, just mortars containing coarse materials exhibit a rise of Rf at 180 days. It is important to note that incorporation of MK did not improve flexural resistance of CZAL20MK mortar, as the final values during all curing ages are alike for CZAL and CZAL20MK (Fig. 4a). FZAL (0.44 MPa) and FZAL20MK (0.48 MPa) mortars show the highest values of Rf at 180 days, predominantly as a result of microstructure and pozzolanic activity of fine zeolite (Figs. 2d and 4a). In general, compared to AL, all the mortars besides AL20MK show higher flexural resistances at 180 days. Fig. 4b presents flexural strength of mortars from the second set. Evolution of Rf with time of curing is different when compared to mortars from the 1st set. Air lime used in these mortars contains less calcite, and metakaolin contains less impurities and above all, no kaolinite peaks were observed what means that calcination was executed in higher level compared to metakaolin from Portugal (Fig. 1b, c and d). Nevertheless, at 28 days of curing AL_2 reaches Rf 0.24 MPa (Fig. 4b) comparable with AL mortar with Rf 0.22 MPa (Fig. 4a). From all the mortars just palygorskite addition improves flexural resistance of air lime mortar of 63% and vermiculite with metakaolin (VAL20MK_2) of 42%. VAL_2, AL20MK_2, PAL20MK_2 and FSAL20MK_2 mortars show lower resistances than AL (Fig. 4b). From 28 to 90 days, Rf of all the mortars increases, but more evidently in case of mortars without MK. Top values reach AL_2 and PAL_2 mortar (0.45 MPa). None of lime–MK mortars caused improvement of flexural strength compared to AL. At 180 days, an upward trend from 90 days in Rf values show mortars without MK. Palygorskite addition to AL mortar provides the best flexural resistance 0.52 MPa, but even AL_2 mortar alone reaches 0.49 MPa. From lime–metakaolin mortars, the highest Rf shows VAL20MK_2. In general, unexpected behaviour was observed in by all the mortars containing metakaolin. According to SEM analysis (Figs. 2i and 3b), metakaolin used in second set
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Fig. 2. Scanning electron images of (a) AL, (b) FSAL, (c) CSAL, (d) FZAL, (e) CZAL, (f) PAL_2, (g) VAL_2, (h) AL20MK, and (i) AL20MK_2 mortar.
produces pozzolanic activity, though, its presence inhibits the growth of flexural strength in opposition to metakaolin (Portugal) (Fig. 2h). Even the time of curing does not effect in a progressive way the development of Rf and all the mortars with metakaolin have comparable values of flexural strength during all testing periods (Fig. 4b). Compared to AL, AL_2 shows even better flexural
resistance at 28, 90 and 180 days (Figs. 4a and 4b). Incorporation of other additives (besides palygorskite) to AL_2 mainly at later ages does not cause any improvement (Fig. 4b). It means that AL_2 mortar already contains just ‘‘accurate’’ amount of binder and other substitutions create a mixture with lack of lime to withstand flexural forces. It is important to note that the air lime used
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Fig. 4b. Flexural strength of mortars from the second set.
Fig. 3. EDAX spectra of (a) AL20MK (Fig. 2h) and (b) AL20MK_2 mortar (Fig. 2i).
Fig. 4a. Flexural strength of mortars from the first set.
in this set (Lusical) has lower bulk density compared to air lime (Calcidrata) (Table 3) and to create volumetric ratio lime:sand 1:3, AL_2 mortar required lower quantity of lime than AL mortar.
4.2.2.2. Compressive strength Rc. Compressive strength values (Rc) of mortars from both sets at 28, 90 and 180 days are shown in Figs. 5a and 5b. Related to the first set at 28 days (Fig. 5a), incorporation of clay minerals to AL mortar caused increase in Rc of 47%, 37% and 47% for CSAL, FZAL and CZAL mortars, respectively, while FSAL (0.32 MPa) mortar shows comparable value with AL (0.30 MPa). Pozzolanic activity of metakaolin manifested in 17% rise of compressive strength when compared to AL mortar, however CSAL20MK and CZAL20MK mortars show alike value as AL20MK (0.35 MPa) meaning that coarse materials at early age of curing do not have a significant impact on Rc increase on lime–metakaolin mortar. On the other hand, combination of fine materials with metakaolin provides an increment of compressive strength (FSAL20MK 0.44 MPa and FZAL20MK 50 MPa) even compared to AL or AL20MK (Fig. 5a). From 28 to 90 days, an increase of Rc was observed in case of all the mortars, following the trend of flexural strength (Fig. 4a). The least resistant mortar under compression is AL with Rc 0.43 MPa. Similarly as flexural strength evolution, all additives provide an improvement of compressive resistance as a result of their morphology and/or pozzolanic activity predominantly at later ages than 28 days. Best endurance (0.74 MPa) was achieved by CZAL and FZAL20MK followed by 0.71 and 0.66 MPa reached in CSAL20MK and FZAL mortar, respectively. AL20MK and CZAL20MK mortars show the same Rc value (0.63 MPa) confirming that coarse zeolite addition does not affect the pozzolanic reaction of MK. CSAL mortar produces at 28 and 90 days of curing the same resistances as FSAL20MK mortar 0.44 and 0.55 MPa, respectively. The lowest impact of 19% on Rc increase has fine sepiolite in AL mortar. At 180 days upward trend in values was observed in all the mortars with exception of AL20MK (Fig. 5a). Again, AL mortar is showing the lowest compressive strength value of 0.47 MPa. Top resistances provide mortars involving fine materials and MK: 0.93 and 1.0 MPa for FSAL20MK and FZAL20MK, respectively. Slightly lower Rc results show CSAL20MK (0.83 MPa) and CZAL20MK (0.88 MPa). Related to coarse zeolite, it is noticeable that its positive effect in AL20MK is evident in late ages of curing as AL20MK mortar reaches 0.52 MPa. In opposition to AL–MK mortars, coarse materials in lime mortar provide higher values than fine ones. Both, coarse sepiolite and zeolite result in similar compressive strength (0.83 MPa), while from fine materials, fine zeolite, like in case of Rf is responsible for higher resistance than fine sepiolite. Even so, FSAL mortar at 180 days exhibits the same strength as AL20MK (0.52 MPa) meaning that needle shaped
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Fig. 5a. Compressive strength of mortars from the first set.
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Fig. 6a. Elasticity modulus of mortars from the first set.
Fig. 5b. Compressive strength of mortars from the second set.
particles of sepiolite fulfil the same role as pozzolanic products of metakaolin. In Fig. 5b is reported compressive strength of mortars from second set. At 28 days, AL_2 reaches 0.34 MPa. A drop in compressive resistance causes VAL_2 mortar (0.24 MPa), but on the other hand, combination of vermiculite and MK (VAL20MK_2) provides the highest Rc result from all the mortars studied at 28 days (0.95 MPa). VAL20MK_2 showed the best flexural resistance from AL–MK mortars, too (Fig. 4b). Addition of palygorskite to lime mortar (PAL_2) causes the same effect as addition of metakaolin (AL20MK_2) giving the value of 0.44 MPa. However, incorporation of palygorskite to AL20MK_2 manifested in increase in Rc of 19% compared to PAL_2 and AL20MK_2. Substitution of lime by fine sepiolite and MK does not appear to be effective as FSAL20MK_2 shows similar value (0.36 MPa) as AL_2 mortar. At 90 days, Rc values increase for all mortars with exception of VAL20MK_2. Nevertheless, this mortar keeps the highest resistance at 90 days from all the mortars and on the contrary, VAL_2 mortar achieves the lowest Rc (0.42 MPa) from both sets at this age of curing (Fig. 5b). The most gradual increase of more than 100% in strength is achieved by AL mortar (0.77 MPa) and none of mortars with additives, besides VAL20MK_2, exceeds this value. From 90 to 180 days, mortars without MK show similar increasing trend as in case of flexural strength, while there is a decline for AL–MK mortars (Figs. 4b
Fig. 6b. Elasticity modulus of mortars from the second set.
and 5b. VAL20MK_2 keeps the best resistance (0.90 MPa) followed by AL mortar (0.79 MPa) as it was at 90 days of curing. Fine sepiolite and palygorskite addition to AL20MK_2 seems not efficient, as PAL20MK_2 and FSAL20MK_2 mortars act during all ages as AL20MK_2 mortar.
4.2.2.3. Dynamic modulus of elasticity E. Evolution of dynamic modulus of elasticity with time of curing, for mortars from the first and second set is displayed in Figs. 6a and 6b, respectively. The measurement of the frequency of ultrasonic pulses is usually used to evaluate the homogeneity/heterogeneity of the mortar, the presence of voids, cracks or additional imperfections. Elasticity modulus usually increases with curing ages, but on the contrary with mechanical resistances, a significant growth of elasticity modulus with curing time is not preferred, because renders with a high elastic modulus have low deformation capability and are therefore unsuitable for use in conservation work. E values of mortars from the first set at 28 days do not vary significantly, between 2.3 and 2.7 GPa showing that the structure of the mortars is alike during early stage of hardening (Fig. 6a). At 90 days, the most noticeable increase show AL (3.3 GPa) and AL20MK (3.7 GPa) mortars, those with more homogeneous
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structure as the frequency of pulses of these mortars if higher compared to heterogeneous mortar matrixes. From 90 to 180 days, elastic modulus of mortars follows a similar evolution as their Rc (Fig. 5a), upward trend of all the mortars besides AL20MK. According to Veiga et al. [36], one of the requests of mortars to be used as renders of ancient buildings is low elasticity modulus with maximum value of E 2–5 GPa. Maximum elasticity modulus achieved is 3.7 GPa (AL20MK and FZAL20MK), it means that these mortars fulfil mentioned requirement. The second set of mortars provides bigger variety in the E values ranging at 28 days between 2.1 (VAL_2) and 3.4 GPa (FSAL 20MK_2), reflecting higher diversities in the core of hardening mortars (Fig. 6b). Behaviour of mortars at 90 days differs from those from first set (Fig. 6a). Raising effect of E exhibit AL_2 (38%), PAL_2 (23%) and AL20MK_2 (8%) mortars, negligible changes show VAL20MK_2 and FSAL20MK_2; while diminishing effect is observed in VAL_2 (24%) and PAL20MK_2 (75%). Nevertheless, at 180 days, elasticity modulus of all the mortars increases reaching the values of 1.9 GPa (belonging to PAL20MK_2, evidently mortar with most voids and heterogeneities) to 4 GPa (AL20MK_2). AL_2 mortar during all ages of curing achieves comparable E values as AL mortar. It is evident that incorporation of additives, mainly to lime–metakaolin_2 mortars causes structural discrepancies responsible for substandard E evolution with ageing (Fig. 6b).
5. Conclusions Two sets of mortars differing in the main mortars components as air lime and metakaolin (20 wt.%) were evaluated in terms of structural changes and mechanical strength, when various natural and commercial clay minerals were added as replacement of air lime (5 wt.%). Both mortar sets were prepared with fixed lime: sand 1:3 volumetric ratio. It was confirmed that: (i) All the additives (fine and coarse sepiolite and zeolite, palygorskite, vermiculite and metakaolins) cause microstructural changes in mortar matrixes. (ii) Both metakaolins (from Portugal and France) together with fine zeolite, exhibit pozzolanic activity. Related to mortars from the first set (air lime Calcidrata and metakaolin Portugal) it was observed that: – From 28 to 90 days, flexural and compressive strength show upward trends for all the mortars. At 180 days, only mortars without metakaolin (AL, FSAL, CSAL, FZAL and CZAL) and those with metakaolin and coarse sepiolite and zeolite provide an increase in flexural strength, while compressive strength of all the mortars shows an increment with the exception of AL20MK mortar. – Incorporation of fine and coarse sepiolite and zeolite and/or their combination with metakaolin creates blended mortars with improved mechanical strengths mainly at latter ages (90 and 180 days) compared to lime mortar without any addition. – The highest flexural strength (0.48 MPa) and compressive strength (1.04 MPa) at 180 days was attained by mortar containing 5 wt.% of fine zeolite and 20 wt.% of metakaolin, both materials exhibiting pozzolanic activity as was proven by scanning electron microscopy. – Elasticity modulus of mortars at latter curing ages show a similar evolution as their compressive strength – upward trend of all the mortars besides AL20MK. Values of all the mortars at 180 days ranging from 2.7 (FSAL) to 3.7 (FZAL20MK) GPa are appropriate for use especially as renders in conservation and restoration work.
Mortars from the second set (air lime Lusical, metakaolin France) provided the following findings: – AL_2 mortar reaches at 180 days higher mechanical resistances compared to AL from first set as AL_2 involves air lime with negligible traces of calcite. – Growing tendency of mechanical resistances from 28 to 90 and later to 180 days are displayed by mortars without metakaolin (AL_2, PAL_2 and VAL_2). Those with metakaolin have comparable values of flexural strength during all testing periods and declining trend in compressive resistance from 90 to 180 days was observed. – The best flexural resistance at 180 days (0.52 MPa) is obtained by PAL_2 mortar (containing 5 wt.% of palygorskite), while AL_2 mortar alone reaches 0.49 MPa. None of mortars containing metakaolin (France) and/or clay mineral (vermiculite, palygorskite and fine sepiolite) exceeds the value of AL_2 pointing out that combination power of pozzolanic activity of metakaolin with specific shape of clay minerals in AL_2 mortars is insignificant. – Top value of compressive strength (0.90 MPa) reaches VAL 20MK_2 (with 5 wt.% of vermiculite and 20 wt.% of metakaolin France), mortar with the highest flexural strength (0.37 MPa) of mortars with metakaolin. – Mortars provide greater variety in the elasticity modulus values (reflecting their structural heterogeneities evolved in progressed way than mortars from first set) during all ages of curing and do not show any conjoint trend with their mechanical resistances. At 180 days, elasticity modulus of all the mortars increases reaching the values of 1.9–4 GPa. For restoration purposes, compatibility between old and new renders should be attained. Present research shows that to create 1:3 (binder:sand) volumetric ratio, besides mineralogical composition also bulk density of air lime and metakaolin plays an important role. From mortars to be used as renders for restoration of cultural heritage it is important that they attain high flexural strength. In this case is it lime mortar with 5 wt.% of fine zeolite and 20 wt.% of metakaolin from the first set and lime mortar with 5 wt.% of palygorskite from the second set. Acknowledgements This research was supported by the Research Projects: PEstC/CTE/UI4035/2011, METACAL – Study of lime–metakaolin mortars for building conservation (PTDC/ECM/100431/2008) financed by the Fundação para a Ciência e a Tecnologia (FCT) and COMPETE Programme. References [1] Lanas J, Alvarez JI. Masonry repair lime-based mortars: factors affecting the mechanical behavior. Cem Concr Res 2003;33:1867–76. [2] Moropoulou A, Bakolas A, Moundoulas P, Aggelakopoulou E, Anagnostopoulou S. Strength development and lime reaction in mortars for repairing historic masonries. Cem Concr Compos 2005;27:289–94. [3] Arıoglu N, Acun SA. A research about a method for restoration of traditional lime mortars and plasters: a staging system approach. Build Environ 2006;41:1223–30. [4] Marques SF, Ribeiro RA, Silva LM, Ferreira VM, Labrincha JA. Study of rehabilitation mortars: construction of a knowledge correlation matrix. Cem Concr Res 2006;36:1894–902. [5] Stefanidou M, Papayianni I. The role of aggregates on the structure and properties of lime mortars. Cem Concr Compos 2005;27:914–9. [6] Lanas J, Sirera R, Alvarez JI. Study of the mechanical behavior of masonry repair lime-based mortars cured and exposed under different conditions. Cem Concr Res 2006;36:961–70. [7] Faria P, Henriques F, Rato V. Comparative evaluation of lime mortars for architectural conservation. J Cult Herit 2008;9:338–46.
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