Waste Management xxx (2014) xxx–xxx

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Waste Management

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Recycled sand in lime-based mortars

http://dx.doi.org/10.1016/j.wasman.2014.09.0050956-053X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +30 2310995635.E-mail address: stefan@civil.auth.gr (M. Stefanidou).

Please cite this article in press as: Stefanidou, M., et al. Recycled sand in lime-based mortars. Waste Management (2014), http://dx.doi.org/1j.wasman.2014.09.005

M. Stefanidou ⇑, E. Anastasiou, K. Georgiadis FilikasDept. of Civil Engineering, A.U.Th, GR-54124 Thessaloniki, Macedonia, Greece

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 February 2014Accepted 9 September 2014Available online xxxx

Keywords:Fine recycled aggregatesLime-based mortarsStrengthPorosity

The increasing awareness of the society about safe guarding heritage buildings and at the same time pro-tecting the environment promotes strategies of combining principles of restoration with environmentallyfriendly materials and techniques. Along these lines, an experimental program was carried out in order toinvestigate the possibility of producing repair, lime-based mortars used in historic buildings incorporat-ing secondary materials. The alternative material tested was recycled fine aggregates originating frommixed construction and demolition waste. Extensive tests on the raw materials have been performedand mortar mixtures were produced using different binding systems with natural, standard and recycledsand in order to compare their mechanical, physical and microstructure properties. The study reveals theimproved behavior of lime mortars, even at early ages, due to the reaction of lime with the Al and Si con-stituents of the fine recycled sand. The role of the recycled sand was more beneficial in lime mortarsrather than the lime–pozzolan or lime–pozzolan–cement mortars as a decrease in their performancewas recorded in the latter cases due to the mortars’ structure.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Historic mortars usually are based on lime and often containsand of river origin, most commonly of 0–4 mm granulometry(Papayianni, 2006). The addition of aggregates to a binding systemhas proved to confer technical advantages as they contribute tovolume stability, durability and structural performance(Stefanidou and Papayianni, 2005). Apart from the different avail-able aggregate types, as far as their mineralogy is concerned, theirvolume content in the mixture, as well as their maximum size andgradation, influence the structure of a binder–aggregate mixture(Baronio et al., 1997). Properties such as rheology, strength, shrink-age, porosity are strongly based on the type, the ratio, and thegradation of the aggregates (Cortes et al., 2008; Westerholmet al., 2008). Sands of different origins have been used in mortarsand different flow ability, strength and stiffness have beenrecorded (Tasong et al., 1998). Additionally, an increased waterdemand was recorded in cases where angular in shape aggregateshave been used, while the binder–aggregates transition zone inthose cases improved even though porosity had increased(Gonçalves et al., 2007). All the sands that are characterized as suit-able for use should be of selected maximum size, present an evengranulometry and be free of organics and soluble salts.

The incentive to use sand from building demolition in repairmortars derives from different needs: natural sand originatingfrom rivers is becoming rare, while the extraction of aggregatesfrom quarries carries an increased administrative cost due tonew legislation, both at National and European level (COM/2008/699; FEK 2076 B, 2009). Both practices are not considered environ-mentally friendly and, thus, the criteria and legislation for sandextraction are becoming more strict and demanding, while in someplaces good quality natural sands are not available. On the otherhand, the increased waste production offers the availability of largevolumes of recycled materials and public concern about the envi-ronment pushes towards their utilization.

Mixed construction and demolition waste (CDW) aggregate isproduced by crushing waste originating from various constructionsites and building rubble and may contain concrete, ceramic, gyp-sum, glass and bituminous particles. As expected, CDW aggregatesshow great variations in quality since the presence and amount ofimpurities depend on the origin of the CDW and the various pro-cesses of the recycling plant. In Greece, the amount of generatedCDW reached 6.8 and 2.1 million tonnes for 2008 and 2010, respec-tively, and the recycling rate is lower than 5% while, at the sametime, natural aggregates are consumed at high rates (Eurostat,2012).

Many researches performed during recent years converge to theresult that the coarse fraction of construction and demolitionwaste can be used to substitute natural aggregates in concrete suc-cessfully (Kou et al., 2012; Lovato et al., 2012; Martín-Morales

0.1016/

2 M. Stefanidou et al. / Waste Management xxx (2014) xxx–xxx

et al., 2011; Rao et al., 2007), despite the expected reducedstrength and increased porosity (Agrela et al., 2011; Guedeset al., 2013). Such findings have led to the adaptation of relevantstandards such as the EN 12620 (CEN, 2002a), in order to includerecycled aggregates as well. However, fine recycled aggregate hasproven more difficult to incorporate into concrete, mainly due towater demand and fresh concrete workability problems (RILEM,1994a), and its use in concrete is limited (Evangelista and deBrito, 2010; Khatib, 2005). Their current main application is inlandfills (Kourmpanis et al., 2008; Rao et al., 2007) and researchtowards increasing the possible uses for such materials is stillneeded.

Efforts have been made to examine the addition of recycledsand in cement-based mortars by either substituting a part of thenatural sand (Lima and Leite, 2012; Miranda and Selmo, 2006a,2006b) or by using 100% recycled fine aggregates (Corinaldesiet al., 2002; Jiménez et al., 2013; Martínez et al., 2013; Nenoet al., 2013). The results show that cement-based mortars withrecycled sand present lower compressive strength, improved val-ues of bond strength, higher porosity, high shrinkage and absorp-tion capacity. Martín-Morales et al. (2011), even proposeimprovement of the quality of the recycled sand by washing andsieving, by manually removing gypsum, or by mixing with naturalsand, in order to upgrade its quality.

The purpose of this study was to test the use of recycled sandfrom mixed construction and demolition waste in traditional mor-tars in comparison to natural and standard sands. Since the sensi-tive approach to historic buildings demands rigorous tests on thesuitability of materials, a detailed characterization of the recycledsand is firstly presented, followed by tests on traditional mortarmixtures with lime and pozzolan as the main binders. The mortarswere tested for their physical and mechanical properties, micro-structure and durability and their performance in respect to com-posites with natural sands was assessed.

2. Materials and methods

2.1. Materials

The binders used in the test mortars were hydrated lime, natu-ral pozzolan and white Portland cement I42.5. A polycarboxylic-based superplasticizer was also used in some of the mortars with

Table 1Physical properties of recycled and natural sand.

Sample App. specific density (kg/m3) Water absorption (%) Sand equ

Procedure EN 1097-6 EN 1097-6 EN 933-8Natural sand 2650 1.5 98.0Recycled sand 2450 8.0 66.6

Table 2Mixture proportions by weight and workability of the test mortars.

Mortar constituents LSS LNS LRS LRSsp PSS

Hydrated lime (L) 1 1 1 1 0.5White cement (C) – – – – –Natural pozzolan (P) – – – – 0.5Standard sand (S) – 3 – – –River sand (N) 3 – – – 3Recycled sand (R) – – 3 3 –Water/binder 0.80 0.80 1.06 0.95 0.84Effective w/b ratio 0.76 0.76 0.82 0.71 0.80Superplasticizer (sp) – – – 0.015 –

Flow table workability (mm) 160 140 148 143 160

L: lime, P: pozzolan, C: cement, SS: standard sand, NS: natural sand, RS: recycled sand,

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recycled sand, in order to compensate for reduced workability.Three different sands were used in this study as fine aggregatesin traditional mortar mixtures: standard AFNOR sand, natural riversand and recycled sand originating from the fine fraction (<4 mm)of CDW aggregates. The CDW aggregates used in the present paperare commercially available and were obtained from a recyclingplant in the area of Thessaloniki, in Macedonia, Greece, withoutany treatment.

The natural sand used originated from the river Axios in thearea of Thessaloniki and it had been cleaned and sieved previouslyin order to be suitable for construction. The standard sand usedwas siliceous of standard gradation and composition conformingto the EN 196-1 standard (CEN, 2005).

A series of tests on the raw aggregate was decided in order todetermine its suitability for use in traditional mortars. The constit-uents of the recycled sand were identified visually, while FTIR (32scans, resolution 2 cm�1) using tablet KBr 1% and XRD analysisusing Philips PW 1710 diffractometer with Ni-filtered Cu Ka radia-tion were carried out in order to determine its mineralogical com-position and the presence of organic impurities. Wet chemicalanalyses on natural river and recycled sands were also carriedout while soluble salts were determined by ionic chromatography(Thermo Scientific, Dionex ICS-1100) which is a fully quantitativemethod. Some of their physical properties, including granulometry,density, water absorption, sand equivalent, fineness and sphericitywere measured according to relevant standards (see Table 1).

2.2. Mortar mixtures design

Three different binding systems, often used in repair works,were selected for the test mortars using hydrated lime (L), a com-bination of lime and natural pozzolan (P) in 1:1 proportion (byweight) and a combination of lime–natural pozzolan and whitecement (C) in 1:0.8:0.2 ratio (also by weight). The three differentsands were used for each of the binding systems in binder/aggre-gate ratio, 1/3 by weight. Water was adjusted in order to achievea consistency of 150 ± 10 mm in the flow table according to EN1015-3:1999 (CEN, 1999a) (Table 2). The mortars with recycledsand showed significantly increased water demand due to theirwater absorption, which was not taken into account when deter-mining the effective (available) water to binder ratio. Since theaggregates used in mortars and concrete are considered to be in

ivalent (%) Fineness modulus (%) Sphericity (measured under microscope)

EN 126203.63 0.8254.97 0.721

PNS PRS PRSsp CSS CNS CRS CRSsp

0.5 0.5 0.5 0.5 0.5 0.5 0.5– – – 0.1 0.1 0.1 0.10.5 0.5 0.5 0.4 0.4 0.4 0.43 – – – 3 – –– – – 3 – – –– 3 3 – – 3 30.80 1.09 1.00 0.85 0.76 1.00 0.920.76 0.85 0.76 0.81 0.72 0.76 0.68– – 0.015 – – – 0.015

154 150 153 160 150 145 150

sp: superplasticizer

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M. Stefanidou et al. / Waste Management xxx (2014) xxx–xxx 3

saturated surface dry (SSD) condition, the water required by thesand in order to reach the SSD condition cannot be consideredavailable for reaction with the binder, as it is absorbed by theaggregate. The remaining water is available for the hydration ofthe binder and forms the effective water to binder ratio. Thisapplies both for natural and recycled sands, but since natural sandshave considerably lower water absorption, taking into account theeffective water to binder ratio is very important when consideringrecycled sands. Still, for similar effective water to binder ratios, themortars with recycled sand had reduced workability, which can beattributed to their surface roughness and higher absorption capac-ity. Consequently, it was decided to produce a fourth mortar mix-ture for each binding system using recycled sand andsuperplasticizer (sp) in order to achieve the required workabilitywithout increasing the water to binder ratio. The addition ofsuperplasticizer at a rate of 1% by weight of binders enabled con-siderable water demand reduction in the mortars with recycledsand without changing workability significantly (a ± 5 mm changein workability is acceptable by the EN 196-1 standard). Neverthe-less, the water content in these mixtures remains high comparedto the mixtures with the natural sands.

Table 5Constituents of the recycled sand.

Materials % of particles

Bituminous materials 6Masonry 21Concrete and concrete products, mortars 31Lightweight 6Unbound aggregate 30Other 6Total 100

2.3. Hardened mortars test procedures

The fresh mortars were cast in 40 � 40 � 160 mm prisms forflexural and compressive strength testing according to EN 1015-11:1999 (CEN, 1999b). The lime based mortars (‘‘L’’ series) werecured at 21 �C and 50% relative humidity (RH) according toEN459-2 section 5.6.3 considering lime as an aerial binder, whilethe mortars with pozzolan and cement (‘‘P’’ and ‘‘C’’ series) werecured at 21 �C and 95% RH until testing based on the EN 459-2standard section 5.1.2.3 which refers to hydraulic binders (CEN,2010). Compressive strength measurements were carried out at28 and 90 days, while flexural strength was recorded at 90 daysonly, due to the low-strength development of traditional and espe-cially lime mortars. In order to determine the bonding strength ofmortars, the crossed-brick test according to ASTM C952-02 (ASTM,2002) was carried out at 28 days, while the open porosity and cap-illary absorption were recorded by the RILEM CPC11.3 (RILEM,1994b) method and EN1015-18:2002 (CEN, 2002b) respectivelyat the ages of 28 and 90 days. Microstructure analysis by stereo-scope (Leica Wild M10) was also used in order to identify the struc-ture of mortars with different fine aggregate, while further analysiswith Scanning Electron Microscope (SEM-JEOL 840A JSM) and EDSanalysis were used in order to identify crystals formation. Finally,regarding the durability of the test mortars, three specimens fromeach mortar, saturated in water, were exposed to 30 freeze–thaw

Table 3Physical properties of the binders.

Binder d (0.1) lm d (0.5) lm

Lime 1.20 3.09Pozzolan 1.50 4.30White cement 1.87 15.57

Table 4Chemical composition of binders, natural and recycled sands (% wt.).

Sample Na2O K2O CaO MgO Fe2O3

Recycled sand 0.92 0.8 29.7 2.19 3.66Natural sand 2.59 1.86 4.70 1.59 5.86Lime 0.22 0.02 73.18 2.85 0.01Pozzolan 2.78 4.05 15.25 8.23 1.89Cement 0.17 0.08 58.35 2.74 0.19

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cycles at temperatures ranging from �15 �C to 30 �C. The pulsevelocity was measured at the beginning and at the end of theexperiment and Mercury Intrusion Porosimetry (MIP Macro,Quantachrome) was applied at a limited number of samples inorder to determine their pore size distribution.

3. Results and discussions

3.1. Material characterization

Some of the physical properties of the binders, regarding parti-cle size, density and surface area were measured and are presentedin Table 3.

The chemical analysis of the CDW sand (Table 4) shows that itcontains lower amount of SiO2 and higher amount of CaO in rela-tion to natural sands, while the amount of sulfate salts is lowerand this is due to the sand constituents as they were identified(Table 5). The considerably higher water absorption of CDW fines,as presented in Table 1 is in accordance with the literature(Gómez-Soberón, 2002; Poon et al., 2004) while the relativelylow sand equivalent value indicates the presence of argillaceousmaterials. According to the EN 12620 categorization though, therecycled sand falls in the best available category regarding the sandequivalent value. The aggregate granulometry of the differentsands used as presented in Fig. 1 confirms that CDW sand is a bitcoarser and at the same time contains a higher amount of fines(<75 lm) in relation to the other two sands.

Fig. 2 shows the FTIR spectrum of the recycled sand, where thepresence of calcite, quartz, kaoline and bassanite are indicated,without any traces of organic or other hazardous materials suchas metals or asbestos. This analysis is confirmed by the XRD dia-gram shown in Fig. 3. In this diagram calcite, quartz and dolomiteare presented to be the main components of the recycled sand.

Overall, the CDW sand is a largely inert material of heteroge-neous constituents, high amount of porous material (as indicatedby the high water absorption and the reduced apparent specific

d (0.9) lm Density g/ml Surface area m2/g

10.80 2.471 2.2511.60 1.403 1.8254.65 2.664 1.37

Al2O3 SiO2 L.I.% Cl- NO3� SO4

2�

6.27 38.1 18.42 0.02 0.01 0.2613.04 67.46 2.91 0.01 0.01 0.350.08 0.07 23.57 0.03 – 0.489.79 49.57 8.45 0.34 – 0.233.89 22.07 11.07 0.02 – –

e-based mortars. Waste Management (2014), http://dx.doi.org/10.1016/

0102030405060708090

100

0.01 0.1 1 10

% p

assi

ng

sieve (mm)

recycled sand

natural sand

standard sand

Fig. 1. Granulometry of standard, natural and recycled sands.

Fig. 2. FTIR analysis on the recycled sand showing no traces of organic material.

Fig. 3. XRD diagram of the recycled sand.

05

1015202530354045

LNS

LSS

LRS

LRS -

sp

PNS

PSS

PRS

PRS-

sp

CN

S

CSS

CR

S

CR

S-sp

%

28-d90-d

Fig. 4. Porosity evaluation at different ages.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 10 20 30 40

g/cm

2

sq root (min)

LSS LNSLRS LRSsp

Fig. 5a. Capillary absorption at the age of 28 days for samples with lime.

4 M. Stefanidou et al. / Waste Management xxx (2014) xxx–xxx

density) and relatively coarser gradation compared to naturalsands, while the high amount of particles in the size fraction <75lm indicates possible presence of soft particles. These propertiesexplain the increased water demand in the produced mortarsand are also expected to increase porosity with a consequentdecrease in strength. The absence of any organic or deleteriousimpurities and the increased surface roughness of the recycledsand, however, are desirable properties that could be beneficialto the hardened mortar properties.

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3.2. Physical and mechanical properties of hardened mortars

3.2.1. Porosity and capillary absorptionThe open porosity of the different mortars is shown in Fig. 4. It

seems that the samples with recycled sand (RS) present higherporosity values in all binding systems which is in accordance withexisting literature (Gómez-Soberón, 2002; Martínez et al., 2013).However, the porosity was systematically reduced when superp-lasticizer was added in the mixture. Even in this case, however,the porosity was higher in comparison to the mortars with naturalsands as discussed earlier.

When the capillary absorption was measured in the differentsystems, the water absorbed in the samples with RS and superp-lasticizer was reduced in relation to the samples with only RS, indi-cating the beneficial role of the superplastisizer (Fig. 5).

3.2.2. Strength developmentThe compressive strength development of the mortars is shown

in Figs. 6a–6c. It can be seen that the recycled sand does not hinderthe rate of strength development. On the contrary, in the ‘‘softer’’lime mortars, recycled sand, seems to improve the strength evenat early ages. This increase is even higher in the mixtures withsuperplasticizer. The same behavior was recorded in lime–pozzo-lan mortars (Fig. 6b) while in a stronger binder (Fig. 6c) the behav-ior was similar to the mortars with natural sand. The positive effectof RS in lime-based mortars is attributed to the reaction betweenthe lime and the silica constituents of the raw materials of thesand. This was also indicated by the microstructure analysis fol-lowing in Section 3.3.

The flexural strength results (Fig. 7) show that natural sands,and especially standard sand, led to higher values in all cases,while the use of superplasticizer slightly improved the flexuralstrength of mortars with recycled sand. This can be attributed tothe reduced water to binder ratio when using superplasticizer. Itshould be noted that the lime-based mortars were cured and

e-based mortars. Waste Management (2014), http://dx.doi.org/10.1016/

0.00.20.40.60.81.01.21.41.6

0 10 20 30 40

g/cm

2

sq root (min)

PSS PNSPRS PRSsp

Fig. 5b. Capillary absorption at the age of 28 days for samples with pozzolan.

0.00.20.40.60.81.01.21.41.6

0 10 20 30 40

g/cm

2

sq root (min)

CSS CNSCRS CRSsp

Fig. 5c. Capillary absorption at the age of 28 days for samples with cement.

0.00.20.40.60.81.01.21.41.6

28 90

Com

pres

sive

str

engt

h(M

Pa)

Age (days)

LSS LNSLRS LRSsp

Fig. 6a. Compressive strength development of lime mortars (L series).

4.04.55.05.56.06.57.07.58.0

28 90

Com

pres

sive

str

engt

h(M

Pa)

Age (days)

PSS PNSPRS PRSsp

Fig. 6b. Compressive strength development of lime–pozzolan mortars (P series).

4.04.55.05.56.06.57.07.58.0

28 90

Com

pres

sive

str

engt

h(M

Pa)

Age (days)

CSS CNSCRS CRSsp

Fig. 6c. Compressive strength development of lime–pozzolan–cement based mor-tars (C series).

0.00.20.40.60.81.01.21.41.6

lime lime + pozzolan lime + pozzolan + cement

90-d

ay fl

exur

al s

tren

gth

Binders

standard sand natural sandrecycled sand recycled sand + sp

Fig. 7. Flexural strength of the different mortars at 90 days.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

lime lime + pozzolan lime + pozzolan + cement

standard sand

natural sand

recycled sand

recycled sand + sp

Fig. 8. Flexural to compressive strength ratio at 90 days.

0.000.020.040.060.080.100.120.14

lime lime + pozzolan lime + pozzolan + cement

Bon

ding

str

engt

h (M

Pa)

Binders

standard sand natural sandrecycled sand recycled sand + sp

Fig. 9. Bonding strength of the different mortars.

M. Stefanidou et al. / Waste Management xxx (2014) xxx–xxx 5

tested in dry conditions. This affects flexural strength in a positiveway and compressive strength in a negative way (Klieger andLamond, 1994). This, along with the low strength values observedand the sensitivity of the flexural test, explains the lower flexuralto compressive strength ratios for mortars with recycled sand(Fig. 8).

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3.2.3. Bonding strengthFig. 9 shows the improvement in bonding strength of the

crossed-brick tests at the age of 28 days when recycled sand was

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Fig. 10. (a) Detachment of the mortar in CNS mortars and (b) strong cohesion of the LRS mortar to the brick.

Fig. 11. Deterioration of the PRS and CRS mortars after 30 freeze–thaw cycles.

Table 6Pulse velocity of the samples after 50 freeze–thaw cycles.

u (initial) (km/s) u (final) (km/s) % Difference

LSS 2.24 1.74 22LNS 1.75 1.70 3LRS 1.68 1.62 3LRS-sp 1.80 1.71 5CSS 2.96 2.21 25CRS-sp 2.63 2.24 15

05

101520253035404550

<0.1 0.1-1 >1

%

mm

LRS

PRS

Fig. 12. Pore size distribution in mortars with recycled sand.

6 M. Stefanidou et al. / Waste Management xxx (2014) xxx–xxx

used in lime-based mortars. In the lime–pozzolan (P) and lime–pozzolan–cement (C) series, the bond was not so strong but itimproved with the addition of the superplasticizer. The positiverole of the RS in L mortars is attributed to the strong cohesion ofthe binder and the aggregates but also to the adhesion of the mor-tar to brick (Fig. 10). Fig. 10 is describing a representative situationregarding in case 10a the detachment of the CNS mortar from thebrick while in 10b the strong cohesion between LRS mortar and thebrick.

3.2.4. Freeze–thaw resistanceAfter 30 cycles, the lime–pozzolan (P) series were all cracked

and a number of samples from the lime–pozzolan–cement (C) sam-ples were also damaged (Fig. 11) while the lime (L) mortars pre-sented adequate durability. The pulse velocity recorded showed

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small variations for most of the LRS samples while the sampleswith cement, recycled sand and superplasticizer presented smallreduction in pulse velocity, compared to cement mortars withstandard sand and no superplasticizer, thus good mass stability(Table 6). This behavior was difficult to explain as the porosity val-ues of the samples were comparable, so further research wasundertaken to study the pore sizes of the samples. Mercury Intru-sion Porosimetry (MIP) was applied at limited number of samples(only on LRS and PRS samples) and some of the results wereenlightening. The pores that prevailed in lime mortars were largein size in relation to the pores of the pozzolanic mortars. The por-ous structure of the lime mortars could thus provide the necessaryspace before the stresses damage the structure (Fig. 12).

3.3. Microstructure

The microstructure analysis indicated a compact structure withfew pores and good coherence between the aggregates and the bin-der in all cases. In the case of RS the presence of crushed bricks andmixed type of aggregates was additionally recorded (Fig. 13). Fur-ther analysis with Scanning Electron Microscope (SEM) revealedthe co-existence of Ca–Si–Al in the crystals of the binder as shownby EDS analysis and the formation of plate-shaped and small fiber-shaped crystals was observed, contributing to the compact struc-ture of the matrix (Fig. 14). The above observation was verifiedin many lime mortar samples tested and can be explained by asmall scale reaction of lime with reactive Al–Si constituents ofthe fine recycled aggregates. These reactions have been identifiedby Moropoulou et al. (2005). The microstructure observation ofthe other samples indicated that the cohesion of the aggregateswith the binder was in most cases strong (Fig. 15).

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Fig. 13. (a) Structure of PSS, (b) structure of PRS, and (c) structure of PNS mortars.

Fig. 14. SEM analysis on the LRS crystals.

Fig. 15. Compact structure in PRS and CRS mortars.

M. Stefanidou et al. / Waste Management xxx (2014) xxx–xxx 7

4. Conclusions

This paper explores the possibility of incorporating fine recy-cled sand originating from construction and demolition waste inlime-based traditional mortars. Laboratory analysis of the recycledsand (RS) showed that it had an even grain distribution, withoutorganic materials and a low content of soluble salts. The resultswere encouraging to produce mortars using this sand.

The mortars mixtures with RS showed increased water demandand reduced workability compared to mortars with natural sands,even when superplasticizer was used. The mechanical strengthmeasured at 28 and 90 days showed good results as the mortarswith lime and recycled sand had higher compressive strength com-pared to mortars with natural sands. The addition of superplasti-sizer increased the compressive strength even at early ages,while the bonding strength of lime mortars with bricks was alsohigh. Microstructure analysis confirmed the dense structure ofthe lime mortars due to the reaction of lime with Si and Al com-pounds of the fine aggregates.

It seems that two competitive mechanisms acted in these mor-tars; high porosity (due to high water content and the nature of theaggregates) which assists towards low strength and the chemicalreaction due to the presence of reactive components which creates

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a strong structure. This chemical reaction is a stronger mechanismin the case of lime mortars and prevails in relation to the compet-itive mechanisms of the higher porosity. In stronger binding sys-tems, such as the combination of lime with natural pozzolan andcement, natural and standard sands seem to perform better thanthe recycled aggregate. The high porosity recorded in these bindingsystems with RS resulted in the reduction of their mechanicalproperties. As a consequence, the high water content and the highporosity affected negatively the strength and the durability ofthese mortars.

The results showed that it is possible to incorporate RS in limemortars used as repair materials in historic structures. The techni-cal characteristics of these mortars, regarding mechanical strength,capillary absorption and porosity properties are comparable tothose achieved by conventional lime mortars used nowadays inrepair works. Additionally, the structure of lime mortars with RSwas coherent due to the formation of C–S–A needle-like crystals,while also the early strength was improved.

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