Construction and Building Materials 24 (2010) 1392–1397

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Construction and Building Materials

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Environmental cycling and laboratory testing to evaluate the significanceof moisture control for lime mortars

Adel El-Turki a,*, Richard J. Ball a, Stafford Holmes b, William J. Allen c, Geoffrey C. Allen a

a University of Bristol, Interface Analysis Centre, Oldbury House, 121 St. Michael’s Hill, Bristol BS2 8BS, UKb Rodney Melville & Partners, 10 Euston Place, Leamington Spa, Warwickshire CV32 4LJ, UKc Ellis and Moore Consulting Engineers, Station House, Ashley Avenue, Bath BA1 3DS, UK

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

Article history:Received 12 June 2008Received in revised form 15 January 2010Accepted 15 January 2010Available online 20 February 2010

Keywords:Lime mortarEnvironmental cyclingMechanical propertiesMicrostructure

0950-0618/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2010.01.019

* Corresponding author. Tel.: +44 117 33 11175; faE-mail address: a.el-turki@bristol.ac.uk (A. El-Turk

This paper, and the research it describes, examines the benefits and importance of good site practice forthe aftercare of lime mortars and renders. The control of moisture is explored in relation to mortar devel-opment. This work shows the way in which proper aftercare in terms of wetting, drying and protectionenables lime mortars and renders to perform at their best. It is important that Architects, Engineers andspecifiers can rely on craftspeople who understand the need for aftercare and ensure that best practice isfollowed. This paper provides a scientific explanation of how the traditional practice to control moisture,before and immediately following the application of building limes, works to improve set and durability.

Scientific reasons for the traditional site practice of damping down the background to application oflime mixes, and the subsequent tending of finished work, are explained by reference to laboratoryresearch developed for the purpose.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Lime mortars are experiencing resurgence in their use due to in-creased awareness of their environmental and structural benefits[1,2]. This trend is relevant for both new build and conservationapplications where lime is replacing some cement products [3].Building limes are available as high calcium (non-hydraulic) lime(CL90) or hydraulic lime for construction. High calcium lime isessentially calcium hydroxide which hardens through the processof carbonation, whereas hydraulic lime contains calcium hydroxideand other components which react with water, typically di-cal-cium silicate [4,5].

Hydraulic lime mortars achieve their strength through chemicalset, hydration and carbonation [6,7]. The extent to which thesereactions occur is directly related to the relative amounts of cal-cium hydroxide and calcium silicates and to a lesser extent calciumaluminates. The calcium aluminates are more reactive and undergohydration before the silicates. In a cementitious binder gypsum isadded to reduce the intensity of this reaction and avoid a falseset however the low aluminate content in the NHL’s studied doesnot require such additions. Limes which contain reactive silicatesformed from impurities, such as clay, during the burning processare referred to as natural hydraulic limes. In general more siliceousimpurities, and higher burning temperatures, result in a greaterproportion of reactive silicates and a higher strength. Hydraulic

ll rights reserved.

x: +44 117 92 55646.i).

limes are classified in ascending order of compressive strength aseither NHL2, NHL3.5 or NHL5.

The hydration reaction observed in lime mortars is rapid incomparison to carbonation. It is widely acknowledged that the rateof carbonation is influenced by a number of factors, includinghumidity, temperature and carbon dioxide concentration [8].Hydration on the other hand is primarily related to temperatureand humidity. Lime mortar has a porous structure and in practicethe conditions within the mortar may differ markedly from theexternal environment [9].

The action of wetting and drying experienced by a mortar used inan external structure is believed to have an important influence onthe carbonation and hydration reactions and, hence, strength devel-opment. It is normally specified and traditional practice to protectnewly placed lime mortar and lime renders with damp cloth or sack-ing to retain moisture levels. This dries out at varying rates depend-ing on ambient conditions, and it is important that it is dampeddown as frequently as necessary, usually over a period of two weeksor more. An alternative and more recent method of damping downnow available is to use a portable pressure pump to mist spray thesurface to prevent rapid drying out in the same way and at similarintervals. This is recognized good practice, particularly in hot anddry conditions, and is termed ‘‘tending”, Fig. 1. This leads to an arti-ficially created wetting and drying environment.

In practice, it is not possible to determine how wetting and dry-ing affects the strength of a mortar joint on site, and indeed,whether an appropriate number of cycles has been undertaken. If

Fig. 1. Tending the first coat of lime render mix (1 part NHL 3.5:2 part sand) bydamping down with a mist spray to the tympanum of Kedleston Hall east wing.

Table 2Chemical composition (weight %) showing main constituents of calcium lime,hydraulic limes and Portland cement.

Compound CL90 NHL2 NHL3.5 NHL5 PC

Calcium hydroxide, Ca(OH)2 97.1 53.6 38.1 20.6 –Di-calcium silicate, C2S – 14.3 25 45 16.3Calcium carbonate, CaCO3 <2.9 21.7 20.9 23.1 0.3Calcium, alumino ferite, C4AF – 1.7 4.5 2 7.8Tricalcium aluminate, C3A – 0.7 3.8 2.1 8.9Di-calcium alumino silicate, C2AS – 3.4 3.9 2.2 4.7Tricalcium silicate, C3S – – – – 55.6Calcium sulphate, CaSO4 – 1.5 0.9 – 4.3

Total 97.1 96.9 97.1 95 97.9

0

10

20

30

40

50

60

70

80

90

100

10000100010010

Aggregate size, microns

Cum

ulat

ive

% p

assi

ng

Fig. 2. Particle size distribution of Croxden sand used to manufacture test mortars.

FeFe

TiCaKS

Si

Al

O

0 2 4 6 8 10

Energy (keV)

Cou

nts

(a.u

.)

Fig. 3. Energy dispersive X-ray analysis from the surface of a Croxden sand grainshowing the elemental composition.

A. El-Turki et al. / Construction and Building Materials 24 (2010) 1392–1397 1393

the mortar or render dries too rapidly, however, binding propertiesof the lime are lost and the mortar is likely to fail. In severe condi-tions, for example a very dry south facing wall in direct sunlightwithout shading, surface cracking and powdering are likely to oc-cur without regular tending to damp down.

This paper describes the laboratory research in which a numberof different hydraulic lime mortar mixes were exposed to wettingand drying cycles to simulate the effect of climatic changes. Forcomparative purposes, high calcium (CL90) lime, hydraulic limeand Portland cement based mortars were also tested. Mechanicalstrengths were determined by compressive testing. Structuraland compositional changes were identified using focused ion beamimaging and Raman spectroscopy. Small cylindrical samples18 mm in diameter and 36 mm in length were used to ensure ashort path for moisture and gaseous diffusion.

2. Experimental method

2.1. Sample mix designs

A total of fourteen mix designs were included in this study, details of which areprovided in Table 1. Hydraulic limes NHL2 and NHL5, calcium lime (CL90) and Port-land cement, PC were all supplied by Hanson Cement Ltd., Clithero. NHL3.5 wassupplied by Hydraulic Lias Limes Ltd (HLL). A chemical composition of the limesand cement used is given in Table 2. Croxden sand with a maximum particle sizeof 5 mm was used to manufacture the specimens. A particle size distribution is gi-ven in Fig. 2. An elemental analysis obtained using energy dispersive X-ray analysisindicated the presence of silicon, aluminium, oxygen, iron, potassium, calcium, sul-phur and titanium (Fig. 3).

2.2. Sample preparation and exposure conditions

A Hobart mixer was used to mix the constituents of each sample for a durationof 10 min. Release oil was applied to the mould prior to casting the mixtures into

Table 1Mix designs.

No. Constituents Ratio Testing days

1 HLL (NHL3.5):Croxden sand 1:1 28, 56, 90, 1802 HLL (NHL3.5):Croxden sand 1:2 28, 56, 90, 1803 HLL (NHL3.5):Croxden sand 1:3 28, 56, 90, 1804 HLL (NHL3.5):Croxden sand 1:4 28, 56, 90, 1805 Hanson (NHL2):Croxden sand 1:2 28, 56, 90, 1806 Hanson (NHL5):Croxden sand 1:2 28, 56, 90, 1807 PC:Croxden sand (FebDH) 1:6 28, 56, 90, 1808 PC:Castle CL90:Croxden sand 1:2:9 28, 56, 90, 1809 Hanson (NHL2):Croxden sand 1:1 1, 3, 7, 14, 2810 Hanson (NHL2):Croxden sand 1:2 1, 3, 7, 14, 2811 Hanson (NHL5):Croxden sand 1:1 1, 3, 7, 14, 2812 Hanson (NHL5):Croxden sand 1:2 1, 3, 7, 14, 2813 Hanson CL90:Croxden sand 1:1 1, 3, 7, 14, 2814 HLL (NHL3.5):Croxden sand (13 months old) 1:2 1, 3, 7, 14, 28

cylinders 18 mm in diameter and 36 mm in length. To reduce the presence of largeair pockets within the samples and to ensure an even distribution of the mix withinthe mould, each mould was half-filled and vibrated for 1 min before filling to justbelow the top and vibrating for a further 1 min. To ensure that the top surface ofeach cylinder was smooth and parallel with the bottom surface a thin layer ofPortland cement was applied to the top of each sample. The samples were then leftto harden under laboratory conditions at 25 �C and 50% relative humidity. To easeextraction of the samples, the mould had a removable bottom and could be splitalong the sample length. After one day, the bottom of the mould was removedand after 6 days the mould was separated to remove the cylindrical samples.

After de-moulding half the samples from each batch were exposed to the labo-ratory atmosphere (20 �C, 50% relative humidity) and the remainder placed in aspecially designed wetting and drying chamber. This consisted of a spraying devicecapable of saturating the samples within 60 s. Two fans positioned to suck airthrough the chamber dried the samples over a period of several hours. Both spray-ing and drying operations were electronically controlled wetting for 10 min anddrying for 20 h, repeatedly for the duration of exposure within the chamber.

Samples mixes 1–8 were examined after 28, 56, 90 and 180 days of exposure.Samples mixes 9–14 were examined after 1, 3, 7, 14 and 28 days of exposure. De-tails are given in Table 1.

Fig. 5. Ion-induced secondary electron micrograph showing needle like di-calciumsilicate based C–S–H phases bridging void in calcite/portlandite matrix on surface of1:1 NHL3.5 lime mortar after 28 days.

1394 A. El-Turki et al. / Construction and Building Materials 24 (2010) 1392–1397

2.3. Mechanical testing

Eight samples from each batch were tested in compression using a Zwick/Ro-well testing machine with stainless steel platens. The bottom platen was fixedbut the top one was allowed movement, about a central ball joint, to accommodatesample irregularities. Samples were compressed using a constant strain rate of 5lm s�1. Values of force and displacement were logged automatically during thetest. A 10 kN load cell allowed force to be recorded to 0.1 N. Cross head displace-ments were recorded with an accuracy of 1 lm. At the beginning of each test thesample platens were moved together at a rate of 167 lm s�1 until a pre load forceof 20 N was obtained.

2.4. Raman spectroscopy

Raman spectra were recorded using a Renishaw Ramascope spectrometer mod-el 2000, with a 488 nm argon laser. Analyses were performed by focussing the laserwith a �50 objective onto the sample surface through an Olympus BH2-UMA opti-cal microscope, giving a laser spot diameter of �4 lm. The laser power at the spec-imen surface was of the order of 4 mW. Prior to analysis, the spectrometer wascalibrated using a monocrystalline silicon standard specimen. Peak fitting of the re-corded spectra was performed using GRAMS32 software.

2.5. Microstructural analysis

Microstructural characterisation was carried out using a FEI FIB201 worksta-tion. The focused gallium ion beam diameter varied between 7–500 nm and thebeam current from 1 pA to 12 nA, at 30 keV primary ion energy. A platinum organo-metallic gas injector allowed ion-assisted deposition of platinum over selected re-gions of the sample where the ion beam removed the previously deposited goldlayer. This allowed coating of the insulating samples under the ion beam. Ion-in-duced secondary electrons were used to produce images.

3. Results

3.1. Long term exposure, 28–180 days

Samples 1–4 showed little change in compressive strength withtime, but a reduction corresponding to amount of sand added wasobserved. For each mix, repeated wetting and drying did not give anoticeable increase in strength between 28 and 180 days. This sug-gested strength increase occurred within the first 28 days (33cycles).

Fig. 4 shows the compressive strength of two Portland cement(PC) and two natural hydraulic lime (NHL2 and NHL5) mortars.No change in strength was observed for a PC mortar, however,the mortar containing CL90 and PC did indicate a small increase.Significant increases in strength were observed for NHL2 andNHL5 mortars, the largest being for NHL5.

The surface of the 1:1 NHL3.5 to Croxden sand mortar whichserved as a control (sample 1) after 28 days is shown in Fig. 5. Aporous calcium carbonate matrix was observed containing hy-drated calcium silicate crystals distributed throughout the matrixsome bridging the pores. Examination of an internal fracture sur-

0

2

4

6

8

10

1:6PC:sand:FebDH

1:2:9PC:CL90:sand

1:2 CastleNHL2:sand

1:2 CastleNHL5:sand

Com

pres

sive

stre

ss (M

Pa) 28 day (control) 28 day (cycled)

56 day (contol) 56 day (cycled)90 day (control) 90 day (cycled)180 day (control) 180 day (cycled)

Error bars +/- 1 standard deviation

Average stress cycledAverage stress control

Fig. 4. Compressive strengths between 28 and 180 days showing increase in mortarstrength attributed to wetting drying cycles on different mortar compositions.

face revealed similar features but in some regions groups of closelypacked hexagonal calcium hydroxide crystals were identified indi-cating incomplete carbonation. In the samples exposed to wettingand drying cycles a layer of crystals was observed covering the sur-face, which increased in size with the number of cycles. Ramanspectroscopy identified these crystals as calcium carbonate fromthe three bands located at 280, 711 and 1085 cm�1. These bandsidentify the main vibrational modes of the free CO2�

3 ion [10]. Atypical Raman spectrum illustrating calcium carbonate peaks inan NHL5 mortar is given in Fig. 6 and an ion-induced secondaryelectron image of the corresponding area is shown in Fig. 7. Inaddition, the band located at 519 cm�1 was also identified, whichwas attributed to silica from the component sand [11]. By contrastthe PC sample showed little change in structure following wettingand drying over a period of 180 days.

3.2. Short term exposure, 1–28 days

Fig. 8 shows the increase of compressive strength with time forNHL 2, NHL 5 and CL90 samples, respectively. Mix ratios of 1:1 and1:2 are given for both hydraulic limes NHL2 and NHL5, the CL90was only tested in the mix ratio 1:1 due to its relatively lowstrength. It should be noted that site practice usually employs‘leaner’ ratios however, the binder rich ratios used in this study

200 400 600 800 1000 1200 1400

Raman shift (cm-1)

Inte

nsity

(a.u

.)

CaCO3

CaCO3

CaCO3SiO2

Fig. 6. Typical Raman spectrum from the surface of a 1:1 NHL 5 mortar showing thepresence of calcium carbonate from the mortar and quartz from the sand grain.

Fig. 7. Ion-induced secondary electron micrograph from the surface of 1:1 NHL5:Sand mix after 90 days of cycling showing calcite crystals.

0

2

4

6

8

10

12

1:1 NHL2 1:2 NHL2 1:1 NHL5 1:2 NHL5 1:1 CL90

Com

pres

sive

stre

ss (M

Pa) 1 day (control) 1 day (cycled)

3 day (control) 3 day (cycled)7 day (control) 7 day (cycled)14 day (control) 14 day (cycled)28 day (control) 28 day (cycled)

Error bars +/- 1 standard deviation

Average stress cycledAverage stress control

Fig. 8. Enhanced development of compressive strength resulting from wetting anddrying of natural hydraulic and calcium lime mortars over a period of 28 days.

Fig. 9. Growth of di-calcium silicate based C–S–H phase within a pore of NHL2mortar subjected to wetting and drying for 28 days.

A. El-Turki et al. / Construction and Building Materials 24 (2010) 1392–1397 1395

increased the strength thereby improving measurement accuracy.The compressive strength of the control samples remained con-stant throughout the 28 days of testing. Those cycled showed aclear increase, the largest value for 1:1 mixes. The highest rate ofincrease in strength occurred during the first 14 days indicatingthat the effect of wetting and drying decreased with time. It hasbeen suggested that wetting and drying hydraulic lime mortarspromotes the hydration of silicate crystals within the structure.The 1:1 Castle NHL2:Croxden sand mortar illustrates the effectrather well in Fig. 9.

3.3. Exposure of aged sample

The variation in compressive strength of control and cycledsamples of mix 14 were found to be in the range of 2–2.5 MPa(N/mm2). These samples were aged for 13 months prior to wettingand drying and significantly very little change was noted. Previouswork on identical mix designs indicates that this ageing time is suf-ficient for the sample to have reached its ultimate compressivestrength suggesting a fully set material [12].

4. Discussion

The process of wetting and drying lime mortars can have a sig-nificant influence on durability and the compressive strength. The

magnitude of this effect is determined by the mix ratio, mix design,number of cycles and age of mortar. Much of the increase instrength was found to occur during the initial 28 days of exposureand was most marked for the lime mortars. Little effect wasobserved for the OPC samples. It is well documented that cementmortars have a reduced water permeability compared to lime mor-tars [13]. The observations derived from this study are consistentwith a mechanism for sample strength increase which relies onthe transport of water through the structure.

Hydraulic lime mortars harden by the combined effect of hydra-tion and carbonation. The hydration reaction usually occurs over anumber of days and involves the formation of a silicate phase. Forexample the hydration of di-calcium silicate, a common phase innatural hydraulic limes, is given by

2Ca2SiO4 þ 4H2O! Ca3Si2O7 � 3H2Oþ CaðOHÞ2 ð1Þ

Concurrently with hydration and over a period of weeks oryears, depending on porosity, carbonation of calcium hydroxidealso occurs. However the process of carbonation requires the disso-lution of carbon dioxide in water to form carbonic acid before reac-tion with calcium ions occurs, as shown by

CO2 þH2O! HCO�3 þHþ ð2ÞHCO�3 ! CO2�

3 þHþ ð3ÞCaðOHÞ2 ! Ca2þ þ 2OH� ð4ÞCa2þ þ CO2�

3 þ ! CaCO3 ð5Þ

4.1. Influence of wetting and drying cycles on hydration andcarbonation

Silicate phases were more readily identifiable in cycled samplescompared to the equivalent control batch. This suggested that thecycling regime encouraged hydration and consequently an in-crease in strength.

The observation of silicates within pores of the cycled samplessuggested that dissolution and re-precipitation of these phases oc-curred during each cycle. At the same time the optimum conditionsfor carbonation prevail. Here this is illustrated in Fig. 10a–c.Fig. 10a represents the pores in a dry mortar, diffusion of carbondioxide can occur but there will be insufficient water present topromote carbonation. During wetting the sample first absorbs

Fig. 10. Schematic representation of porosity in lime mortar cross section showing; (a) free flow of carbon dioxide into pores, (b) saturated pores completely filled with water,(c) pores coated in a thin layer of water. s Gas, Lime matrix, Water, ? Diffusion path of CO2.

Fig. 11. Completed repairs to Warwick Castle’s South Front retaining wall in newashlar masonry.

Fig. 12. Close up of new ashlar masonry showing preferential drying of stoneadjacent to lime mortar joint. Dark areas are wet, light areas are dry.

1396 A. El-Turki et al. / Construction and Building Materials 24 (2010) 1392–1397

water and then becomes saturated as shown in Fig. 10b. Waternow blocks the pores and the diffusion rate of carbon dioxide isapproximately 10,000 times slower than that in air, thus the rateof carbonation is low [7]. During the drying process the pore watergradually evaporates. At a critical point the pores will contain athin film of water and allow the diffusion of carbon dioxide. Thiscritical point also occurs during the initial process of wetting andabsorption. These are the optimum conditions for carbonation,Fig. 10c. It is noteworthy that a pumping effect of air through thesystem of pores may be caused by the movement of water throughthe saturated structure.

4.2. Influence of mix design on strength development

Variations in the rate of increase in compressive strength duringthe initial 28 days are determined by the mix design as well as theextent of water absorption and evaporation. The rapid increase incompressive strength during the first 7 days of cycling of theCL90 sample is due to carbonation. When considering the naturalhydraulic limes however the increase occurs between 7 and14 days. This delay would be consistent with a competing reaction,such as hydration, taking priority. The formation of silicates withinthe structure is also expected to limit the water permeabilitywhich may reduce the rate of carbonation. The results reportedin this paper provide evidence for the fact that the process of wet-ting and drying influences both reactions.

4.3. Influence of sample age

The influence of sample age on the effect of wetting and dryingis evident by considering sample mix 14 which was aged for13 months. The absence of any significant change in strength ofthe NHL3.5 hydraulic lime mortar suggests that sample age playsa very important role in determining the susceptibility of a sam-ples structure to changes from wetting and drying treatmentsand that this reduces with age. The strength increasing mecha-nisms are most prevalent in fresh mortars where unreacted phasesare present. The aged sample was expected to be fully hardenedand therefore would not have contained any unreacted silicatephases or calcium hydroxide.

5. Case study

The results presented demonstrate that the action of wettingand drying increases at lime mortar joints. Observations made onsite during masonry repairs to Warwick Castle’s South Front showwetting and drying taking place in a number of locations, Fig. 11shows the completed repairs to one area of the Warwickshire Tri-assic sandstone South Front retaining wall with new Grinshill Tri-assic sand stone ashlar masonry. Very close examination of themortar joints shows evidence of the strong capillary potential gen-

erated by the lime mortar, and indicates that water is absorbedinto the mortar joint and evaporates from it in preference to themasonry unit, Fig. 12. The figure illustrates this during a drying cy-cle where the edge of the stone has dried faster than the middle asa consequence of being adjacent to the mortar. This action wouldextend the period of evaporation and time the mortar is partly sat-urated thus enhancing carbonation, see Fig. 10c.

A similar process of extended drying will occur where masonryhas become saturated. During masonry repairs adjacent to a hop-per head, that had been in place for over 150 years and was extre-mely difficult to access, it was discovered blocked. The blockage isbelieved to have been present for a number of years and lead tocomplete saturation of the wall at this point. Green algae and li-

Fig. 13. Green algae and lichen growing on surface of mortar joint indicating watertransport to a greater extent through the lime mortar in preference to the masonryunits.

Fig. 14. Saturated masonry at lower level resulting from high water levels on thewest wall of Warwick Castle Mill following flooding of the Avon.

A. El-Turki et al. / Construction and Building Materials 24 (2010) 1392–1397 1397

chen were observed on the face of the saturated stone surface be-low the hopper head and adjacent to the downpipe.

Following clearance and repair of the downpipe, the masonrywas repointed with lime mortar. The vast scale of the South Frontis such that in this phase of work the scaffold was in place for oversix months following placement of the new mortar. During thecourse of this time the excess water remaining within the core ofthe wall evaporated slowly, Fig. 13.

If differences between the mortar and stone such as pH have anegligible effect on lichen growth its presence on the mortar joints,and absence on the face of the stone, where it had previouslygrown, may be attributed to greater water transport through thelime mortar in preference to the masonry units. Again this processwould be expected to promote carbonation of the lime mortarsince the transport of moisture would have occurred for a longerperiod to that for a similar repair to a dry wall.

Extended wetting and drying conditions often occur naturallynot only due to precipitation but also in structures associated with,or close to, water; such as mills, locks, canals and river embank-ments. Hard dense materials trap moisture, often where it can beharmful to adjacent building fabric. Porous materials, which in-

clude lime mortars, allow the moisture to evaporate. It is helpfulto know that the durability of lime mortar can be improved bymoisture movement in this way. Fig. 14 shows the result of highwater levels on the Warwickshire Triassic sand stone west wallof Warwick Castle Mill following flooding of the Avon. The wall be-neath the water level is made from Stancliffe Darley Dale sandstone. This happens to varying degrees on numerous occasions,but the compatible and permeable nature of the lime mortar andmasonry assists moisture movement and evaporation which re-sults in an even and beneficial drying out. It appears, therefore, thatlime mortars in buildings constructed in areas subject to intermit-tent damp conditions may benefit from this moisture movementand initially develop a greater durability.

6. Conclusions

The following conclusions can be drawn from the resultspresented:

(1) Wetting and drying increases the compressive strength ofnewly manufactured calcium (CL90) and natural hydrauliclime mortars of classifications 2, 3.5 and 5 by increasingthe rate of hydration and carbonation.

(2) An increase in compressive strength with wetting and dry-ing was not observed for (13 months) aged NHL3.5 limemortar.

(3) Wetting and drying had no effect on 1:6 Portland cement tosand mortar.

(4) The study suggested that wetting and drying can increasestrength by influencing the hydration and carbonationreactions.

(5) Wetting and drying promoted the growth of portlanditecrystals on the sample surface, where saturated solutionevaporates.

Acknowledgements

The authors would like to thank the EPSRC, STI programmegroup and DTI for supporting this work.

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