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Effect of Portuguese metakaolin on hydraulic lime concreteusing different curing conditions

Paulo Cachim a,*, Ana Luísa Velosa b, Fernando Rocha c

a DECivil and LABEST, Universidade de Aveiro, Portugalb DECivil and Geobiotec, Universidade de Aveiro, Portugalc Departmento de Geociências and Geobiotec, Universidade de Aveiro, Portugal

a r t i c l e i n f o

Article history:Received 15 May 2009Received in revised form 10 July 2009Accepted 5 August 2009Available online 8 September 2009

Keywords:Hydraulic limeConcreteMetakaolinCuring conditions

a b s t r a c t

Hydraulic lime concrete with the addition of metakaolin in various percentages and cured under differentconditions was studied in order to evaluate the effect of the pozzolanic additions over time (7, 28, 90 and360 days). This study was performed using NHL 5, a metakaolin of Portuguese origin, siliceous sand andcalcareous coarse aggregate, with increasing percentages of substitution of hydraulic lime by metakaolinin weight. In order to evaluate the pozzolanic effect, compressive strength test were performed and ana-lysed by relative strength–time plots and by the application of different indexes such as the index of spe-cific strength and the pozzolanic index. Thermogravimetric analysis was performed at the age of 28 daysin order to evaluate the phases present in the concrete specimens. As main conclusions metakaolin is anadequate material for application in concrete with hydraulic lime binder as a pozzolanic addition provid-ing an increase in mechanical strength for the studied substitutions of 20% and 30%. Testing should beperformed at 28 days when portlandite consumption is complete.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Although currently concrete is made with the use of Portlandcement as a binder, it is known that in ancient times the bindersused in concrete were air lime and pozzolans. Taking into ac-count this possibility and the fact that natural hydraulic limeand metakaolin are produced in Portugal, a concrete with thesetwo products was designed and tested for its mechanical proper-ties. Although it is expected that this concrete will have a lowerperformance and therefore limits to its use, it may be advanta-geous in the construction business due to lower CO2 emissionsand energy consumption in the production process. Sulphateattack caused by exposure to sulphate solutions is a commondegradation factor in mortars and concrete. In the case ofhydraulic lime concrete the absence of sulphates in the binderdoes not contribute to delayed ettringite formation in humidenvironments.

The use of metakaolin as a pozzolanic addition is growing andits influence on concrete with Portland cement as the main binderis well known, both in terms of mechanical strength and durability[1,2]. The reaction products of cement/metakaolin concrete ormortar have also been prone to several studies and it is known that

apart from CSH there is a formation of calcium aluminate hydratesand also calcium silicate aluminate hydrates [3,4]. The reaction ofair lime and metakaolin has also been the object of study [5,6].However the use of pozzolanic additions with a hydraulic lime bin-der is complex as carbonation reaction is relevant and hydraulicreaction is slower than in ordinary Portland cement, due to thepredominance of dicalcium silicate and very low contents of trical-cium silicate.

This study aims at identifying the mechanical evolution of con-crete with natural hydraulic lime and the influence of metakaolinin its performance at various ages. It was executed using variouscuring conditions in order to evaluate their effect on the mechan-ical properties of hydraulic lime concrete.

2. Experimental

2.1. Composition and curing

Composition of hydraulic lime concrete was determined using a water/binderratio of 0.45 with aggregates in a saturated condition. Aggregates and cement rela-tive proportions were studied using a modified Faury method that takes into ac-count the differences between cement (that served as basis of the method) andhydraulic lime. Three different compositions were studied. Mixture M0 is the refer-ence mixture and uses only hydraulic lime as binder. Mixtures M2 and M3 had apartial replacement of hydraulic lime by metakaolin. This replacement, by weight,was 20% for composition M2 and 30% for composition M3. Grading of aggregates ispresented in Fig. 1 that also shows the resulting final grading curve. The final mixproportions for all types of studied concrete are shown in Table 2 including hydrau-lic lime percentage in the binder (p).

0950-0618/$ - see front matter � 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.conbuildmat.2009.08.010

* Corresponding author. Tel.: +351 919397727.E-mail addresses: pcachim@ua.pt (P. Cachim), avelosa@ua.pt (A.L. Velosa).

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The developed experimental program was designed in order to incorporate theassessment of the effect of different curing conditions on the properties of hydrauliclime concrete with metakaolin. Three different curing conditions at atmosphericpressure were used where the relative humidity, RH, of the environment was chan-ged. Concrete specimens were cured immersed in water, at 95% RH and at 65% RH.The temperature was kept constant at 20 �C in all cases. Specimens were preservedin the same curing conditions until the date of testing (up to 1 year).

2.2. Properties in fresh and hardened state

Fresh properties of concrete such as density and workability (measured usingthe slump test according to NP EN 12350-2:2002 standard) were measured. Inthe hardened state, the compressive strength (tests on 15 cm cubes) were per-formed at 7, 28, 90 and 360 days for original specimens.

In order to evaluate the formation of different phases with metakaolin contentof these concrete specimens, Differential Thermal Analysis was performed at theage of 28 days, using a Netzch STA 409 �C and a temperature range between20 �C and 1000 �C, with a heating rate of 10 �C/min.

2.3. Materials

All the materials used in this study are commonly available in Portugal,although their use is not widespread due to the preferred use of Portland cementas a binder and fly ash that is still abundant as a pozzolan. Binders were character-ised in terms of their chemical composition, taking into account major elements, byX-ray Fluorescence using a Philips PW 1400 X-ray Fluorescence Spectrometer andin terms of their mineralogical composition by X-ray Diffraction using a PhilipsX-Pert Pro X-ray Diffractometer. Aggregates were characterised in terms of theirparticle size distribution following standard EN 12620:2002, Aggregates forconcrete.

2.3.1. Hydraulic limeThe binder used was hydraulic lime NHL 5 that is currently the only hydraulic

lime produced in Portugal. Additionally, its compressive strength of 5 MPa at theage of 28 days is sufficient to be improved by the addition of a pozzolan, allowingthe use of this type of concrete in low demanding structural applications such asurban equipment, cycling roads, blocks, among others. The predominant crystallinephase present in its composition is portlandite and other phases present are dical-cium silicate, tricalcium silicate, calcite and quartz. Chemical composition of Portu-guese NHL 5 is shown in Table 1.

2.3.2. MetakaolinMetakaolin is an artificial pozzolan obtained by the calcination of kaolinitic

clays over a specific temperature range.

It is a very fine material, with a high specific surface that enhances pozzolanicreactivity. Pozzolanic reactivity was measured following ‘‘NP EN 196–5: methods oftesting cement. Pozzolanicity test for pozzolanic cement”, that is a testing proce-dure intended for application in pozzolanic cements, measuring Ca(OH)2 consump-tion in a solution with a standard quantity of cement and pozzolan. However, andtaking into account the various available methods for measurement of pozzolanicreactivity, it has proven to be the most adequate for this purpose when dealing withpozzolanic materials. When applied to metakaolin, this testing procedure classifiedthe material as an active pozzolan. XRD results revealed the presence of quartz andkaolinitic minerals whilst in terms of chemical composition, shown in Table 1, thereis a predominance of silica and alumina.

2.3.3. AggregatesAggregates used in this study were natural siliceous sand and calcareous coarse

aggregate. The sand grading curve is in the range of 0–4 mm. Coarse aggregateswere divided into two groups; one in the range 5–10 mm (CA1) and the other inthe range 10–25 mm (CA2). Grading curves of sand and coarse aggregate are plottedin Fig. 1.

2.4. Assessment of the effect of metakaolin

Some strength indexes were used to assess the effect of metakaolin. Pozzolanicstrength indexes as defined in [7] for cement concrete and previously used forhydraulic lime concrete [8] were used to assess the pozzolanic effect of metakaolin,while k-value efficiency factor [9] was calculated from compressive strength tomeasure the effect of metakaolin addition on the strength of concrete.

The specific strength ratio, R, is defined [7] as the contribution to concretestrength from unit binder and unit mineral admixture and is defined by

R ¼ fc=p ð1Þ

where fc is the concrete compressive strength and p is the hydraulic lime or mineraladmixture percentage of the cementitious materials (see Table 2). RHL expresses thecontribution of unit hydraulic lime to concrete strength without any mineral admix-ture, RM expresses the contribution of unit hydraulic lime, when metakaolin is used,to concrete strength, and RP is the contribution of the pozzolanic effect of metakaolinto concrete strength due to metakaolin, expressed by the equation:

RP ¼ RM � RHL ð2Þ

The index of specific strength, K, is the ratio of RM to RHL, and the contribution ofpozzolanic effect to strength, P, can be assessed by the percentage value of the con-tribution of pozzolanic effect to concrete strength, which can be written as:

P ¼ 100ðRP=RMÞ ð3Þ

The pozzolanic index, P, relates to the favourable contribution of the pozzolanrelatively to the usual binder.

The estimation of the k-values [9], follows the procedure described next. Thecompressive strength of a Portland cement concrete and generalized here forhydraulic lime concrete, can be estimated by the following empirical equation:

fc ¼ S1

W=HL� a

� �ð4Þ

where fc is the compressive strength (MPa), W is the water content in the initial con-crete mix (kg/m3), HL is the hydraulic lime content in concrete (kg/m3), S is a param-eter depending on the binder type (MPa) and a is a parameter depending mainly ontime and curing. In the case of metakaolin concrete, expression Eq. (4) can be gener-alized and the following expression for compressive strength can be used, which in-volves the concept of k-value:

Fig. 1. Aggregate grading curves (sand, CA1, CA2) and final grading curve.

Table 2Mixture composition including hydraulic lime percentage in the binder (p).

Name Constituents (kg/m3) p (%)

CA1 CA2 Sand NHL 5 MK Water

M0 619 455 321 550 – 247.5 100M2 619 455 321 440 110 247.5 80M3 619 455 321 385 165 247.5 70

Table 1Chemical composition of metakaolin and hydraulic lime (percentage in weight).

Oxides CaO SiO2 Al2O3 Fe2O3 K2O MgO Na2O TiO2 Na2O LOI

MK 0.04 59.90 32.29 1.28 2.83 0.17 0.24 0.36 2.83 2.80NHL 5 43.13 22.45 7.07 2.82 1.67 2.13 0.59 0.38 0.59 18.6

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fc ¼ S1

W=ðHLþ kMÞ � a� �

ð5Þ

where M is the metakaolin content in the concrete (kg/m3) and k is the k-value. Thus,the k-value measures the relative effect of metakaolin when compared to that ofhydraulic lime.

3. Results and discussion

3.1. Fresh concrete

Slump results gave results of 1.5 cm, 0.8 cm and 1.6 cm, respec-tively, for series M0, M2 and M3 (class S1, according to NP EN 206–1:2007), that indicates a rather small workability of concrete. Therather small workability was not considered a very important issueat this stage because the aim of this study was the investigation ofthe effect of metakaolin and of curing conditions. However, this is avery important aspect that must be accounted for in the subse-quent development of the study that may be solved by the additionof a plasticizer.

Density of fresh concrete was 2240, 2220 and 2160 kg/m3,respectively, for series M0, M2 and M3.

3.2. Hardened concrete

3.2.1. StrengthThere are three basic factors that influence the contribution of

metakaolin to concrete strength when it partially replaces hydrau-lic lime in concrete: (i) the filler effect; (ii) the acceleration of

hydraulic lime hydration and (iii) the pozzolanic reaction of MKwith CH [10].

The results obtained for compressive strength at 7, 28, 90 and360 days are shown in Table 3. It can be observed that the additionof metakaolin has a beneficial effect on strength at 28 days and be-yond for all types of curing conditions. Only after 7 days of curingand low air relative humidity (RH = 65%), the influence of metaka-olin is not beneficial for strength.

The results of compression tests show that up to 28 days,the specimens immersed in water have smaller strength thanthose cured at 65% and 95% of relative humidity. In the longterm the differences between curing conditions progressivelydisappear.

Relative strength is defined by the ratio of the strength of a spe-cific concrete to the strength of the control concrete at each partic-ular curing time. The control concrete considered for allcompositions was mixture M0 at 95% RH curing conditions. Rela-tive strength for mixtures M0, M2 and M3 and for all curing condi-tions is plotted in Fig. 2. It can be seen that up to 28 days of curing,concrete with metakaolin cured under water always has lower rel-ative strength than concrete cured at 95% RH. With time, at ages 90and 360 days, this trend is reversed. For the case of concrete with-out metakaolin, relative strength of specimens cured under wateris always less than strength of specimens cured at 95% RH. It is alsointeresting to note that for hydraulic lime concrete withoutmetakaolin, curing conditions at 7 and 28 days severely affect thefurther development of strength.

The influence of metakaolin in hydraulic lime concrete onstrength of concrete increases until 28 days and decreases after-

Table 3Compressive strength, standard deviation of compressive strength, pozzolanic indexes Rm, Rp, K, P and efficiency factor parameters a and k.

Days of curing Mixture fc (MPa) Standard deviation (MPa) Rm Rp K P a k

7 M0-65 6.6 0.16 0.066 0.0 1.0 0.0 0.902 0.0M0-95 5.0 0.24 0.050 0.0 1.0 0.0 1.222 0.0M0-W 3.3 0.15 0.033 0.0 1.0 0.0 1.566 0.0M2-65 6.5 0.15 0.082 0.0161 1.24 19.7 0.902 0.988M2-95 7.2 0.15 0.090 0.0399 1.80 44.5 1.222 1.994M2-W 6.1 0.21 0.076 0.0432 2.33 57.0 1.566 2.271M3-65 6.8 0.10 0.097 0.0315 1.48 32.5 0.902 1.072M3-95 6.8 0.21 0.097 0.0470 1.95 48.6 1.222 1.542M3-W 6.0 0.12 0.085 0.0526 2.61 61.8 1.566 1.817

28 M0-65 8.9 0.21 0.089 0.0 1.0 0.0 0.438 0.0M0-95 8.3 0.57 0.083 0.0 1.0 0.0 0.558 0.0M0-W 5.6 0.21 0.056 0.0 1.0 0.0 1.089 0.0M2-65 15.7 0.47 0.196 0.1072 2.21 54.7 0.438 4.078M2-95 17.5 0.21 0.218 0.1356 2.64 62.1 0.558 5.163M2-W 16.8 0.81 0.210 0.1533 3.72 73.1 1.089 6.042M3-65 13.4 0.49 0.191 0.1023 2.15 53.6 0.438 2.358M3-95 12.8 0.68 0.182 0.0997 2.21 54.7 0.558 2.357M3-W 12.8 0.20 0.183 0.1266 3.25 69.2 1.089 3.164

90 M0-65 10.2 0.81 0.102 0.0 1.0 0.0 0.170 0.0M0-95 11.1 0.27 0.111 0.0 1.0 0.0 �0.004 0.0M0-W 10.5 0.01 0.105 0.0 1.0 0.0 0.104 0.0M2-65 16.5 0.21 0.206 0.1038 2.02 50.4 0.170 3.837M2-95 19.3 0.50 0.241 0.1306 2.18 54.1 �0.004 4.727M2-W 20.0 1.28 0.250 0.1447 2.37 57.9 0.104 5.287M3-65 14.5 0.38 0.207 0.1047 2.03 50.6 0.170 2.288M3-95 15.1 0.40 0.216 0.1050 1.95 48.7 �0.004 2.217M3-W 15.7 0.31 0.224 0.1185 2.13 53.0 0.104 2.550

360 M0-65 11.8 0.72 0.118 0.0 1.0 0.0 �0.152 0.0M0-95 10.1 0.37 0.101 0.0 1.0 0.0 0.190 0.0M0-W 9.6 0.28 0.096 0.0 1.0 0.0 0.291 0.0M2-65 12.2 0.64 0.153 0.0345 1.29 22.6 �0.152 1.121M2-95 13.3 0.66 0.166 0.0653 1.65 39.2 0.190 2.449M2-W 14.5 0.83 0.181 0.0853 1.89 47.0 0.291 3.218M3-65 12.2 0.53 0.174 0.0563 1.48 32.3 �0.152 1.121M3-95 11.9 0.29 0.170 0.0690 1.68 40.6 0.190 1.543M3-W 12.0 0.21 0.171 0.754 1.79 44.0 0.291 1.724

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wards. This is in agreement with the results of [10,11] for ordinaryPortland cement concrete and mortars where the influence ofmetakaolin for strength was found to diminish beyond 14 days.Another conclusion is that when metakaolin content increasesfrom 20% to 30% there is a reduction of the beneficial effect ofthe addition of metakaolin. These results were also in agreementwith [10,11]. Furthermore, the acceleration of hydration reactionmay be linked with the peak verified in Fig. 2.

Relative strength–time plots allow for an overview of reactionrates in a complex system that involves three different chemicalreactions developing at the same time (carbonation, hydrationand pozzolanic reaction).

3.2.2. Pozzolanic indexPozzolanic indexes Rm, Rp, K and P are shown in Table 3, for

each mixture, at ages 7, 28, 90 and 360 days.

The contribution of unit hydraulic lime, when metakaolin isused, to concrete strength, measured by index K, is plotted in Fig. 3.

It can be seen from Fig. 3 that the use metakaolin always pro-duces a positive effect in terms of strength improvement. This isparticularly evident at age 28 days and decreases with time. Theinfluence of the addition of 30% metakaolin does not produce sucha significant initial impact as the addition of 20% metakaolin, but inthe long term the influence of both percentages of pozzolanicmaterial addition is similar. Curing in water is always beneficialin terms of K factor.

The contribution of pozzolanic effect to concrete strength mea-sured by P is plotted in Fig. 4. The values of P are maximum at28 days and progressively decrease until 1 year of curing. Exceptfor the case of mixture M3 at 90 days (and partially at 28 days),curing at saturated environment increases the pozzolanic effectof metakaolin.

Fig. 2. Relative strength at various concrete admixtures with curing time.

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Fig. 4. Pozzolanic index P at various ages and under different curing conditions.

Fig. 3. Pozzolanic index K at various ages and under different curing conditions.

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Fig. 5. k-factor at various ages and under different curing conditions.

Fig. 6. TGA of hydraulic lime concrete specimen with 20% metakaolin (M2-95) at age 28 days.

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3.2.3. k factorFor the hydraulic lime used in this work, the S was calculated as

4.970 MPa. Using the values of the compressive strength of thecontrol sample (Table 3) and considering as absolute reference cur-ing conditions at RH = 95%, parameter a was estimated for 2, 7, 28,and 90 days for each curing condition and given in Table 3. The re-sults of the estimated k-value are plotted in Fig. 5.

Similarly to what happens to relative strength, there is a pro-nounced increase of k-value from 7 to 28 days and then a progres-sively decrease measure up to 1 year for all curing conditions. Itcan be seen that saturated conditions (95% RH and immersion inwater) lead to improved efficiency of pozzolanic effect both at20% and 30% of hydraulic lime replacement.

3.2.4. Evaluation of the pozzolanic reaction by thermal analysisThe application of thermal analysis on samples taken from the

various specimens was performed in order to evaluate the phasespresent in the concrete and the effect of the addition of metakaolin.Generally an initial weight loss is observed in all specimens due tothe loss of adsorbed water until a temperature surrounding 200 �Cis attained. Between 200 �C and 700 �C a continuous mass loss isobserved which may be attributed to the dehydroxilation of sili-cate and aluminate hydrates. All samples present a substantialmass decrease in the temperature range of 700–900 �C due to thedecarbonation of calcite. This particular mass decrease is an indica-tor of the carbonation degree of various materials. At the age of28 days it was verified that no specimens presented mass lossesthat could be linked with the presence of portlandite, indicatingthe total consumption of portlandite in the system, either by car-bonation or pozzolanic reaction. Figs. 6 and 7 depict the changesin calcite quantity in specimens cured at 95% RH and containing20% and 30% of metakaolin, respectively (Specimens M2-95 andM3-95). Calcite, calcium silicates from the hydration reaction andcalcium aluminates from the pozzolanic reaction all contribute to-

wards mechanical strength gain that is also influenced by level ofcompactness of the concrete specimen. It is verified that there isno direct link between the amount of calcite present in the samplesand the attained compressive strength.

4. Conclusions

This study aims at improving the knowledge on the effect of theuse of metakaolin as a binder in partial substitution of hydrauliclime and the influence of curing conditions on hydraulic lime con-crete. The following conclusions can be drawn:

1. Metakaolin is a suitable material for application in concretewith hydraulic lime binder as a pozzolanic addition. A substi-tution of hydraulic lime by 20% and 30% of metakaolin pro-duced increased compressive strength.

2. Substitution around 20% of hydraulic lime by metakaolinseemed to give the best results.

3. As results indicate, curing conditions produce changes inmechanical strength due to the development of chemical reac-tions in time.

4. Curing conditions also seemed to affect the effect of metakao-lin in hydraulic lime concrete that is improved when speci-mens are cured under water.

5. Testing at early ages (7 days) does not allow attaining knowl-edge on future behaviour of limecrete, contrarily to currentcement concrete. The age of 28 days, with the complete con-sumption of portlandite seems to be the adequate testing age.

6. The simultaneous use and combined analyses of differentindexes together with mechanical strength results show thereal contribution of the pozzolanic material towards themechanical properties of hydraulic lime concrete, allowing abetter understanding of this phenomenon.

Fig. 7. TGA of hydraulic lime concrete specimen with 30% metakaolin (M3-95) at age 28 days.

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7. All three chemical reactions (carbonation, hydration and poz-zolanic reaction) contribute towards strength development ina complex system.

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