Effects of Metakaolin on Durability of Reinforced Mortars used to strengthen Masonry Walls

8/18/2019 Effects of Metakaolin on Durability of Reinforced Mortars used to strengthen Masonry Walls

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Effects of Metakaolin on Durability ofReinforced Mortars used to strengthen

Masonry Walls

J. M. F. MotaFederal University of Pernambuco

[email protected]

R. A. OliveiraCatholic University of Pernambuco

[email protected]

A. M. P. CarneiroFederal University of Pernambuco

[email protected]

ABSTRACTThe goal of this study was to discuss the contribution of pozzolan to the durability ofreinforced mortars with addition of metakaolin used to strengthen non-load bearing walls. It isknown that, from the 1960s, around 6,000 buildings have been erected with masonry sealinghaving structural function in the metropolitan region of Recife. They have high rates of

pathologies and even fatal accidents. This way, we sought to establish criteria forstrengthening with reinforced mortar within the matrix that meets three essential conditions:(i) support capacity; (ii) ductility; and (iii) durability. In this work, we draw conclusions ondurability, in which the experimental arrangement encompassed the specification of two

proportions [(1:1:6:1.5 and 1:0.5:4.5:1.5) - cement, lime, sand, and water/cement ratio], withtwo contents of addition (8 and 15%) and two ways of addition (replacing part of the cementand with no cement replacement). Tests of compressive strength, elasticity modulus, velocityof ultrasonic wave propagation, absorption of water by immersion, and accelerated agingmethods were conducted to assess the performance regarding carbonation and chlorides inmortar specimens. The results showed that metakaolin strongly reduces the effects ofdeleterious agents.

KEYWORDS: structural masonry; non-load bearing walls; reinforced mortar;metakaolin.

INTRODUCTION

Several "box-type buildings" built in the metropolitan region of Recife have masonry sealing

as structure. These buildings–about six thousand catalogued–have walls built with sealing blocks,mostly ceramic with horizontal holes. In a few cases, concrete blocks with average thickness of9.0 cm and compressive strength of 2.5 MPa were used. Knowing that slenderness for standardceiling height of 2.60 m is close to 30, it implies an additional reduction of the support capacity ofthe walls, considering it to be well above than acceptable in structural masonry, i.e., 20 (MOTA;ARAÚJO; OLIVEIRA, 2006).

Assessing the ratio between the number of accidents and the number of existing buildings, the probability of failure is estimated at approximately 1:500, i.e., well above the ratio socially

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acceptable when it involves the safety of human lives, which, at the most, is 1:10,000. However,in spite of total lack of control in the production of materials, it is worth noting that most buildings with these characteristics are still used and some are aged over 40 years.

In a doctoral research, as strengthening model was developed with reinforced mortar andaddition of metakaolin (MK) to meet the need of load bearing, ductility, and durability of wallssimilar to those of box-type buildings. However, the goal of this work was to assess the influenceof MK on the durability of reinforced mortars used for strengthening. It can be affirmed that thedurability of this strengthening derives from the packing condition of mortars, since thereinforcing bars placed inside carry out fundamental action in support and ductility, thus requiringgreater protection from deleterious agents.

Various scientific studies were developed in support of structural repairs concerning supportability; however, it should be noted that it is also important to assess the durability of thestrengthening used. It is also worth noting the contribution of MK to durability of mortars usedfor strengthening, mainly in the metropolitan region of Recife, where most of these buildings arelocated. Only about 25% of the households are connected to sewage systems; therefore, there is

contamination of the water table, action of chlorides in marine environments, and microclimate ofurban areas (sulfates, chlorides, CO2, and others).

The NBR 12653 (regulation of the Brazilian National Standards Organization) defines pozzolans as siliceous alumina-like materials that have little binding activity, but, when finelydivided and in the presence of water, react with calcium hydroxide at room temperature to formcompounds with binding properties. When pozzolans originate from high-purity kaolin, after thecalcination and grinding process of the particles, high-reactivity MK(HRM)is created NBR 13279(ABNT, 2005).

In the chemical formation of MK, when kaolinitic clays and kaolin are subjected to heattreatment, they have their atomic arrangement destroyed due to the removal of hydroxyl ions(Al2O3.2SiO2). This chemical process can be represented as follows:

[Al2Si2O5(OH)4]600 to 850 ºC [Al2O3.2SiO2] + [2H2O]

MK consists of continuous plain plates stacked in the perpendicular direction, and thethicknesses of the units are approximately 7.2 Ǻ. These units are held together by hydrogen bonds between the layers, so that the mineral is not dispersed in water. Their particles are very small,with maximum lateral dimensions ranging between 0.3 and 4.0 µm and thickness between 0.05and 2 µm (MURAT, 1983). In turn, Nita; John (2007) stated that the size of MK particles ranges between 0.2 and 15 µm, and the values of their specific area are greater than 12,000 m²/kg. HRM

has specific area with values up to 60,000 m²/kg and its particles have a lamellar structure.Figures 1 (a) and 1 (b) show the morphological aspects of MK particles.

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Figure 1: Micrograph: (a) metakaolin particle with 1,521 nm; (b) stacked metakaolin particles (NINA; JOHN, 2007).

Therefore, lamellar structures of MK have strong binding along the layers; however, the bindings between layers are soft, thus facilitating slippage between the layers.

The judicious use of MK and HRM increases the mechanical property of concrete sand other products that use Portland cement as a binder. The ideal dosage of these pozzolans, aiming atmaximum mechanical property, are between 6 and 15%with respect to the amount of cementmass; however, it can reach up to 50% in special cases, depending on the application and othermaterials used in the mixture (MALHORTA; MEHTA, 1996).

The service life of mortars and concretes depends on the structure of the pores, wheremoisture, oxygen, CO2, sulfates, and chlorides (deleterious agents) penetrate. When the matrix isexposed to these deleterious agents, its durability depends mainly on the permeability (MEHTA;MONTEIRO, 1994).

The adherence strength of mixed inorganic mortar with addition of pozzolan (silica)increased by as much as 45%after 180 days, which is the ideal content to replace cement by 20%silica fume for the mixture used, i.e., 1:1:6 (cement, lime, and sand) (TAHA; SHRIVE, 2001).

It was possible to observe high performance of mortars with addition of MK and replacementof part of the cement. The content of 15%had better results in protection against chloride ions andcarbonation, and greater compressive strength, elasticity modulus, and diametric tensile strength,always surpassing micro-silica. Regarding water absorption by immersion, no significantdifferences were found with respect to the samples without addition of MK (GALVÃO, 2004).However, addition of 5% Active Silica and Metakaolin in the level of 10% represented theadditions whose concretes showed the best results (ALMEIDA et al., 2015).

Mota et al., (2011) assessed mortars with MK in place of cement (4 samples) with proportions of 0 (reference), 5, 10, and 15%, and all samples with a mixture of 1:1:6:1.5 (cement,lime, sand and water/cement ratio). Their goal was to establish the best replacement content thatincreased the properties of the mortar. The authors concluded that the ideal content to replacecement by MK was 15%, since they observed the best mechanical results (compressive strength,diametric tensile strength, and adherence strength) and durability (total absorption andcapillarity)using this proportion.

Research with four different types of metakaolin and microsilica, in substitution of 15% ofthe cement mass in high performance mortar, showed the influence of fineness on compressive



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strength. It appears that at initial ages, mortars containing metakaolin with higher specific surface,exhibit resistance values higher than those obtained for the reference mortar with microsilica.However, at advanced ages the samples with microsilica and metakaolin, present similarresistance values (CURCIO; DE ANGELIS; PAGLIOLICO, 2003).

Concretes and mortars with addition of MK have their elasticity modulus increased by asmuch as15% due to the fact that pozzolanic reactions reduce the porosity, making the matrixdenser (LACERDA; HELENE, 2005). Therefore, the difficulty of mass transport in cementitiousmaterials (leaching, carbonation, corrosion) is the key to durability and, consequently, the servicelife. The size, volume, and pore continuity imply greater or lesser means of transport of materialsin the microstructure of cementitious materials, from which the performance derives.

Addition of pozzolans in materials whose matrices are cimentitious causes greater density ofthe mixtures, creating natural porosity reduction from the interface (due to wall effect) to thesurface Neville (1997), Mehta; Monteiro (1994), reported that pozzolanic additions–replacing part of the cement mass or addition with no cement replacement–increase the service life ofmortars and concretes, because the calcium hydroxide of the cement react with the silica added

forming products such as hydrated calcium silicates that fill the empty capillaries. These productsrefine the structure of the pores, and reduce the permeability of the system to agents like sulfates,chlorides, and alkali-aggregate reaction, thus leading to high mechanical strength and durability.

Souza (2003) studied mortars with addition of HRM (5, 10, 15, and 20%) and found that theamount of water influences the strength regardless of the content. The best results were close to0.25 water/cement and replacement of 20% of part of the cement, with compressive strengtharound 80 MPa. Therefore, greater water/cement ratio requires greater contents of addition inorder to obtain more significant strength.

Vu; Stroeven; Bui (2001) assessed MK from a quarry in northern Vietnam replacing 10 to30% of part of the cement in mortars and 10 to 20% in concretes. The results showed that mortarswith contents between 20 and 25% had greater strength results. It was also found that greater

water/binders ratio requires greater content of MK in order to obtain more strength. Neville (1997) reported that the benefits of MK for concretes are important due to the

significant reduction of the diffusion coefficient and penetration of chloride ions. Gruber et al.,(2001) studied concretes with 0.3 and 0.4 water/cement ratio, and 0, 8, and 12% HRM massreplaced by cement, in order to assess diffusion of chlorides. They concluded that 8 and12%HRM decreased the diffusion coefficient in 50 and 60%, respectively.

A study conducted by Courard et al., (2003) showed that mortars with addition of MK andreplacement of 10 and 15% of part of the cement exhibited better results regarding the diffusionof chlorides and sulfate degradation. However, the authors assessed water absorption byimmersion and did not find satisfactory results in samples with addition of MK, observing lesserabsorption in the mortar with no MK addition; however, the values were very similar to those of

mortars with MK additions.

MATERIALS AND METHODSMaterials

The materials used were cement code CP II Z – 32; hydrated lime (type I); HP ULTRAMetakaolin®, and medium sand. These binders are widely used in the region due to the proven



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performance. Table 1 shows the characteristics of the cement used and Table 2 shows thecharacteristics of the lime used.

Table 1: Characteristics of the cement used (Source: the manufacturer)

Determination (CP II Z – 32) Results

P h y s i c a l c h a r a c t e r i z a t i o n

Blaine specific surface (cm2/g) 3,640Specific mass (g/cm3) 3.04Apparent density (g/cm3) 1.20

FinenessWaste in the sieve #200 (%) 2.20Waste in the sieve #325 (%) 15.60

Setting time Beginning (min) 150End (min) 220

Compressive strength3 days (MPa) 26.407 days (MPa) 32.10

28 days (MPa) 39.5

C h e m i c a l c h a r a c t e r i z a t i o n ( % ) Potential composition of

clinker

C3S 67.00C2S 7.80C3A 7.80C4AF 10.50

Loss on ignition 4.39Insoluble waste 6.89Al2O3 5.20SiO2 20.60Fe2O3 3.50CaO 65.00

2.66/3.26 Na2O/K 2O 0.3/0.8Free CaO 1.44

Table 2: Characteristics of the lime used (Source: the manufacturer)Test Hydrated lime (calcitic - type 1)

P h y s i c a l

c h a r a c t e r i

z a t i o n

Fineness(% etained)

#30 (0.600 mm) .3# 200 (0.075 mm) 1.8

Apparent density (g/cm3) .56Moisture (%) 1.26

C h e m i c a l

c h a r a c t e r i z a t i o n ( % )

Carbonic anhydride – CO2 2.21

Sulfuric anhydride – SO3 .05Loss on ignition 24.15

Silica and insoluble waste .84 Non-hydrated oxide 7.3

CaO 73.72 Non-hydrated MgO .71

Non-volatile total oxides 98.1



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Tables 3 (a) and 3 (b) show the characteristics of metakaolin.

Table 3 (a): Chemical characteristics of HP Metakaolin® (%)SiO2 51.57

Al2O3 40.5

Fe2O3 2.8

CaO -

MgO -

SO3 -

Na2O .08

K 2O .18

Moisture .6

Loss on ignition 2.62

Alkaline equivalent .20

SiO2 + Al2O3 +Fe2O3 94.87

Odor/pH Odorless/5.0 to 6.5

Source: (HELENE; MEDEIROS, 2004)

Table 3 (b): Physical characteristics HP Metakaolin® Average diameter of the particles 12.4 µm

Specific mass density 2,650 kg/m3

Unitary mass 600 kg/m3

Specific surface area – BET method 327,000 cm2

/gResult de pozzolanic activity at 771.2 mg CaO/g sample

Physical state/Form Solid / dry powder

Storage Filter with efficient containment of

Source: (HELENE; MEDEIROS, 2004)

The fine aggregate was natural quartz sand. Table 4 illustrates some characteristics.

Table 4: Characteristics of natural sandMaximum characteristic diameter (mm) 2.4

Fineness modulus 2.52

Unitary mass (g/cm3) 1.48

Specific mass (g/cm³) 2.62

Swelling 1.23

Content of powdery material (%) 2.13

Moisture (%) 3.4

Organic matter Light



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Methods

Table 5 shows the mortars used in this study. The proportions were established according toASTM C 270 (1982) ["N": (1:1:6) and "S": (1:0.5:4.5). The contents of metakaolin were 8 and

15%, with addition and replacement of part of the cement and addition with no cementreplacement.

Table 5: Samples assessed in the durability testsMedium mixture – 1:1:6:1.5 Strong mixture – 1:0.5:4.5:1.5

0% Metakaolin 0% metakaolin

Replacement of 8% of part of thecement

Replacement of 8% of part of thecement

Replacement of 15% of part ofcement

Replacement of 15% of part ofthe cement

Addition of 8% Addition of 8%

Addition of 15% Addition of 15%

The water/cement ratio was set through several attempts with the purpose of establishing the best workability. A matrix of 200±20 mm consistency was created as ideal to be applied to themortar coatings studied. There was no need of surfactant additive in any sample.

Cylindrical mortar specimens of 5 x 10 cm NBR 13749 (ABNT, 2013) were molded in orderto test: (a) compressive strength NBR 13279 (ABNT, 2005); (b) dynamic elastic modulus NBR

15630 (ABNT, 2008); (c) rate of water absorption by immersion NBR 9778 (ABNT, 2009); (d)velocity of ultrasonic wave propagation NBR 8802 (ABNT, 2013) and accelerated aging methodsfor performance assessment; (e) carbonation (RILEM, 1998); and (f) soluble chlorides (ASTM C1152, 2012).

The tests of compressive strength, modulus of elasticity, and water absorption wereconducted after 90 days and the other tests after 300 days, using 12 replicas by samples forcompressive strength, six for total absorption and dynamic elastic modulus, three for carbonation,and three for chlorides.

Based on NBR 15630 (ABNT, 2008) and NBR 8802 (ABNT, 2013), it was possible tocalculate the dynamic elastic modulus through ultrasound test with the following equation:

= (1 + )(1 − 2)

1 − 2

where Ed = dynamic elastic modulus; v = velocity of wave propagation (mm/μs); ρ = density ofapparent mass (kg/m³); and ν = Poisson coefficient (this value was considered constant for thedifferent types of mortars and equal to 0.20).

The wave velocity points to internal voids, cracks, and failures of thickening, since thevelocity for well-packed concrete is greater than 2,500 m/s. However, the velocity of wave propagation due to steel in reinforced concrete is 6,000 m/s (HELENE; REPETTE, 1989).



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The specimens of mortar had the following conditions of exposure to carbonation: (a) timematuration (10 months); (b) previous condition, plus 24-hour exposure in carbonation chamber;and (c) previous condition, plus 72-hour exposure in the chamber. The readings were conductedshortly after sprinkling with phenolphthalein.

The specimens of mortars were subjected to attack by CO2 in the carbonation chamber withmoisture, temperature, and the following concentrations: CO2 (66±5) %;(25±3) °C, and (10±2) %,respectively. The procedure Rilem (1998) was carried out to determine the carbonation front.Carbonation depths were measured visually using a caliper, after spraying the phenolphthaleinsolution on the freshly cut surface with a disc and a suitable machine.

Figures 2 (a) and 2 (b) show, respectively, the cut in the specimens and the specimens afterthe withdrawal from the carbonation chamber and sprinkling with phenolphthalein solution. Thespecimens positioned at the top refer to the mixture 1:1:6, and at the bottom to the mixture1:0.5:4.5. Figures 3 (a) and 3 (b) show the carbonation chamber and one of the readings,respectively.

(a) (b)

Figure 2 (a): Cutting of the specimens; (b) - Specimens after withdrawal from thecarbonation chamber and sprinkling with phenolphthalein solution [condition (c)].

(a) (b)Figure 3 (a): Carbonation chamber; (b) - Reading of the specimens of strong mixture

with replacement of 8% of part of the cement [condition (b)].



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The chlorides are present in three forms in concretes and mortars: (i) chlorides chemicallylinked to tricalcium aluminate (Friedel's salt); (ii) adsorbed in the surface of the pores; and (iii)free chlorides. The latter are indeed deleterious, because they depassivate the reinforcing bars(CASCUDO 2008).

Mineral additions play a fundamental role in protection against action of chlorides inside the pores. Therefore, it can be affirmed that the determination of the contents of chlorides isfundamental, because they indicate the actual condition of their transport inside the mortar(NEVILLE, 1997).

The mortar samples were submitted to the chloride chamber for 192 hours. We used sodiumchloride (pure for analysis) and distilled water in the chamber. This period was estimated basedon the time specified for other materials (metals) by the manufacturer and to the highcement/water ratio and porosity of mortars compared to concretes. Moisture, temperature, andconcentration of chlorides (content with respect to distilled water) were around (95±4) %; (55±5)°C, and 5%, respectively. Pressure injection (1 bar) of compressed air was used to create the saltfog in the chamber.

After the withdrawal of the specimens, silver nitrate was used to verify the presence of freechlorides. A darkened color indicative of chlorides was found. Subsequently, 50 g of powder withapproximate depth between 1 and 2 cm were collected in each sample, since the reinforcing barsfor the structural strengthening of the walls are placed in this region. Figures 4 (a) and (b) showspecimens with silver nitrate and in the chloride chamber, respectively.

(a) (b)

Figure 4 (a): Specimens in the test after the chloride chamber with silver nitrate (mixture1:1:6); (b) - Chloride chamber working with the specimens.

Figure 4 (b) shows the opening of the chamber made just for the photograph during the test.The salt fog stands out in the environment shortly after opening the chamber. Finally, the contentof soluble chlorides was determined in a specific laboratory by potentiometric titration using a

selective electrode for chlorides, according to the ASTM C 1152 method.

RESULTS AND DISCUSSION

Table 6 shows the average values and standard deviation of compressive strength ofspecimens of mortar used for strengthening.



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Table 6: Compressive strength of mortar used for strengtheningCompressive strength – [average/SD - (MPa)]

Medium mixture – 1:1:6:1.5 Strong mixture – 1:.5:4.5:1.5

0 % metakaolin 7.53/.24 0 % Metakaolin 12.80/.43Replacement of 8%of part of the cement

7.31/.31Replacement of 8%of part of the cement

13.1/.42

Replacement of 15%of part of the cement


13.87/.45

Addition of 8% 8.97/.29 Addition of 8% 16.6/.50Addition of 15% 10.43/.34 Addition of 15% 17.57/.44

SD = Standard deviation

It can be observed that the sample with strong mixture and addition of 15% of metakaolinwith no cement replacement exhibited greater strength. On the other hand, the sample of medium

mixture with replacement of 8% of part of the cement by MK exhibited the lowest performance.Possibly, pozzolanic reactions are more susceptible and increase with cement content. Anotherimportant aspect is the supremacy of the sample with addition and no cement replacement withrespect to the sample with cement replacement in the two mixtures assessed. Table 7 shows theaverages and standard deviation of the elasticity modulus of the mortars used for strengthening.

Table 7:- Dynamic elasticity modulus of mortars used for strengtheningDynamic elasticity modulus [average/SD - (GPa)]

Medium mixture– 1:1:6:1.5 Strong mixture – 1:.5:4.5:1.5

0 % metakaolin 10.22/.44 0% Metakaolin 13.03/.48Replacement of 8%of part of the cement


12.75/.49

Replacement of 15%of part of the cement 9.91/.39 Replacement of 15%of part of the cement 11.99/.47

Addition of 8% 11.89/.41 Addition of 8% 14.52/.58Addition of 15% 12.58/.39 Addition of 15% 15.09/.43

SD = Standard deviation

Samples with addition and no cement replacement exhibited better results compared to thesamples without addition and with addition and replacement of part of the cement. The samplewith strong mixture and addition of 15% with no cement replacement exhibited the best resultamong the mortars assessed. This way, it can be concluded that the refinement of the pores wasrelevant in view of the greater packing of the mixture provided by MK.

Figure 5 shows the velocity of the linear ultrasonic wave propagation, in which its growth points to the compact condition of the material. This way, it is possible to observe the growthrelated to the enrichment of the mixture and the addition of 15% with no cement replacement.



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Figure 5: Velocity of ultrasonic wave in mortars for strengthening. RC = replacement of part of the cement; NCR = no cement replacement.

It is concluded that the mortar with strong mixture and addition of 15% of MK with nocement replacement can be classified as "good" by being within the range of 3,000 to 3,500 m/sWhitehurst (1996) and the sample with the best result. The other samples were classified as "fair" by being within the range of 2,000 to 3,000 m/s. Cylindrical specimens were submitted to waterabsorption test. The average results and standard deviation are shown in Table 8.

Table 8: Absorption of water by immersion of samples used for strengtheningAbsorption (%) – average/SD

Medium mixture – 1:1:6:1.5 Strong mixture – 1:.5:4.5:1.5

0% metakaolin 1046/.18 0% Metakaolin 9.86/.13Replacement of 8%of part of the cement


10.86/.16



11.07/.14

Addition of 8% 10.01/.14 Addition of 8% 9.67.13Addition of 15% 10.06/.13 Addition of 15% 9.58/.13

The results of absorption of water by immersion showed that there was no significant effectof samples with additions compared to the samples without addition. However, the samples withaddition of MK and no replacement of part of the cement exhibited the best performance withrespect to the samples with replacement of part of the cement by MK. Tables 9 (a) and 9 (b) showthe results of carbonation of the mortars for strengthening with medium and strong mixtures,respectively, in the three maturation conditions.

0

2000

4000

0%8% RC

15% RC8% NCR

15% NCR

25452551 2481 2711

2841

2840 28172747 2941

3086

L i n e a r p r o p a g a t i o n v e l o c i t y ( m / s

)

Samples: 0% (metakaolin); 8% (replacement of part of the cement by

metakaolin); 15% (replacement of part of the cement by metakaolin); 8%

(addition); 15% (addition)

Ultrasonic wave velocity

Medium mixture Strong mixture



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Table 9 (a): Carbonation of the mortars (medium mixture) in the three maturationconditions

Samples – medium

mixture (1:1:6:1.5)

Dry transverse

section

Condition

(a):

Natural 10-

month

exposure

Condition

(b): Prior +

24-hour

exposure in

the chamber

Condition (c):

Prior + 72-

hour exposure

in the

chamber

Carbonated thickness (mm)/SD

0% metakaolin15 mm, 30 mm, 45 mm

– from top5.48 7.00 14.00/1.08


15 mm, 30 mm, 45 mm – from top

6.41 7.12 14.30/1.10

Replacement of 15%

of part of the cement

15 mm, 30 mm, 45 mm

– from top 8.45 9.53 14.20/1.10Addition of 8%

15 mm, 30 mm, 45 mm – from top

1.83 4.04 10.53/.80

Addition of 15%15 mm, 30 mm, 45 mm

– from top1.6 3.89 9.82/.75

It can be observed that the samples with addition and no replacement of part of the cement stoodout in the cases of mortars with medium mixture.

Table 9 (b): Carbonation of the mortars (strong mixture) in the three maturationconditions

Samples - strong

mixture

(1:.5:4.5:1.5)

Dry transverse

section

Condition (a):Natural 10-month

exposure

Condition (b): Prior+ 24-hour exposure

in the chamber

Condition (c):Prior + 72-hour

exposure in the

chamber

Carbonated thickness (mm)/SD

0% metakaolin15 mm, 30 mm, 45

mm – from top2.7 4.08 9.49/.74

Replacement of 8%of part of the

cement

15 mm, 30 mm, 45mm – from top

4.85 5.54 9.64/.74

Replacement of15% of part of the

cement

15 mm, 30 mm, 45

mm – from top7.56 8.17 13.40/ 1.0

Addition of 8%15 mm, 30 mm, 45

mm – from top4.45 4.94 9.08/.70

Addition of 15%15 mm, 30 mm, 45

mm – from top1.26 3.04 7.55/.50

The sample with strong mixture and addition of 15%of MK exhibited better results in thethree maturations conditions. However, samples with replacement of part of the cement and



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samples with addition of 8% and no cement replacement exhibited results of carbonation frontthickness similar to those of the samples without addition, showing that, for certain contents,mineral additions do not reduce carbonation.

The great importance (performance) of cement should not be disregarded. Replacing part of thecement by pozzolan as may increase mechanical properties, but not necessarily those relating todurability, even knowing that there is a close relationship between them.

In turn, the samples with addition of metakaolin and no cement replacement of both mixturesexhibited the best performance with respect to carbonation. This fact results from the chemicaland mineralogical aspects of the addition, the pozzolanic activity, and fineness, i.e., the potentialto change the cement paste physically and chemically.

Regarding the proposed contents of addition with no cement replacement, the consumptioncondition of alkaline "reserve" by the silica of the pozzolan did not stand out with respect to the benefits of these additions to the cementitious matrix. However, carbonation increases when pozzolan replaces part of the cement due to alkalinity reduction.

Therefore, the samples with the highest degree of maturation(c) were classified as having"medium" carbonation condition. On the other hand, regarding the intermediate maturitycondition (b), all samples were classified as having low carbonation condition. With respect tocarbonation depth, Medeiros (2002) suggests the following limits: low15 mm. Samples with replacement of part of the cement had no significant performance in combating carbonation.

Table 10 shows the test results of soluble chlorides (ASTM C 270, 1982).

Table 10: Results of chloride contents Cl- / SDMedium mixture – 1:1:6:1.5 Strong mixture – 1:.5:4.5:1.5

0 % metakaolin 0,5178 / 0,039 0 % Metakaolin 0,3950 / 0,0311

Replacement of 8% of part of the cement

0,4763 / 0,047 Replacement of 8% of part of the cement

0,5202 / 0,0466


0,6064 / 0,0366Replacement of 15%of part of the cement

0,5956 / 0,0410

Addition of 8% 0,2971 / 0,0121 Addition of 8% 0,2616 / 0,0116Addition of 15% 0,2520 / 0,0225 Addition of 15% 0,2171 / 0,0201

It can be observed that samples of strong mixture with addition of 15%of metakaolin and nocement replacement exhibited the best performance. In contrast, the samples with replacement of part of the cement exhibited the worst results (higher concentration of free chlorides).

Finally, in all cases assessed concerning durability (carbonation, chlorides, and velocity ofultrasonic wave propagation), it was found that the mortar with strong mixture and addition of15%with no cement replacement exhibited the best condition to reduce the effects of deleteriousagents. That conclusion is in line with the mechanical results.



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CONCLUSIONS

All tests related to durability (carbonation, chlorides, velocity of ultrasonic wave propagation,and absorption of water) demonstrated that the mortars with addition of metakaolin exhibited the

best condition to reduce the effects of deleterious agents.

The results are in line with those found by various authors due to the verification of the benefits promoted by the addition of pozzolan in mortars and concretes, in view of the increase inmechanical strength, mainly due to the refinement of pores. The chemical attacks (sulfatedwaters, alkali-aggregate expansion, acid waters, and waters with organic or inorganiccontamination) are mitigated and the addition also promotes good performance with respect to theaction of chlorides and attack of carbon dioxide.

It was found that the sample with "strong" mixture and addition of 15% of MK with nocement replacement stood out with respect to the other samples in all tests. This fact allows thefollowing assumptions:

(a) greater compressive strength and modulus of elasticity, indicative of lower porosity;(b) greater velocity of ultrasonic wave, classified as "good" according to the reading and

indicative of satisfactory sample compactness;

(c) less carbonated thickness, classified as "medium" according to the reading, which is aninteresting condition of protection in urban centers;

(d) lower content of soluble chlorides in the thickness of 1 to 2 cm of the surface of thespecimens, which is important in regions close to marine areas.

The test for absorption of water by immersion did not show significant difference betweenthe reference samples and those with addition with no cement replacement. The samples withaddition and replacement of part of the cement exhibited more unfavorable results.

It should be noted that, concerning durability, the samples with replacement of part of thecement by MK always showed lower results compared to samples with addition and no cementreplacement and, in most cases, with lower performance than the samples without addition, thuscharacterizing the importance of cement.

REFERENCES

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Effects of Metakaolin on Durability of Reinforced Mortars used to strengthen Masonry Walls

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