Materials andStructures I Matériaux et Constructions, Voi. 36, December 2003, pp 702-708

Environmentally-friendly mortars: a way to improve bond between mortar and brick

G. Monconi1, V. Corinaldesi1 and R. Antonucci2

(1 ) Department of Materials and Environment Engineering and Physics, University of Ancona, Ancona, Italy (2) Institute of Structural Engineering, University of Ancona, Ancona, Italy

ABSTRACT

In order to find new application fields for either fine materials coming from building demolition or industriai by-products, some mortars, in which fine recycled materials, obtained from a plant where rubble from building demolition are ground, are substituted to naturai sand, were tested.

Moreover, mortars containing either fly ash or ground brick powder as partial cement replacement were studied.

Based on characterization results and performance evaluations, recycled-aggregate mortar seems to be superior in terms of mortar-brick bond strength, mainly because of its rheological properties.

In addition, the use of fine recycled aggregate instead of naturai sand is in accordance with the sustainable development concept, with recycling and reuse of building rubble playing a key role in meeting the need to complete the building life cycle.

RÉSUMÉ

En vue de trouver de nouveaux domaines d'application pour les déchets de demolition ou pour quelques sous-produits industriels, des mortiers ont étè produits. C'est dans ceux-ci que, par rapport à un mortier traditionnel à base de ciment, on a étudié le remplacement du sable naturel par la fraction fine recyclée obtenue d'une installation de recyclage, dans laquelle les déchets de demolition sont concassés.

En outre, on a étudié des mortiers contenant soit des cendres volantes, soit de la poudre de briques concassées au lieu du ciment. En les comparant selon les résultats de la caractérisation et selon l'évaluation des performances, les mortiers avec granulats recyclés semblent les meilleurs aux termes de la tension d'adhérence entre le mortier et la brìque, due à ses propriétés rhéologiques.

De plus, l'usage de la fraction fine des granulats recyclés au lieu du sable naturel correspond à la notion du développement soutenable et le recyclage et le réemploi des déchets de demolition jouent un róle clé dans la fermeture du cycle de vie des bàtiments.

1. INTRODUCTION

Because of increasing waste production and public concerns about the environment, it is desirable to recycle materials from building demolition [ 1 ] . I f suitably selected, ground, cleaned and sieved in appropriate industriai crushing plants, these materials can be profitably used in concrete [2].

Nevertheless, the presence of masonry in concrete rubble is particularly detrimental to the mechanical performance and durability of recycled-aggregate concrete, and the same negative effect is detectable when naturai sand is replaced by fine recycled aggregate fraction [3-4]. These strength losses can be counteracted by adopting appropriate measures, such as

Editorial Note Prof. Giacomo Monconi is a RILEM Senior Member.

the reduction of water to cement ratio and the addition of minerai admixtures [5], as well as proper adjustment of the mixture proportions design in order to take into account actual physical properties of recycled aggregate. However, ali these actions lead to reduced use of fine recycled material, which by this way turns out to be only partially employable in concrete.

An alternative use of both masonry rubble and surplus fine recycled material could be in mortars. These could contain either recycled instead of naturai sand or powder obtained by brick grinding as partial cement substitution.

Actually, the aim of this work was to produce mortars incorporating, instead of cement and naturai sand, either industriai by-products or fine materials coming from building

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Materials and Structures I Matériaux et Constructions, Voi. 36, December 2003

demolition, in order to gain an environmental benefit without possibly impairing the mortar performance.

Any existing pozzolanic property of brick powder was investigated in order to evaluate the possibility of using crushed masonry rubble as cementitious material replacing cement. A similar attempt had been made also by Winkler and Mueller [6], in which the effect of different brick powders on cement hydration and mortar compressive strength development was studied.

Moreover, a partial substitution of cement with fly ash was tested, so that a comparison between fly ash and brick powder in terms of pozzolanic activity could be made. Because fly ash is an industriai by-product, its reuse is in line with sustainable development.

Certainly, from a mechanical performance point of view, these mortars will be weaker than a refcrcnce cementitious mortar but the same could not be valid when mortars are used in a masonry wall. In effect, the masonry mechanical behaviour depends much more on the bond strength between brick and mortar than on the intrinsic mechanical properties of the mortar. On the other hand, the bond strength is related to the adhesion of fresh mortar to the brick [7-9], and thus to the rheological properties of the mortar. For this reason attention was focused on rheological properties of cement mixes prepared with the same ingrediente as those employed in mortars, forecasting a strong influence of the mortar rheology on the interfacial zone quality.

Finally, some tests on bond strength of mortar to masonry units were carried out, by varying brick type, and results obtained were related to rheological test results.

Chemical composition and Blaine fineness of ali the ingredients are shown in Table 1.

The proportions of the paste mixtures are shown in Table 2. The water content of each paste was set to achieve a given flow rate, corresponding to a time of 30 seconds for 500 mi of cement paste to flow through the Marsh cone (8 rnm inner diameter).

Table 1 - Chemical composition and Blaine fineness of materials passing the sieve ASTM No.170

Cement Fly ash Brick powder

Rubble powder

Blaine fineness (m2/g) 0.42 0.45 0.33 0.73

Oxide (%)

Si0 2 29.67 59.94 71.18 84.99

Oxide (%)

A1 2 0 3 3.74 22.87 10.89 4.47

Oxide (%)

Fe 2 0 3 1.80 4.67 4.95 3.91

Oxide (%)

T i 0 2 0.09 0.94 0.28 0.11

Oxide (%) CaO 59.25 3.08 6.48 2.94 Oxide (%) MgO 1.15 1.55 2.86 1.10

Oxide (%)

S0 3 3.25 0.35 0.60 1.30

Oxide (%)

K 2 0 0.79 2.19 1.58 0.77

Oxide (%)

Na 2 0 0.26 0.62 1.18 0.41

Oxide (%)

L.O.I.* 11.62 3.34 1.83 26.57 loss on ignition

2. MATERIALS AND PROCEDURES

Table 2 - Paste mixture proportions Mixture Proportions, g

M i x t u r e W / C M Water Cement

Fly Br ick Rubble Sand Water Cement Ash Powder Powder Powder

C e m 0.50 50 100 - - - -C e m + F A 0.60 60 70 30 - - -C e m + P B 0.50 50 70 - 30 - -C e m + P R 0.55 55 70 - - 30 -Cem+PS 0.45 45 70 - - - 30

2.1 Pastes A commercial portland-limestone blended cement type

CEM II /A-L 42.5 R according to EN-197/1 was used. A 30% substitution (by mass) of a low calcium fly ash

(ASTM C 618 Class F) for the cement was studied. The Blaine fineness of the fly ash was 0.45 m 2/g, and its specifìc gravity was 2250 kg/m 3.

A 30% substitution (by mass) of ground brick powder for cement was also studied. According to Winkler and Mueller [6] there is a limit of 40% in the substitution of ground brick powder for cement, because with higher percentages there is insufficient Ca(OH)2 for a complete pozzolanic reaction. The brick powder was obtained by grinding some red high-burnt bricks until a Blaine fineness of 0.33 mVg was achieved which passed through the ASTM No.170 sieve with opening of 90 uni.

In order to compare changes induced on the rheological behaviour of the mortars by the different additions and substitutions, a 30% substitution (by mass) of powdered either rubble or sand for cement were also studied. The powdered rubble and sand were obtained by sieving them through the ASTM No.170 sieve with opening of 90 urn, achieving a Blaine fineness of 0.73 and 0.30 m2/g respectively.

2.2 Mortars Naturai sand or a fine fraction of recycled aggregate

were used as fine aggregate to manufacture mortars. The fine recycled aggregate fraction was directly supplied by an industriai crushing plant in Villa Musone, Italy, in which rubble from buildings demolition are suitably selected, ground, cleaned and sieved. The particle size distribution of naturai and recycled aggregates, both with maximum size of 6 mm, are shown in Fig. 1.

The specifìc gravity in saturated-surface-dried condition was 2620 kg/m 3 and 2150 kg/m 3 for the naturai sand and the fine recycled fraction respectively, while the water absorption was about 3% and 10% respectively.

The mortar mixture proportions are given in Table 3. The cement to sand ratio was 1:3 (by mass); the water content of each mortar was set to achieve the same consisterne of 110±5 mm, evaluated according to EN 1015-3.

When recycled sand was used, a higher water dosage was necessary to achieve the same consistence as that of the other mortars, because of the higher water absorption of the recycled sand with respect to the naturai one.

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about 380 Pa. The experimental apparatus was a rotating rheometer based on coaxial rotary cylinders with a slowly increasing shear rate (D) ranging from 0 to 100 s"1. The walls of concentric cylinders were not smooth but roughened in order to reduce ( i f not totally eliminate) the "slip" phenomenon Le. the development of a water-rich layer dose to the rotating cylinder which produces a lubricating effect, making flow easier and not representative of the bulk material [10].

The rheological behaviour was described by means of Bingham flow model:

0.01 0.1 1 Sieve opening (nini)

10 (1)

Fig . 1 - Grain size distribution o f the aggregates used.

Table 3 - Mortar mixture proportions Mixture Proportions, kg/m 3

Mixture W/CM Water Cement Naturai

Sand Recycled Aggregate

Fly Ash

Brick Powder

Ref 0.50 225 450 1350 - -RA 0.60 300 450 - 1350 - -PB 0.50 250 315 1350 - - 135 FA 0.45 225 315 1350 - 135 -

2.3 Bricks Two kinds of brick were tested: a red and a yellow high-

burnt brick. Some absorption and porosity characteristics of these bricks are shown in Table 4; also some data referred to brick physical behaviour are reported in the same table. The Initial Rate of Absorption (IRA) represents the mass of water absorbed per unit area in 1 minute by the brick face in contact with the mortar when immersed to a depth of 3 mm in water; generally the IRA is expressed in kg/m2/min.

where x is the shear stress (Pa), xy is the yield stress (Pa), r) is the plastic viscosity (Pa-s) and D is the shear rate (s"1).

In Fig. 2 are shown the Bingham curves as extrapolated from experimental tests carried out at 35 minutes since ingredients mixing. The slope of the down-curve (decreasing shear rate) was used to calculate the plastic viscosity, while the intercept at zero shear rate was used to calculate the yield stress.

In Fig. 3 the measured yield stress values are plotted as a function of time. It is quite evident that the paste containing powder from rubble maintains the minimum yield stress value throughout the test; in addition, this paste could

maintain its workability for longer because the time to achieve

240

« 180 e ,

| 120 m

- • -Cem O C c m + F A -ó-Cem+PB - -Cem+PS -O-Cem+PR

Table 4 - Absorption, porosity and mechanical characteristics of the bricks

Yellow Brick Red Brick Water Absorption after 24 h

(weight % ) 35 19

I R A * (kg /m 2 /min ) 5.94 2.42 **

Total Open Porosity (%) 60 43

Average Pore Diameter ( u m )

0.6 1.8

Compressive Strength (MPa) 8.28 10.89 Dynamic Elastic Modulus

(MPa) 8850 9150

20 40 60 Shear Rate D ( sec 1 )

80 100

Fig. 2 - Bingham curves after 35 minutes since ingredients mixing.

160

120 --

IRA: Initial Rate of Absorption determined by usìng mercury intrusion technique

3. R E S U L T S AND DISCUSSION

3.1 Rheological characterization of pastes Rheological behaviour of the cement pastes was tested at

5 minutes after ingredients mixing and then each 15 minutes until shear stress reached the machine limit of

-"O—Cem+FA. —•Èr— Cem+PB —O— Cem+PR —0—Cem+PS

0 50 100 150 200 Time (min)

Fig. 3 - Y i e l d Stress values plotted as a function o f t ime.

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Materials and Structures I Matériaux et Constructions, Voi. 36, December 2003

9000 Cent —O— Cem-HFA

-O—-Cem+PR —0—• Cem+PS

50 to 10.0 t v 150 l u n e (mm)

Fig. 4 - Thixotropy values plotted as a function o f t ime.

the threshold value of shear stress for test interruption (as high as 380 Pa) was doublé with respect to the other mortars. Better adhesion between mortar and brick should be expected as a consequence of this particular rheological behaviour of the mortar containing powdered rubble.

Thixotropy is the property of certain gels, such as cement paste, which are rigid when left standing but increase their own fluidity when put into movement. In Fig. 4 the measured thixotropy values are plotted as a function of time, where thixotropy was calculated as the area included between the up-curve and the down-curve. When powdered rubble was added to the paste, the thixotropy value was very close to zero for substantially longer than average time. During this period, the rheological behaviour of that paste could be considered pseudo-plastic without hysteresis.

3.2 Chemical characterization of mortars The evolution in time of calcium hydroxide in each

mortar was detected in order to point out pozzolanic activity, i f any. Calcium hydroxide content was also corrected in order to keep into account carbonation effect [11]. Calcium hydroxide, as well as calcium carbonate, in the mortars were determined by differential thermal analysis; the results are reported respectively in Figs. 5 and 6 as a function of curing time up to 70 days.

Pozzolanic activity is evident in Fig. 5 only for the mortar containing fly ash and labelled "FA".

The different nature of the aggregate used is quite evident in Fig. 6: the limestone content in recycled-aggregate mortar (RA) is about half of the other mortars. An X-ray diffraction analysis of the recycled-aggregate mortar confirmed both a lower content of limestone and a higher presence of silica from cementitious products in concrete rubble, as shown in Fig. 7, after 70 days of curing.

Hazardous substances were not found in any of the samples.

3.3 Pore structure characterization of mortars Three specimens for each mortar were tested after 1,3,

7, 14 and 28 days of wet curing by means of the mercury intrusion technique.

The total open porosity values calculated as average of the test results on three dried specimens are included in

20 40 60 C u r i n g t ime (days)

80

Fig. 5 - Calcium hydroxide content o f mortars as a function o f t ime up to 70 days o f curing.

U

i

- ù - R A -O-Ref - O F A -O-PB

20 40 60 C u r i n g t ime (days)

80

Fig. 6 - Calcium carbonate content o f mortars as a function o f t ime up to 70 days o f curing.

O

8000

7000

6000

5000

4000

3000

2000

1000

0

Silica Calcite

" W ^ w - . _ ! ^ . L . À..r *-~~«AÀ L i AL FA

—--r—L r j ^ À ^ X A A J L i ^ 10 20 30

20 40 50 60

Fig . 7 - Results o f the X-ray diffraction analysis o f the mortars.

Fig. 8; for each curing time the mortar containing the fine recycled fraction (RA) showed the higher total open porosity owing to a more porous aggregate.

3.4 Compressive and flexural strengths of mortars

Prismatic specimens (40 x 40 x 160 mm) were manufactured, cast and wet cured at 20°C. The compressive

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Monconi, Corinaldesi, Antonucci

20

5 10 15 20 25 Curing time (days)

30

Fig. 8 - Pore structure evolution up to 28 days o f curing.

and flexural strengths were evaluated according to EN 196-1. The results are reported in Figs. 9 and 10 respectively.

The tangent modulus of elasticity was measured from the stress-strain curve at the point of interest, which corresponds to about half of the compressive strength of the mortar. The results are shown in Fig. 11 : it is quite evident that mortar containing fly ash (FA) has roughly the same stiffness as reference mortar (Ref), and mortars containing brick powder (PB) and recycled aggregate (RA) have a lower stiffness coherently with a lower compressive strength.

3.5 Bond strength of mortar to masonry units In Fig. 12 the two masonry failure mechanisms where

only joints (and not bricks) are involved are shown [12]: the first (i) and second (ii) mechanisms are respectively the fracture of the joint and the sliding along the mortar bed at low values of normal stress. The Standard Test Method suggested by ASTM C 952 evaluates the bond strength developed according to the first mechanism.

In this work a test method, derived from the draft prEN 1052-3 [13], was adopted in order to evaluate the bond strength developed during the shearing of a unit with respect to the other along a mortar layer 10 mm thick. In this way the masonry behaviour in the absence of normal stress was investigated, corresponding to the Constant term in the Mohr-Coulomb friction law.

The tested model, shown in Fig. 13, is composed of three bricks; it has a symmetric structure thus avoiding eccentric loads. For this purpose the geometry of the model was always kept under carenai control. The configuration of the model was chosen to avoid the influence of the lateral deformation of the brick portions emerging from the joined portion. The applied load (L) was measured and at the same time the vertical displacement of the centrai brick (8) was also monitored.

Usually at the end of the test only one joint was cracked, so the bond strength was calculated dividing the maximum load by twice the fracture area where brick and mortar were in contact (approximately 120 x 200 mm); test results are included in Fig. 14.

In particular, very high bond strength was obtained by coupling red bricks and recycled-aggregate mortar. Observing data reported in Table 4 is not surprising that red brick performs well, mainly owing to an Initial Rate of Absorption

20 40 60 Curing time (days)

80

Fig. 9 - Compressive strength development as a function o f curing t ime.

20 40 60 Curing time (days)

80

Fig. 10 - Flexural strength development as a function o f curing t ime.

7 14 28 Curing Time (days)

F ig . 11 - Tangent elastic modulus values at different curing times.

value very dose to 2kg/m2/min, in correspondence to the recommended value in [14] for obtaining the maximum bond strength value.

The pore size distribution for the red brick is shifted towards large pores; it allows mortar to penetrate the brick surface well, in addition to allowing the water flow caused by the suction of brick pores, to ensure a suitable water to cement ratio in the interfacial zone, as expected from capillary pressure theories [14].

A good mortar-brick adhesion depends mainly on the quality of the interfacial zone; in fact, recycled-aggregate

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Materials and Structures I Matériaux et Constructions, Voi. 36, December 2003

FI brick A k A A

I I ! I

0) (ii)

Fig. 12 - Masonry failure mechanisms: ( i ) j o in t cracking under tension; ( i i ) j o in t slipping.

m or la­

bri ck

250 mai 200 mm

50 min

Fig . 13 - Masonry model for bond strength evaluations.

Ret FA PB Mor ta r Type

: /

Fig. 14 - M a x i m u m bond strength values.

mortar, in spite of having the worst mechanical behaviour (see Figs. 9 and 10), showed the best mortar-brick bond strength. The presence of recycled material lowers the yield stress value keeping it low for longer times, as previously pointed out by means of rheological tests; in this way mortar could better permeate the brick surface assuring a physical interlock and, as a consequence, an improved bond. Also mortars containing fly ash or brick powder showed better adhesion properties with brick than simply cementitious mortar, but bond strength was not so high as in the case of recycled-aggregate mortar. This fact could be related with the

yield stress values measured in the corresponding pastes; as obtainable from the comparison of the curves reported in Fig. 3 with the histograms reported in Fig. 14, a lower yield stress of pastes corresponds to an higher bond strength of mortars. Also the lower stiffness of mortars "RA" and "PB", as shown in Fig. 11, could play a role in improving mortar behaviour when submitted to bond strength test.

4. CONCLUSIONS

The presence of masonry in recycled aggregates is known to be detrimental to the concrete mechanical properties, as is the presence of the fine fraction of recycled aggregates.

However, experimental results showed the feasibility of using either recycled instead of naturai sand or powder obtained by bricks grinding as partial cement substitution for the production of mortars.

In this way, the alternative use of undesirable fractions of the recycled aggregate for the production of mortar has the added effect of improving the quality of the recycled aggregate for the production of concrete.

Moreover, in the case that a high masonry resistance to external actions is one of the design requirements, these mortars, in particular those containing recycled aggregate, could be of great benefit in terms of mechanical performance. In fact, an excellent bond strength was found, in particular when recycled-aggregate mortar and red bricks were coupled, due to the low thixotropy showed by that mortar and the appropriate pore size distribution of that bricks. As a matter of fact, mortar-brick bond strength evaluations showed a strong influence of both pore size distribution of brick and rheological behaviour of mortar on their mutuai adherence.

Future development of the research will be a two-fold approach: on one hand suitable procedures wil l be determined

rin order to guarantee standard characteristics of fine recycled aggregate independent of the recycling process. On the other hand, applied research wil l be conducted on the performance evaluation of masonry elements built by using fine recycled-aggregate mortars.

R E F E R E N C E S

[1] Corinaldesi, V . , Ti t tarel l i , F., Coppola, L . and M o n c o n i , G., 'Feasibili ty and performance o f recycled aggregate i n concrete containing f ly ash for sustainable bu i ld ing ' , i n 'Sustainable Development and Concrete Technology' , Proceedings o f a Three-Day International Symposium, ( A C I , Farmington Hi l l s , Michigan , U.S.A. , 2001), SP 202-11, 161-180.

[2] Corinaldesi, V . , Isolani, L . and M o r i c o n i , G., 'Use o f rubbles f rom bui ld ing demoli t ion as aggregates for structural concretes', in 'Valor izat ion and Recycling o f Industriai Wastes', Proceedings o f the 2 n d National Congress, (Edizioni Graphic Press, L ' A q u i l a , I taly, 1999), 145-153.

[3] R I L E M Technical Committee 37, Demol i t ion and Reuse o f Concrete, 'Recycl ing o f Demolished Concrete and Masonry ' , edited by T.C. Hansen (E & F N Spon, London, Great Br i ta in , 1992).

[4] 'Use o f Recycled Concrete Aggregate' , Proceedings o f the International Symposium, London, Great Br i ta in , (Thomas Telford Publishing, London, Great Br i ta in , 1998).

[5] Corinaldesi , V . and M o r i c o n i , G., 'Role o f chemical and minerai admixtures on performance and economics o f

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Moriconi, Corinaldesi, Antonucci

recycled aggregate concrete', i n 'Sustainable Construction: Use o f Recycled Concrete Aggregate ' , Proceedings o f the Seventh C A N M E T / A C I International Conference, ( A C I , Farmington H i l l s , Mich igan , U .S .A. , 2001), SP 199-50, 869-884.

[6] Winkler, A . and Mueller, H.A. , 'Recycling o f fine processed building rubble materials', i n 'Use o f recycled concrete aggregate', Proceedings o f an International Symposium, (Thomas Telford Publishing, London, Great Britain, 1998) 157-168.

[7] McGin ley , W . M . , T R A and the flexural bond strength o f clay br ick masonry' , i n 'Masonry: Components to Assemblages', A S T M Special Technical Publication n.1063, ( A S T M , Philadelphia, PA, U.S.A. , 1990), 217-234.

[8] Robinson, G.C., 'Adhesion mechanisms i n masonry', American Ceramic Society Bulletin 75(2) (1996) 81-86.

[9] Brocken, H.J.P., Spiekman, M . E . , Pel, L . , Kopinga, K . and Larb i , J.A., 'Water extraction out o f mortar during br ick laying: A N M R study', Mater. Struct. 31(205) (1998) 49-57.

[10] Saak, A . W . , Jennings, H . M . and Shah, S.P., 'The influence o f W a l l Slip on Y i e l d Stress and Viscoelastic Measurements o f Cement Paste', Cement and Concrete Research 31 (2001) 205-212.

[11] Chatterji, S., Collepardi, M . and Mor i con i , G., Tozzolarne property o f naturai and synthetic pozzolans: a comparative study', in 'The Use o f F ly Ash, Silica F u m é , Slag and Other Minera i By-Products in Concrete', Proceedings o f the First International Conference, ( A C I , Detroit , Mich igan , U.S.A. , 1983), V o i . I , SP 79-10, 221-233.

[12] Lourenco, P.B., Rots, J.G. and Blaauwendraad, J., ' T w o approaches for the analysis o f masonry structures: micro and macro-modeling' , Heron 40(4) (1995), 313-339.

[13] p r E N 1052-3, 'Methods o f test for masonry; Part 3: Determination o f in i t ia l shear strength', Draft for public comment, 1996.

[14] Groot, C. and Larb i , J., 'The influence o f water f l o w (reversai) on bond strength development in young masonry' , Heron 44(2) (1999) 63-77.

Paper received: August 6, 2002; Paper accepted: January 7, 2003

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