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THE PROPERTIES OF LIME AND CEMENT MORTARS MODIFIED BY METAKAOLINITE

W. [)()MASLOWSKI

NichOlaS Copernicus University

SUMMARY The contribution presents results of investigation on the influence of various hydraulic additives (crude and after activation at elevated temperature) on the properties of lime, cement and mixed

cement-lime mortars. It was established that the best mechanical properties could be ascribed to lime mortars (their mechanical strength is many times higher than of "virgin" ones) and cement-lime mortars (their mechanical strength increases from tens to over 100%) modified by means of metakaolinite only. Their water resistance increases too while their capillarity decreases. Differentiated physical and mechanical properties can be achieved by mixing of aggregates with lime

and cement-lime binders (with metakaolinite) in a selected ratio. Such mortars can find a wide application to conservation. The addition of metakaolinite to cement mortars based on a high-grade cement brand 45 does not upgrade their mechanical properties. Nevertheless, the application of such mortars may be positive because of immobilisation of calcium hydroxide.

INTRODUCTION

Lime mortars have been applied for thousands years. Written documents from the past tell that Greeks got the knowledge of lime firing from Persia and later used that process in a large scale /1 /. Despite that historic fact, we have memorised almost only roman mortars because Romans overtook the lime mortars technology from Greeks and upgraded it getting mortars of specific properties. They increased not only the mechanical strength of lime mortars but made them waterproof and capable to setting under water. Possessing such mortars Romans could construct huge buildings and hydrotechnic installations like bridges, aqueducts, baths etc. Ancient Romans are not only the

inventors of waterproof mortars. They also established the fundamentals of concrete technology that are u~to-date nowadays, too. 121 The performed investigation of roman mortars /3/ revealed that the mortars could achieve compressive strength ca. 20 MPa while standard mortars exhibited compressive strength of 0.8 MPa

(after 28 days) or 2.5 MPa if one used hydraulic lime. As we all know, the waterproof mortars were manufactured by Romans from a specific lime and aggregate but they also applied some additives like pozzolanas i.e. special hydraulic substances /4/. The additives were: ashes and fine deminished tufts of volcanic origin and some specific ceramic components. The effect of under water setting of mortars with hydraulic additives results from the

presence of so called active silica. It can react with calcium hydroxide at room temperature (xCaO • Si02 • yH20). Therefore one expects the formation of calcium aluminate and aluminosilicate. Nowadays, we know numerous sources of active silica and despite their origin (natural or artificial) one calls the substances as pozzolanas, too. The substances are applied as additives to lime and to

Portland cement as well. The activity of almost all hydraulic additives can be upgraded by burning. Diatomite is usually burned at 500 °c while other pozzolanas undergo firing up to 900 °c. The currently applied hydraulic substances are: ash and tuft of volcanic origin, pumice-stone, diatomite, tripolis, gaize, crude and

fired clays, rocks' dust, fired schist, blast furnace slag, furnace slag and plain ash.

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EXPERIMENTAL

The performed experiments were aiming at the elaboration of hydraulic lime mortars technology applicable to conservation works. One assumed that such mortars had to exhibit adjustable properties like mechanical strength and water transport ability. Despite the field of application, the mortars were supposed to have mechanical strength not higher than a selected monument's material. The mortars were also expected to exhibit a better developed ability to water transport (capillary suction and

drying). One applied hydrated lime, white Portland cement (DyPt45F manufactured by Dyckerhoff, Germany), glass-making sand (0.25-0.50 mm fraction) and metakaolinite obtained from kaolinite at 900 oc. Additionally, one performed modification attempts by means of some hydraulic additives like clay, bentonite, diatomite, colloidal silica and sol of silicitic acid called "Syton". One prepared the mortars for testing in different binder to aggregate ratio. In such a case, the hydraulic additives were regarded as a fraction of binding material. The tested mortars were of good plasticity. Those materials were used for filling standardised forms. The obtained samples were tested after 28 days long water bath.

RESULTS

1. The influence of hydraulic additives on selected physical and mechanical properties of lime mortars As mentioned, one applied kaolinite, clay, bentonite and colloidal silica, crude and fired at 900 oc as well as crude and fired (500 and 900 OC) diatomite. Unfortunately, the mortars containing crude kaolinite, clay and bentonite, colloidal silica (crude and fired) were very weak and did not resist the destructive action of water. Therefore, Table 1 summarises only the results dealing with the remaining hydraulic additives. It is clear (Table 1) that the mortars with metakaolinite (No. 1) exhibit much more higher compressive strength if comparing to the rest of the mortars. On the other hand these mortars have better mechanical strength than the typical lime-sand mortars (No. 7). Mechanical strength of these samples is too low to be measured by a hydraulic press of 1 O ton-load (too low sensitivity of the press). Regarding the standard value of mechanical strength of lime-sand mortars (0.8 MPa) one may state that the tested mortars with metakaolinite have 1372% higher strength while 281-420 % better strength is noticed for the remaining mortars. The all described mortars are differentiated considering their water resistance. The best water resistance is discovered for the mortars with diatomite (strength decrease from 15-22%) then for the mortars with metakaolinite (29%) and finally for the mortars with clay and bentonite (38 and 42%).

All the mortars are specific because of a considerably intensive shrinkage. The most intensive takes place for the mortars with pure lime and the mortars with diatomite. Less intensive shrinkage than for the lime-sand mortars (32-40% less) occurs for the remaining mortars with hydraulic additives. It was also found that the hydraulic additives extended the time of capillary rise of water. The time is six time longer for the mortars with metakaolinite that have the highest bulk density, too. For the remaining samples the capillary rise of water was only 2-3.5 times slower. Water absorption of all modified mortars is comparable to typical lime-sand mortars.

2. The influence of sand quantity on the properties lime mortars with metakaolinite.

As it can be concluded from Table 2, one can drive mechanical and physical properties of the mortars in a wide range by means of different amounts of sand. Thus, the properties are adjustable to the specific requirements of conservation works. Mechanical strength and water resistance of the mortars decrease upon the increasing quantity of sand while the magnitude of shrinkage is less intensive. In the same order the speed of capillary rise of water is higher. Mechanical strength of these mortars is

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comparable to the strength of cement mortars made of cement brand 45. Unfortunately their shrinkage and water absorption are higher.

Table 3a. The influence of aggregate type on the properties of mortars with metakaolinite.

One prepared aggregate from natural stones of the following properties:

Property limestone limestone sandstone sandstone "Pil'lcz6w'' "Karsy" "Nietulisko" "Zerkowice"

bulk density 1.64 1.41 1.78 1.97 [g/_cm31 water absorption 19.7 28.2 12.1 7.6 ~l porosity 30.3 43.1 21 .6 14.3 %1 specific surface area 2.9 15.4 0.3 -

J.m:ll91

The above Table 3a indicates the fact that the applied aggregates are differing very much from each other regarding some important properties like specific surface area and water absorption. These factors influence such specific feature of mortars as affinity to water. From Table 3 one may also conclude that there is a relationship between the properties of prepared mortars and the kind of aggregate used. The dependence however does not result from the mineralogical composition of the stones. It is more probably that bulk density of the mortars is a more important factor. Bulk density of the mortars is dependent on grains' morphology of the aggregates what controls the amount of water included in the mortar. Thus, the limestone containing mortars exhibit the most intensive shrinkage and the lowest mechanical strength. Capillary rise of water is faster in the case of lime aggregate than for the sandstone aggregate containing mortars. The resistance to water action is also differentiated: from 10% for sandstone "Nietulisko" to 40% in the case of limestone "Karsy" and sandstone "Zerkowice". It should be emphasised that water absorption is better for the mortars with limestone aggregate than for those containing sand or sandstone aggregate. Water absorption is particularly high for the mortars with the limestone "Karsy" aggregate (41%).

4. The influence of metakaolinite on the properties of cement mortars Cement and metakaolinite were premixed in the 1 to 0.5 ratio and used for the preparation of modified cement mortars. It was established that the above proportion was an optimal one. The properties of such modified cement mortars were compared to analogous properties of cement-sand

mortars without any additives. The results of the performed tests are presented in Table 4. One may conclude that the presence of metakaolinite depresses mechanical strength of such modified cement mortars (the mortar No. 4 does not apply). Simultaneously, one observes a worsening of capillary properties and an intensification of shrinkage. Water absorption of the pairs of mortars being compared is almost equal. One observes that water resistance of the metakaolinite modified cement mortars with small amount of aggregate (the samples no. 1 - 4) is comparable to that one of unmodified mortars. Water resistance of the remaining mortars (the samples 5 - 10), is always lower after the metakaolinite

modification.

5. The influence of metakaolinite on the properties of cement-lime mortars The mortars were modified by means of a metakaolinite-lime-cement binder premixed in the 1.5 : 1 : 1 ratio respectively. The properties of such modified mortars were compared to the properties of

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analogous mortars but without the hydraulic additive. The data in Table 5 let one to conclude that all

the mortars with metakaolinite exhibit better compressive strength when comparing to the unmodified samples of cement-lime type: 79% higher for the 1 : 2 mortar, 112% better for the 1 : 3 mortar and

68% better for the 1 : 4 mortar. The mentioned increase of mechanical strength is in part resulting

from higher density of the kaolinite containing mortars (higher bulk density of the samples). The strengthening effect is not questionable particularly if one regards the samples 5 and 6 of similar bulk

density. The mortars with metakaolinite are more water resistant, too. Their water absorption is slightly lower while shrinkage becomes larger but less intensive than for pure lime mortars.

Metakaolinite dramatically depresses capillary properties of the modified mortars. The time of 5 cm

high capillary rise of water is usually 2-3 times longer.

DISCUSSION

The performed investigation revealed that metakaolinite was the most active among all hydraulic additives tested. It increases efficiently mechanical strength of the mortars of high (common lime)

and medium (lime and cement) hydraulic index. The mentioned high mechanical strength results in part from higher density of the mortars with metakaolinite i.e. the mortars exhibit high bulk density. On the other hand, metakaolinite does not influence the mechanical strength of the cement mortars made of high quality cement (45 MPa). The influence of other pozzolanas (clay, fired bentonite, diatomite) on the mechanical strength of lime mortars is less intensive. Nevertheless, the influence has to be taken into consideration, too. The fact that mechanical strength of such mortars is 300-400% higher than of common lime mortars can be practically used in conservation works, too. The hydraulic additives change other properties of the mortars. The additives slightly reduce shrinkage of the lime mortars (diatomite does not apply) but cause more intensive shrinkage of the cement-lime and cement mortars. The capillary properties of such modified lime mortars are considerably worse, dramatically inferior in the case of the cement-lime mortars and only slightly

worse for the cement mortars. The additives does not influence water absorption of the cement mortars (comparable bulk density) but reduce water absorption of the cement-lime mortars particularly low water absorption occurs if one examines the lime mortars (bulk density of the metakaolinite modified mortars increases).

The hydraulic additives increase water resistance of the lime and cement-lime mortars but decrease water resistance of the cement mortars of high cement to aggregate ratio (starting from 1 :6 value).

Mechanical properties of the lime mortars with metakaolinite can be altered in a wide range by means of various amounts of aggregate added to binder (Table 2). For high aggregate content, one observes lower mechanical strength and shrinkage but capillary properties become better.

The application of aggregates other than sand does not change mechanical and physical properties of the kaolinite modified lime mortars in any significant manner.

CONCLUSIONS

Metakaolinite is particularly recommended for the modification of lime mortars. It increases mechanical strength and water resistance of such modified mortars. By means of different aggregate quantities one can drive in a wide range such properties of the mortars like mechanical strength (4 -15 MPa) and capillarity (water suction to 5 cm level needs from 60 to 200 minutes).

There is no reasonable need to modify cement-lime mortars by means of metakaolinite. As the matter

of fact, metakaolinite upgrades mechanical strength of the mortars but this effect is accompanied by a dramatic worsening of capillary properties of the material.

There is no reason to modify cement mortars by means of metakaolinite since it's addition does not influence significantly mechanical and physical properties of the mortars. However, metakaolinite can

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find an application also to cement mortars provided there is a clear need to immobilise calcium

hydroxide by it's chemical fixing. It can prevent the formation of a characteristic white efflorescence on the surface. Lime mortars modified by metakaolinite should be applied to such conservation works where cement

mortars are not acceptable. The modified mortars can be widely used to bricklaying, pointing and plastering of walls, filling of missing parts in natural and artificial stones and in pottery reconstruction.

It must be particularly emphasised that the high mechanical strength of the mortars can be achieved

just after few days long seasoning in liquid water. This fact makes them similar to standard cement mortars. Contrary to cement mortars, the lime mortars with metakaolinite are not a source of water­soluble salts provided one uses pure raw materials.

LITERATURE

1. M. Wirska-Parachowiak, Z historii material6w budowlanych, Ochrona Zabytk6w, 1968, 4, 19, Z. Brochwicz, Materialy wiazace w budownicywie starozytnym i wczesnosredniowiecznym, Materialy Zachodnio-Pomorskie, 1971 , 14, 775.

2. M. Wirska-Parachowiak, op. cit. , 21 . 3. B. Bukowski, Technologia beton6w i zapraw, 1947, 384. 4. Witruwiusz, O architekturze ksiag dziesiec, 1956, 99,

V. Furlan, Y. Houst, Les materiaux pouzzolaniques et leur utilisation, Chantiers, 1980, 7.

5. B. Bukowski, op. cit. , 164.

Bulk Water Shrinkage Time of5 Compressive No Components of mortar Component density absorptio cm capillary stregth (dry)

ratio n rise of water Jgl_cm1 _lo/tl _lo/tl [min] [Mpa)

I. lime:metakaolinite:sand I : I : 6 1.73 15.7 1.18 198 11.78

2. lime:clay (900°C):sand I : I : 6 1.66 17.6 1.2 1 72 3.44

3. lime:bentonite(900°):sand I : I : 6 1.65 16.2 1.05 121 3.09

4. lime:diatomite(900°C}:sand I : 0.5 : 4.5 1.55 19.7 1.42 102 4. 16

5. lime:diatomite(500°C):sand I : 0.5 : 4.5 1.55 19.6 1.43 63 3.05

6. lime:diatomite(raw):sand I : 0.5 : 4 .5 1.55 19.4 1.73 98 4 . 11

7. lime:sand I : 3 1.58 18.9 1.74 34 -

Compressive strength (wet)

_lMp~J

8.3 1

2.12

1.79

3.54

2.38

3.30

-

Table I. l it e: inll uence or hyd t au lie additi ves on selected physical and mechanical properties of lime mo11ars.

Composition of mortar Bulk Water Shrinkage Time of5 cm Compressive Compressive

No lime: metakaohnite: sand density absoption capillary rise of strength (dry) strength water (wet)

J g_/ cm3l __[ % ] _[ %j JmirD_ _lMP~ [MP~

I I : I : 4 1.68 15.4 1.35 249 14.79 12.08

~ I : I : 6 1.73 15.7 1.18 198 11.78 8.31

3. I : I : 8 1.72 14.9 0.75 80 7.76 5.08

4. I : I : 10 1.69 14.4 0.36 57 4 . 15 2.53

Table 2. Tlte i11 11uc11ce of sand to lime (including metakaolinite) ratio on the prope1t ies of mortars.

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Grain size Bulk Water Shrinkage TimeofS cm Compressive Compressive

No Aggregate type density absorption capillary rise of stregth strength

[mm] water

jg[cmJ _lo/_tl [%] _lmin} [Mpa] JMPa]

I. limestone "Pincz6w" 0.25-0.50 1.44 20.9 1.62 163 5.94 4.75

2. limestone "Karsy" 0 .25-0.50 1.08 41.1 1.66 64 3.69 2.28

3. sandstone "Nietulisko" 0 .125-0.25 1.62 15.9 1.11 252 8.99 8.09

4. sandstone "Zerkowice " 0.25-0.50 1.70 13.4 1.41 243 12.64 7.76

5. sand 0.25-0.50 1.73 15.7 1.18 198 11.78 8.31

Ta hie 3. The in llucncc of aggregate on the properties of lime mortars with metakaolinite.

Composi tion of mortar Binder to Bulk Water Shrinkage Time of5 cm Compressive Compressive No cement : metakaolinite : aggregate density absoption capillary rise of ~trength (dry) strength

sand ratio water (wet) Jg_/ cm] _l%J_ _l%J_ [min] fMPa] [MP~]

I. I : 0 : 4 I : 4 1.84 7.6 no 134 18.87 14.15

') I : 0.5 : 6 1.84 7.9 0.04 168 16.04 12.20

3. I : 0 : 5 I : 5 1.83 8.8 no 199 10.45 8.95

4. I : 0.5 : 7.5 1.81 8.7 0.04 119 12.05 9.77

5. I . 0 : 6 I : 6 1.74 10.3 no 48 9.02 5.93

6. I : 0.5 : 9 1.75 11.2 0.08 91 5.78 3.05

7. I : 0 8 I : 8 1.67 13 .1 no 4 5.96 4.48

8. I : 0.5 . 12 1.68 12.9 0 .03 21 3 .23 1.22

9 . I : 0 : 10 I 10 1.62 14.8 no 3 2.46 1.99

10. I · 0.5 : IS l.64 14.2 no 9 2.10 0.85

Table 4. Properties of the cement mortars modified by metakaolinite.

Composition of mortar Binder to Bulk Water Shrinkage Time of5 cm Compressive Compressive No cement : lime : aggregate density absoption capillary rise of ~trength (dry) strength (wet)

mctakaolinite : sand ratio water lJg_! cm] (%] [%] _lmin} [MPa] fMPal

I. I : I : 0 : 4 I : 2 1.64 17.0 0.19 81 8.31 5.26

') I : I : 1.5 : 8.5 I : 2.4 I. 76 14.4 0.23 235 14.89 13.29

3. I : I : 0 : 6 I : 3 1.69 16.0 0 .05 60 6.40 3.40

4. I : I : 1.5 : 10.5 I : 3 1.76 14.0 0.17 218 13 .59 11 .07

5. I : I : 0 : 8 I : 4 l.72 14.6 0.01 48 4.59 3.09

6. I : I : 1.5 : 14.5 I : 4 1.73 14.0 0 .12 138 7.73 5.63

Tnble 5. Properties of the cement-lime mo11ars modified by metakaolinite.