Effect of admixtures on the fresh and hardened properties of modern rendering systems Carl-Magnus J. Capener, Mr, Division of Building Technology, Chalmers University of Technology, Göteborg, Sweden; [email protected] http://www.bm.chalmers.se KEYWORDS: mortar, rendering, admixtures, polymers, chemistry, durability, and microstructure. SUMMARY: In this paper, the first part of a doctoral project studying the chemistry and mechanism of modern rendering systems is presented. This study is focused on polymer modified renders, a type of material that possesses several superior properties to conventional renders, such as better crack resistance, adhesion to substrate and flexural strength. This improves both the workability and the durability of the material. The main part of the project consists of experimental studies focused on the interaction between composition, hardening process, microstructure and transport properties. These studies concentrate on a small but most relevant selection of rendering systems with different binders and admixtures. Different studies such as vapour transport, capillary water absorption and the drying process give good prerequisite durability of rendering systems. The results of the project will give a better understanding of rendering systems formulated and their constitutive components.

1. Introduction This study is focused on polymer modified renders, a type of material that possesses several superior properties to conventional renders, such as better crack resistance, adhesion to substrate and flexural strength. This improves the durability of the material. The main part of the project consists of experimental studies on the interaction between composition, hardening process, microstructure and transport properties. These studies concentrate on a small but most relevant selection of rendering systems with different binders, admixtures and substrates. Studies on transport properties include transport properties relevant for both water uptake and for drying, i.e. the performance creating the moisture balance of the rendering systems. The influence of different admixtures and additions with the special focus on organic polymers, air entraining agents and fibres is studied. The study includes the mechanism of the hardening process, from plastic to hardened state, moisture transfer and obtained structure. In the plastic state the rheology and plastic shrinkage is studied as well as water suction. Among the decisive properties of the hardened mortars the focus is kept on water transport and ion diffusion that are important for durability. The influence of the decisive property profile on durability of the hardened render during exposure to environment parameters is included. The results of the project will give a better understanding of rendering systems and their constitutive components. The chemistry and mechanism created by a certain component or combination/variation of component including water/binder ratio is examined and analysed. Composite mineral–organic materials and in particular cement-organic materials are increasingly used in today’s construction applications and civil engineering projects (Ohama, 1995). The importance of understanding the mechanisms behind the materials behaviour is crucial when designing new and advanced products for the industry (Holmberg 2004). There are many factors influencing the behaviour of a material and it is necessary to build up a fundamental knowledge on the chemistry and mechanisms affecting it.

2. Experimental study 2.1 Introduction The main part of this licentiate consists of experimental studies on the interactions between composition, hardening process, microstructure and transport properties. These studies are concentrated on a small but most relevant selection of rendering systems with different binders and admixtures. Studies on transport properties include vapour transport and water absorption. The transport is relevant for both water uptake and drying, i.e. the performance creating the moisture balance of the rendering systems. The influence of different admixtures, especially organic polymers and air entraining agents, was studied. The study includes the mechanism of the hardening process, from plastic to hardened state, moisture transfer and obtained structure. In the plastic state the consistency and plastic shrinkage as well as water suction are studied. Among the decisive properties of the hardened mortars the focus is kept on vapour transport, moisture transport properties and drying shrinkage (Sandin, 1980). The influence of the decisive property profile on durability of the hardened render during exposure to environment parameters is investigated.

2.2 Experimental setup In order to get a survey of different compositions modified by admixtures, four reference compositions were used. One group of compositions was based on constant consistency while another group had a fixed water/binder ratio. In both particular groups both cement and lime/cement binder were studied, creating the four reference compositions, see Fig. 1. All of the compositions had a constant ratio binder/aggregate content.

Compositions Constant consistency

Constant w/b-ratio

Comp. A:

Comp. B:

Comp. C:

Comp. D:

Cement

Lime/cement

Cement

Lime/cement

FIG. 1. Set up of the four reference compositions depending on decisive criterion and binder base. The compositions above, without any admixtures, form the reference mortars, A-D. These reference compositions were used for studying the effect of different admixtures. The four “pure” reference compositions were then developed further. Two fixed admixtures then modified each reference composition; creating four new compositions, A1-D1. After the expansion of compositions by the addition of two fixed admixtures, there are 8 different compositions: A-D and A1-D1. The two fixed admixtures used were: •

Hostapur OSB (“Admixture 1”) C14/C16-alpha olefin sulphonate

•

Walocel MKX (“Admixture 2”) Methyl-hydroxyethyl cellulose (MHEC)

The series now expanded further as the second group of compositions, A1-D1, were modified by a new admixture, “Admixture 3”. A fourth admixture, “Admixture 4”, added to compositions A1-D1, created the last four compositions, A3D3, thus giving a total of 16 different compositions. The two new admixtures introduced are:

•

Vinnapas RE 5044 N (“Admixture 3”) Vinyl Acetate/Ethylene Copolymer (Et/VAc)

•

Acronal S 695 P (“Admixture 4”) Styrene-Butylacrylate Copolymer (SBA)

In Table 1 all 16 compositions are listed and grouped according to whether they have constant consistency or fixed water binder ratio. They are also grouped depending on binder used, cement or lime/cement, which gives a total of four groups A-A3, B-B3, C-C3 and D-D3. Table 1. Summary of mortar mix compositions. Constant consistency

Constant w/b-ratio

Cement

Lime/cement

Cement

Lime/cement

Reference

A

B

C

D

Reference + Adm. 1 &2

A1

B1

C1

D1

Reference + Adm. 1, 2 & 3

A2

B2

C2

D2

Reference + Adm. 1, 2 & 4

A3

B3

C3

D3

2.3 Constituents used in formulations For all of the compositions the following formulation was used as a base: (percentage by weight) Cement binder base: CEM II/A-L 42,5 R: 16.67% (“Byggcement/Slite”) Lime/cement binder base: CEM II/A-L 42,5 R: 8.33% (“Byggcement/Slite”) German Technical Lime, Scheaffer: 8.33% (”Släckt Murkalk E”) Aggregate: Natural silica sand: 83.33% For the sand, the sieve curve shown in Table 2 was used. Table 2. Passing percentage for sand sieve curve.

Sieve curve

Passing percentage [%]

2 mm

100

1 mm

80

0.5 mm

50

0.25 mm

25

The four admixtures were weighed as a ratio to the binder, the polymer/binder ratio:

•

C14/C16-alpha olefin sulphonate 0.02 %

•

A methyl-hydroxyethyl cellulose (MHEC) 0.10 %

•

Ethylene-Vinyl Acetate Copolymer (Et/Vac) 3.00 %

•

Styrene-Butylacrylate Copolymer (SBA) 6.00 %

The admixture dosages were all within the range recommended by the manufacturers.

2.4 Water content in the compositions For groups A and B, that is, the groups with constant consistency the water content is shown in Table 4. Groups C and D have a constant water/binder ratio and therefore constant water content. For Group C, the water content = 13.3% and for Group D, the water content = 16.7%. Table 4. Water content for compositions A-A3 and B-B3. Composition A A1 A2 A3 B B1 B2 B3

Water content [%] 14.0 13.7 13.4 11.3 17.9 17.5 16.7 15.3

2.5 Measurements of relevant properties – fresh mix 2.5.1 Density and air content The air contents of the fresh mortars were measured using the pressure method, EN 1015-7. The density was measured using the same container used for measuring the air content of exactly 1 dm3. 2.5.2 Consistency using the flow table This procedure was performed in accordance with EN 1015-3. After the material had spread on the table, the average diameter was measured and round off to the nearest 5 mm. 2.5.3 Water retention The water retention tests were performed in accordance with EN 1015-8. The principle is to measure the water retained in a sample of fresh mortar when subjected to suction. 2.5.4 Plastic shrinkage In this test, the shrinkage of the render was measured at a very early age, that is, when the material was still in a plastic state. The first measurements started roughly 30 minutes after mixing and the shrinkage was then monitored for 48 hours. The equipment used for the measurements was the Delta-L developed by Cementa Research in Sweden (former Scancem Research). This equipment measures length change during a specified time period at intervals set by the user. The precision of the length measuring gauges used was 1/1000 mm length change. The form was custom made for this series of experiments. The inner size of the form was 150 mm in length and 100 mm width, see Fig. 2. The depth of the cast specimen was 10 mm, making the experiments more realistic for thin coat renders.

0,19

0,10

FIG. 2. Simple sketch of the form used for measuring plastic shrinkage.

2.6 Measurements of relevant properties – hardened state 2.6.1 Steady state moisture transport properties Vapour transport by the “wet-cup”-test was performed. The set up was simple; the material shaped as a cylinder (diameter, d=100 mm and thickness, lmaterial=25 mm) was attached to a cup with inner diameter 100 mm and containing pure water. As the gap between the specimen tested and the water’s surface, lairgap, also contribute to the vapour resistance, it was also measured. The attachment connecting the cup and sample was sealed as well as the samples mantle surface. The “cups” were stored in a climate room with constant temperature (T=20ºC) and relative humidity (RH=50%). The flow of vapour, g [kg/m2s], can be calculated according to Equation 1 where m1 and m2 is the weight of the cup at two different times, t 1 and t2. A is the area of the cylinder surface.

g=

m1 − m2 1 ⋅ t 2 − t1 A

(1)

This can also be written as:

g=

∆v Z material + Z airgap

(2)

Where Z is the vapour resistance in the air and material and ∆v = 8,66 kg/m3, is the difference in vapour concentration. Zairgap can be calculated as:

Z airgap =

d airgap δ air

(3)

δ air is the transport coefficient for vapour transport, 25·10-6 [m2/s] at 20ºC. The materials vapour transport can now be calculated according to Equation (4):

δ material =

l material Z material

(4)

The same set up as above was used but this time the cup was turned upside down to measure the water transport. This is called the “upside/down-cup”-test. In this case g in Equation 1 is related to:

g=

∆v perm.

(5)

Z perm.

Water absorption by capillary suction was also performed on cylindrical specimens with the same dimensions as above. The specimens, which had been dried at 65ºC and 105ºC respectively, were weighed and then placed on a wet cloth that enabled an abundant amount water to reach the specimens lower surface. The increase in weight, as the sample took up water, was measured and plotted as a function of the square root of time,

t.

2.6.2 Thermogravimetric analysis The thermo balance used in this study is a LECO MAC-500. It can be programmed in five temperature steps in the range from room temperature up to 1000ºC (Helsing-Atlassi, 1993). It can analyse up to 19 samples simultaneously so it was possible to run all 16 compositions at one time. In the thermogravimetric analysis performed, the specimens were fully saturated with water before the start of the analysis and then placed inside the furnace. Starting at room temperature, the heating rate was set for 10ºC/minute until the next temperature step was reached. Here the temperature remained until all the compositions had reached weight equilibria. Once they were all stable, the heating rate proceeded until the next level and so on. In table 5 the temperature ranges and corresponding decomposing compounds are shown. Table 5. Temperature ranges for the thermogravimetric analysis. Temperature range -105 105-380 380-450 450-600 600-975

Decomposing compound Evaporable water Most water in the CSH and CAH phases Hydroxyl in Ca(OH)2 Some hydrates + carbonation products other than calcite CaCO3 (calcite) + Other non-defined components

2.6.3 Other tests performed In addition to the test above, a number of other tests were carried out. The moisture desorption isotherm was determined for all compositions and the apparent and capillary pore volume was also measured along with the flexural strength and drying shrinkage.

3. Main results and discussion 3.1 Air content, consistency and workability Adding air entraining agents and cellulose ethers to the reference compositions dramatically increases the air content in the mortar. However, for the compositions based on a lime/cement binder, the effect is somewhat reduced. This can probably be attributed to the hard ions in calcium-enriched samples, which depress the foam. When introducing Styrene/Butylacrylate copolymers (SBA) the air content is further reduced for the lime-enriched samples. The water demand is lower for polymer-modified renders. Most of the effect comes from the surfactants introducing air into the system. The increase in workability is often ascribed to some sort of “ball bearing” action of the air bubbles. The highly numerous air bubbles are compressible compared to the other constituents of the mortar and allow for easier deformation when the mortar is worked, resulting in an improvement in workability.

3.2 Water retention It is expected that water absorption of methylcellulose-modified systems should increase with rising polymer/cement ratio. Methylcellulose causes a considerable swelling due to water absorption, and seals capillary cavities in the modified systems. This would decrease bleeding and keep the water inside the material, hence retain water. Therefore, the introduction of the cellulose ether into the system was expected

to generate a much higher effect on the water retention. The test results were, however, somewhat surprising. For the compositions without lime, an increase in water retention was found when adding the cellulose derivative, but not as much as expected, see Fig. 3. Further study is needed for clear explanations to these phenomena. The other admixtures also improved the water retention to some extent. For the SBA, this could probably be explained by the hydrophilic colloidal properties and the inhibited water evaporation due to the filling and sealing effects of impermeable polymer films formed. Water retention

Water retention [%]

100 95 90

Group A Group B Group C Group D

85 80 75 70 65 Ref.

Ref. + Adm. 1&2

Ref. + Adm. 1, 2 & 3

Ref. + Adm. 1, 2 & 4

FIG. 3. Effect of admixtures on water retention.

3.3 Plastic shrinkage Compared to the reference compositions, plastic shrinkage was generally reduced for all compositions modified by admixtures. SBA however, increased plastic shrinkage for all groups except Group C. Also interesting is the effect of SBA on the dormant period in Group A, see Fig. 4. The shell formation of cement grains seems to be prevented by the polymers, resulting in an unclear dormant period, probably due to continued access to mixing water. This effect is not seen for the lime-enriched samples, where the shell formation in cement grains is not clear, see Fig. 5. For Group C, the reduced amount of mixing water prevents this effect. Plastic shrinkage, Group B

Plastic shrinkage, Group A

A

2

A1

1,5

A2

1

A3

0,5 0 0

10

20

30

40

50

Time [Hours]

60

Shrinkage [‰]

Shrinkage [‰]

3 2,5

8 7 6 5 4 3 2 1 0

B B1 B2 B3

0

10

20

30

40

50

60

Time [hours]

FIG. 4 and 5. Effect of admixtures on plastic shrinkage, Group A, cement based renders with constant consistency, and Group B, lime/cement based renders with constant consistency.

3.4 Moisture transport properties As expected, the vapour transport is linked to the air content of the system, with the highest transport found in the air entraining/MHEC modified system (A1). Water transport was, however, not related to the air content but to the sizes of capillary pores, their continuity and surface characteristics. In general, the capillary suction was slowed down by the addition of polymers, and the amount of water taken up reduced. This is in agreement with the effect on water transport. The significant decrease in water transport coefficient and capillary suction rate could be found in the cement-based systems modified by the SBA, Styrene/Butylacrylate, indicating a strong hydrophobic effect, see Figure 6. In the cement-lime binder systems, this hydrophobic effect seems not significant.

Water transport 1,20E-06 1,00E-06 Group A

[m2/s]

8,00E-07

Group B

6,00E-07

Group C

4,00E-07

Group D

2,00E-07 0,00E+00 Ref.

Ref. + Adm. Ref. + Adm. Ref. + Adm. 1&2 1, 2 & 3 1, 2 & 4

FIG. 6. Effect of admixtures on water transport.

4. Concluding remarks •

From the results obtained it is clear that admixtures have a great influence on the behaviour and properties of mortar both in the fresh and hardened states.

•

For the compositions based on a lime/cement binder, the effect of the air entrainers is somewhat reduced. This can probably be attributed to the hard ions in calcium-enriched samples, which depress the foam.

•

Plastic shrinkage was generally reduced by the introduction of admixtures. There were however, some exceptions.

•

The drying shrinkage was increased by the introduction of admixtures, probably due to the higher air content in the mortar.

•

The hydrophobic effect, found by the addition of SBA, Styrene/Butylacrylate, was significant in the cement base mortars, whereas, in the lime/cement base mortars, the effect was negligible.

•

SBA, Styrene/Butylacrylate copolymers increase the flexural strength for both cement and lime/cement base mortars.

•

SBA also show a higher amount of chemically bound water, indicating that the polymer binds chemically to the mixing water and thus facilitate for water retention.

5. References Capener C-M. (2004). Mechanism and Chemistry of Modern Rendering Systems – Effect of Admixtures on the Fresh and Hardened Properties, Department of Building Technology, Chalmers University of Technology, Göteborg, Sweden. Helsing-Atlassi E. (1993). A quantitative thermogravimetric study on the nonevaporable water in mature silica fume concrete. Department of Building Materials, Chalmers University of Technology, Göteborg, Sweden. Holmberg K., Jönsson B., Kronberg B. and Lindman B. (2004). Surfactants and polymers in aqueous solution, 2nd Edition, John Wiley & Sons Ltd, West Sussex, England. Ohama Y. (1995). Handbook of polymer modified concrete and mortars. Noyes Publications, New Jersey 07656, USA. Sandin K. (1980). The effect of the rendering on the moisture balance of the façade – Main report. Division of Building Materials, Lund Institute of Technology, Sweden. Taylor H.F.W. (1997). Cement Chemistry. 2nd Edition, Thomas Telford Services Ltd, London, England.