Water-soluble polymeric modifiers for cement mortar and concrete E. Knapen 1), A. Beeldens 2) and D. Van Gemert 1) 1) Reyntjens Laboratory, Department of Civil Engineering, K.U.Leuven, Leuven, Belgium 2) Belgian Road Research Centre, Belgium
Abstract Usually, polymer-modified cement concrete or mortar is prepared by mixing polymer dispersions or redispersible polymer powders with the fresh mixture. The surface active agents, added to allow emulsification and stabilization of the dispersion during storage, hinder the polymer film formation, proceeding from the dispersion, and the cement hydration. In this paper, the addition of polymers in an aqueous solution is studied. Water-soluble polymer molecules are supplied on a molecular scale, improving the approach of the relatively large cement grains by the polymers. In the absence of surface active agents, the film formation on the hydrate crystals may proceed more easily and uniformly and the material properties can be better tuned and modelled. The addition of very small amounts of water-soluble polymers results in an improvement of the durability and the adhesion strength of the cementitious materials, which makes them appropriate as repair materials. Keywords: polymer modification, water-soluble polymers, microstructure building.
Elke Knapen Katholieke Universiteit Leuven Kasteelpark Arenberg 40 3001 Heverlee Belgium Email: [email protected]
Tel: +32 (0) 16 321678
1.0 Introduction Polymer modification is a frequently used technique to overcome some of the shortcomings of conventional mortars and concretes such as poor tensile and impact strength, limited resistance to corrosion, poor behaviour under severe conditions and poor adhesion of fresh mortar or concrete to old concrete. Polymer-modified mortar and concrete are prepared by mixing a small amount of polymer with the fresh cement mortar and concrete mixture. These modified cement mortar and concrete contain two types of binder: the system based on hydraulic cement and the polymer system. An interpenetrating network of polymer and cement hydrates is generated in which the aggregates are embedded. Polymer-modified cement concrete or mortar is produced by mixing polymer dispersions, redispersible powders, water-soluble polymers or liquid polymers with the fresh mixture. Even monomers can be added to the mixture on the assumption that polymerization and adequate film formation take place in combination with the cement hydration process . Each of the application forms has its typical advantages and disadvantages. Generally, the polymers are added to the fresh mixture as an aqueous dispersion or as a redispersible powder. The major disadvantage of these two types of application forms is the presence of surface active agents, which disturb the polymer film formation and retard the cement hydration . Water-soluble polymers are supplied on a molecular scale, improving the approximation of the relatively large cement grains by the polymers. In the absence of surface active agents, the film formation on the hydrate crystals will proceed more easily and uniformly and the material properties can be better tuned and modelled . A disadvantage of the use of water-soluble polymers may be their sensitivity to humidity and water. In this paper, the use of water-soluble polymers in cement concrete and mortar is discussed. Research on the influence of water-soluble polymers on the microstructure building and on the cement hydration reactions is running at the Katholieke Universiteit Leuven (Belgium) and the first results are presented here.
2.0 Water-soluble polymers in cement concrete and mortar Polymer modification of mortars or concrete by adding water-soluble polymers to the fresh mixture is less frequently applied, although recent studies list some advantages, as will be reported in this paragraph. First, an overview of water-soluble polymers that can be used for the modification of mortars and concrete is given. 2.1 Classification of water-soluble polymers There are various classes of water-soluble polymers that can be used for the modification of cement mortars and concrete. The first class consists of non-ionic polymers with an oxygen or nitrogen in the backbone of the polymer. Examples are polyethylene oxide (PEO) and polyethylene imine (PEI). These polymers can be synthesized with molecular weights up to the millions. Secondly, there are water-soluble non-ionic polymers containing an acrylic group, e.g. polyacrylic acid (PAA) and polyacrylamide (PAAm). The water-soluble polyvinyl alcohol, frequently used for the modification of cement mortars and concrete, belongs to the class of the water-soluble non-ionic polymers containing a vinyl group. This polymer is synthesized by the hydrolysis of polyvinyl acetate. A fully (PVA) or partially hydrolyzed (PVAA) polyvinyl acetate
may be used. Finally, cellulose derivates, as methylcellulose (MC), and polyelectrolytes, as polystyrene sulphate (PSS), can be added to the fresh mixture. 2.2 Properties of mortars and concrete modified with water-soluble polymers Water-soluble polymers are generally used at very low polymer/cement ratios. The addition of small amounts of water-soluble polymers changes the properties of the fresh mortar mixture. The workability of the fresh mixture is markedly improved over that of ordinary cement mortar and concrete, because of plasticizing and air-entraining effects of the polymers. The modified system shows higher water retention than the ordinary cement systems. This may contribute to an improvement in the workability and the prevention of dry-out, and it also leads to superior adhesion to porous substrates such as ceramic tiles, mortars and concrete . Addition of watersoluble polymers reduces bleeding and improves the homogeneity of mortar or concrete by controlling segregation of fresh mixtures . Also, a better dispersion of carbon fibers in the cement paste is realized . On the other hand, water-soluble polymers have often been found to decrease the mechanical properties of mortar and concrete because of the increased air entrainment . Although, recently, an increase of the tensile strength  and an increase of the bond strength between cement paste and aggregate [7, 8], between cement paste and steel fibers  and between cement paste and carbon fibers  has been reported. Fu and Chung studied the effect of small additions of methylcellulose (0.2% - 0.8% by weight of cement) [6, 9, 10]. The tensile strength was found to be increased by up to 72%, the tensile ductility by up to 620%, while the compressive strength was decreased by up to 30% and the compressive ductility by up to 34% . The addition of methylcellulose (0.4% by weight of cement) or latex (20% by weight of cement) to cement paste gave similarly significant increases of the shear bond strength between stainless steel fibers and cement paste, in spite of the low concentration and therefore lower cost of methylcellulose compared to latex . The microstructure of the interfacial transition zone (ITZ) is influenced by modification with water-soluble polymers. Because of the alteration of the microstructure of the ITZ, the admixing of small amounts of water-soluble polyvinyl alcohol was reported to have doubled the pullout strength and friction of steel fibres in cement paste by Najm et al. . Kim and Robertson  found an increase of the aggregate-paste bond strength after the addition of polyvinyl alcohol. They noticed a significant reduction in the thickness of the ITZ around sand grains and coarse aggregates and a significant reduction or even elimination of the Ca(OH)2 crystals that normally coat the aggregate surface and their possible replacement by C-S-H crystals. After addition of methylcellulose, Hayakawa et al.  also found an improved bond strength between cement matrix and aggregate. They attributed this to a decreased bleeding and a subsequent decreased defect under aggregates caused by bleeding. Singh and Rai  report a possible chemical interaction between polyvinyl alcohol and cement which results in the formation of some new compounds. The compounds are thought to be amorphous in nature, because they are not detected by X-ray diffraction techniques, only by the UV-visible spectra and differential thermal analysis. Singh et al.  also report a possible interaction of hydroxyethyl cellulose with the ettringite phase and with the C-S-H phases of the hydrated cement and an indication of the formation of a new product. Their results are obtained by IR spectra and X-ray diffraction.
As illustrated above, polymer modification with water-soluble polymers improves some properties of mortars, such as the workability and the adhesiveness, rendering them appropriate as adhesive mortars. There still needs to be carried out a lot of research on the influence of these polymers on the cement hydrates that are formed, on the microstructure building and on the durability of the modified systems.
3.0 Research program A research program was set up at the Katholieke Universiteit Leuven (Belgium) to study the film formation in mortars modified with water-soluble polymers. The influence of the curing conditions on the flexural strength was determined. If film formation takes place, the durability of the polymer film is important. Therefore, the influence of moisture and water on the mechanical properties of mortar beams is studied. By means of SEM investigation, the microstructure at the air void interfaces is analyzed. 3.1 Materials An ordinary Portland cement (CEM I 52.5 N) is used. Different types of polymers are added to the fresh mortar mixtures: a polyvinyl alcohol (PVAA) which is a 87-89% hydrolyzed polyvinyl acetate; two cellulose derivatives, methylcellulose (MC) or hydroxyethylcellulose (HEC); or a polyethylene oxide (PEO). The polymer/cement ratio is 1%, the water/cement ratio 0.45 and the sand/cement ratio 3. The polymer powders are first dissolved in the mixing water before adding to the sand and cement in the mixer. Mortar beams (40*40*160mm) are prepared and covered for 2 days. The standard curing implies a storage, after demoulding, in a moist room for 5 days (20°C, 85-90 %), followed by a dry curing of 21 days (20°C, 63% RH). 3.2 Influence of the curing conditions Possible film formation is studied by examining the influence of the curing conditions on the flexural strength of mortar beams. Cement hydration requires a wet or moist curing, while a dry curing promotes polymer film formation. Figure 1 shows the flexural strength for mortar beams subjected to the standard curing, as described above, and beams subjected after demoulding to a 26-day wet curing (85-90% RH). The flexural strength of the mortars beams modified with PEO and HEC is not influenced by the curing conditions. On the other hand, the flexural strength of the mortars modified with PVAA and MC is increased with 21% and 27% resp. if a dry curing period is included (standard curing). This is an indication of the formation of a PVAA and MC film which contributes to the flexural strength of the mortars. In the HEC- en PEO-modified mortars, no polymer film is formed or the polymer film does not contribute to the tensile properties of the mortars. 3.2 Influence of moisture and water on polymer-modified mortars Films of water-soluble polymers are quite hygroscopic. The equilibrium moisture content of a polyvinyl alcohol film increases exponentially with increasing relative humidity. The mechanical properties of these films are again very much a function of the water content of the film. At high relative humidities, the mechanical properties of water-soluble polymer films decrease dramatically . Furthermore, films of water-soluble polymers dissolve in water. Holzer et al.  have shown by means of in-situ investigations in an Environmental Scanning Electron Microscope (ESEM) that cellulose ether and PVA may become mobile in mortars during wetting and drying cycles, whereas latexes remain immobile.
Flexural strength [MPa]
standard curing (2d wet, 5d moist, 21d dry)
wet curing (2d wet, 26d moist)
Figure 1. Influence of the curing conditions on the flexural strength of unmodified mortars (REF) and mortars modified with 1% PVAA, 1% MC, 1% HEC and 1% PEO. To investigate the influence of moisture on the mechanical properties, mortar beams modified with water-soluble polymers are subjected, after standard curing, to a moist curing of 7 days (85-90% RH) or a moist curing of 14 days (85-90% RH). No influence of the moist curing on the splitting tensile strength of the mortar beams is noticed. Further research is planned to study the influence of longer storage at a high relative humidity (> 95%). Additionally, splitting tensile tests are carried out on mortar beams before and after vacuum saturation and storage under water for 7 days. The results are shown in Figure 2. The splitting tensile strength of PVAA-modified mortars decreases with almost 40% after storage under water. The decrease in splitting tensile strength of HEC-modified mortars is not significant. These results confirm the results of the previous paragraph. A PVAA film is formed which contributes to the tensile strength of the mortar beams. A contribution of a HEC film to the strength properties is not noticed. Further research is needed to the possible degradation of mechanical properties by storage under water, especially with respect to practical applications of mortars modified with watersoluble polymers. 3.3 Microscopic investigation Fresh fracture surfaces of the mortar beams, cured as described above, are investigated using a JEOL JSM-6400 type of Scanning Electron Microscope (SEM) in the SEI mode. Samples are coated by evaporation with gold. Special attention was paid to the air void interfaces in the mortar specimens. At these interfaces, the presence of water-soluble polymers is expected because of their strong affinity to the gas-water phase. The polymers serve as surface-active agents that are initially dissolved in the mixing water. During mechanical mixing, they become attached to the air void interface and start to stabilize the entrained air voids in the fresh mixture. So, an enrichment of polymer at the interface between air void and wet cement paste may be detected , depending on the surface activity of the polymer.
Splitting tensile strength [MPa]
after storage under water
Figure 2. Influence of storage under water on the splitting tensile strength of unmodified mortar beams and mortar beams modified with 1% PVAA and 1% HEC. The detection of water-soluble polymer films is not easy because of the very low concentrations of the polymers in the cement paste, their possible integration in the polymer-cement matrix at a submicron scale and their sensibility to electron beam damage. Jenni et al.  report thicknesses of cellulose ether films of less than 1 µm. The microstructure of cement hydrates formed at the air void interfaces is strongly influenced by polymer modification, as shown in Figure 3, although polymer film formation is not detected. The air void interfaces of unmodified mortar beams have a smooth surface at the micrometer scale. Microcracks are due to shrinkage of the mortars. In the case of polymer modification, Figures 3-b and 3-c show a similar microstructure at the air void interface for the PVAA-modified and the HEC-modified mortar. C-S-H fibres cover the walls of the air voids, disturbing the smooth surface that was found in unmodified mortars. The presence of polymer particles or polymer films is expected at the air void interfaces. Water molecules are bounded to the polymer particles until sufficient forces are exerted, e.g., by cement hydration. In this aqueous environment, crystal formation is possible. The air void interface of the MC-modified mortar (Figure 3-d) is characterized by an abundant efflorescence of Ca(OH)2 crystals, surrounded by C-S-H fibres. The Ca(OH)2 plates are formed parallel towards each other and they are almost undistorted. MC has a high swelling capacity and can swell to forty times its dry volume in water . The presence of MC at the air void interfaces results in a very aqueous environment which promotes crystal formation.
Figure 3. SEM images at the air void interfaces: unmodified mortar (a), mortar modified with 1% PVAA (b), 1% HEC (c) and 1% MC (d).
4.0 Further research Further research on the mechanical properties of mortars, modified with water-soluble polymers, after a long storage in a moist atmosphere or under water is needed for two reasons. From a practical point of view, the knowledge of these properties is essential for the application of these materials under such conditions. Secondly, studying the behaviour of mortars in moist or water conditions can give an indication of the contribution of the polymer film to the properties of the material. Does the polymer film really contribute to the durability and the mechanical properties of the material or does the polymer only influence the cement hydration products that are formed? For a better understanding of the influence of water-soluble polymers on the cement hydrates, a thorough characterization of the microstructure building is needed. In the research project planned at the Katholieke Universiteit Leuven (Belgium), the formation of cement hydrates at different ages will be followed by some analytical techniques, such as thermal analysis, X-ray diffraction and microscopic investigation. The evolution of the microstructure at an early stage will be examined and the interaction between cement hydrates and polymers will be clarified.
5.0 Conclusions Adding polymer dispersions, redispersible powders, water-soluble polymers or liquid polymers to the fresh mixture produces polymer-modified cement concrete or mortar. Generally, polymers dispersed in water by surfactants are used. These surfactants badly influence the cement hydration and the polymer film formation. In the absence of surfactants, water-soluble polymers are supplied on a molecular scale, allowing lower amounts of polymer to be used and an easier and more uniform film formation on the hydrate crystals. By studying the influence of the curing conditions and the influence of moisture and water on the mechanical properties, polymer film formation is assumed in mortars modified with PVAA and MC. A disadvantage of the use of water-soluble polymers might be their sensitivity to humidity and water. The effect of a storage in a moist atmosphere and of an under water storage on the mechanical properties of mortars should be examined.
6.0 Acknowledgement The grant offered by the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT-Vlaanderen) is gratefully acknowledged.
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