Determining the physical properties of polymer in different admixtures used for self-compacting cement paste by ESEM

Highlights

• ESEM will enable the examination of fresh cement pastes thanks to allowing to capture images of fluid materials.

• The properties of the polymers in polycarboxylate based plasticizer additives were screened in fresh cement paste using ESEM.

• The lengths and diameters of the polymers in the polycarboxylate based plasticizer additive were measured.

• The presence of electrostatic effect in the cement paste was observed from the brightness formed around the cement particles.

• No electrostatic effect was observed in lignin and naphthalene based additives.

Abstract

The aim of this study is to determine the physical properties of the polymer in plasticizers used for self-compacting cement pastes by environmental scanning electron microscopy (ESEM) method. Images of fresh cement paste containing cement, water, and plasticizer prepared by using 3 different types of plasticizer were examined by ESEM. Lignosulfonate-based, naphthalene sulfonate-based, and polycarboxylate-based plasticizers were added to the cement paste at a rate of 1% by weight of the cement. Images of the fresh cement pastes were obtained using ESEM within the first 3−5 min after mixing. As a result, the presence of electrostatic and steric effect forces of cement pastes prepared with the polycarboxylate-based additive was determined by ESEM analysis. In the ESEM analysis of the self-compacting cement paste, the diameter of the polymer that forms the steric force effect of the polycarboxylate-based additive was measured as 100−160 nm and the polymer length was 1−2 μm. The presence of an electrostatic force was determined from the brightness around the cement particles by ESEM image analysis.

Introduction

In concrete technology, chemical admixtures began to enter concrete structures intensively and new products were developed according to the needs arising from their use beginning in the 1970s. The search for solutions for the problems experienced with concrete and mortar accelerated the development of chemical additives.

According to the produced concrete class, aggregate accounts for 60–80 % of the concrete volume. In addition, in the structure of concrete there are cement, mixture water, and mineral additives within ranges of 10–15 %, 15–20 %, and 2–5 % of the total volume of concrete, respectively. The percentage of chemical additives is 0.2−0.5% of the total volume of concrete (Fig. 1).

Chemical additives have a relatively insignificant volume in concrete; however, they have an important influence on the properties of fresh and hardened concrete. Chemical additives’ influence is estimated to be approximately 50 % (Fig. 2). Thus, they are an indispensable concrete component with such a large effect in spite of being added in such small amounts.

The first-generation additives are lignosulfonate (LS)-based additives. In this type, wood is the main raw material, being a byproduct of paper production. Different types of lignosulfonates can be obtained depending on the type of wood and chemical processes. Since the obtained lignosulfonates contain high amounts of sugar, some disadvantages such as delay in setting time and air entrainment have been encountered. Efforts were made to resolve these problems by reducing the sugar content in the lignosulfonates.

The second generation of additives includes melamine- and naphthalene-based superplasticizers. They were first used in Japan in the late 1960s and in North America in 1974. In addition, some modified LS types can also be classified as superplasticizer additives (Ramachandran and Malhotra, 1974). The new generation of polycarboxylate-based additives was first introduced in Japan in the late 1980s to prevent loss of consistency, improve workability, and solve vibration problems in concrete. Self-compacting concrete (SCC) is produced by using polycarboxylate-based admixtures. SCC can spread homogeneously with its own weight to the desired cross-section without the need for vibration (Felekoğlu et al., 2003).

Lignosulfonates are complex, natural polymeric compounds. Together with cellulose, they form the structure of trees. Lignosulfonate is separated from cellulose by hydrolysis and sulfonation. In the optimal use of lignosulfonates as a raw material in concrete admixtures, they reduce the water requirement of concrete by 10 %. From a toxicological point of view, lignosulfonates are safe chemicals and have not caused any harm to different types of organisms in ecotoxicity studies. They are also biodegradable. Naphthalene and melamine sulfonates are used as formaldehyde condensates.

No irritant effects on the skin or eyes and no genetic code-altering properties have been identified. In ecotoxicity studies on fish, it has been found that such condensates do not have harmful effects. However, they are not biodegradable, and the products obtained by the condensation of melamine and naphthalene, due to the production processes, normally contain a small amount of formaldehyde. The necessary precautions should be taken in production to ensure that this quantity is below the limit values ​​that do not require labeling. There is a danger of release of free formaldehyde from the concrete. The release of the admixture from hardened concrete becomes important when the concrete in normal structures is in contact with drinking water and can affect the environment (State-of-the-Art Report, 2011).

Polycarboxylates are water-soluble polymers of high molecular size, branched by the attachment of carboxyl and polyether groups on the anionic carbon-carbon skeleton. As concrete admixture raw material, they reduce the water requirement of concrete by 40 %. These products have been used for many years as performance enhancers in the cosmetics and detergent industries. They are degradable in nature. In the 1990s, a new era in SCC technologies began with the synthesis of the types used in the concrete sector in Japan. The development of green environmental contributions alone or in combination with lignin occurred during this period.

The basic behavior of the polymer type is very important in the design of SCC. The most important effect expected from superplasticizers in this type of concrete is having high water-reducing properties, as well as rapid adsorption features of the additive to cement particles. The desired workability of concrete depends on the content of the structural elements where the concrete will be applied. Easier processability is preferred in precast concrete applications, while it may be desirable to have relatively long processability in self-compacting ready-mixed concrete production. Regarding the flowability and mixing time of the concrete, different polymers exhibit different behaviors related to the dispersing effect of the polymer. In order to understand the dispersion effect of superplasticizers, it is necessary to investigate the adsorption behavior and the shape of the adsorbed polymer. The form of adsorption is influenced by the molecular mass, side chain density, and anionic charge density of the polymer as well as the ionic strength of the liquid phase (Biesalski and Rühe, 2002).

Polycarboxylate-based superplasticizer additives are called comb-type superplasticizers in some works because of their macromolecular structure (Compater et al., 2003). Although their macromolecular structures are approximately the same, they perform completely differently in concrete. To understand these differences, the structures of the polymers should be reviewed. All polymers studied include carboxylate and polyoxyethylene groups. The presence of the carboxylate and polyoxyethylene groups creates ionic repulsion and steric inhibition events. Steric inhibition plays an important role in the dispersion mechanism of polymers. The dispersion effect starts with the adsorption of a macromolecule to the fine materials and there is a relationship between the number of carboxyl groups and the adsorption process. The high carboxyl group in the main chain enables rapid adsorption. In fact, this situation can be explained by basic rules of physics. Due to the carboxyl group, all polycarboxylate-based polymers are anionic and the formed cement particles are positively charged. Although all carboxyl ions are likely to be reverse ions, these ions will be replaced by the calcium ion on the cement surface in the cement paste. As a result of this interaction, the backbone of the macromolecule is adsorbed onto the surface and the side chain goes towards the water phase. This structure ensures that the cement particle is dispersed. Any attempt to increase the molarity ratio of carboxyl units on the copolymer will result in increasing the adsorption rate of the polycarboxylate-based superplasticizer on cement particles (Felekoğlu and Sarıkahya, 2007).

A modified polycarboxylate-based superplasticizer has been developed using chemicals having wetting characteristics to improve the effects of the polycarboxylate type on cement paste properties such as water reduction, consistency protection, and resistance to decomposition. This method is easier and also more economical than the search for polymers with different chemical structures. The objective here is to ensure that ionic repulsion and steric inhibition processes are continued by activating the adsorption ability of the polycarboxylate structure by allowing the auxiliary wetters to absorb particles in the cement paste structure.

Plasticizer chemical additives reduce the essential water requirement for the workability and application of concrete and provide maintained consistency of the concrete for a certain period of time. The dispersion mechanism of these additives depends on two different types of thrust forces between the cement particles. These are electrostatic and steric repulsion effects. Electrostatic repulsion occurs due to the presence of a negative charge given by carboxyl groups, while steric repulsion occurs due to long-sided polymers (Fig. 3) (Yılmaz, 2003).

Additive molecules are attracted by soft cement granules and ıjwrap around the cement during mixing. This formation increases the negative charges of the cement particles on the surface and causes electrostatic repulsion (Fig. 3a). The dispersion of a large quantity of cement granules is the result of this. This leads to a significant increase in the workability of the concrete, although the water content is low (Yılmaz, 2003).

Molecules of polycarboxylate-based additives have long-sided chains. These form a steric barrier, leading to increased ability of the cement particles to maintain their distance from each other, thus enabling an excellent dispersion effect (Fig. 3b) (Yılmaz, 2003).

The analysis of the polymeric film formed in ethylene-co-vinyl acetate-modified cement paste was observed by environmental scanning electron microscopy (ESEM) in a previous study. The image analysis of the polymeric film that formed in the prepared cement paste with ethylene-co-vinyl acetate at the rate of 20 % by weight of cement was performed using ESEM (Silva and Monteiro, 2005).

ESEM observations were made on some aspects of latex drying in another study, wherein ESEM was used to form a polymethyl methacrylate-based latex system and to examine the nonfilm drying behavior. It was possible to draw observations and conclusions on the structural development of the latex by allowing it to film initially (Islam et al., 2012).

In basalt fiber reinforced asphalt concrete, ESEM was used to analyze the microscopic properties of the basalt. As a result, it was found that the chemical bonding connections between the fiber and the asphalt binder were strong (Gao and Wu, 2018).

In a study in which the interaction of polycarboxylate ether with silica fume was investigated, the interaction of silica fume with negatively charged polycarboxylate ethers compared to one polyacrylate and two nonionic polyethylenes was explored. It was stated that the polyacrylate negatively charged the silica surface and the carboxylate groups adsorbed in a very small amount due to the mutual electrostatic repulsion (Hommer, 2009).

A new stellate polycarboxylate superplasticizer was developed in a previous study. The performance and working mechanism of the new polycarboxylate were investigated. The stellate polycarboxylate additive was compared with a comb-shaped polycarboxylate additive. It was aimed to increase the dispersing effect and improve the working efficiency with polycarboxylate structures ranging from comb-shape to stellate (Liu et al., 2017).

In a study on the characterization and working mechanism of a polycarboxylate-based superplasticizer admixture for viscosity-reduced concrete, it was found that a reduction in molecular weight was effective on the improvement of dispersion capacity and exhibited viscosity reduction effects (Qian et al., 2018).

In a study in which the interaction of superplasticizers with mineral particles was investigated, it was possible to measure the adsorption of superplasticizers to particles by fluorescence microscopy. As a conclusion, the importance of fluorescence microscopy for the superplasticizer in cementitious systems was emphasized (Arend et al., 2018).

The interaction of superplasticizer in cement systems was examined by atomic force microscopy, zeta potential, and adsorption measurements in a previous study. The work focused on steric action forces responsible for providing electrostatic and dispersion of chemical additives. As a result, the authors indicated that polycarboxylates were strongly adsorbed by positively charged materials (Ferrari et al., 2010).

In a study in which the interaction between organic latex polymers and a hydrating cement surface was investigated, the zeta potential and the amount of adsorbed polymer in cement were also examined. Zeta potential was measured by electroacoustic method. Photographs by electron microscopy confirmed that the loaded latex polymers were adsorbed (Plank and Gretz, 2008).

In research that investigated the use of ESEM in the study of cement-based materials, it was emphasized that ESEM enables the examination of cement paste in fresh form. In addition, it was stated that the structural distortions caused by drying that occurred using other microscopes were not experienced in ESEM analyses (Neubauer and Jennings, 1996).

In another study, researchers used uncontrolled burning of rice husk ash (RHA) as a partial replacement to cement in SCC. The experimental studies indicated that RHA-based SCC developed from uncontrolled burning has significant potential for use when normal strength is desired (Rahman et al., 2014).

Researchers also examined the effect of superplasticizer, RHA, and other mineral admixtures in high-performance SCC. As a result, the combination of RHA and fly ash improved not only the self-compactability but also the compressive strength (Le and Ludwig, 2016).

Researchers have examined the properties of SCC containing class F fly ash. Investigation has shown that it is possible to design SCC mixes incorporating fly ash content of up to 35 % and that SCC mixes made with fly ash reduced the rapid chloride ion penetrability (Siddique, 2011).