CHARACTERIZATION OF CEMENT-POLYMER COMPOSITES

ByMohamed M. Shoaib and El-Taher M. Hassan*

Department of Chemistry, Faculty of Science, Zagazig University, Zagazig, Egypt*Department of Chemistry, Faculty of Science, Sebha University, Sebha, Libya

ABSTRACT

Nowadays, research on cement matrix materials is focused on the inclusion of additives, admixtures and short fibers, to improve certain physical and mechanical properties of cement and concrete. Polymeric concrete admixtures play an important role in the properties and handling of building materials. It is very important to understand the role of admixtures on cement hydration in the first few hours of its contact with water and also the effect of these materials on the properties of the hardened products. The present investigation deals with the influence of different percentages of Carboxymethyl Cellulose and Polyvinyl Alcohol as cement admixture, in water-to-cement ratio (W/C), initial and final setting times, physical and chemical properties of fresh and hardening cement pastes. The electrical conductivity technique was used to reflect the physico-chemical changes though hydration progressing. Bulk density measurements, free lime contents and the chemically combined water contents were used to evaluate the main significant changes in the character of the prepared samples with each polymeric material in fresh and hardened specimens.

Key Words: Polymer, CMC, PVA, Cement, Hydration, Electrical conductivity.

1- INTRODUTION: The use of organic additives is common in cement chemistry with

keeping or improving its strength, low cost and capacity to fill almost any shape. Adherence, permeability, thermal and acoustical insulation, ductility, flexural strength, fire performance and viscous damping are some of the main research lines on cement matrix materials. Polymeric admixtures are defined as polymers used as a main ingredient effective at modifying or improving cement- based material properties [1–7].Polymeric concrete admixtures have been used to reduce the water of consistency and modify the handling and workability of cement pastes, mortars or concretes, leading to improve the mechanical properties, the resistance to welds environmental deterioration, chemical attack and moistures [8]. According to Chiocchio and Paolini [9], improvement in the fluidity of the mix depends upon many factors; e.g., the nature and concentration of and optimum time of admixtures addition.

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Some of polymeric compounds such as polymer latex, redispersible polymer powder, water-soluble polymer or liquid polymer. Among the different presentations of polymer admixtures, polymer latex is in most widespread use [10]. Floor and bridge overlays, repairing mortars and bonding ceramic tile agents and naturally treated polymer are used. Recently, new uses have been proposed in precast elements and as precast elements joining material [11-14].

Portland cement is a multi-component system; its hydration is a chemical process that, from the anhydrous material through several chemical reactions, leads to the formation of hydrates. This complex is consisting of individual chemical reactions having series thermodynamic and kinetic characters. Which depend on both chemical and physical parameters [15]. The change in the physical state of the cement paste accompanied by a change in water value and ionic concentration within the paste, this is reflect the change in the electrical conductivity of cement paste.

The electrical conductivity is an important parameter to study the hydration process of cement at early stages [16,17].The concentration and mobility of ions, as well as, the porosity and pore size distribution, which control the electrical conductivity in cement were studied [18]. The initial hydrolysis of cement and later formation of hydration products has been also studied using the conductivity techniques in many studies [19-20]. Electrical conductivity of cementitious materials with a wide range of chemical compositions has been studied during the first 24 h of hydration.. Results are tentatively explained in terms of some physical and chemical parameters. Further evidence that the electrical conductivity of hydrating cement pastes is related to the hydration mechanisms operating in these systems was obtained. The order in which the cations of inorganic admixtures (chlorides and hydroxides) were found to increase the peak and rate of development of the electrical conductivity is the same order that they have been found to increase the heat liberated upon hydration of systems containing these admixtures [21].

Polymeric concrete admixtures which acting as water reducer decrease the total porosity, the relationship between porosity and the electrical resistively in cementitous systems were studied [22]. The electrical conductivity measurements also were used as a method for measuring the effect of additive diffusitives in Portland cement pastes [23]. Several studies on the effect of admixtures on the electrical conductivity of Portland cement were also investigated [24, 25]. Xuli et al [26], studied the effect of polymer admixtures to cement on the bond strength and electrical contact receptivity between steel fiber and cement; and the degree of dispersion of latex particles in cement paste, as assessed by electrical receptivity measurement. The interactions of polymers with

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portland cement during the hydration process were investigated [27]. It was concluded that polymers and organic admixtures interact with the component of portland cement when they come in react with water.

Carboxymethyl cellulose (CMC) is an anionic water soluble polymer. It is manufactured by reacting insoluble cellulose with sodium hydroxide and chloroacetic acid during which the hydroxyl groups are etherified and converted into carboxymethyl groups. These polymers are characterized by the degree of substitution (DS) and degree of polymerization (DP). They are used as viscofiers and absorbents in food, food packaging, personal care products, cement, plaster, water-based paints, wallpaper, adhesives, detergents [28-30].

Poly Vinyl Alcohol (synonyms, vinyl alcohol polymer, PVA, ethanol homopolymer) is a water-soluble synthetic resin, prepared by the polymerization of vinyl acetate, followed by partial or complete catalyzed hydrolysis, and it is one of the few high molecular weight commercial polymers, which is water soluble, dry solid [31]. It is commercially available in a granular or powder form. The properties of polyvinyl alcohol vary according to the molecular weight of the parent poly vinyl acetate and the degree of hydrolysis. The physical characteristics of polyvinyl alcohol vary depending on the degree of polymerization and hydrolysis. Polyvinyl alcohol is classified into grades of partially and fully hydrolyzed polymers with different intended functional uses. Polyvinyl alcohol has a history of use in cosmetic, food packaging materials, pharmaceutical and medical applications [32-33]. Polyvinyl alcohol acts a good binder for the solid particles including pigments, ceramics, and cement and textile fibers. Polyvinyl alcohol is used as a binder for the stabilization of soil so as to control erosion. It can also be used as a binder in catalyst pellets, cork compositions and variety of other materials [34].

Effect of polymer admixtures to cement on the bond strength and electrical contact resistively between steel fiber and cement was studied [35-36]. 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 fiber and cement paste, in spite of the low concentration of methylcellulose compared to latex. The methylcellulose addition did not affect the contact electrical resistively between fiber and cement, whereas the latex addition increased this resistivity. Hence, for low cost and low contact resistivity, methylcellulose is preferred to latex. For a given cement paste composition, the bond strength was increased linearly with the contact resistivity.

Two different ways of adding polymers to cement composites have been described [37]. With keeping the water-to-cement ratio (W/C)

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constant to obtain a similar hydration of the cement paste. Fitting the water of consistency of the composite, was carried out by adjusting the W/C or the inclusion of polymers. It is a trial-and-error procedure, but the results are of direct practical application.

The present investigation was directed to study the effect of some polymeric materials on the physico-chemical properties of the prepared samples. Water of consistency, initial and final setting times, bulk density measurements, free lime contents and the chemically combined water contents were used as techniques to evaluate the main significant changes in the character of the prepared samples with each polymeric material in fresh and hardened specimens.

2.Materials and Experimental Techniques

The materials used in this investigation are moderate sulphate-resisting cement (Cement Company., Beni-Gazi, Libya) and water soluble polymeric materials as Carboxymethyl cellulose (CMC) which provided by El-gouf chemical company, Libya, in compared with BHD products, and polyvinyl alcohol (PVA). The chemical oxide compositions of the used cement as provided from the company are given in table (1).

Table (1): The chemical oxide compositions of the used cement:

Oxide SiO2 Al2O3 Fe2O3 CaO MgO SO3 R2O I.L. Free lime

SRC 20.63 1.54 6.88 64.62 1.62 1.21 0.27 3.01 1.09

The specific surface area is about 3000 Cm2/gm. Series of mix composition were prepared with admixing polymer by 0, 0.1, 0.2, 0.4, 0.5, 1.0, 1.5, and 2.0 % of the neat cement used named as the following table.

Table (2): The mix composition of the prepared samples:Mix No. I II III IV V VI VII VIII IV

Polymer content, % Zero 0.1 0.2 0.4 0.6 0.8 1.0 1.5 2.0

Water of consistency, initial, and final setting times were determined using VICAT apparatus [38]. The mixing of cement pastes was carried out with the standard water of consistency as describer by ASTM specifications as well as setting times [39].

The electrical conductivity measurements of the fresh cement pastes, prepared with suitable mixing water content of distilled water containing a selected polymer doses were carried [40]. The kinetics of hydration was followed by the determination of free lime as well as the chemically combined water contents. The combined water content was determined

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by the ignition loss of the dried paste on ignited weight losses minus the amount of water held by free lime and the loss of the anhydrous blend. The combined water content was calculated as Wn using the following equation:

Wn = 1W 2W

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W L

Where: Wn is the non-evaporable water, W1 is the weight of sample before ignition (g), W2 is the ignited weight of specimen (g) and L is the ignition loss of unhydrated specimen. The free water content, We, was taken as the difference between the combined water content, Wn, and the total water content, Wt, i.e.

We = Wt - Wn

Where: We is the free water content, Wt is the total water content and Wn is the chemically combined water content. The total porosity of the hardened cement pastes could be calculated from the equation:

=

where : 0.99 is the specific volume of free water in cm3/g, dp is the bulk density of the paste in g/cm3, Wt is the water content of saturated hardened paste and We is the free water content of the paste [42]. The ratio of the volume of the pores to the volume of the pastes gives the total porosity, of the hardened cement pastes.

3. RESULTS AND DISCUSION When the cement is in contact with water, Ca (OH)2 is liberated

during the hydration; this reacts with the colloidal acid hydrates to form hydrated calcium-aluminates-silicates [43]. The reactions of C3A and C4AF predominate at the early ages of hydration. The reaction of calcium silicate phases predominate from about the time of initial set onward [44]. The setting and hardening of the cement paste is due to several chemical reactions which begin as soon as the water is mixed with cement. The reactions are summarized as follows.

2(3CaO.SiO2) + 6H2O 3CaO.2SiO2.3H2O + 3Ca(OH)2

2(-2CaO.SiO2) + 4H2O 3CaO.2SiO2.3H2O + Ca(OH)2

4CaO.Al2O3.Fe2O3 + 10H2O + 2Ca(OH)2 6CaO.Al2O3.Fe2O3.12H2O

3CaO.Al2O3 + 12H2O + Ca(OH)2 3CaO.Al2O3.Ca(OH)2.12H2O

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So using water soluble polymeric are needed or must be needed to enhancement some desirable cement paste or concrete properties. Figures (1-a and 1-b) show the effect of different doses from Carboxymethyl cellulose and Polyvinyl alcohol on the water of consistency. The results show that with increasing the polymer mixing percent the amount of water required to obtain standard plastic paste increases and this attributed to the ability of polymer to hold some of mixing water molecules, it is observed that amount of water of consistency consumed by CMC are more than that of PVA for each mix composition this is due to more adsorption characteristic of cellulose based material, by means, Carboxymethyl cellulose show that more water is required to achieves given consistency that that observed with Polyvinyl alcohol, and relatively high concentration of CMC shows an increase in water/cement ratio, this is brought about by the reduction of the free water in the fresh mixture since some of the free water becomes bound by CMC molecules. So it can be recommended to use CMC as concrete polymeric materials especially in hot weather.

The result of the initial and final setting times for these mixes are graphically plotted in Figures (2-a and 2-b). Results show that with the admixed polymer to the cement pastes has a longer setting time than that with non or low percents of Carboxymethyl Cellulose and Polyvinyl alcohol samples; and also with increasing the admix content the rate of hydration of cement compounds was decreased at the first ages of the hydration reaction for all mix compositions. These results suggest that slow hydration rate due to the effect of Carboxymethyl Cellulose and Polyvinyl alcohol whose acting as retarding agents at which related to its dispersion abilities and many hydroxyl containing groups in both polymer. So, the initial and final setting times were increased, this attributed to the contribution and dispersion action at cement particles and reduces the concentration of contact points between the different cement grains. Also as a general trade the moderate sulphate resistance cement shows slow hydration rate, this due to the lower content of C3A and alkali sulphate which causes rapid setting and hydration activities. Results reflect that the polymeric materials used decrease the water of consistency and increase the initial and final setting times. In deed the both CMC or PVA are adsorbed by C3A phase for the first minutes of mixing and interfere with the early hydration of ettringite and CSH phases. The polymer adsorption depends on the amount of C3A and on the presence of soluble alkali sulphates. Formation of ettringite may accelerated or retarded by the amount of alkali sulphate in the cement.

The hydration reaction progress can be evaluated from the result of the chemically combined water content as shown in Figures(3-a and 3-b). It is concluded that the amount of the chemically combined water

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contents increases with the hydration time this mainly due to the more hydration reaction resulting more hydration products mainly silicate hydrates including the combined water contents. The combined water contents as a function of hydration rate shows that the rate of hydration in case of PVA is more than observed with CMC for all polymeric mixes.

It is known that the cement paste bulk density and total porosity gives an indication about the hydration reaction development. Figures (4-a and 4-b) shows the bulk density values of the cement pastes, it is observed that the values of cement paste bulk densities increases with the hydration time increasing for all mix composition this is attributed to that the hydration products would fill a part of the pore volume, then the total porosity decreases and the bulk density increases.

Also, it is found that the increase of polymer content, the bulk density of hardened blended cement pastes decreases while the total porosity increases. This is due to the increase of the initial water/cement ratio which plays an important role in the values of bulk density as well as total porosity. The bulk density is inversely proportional to the water of consistency and the total porosity of the hardened cement paste.

The Conductance-Time curves of CMC and PVA polymeric admixed samples as shown in Figures (5-a and 5-b), show that an initial increase in the conductivity in all curves at the first few minutes in contact with water this is mainly due to hydrolysis of paste constituents which accompanied by an increase of ionic concentration and mobility of ions specially calcium, sulphate, hydroxide and all water soluble alkali ions, which acts as charge carriers cause the initial increases in the conductivity. A slow decreasing in the conductivity were observed; this attributed to the formation of electrically insulating layer around the cement grains and also decreasing in mobile ions which readily adsorbed by the hydrated products.

Results show that with increasing the amount of polymeric admixtures, the conductivity maximum becomes more broader and was shifted to long time. This can be explained by the formation of ionic bonds between Ca2++ liberated from cement hydration and the added materials, which causes a lowering in Ca-ion concentration in the paste, who's responsible for continued hydration of cement, and/or the stability of the colloidal particles is usually due to the development of charge as a result of adsorption of ions. These particles development the same charge, repel each other and prevent agglomeration or precipitation. Hydrated cement, especially the calcium silicate hydrates are in the form of extremely small interlocking particles and in the presence of some admixtures are dispersed. Hence, colloidal chemical principles have been applied to Cement-Water-Admixture system. A slowly decreasing in electrical conductivity in most samples due to the formation gel-thin

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layer which acting as electrical double layer of adsorbed ions around cement grains. And also formation and accumulation of hydrated cement phases. This means that supersaturated solution was transformed into more dense material, therefore, losses in ions concentration and nobilities which gave the observed decrease in the conductivity. Polymers also show a decrease in the conductance reading, because of, polymer dispersions are two- phase systems, consisting of solid polymer particles in water, which lowering also the mobility of charge carrier ions.

4. CONLUSION:From the above findings it can be concluded that

1. The admixture is a chemical agent as polymer used to increase or decrease the rate of hydration, setting times and early strength development in cement or concrete.

2. Polymers interact with the component of cement when it contact with water by ionic bonding lowering the ionic solution strength.

3. An increase in the water of consistency was attained with CMC or PVA it is higher in case of CMC than that with PVA.

4. The polymeric materials prolonged the initial and final setting times as well as these times increased with both polymer content increasing.

5. It is found that the increase of polymer content, the bulk density of hardened blended cement pastes decreases while the total porosity increases.

6. The amount of the chemically combined water contents increases with the hydration time this mainly due to the more hydration reaction resulting more hydration products

7. The change in the electrical conductivity reflects the physical and chemical changes in the cement paste during the hydration and can be used to monitor setting and hardening processes.

8. Adding and increasing the polymeric materials the conductivity maximum were shifted and breaded to longer hydration times specially in with using CMC.

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Figures Capture

Figure (1-a): Water of consistency of cement paste with different doses of CMC polymer.

Figure (2-a): Initial and Final Setting Times of cement pastes with different doses of CMC.

Figure (3-a): The chemical combined water content of cement pastes with different doses of CMC polymer.

Figure (4-a): Bulk density values of cement pastes at different hydration ages with different percents of CMC polymer.

Figure (5-a): Electrical conductivity values of Cement pastes at different hydration times with different doses of CMC polymer.

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Figure (1-b): Water of consistency of cement pastes with different percents of PVA polymer

Figure (2-b): Initial and Final setting times of cement pastes with different doses of PVA polymer.

Figure (3-b): The chemical combined water content of cement pastes at different hydration ages with different PVA doses.

Figure ( 4-b): Bulk density values of cement pastes at different hydration ages with different doses of PVA polymer.

Figure ( 5-b): Electrical Conductivity values of cement pastes at different hydration ages with different doses of PVA polymer.

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