Physico-chemical characteristics of some polymer cement composites

Materials Chemistry and Physics 71 (2001) 76–83

Physico-chemical characteristics of some polymer cement composites M. Heikal a,∗ , I. Aiad b , M.M. Shoaib c , H. El-Didamony c a

Institute of Efficient Productivity, Zagazig University, Zagazig, Egypt b Egyptian Petroleum Research Institute, Cairo, Egypt c Faculty of Science, Zagazig University, Zagazig, Egypt

Received 30 January 2001; received in revised form 10 February 2001; accepted 6 March 2001

Abstract The electrical conductivity of cement pastes can give an indication of the initial hydration of the cement pastes and early formation of products. In this study, sulphate-resisting cement (SRC) pastes were prepared with different doses of two synthesised admixtures, namely phenol formaldehyde sulphonate (PhFS) and melamine formaldehyde sulphonate (MFS). This work is aimed to evaluate the effect of PhFS and MFS on the hydration reaction of cement pastes during the first 24 h by determining the initial, final setting times and the electrical conductivity changes, as well as the effect of these polymers on the hydration progressing up to 90 days by determining the chemically combined water content and gel/space ratio of each paste at different intervals of time (1, 3, 7, 28 and 90 days). © 2001 Elsevier Science B.V. All rights reserved. Keywords: Sulphate-resisting cement (SRC); Setting time; Electrical conductivity; Polymer and gel/space ratio

1. Introduction Portland cement is a multi-component system, its hydration is a rather complex process consisting of individual chemical reactions having series of thermodynamic and kinetic characters, which depend on both chemical and physical parameters. Hydration is a chemical process that from the anhydrous material through several chemical reactions leads to the formation of hydrates. The reaction of cement with water proceeds at different rates for the various mineral phases and involves both hydrolysis and hydration process, then its four major phases start to hydrate. Some phases such as calcium aluminoferrite (C4 AF) and tricalcium aluminate (C3 A) are the hydrates at very early stages of hydration. The silicate phases such as ␤-dicalcium silicate (␤-C2 S) and tricalcium silicate (C3 S) need some time for initial setting [1]. Abbreviations in cement chemistry : C = CaO, S = SiO2 , A = Al2 O3 , F = Fe2 O3 The production of Portland SRC is expected to increase because of the attack of sulphates in soil and ground water. SRC is exposed to severe sulphate action when the construction is exposed to aggressive media such as sulphates, ∗

Corresponding author.

seawater and/or ground water. Low C3 A and comparatively high C4 AF contents of SRC means that it has high strength but because ␤-C2 S represents a high value, the early strength is low. The heat developed by SRC is not much higher than the heat of Portland cement. Therefore, it could be argued that it is theoretically an ideal cement for massive structures exposed to seawater or ground water. Superplasticizers are now widely used in the production of concrete with excellent workability, for easy placement without reduction in cement content and strength. These admixtures are extremely effective for dispersing cement particles in water. The dispersion mechanism has been described in terms of electrostatic repulsive forces between the cement particles followed by adsorption of charged superplasticizer molecules. Several reports have been concerned with the improvement of the strength and development of Portland cement using admixtures [2–4]. Concrete admixtures or superplasticizers have been used to reduce the water of consistency and to improve the workability of cement pastes and consequently concrete, leading to improvement in mechanical properties and resistance towards environmental deterioration, chemical attack and moistures. Polymeric concrete admixtures which act as water reducer decrease the total porosity, therefore the relationship between porosity and electrical resistivity in cementitious systems were studied [5]. Electrical conductivity is an important parameter to

0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 3 4 6 - 7

M. Heikal et al. / Materials Chemistry and Physics 71 (2001) 76–83 Table 1 Elemental analysis of MFS and PhFS Polymer C (%)

H (%)

Table 2 FT-IR characteristic bonds of the synthesised polymers (cm−1 ) N (%)

S (%)

Theo. Found Theo. Found Theo. Found Theo. Found MFS PhFS

23.35 20.20 43.40 38.10

3.17 3.15

3.70 3.30

77

29.58 23.6 – –

10.81 9.00 13.00 10.70

study the hydration process of cement pastes at early stages [6,7]. Electrical conductivity was used as a method for measuring the effect of additives on effective diffusitives in Portland cement pastes described [8]. Several studies on the effect of admixtures on the electrical behaviour of Portland cement were also investigated [9,10]. Hansson [11] discussed the factors which control electrical conduction in cement based materials, such as concentration and mobility of ions in the pore solution, porosity and pore size distribution. A high water/cement ratio leads to a greater ability to conduct electricity. Morsy [12] studied the effect of temperature on the electrical conductivity of blended cement pastes. The previous study was directed to investigate the effect of polycarboxylate additives on the electrical conductivity of fresh cement pastes at the first few hours of the hydration [13]. The aim of the present investigation is to study the influence of different doses of MFS and PhFS as synthetic polymers on the hydration characteristics of SRC at the first few hours, as measured by the determination of water of consistency, initial and final setting times and the electrical conductivity changes of fresh cement pastes. The hydration progressing up to 90 days was followed by determining the

Polymer

OH and NH

C–O–C

C=N

C =C

S=O

C–H

C–N

MFS PhFS

3432–3319 3445–3408

1043 1045

1573 –

– 1609

1357 1182

2986 –

1043 1049

chemically combined water contents and gel/space ratio at interval times 1, 3, 7, 28 and 90 days. 2. Experimental Materials used in this investigation are sulphate-resisting cement (SRC) provided from Helwan Cement, Helwan, Egypt. The chemical oxide compositions as determined by using X-ray fluorescence and chemical analysis was found to be: CaO, 63.83%; SiO2 , 21.63%; Al2 O3 , 2.02%; Fe2 O3 , 4.48%; MgO, 1.92%; SO3 , 1.61% and the loss of ignition was 2.2%. The specific surface area determined by the Blaine air-permeability method was found to be 3300 cm2 g−1 . Laboratory synthetic water-soluble polymeric materials, namely melamine formaldehyde sulphonate condensate (MFS) and phenol formaldehyde sulphonate (PhFS) were prepared according to the four steps methods [14,15]. In this method the polymer formed were through (a) addition reaction; (b) sulphonation reaction; (c) condensation reaction; (d) rearrangement reaction. The elemental and FT-IR spectroscopic analyses were performed to investigate the structure of the synthesised polymers as shown in Tables 1 and 2. The structure of these prepared polymers was

Fig. 1. The FT-IR spectrum of MFS and PhFS polymers.

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confirmed by Fourier Transformation Infrared ATI Mattson Gensis spectra of these polymers (Fig. 1). The average molecular weight (MW) of MFS and PhFS were 1 040 000 and 33 000 respectively. Effect of different percentage of these polymers on the water of consistency, initial and final setting times were determined using Vicat apparatus [16,17]. The electrical conductivity measurements of the fresh cement pastes, prepared with water of consistency of distill water containing a selected polymer dose, were made using cylindrical plastic container with two stainless steel electrodes [18]. The cell has been designed to give good contact with cement paste during the hydration, stiffening, and final rigidity and overcome changes in the paste volume during hydration. The electrodes were polished before each experiment. Measurements commenced exactly 3 min after the first contact with water. Temperature of the paste was kept constant at 27◦ C during the experiment period. The conductivity of the paste was measured using digital conductometer “Consort-K120” with an accuracy of 0.01 ␮S. Conductivity (i.e. specific conductance) value was calculated from the conductance by the known dimensions of the cell. The hydration reaction was stopped by employing alcohol–ether method [19]. The chemically combined water contents and gel/space ratio were determined [20].

3. Results and discussion 3.1. Water of consistency, initial and final setting times Water of consistency is the gauging water required to bring the cement paste to a standard consistency. The setting terms are used to describe the stiffening of the cement paste. At a certain stage, however, the paste begins to stiffen to such a degree that, although still soft, it becomes unwork-

able. This is known as “initial set” and the time required for the paste to reach this stage as the “initial setting time”. The setting period follows, in which the paste continues to stiffen until a stage is reached, when it may be regarded as a rigid solid [20]. This is known as the “final set” and the time required for the paste to reach this stage as the “final setting time”. The use of water reducer or superplasticizers are needed or must be needed to eliminate defects from high per cent of mixing of water during the preparation of cement pastes, mortars or concrete. The high water–cement ratio produces a relatively porous material, which directly affects mechanical properties, as well as the chemical performances. Figs. 2–4 shows the effect of different doses (0.00, 0.25, 0.50, 0.75 and 1.00%) from MFS and PhFS on the water of consistency, initial and final setting times. Results reflect that the polymeric materials decrease the water of consistency for all samples. Superplasticizer admixtures are effective in breaking the cement particle networks and dispersing them, preventing premature linkage and minimizing the amount of water required to suspend the particles and render the mix workable. The admixtures control the forces between particles and increase fluidity of the mix. As a result of this, less water is required to achieve a given consistency with MFS and PhFS. Generally, these materials act as a water reducer to maintain good consistency. These polymeric materials are adsorbed on cement particles, forming a good ionic repulsion raised from the negative charge of sulphonate groups, consequently increasing its fluidity. The water reduction due to addition of 1.0% MFS and PhFS is 42.90 and 34.70%, respectively. The MFS is a more powerful superplasticizer than PhFS. Cement pastes with 0.25 and 0.5% polymer shows a relative decrease in the initial and final setting time than that of cement pastes free from polymer and this may be attributed to water reduction effect of the used materials. Whereas, in cement paste with high per cent of polymer 0.75

Fig. 2. Water of consistency of cement pastes with different doses of polymer.

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Fig. 3. Initial and final setting times of cement pastes with different doses of MFS polymer.

and 1.00%, the initial and final setting times were elongated. This may be attributed to the dispersion action on flocculated cement particles and reduction in the concentration of contact points between different grains. Also, SRC shows slow hydration rate. This is due to the lower content of C3 A, which causes rapid setting and hydration. 3.2. Electrical conductivity Conductance–time curves are represented in Figs. 5 and 6. As soon as the cement comes in contact with water, its four major phases start to hydrate. The reactions of tricalcium

aluminate (C3 A) and ferrite phases (C4 AF) were predominant at very early stages of hydration. Ettringite is the usual product in the early stages of hydration. The reaction of the calcium silicate phases (C3 S and ␤-C2 S) was predominant from about the time of initial set onward. Two conductivity peaks were obtained during the initial hardening of cement pastes. The first conductivity peak takes place within 0.5–1 h. The first maximum was due to the hydrolysis of cement pastes. The ions produced, such as Ca2+ , OH− , SO4 2− and alkali ions, act as charge carriers, leading to rapid increase in the conductivity. Once the concentration of these ions in the solution becomes very high, ionic association starts

Fig. 4. Initial and final setting times of cement pastes with different doses of PhFS.

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Fig. 5. Conductogram of cement pastes with different doses of MFS.

taking place and as a result the concentration of ions starts decreasing. This occurs during the induction period and the conductivity decreases. During this period, some of the ions get adsorbed or precipitated with the formation of calcium silicate hydrate (C–S–H) and ettringite, and also due to the formation of electrical insulating layer around the cement grains leading to the decrease in the mobility of ions, which is readily adsorbed by the hydration products. So the electrical conductivity decreases again. The second maximum on the conductogram shows a very flat maximum, which appears at the hydration times 6–7 h. The second maximum peak is due to an increase in the number

of ions and internal pressure development, around the cement grains, due to ettringite-monosulphate transformation [21]. In the presence of the polymer, it appears that only a few number of Ca2+ ions go into the solution and it does not become supersaturated with respect to Ca(OH)2 . The dissolution of Ca2+ ions increases with time and accordingly increases with conductivity. The increase continues until the Ca2+ ions becomes saturated with respect to Ca(OH)2 . As the amount of admixture increases, the conductivity maximum becomes more broad and shifted upon giving longer time.

Fig. 6. Conductogram of cement pastes with different doses of PhFS.

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Fig. 7. Effect of MFS polymer on the chemically combined water of the hydrated cement pastes.

MFS and PhFS are anionic surfactants having negative charges. From above, it is well known that when cement is in contact with water, Ca2+ ions go into the solution and then adsorb at the surface becoming positively charged. The polymer (superplasticizer) in the solution gets adsorbed over the cement surface and blocks the hydration. Since, the superplasticizer has a depressive effect, it increases the workability of the paste, and during this, the solution diffuses inside the cement through the polymer coating. This causes an increase in the osmotic pressure in the polymer–paste interface, leading to the rupturing of the coating and the hydration is accelerated.

3.3. Non-evaporable water content Non-evaporable water content (chemically combined water) Wn of cement pastes was used as a measure of the degree of hydration [22]. Data of the combined water contents are graphically represented as a function of curing time in Figs. 7 and 8. It is clear that the chemically combined water content increases gradually with curing time for all hardened cement pastes. This is due to the progress of the hydration with curing time and accumulation of hydration products. Figs. 7 and 8 also show that 0.5% gives the higher rate of hydration with MFS and PhFS, especially at early ages, but

Fig. 8. Effect of PhFS polymer on the chemically combined water of the hydrated cement pastes.

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Fig. 9. Effect of MFS polymer on the gel/space ratio.

with the increase in the polymer content from 0.75 to 1.00%, a decrease in the hydration rate is observed indicating its retarding effect. It was observed that PhFS has an enhancement effect slightly more than MFS. As the curing time increases from 3 to 90 days, the chemically combined water content increases with increase in the polymer content. 3.4. Gel/space ratio Gel/space ratio is the ratio of the volume of cement hydrates to the sum of the volumes of cement hydrates and the capillary pores. The volume of the hydration products

per g of cement equals 0.490 cm3 ; therefore the volume of gel equals 0.490 + 0.198 = 0.688 cm3 . Accordingly, gel porosity is 0.198 × 100/0.688 ∼ = 28%, where the pore volume per one gram of cement is 0.198 cm3 g−1 [20]. The ratio of the gel volume to that of the anhydrous cement will be 0.688/0.32 ∼ = 2.2. In other words, the volume of gel is 2.2 times the original volume of the cement. This increase in volume implies that paste porosity decreases as the hydration proceeds. Figs. 9 and 10 show the gel/space ratio of the cement paste admixed with different doses of MFS and PhFS. Data showed that the gel/space ratio increases with curing time and which is mainly due to the increase of the

Fig. 10. Effect of PhFS polymer on the gel/space ratio.

M. Heikal et al. / Materials Chemistry and Physics 71 (2001) 76–83

amount of hydration products. The gel/space ratio increases with increasing percent of the polymer. This is attributed to the effect of these polymers on the reduction of the total water of consistency, which is accompanied by lowering the total porosity. Relatively, MFS gives a higher gel/space ratio values than that with PhFS. These results are in a good agreement with water of consistency values.

4. Conclusion 1. MFS and PhFS act as a water reducer admixture and also they have a retarding property by decreasing the initial setting time and delaying the final setting. 2. MFS is a better superplasticizer than PhFS. 3. The change in the electrical conductivity reflects the physico-chemical changes in the cement paste and it can be used to monitor setting and hardening processes. 4. Significant changes in electrical conductivity occur when the paste gains its rigidity. 5. On increasing the amount of polymeric materials, the conductivity maximum was found to have shifted to longer hydration times. References [1] P.C. Hewlett, Lea’s Chemistry of Cement and Concrete, 4th Edition, Edward Arnold, London, 1998. [2] V.S. Ramachandran, Concrete admixtures: Properties, Science and Technology, NOYES Publications, Park Kidge, NJ, USA, 1984. [3] A. Larbi, J.M. Bijen, Interaction of polymers with Portland cement during hydration: a study of the chemistry of the pore solution, Cem. Concr. Res. 20 (1990) 139. [4] S. Chandra, P. Flodin, Interaction of polymers and organic admixtures of Portland cement hydration, Cem. Concr. Res. 17 (1987) 875. [5] P.J. Tuimdajski, A.S. Schumacher, S. Perron, P. Gu, J.J. Beaudoin, On the relationship between porosity and electrical resistivity in cementitious systems, Cem. Concr. Res. 26 (4) (1996) 539.

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