Reactions and early-age properties in in-situ polymerized gypsum-acryl amide composites

Transcription

1 Materials and Structures/Matériaux et Constructions, Vol. 29, July 1996, pp SCIENTIFIC REPORTS Reactions and early-age properties in in-situ polymerized gypsum-acryl amide composites E. Stav and A. Bentur National Building Research Institute-Faculty of Civil Engineering, Technion, Israel Institute of Technology, Haifa, Israel A B S T R A C T The development of the reactions and early-age properties of a system consisting of calcium sulfate hemihydrate and acrylamide monomer was studied. The hydration of the hemi-hydrate and the polymerization of the acrylamide were determined at various time intervals, and the expansion and rise in temperature were continuously recorded. Setting times and flow were determined using standard test methods. It was shown that acrylamide and hemi-hydrate could react simultaneously to form gypsum and polyacrylamide composite in which the reactions to form both components advanced almost until completion, for mixes containing up to 10% acrylamide. Increase in the content of acrylamide improved the flow properties of the mix, but delayed the setting. At contents of 2% acrylamide, the hemi-hydrate reaction occurred prior to the polymerization process. Above 6% acrylamide, the polymerization reaction preceded the hemi-hydrate reaction. This sequence of reactions can account for the observations that above 6% acrylamide two peaks in the heat liberation curves could be detected as well as that expansion was smaller at the higher acrylamide content, due probably to separation between gypsum crystals forming when the polymer is already present. R É S U M É On a étudié le développement des réactions et des propriétés au jeune âge d un système composé de monomères d acrylamide et de semi-hydrates de sulfate de calcium. L hydratation du semihydrate et la polymérisation de l acrylamide ont été déterminées à différents intervalles de temps, tandis que l expansion et les élévations de température étaient enregistrées de façon continue. Le temps de prise et la fluidité ont été mesurés à l aide d essais normalisés. Les résultats obtenus montrent que l acrylamide et le semi-hydrate peuvent réagir simultanément et forment un système composé de gypse et de polyacrylamides. Les réactions nécessaires à la formation de ces deux constituants sont presque complètes pour des mélanges contenant jusqu à 10% d acrylamide. Une augmentation de la quantité d acrylamide a amélioré la fluidité des mélanges, mais a retardé le temps de prise. Pour des contenus en acrylamide de 2%, le semi-hydrate a réagi avant que la polymérisation se soit produite. Pour des contenus en acrylamide supérieurs à 6%, la réaction de polymérisation a précédé celle du semi-hydrate. Cette succession de réactions expliquerait la présence des deux pics observés dans les courbes de chaleur libérée, ainsi que le fait que l expansion était plus faible pour le plus haut contenu en acrylamide. Cela est probablement dû à la séparation entre les cristaux de gypse qui se forment lorsqu un polymère est déjà présent. 1. INTRODUCTION Production of gypsum-polymer composites can be readily achieved by a process of mixing the calcium sulfate hemi-hydrate/water system with polymer emulsion (e.g. acrylic, PVA, EVA, SBR) which later on coalesces into a film [1,2], or by impregnation of the hardened calcium sulfate di-hydrate (gypsum) with monomer, followed by polymerization [3,4]. The microstructure of the hardened system with emulsion consists of a gypsum network interlaced with a tri-dimensional network of a polymer film, which coats the gypsum particles and fills part of the pores ; the setting process in this system is affected by the presence of the polymer which also causes modifications in the nature of the gypsum crystals [5,6]. The polymer in the impregnated system is filling the pore space in between the gypsum crystals, thus leading to lower porosity and higher strength. A different approach to the formation of a gypsum-polymer composite has been proposed based on an in-situ polymerization of a monomer of acrylamide which is added to the fresh hemi-hydrate/water mix and undergoes polymerization at the same time as the hemi-hydrate reacts to form gypsum [7,8]. In this system, the radical polymerization was initiated by the redox pair K 2 S 2 O 8 NaHSO 3 and could Editorial note Prof. Arnon Bentur and Dr. E. Stav are working at the Technion, Isarel Institute of Technology, Haifa, which is a RILEM Titular Member. Prof. Arnon Bentur is the RILEM Delegate in Israel. He has been appointed chairman of the Technical Committee ETC on Engineering of the interfacial Transition zone in Cementitious composites /96 RILEM 366

2 Stav, Bentur take place in water at room temperature. The present paper presents the results of a detailed study to characterize this system in terms of the sequence and extent of polymerization and the hydration reactions taking place in mixes with various contents of monomer. It also evaluates the setting and hardening characteristics as influenced by the two reactions. A future publication will deal with the microstructure and mechanical properties of the hardened composite. 2. EXPERIMENTATION 2.1 Materials The α- calcium sulfate hemi-hydrate used was an analytical grade product of the British Drug House (BDH) with 99% purity. The acrylamide monomer was a product of Fluka (purum grade) purified from chloroform [9]. The initiators were potassium persulfate (product of Merck- pro-analysis grade) and sodium bisulfite (product of Baker- analytical reagent grade). 2.2 Specimen composition, preparation and curing The specimens prepared were all mixes having water/solids ratio of The monomer and initiators were dissolved in water prior to mixing. The monomer contents were 0% (control), 2%, 6% and 10% by weight of the calcium sulfate hemi-hydrate, and the contents of the initiator pairs (K 2 S 2 O 8 NaHSO 3 ) used were the optimal ones as reported in previous publications [7,8] : 1.08% and 0.436% of K 2 S 2 O 8 and NaHSO 3, respectively, by weight of the monomer. After mixing, the slurry was molded into 25 x 25 x 120 mm-size beam specimens. The specimens were sealed and demolded after 24 hours at which time the polymerization and hydration reactions were largely completed. Thereafter, they were kept at 20 C/50% RH for 4 days and then dried in an oven at 30 C until constant weight was achieved. The workability of the fresh mix was determined by a flow test in which the spread of a slurry cast in a ring (the ring used for the setting test in accordance with ASTM 4790) was measured after lifting the ring and letting the slurry spread under its own weight. Expansion and temperature rise during the hydration process were measured continuously by a computerized system described in reference 10. Setting times were determined by two standard test methods : the knife test (DIN 1168 Part 2) and the Vicat needle test (ASTM C472). The degree of hydration and polymerization were determined periodically. Reactions were stopped by a drying treatment from methanol from which a precipitate was also obtained, consisting of hemi-hydrate, gypsum and polymerized acrylamide (the acrylamide monomer dissolves in the methanol which is then removed by filtering). The extent of the hydration reactions was than evaluated by means of the DTA test [10], and the degree of polymerization was evaluated on the basis of the determination of the polymer content in the precipitate [7,8]. The beam specimens were tested for flexural strength and equivalent compressive strength, using the two halves of the beam. The results were based on the average of at least six specimens. The broken fragments were also used to determine both the pore structure by mercury porosimetry and the microstructure by SEM. The results of these characteristics of the hardened composite will be the subject of a future publication. 3. RESULTS The effect of acrylamide (A.Amide) content on the flow properties of the fresh system as measured by the spread values is shown in Fig.1. Increase in the monomer content is accompanied by monotonous increase in the flow properties as might be expected from an organic material acting as a surface active agent. Examples of curves of degree of reaction (hydration and polymerization), resistance to the Vicat needle penetration, development of temperature and expansion of the systems with and without A.Amide are shown in Figs. 2 and 3. The time of initial setting as determined by the knife test is superimposed as a triangular point on the Vicat needle resistance curve. It was seen that in all the systems, this initial setting value coincided with the time at which the hydration reaction initiated. The characteristic values of each system (initial setting time, expansion, temperature rise at peak and peak time) determined from curves, such as those in Figs. 2 and 3, will be presented later on as a function of the A.Amide content. The initial setting times determined by the knife and Vicat test methods were delayed in the presence of monomer, with the extent of the delay 2.3 Tests Fig. 1 Effect of A.Amide content on the workability of the fresh mix as estimated by spread values. 367

3 Materials and Structures/Matériaux et Constructions, Vol. 29, July 1996 Fig. 2 Hemi-hydrate reaction, resistance to Vicat needle penetration, heat liberation and expansion curves for hemihydrate system. Fig. 3 Hemi-hydrate and monomer reaction, resistance to Vicat needle penetration, heat liberation and expansion curves for hemi-hydrate-10% A.Amide system. Fig. 4 Effect of A.Amide content on the initial setting times determined by the knife and Vicat tests. increasing with increases in monomer content (Fig.4). The temperature curves show a single peak in the systems without A.Amide (Fig.2) and with 2% A.Amide, and two peaks on the higher A.Amide content curves (Fig.3). Maximum expansion decreased considerably with increases in A.Amide content, from a value of about 0.45% in the system without A.Amide to about 0.05% in the system with 10% A.Amide (Fig. 5). Fig. 5 Effect of A.Amide content on expansion after completion of tests (60 ). 4. DISCUSSION The physical behavior of the hemi-hydrate-a.amide systems changed considerably with increases in A.Amide content. In order to understand the causes of these differences and their significance, one must first analyze the sequence of the hydration and polymerization reactions (Fig.6). In the hydration of the hemi-hydrate, four 368

4 Stav, Bentur Fig. 6 Hydration and polymeric reactions in systems with different contents of A.Amide. Fig. 7 Hydration curve of hemi-hydrate showing four stages of reaction. stages can be identified depending on the rate of reaction (Fig.7), and each of them can be accounted for by proposing a different mechanism of reaction [10] : Stage I Dormant period during which there is a gradual build-up of ions in the solution as the hemihydrate dissolves, but the concentration is too low for the nucleation and formation of gypsum to occur. Stage II Formation of gypsum begins but the growth of the crystals is at a slow rate. It was shown that the initial setting as measured by the knife test coincided with the beginning of this stage [10]. Stage III The rate of gypsum formation is accelerated when the size of the gypsum crystals growing slowly in stage II has become larger than a critical value, thus setting the conditions for their fast growth. This stage begins when the degree of reaction is about 10%, and it coincides with the time of initial setting as determined by the Vicat needle test. The rapid reaction at this stage is accompanied by a build-up of temperature which peaks at the time when this rapid stage terminates. Stage IV When stage III reaches a high degree of reaction near 85%, the growth rate is slowed and levels off, since the gypsum crystal network is sufficiently dense to reduce the mobility of ions, and the process becomes diffusion-controlled. Initially, during the second stage, the reaction is accompanied by a reduction in the volume of the constituents, and since the system has no rigidity, shrinkage is 369

5 Materials and Structures/Matériaux et Constructions, Vol. 29, July 1996 (a) Fig. 8 Effect of A.Amide content on the intensity (a) and time (b) of the heat liberation peak characteristic of the hemi-hydrate and A.Amide reactions. (b) obtained (expansion curve in Fig. 2). In the third stage, the system is already set, and the growth of the needle-like gypsum crystals leads to expansion which tends to slow and level off as the reaction rates slow down in stage IV. This sequence of events and reaction rates is affected by the presence of A.Amide; its influence should be addressed on the basis of both its effect on the reaction of the hemi-hydrate and the effect of the presence of a polymeric phase alongside the gypsum crystal network. The most apparent influence of the A.Amide is the slowing of the hydration reaction of the hemi-hydrate, which shows up as an increase in the dormant period and a reduction in the rates afterwards (Fig. 6). This phenomenon accounts for the delay in the setting times (Fig. 4). Two possible mechanisms can lead to this retardation: adsorption of monomers on the surface of the hemihydrate particles and perhaps on the nucleating sites, and the presence of a polymer network which slows the diffusion of ions during the rapid growth stage of the gypsum crystals. Observations of the sequence of reactions in Fig. 6 indicate that with 2% A.Amide, the rapid stage of the hemi-hydrate reaction has almost been completed before the polymeric reaction has started, thereby suggesting that in this case the retardation was probably largely due to the adsorption of monomers. Adsorption of monomers, which are surface active agents, can account for the improved flow properties reported in Fig.1. At higher A.Amide contents of 6 and 10%, the polymerization reaction starts prior to the termination of the dormant period of the hemi-hydrate system. This may be due to two interrelated influences : higher content of monomers which lengthens the dormant period of the hemi-hydrate, and acceleration of the polymerization process which can be more readily initiated when the concentration of the monomer is higher. Thus, the hemi-hydrate reaction may be slowed to a greater extent by a combination of monomer adsorption and the presence of a polymer network, where the latter has an influence on slowing down stages II and III of the hemi-hydrate reaction and preventing completion of hydration (Fig.6). On the basis of this sequence of events, an explanation can be given to account for the temperature and expansion curves. From comparison of the temperature and reaction curves, it is possible to resolve the temperature peaks in terms of the reactions that generated them. At the low A.Amide-content systems, 0 and 2%, there is a single temperature peak which may be related to the reaction of the hemi-hydrate whose rapid stage III termination time coincides with the time of occurrence of the temperature peak (Fig. 2). No separate peak is observed for the polymerization of the A.Amide in the 2% system, as it may be overlapping with the descending branch of the hemi-hydrate peak and the low content of the monomer is perhaps too small to provide even a shoulder on the curve. In the systems with 6 and 10% A.Amide, the termination of stage III of the hemi-hydrate reaction coincides with the time of occurrence of the second temperature peak (Fig. 3), suggesting that it represents the hemi-hydrate reaction. The first peak in these systems is apparently related to the polymerization reaction, and it occurs at a time when considerable polymerization takes place with only a much smaller degree of reaction for the hemi-hydrate. Although this resolution is not ideal, as there is overlap between the two chemical processes, one may use these peaks as an estimate for the intensity and timing of the two reactions, as shown in Fig. 8. The reduction in intensity and delay in time of the peak characteristic for the hemi-hydrate reaction corresponds qualitatively to the influences observed in the degree of hydration curves (Fig. 6). Similar relations can be deduced for the peak characteristic for the polymerization reaction. The slightly higher peak temperature for the 2% A.Amide system in Fig. 8a may be due to some contribution of the A.Amide polymerization. The marked reduction in expansion observed in Fig. 5, in particular at the 6 and 10% A.Amide-content levels, can be accounted for on the basis of the sequence of events discussed previously : at these A.Amide contents, much of the hemi-hydrate reaction occurs 370

6 Stav, Bentur advanced almost to completion. Thus, a composite of gypsum and polymer could be produced by in-situ polymerization. The properties of the hardened composite will be reported in a future publication. 5. CONCLUSIONS Fig. 9 Effect of A.Amide content on the degree of hemihydrate and A.Amide reactions needed to obtain initial setting as measured by the Vicat needle. when considerable polymer has been formed, and it acts to restrain the expansion of the gypsum crystals. In view of these considerations, there is a need to assess the contribution to the rigidity of the structure being formed as a function of the two phases. When considering the initial setting by the Vicat test, it seems that in the presence of the polymer phase the degree of hemi-hydrate reaction needed to obtain the initial setting is not reduced and even increases for the system with 2 and 6% A.Amide (Fig. 9). This suggests that the presence of the polymer phase is not contributing to the initial rigidity, and perhaps it is even detrimental. Possible explanations are that the polymer present separates between the gypsum crystals which cannot interact with each other readily, or that in its presence the size and morphology of the gypsum crystals are changed, both of which cause the need for more crystals to be formed in order to generate an effective tridimensional gypsum crystal structure required for providing the rigidity essential for initial setting. This ties in with the explanation for the reduction in expansion on increases of A.Amide content. It is of interest to note the effect of the A.Amide content on the shape of the resistance to penetration of the Vicat needle (Figs. 2 and 3), which shows its slope becoming milder, and the time span between initial and final setting longer. This may reflect both the slower rate of hemi-hydrate reaction and the greater difficulty it has in generating a rigid structure during the setting process. The latter influence is a hypothesis based on indirect evidence like that of Fig. 9. It needs verification by systematic microscopic observations of the structure developed during the setting process. As a conclusion to this discussion, attention should be drawn to the fact that under the conditions studied herein, it was possible to obtain both reactions, the inorganic and the organic, to occur during the same time range, and although they influenced each other, they could be 1. A.Amide and hemi-hydrate could react simultaneously to form a gypsum and polyacrylamide composite in which the reactions to form both components advanced almost until completion, for mixes containing up to 10% A.Amide. 2. Increases in the content of A.Amide improved the flow properties of the mix, but delayed the setting. 3. At contents of 2% A.Amide, the hemi-hydrate reaction occurred prior to the polymerization process. Above 6% A.Amide, the polymerization reaction preceded the hemi-hydrate reaction. 4. The sequence of reactions described in Conclusion Point 3 can account for the observations that above 6% A.Amide two peaks in the heat liberation curves could be detected ; also expansion was smaller at the higher A.Amide content, due probably to separation between gypsum crystals forming when polymer is already present. REFERENCES [1] Zakaria, M.R., Johnston,W.M., Reisbisck, M.H., and Campagni, W.V., The effects of a liquid dispersing agent and a microcrystalline additive on the physical properties of type IV gypsum, J. Prosthetic Dentistry, 60 (5) (1988) [2] Sherr, E.A. and Roshel, J., Plaster compositions, I&Ec Product Research and Development, 8 (2) (1969) [3] Sand, M.E.A., Combe, E.C. and Grant, A.A., Hardening model and die materials by epoxy resins, J. Dentistry, 8 (2) (1969) [4] Whittaker, J. and Jacobson, P.H., Improvement in the method for the impregnation of gypsum casts, J. Materials Science, 13 (1978) [5] Smith, B.R. and Alexander, A.E., The effect of additives on the process of crystallization, J.Colloid and Interface Science, 34 (1) (1970) 81. [6] Kuntze, R.A., Retardation of crystallization of calcium sulfate dihydrate, Nature, 23 (211) (1966) 406. [7] Stav, E., Kohn, D.H. and Bentur, A., Polymerization of acrylamide in the presence of calcium sulfate dihydrate and calcium sulfate hemihydrate, J. Applied Polymer Science, 45 (12) (1992) [8] Stav, E., Bentur, A. and Kohn, D.H., Polymerization of acrylamide in the presence of calcium sulfate hemi-hydrate : compressive strength and microstructure of the composite, Advances in Cement Research, 4 (14) (1991/1992) [9] Thomas, W.M., Acrylamide Polymers pp in Encyclopedia of Polymer Science and Technology, Vol. 1, Wiley - Interscience, New York, (1964). [10] Stav, E. and Bentur, A., Characterization of hydration process of calcium sulfate hemi-hydrate by simultaneous evaluation of chemical and physical parameters, accepted for publication, Advances in Cement Research. 371