Mineralogical study of polymer modified mortar with silica fume

Experimental investigation on the effects of styrene acrylic polymer and silica fume on the mineralogical composition of pastes of high-early-strength portland cement after 28 days of casting are presented in this paper. Thermogravimetry and derivative thermogravimetry were used to study the interaction between polymers and cements, and the extent of pozzolanic reaction of mortars with silica fume. Differential scanning calorimetry and X-ray diffraction were also used to investigate the cement hydration according to the additions. The results showed that the addition of silica fume and polymer reduces the portlandite formation due to delaying of portland cement hydration and pozzolanic reaction.

Construction and Building MATERIALS Construction and Building Materials 20 (2006) 882–887 www.elsevier.com/locate/conbuildmat Mineralogical study of polymer modified mortar with silica fume Alessandra E.F.de.S Almeida *, Eduvaldo P Sichieri School of Engineering of Sa˜o Carlos, University of Sa˜o Paulo, Av Trabalhador Sa˜o Carlense, 400 - CEP: 13566-590, Sa˜o Carlos, Sa˜o Paulo, Brazil Received 29 September 2004; received in revised form 16 June 2005; accepted 30 June 2005 Available online September 2005 Abstract Experimental investigation on the effects of styrene acrylic polymer and silica fume on the mineralogical composition of pastes of high-early-strength portland cement after 28 days of casting are presented in this paper Thermogravimetry and derivative thermogravimetry were used to study the interaction between polymers and cements, and the extent of pozzolanic reaction of mortars with silica fume Differential scanning calorimetry and X-ray diffraction were also used to investigate the cement hydration according to the additions The results showed that the addition of silica fume and polymer reduces the portlandite formation due to delaying of portland cement hydration and pozzolanic reaction Ó 2005 Elsevier Ltd All rights reserved Keywords: Silica fume; Polymer; Thermal analysis; X-ray diffraction Introduction Polymers have been used for improving mechanical properties, adhesion with substrates, or waterproofing properties of mortars and concretes Pozzolanic materials can partially substitute Portland cement in order to enhance the properties of concrete and mortars such as durability and mechanical properties Polymer modified mortars are known as a popular construction material because of their excellent performance The fundamentals about polymer modification for cement mortar and concrete have been studied for the past 80 years or more The cement mortar and concrete made by mixing with the polymer-based admixtures are called polymer-modified mortar (PMM) and polymer-modified concrete (PMC), respectively [1,2] Polymeric admixture, or cement modifier, is defined as an admixture which consists of a polymeric compound that acts as a main ingredient at modifying or * Corresponding author Tel.: +55 16 33 64 5788 E-mail addresses: aefsouza@ig.com.br, aefsouza@sc.usp.br (A.E.F.de.S Almeida) 0950-0618/$ - see front matter Ó 2005 Elsevier Ltd All rights reserved doi:10.1016/j.conbuildmat.2005.06.029 improving the properties such as strength, deformation, adhesion, waterproofing and durability of mortars and concretes Polymer latex is a colloidal dispersion of small polymer particles in water, which is obtained by the emulsion polymerization of monomers with emulsifiers [3,4] The physical properties of a latex-modified cement mortar are affected by those same variables that can affect unmodified Portland cement mortars and concretes, and by polymer typical properties, such as solids content, pH, density and minimum film formation temperature [3,4] Acrylic polymers used with Portland cement are composed mainly of polyacrylates and polymethacrylates, resulting from the polymerization of derivatives of acrylic acids [4] The literature agrees that the properties of polymermodified mortar and concrete depend significantly on the polymer content or polymer–cement ratio, that is, the mass ratio of the amount of polymer solids in a polymer-based admixture to the amount of cement in a polymer-modified mortar or concrete [1,2,5] Silica fume or microsilica is an industrial by-product from electric arc furnace producing silicon and A.E.F.de.S Almeida, E.P Sichieri / Construction and Building Materials 20 (2006) 882–887 ferrosilicon alloys It has been widely used as a concrete and mortar mixture, mainly to improve the mechanical properties and reduce porosity, due to pozzolanic activity [6,7] Finely ground material such as silica fume can increase the water required for a given degree of workability at low water–cement ratio, thus water reducing admixture (or superplasticizer) is often used to improve the workability of mortars with silica fume [6] The correct combination of silica fume, superplasticizer and polymeric emulsions may have the synergistic effects of these three admixtures, resulting in good performance of construction material to many applications, for example, a high quality repairing and overlaying materials for the application of the concrete structures [8,9] In the previous work, the author studied the effects of silica fume and acrylic polymer on the mortar properties, specifically to fix porcelain tiles [10] This work showed the improvement of the adherence strength of mortars using such additions For this reason, aim of this work is to study the influence of such admixtures concerning the hydration of Portland cement by means of the mineralogical study of the pastes with the same composition used in the mentioned work [10] The interaction between polymers and cement portland can be investigated through several techniques such as thermal analysis and X-ray diffraction Thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) are considered important tools for evaluating the nature of hydrated products according to different stages of cement hydration, in addition to quantifying the different phases [11–14] When cement is hydrated, its main components are transformed into hydration products, mainly calcium silicate hydrate (C–S–H gel) and portlandite The hydration can be evaluated by measuring the mass loss of hydrated compounds up to 900 °C The following peaks and temperature ranges have been studied when hydrated cement is heated in thermobalance and they are interpreted as described below [12,13]:  $100 °C: dehydration of pore water,  100–300 °C: different stages of C–S–H dehydration,  $500 °C: dehydroxylation of Ca(OH)2,  $700 °C: decarbonation of CaCO3 This study reports the results of investigations in which methods of thermal analysis, TG, DTG and DSC, were applied to investigate the effects of polymer modification on the process of hydration of portland cement by estimating Ca(OH)2 content and calcium hydrate content X-ray diffraction was carried out to study the hydrate products of cement [15] 883 Materials 2.1 Cement and silica fume The mortars were prepared using high-early-strength Portland cement (CPV-ARI Plus according to NBR 5733; and Type III cement according to ASTM C150), chemical and physical properties of cement are shown in Tables and 2, respectively, according to the manufacturer The silica fume used was marketed by Microssilica Brazil, with specific surface area of 27.74 m2/g obtained by BET test, and 94.3% SiO2 content Table shows the chemical properties of the silica fume, according to the manufacturer 2.2 Superplasticizer The superplasticizer marketed by MBT Brazil I.C was used, presenting chemical base sulfonated melamine, liquid aspect, density 1.11 g/cm3 (± 0.02), pH: 8.5 ± 16.49% solids content 2.3 Polymer latex  Aqueous dispersion of styrene-acrylate copolymer with 49–51% total solids content; viscosity Brookfield (RVT 415 °C): 1000–2000 mPas; density: 1.02 g/cm3; pH value: 4.5–6.5  Minimum film-forming temperature: 20 °C  Mean size of particles: 0.1 lm  Film properties: clear and transparent  Stability to ageing: good Table The chemical composition of cement Chemical composition CPV-ARI-Plus % Loss on ignition SiO2 Al2O3 Fe2O3 CaO total MgO SO3 Na2O K2O CO2 RI CaO 3.10 18.99 4.32 3.00 64.7 0.68 3.01 0.03 0.85 1.81 0.26 1.63 Table Physical properties of cement Setting time (min) Initial 150.78 Blaine surface area (m2/kg) Final 226.25 467.9 Compressive strength (MPa) NBR 7215 day days days 28 days 27.87 43.57 48.69 56.16 884 A.E.F.de.S Almeida, E.P Sichieri / Construction and Building Materials 20 (2006) 882–887 Table The chemical compositions of silica fume Chemical composition % SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O 94.3 0.09 0.10 0.30 0.43 – 0.83 0.27 The experimental conditions were: N2 gas dynamic atmosphere (40 ml minÀ1); heating rate (10 °C minÀ1); platinum top-opened crucible The samples were heated in the range À25 to 500 °C at a constant rate XRD was used to identify the polycrystalline phases of cement and hardened cement paste by means of the recognition of the X-ray patterns that are unique for each of the crystalline phases The qualitative XRD investigation was performed in a Carl Zeiss-Jena Universal Diffractometer, URD6 model Experimental program Results and discussion Six mixtures were prepared as described in Table 4, which are the pastes with the same proportions used in the previous work [10], as explained in the introduction The materials were weighed and mixed in a planetarytype mortar mixer The total quantity of water was maintained, taking into account the water from the latex The superplasticizer was also added in the ratio of 1% of the weight of cement The preparation for TG, DSC and X-ray diffraction was carried out using agate crucible, in which the paste was manually ground until the size of particles was lower than 0.063 mm For the prevention of carbonation and maintenance of relative humidity, all specimens were stored in the vacuum up to the time when the test started The analyses were performed in the Institute of Chemistry of Sa˜o Carlos, University of Sa˜o Paulo, using a TGA 2050 Thermogravimetric Analyzer V5.1A equipment The experimental conditions were: N2 gas dynamic atmosphere (40 ml minÀ1); heating rate (10 °C minÀ1); platinum top-opened crucible The samples were heated in the range of 20–900 °C at a constant rate The Ca(OH)2 was estimated from the weight loss measured in the TG curve between the initial and final temperature of the corresponding TG peak Differential scanning calorimetry (DSC) has been employed to investigate the combined effect of silica fume and polymer on heat development in the pastes A DSC 2010 differential scanning calorimeter was used Figs and show the TG curves of pastes with silica fume content of 5% and 10%, respectively (P5 and P6) It can be seen that TG curves for these pastes consist of four zones: $25–123.3 °C: dehydration of pore water, $123.3–420 °C: dehydration of calcium silicate hydrates, $420–480 °C: dehydroxylation of calcium hydroxide, $480–730 °C: decarbonation of CaCO3 Figs and show TG curves of pastes with 5% silica fume and polymer addition of 5.2% and 10.4% (polymeric solids) Figs and present TG curves of pastes with 10% silica fume and polymer addition of 5.2% and 10.4% (polymeric solids) The TG curves obtained in these tests are typical of hydrated cement pastes containing carbonate phases and polymeric admixtures As it is shown, the curves can be divided into five major parts, according to different reactions: $25–123.3 °C: dehydration of pore water, $123.3–345 °C: dehydration of calcium silicate hydrates, $345–427 °C: weight loss due to polymer pyrolysis, Table Mixture proportion of the mortars Designation of paste Silica fume content (%)a Polymer latex content (%)a Solids of polymer content (%)a Water/ cement ratio P1 P2 P3 P4 P5 P6 5 10 10 10 10 20 10 20 0 5.2 10.4 5.2 10.4 0 0.36 0.31 0.36 0.31 0.36 0.36 a By weight of cement Fig TG curve of the paste without polymer and 5% of silica fume P5 A.E.F.de.S Almeida, E.P Sichieri / Construction and Building Materials 20 (2006) 882–887 885 0,02 0,02 TG P6 DTG P6 100 DTG P3 100 TG P3 0,00 95 0,00 95 -0,02 -0,02 -0,04 -0,06 85 -0,08 80 90 TG (%) TG (%) 90 85 -0,06 80 -0,10 -0,12 75 -0,04 -0,08 75 -0,10 -0,14 70 70 100 200 300 400 500 600 700 800 900 1000 100 200 300 Temperatureo( C) 500 600 o 700 800 900 -0,12 1000 Temperature ( C) Fig TG curve of the paste without polymer and 10% of silica fume P6 TG P1 DTG P1 100 400 Fig TG curve of the paste P3 0,02 0,01 DTG P4 100 TG P4 0,00 0,00 95 95 90 -0,04 85 -0,06 -0,08 80 -0,10 TG (%) TG(%) -0,02 -0,01 90 85 -0,02 80 -0,03 75 75 -0,04 -0,12 100 200 300 400 500 600 700 800 900 1000 o Temperature ( C) 0,02 TG P2 0,00 95 -0,02 TG (%) 90 -0,04 85 -0,06 80 -0,08 75 -0,10 70 100 200 300 400 500 600 100 200 300 400 500 600 700 800 900 1000 Fig TG curve of the paste P4 DTG P2 0 Temperature (oC) Fig TG curve of the paste P1 100 70 700 800 900 -0,12 1000 Temperature (oC) Fig TG curve of the paste P2 $427–475 °C: dehydroxylation of calcium hydroxide, $475–711 °C: decarbonation of CaCO3 The weight loss for each temperature range can be seen in Table For pastes with polymer addition, the weight loss related with the dehydroxylation of calcium hydroxide is lower than pastes with silica fume addition alone Fig shows the DSC curves obtained for the pastes studied From these results, it is clear that pastes with polymer present different results from the pastes which not contain polymer All curves show an endothermic peak around 480 °C, but they are more intense for pastes with silica fume alone because of the higher Ca(OH)2 content Pastes with silica fume and polymer presents an exothermic peak around 350 °C, indicating polymer pyrolysis, as it was found from the TG/DTG analyses The pastes modified with polymer presented higher heat absorption between 100 and 200 °C, suggesting that these pastes contain more free water resulting from the delaying of hydration, and that these have a bigger amount of calcium silicate hydrates The XRD results show some qualitative differences in the hydration rate due to the incorporation of silica and polymer Figs 8–10 show the X-ray patterns of the pastes with 5%, 10% of silica fume, and pastes with polymer The main compounds observed are Ca(OH)2 in the 6.544 5.175 4.663 1.707 6.289 74.88 S+C P+S P P C P P S+C C P 150 55 60 100 10 15 20 25 30 35 40 45 50 65 70 75 2θ (degrees) Fig XRD patterns of the P1 and P2 pastes after 28 days of hydration P, portlandite (Ca(OH)2); CC, calcium carbonate (CaCO3); E, ettringite (Ca6[Al(OH)6]2 (SO4)3 Ỉ 26H2O); S, silicates; F, ferrite P P 150 P S+C C P 200 P4 P 250 P+S S+C 300 S+E+F 350 100 250 P3 P 150 P S+C C P P 200 S+E+F S+C 300 P+S 50 100 50 0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 2θ (degree) Fig XRD patterns of the P3 and P4 pastes after 28 days of hydration P, portlandite (Ca(OH)2); CC, calcium carbonate (CaCO3); E, ettringite (Ca6[Al(OH)6]2(SO4)3 Ỉ 26H2O); S, silicates; F, ferrite -1,1 P+S 250 P P P C P5 P 150 P S+C P 200 C 300 Intensity (cps) -0,5 -1,0 S+C 100 50 P1 P4 P5 P6 S+C 150 -0,4 -0,6 P+S 200 P6 S+E+F -0,3 250 S+E+F -0,2 300 E ntensity (cps)I -0,1 -0,9 P1 0,0 -0,8 S+C 200 S+E+F 250 50 0,1 Heat flux (mW/mg) 300 P Intensity (cps) 50 form of portlandite, a small amount of CaCO3 resulting from carbonation of Ca(OH)2 and calcium silicate anhydrous The peak intensity in the region 2h = 18° has been -0,7 P 24.6–123.3 123.3–344.6 344.6–429.6 429.6–476.5 476.5–713.5 Residue above 850 °C P 5.695 4.949 2.53 1.912 6.308 77.89 P P 25.2–123.3 123.3–344.6 344.6–427.3 427.3–474.3 474.3–711.3 Residue above 850 °C C 5.236 4.767 5.175 2.301 7.357 74.12 S+C C 26.1–123.3 123.3–333.4 333.4–422.9 422.9–478.8 478.8–713.5 Residue above 850 °C S+E+F 5.922 5.037 2.665 2.095 7.412 75.04 P 25.9–123.3 123.3–337.9 337.9–429.6 429.6–481 481–711.3 Residue above 850 °C P2 100 S+C P4 8.43 5.238 1.876 8.436 75.23 150 P P3 25.2–123.3 123.3–420.6 420.6–478.8 478.8–729.2 Residue above 850 °C 200 P P2 7.487 5.75 2.056 5.258 78.9 250 P P1 28.3–123.3 123.3–416.2 416.7–472.1 472.1–702.3 Residue above 850 °C Intensity (cps) P6 Weight loss (%) Intensity (cps) P5 Temperature range (°C) 300 P Table Weight loss of the pastes according to the temperature P+S A.E.F.de.S Almeida, E.P Sichieri / Construction and Building Materials 20 (2006) 882–887 Intensity (cps) 886 100 50 0 -1,2 10 15 20 25 30 35 40 45 50 55 60 65 70 75 2θ (degree) -1,3 -50 50 100 150 200 250 300 350 o 400 450 500 Temperature ( C) Fig DSC curves of the pastes P1, P4, P5 and P6 550 Fig 10 XRD patterns of the P5 and P6 pastes after 28 days of hydration P, portlandite (Ca(OH)2); CC, calcium carbonate (CaCO3); E, ettringite (Ca6[Al(OH)6]2(SO4)3 Ỉ 26H2O); S, silicates; F, ferrite A.E.F.de.S Almeida, E.P Sichieri / Construction and Building Materials 20 (2006) 882–887 considered as a measure of the quantity of Ca(OH)2 [15] Therefore, Figs 8–10 show that 10% of silica fume replacement and polymer addition resulted in the lowest peak intensity for the portlandite Conclusions From the thermogravimetric investigations performed, showed in the TG and DSC curves, it is possible to conclude that mineral admixtures and polymeric additions have influenced the cement hydration, mainly when added simultaneously Both the pozzolanic reaction and the delaying of hydration due to polymer addition appear to cause a decreasing on the Ca(OH)2 content The qualitative XRD investigation revealed that a lower intensity of Ca(OH)2 (in the region 2h = 18°) was obtained in the presence of latex, compared to pastes without polymer Similarly, we found a decrease in the Ca(OH)2 content in the TG analyses for the pastes with polymer addition As it can be seen, pastes with polymer and 10% silica fume content presented the lowest Ca(OH)2 compared with the other pastes The additions studied in this work resulted in the decrease of the portlandite (Ca(OH2)) content, which can justify the improvement of the mortarsÕ performance studied earlier by the authors Acknowledgments The authors acknowledge the financial support from FAPESP 887 References [1] Ohama Y Polymer based admixtures Cement Concrete Compos 1998;20:189–212 [2] Fowler DW Polymers in concrete: a vision for the 21st century Cement Concrete Compos 1999;21:449–52 [3] Walters DG What are latexes? 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