Setting Additives Influence on the Thermomechanical Properties of Wet Shotcrete Refractory Castable Matrices

REFRACTORY CERAMICS Setting Additives Influence on the Thermomechanical Properties of Wet Shotcrete Refractory Castable Matrices The effect of coagulants and setting admixtures on the thermomechanical properties of wet shotcrete refractory castable matrices was evaluated and discussed. Y.A. Marques, R.G. Pileggi, F.A.O. Valenzuela, M.A.L. Braulio and V.C. Pandolfelli Dept. of Materials Engineering, Federal University of São Carlos, São Carlos, S.P., Brazil Sprayed concretes were originally developed for civil construction in the early 20th century. 1 The advances and benefits that have been attained since then are responsible for the current widespread use of this placing technique. High installation rates, lower costs, automation capabilities and performances similar to preshaped refractories are some of the benefits of this technique. 1 4 Wet shotcrete consists of pumping the castable suspension out of the mixer and onto the target surface (Figure 1). 1,5 Compressed air is injected into the tip of the pipeline nozzle to generate the castable spray that lines the surface. The sprayed castable is consolidated by an additive, also injected at the nozzle tip, which causes sudden loss of fluidity in the concrete. This technique results in low rebound and does not require molds. This process also is characterized by low porosity of the placed material because of the high shearing rates imposed by the process. Wet shotcrete has evolved as a result of the advances in pumpable castables, rheometric analyses, 2,5 equipment and additives. 2 These developments have led to the introduction of wet shotcrete in the refractory industry. 1 Despite its simple concept, the task of preparing refractory shotcrete is complex. It involves particle-size design, dispersion, mixing, pumping, spraying and setting. 2,6 8 Refractory castables can be prepared in a broad range of particle sizes. 5,8 They consist of a matrix (particle size of <100 µm, controlled by surface forces and interactions with the aqueous media) and aggregates (particle size of >100 µm, controlled by mass forces). For better sprayability, the pumped castable should be homogeneous, dispersed and segregation-free (matrix/aggregate separation). 1,2 The design of pumping castables has been mastered to a considerable extent. However, other aspects of wet shotcrete that require research are the generation of spray at the conical nozzle (Figures 1 and 2) and the proper selection and quantity of setting additive. The granules formed during spraying can become brittle or plastic during the short trajectory from the nozzle to the target. These results depend on the setting additive and its interaction with castable constituents. The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9201

Figure 1 Schematic description of the wet shotcrete process, showing mixing, pumping and spraying stages. 5 Figure 2 Schematic illustration of the wet-shotcrete spraying technique of refractory castables. 7 The formation of spray is influenced by the equipment (pump, pipeline and nozzle design), operational proceedings (compressed air pressure, additive injection, placing direction and spray opening angle), castable rheological behavior and matrix composition. The setting additives directly affect the rheology of the material. 4 Therefore, spraying and consolidation of the castable also depend on how the additive interacts with the matrix. Cement accelerators are normally used in wet shotcrete applications to make the castable set instantaneously. 1,2,4 Most of these additives are based on alkaline compounds. Their performance depends on their chemical composition and cement particle-size distribution. 3 However, a disadvantage of this class of additives is that they decrease castable mechanical strength. Moreover, they can decrease material refractoriness in high-temperature applications. These effects scale with the amount of additive. 2,3 To minimize these effects, alkali-free additives have been developed. 2,3 They also decrease the risks related with the toxic nature of alkaline substances. The setting mechanism of admixtures usually derives from an increase in the ionic strength and a change in suspension ph levels. This promotes greater attracting forces between the particles and, thus, directly affects material packing structure. 3,4,9 A novel class of organic viscosityenhancing admixtures has been developed recently for hydraulically bound materials (cement based). 10 The admixtures in this castable generate water-trapping gels that increase viscosity of the solution, cohesion and material adhesion. Nevertheless, these additives may retard the drying of refractory castables, which extends their processing time. Other admixtures form 3-D network gels as a result of crosslinked bonds with cement calcium ions. 10,11 This mechanism decreases water retention, because cohesion also is promoted by the newly generated chemical bonds. Organic polyelectrolytes that have long polymeric chains also have been proposed as admixtures for wet shotcrete refractory castables. 4,9 Although the long polymeric molecule chain establishes bridges between the particles, 12 the steric effect prevents the particles from approaching each other too closely.therefore, the permeability and drying time of the castable is not overly affected. 9,12 Setting additives also can affect other castable properties. Several recent studies 4,5,7 have focused on the rheological aspects of wet shotcrete applications. Another study reports the influence of various coagulation mechanisms on the permeability and drying behavior of refractories. 9 However, the impact of coagulant admixtures on the thermomechanical properties of refractory castables remains unclear. The major problems involved in studies of the postsetting properties of shotcrete are associated with sampling. In the field, the panel waterdrilling technique can damage the samples. In the laboratory, the rapidly decreasing fluidity of the castable makes shaping a difficult task. This inconvenience has been overcome by pressing the material. Nevertheless, the particle-packing structure may undergo alterations and, therefore, not reproduce the actual process. 3,9 Moreover, the various coagulation mechanisms of additives can directly influence particle packing, which affects material microstructure. Previous research 8 has shown that the creep behavior and elastic modulus of refractory castables, based on different Andreasen s packing coefficients, are directly related to the nature and content of the matrix. Moreover, the maximum deformation during the creep test is only slightly The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9202

Figure 4 Castable coagulation mechanisms: (A) castable without additives; (B) agglomeration caused by attraction forces among the particles; (C) bridging effect caused by PAS molecules; and (D) bridging effect and gel formation. 9 dependent on the maximum particle diameter. The chemical and structural effects of the coagulation mechanisms on refractory castables can be isolated by characterizing the fine fraction of the particle-size distribution that corresponds to the castable matrix (<100 µm).therefore, the main purpose of this work is to evaluate the impact of setting additives on the thermomechanical properties of wet shotcrete refractory castable matrices. Design/Evaluation Tests Castables for shotcrete applications are pumped before they are sprayed. Therefore, a high-alumina, ultra-lowcement refractory composition that had pumpable characteristics was first formulated (Figure 3(A)). 5 It was based on the Andreasen model and had a packing coefficient of q = 0.26. The formulation had an original composition of 78.9 wt% white fused alumina (Elfusa, Brazil), 20.6 wt% calcined alumina (A1000-SG and A3000-FL, Almatis, U.S.) and 1 wt% aluminous cement (CA-14M, Almatis, U.S.). The castable was reformulated so that the entire particle-size distribution was equivalent to that of the castable matrix (<100 µm) (Figure 3(B)). The matrix formulation also had an Andreasen packing coefficient of q = 0.26.The interparticle spacing (IPS) 5 was 0.077 µm, similar to that of the castable (0.088 µm).the matrix suspension was prepared with 60 vol% of solids and 40 vol% of water.this was equivalent to a water content of 18 vol% in the concrete. A polycarboxylate ether (0.15 wt%) (SKW, Germany) was used as the dispersant. The matrix was mixed in a lab mixer (Etica SA, Brazil), and the additive was incorporated as follows: powder dispersion in water at a constant mixing speed; injection of setting additive; 20 s of homogenization; molding of samples for the mechanical strength and creep tests; curing in a saturated atmosphere (100% relative humidity) at 50 C for 72 h; and further drying for another 72 h, embedded in silica gel (50 C). Additives Three distinct classes of setting additives were used: inorganic (sodium silicate (SS) (Aldrich, Brazil)); calcium chloride (CC) (Synth, Brazil); organic polyelectrolyte (sodium polyacrilate (PAS) (BASF, Germany)); and viscosityenhancing polymer (hydroxyethyl cellulose QP90 (HEC) (Union Carbide, Brazil)). Two systems were studied: 0.6 wt% of each of these additives added separately; and combined 0.075 wt% of alginic acid salt (Alg) (Fluka, Switzerland) and 0.6 wt% sodium polyacrilate. Various additive coagulation mechanisms were chosen to consolidate the castables (Figure 4). SS and CC are inorganic admixtures commercially used. Basically, they increase system ionic strength, alter the potential energy balance, and promote particle attraction and agglomeration. 4,9,13 The PAS used in this work is a highmolecular-weight organic polyelectrolyte (M W = 15,000 g/mol). It flocculates/coagulates the particles in suspension by promoting bridging, depletion and ionic strength increase. 12 However, the steric effect associated with its molecules keeps the particles apart, which preserves their original positions. Alg is a high-molecular-weight polymer (M W = 48 186 kg/mol) derived from brown seaweed. 4,9,10 This additive gels in water by crosslinking its molecules with the calcium ions in the cement. Consolidation promoted by Alg does not alter particle positions in the matrix. HEC is a water-soluble, nonionic, semisynthetic organic polymer that increases liquid viscosity and yield stress by generating a thixotropic lubricant gel. The gel does not affect system ph or ionic strength. 4,10 Porosity Tests The total matrix porosity of the samples cured and dried at 50 C and prefired at 500 C for 5 h was evaluated. Kerosene was used as the immersion liquid (ASTM C 20-87). The mean pore-size diameters of these samples also were evaluated using mercury porosimetry (Model EUA, Aminco Winslow). The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9203

Strength The splitting strength technique (ASTM C 496-90) was used to evaluate the mechanical strength of samples (40 mm in diameter and height). The splitting strength was evaluated for five samples of each experimental set after they were cured (72 h) and dried (50 C) or prefired (500 C for 5 h after they were dried). CPI Hg (%) Refractoriness under load (RUL) was evaluated for cylindrical specimens (50 50 mm) that had a central hole (12.4 mm in diameter). Tests were conducted on prefired samples (500 C for 5 h) to ensure that all possible residual hydrates were eliminated. This decreased the likelihood of explosion. For RUL and creep evaluations, the samples were heated to 1500 C in 5 C/min steps, under a compressive load of 0.2 MPa (Model 421, Netzsch). For the creep tests, the load was maintained for 12 h at 1500 C. 8,14 Shotcrete Additive Selection Apparent porosity and density results (Figure 5) showed that these properties of the castable matrix were not greatly affected by the setting additives, particularly after they were prefired at 500 C for 5 h. Figure 6 σ f (MPa) Pore diameter (µm) Influence of shotcrete additives on pore-size distribution of castable matrix after thermal treatment at 500 C for 5 h (CPI Hg is cumulative percentage of intruded mercury). The total pore volume and pore mean diameter were similar in all the compositions. Therefore, the cumulative percentage of intruded mercury (CPIHg) as a function of the pore diameter for the various setting additives showed no significant influence (Figure 6). These results are congruent with previous research 15 that shows these characteristics are governed by the amount of water in the matrix. Therefore, the mechanical strength, RUL and creep values in this study are less likely to be influenced by the apparent porosity of the matrix. The present results demonstrate that shotcrete admixtures affect the Figure 7 Influence of shotcrete additives on mechanical strength of castable matrix. mechanical strength of the castable matrix in various ways (Figure 7). The influence of organic and inorganic additives is clearly distinguished. Inorganic additives SS and CC decreased the mechanical strength of the matrix dried at 50 C. The combination of SS and high-alumina cement (HAC) increased the ph level of the matrix, 4,12 which led to intense particle agglomeration. As a result, the bonding force of the matrix was directly affected, which weakened the structure of the matrix after drying at 50 C. 4 Furthermore, SS retarded HAC setting, which promoted the formation of compounds, such as 2CaO Al 2 O 3 SiO 2 8H 2 O, that prevented cement hydration. 13 The low mechanical strength of the castable matrix that contained CC is attributed to its agglomerating effect. CC is a cement hydration accelerator 2,3 that promotes a lesspacked structure. The organic additives PAS, HEC and the combination of PAS and Alg increased castable mechanical strength after drying at 50 C. This effect was caused mainly by the formation of polymeric chains (drying at 50 C) and gel drying (at 50 C and thermal treatment at 500 C). These The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9204

T ( C) dl/l 0 (%) admixtures. PAS and its combination with Alg showed a similar behavior in the RUL and creep tests. Both additives shifted the onset of deformation to higher temperatures (Figure 9). Moreover, these organic admixtures decreased the maximum creep deformation attained by the castable matrix, which improved the performance of the material. A comparison of the mechanical strength of the samples dried at 50 C and the creep results (Figure 10) revealed a definite correlation. Figure 9 Starting temperature deformation of matrix, T, with various additives, and maximum percentual deformation, dl/l 0, attained by matrix during creep test. Particle packing and pore-size distribution did not substantially affect the mechanical strength after drying. Therefore, the different values obtained were related to the additive binding property and the RUL and creep results to its chemistry. dl/l 0 (%) Figure 10 Maximum creep strain, dl/l 0, vs mechanical strength, σ f, of unfired castables cured and dried at 50 C. resulted in a 3-D particle-bound structure. The refractoriness under load and creep behavior of the matrix can be evaluated by the deformation (dl/l 0 ) as a function of temperature (up to 1500 C) and time (Figure 8). The results indicated that the shotcrete additives used here influenced these properties significantly. σ f (MPa) The addition of SS resulted in the greatest deformation of the castable matrix. Moreover, the SS-containing samples presented the lowest starting deformation temperature (Figure 9). This result may have been caused by the lower-temperature melting phases in the Na 2 O SiO 2 Al 2 O 3 CaO system. CC led to greater deformation than did the material without additives. However, its effect was less intense than the SS-containing samples. Based on these results, the inorganic admixtures yielded the poorest results in the RUL and creep tests. HEC, a gelling additive, resulted in a lower creep than the inorganic The amounts of additive in the matrix can be greater under terms of field performance. 1,3 Therefore, their effect on the properties of the castable can be intensified. Moreover, a distinct criterion for the selection of setting additives besides rebound loss 2 4 and drying behavior 9 should influence the thermomechanical properties of the castable matrix. Admixtures Influence Properties The introduction of coagulant admixtures greatly influenced the thermomechanical properties of wet shotcrete refractory castable matrix. Organic additives that contained sodium PAS and PAS + Alg improved matrix performance in all properties evaluated. Commercial inorganic additives that promote particle agglomeration (SS and CC) decreased the mechanical strength of green samples. Moreover, CC and, to a greater extent, SS resulted in a higher creep and decreased the starting temperature deformation. The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9205

PAS and PAS + Alg increased the mechanical strength of green and prefired matrix samples. These additives also promoted less creep and higher starting deformation temperatures. Additives with these coagulant/flocculation mechanisms should be taken into greater consideration for wet shotcreting. Acknowledgments The authors are grateful to the Brazilian research funding agencies FAPESP and CNPq. They also are grateful to ALCOA- Brazil and Magnesita for supporting this research and to D. Vasques Filho for helping on the experimental procedure. References 1 I.L. Glassgol, Refractory Shotcrete The Current State of Art, Shotcrete Magazine, [Summer] 24 32 (2002). 2 M. Jolin, D. Beaupré and S. Mindess, Tests to Characterize Properties of Fresh Dry-Mix Shotcrete, Cem. Concr. Res., 29, 753 60 (1999). 3 L.R. Prudêncio Jr., "Accelerating Admixtures for Shotcrete," Cem. Concr. Res., 20, 213 19 (1998). 4 R.G. Pileggi, Y.A. Marques, D. Vasques Filho, A.R. Studart and V.C. Pandolfelli, Wet-Shotcrete Additives, Am. Ceram. Soc. Bull., 81 [6] 51 57 (2002). 5 R.G. Pileggi and V.C. Pandolfelli, Rheology and Particle-Size Distribution of Pumpable Refractory Castables, Am. Ceram. Soc. Bull., 80 [10] 52 57 (2001). 6 R.G. Pileggi,Y.A. Marques, D. Vasques Filho, A.R. Studart and V.C. Pandolfelli, Shotcrete Performance of Refractory Castables, Refract. Appl., 8 [3] 15 20 (2003). 7 D. Vasques Filho, Y.A. Marques, R.G. Pileggi and V.C. Pandolfelli, Influence of the Polymeric Fibers on Shotcrete Refractory Castables (in Portuguese), Ceramica, 50, 69 74 (2004). 8 R.G. Pileggi, F.T. Ramal Jr., A.E. Paiva and V.C. Pandolfelli, High-Performance Refractory Castables: Particle Size Design, Refract. Appl., 8 [5] 17 21 (2003). 9 Y.A. Marques, D. Vasques Filho, R.G. Pileggi and V.C. Pandolfelli, Influence of Additives on the Permeability and Drying Behavior of Wet Shotcrete Refractory Castables (in Portuguese), Ceramica, 50, 7 11 (2004). 10 K.H. Khayat, Viscosity-Enhancing Admixtures for Cement-Based Materials: An Overview, Cem. Concr. Res., 20, 171 88 (1998). 11 A.R. Studart, V.C. Pandolfelli, E. Tervoot and L.J. Gauckler, Gelling of Alumina Suspensions Using Alginic Acid Salt and Hydroxyaluminum Diacetate, J. Am. Ceram. Soc., 85 [11] 2711 18 (2002). 12 I.R. Oliveira, A.R. Studart, R.G. Pileggi and V.C. Pandolfelli, Dispersion and Particles Packing: Fundamental Aspects and the Application in Ceramic Processing (in Portuguese), Fazendo Arte Editorial, São Paulo, 2000; 224 pages. 13 J. Ding, Y. Fu and J.J. Beaudoin, Study of Hydration Mechanisms in High-Alumina Cement Sodium Silicate System, Cem. Concr. Res., 26 [5] 799 804 (1996). 14 D.J. Bray, J.R. Smyth and T.D. McGee, Creep of 90+% Alumina Concrete, Am. Ceram. Soc. Bull., 59 [7] 706 10 (1980). 15 F.T. Ramal Jr., R. Salomão and V.C. Pandolfelli, Water Content and Its Effect on Refractory Castables Drying Behavior, Refract. Appl., 10 [3] 10 13 (2005). Equations Andreasen Packing Model 5,12 Equation CPFT = 100 (D/D L ) q where CPFT is the cumulative percentage of particles smaller than diameter D, D L the maximum diameter (CPFT = 100% when D = D L ) and q the distribution coefficient. Splitting Tensile Strength Equation σ f = 2F/πDh where σ f is the splitting tensile strength, F the maximum force (in newtons) applied, D the sample diameter and h the sample height. The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9206