©The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9201

Y.A. Marques, R.G. Pileggi,F.A.O. Valenzuela, M.A.L. Braulio and V.C. PandolfelliDept. of Materials Engineering, FederalUniversity of São Carlos, São Carlos, S.P., Brazil

prayed concretes wereoriginally developed forcivil construction in theearly 20th century.1 Theadvances and benefits that

have been attained since then areresponsible for the current widespreaduse of this placing technique. High instal-lation rates, lower costs, automationcapabilities and performances similar topreshaped refractories are some of thebenefits of this technique.1–4

Wet shotcrete consists of pumping thecastable suspension out of the mixer andonto the target surface (Figure 1).1,5

Compressed air is injected into the tip ofthe pipeline nozzle to generate thecastable spray that lines the surface. Thesprayed castable is consolidated by anadditive, also injected at the nozzle tip,which causes sudden loss of fluidity inthe concrete. This technique results inlow rebound and does not require molds.This process also is characterized by lowporosity of the placed material becauseof the high shearing rates imposed bythe process.

Wet shotcrete has evolved as a result ofthe advances in pumpable castables,

rheometric analyses,2,5 equipment andadditives.2 These developments have ledto the introduction of wet shotcrete inthe refractory industry.1

Despite its simple concept, the task ofpreparing refractory shotcrete is com-plex. It involves particle-size design, dis-persion, mixing, pumping, spraying andsetting.2,6–8

Refractory castables can be prepared ina broad range of particle sizes.5,8 Theyconsist of a matrix (particle size of <100µm, controlled by surface forces andinteractions with the aqueous media)and aggregates (particle size of >100 µm,controlled by mass forces). For bettersprayability, the pumped castable shouldbe homogeneous, dispersed and segregation-free (matrix/aggregate separation).1,2

The design of pumping castables hasbeen mastered to a considerable extent.However, other aspects of wet shotcretethat require research are the generationof spray at the conical nozzle (Figures 1and 2) and the proper selection andquantity of setting additive. The granulesformed during spraying can become brit-tle or plastic during the short trajectoryfrom the nozzle to the target. Theseresults depend on the setting additiveand its interaction with castable constituents.

Setting Additives Influence on theThermomechanical Properties of

Wet Shotcrete Refractory CastableMatrices

The effect of coagulants and setting admixtures on the thermo-mechanical properties of wet shotcrete refractory castablematrices was evaluated and discussed.

S

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The formation of spray is influencedby the equipment (pump, pipelineand nozzle design), operational pro-ceedings (compressed air pressure,additive injection, placing directionand spray opening angle), castablerheological behavior and matrixcomposition. The setting additivesdirectly affect the rheology of thematerial.4 Therefore, spraying andconsolidation of the castable alsodepend on how the additive inter-acts with the matrix.

Cement accelerators are normallyused in wet shotcrete applications tomake the castable set instantaneous-ly.1,2,4 Most of these additives arebased on alkaline compounds. Theirperformance depends on theirchemical composition and cementparticle-size distribution.3 However, adisadvantage of this class of addi-tives is that they decrease castablemechanical strength. Moreover, theycan decrease material refractorinessin high-temperature applications.These effects scale with the amountof additive.2,3

To minimize these effects, alkali-freeadditives have been developed.2,3

They also decrease the risks relatedwith the toxic nature of alkaline sub-stances. The setting mechanism ofadmixtures usually derives from anincrease in the ionic strength and achange in suspension pH levels. This

promotes greater attracting forcesbetween the particles and, thus,directly affects material packingstructure.3,4,9

A novel class of organic viscosity-enhancing admixtures has beendeveloped recently for hydraulicallybound materials (cement based).10

The admixtures in this castable gen-erate water-trapping gels thatincrease viscosity of the solution,cohesion and material adhesion.Nevertheless, these additives mayretard the drying of refractory casta-bles, which extends their processingtime. Other admixtures form 3-D net-work gels as a result of crosslinkedbonds with cement calcium ions.10,11

This mechanism decreases waterretention, because cohesion also ispromoted by the newly generatedchemical bonds.

Organic polyelectrolytes that havelong polymeric chains also have beenproposed as admixtures for wet shot-crete refractory castables.4,9 Althoughthe long polymeric molecule chainestablishes bridges between the par-ticles,12 the steric effect prevents theparticles from approaching eachother too closely.Therefore, the per-meability and drying time of thecastable is not overly affected.9,12

Setting additives also can affectother castable properties. Several

recent studies4,5,7 have focused onthe rheological aspects of wet shot-crete applications. Another studyreports the influence of variouscoagulation mechanisms on the per-meability and drying behavior ofrefractories.9 However, the impact ofcoagulant admixtures on the ther-momechanical properties of refrac-tory castables remains unclear.

The major problems involved instudies of the postsetting propertiesof shotcrete are associated with sam-pling. In the field, the panel water-drilling technique can damage thesamples. In the laboratory, the rapid-ly decreasing fluidity of the castablemakes shaping a difficult task. Thisinconvenience has been overcomeby pressing the material.Nevertheless, the particle-packingstructure may undergo alterationsand, therefore, not reproduce theactual process.3,9 Moreover, the vari-ous coagulation mechanisms ofadditives can directly influence parti-cle packing, which affects materialmicrostructure.

Previous research8 has shown thatthe creep behavior and elastic modu-lus of refractory castables, based ondifferent Andreasen’s packing coeffi-cients, are directly related to thenature and content of the matrix.Moreover, the maximum deformationduring the creep test is only slightly

©The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9202

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 refractorycastables.7

dependent on the maximum particlediameter.

The chemical and structural effectsof the coagulation mechanisms onrefractory castables can be isolatedby characterizing the fine fraction ofthe particle-size distribution that cor-responds to the castable matrix (<100µm).Therefore, the main purpose ofthis work is to evaluate the impact of

setting additives on the thermome-chanical properties of wet shotcreterefractory castable matrices.

Design/Evaluation TestsCastables for shotcrete applicationsare pumped before they are sprayed.Therefore, a high-alumina, ultra-low-cement refractory composition thathad pumpable characteristics wasfirst formulated (Figure 3(A)).5 It wasbased on the Andreasen model andhad a packing coefficient of q = 0.26.The formulation had an originalcomposition of 78.9 wt% white fusedalumina (Elfusa, Brazil), 20.6 wt% calcined alumina (A1000-SG andA3000-FL, Almatis, U.S.) and 1 wt%aluminous cement (CA-14M, Almatis,U.S.). The castable was reformulatedso that the entire particle-size distri-bution was equivalent to that of the castable matrix (<100 µm)(Figure 3(B)).

The matrix formulation also had anAndreasen packing coefficient of q =0.26.The interparticle spacing (IPS)5

was 0.077 µm, similar to that of thecastable (0.088 µm).The matrix sus-pension was prepared with 60 vol%of solids and 40 vol% of water.Thiswas equivalent to a water content of18 vol% in the concrete. A polycar-boxylate ether (0.15 wt%) (SKW,Germany) was used as the dispersant.

The matrix was mixed in a lab mixer(Etica SA, Brazil), and the additivewas incorporated as follows: powderdispersion in water at a constantmixing speed; injection of settingadditive; 20 s of homogenization;molding of samples for the mechani-cal strength and creep tests; curingin a saturated atmosphere (100% rel-ative humidity) at 50°C for 72 h; andfurther drying for another 72 h,embedded in silica gel (50°C).

AdditivesThree distinct classes of setting addi-tives were used: inorganic (sodiumsilicate (SS) (Aldrich, Brazil)); calciumchloride (CC) (Synth, Brazil); organicpolyelectrolyte (sodium polyacrilate

(PAS) (BASF, Germany)); and viscosity-enhancing polymer (hydroxyethylcellulose QP90 (HEC) (Union Carbide,Brazil)). Two systems were studied:0.6 wt% of each of these additivesadded separately; and combined0.075 wt% of alginic acid salt (Alg)(Fluka, Switzerland) and 0.6 wt%sodium polyacrilate.

Various additive coagulation mecha-nisms were chosen to consolidate thecastables (Figure 4). SS and CC areinorganic admixtures commerciallyused. Basically, they increase systemionic strength, alter the potentialenergy balance, and promote particleattraction and agglomeration.4,9,13

The PAS used in this work is a high-molecular-weight organic polyelec-trolyte (MW = 15,000 g/mol). It floccu-lates/coagulates the particles in sus-pension by promoting bridging,depletion and ionic strengthincrease.12 However, the steric effectassociated with its molecules keepsthe particles apart, which preservestheir original positions.

Alg is a high-molecular-weight poly-mer (MW = 48–186 kg/mol) derivedfrom brown seaweed.4,9,10 This addi-tive gels in water by crosslinking itsmolecules with the calcium ions inthe cement. Consolidation promotedby Alg does not alter particle posi-tions in the matrix.

HEC is a water-soluble, nonionic,semisynthetic organic polymer thatincreases liquid viscosity and yieldstress by generating a thixotropiclubricant gel. The gel does not affectsystem pH or ionic strength.4,10

Porosity TestsThe total matrix porosity of the sam-ples cured and dried at 50°C andprefired at 500°C for 5 h was evaluat-ed. Kerosene was used as the immer-sion liquid (ASTM C 20-87). The meanpore-size diameters of these samplesalso were evaluated using mercuryporosimetry (Model EUA,Aminco–Winslow).

©The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9203

Figure 4 Castable coagulation mechanisms: (A) castable without additives; (B) agglomeration caused by attractionforces among the particles; (C) bridgingeffect caused by PAS molecules; and(D) bridging effect and gel formation.9

StrengthThe splitting strength technique(ASTM C 496-90) was used to evalu-ate the mechanical strength of sam-ples (40 mm in diameter and height).The splitting strength was evaluatedfor five samples of each experimen-tal set after they were cured (72 h)and dried (50°C) or prefired (500°Cfor 5 h after they were dried).

Refractoriness under load (RUL) wasevaluated for cylindrical specimens(50 � 50 mm) that had a central hole(12.4 mm in diameter). Tests wereconducted on prefired samples(500°C for 5 h) to ensure that all pos-sible residual hydrates were eliminat-ed. This decreased the likelihood ofexplosion. For RUL and creep evalua-tions, the samples were heated to1500°C in 5°C/min steps, under acompressive load of 0.2 MPa (Model421, Netzsch). For the creep tests, theload was maintained for 12 h at1500°C.8,14

Shotcrete Additive SelectionApparent porosity and densityresults (Figure 5) showed that theseproperties of the castable matrixwere not greatly affected by the set-ting additives, particularly after theywere prefired at 500°C for 5 h.

The total pore volume and poremean diameter were similar in all thecompositions. Therefore, the cumula-tive percentage of intruded mercury(CPIHg) as a function of the porediameter for the various settingadditives showed no significantinfluence (Figure 6).

These results are congruent withprevious research15 that shows thesecharacteristics are governed by theamount of water in the matrix.Therefore, the mechanical strength,RUL and creep values in this studyare less likely to be influenced by theapparent porosity of the matrix.

The present results demonstratethat shotcrete admixtures affect the

mechanical strength of the castablematrix in various ways (Figure 7). Theinfluence of organic and inorganicadditives is clearly distinguished.

Inorganic additives SS and CCdecreased the mechanical strengthof the matrix dried at 50°C. The com-bination of SS and high-aluminacement (HAC) increased the pH levelof the matrix,4,12 which led to intenseparticle agglomeration. As a result,the bonding force of the matrix wasdirectly affected, which weakenedthe structure of the matrix after dry-ing at 50°C.4 Furthermore, SS retard-ed HAC setting, which promoted theformation of compounds, such as

2CaO·Al2O3·SiO2·8H2O, that preventedcement hydration.13

The low mechanical strength of thecastable matrix that contained CC isattributed to its agglomeratingeffect. CC is a cement hydrationaccelerator2,3 that promotes a less-packed structure.

The organic additives PAS, HEC andthe combination of PAS and Algincreased castable mechanicalstrength after drying at 50°C. Thiseffect was caused mainly by the for-mation of polymeric chains (dryingat 50°C) and gel drying (at 50°C andthermal treatment at 500°C). These

©The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9204

CPI

Hg

(%)

Figure 6 Influence of shotcrete additives on pore-size distribution of castable matrix after thermal treat-ment at 500°C for 5 h (CPI Hg is cumulative percentage of intruded mercury).

Pore diameter (µm)

Figure 7 Influence of shotcrete additives on mechanical strength of castable matrix.

σ f(M

Pa)

resulted in a 3-D particle-boundstructure.

The refractoriness under load andcreep behavior of the matrix can beevaluated by the deformation (dL/L0)as a function of temperature (up to1500°C) and time (Figure 8). Theresults indicated that the shotcreteadditives used here influenced theseproperties significantly.

The addition of SS resulted in thegreatest deformation of the castablematrix. Moreover, the SS-containingsamples presented the lowest start-

ing deformation temperature (Figure9). This result may have been causedby the lower-temperature meltingphases in the Na2O–SiO2–Al2O3–CaOsystem.

CC led to greater deformation thandid the material without additives.However, its effect was less intensethan the SS-containing samples.Based on these results, the inorganicadmixtures yielded the poorestresults in the RUL and creep tests.

HEC, a gelling additive, resulted in alower creep than the inorganic

admixtures. PAS and its combinationwith Alg showed a similar behaviorin the RUL and creep tests. Bothadditives shifted the onset of defor-mation to higher temperatures(Figure 9). Moreover, these organicadmixtures decreased the maximumcreep deformation attained by thecastable matrix, which improved theperformance of the material.

A comparison of the mechanicalstrength of the samples dried at50°C and the creep results (Figure10) revealed a definite correlation.

Particle packing and pore-size dis-tribution did not substantially affectthe mechanical strength after drying.Therefore, the different valuesobtained were related to the addi-tive binding property and the RULand creep results to its chemistry.

The amounts of additive in thematrix can be greater under terms offield performance.1,3 Therefore, theireffect on the properties of thecastable can be intensified.Moreover, a distinct criterion for theselection of setting additives—besides rebound loss2–4 and dryingbehavior9—should influence thethermomechanical properties of thecastable matrix.

Admixtures Influence PropertiesThe introduction of coagulantadmixtures greatly influenced thethermomechanical properties of wetshotcrete refractory castable matrix.Organic additives that containedsodium PAS and PAS + Alg improvedmatrix performance in all propertiesevaluated.

Commercial inorganic additivesthat promote particle agglomeration(SS and CC) decreased the mechani-cal strength of green samples.Moreover, CC and, to a greaterextent, SS resulted in a higher creepand decreased the starting tempera-ture deformation.

©The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9205

Figure 9 Starting temperature deformation of matrix, T, with various additives, and maximum percentualdeformation, dL/L0, attained by matrix during creep test.

T (°

C)

dL/L

0(%

)

Figure 10 Maximum creep strain, dL/L0, vs mechanical strength, σf , of unfired castables cured anddried at 50°C.

dL/L

0(%

)

σf (MPa)

©The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org August 2005 9206

PAS and PAS + Alg increased themechanical strength of green andprefired matrix samples. These addi-tives also promoted less creep andhigher starting deformation temper-atures. Additives with these coagu-lant/flocculation mechanisms shouldbe taken into greater considerationfor wet shotcreting. �

Acknowledgments

The authors are grateful to the Brazilianresearch funding agencies FAPESP andCNPq. They also are grateful to ALCOA-Brazil and Magnesita for supporting thisresearch and to D. Vasques Filho for help-ing on the experimental procedure.

References

1I.L. Glassgol,“Refractory Shotcrete—TheCurrent State of Art,” Shotcrete Magazine,[Summer] 24–32 (2002).

2M. Jolin, D. Beaupré and S. Mindess,“Tests to Characterize Properties of FreshDry-Mix Shotcrete,” Cem. Concr. Res., 29,753–60 (1999).

3L.R. Prudêncio Jr., "AcceleratingAdmixtures for Shotcrete," Cem. Concr.Res., 20, 213–19 (1998).

4R.G. Pileggi, Y.A. Marques, D. VasquesFilho, A.R. Studart and V.C. Pandolfelli,“Wet-Shotcrete Additives,” Am. Ceram.Soc. Bull., 81 [6] 51–57 (2002).

5R.G. Pileggi and V.C. Pandolfelli,“Rheology and Particle-Size Distributionof Pumpable Refractory Castables,” Am.Ceram. Soc. Bull., 80 [10] 52–57 (2001).

6R.G. Pileggi,Y.A. Marques, D.Vasques Filho,A.R. Studart and V.C. Pandolfelli,“ShotcretePerformance of Refractory Castables,”

Refract. Appl., 8 [3] 15–20 (2003).

7D. Vasques Filho, Y.A. Marques, R.G.Pileggi and V.C. Pandolfelli,“Influence ofthe Polymeric Fibers on ShotcreteRefractory Castables” (in Portuguese),Ceramica, 50, 69–74 (2004).

8R.G. Pileggi, F.T. Ramal Jr., A.E. Paiva andV.C. Pandolfelli,“High-PerformanceRefractory Castables: Particle SizeDesign,” Refract. Appl., 8 [5] 17–21 (2003).

9Y.A. Marques, D. Vasques Filho, R.G.Pileggi and V.C. Pandolfelli,“Influence ofAdditives on the Permeability and DryingBehavior of Wet Shotcrete RefractoryCastables” (in Portuguese), Ceramica, 50,7–11 (2004).

10K.H. Khayat,“Viscosity-EnhancingAdmixtures for Cement-Based Materials:An Overview,” Cem. Concr. Res., 20,171–88 (1998).

11A.R. Studart, V.C. Pandolfelli, E. Tervootand L.J. Gauckler,“Gelling of AluminaSuspensions Using Alginic Acid Salt andHydroxyaluminum Diacetate,” J. Am.Ceram. Soc., 85 [11] 2711–18 (2002).

12I.R. Oliveira, A.R. Studart, R.G. Pileggi andV.C. Pandolfelli,“Dispersion and ParticlesPacking: Fundamental Aspects and theApplication in Ceramic Processing” (inPortuguese), Fazendo Arte Editorial, SãoPaulo, 2000; 224 pages.

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14D.J. Bray, J.R. Smyth and T.D. McGee,“Creep of 90+% Alumina Concrete,” Am.Ceram. Soc. Bull., 59 [7] 706–10 (1980).

15F.T. Ramal Jr., R. Salomão and V.C.Pandolfelli,“Water Content and Its Effecton Refractory Castables’ Drying Behavior,”Refract. Appl., 10 [3] 10–13 (2005).

Equations

Andreasen Packing Model5,12 Equation

CPFT = 100 � (D/DL)q

where CPFT is the cumulative percentage of particles smaller than diameter D, DLthe maximum diameter (CPFT = 100% when D = DL) and q the distribution coefficient.

Splitting Tensile Strength Equationσf = 2F/πDh

where σf is the splitting tensile strength, F the maximum force (in newtons) applied,D the sample diameter and h the sample height.