Mortars with nano-SiO2 and micro-SiO2 investigated by experimental design

Mortars with nano-SiO 2and micro-SiO 2investigated by experimental designLuciano Senff a,*,Dachamir Hotza a ,Wellington L.Repette b ,Victor M.Ferreira c ,João brincha daMaterials Science and Engineering Graduate Program (PGMAT),Federal University of Santa Catarina (UFSC),88040-900Florianópolis,SC,Brazil bDepartment of Civil Engineering (ECV),Federal University of Santa Catarina (UFSC),88040-900Florianópolis,SC,Brazil cDepartment of Civil Engineering/CICECO,University of Aveiro (UA),3810-193Aveiro,Portugal dDepartment of Ceramics and Glass Engineering/CICECO,University of Aveiro (UA),3810-193Aveiro,Portugala r t i c l e i n f o Article history:Received 8April 2009Received in revised form 30December 2009Accepted 15January 2010Available online 6February 2010Keywords:Nanosilica Microsilica MortarUnrestrained shrinkagea b s t r a c tThis paper reports the effects of nanosilica (nS)and silica fume (SF)on rheology,spread on flow table,compressive strength,water absorption,apparent porosity,unrestrained shrinkage and weight loss of mortars up to 28days.Samples with nS (0–7wt.%),SF (0–20wt.%)and water/binder ratio (0.35–0.59),were investigated through factorial design experiments.Nanosilica with 7wt.%showed a faster forma-tion of structures during the rheological measurements.The structure formation influences more yield stress than plastic viscosity and the yield stress relates well with the spread on pressive strength,water absorption and apparent porosity showed a lack of fit of second order of the model for the range interval studied.In addition,the variation of the unrestrained shrinkage and weight loss of mortars do not follow a linear regression model.The maximum unrestrained shrinkage increased 80%for nS mortars (7days)and 54%(28days)when compared to SF mortars in the same periods.Ó2010Elsevier Ltd.All rights reserved.1.IntroductionThe use of mineral additions in cement-based materials has grown in recent years,due to sustainability action and environ-mental issues or due to the technical advantages reached on the fi-nal product [1–3].When used as a partial replacement of cement,suited additions can improve the performance of materials through modifications in the fresh and hardened states,physical and chem-ical properties,as well as the hydration and microstructure.More recently,nanosilica (nS)is receiving special attention because of its better performance when compared with traditional mineral additions [4].However,they exhibited stronger tendency for adsorption of ionic species in the aqueous medium and the forma-tion of agglomerates should be expected [5–12].In this case,it is necessary to use a dispersing additive to minimize this effect [13].In the fresh state,mortars and concrete can be considered as a fluid,in which the rheological parameters are very important to its processing and final quality.A traditional method used in Civil Engineering to evaluate the properties of mortar (workability)in the fresh state,is known by consistency test or flow table test [14].However,recently the rheometric characterization is becom-ing usual for cement-based materials,because it is possible to de-fine simultaneously plastic viscosity and yield stress for different torque applied [15].The rheological behavior of those mixtures is often represented by the Bingham equations ¼s o þl Ácð1Þwhere s (Pa)is the shear stress,s o (Pa)is the yield stress,l (Pa s)is the plastic viscosity and c (s À1)is the shear rate.Bingham model can be represented as a torque (T )as a function of rotation speed (N ).In this case,the model is expressed byT ¼g þhN ð2Þwhere g (N mm)and h (N mm min)are directly proportional to the yield stress and plastic viscosity,respectively.In the hardened state,the cement paste is not dimensionally stable,when it is exposed in the environment humidity below than 100%[16].The material will begin to lose water and shrink and,consequently,the stress generated can be responsible for cracking [17].The use of multiple regression models in cement-based materi-als is not usual as the linear regression model.However,this meth-od allows identifying the main effects and the interactive effect between the factors,although using a reduced number of experi-mental trials [18].Some authors [19–29]tested the effect of nS on cement pastes,mortars and concrete.However,rheological and flow table measurements have not been used as parameter to formulate compositions,using an experimental design ap-proach.Therefore,the main purpose of this paper is to report the effect of nS (0–7%),SF (0–20%)and W /B ratio (0.35–0.59)on rheo-logical properties,compressive strength,water absorption,appar-ent porosity,unrestrained shrinkage and weight loss of mortars using a factorial design of experiments.0950-0618/$-see front matter Ó2010Elsevier Ltd.All rights reserved.doi:10.1016/j.conbuildmat.2010.01.012*Corresponding author.E-mail address:lsenff@ (L.Senff).Construction and Building Materials 24(2010)1432–1437Contents lists available at ScienceDirectConstruction and Building Materialsjournal homepage:www.e l s e v i e r.c o m /l o ca t e /c o n b u i l d m at2.Experimental procedures 2.1.MaterialsThe Portland cement used was CEM I –52.5R (Cimpor,Portugal),as classified by the EN 197-1[30]standard.Its chemical composition is shown in Table 1.Silica fume (SF,920D,Elkem Microsilica,Norway)presented a specific area of 18.41m 2/g.A nanosilica slurry (nS,Levasil 300/30%,H.C.Starck,Germany),with 1.21g/cm 3density,contains 30wt.%solids,an average size of 9nm,a specific sur-face area of 300m 2/g and a chemical composition that is shown in Table 2.The superplasticizer (SP)admixture was a polycarboxylic acid based (Glenium 51,BASF,Germany)with solids content between 28.5and 31.5wt.%and density between 1.067and 1.107g/cm 3.A commercial sand (Weber Cimenfix,Saint Gobain,Portu-gal),composed by four size fractions (1.2,0.6,0.3and 0.15mm),each one corre-sponding to 25wt.%,was used as aggregate in the mortar.The flow table test (Fig.1a)as specified by EN 1015-3[31]was performed after mixing.The rheological behavior of fresh mortars was evaluated in a rheometer (Viskomat PC,Germany)(Fig.1b).The maximum speed was set to 100rpm and at every 15min the speed was brought to zero,kept during 30s,and later increased during 30s until reaching again 100rpm.This procedure allowed the construction of flow curves from which g and h values were extracted.For compressive strength,the 40Â40Â160mm prism samples were cast according to EN 196-1[32]and stored in plastic bags.After 24h they were demolded and immersed in water for 28days.The compressive strength was then determined by EN 1015-11[33]and the water absorption and apparent porosity were also determined by the immer-sion techniques [34].For unrestrained shrinkage and weight loss (CSTB 2669-4)[35]the 40Â40Â160mm prism samples was kept in relative humidity (55%)and temperature (23°C)and measured after 1,7and 28days.2.2.Methodology and mortar mixingSamples of mortars were prepared with a binder/aggregate weight ratio of 1:2to study rheology,flow table,compressive strength (28days),water absorption (28days),apparent porosity (28days),unrestrained shrinkage and weight loss (1,7and 28days).The preparation of mortars involved:(a)weighing of the compo-nents,(b)dry mixing solid components inside a plastic bag for 1min,(c)adding superplasticizer into water,(d)pouring the solid components into water,(e)mechanical mixing for 3.5min.The rheological and flow table measurements allowed us to define the lower and upper limits of nS,SF,W /B and SP.This methodology guarantees that all mor-tars have suitable working conditions in both equipments,since different plasticity levels are required.In addition,a factorial design method was used as a tool to pre-pare SF and nS containing formulations for the tests.The factorial 2k is one of the used designs to research the effects of different factors in a particular response,where k stands for the factors (nS,SF,SP or W /B )and the base 2represents the level of treatments for each factor.When the central point is added,the effect estimates,interactions and curvature of surface response can be estimated without to carry out the full factorial design,decreasing the number of the experiments [18].The range of W /B ratio was set up using mortars with nS,since nS requires much more water in the mixture,when compared to SF.For compressive strength,water absorption and apparent porosity,the factorial design 22with a central point was used to check the curvature,followed by factorial 32to determine the surface re-sponse.For unrestrained shrinkage and weight loss,the factorial 22with a central point was applied on interval relative to higher compressive strength (0.35W /B ).A proper number of replications were performed,for testing compressive strength (four replicas),water absorption (two replicas),apparent porosity (two replicas),unrestrained shrinkage (three replicas)and weight loss (three replicas).All design experiments of mortars formulated are shown in Table 3.3.Results and discussion3.1.Rheometry and flow table spread testThe rheology test of mortars with low W /B ratio or high amount of nS added,exhibited high values in the initial or end torque,and after 120–135min,mortars showed lower plasticity (Fig.2a).The structure formation,particle friction and free water available in mixtures can be responsible for this behavior,because such factors are altered with the evolution of cement hydration.In this case,the shear stress was not sufficient to breakdown the structures forma-tion,during the decrease of speed up to zero,and the Bingham’s model adjusted becomes poorer.With 7%nS,mortar exhibited fas-ter formation of structure during these resting periods as repre-sented by the peak in torque when the speed increases from zero back to the test speed (vertical line above the torque line).This line is set up during the resting time of the test procedure (30s),mean-ing that an additional torque is necessary to breakdown formed structure.After 15min of test,the yield stress increased,while the plastic viscosity remained almost constant (Fig.2b and c).Table 1Chemical composition of Portland cement CEM I –52.5R.Constituents Content (wt.%)SiO 220.9Al 2O 3 4.60Fe 2O 3 3.15CaO 62.0MgO 2.00SO 33.60Table 2Chemical composition of nS.Constituents Content SiO 2(wt.%)99.4Na 2O (wt.%)0.45Al 2O 3(wt.%)0.075Sulphate (wt.%)<0.1Fe (ppm)25Ca (ppm)10Zn,Pb and Cu (ppm)<0.1(a)(b)Fig.1.(a)Spread on table test and (b)rheometer.L.Senff et al./Construction and Building Materials 24(2010)1432–14371433Therefore,the yield stress is the rheological parameter that is more affected by the three-dimensional structure formation and plastic-ity loses of the mortar.Mortar with SF showed higher initial yield stress,while nS up to the end of the test.The amount of SP avail-able in the mixture with nS can be considered to lower the initial yield stress,however due to the higher reactivity of nS the mixture became lessfluid along the period of time the test was performed.The spread on theflow table after15strokes showed that the mortars had different deformability(Fig.3).The spread for samples with3.5%nS were higher than for20%SF.In this case,if one con-siders the amount of SP expressed in grams per surface particle area of each addition,for the nS results a value of approximately 0.00714g/m2,while for SF the amount is0.00476g/m2.Therefore,the amount of SP per surface area used in mixtures with nS was about1.5higher than with SF,what could explain the higherflow values obtained for the mixtures with nS.When the W/B of mortars with3.5%and7%nS are compared,the amount of water expressed in grams per superficial particle area of each addition,showed that mortar with3.5%nS was approximately1.5higher than mortars with7%nS.Therefore,mortar with7%nS shows more cohesion than for3.5%nS.The yield stress is the rheological parameter (see Fig.2b and c)that relates better with to the spread on table (see Fig.3),since that the plastic viscosity increases,when the spread on table also increases.pressive strengthWhen the values of compressive strength are analyzed,the fac-torial design22with central point showed that the curvature is sig-nificant,suggesting that the compressive strength variation did not follow the linear regression model.Therefore,the factorial design 32with the levels of treatment of SF(0%,10%and20%)or nS(0%, 3.5%and7%)and W/B(0.35,0.47and0.59)were used(Table3). The surfaces response areas with distinct curves are illustrated in Fig.4.The concavity of curve is positive when W/B varies and nS or SF is kept constant,while negative curve concavities were ob-tained when nS or SF vary and W/B is kept constant.Whenfine par-ticles of amorphous silica are added,the physical and pozzolanic effect can be expected.As consequence,the values of compressive strength increased when compared to mortars without mineral additions.However,when higher amount of cement is substituted (SF=20%)or the mixtures become difficult to mould(nS=7%)the performance is decreased.When the models were tested for lack offit,the result was sig-nificant and,therefore,other regression models or working inter-vals should be used.In this case,when different types of concavity(concave and convex)are present on the same response surface,along with wide composition ranges,the lack offit of the model can be significant.In this way,the authors[36]showed thatTable3Design of mortar formulations.Design experiments Mortars nS(wt.%)SF(wt.%)SP(wt.%)W/BRheology andflow table test nS 3.5;7–30.35;0.47SF–200.70.35 Compressive strength,water absorption,apparent porosity nS0;3.5;7–30.35;0.47;0.59SF–0;10;200.7 Shrinkage,(%)weight loss nS0;1.75;3.5–3;3.3;3.60.35SF–0;10;200.7;0.95;1.2Fig.3.Spread on table of mortar after15strokes.1434L.Senff et al./Construction and Building Materials24(2010)1432–1437it is possible to adjust the data to a 2nd order model,when the W /B was reduced (0.59–0.47).The authors also identified the interac-tive effects between the factors (W /B ,SF or nS),which are ex-plained in detail elsewhere [36].In addition,the maximum compressive strength of mortars found corresponds to 12.2%SF and 3.3%nS [36].A comparable added amount (10±5%SF and 3.5±0.5%nS)can be seen on the response surface illustrated in Fig.4.3.3.Water absorption and apparent porosityWhen mortars are composed with high amount of water (0.59W /B )or exhibited poorer plasticity condition (0.35W /B )to molding,the water absorption and apparent porosity can increase (Figs.5and 6).In this investigation,the plasticity loss of mortar be-comes more evident in mixtures with nS,due to its higher surface area.In addition,the higher surface reactivity of nS hinders the maintenance of the adequate conditions for molding in a longer time.As a consequence,the final densities of the samples become smaller,as observed in mortar with 7%nS (Fig.7).Mortars with 7%nS,exhibited the highest values of water absorption and apparent porosity (between samples with 0.35W /B ),while these values de-creased for mortars with 20%SF.In the latest,the samples with SF show still an adequate condition to be casting.For surface re-sponses,the lack of fit of the 2nd order of the model was significant and the previous comments on compressive strength (3.2)are valid here as well.3.4.Unrestrained shrinkage and (%)weight variationThe results of the weight loss and unrestrained shrinkage of mortars are illustrated in Table 4.In general,time and SP increased the weight loss,while nS and SF decreased it.For unrestrained shrinkage,nS exhibited higher values than SF.When the maximum unrestrained shrinkages are compared,nS mortars presented val-ues 80%(7days)and 54%(28days)higher than SF mortars for the same periods.In this case,when the minerals are added,theypressive strength of mortars with:(a)SF and (b)nS after 28days.Fig.5.Water absorption of mortars with:(a)SF and (b)nS after 28days.Fig.6.Apparent porosity of mortars with:(a)SF and (b)nS after 28days.L.Senff et al./Construction and Building Materials 24(2010)1432–14371435can refine the porous and produce higher shrinkage [16].On one hand,the higher reactivity of nS generates hydrate products in a shorter time when compared to SF;on the other hand,higher shrinkage is also obtained.In this case,the shrinkage can lead the materials to cracking in certain applications.Such cracking can be responsible for the reduction of the strength and the stiff-ness of concrete [37].For SP,the unrestrained shrinkage is also in-creased,since it improves the cement particle dispersion in water and also makes the pores finer [16].Table 5shows the individual effect estimates and the significant factors indicated by (Ã).All factors (nS,SF and SP)were significantto unrestrained shrinkage and weight loss of the mortar.However,the interactive effect between SF and SP was not significant.In addition,all experimental design showed that the curvature is sig-nificant,meaning that they did not follow a linear regression mod-el.When the individual effect estimates are analyzed,SF,nS and SP have a positive value,indicating a direct increase on the unre-strained shrinkage.For the weight loss,nS and SF are negative (de-creased),while SP is positive (increased)the results.4.ConclusionsMortar with 7%nS showed faster structures formation during the rheological test.The structure formation limited the open test time and influenced the yield stress more significantly when com-pared to plastic viscosity.Mortars with 3.5%nS and 3%SP showed spread on table values higher than the ones with 20%SF and 0.7%SP.In addition,yield stress is the rheological parameter better re-lated to the spread on table.For compressive strength,water absorption and apparent porosity,the lack of fit of 2nd order of the model was significant,when the composition ranges were 0–7%nS,0–20%SF,and 0.35–0.59W /B .It is suggested that a more restricted interval should be used.For mixtures with 0.35W/B ,the water absorption and apparent porosity reached the maximum values for mortars with 7%nS.The factorial design showed that the unrestrained shrinkage and weight loss of mortar did not follow a linear regression model and the mortars with nS showed higher values than SF.With 7days the shrinkage increased 80%,while at 28days it increased 54%.AcknowledgementsThe authors acknowledge the support of CAPES,Brazil.The authors also thank Weber Portugal,BASF,Elkem,Cimpor for 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