Effects of Sodium Hexametaphosphate Addition On

materials

Article Eﬀects of Sodium Hexametaphosphate Addition on the Dispersion and Hydration of Pure Calcium Aluminate Cement

Benjun Cheng 1, Can Yao 1 , Jian Xiong 1, Xueyin Liu 2, Haijun Zhang 3,* and Shaowei Zhang 4,*

1 School of Energy Science and Engineering, Central South University, Changsha 410083, China; [email protected] (B.C.); [email protected] (C.Y.); [email protected] (J.X.) 2 College of Civil Engineering and Architecture, Quzhou University, Quzhou 324000, China; [email protected] 3 The State Key Laboratory of Refractory and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China 4 College of Engineering, Mathematics and Physical Science, Exeter University, Exeter EX4 4QF, UK * Correspondence: [email protected] (H.Z.); [email protected] (S.Z.); Tel.: +86-1569-718-1003 (H.Z.); +86-1300-712-2491 (S.Z.) Received: 2 October 2020; Accepted: 17 November 2020; Published: 19 November 2020

Abstract: The eﬀects of sodium hexametaphosphate (SHMP) addition on the dispersion and hydration of calcium aluminate cement were investigated, and the relevant mechanisms discussed. The content of SHMP and the adsorption capacity of SHMP on the surface of cement particles were estimated using plasma adsorption spectroscopy and the residual concentration method. The rheological behavior of hydrate, ζ-potential value of cement particles, phase transformation and the microstructure of the samples were determined by coaxial cylinder rheometer, zeta probe, X-ray diﬀraction (XRD) and scanning electron microscopy (SEM). The results indicate that SHMP readily reacted with 2+ 2 Ca , forming complexes [Ca2(PO3)6] − ions which were subsequently adsorbed onto the surfaces of cement particles. When the content of SHMP was 0.05%, the adsorption ratio reached 99%. However, it decreased to 89% upon further increasing the addition of SHMP to 0.4%. The complexes 2 2+ [Ca2(PO3)6] − adsorbed onto the surfaces of cement particles inhibited the concentration of Ca and changed ζ-potential, resulting in enhanced electrostatic repulsive force between the cement particles and reduced viscosity of cement-water slurry. The experimental results indicate that the complexes 2 [Ca2(PO3)6] − covering the surfaces of cement particles led to a delayed hydration reaction, i.e., they extended the hydration time of the cement particles, and that the optimal addition of SHMP was found to be about 0.2%.

Keywords: pure calcium aluminate cement; sodium hexametaphosphate; adsorption; ζ-potential; rheological properties

1. Introduction Superplasticizers around the world can be divided into several distinct types based on diﬀerent chemicals; they have a similar function, that is, to reduce the water content of concrete without loss of workability [1–3]. Normally, most of these chemicals are high molecular weight organic compounds, some are synthetic and others are derived from natural products. The main water- reducing agents are based on naphthalene, melamine, lignosulfonate and other natural compounds, such as glucose, sucrose and hydroxylated polymers, and salts of organic hydroxycarboxylic acids, etc. [4–6]. Water reducer is usually a kind of surfactant, which belongs to anionic surfactant. After the

Materials 2020, 13, 5229; doi:10.3390/ma13225229 www.mdpi.com/journal/materials Materials 2020, 13, 5229 2 of 11 superplasticizer dissolves into water, it will dissociate negative ions, which will be adsorbed on the surface of cement particles. When the adsorption reaches a certain degree, the cement particles show electrical properties. The charge between cement particles is the same. Under the action of electrostatic repulsion, the particles are dispersed, and a solvated water film is formed on the surface of the particles to reduce the surface tension of water, so that the fluid can penetrate into the solid particles and the dense mixture has enough water permeability. Due to the electrostatic repulsive force and the lubricating effect of the solvent water film, the cohesive structure (flocculation structure) between the cement particles is broken up, releasing the free water that it encloses. A small amount of superplasticizer can reduce the water content of cement, but it does not affect the fluidity or workability of cement [7,8]. On the other hand, because of the addition of water-reducing agent, the surface of cement particles forms an adsorption film, which affects the hydration speed of cement, makes the growth of cement stone crystal more perfect, reduces the capillary gap of water evaporation, and makes the network structure more compact, which improves the hardness and structural densification of cement mortar [9–11]. Fluidity is considered one of the basic properties of cement-based materials, because it plays an important role in determining the workability of construction and various properties of hardened materials [12]. Superplasticizers are usually added to improve the fluidity of cement slurry [13]. In recent decades, a variety of superplasticizers with different dispersing capacity have been developed and applied in cement-based materials. Among them, polycarboxylate (PCE), lignosulfonate, etc., superplasticizers are commonly used in Portland cement used in building materials [14–16]. However, the aluminate cement used as binder in refractory pouring materials generally uses sodium hexametaphosphate (SHMP), sodium tripolyphosphate (STP), etc., superplasticizers [17]. In the unshaped refractory industry, calcium aluminate cement (mainly composed of CA and CA2) is an excellent adhesive for the preparation of high-performance refractories because of its good early strength, high fire resistance and good wear resistance [18]. Sodium hexametaphosphate (SHMP) is commonly used as a superplasticizer for calcium aluminate cement (CAC) composite castables with a silica fume content of 2–8%; because of its long chain, it can ensure that the refractory castable can achieve the required fluidity and easy pouring construction performance, and promote the refractory casting construction lining to form a low porosity and high-mechanical-strength structure [19,20]. In recent years, the behavior and performance of SHMP in different systems has been studied by several researchers. Xu et al. [21] tried to develop a kind of chemically bonded refractory castable based on acid-based reaction between calcium aluminate cement (CAC) and polyphosphate-based reactants. It was found that the addition of sodium hexametaphosphate (SHMP) significantly improved the fluidity of the resulting binder, which was due to its good anti flocculation effect and sufficient hydration of CAC particles [22]. Ma and Brown used sodium phosphate to modify the hydration of CAC, thereby inhibiting the formation of metastable CAH10 and C2AH8, and avoiding subsequent conversion reactions to become stable C3AH6 [23–25]. Although, many scholars have done many studies on SHMP, only limited work has been done on the dispersing/retarding effect of SHMP on pure calcium aluminate cement. For example, the amount of SHMP adsorbed on cement particles is related to what factors, how to achieve the water reducing effect, how the cement hydration process depends on the amount of SHMP added and a series of related issues. The relevant mechanism still has not been fully understood. To address this, in the present work, ζ-potential, the adsorption amount of SHMP, the concentrations of P element and Ca2+ and rheological properties corresponding to different SHMP additions were examined, based on which, and the hydration behavior of SHMP investigated, the relevant mechanism underpinnings were discussed. The work would provide an insight into the effects of SHMP on the dispersion and water reduction in a PCAC (pure calcium aluminate cement)-bonded castable system. Materials 2020, 13, 5229 3 of 11

2. Experimental

2.1. Raw Materials Pure calcium aluminate cement (SECAR 71) was supplied by the Sinosteel Luoyang Institute of Refractories Research (Luoyang, China), the chemical composition of which is listed in Table1. Analytical-grade SHMP was purchased from Luoyang Hexin Refractories Co., Ltd (Luoyang, China).

Table 1. Chemical composition of PCAC.

Al2O3 CaO SiO2 Fe2O3 MgO TiO2 SO3 w/% 68.7 28.5 0.4 0.2 0.25 0.2 0.15

2.2. Sample Preparation and Characterization

2.2.1. Adsorption of SHMP onto PCAC Cement A total of 2.5 g PCAC was mixed with 50 mL SHMP solution with a concentration of 0%, 0.05%, 0.1%, 0.2%, and 0.4%, respectively, and stirred with a glass rod for 5 or 30 min. A total of 5 mL of the ﬁltrate was taken, and its P element and Ca2+ concentrations were determined by plasma adsorption spectroscopy (ICAP 6000 SERIES, Cambridge, UK) and used to estimate the SHMP content in it. The amounts of SHMP adsorbed onto the surfaces of cement particles were evaluated by the residual concentration method [26].

2.2.2. ζ-Potential of Cement Particles ζ-potential values of cement particles were measured by zeta probe (ZetaProbe, LWL Development Limited, Hong Kong, China). A total of 10 g PCAC was mixed with 200 mL SHMP solution with a concentration of 0%, 0.05%, 0.1%, 0.2%, and 0.4%, respectively, and the suspension was stirred. After 5 min, the zeta potential of the cement particles was measured by a potentiometer [27]. During the experiment, the pH value of the system was around 11.2, decreasing slightly with the test time increasing, reaching the standard pH value (11–12) in the CAC system.

2.2.3. Rheological Behavior of Cement-Water Slurry PCAC was also mixed with water in a given weight ratio of 7/3, followed by the addition of respectively 0%, 0.05%, 0.1%, 0.2% and 0.4% SHMP. A rheometer (MCR301, Anton Paar, Styria, Austria) with a coaxial cylinder geometry (ST22-4V-40 system) was used to examine the rheological behavior of each slurry at 37 ◦C. (This experiment is carried out at room temperature, in summer.) For the static 1 1 state ﬂow test, the shear rate used was in the range from 0.1 s− to 1000 s− .

2.2.4. Hydration Behavior of PCAC A total of 2 kg PCAC was mixed with, respectively, 0%, 0.05%, 0.1%, 0.2% and 0.4% SHMP, followed by the addition of appropriate amounts of water under stirring. The resultant wet mix was cast into a mould of 40 mm 40 mm 160 mm. The samples were demoulded after natural curing × × for 24 h in a thermostat at 37 ◦C (simulating normal summer temperature). Phase changes in the samples were analyzed by X-ray diﬀraction (XRD) (PANalytical, Empyrean, The Netherlands), and their microstructures were observed by using a scanning electron microscope (SEM) (Nova400NanoSEM, Amsterdam, The Netherlands). Materials 2020, 13, 5229 4 of 11

3. Results and Discussion

3.1. Adsorption Behavior of SHMP Based on the residual concentration method, the concentrations of P element determined by Materials 2020, 13, x FOR PEER REVIEW 4 of 11 ICAP canMaterials be used 2020, 13 to, x estimateFOR PEER REVIEW the amounts of SHMP in the ﬁltrate and adsorbed onto the4 of surfaces 11 of cement particles. At the beginning of the experiment, the cement particles sank to the bottom, and the Based on the residual concentration method, the concentrations of P element determined by adsorption amountBased on ofthe SHMP residual on concentration its surface was method, very little,the concentrations almost none. of TheP element concentration determined of by p element ICAP can be used to estimate the amounts of SHMP in the filtrate and adsorbed onto the surfaces of was measuredICAP can and be used compared to estimate after the standingamounts of for SHMP 5 min in the and filtrate 30 min, and adsorbed and it was onto found the surfaces that thereof was cement particles. At the beginning of the experiment, the cement particles sank to the bottom, and no change. Therefore, stirring was adopted to enhance its adsorption capacity. Figure1 shows these the adsorption amount of SHMP on its surface was very little, almost none. The concentration of p two amountselement aswas a functionmeasured ofand total compared addition after of standing SHMP for and 5 min stirring and 30 time, min, revealingand it was thatfound the that amounts of SHMPthere adsorbed was no change. onto theTherefore, surfaces stirring ofcement was adop particlested to enhance increased its adsorption almost capacity. linearly Figure with 1 the total additionshows of SHMP these (Figuretwo amounts1c,d). as Figure a function2 further of total illustrates addition of the SHMP change and of stirring the adsorption time, revealing ratio that of SHMP the amounts of SHMP adsorbed onto the surfaces of cement particles increased almost linearly with (i.e., thethe ratio amounts of the of amount SHMP adsorbed of SHMP onto adsorbed the surfaces onto of cement cement particles particles increased to the almost total additionlinearly with of SHMP) the total addition of SHMP (Figure 1c,d). Figure 2 further illustrates the change of the adsorption with thethe total total addition addition of of SHMP SHMP (Figure and stirring1c,d). Figure time. 2 further When illustrates 0.05% SHMP the change was of added, the adsorption the adsorption ratio of SHMP (i.e., the ratio of the amount of SHMP adsorbed onto cement particles to the total ratio after 5 min stirring reached 99.6%. However, it decreased to 89.2% upon increasing the SHMP addition of SHMP) with the total addition of SHMP and stirring time. When 0.05% SHMP was added, additionthe to adsorption 0.4%. Furthermore, ratio after 5 comparison min stirring ofreache Figured 99.6%.2a,b revealsHowever, that, it de forcreased a given to addition89.2% upon of SHMP, the adsorptionincreasing ratio the SHMP after 30 addition min stirring to 0.4%. was Furthermore, greater than comparison that after of Figure 5 min 2a,b stirring, reveals indicating that, for a that the stirringgiven assisted addition the adsorptionof SHMP, the of adsorption SHMP onto ratio the after cement 30 min particles,stirring was as greater the stirring than that time after increased 5 min from stirring, indicating that the stirring assisted the adsorption of SHMP onto the cement particles, as the 5 min tostirring, 30 min, indicating the adsorption that the stirring capacity assisted was the enhanced. adsorption of SHMP onto the cement particles, as the stirring time increased from 5 min to 30 min, the adsorption capacity was enhanced.

Figure 1. Amounts of SHMP in the filtrate after respectively: 5 min (a) and 30 min (b) stirring, and Figure 1.FigureAmounts 1. Amounts of SHMP of SHMP in the in ﬁltratethe filtrate after after respectively: respectively: 5 5 min min (a)(a) andand 30 min (b) (b) stirring, stirring, and and those those adsorbed onto cement particles after respectively: 5 min (c) and 30 min (d) stirring. adsorbedthose onto adsorbed cement onto particles cement afterparticles respectively: after respectively: 5 min 5 (c)min and (c) and 30 30 min min (d) (d) stirring. stirring.

Figure 2. Adsorption ratio of SHMP after 5 min (a) and 30 min (b) stirring, respectively, as a function Figure 2. Adsorption ratio of SHMP after 5 min (a) and 30 min (b) stirring, respectively, as a function Figure 2.of theAdsorption addition amount ratio of of SHMPSHMP. after 5 min (a) and 30 min (b) stirring, respectively, as a function of of the addition amount of SHMP. the addition amount of SHMP. Materials 2020, 13, 5229 5 of 11 Materials 2020, 13, x FOR PEER REVIEW 5 of 11

FromFrom Figure 11a,b,a,b, itit alsoalso cancan bebe seenseen thatthat onlyonly minorminor amountsamounts ofof PP elementelement werewere detecteddetected inin thethe 2+ filtrates.filtrates. This This might might be be due due to toone one of the of the follo followingwing reasons: reasons: (1) the (1) chelation the chelation of SHMP of SHMP with Ca with in Ca the2+ 2+ solutionin the solution resulted resulted in insoluble in insoluble phosphates, phosphates, and (2) andthe chelation (2) the chelation of SHMP of with SHMP Ca with in the Ca 2solution+ in the resultedsolution in resulted soluble in complexes soluble complexes which were which adsorbed were adsorbed onto the ontosurfaces the surfacesof cement of particles, cement particles, and re- precipitatedand re-precipitated with cement with particles. cement particles. Previous Previousstudies [28–30] studies showed [28–30 ]that showed the phosphates that the phosphates generated 2+ fromgenerated the strong from chelation the strong of chelation SHMP with ofSHMP Ca were with water Ca2+ soluble,were water so the soluble, first case so could the first be caseruled could out. 2+ Therefore,be ruled out. it can Therefore, be considered it can bethat considered the complexes that theformed complexes from the formed chelation from of the SHMP chelation with ofCa SHMP (see Reactionwith Ca2 +(1)(see in Section Reaction 3.2 (1) below) in Section were 3.2 adsorbed below) wereonto the adsorbed surfaces onto of cement the surfaces particles, of cement leaving particles, only a littleleaving P element only a little in the P filtrates. element in the filtrates.

2+ 3.2.3.2. Ca Ca2+ ConcentrationConcentration in in Filtrate Filtrate

2+ FigureFigure 33 illustratesillustrates thethe changechange ofof CaCa2+ concentrationconcentration in in the the filtrate filtrate with with the the total total addition of of 2+ SHMP.SHMP. After 5 min stirring, the Ca2+ concentrationconcentration in in the the case case of of no no SHMP SHMP was was 497.3 497.3 mg/mL, mg/mL, but but it it 2+ decreaseddecreased toto 186.4186.4 mg mg/mL/mL upon upon increasing increasing the SHMPthe SHMP addition addition to 0.2%. to The0.2%. Ca 2The+ concentration Ca concentration became becamealmost constantalmost constant as the total as the addition total addition of SHMP of SHMP was increased was increased to >0.2%. to >0.2%. Comparison Comparison of Figure of Figure3a,b 2+ 3a,breveals reveals that that the Cathe2 +Caconcentration concentration decreased decreased with with increasing increasing the the stirring stirring time. time. TheThe above results 2+ indicatedindicated that the addition addition of SHMP inhibited the dissolution ofof CaCa2+,, which which was was additionally additionally assisted assisted byby stirring, and and with the increase of stirring time, the inhibitory effect effect is better. This was consistent with that found from the adsorption tests presen presentedted above (Section 3.1),3.1), and and could could be be similarly similarly explained. In the presence of SHMP, it combined with Ca2+2+ to form water-soluble [Ca2(PO3)6]22− ions explained. In the presence of SHMP, it combined with Ca to form water-soluble [Ca2(PO3)6] − ions (Reaction(Reaction (1)) (1)) [30–32], [30–32], which were adsorbed onto the surfaces of cement particles and subsequently 2+ re-precipitatedre-precipitated with with them, resulting in the reducedreduced CaCa2+ concentrationconcentration in in the filtrates.filtrates. The The stirring 2+ promoted the the chelation chelation of of SHMP SHMP with with Ca Ca2 +andand thus thus the the whole whole process process stated stated above. above. According According to 2+ Figureto Figure 3, the3, theCa Caconcentration2+ concentration reached reached the minimum the minimum upon addition upon of addition about 0.2% of about SHMP, 0.2% indicating SHMP, theindicating best water the bestreducing water effect. reducing effect.

22++ 22− + + (NaPO(NaPO)3)6+ +2Ca 2Ca → [Ca[Ca(PO2(PO3))6]] − ++ 6Na6Na (1)(1) 3 6 → 2 3 6

FigureFigure 3. EffectEﬀect of of SHMP SHMP addition addition on on Ca Ca2+2 +concentrationconcentration in in the the filtrate ﬁltrate af afterter stirring stirring for for respectively: respectively: 5 min5 min (a) (a) and and 30 30 min min (b). (b).

3.3.3.3. ζζ-Potential-Potential of of Cement Cement Particles Particles FigureFigure 44 showsshows the ζζ-potential-potential of of cement cement particles particles as as a a func functiontion of of the the addition addition amount amount of of SHMP. SHMP. ζ-potential in the case of without SHMP was 8.5 mV. However, it changed to 14.8 and 20.2 mV upon ζ-potential in the case of without SHMP was 8.5 mV. However, it changed− to −14.8− and −20.2 mV uponadding, adding, respectively, respectively, 0.1% and0.1% 0.2% and SHMP.0.2% SHMP. Upon Upon further further increasing increasing the SHMP the SHMP addition addition from from 0.2%

Materials 2020, 13, 5229 6 of 11 Materials 2020, 13, x FOR PEER REVIEW 6 of 11 to0.2% 0.4%, to the0.4%,ζ-potential the ζ-potential almost almost did not did change. not change. These results These and results their and signiﬁcance their significance can be discussed can be asdiscussed follows. as follows.

Figure 4. ζζ-potential-potential of of cement cement particles particles as as a a fu functionnction of addition amount of SHMP.

As indicated by Reaction (1) and mentioned above, when SHMP was added, water soluble 2 2+ [Ca2(PO(PO33))6]62]− ions− ions were were formed formed due due to its to itschelation chelation with with Ca2+ Ca, which, which were were subsequently subsequently adsorbed adsorbed onto ontothe surfaces the surfaces of cement of cement particles, particles, resulting resulting in a reduced in a reduced concentration concentration of P element of P element in the in filtrate. the filtrate. The 2 Theadsorption adsorption of [Ca of2(PO [Ca3)26(PO]2− ions3)6] on− ionsthe surfaces on the surfacesof cement of particles cement led particles to in the led change to in the of ζ change-potential of ζfrom-potential positive from to positivenegative. to With negative. increasing With increasing the amount the of amount SHMP of from SHMP 0 to from 0.2%, 0 to more 0.2%, and more more and 2 more[Ca2(PO [Ca3)26(PO]2− ions3)6] were− ions formed were formed and accumulated and accumulated onto ontothe surfaces the surfaces of cement of cement particles, particles, which which led led to tothe the significant significant increase increase in the in the absolute absolute value value of ζ of-potential.ζ-potential. When When the theSHMP SHMP addition addition was was >0.2%,>0.2%, the 2 thesurfaces surfaces of cement of cement particles particles became became “saturated” “saturated” with complexes with complexes [Ca2(PO [Ca3)62]2(PO− ions,3)6 ]and− ions, the negative and the negativeelectric strength electric of strength cement of particles cement reached particles th reachede maximum the maximum value; thus, value; the electrostatic thus, the electrostatic repulsion repulsionbetween the between cement the particles cement particles became became the largest. the largest. Consequently, Consequently, further further increasing increasing the theamount amount of ofSHMP SHMP to toabove above 0.2% 0.2% did did not not lead lead to to any any obvious obvious change change in in the the ζζ-potential-potential value. value. The The great great increase increase in the ζ-potential value with the SHMP SHMP addition addition implied that the dispersion of cement particles could be significantlysignificantly improved. When the concentration of SHMP is greater than 0.2%, the dispersibility of PCAC is not further improved. SHMP contains sodium ions, so when the concentration of SHMP + is increased, the concentrationconcentration ofof NaNa+ inin the solution will increaseincrease accordingly.accordingly. Keita Irisawa and others mentioned that, in the formulationformulation of refractory castables, th thee main refractory raw materials, cement and sodium salt additives all contain solublesoluble sodium, which is easy to form into low-melting sodium salt, which is easy to collapse at high temperatures,temperatures, which affects affects the refractoriness of the castable and other other high-temperature high-temperature performance performance prop propertieserties [24]. [24]. The The purpose purpose of ofthis this research research is to is toimprove improve the the performance performance of ofPCAC PCAC by by adding adding SHMP, SHMP, so so that that it it can can better better act act as as a a binder binder in the castable. Based Based on on Figure Figure 33 andand thatthat discusseddiscussed above,above, thethe optimaloptimal additionaddition ofof SHMPSHMP forfor achievingachieving the best dispersion eeffectffect was around 0.2%. The ex excessivecessive addition of SHMP beyond 0.2% would not make further improvement in the di dispersibilityspersibility of PCAC. On the contrary, due to the increase in the + concentration of Na+,, it it will will ultimately ultimately affect affect the the refractori refractorinessness and and other other high-temperature high-temperature properties of castable products.

3.4. Rheological Properties 3.4. Rheological Properties Figure5 demonstrates the relationship between shear stress and shear rate in the cases of Figure 5 demonstrates the relationship between shear stress and shear rate in the cases of cement-water slurries added with diﬀerent amounts of SHMP [33]. According to the rheological cement-water slurries added with different amounts of SHMP [33]. According to the rheological theory, all the slurries belonged to non-Newtonian ﬂuids. As can be seen from Figure5, the shear theory, all the slurries belonged to non-Newtonian fluids. As can be seen from Figure 5, the shear stress generally increased with increasing the shear rate. In the case of without SHMP, the shear stress stress generally increased with increasing the shear rate. In the case of without SHMP, the shear stress decreased rapidly as the shear rate increased from 0.1 to 16 s−1, which was probably caused by the

Materials 2020,, 13,, 5229x FOR PEER REVIEW 7 of 11 Materials 2020, 13, x FOR PEER REVIEW 7 of 11 destruction of the flocculated structures due to the rotor rotation. Furthermore, at a given shear rate, 1 decreasedthedestruction shear stress rapidly of generally the as flocculated theshear decreased rate struct increased withures increasing due from to the 0.1 therotor to addition 16 rotation. s− , which amount Furthermore, was of probablySHMP, at asuggesting given caused shear by that the rate, destructionSHMPthe shear was ofstressvery the flocculatedgenerallyeffective decreasedin structures r reducing/avoiding with due increasing to the rotor the the rotation. agglomeration addition Furthermore, amount of offine SHMP, at aparticles given suggesting shear and rate,the that thegenerationSHMP shear stresswas of flocculation very generally effective decreasedstructure. in r reducing/avoiding with increasing the the addition agglomeration amount of of SHMP, fine suggestingparticles and that the SHMPgeneration was very of flocculation effective in r structure. reducing/ avoiding the agglomeration of fine particles and the generation of flocculation structure.

Figure 5. Shear stress versus shear rate curves for cement-water slurries added with different amounts Figureof FigureSHMP: 5. Shear5. (a) Shear without stress stress versusSHMP, versus shear (b) shear with rate rate 0.1% curves curves SHMP, for for cement-water (c)ceme withnt-water 0.2% slurries SHMP, slurries added and added (d) with with di ffdifferent0.4%erent SHMP. amounts amounts of SHMP:of SHMP: (a) (a) without without SHMP, SHMP, (b) (b) with with 0.1% 0.1% SHMP, SHMP, (c) with(c) with 0.2% 0.2% SHMP, SHMP, and and (d) with(d) with 0.4% 0.4% SHMP. SHMP. Figure 6 further displays viscosity values of cement-water slurries added with different amounts of SHMP,FigureFigure 6versus further 6 further shear displays displays rates. viscosity The viscosity viscosity values values ofin cement-waterofgeneral cement-water decreased slurries slurries with added added increasing with with di ffdifferenttheerent shear amounts amounts rate. ofFurthermore,of SHMP, SHMP, versus versusat a sheargiven shear rates. shear rates. Therate, The viscosity itviscosity decreased in in general withgeneral increasing decreased decreased the with with addi increasing increasingtion amount the the shearof shear SHMP, rate. rate. Furthermore,showingFurthermore, the shear-thinning at at a givena given shear shearbehavior rate, rate, in it it decreasedmost decreased cases with(except with increasing increasingfor in the the case the addition ofaddi additition amounton amount of 0.4% of of SHMP, SHMP, SHMP, showingwhereshowing the the viscosity, the shear-thinning shear-thinning changed behavior very behavior little in with in most most increasi cases cases (exceptng (except the shear for for in rate). in the the caseThe case decrease of of addition additi inon viscosity of of 0.4% 0.4% SHMP, could SHMP, wherebewhere attributed the the viscosity, viscosity, to the changeddestruction changed very very of little the little withflocculate with increasing increasid structures.ng the the shear shear The rate). rate).results The The decreaseshown decrease in in Figures in viscosity viscosity 5 and could could 6 beindicatedbe attributed attributed that tothe to the additionthe destruction destruction of appropriate of of the the flocculated flocculate amountsstructures. dof structures. SHMP (around The The results results 0.2% shown in shown this inwork) in Figures Figures could5 and make5 and6 6 indicateda significantindicated that that improvement the the addition addition ofin appropriateofthe appropriate rheological amounts amounts proper ofties SHMP of SHMPof cement-water (around (around 0.2% 0.2% systems, in this in this work) which work) could is could believed make make a significantto abe significant beneficial improvement improvement to the improvements in the in rheological the rheological in rheolo properties gicalproper ofpropertiesties cement-water of cement-water and workability systems, systems, which of a is whichPCAC-bonded believed is believed to be beneficialcastable.to be beneficial to the improvements to the improvements in rheological in rheolo propertiesgical properties and workability and workability of a PCAC-bonded of a PCAC-bonded castable. castable.

Figure 6.6. Viscosity–shearViscosity–shear rate rate curves curves for for cement-water cement-water slurries slurries added added with diwithﬀerent different amounts amounts of SHMP: of (a)SHMP:Figure without (a) 6. without SHMP, Viscosity–shear (b)SHMP, with (b) 0.1% rate with SHMP, curves 0.1% (c) SHMP,for with cement-water 0.2%(c) with SHMP, 0.2% slurries and SHMP, (d) withadded and 0.4%(d) with with SHMP. different 0.4% SHMP. amounts of SHMP: (a) without SHMP, (b) with 0.1% SHMP, (c) with 0.2% SHMP, and (d) with 0.4% SHMP.

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3.5.3.5. EEffectﬀect of SHMP Addition on Hydration BehaviorBehavior ofof CalciumCalcium AluminateAluminate CementCement AsAs isis wellwell documented,documented, the hydrationhydration of calciumcalcium aluminate cement involves three main steps: 2+ dissolution, nucleation and setting. Upon combination with water, Ca and2+ Al(OH) are− released from dissolution, nucleation and setting. Upon combination with water, Ca and Al(OH)4− 4 are released thefrom cement. the cement. When When their concentrations their concentrations reach a reach critical a level,critical hydration level, hydration products products will start will to nucleate start to andnucleate then and precipitate then precipitate [34,35]. These[34,35]. hydration These hydration steps are steps closely are closely related related to temperature, to temperature, CAH10 CAHis the10 mainis the hydration main hydration products products at temperatures at temperatures less than less 20 ◦ Cthan and 20 C 2°CAH and8 and C2 AHAH38 areand the AH main3 arehydration the main productshydration above products 20 ◦C; above however, 20 °C; at however, a temperature at a temperature above 35 ◦C above (the test 35 temperature°C (the test temperature in this work wasin this at aboutwork was 37 ◦ C),at about CA and 37 CA°C),2 CAin the and cement CA2 in react the cement with water react according with water to according Reactions to (2) Reactions and (3) [35 (2)–37 and]. (3) [35–37]. 3CA + 12H C3AH6 + 2AH3 (2) 3CA + 12H→ → C3AH6+2AH3 (2)

3CA2 + 21H C3AH6 + 5AH3 (3) 3CA2 + 21H→ → C3AH6+5AH3 (3) However,However, thesethese hydrationhydration reactionsreactions werewere significantlysignificantly aaffectedffected byby the addition of SHMP, asas willwill bebe describeddescribed andand discusseddiscussed below.below. FigureFigure7 7 illustrates illustrates thethe eeffectffect ofof SHMPSHMP additionaddition onon thethe hydrationhydration extentextent of PCACPCAC (after(after 1 dd hydration), revealing that 3CaO Al O 6H O (C AH ) was formed as the main hydration product, hydration), revealing that 3CaO·Al· 2O2 3·6H3· 2O2 (C3AH3 6) was6 formed as the main hydration product, and andits diffraction its diffraction peaks peaks in the in sample the sample without without SHMP SHMP (Figure (Figure 5a) were5a) werehigher higher than in than the insamples the samples added added with different amounts of SHMP (Figure7b–e). In the latter, Al (PO ) (OH) 9H O (Vantasselite) with different amounts of SHMP (Figure 7b–e). In the latter, Al4(PO44)3(OH)4 3 3.9H23O· (Vantasselite)2 was wasalso alsodetected detected [38], [and38], it and decreased it decreased with withincreasing increasing the addition the addition amount amount of SHMP. of SHMP. The results The results of XRD of XRDcan only can onlyjudge judge the type the type of substance, of substance, not not the the specif specificic quantity, quantity, and and the the integrated integrated area area of of the peak lineline cancan representrepresent thethe relativerelative content.content. In severalseveral samples,samples, exceptexcept for thethe didifferentfferent concentrationconcentration ofof SHMP, other conditions are the same. We compared the peak-line integrated areas of C AH , CA, CA SHMP, other conditions are the same. We compared the peak-line integrated areas3 of C63AH6, CA,2 and other substances in different samples to determine the relative change of their contents in different CA2 and other substances in different samples to determine the relative change of their contents in samples. With increasing the addition amount of SHMP, C AH decreased whereas both CA and CA different samples. With increasing the addition amount of3 SHMP,6 C3AH6 decreased whereas both CA2 increased. Figure8 further gives SEM images of the samples after 1 d hydration. Granular C AH was and CA2 increased. Figure 8 further gives SEM images of the samples after 1 d hydration.3 Granular6 formed in the hydrated microstructure, but its amount decreased after increasing the addition amount C3AH6 was formed in the hydrated microstructure, but its amount decreased after increasing the of SHMP, especially when the addition was 0.1%. Combination of the results shown in Figures7 addition amount of SHMP, especially when≥ the addition was ≥0.1%. Combination of the results andshown8 reveals in Figures that the 7 and addition 8 reveals of SHMP that the inhibited addition the of hydration SHMP inhibited processes the of hydration CA and CA processes2. This canof CA be explained as follows: and CA2. This can be explained as follows:

Figure 7. X-ray diﬀraction (XRD) patterns of PCAC after 1 d hydration: (a) without SHMP, (b) with Figure 7. X-ray diffraction (XRD) patterns of PCAC after 1 d hydration: (a) without SHMP, (b) with 0.05% SHMP, (c) with 0.1% SHMP, (d) with 0.2% SHMP, and (e) with 0.4% SHMP. 0.05% SHMP, (c) with 0.1% SHMP, (d) with 0.2% SHMP, and (e) with 0.4% SHMP.

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FigureFigure 8.8.Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) images images of PCAC of PCAC after 1after d hydration: 1 d hydration: (a) without (a) without SHMP, (bSHMP,) with ( 0.05%b) with SHMP, 0.05% ( cSHMP,) with 0.1%(c) with SHMP, 0.1% and SHMP, (d) with and 0.2%(d) with SHMP. 0.2% SHMP.

2−2 2+ AsAs mentionedmentioned earlier in this this paper, paper, [Ca [Ca22(PO(PO3)36)]6] ions− ions formed formed by bythe thechelation chelation of SHMP of SHMP with with Ca Cawere2+ were adsorbed adsorbed onto onto the thesurfaces surfaces of cement of cement part particles,icles, which which reduced reduced the the contact contact area area between thethe cementcement particles particles and and water, water, leading leading to the to significant the significant delay indelay the hydrationin the hydration of CA and of CA CA2. Moreover,and CA2. theMoreover, nucleation the and nucleation precipitation and ofprecipitation hydration product of hydration requires product sufficiently requires high concentrationssufficiently high of 2+ 2+ − Caconcentrationsand Al(OH) of 4−Ca[14 and]. However, Al(OH)4 this[14]. wasHowever, not the this case was when not the SHMP case when was added. SHMP was As discussedadded. As above,discussed the Caabove,2+ concentration the Ca2+ concentration was very low was when very SHMP low waswhen present. SHMP Therefore, was present. the nucleationTherefore, and the precipitationnucleation and of precipitation hydration products of hydration would products be inhibited. would Forbe inhibited. the above For reasons, the above when reasons, SHMP when was usedSHMP as was a dispersing used as a agentdispersing for calcium agent for aluminate calcium cement,aluminate its cement, addition its amount addition should amount be should carefully be controlled.carefully controlled. Otherwise, Otherwise, its excessive its excessive addition couldaddition lead could to an lead extended to an extended hydration hydration time of calcium time of aluminatecalcium aluminate cement, andcement, significant and significant degradation degradation in the high-temperature in the high-temperature properties. properties.

4.4. ConclusionsConclusions 2 (1)(1) Complexes Complexes [Ca 2(PO[Ca32(PO)6] −3)ions6]2− initiallyions initially formed fromformed the reaction from of sodiumthe reaction hexametaphosphate of sodium 2+ (SHMP)hexametaphosphate with Ca ions (SHMP) were with adsorbed Ca2+ ontoions were the surfaces adsorbed of cementonto the particles. surfaces Whenof cement the additionparticles. amountWhen the of SHMPaddition was amount 0.05%, of the SHMP adsorption was 0.05%, ratio reachedthe adsorption 99%. However, ratio reached when 99%. the addition However, amount when wasthe addition increased amount to 0.4%, was the increased adsorption to ratio 0.4%, adversely the adsorption decreased ratio to adversely 89%. decreased to 89%. 2 (2)(2) TheThe adsorptionadsorption of complex complex [Ca [Ca22(PO(PO3)36)]62−] ions− ions onto onto the the surfaces surfaces of cement of cement particles, particles, in the in case the caseof SHMP of SHMP addition, addition, could could change change their their electroc electrochemicalhemical properties properties and and improve thethe rheologicalrheological propertiesproperties andand workability of of a a cement-bonded cement-bonded cast castable.able. When When the the addition addition of SHMP of SHMP was wasless than less than0.2%, 0.2%, the absolute the absolute value value of ζ of-potentialζ-potential increased increased with with the theadditi additionon amount amount of ofSHMP. SHMP. However, However, it itbecame became almost almost constant constant upon upon increa increasingsing the the addition addition of SHMP to >>00.2%,.2%, suggesting thatthat thethe optimaloptimal additionaddition ofof SHMPSHMP waswas aboutabout 0.2%.0.2%. 2 (3)(3) TheThe complexes complexes [Ca [Ca2(PO2(PO33))66]]2− ionsions adsorbed adsorbed onto onto the the surfaces surfaces of cement particlesparticles couldcould inhibitinhibit 2+ thethe dissolutiondissolution ofof CaCa2+ ions,ions, andand retardretard thethe hydrationhydration ofof CACA andand CACA22,, resultingresulting inin anan extendedextended hydrationhydration timetime ofof cementcement particles.particles.

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Author Contributions: C.Y. and J.X. drafted the manuscript and performed the experiments; B.C. contributed to the conception of the study; X.L. performed the calculations; H.Z. and S.Z. performed the data analyses and revised the manuscript. All co-authors contributed to manuscript. All authors have read and agreed to the published version of the manuscript. Funding: The study was financially supported by The State Key Laboratory of Refractories and Metallurgy (Wuhan University of Science of Technology) (ZR201601), and National Natural Science Foundation of China (Grant No. 51902180). Conflicts of Interest: The authors declare no conflict of interest.

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