Cement and Concrete Research 78 (2015) 24–37

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Cement and Concrete Research

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Research review of cement clinker chemistry

Horst-Michael Ludwig a,⁎, Wensheng Zhang b

a Finger-Institute for Building Materials Science, Bauhaus-Universität Weimar, Weimar, Germanyb State Key Laboratory of Green Building Materials, China Building Materials Academy, Beijing 100024, China

⁎ Corresponding author.E-mail address: horst-michael.ludwig@uni-weimar.de

http://dx.doi.org/10.1016/j.cemconres.2015.05.0180008-8846/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 May 2015Accepted 13 May 2015Available online 3 June 2015

Keywords:ClinkerCa3SiO5Ca3SiO4SulfoaluminateBlended cement

This paper aims to review the progress in cement clinker chemistry since the last International Conference on theChemistry of Cement in 2011. Although Portland cement clinker is still, by far, the most important compound ofmodern cementswe show that there is a strong development of alternatives. This ismainly due to the emission ofcarbon dioxide during the calcination of calcium carbonate as rawmaterial whose reduction is the goal of inter-national activity due to anthropologically caused climate change. Furthermore, it is an objective to use bothmorerawmaterials that are located close to the concrete plants and alternative fuels. Developments in the field of ce-ment clinker chemistry show a potential for alternatives. Thereby we discuss both old and new ideas. But it hasbeen shown that the substitution of Portland cement clinker has to consider not only reduction in CO2 emissionduring fabrication: For practical solutions the performance in both in terms of strength development and dura-bility has to be adequate compared to the ordinary Portland cement clinker.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Since the first Portland cement with the present definition was pro-duced in 1843byWilliamAspdin there has been a continuous process ofevolution in cement process technology and cement itself. Comparedwith other industries the speed of development has been moderate,which can certainly be explained by the huge efforts needed to achievesafe and standardized concrete construction. However, since someyears there has been a significant increase in the development rate ofnew cementitious binders. The main impulse for this development isgiven by the international agreement linked to the United NationsFramework Convention on Climate Change to reduce global CO2 emis-sion (i.e. Kyoto Protocol). Due to the fact that cement production isresponsible for 5% of the global CO2 emission (i.e. 0.95 tons of CO2 perton of Portland cement) and also the introduction of the EuropeanUnion Emission Trading System the cement industry is facing the chal-lenge to reduce CO2 emission. Furthermore concrete production hasjoined the debate on sustainability of the material and productionprocesses.

To date the most effective way to reduce CO2 emission of cementproduction is to reduce the clinker content by blending cements withSupplementary Cementitious Materials (SCMs). These SCMs typicallyare ground granulated blast furnace slag, fly ash, silica fume and lime-stone. In several countries and for several applications these Portland-composite cements (CEM II according to EN 197-1 and type IS, IP andIT according to ASTM C595/C595M) have already partly replaced the

(H.-M. Ludwig).

classical Portland cement. The main component of Portland-compositecements remains the Portland cement clinker that is burnt in a rotarykiln.

In the light of these facts, the fundamental understanding of clinkerchemistry remains an important issue and will become even more im-portant if, as often desired, the SCM percentage further increases. Themain drawback of cements containing a high percentage of SCM is thelow early strength. Additionally, the durability of concretes is affectedby SCMs. Optimisation of SCM characteristics can only partly improvethese issues. A more effective option could be to target an increase inperformance of the clinker fraction in blended cements. Thus, eitherthe search for SCM specific accelerators or improved clinker reactivityis needed. To improve clinker reactivity a detailed characterization ofclinker phases starting from chemical composition of clinker phasesincluding minor components, crystallographic and microstructuralcharacterization is a fundamental prerequisite.

Aside from the development of cement composition also theproduction conditions of Portland cement clinker have changed sig-nificantly. Following the rising social demand for CO2 reduction andsustainability an increasing amount of secondary fuels and raw ma-terials are used in the cement production process. Who would havethought two decades ago that today some cement plants use upto 100% secondary fuels for production and in some countries (e.g.Germany, Switzerland, Austria) the mean proportion of secondaryfuels reaches 70%. These developments have induced important chang-es not only to process technology but also to product composition(chemical and mineralogical composition of clinkers). To ensure thequality of the product the potential changes with respect to reactivityhave to be understood. In this way it may also be possible to identify

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advantages of secondary raw and fuel materials (e.g. increased reactiv-ity to due incorporation of minor components introduced by secondarymaterials).

In addition to reduction of cement clinker content by blendingcements with SCMs a couple of other alternative ideas to reduce theCO2 footprint of cements exist. There are binary or ternary cementitiousmaterials composed of SCMs and activators (geopolymers, sulfatecontaining slag cements etc., which is the subject of another paper inthis volume) or cements based on complete different materials.Intensive research and development is currently carried out onCalcium Sulfoaluminate and binders based on reactive C2S polymorphs(Ca-Si-Bi) or hydraulic calcium hydro silicates (Celitement).

Our review starts with themain compounds of OPC clinker alite andbelite. Then we focus on the alternative fuels and raw materials (AFR)for the cement production. Finally we report on alternative binders.

Fig. 1. T1 structure of pure C3S.

2. Research on tricalcium silicate (alite)

2.1. Polymorphism, crystal structure and stabilization of alite

2.1.1. Polymorphism and crystal structure of aliteTricalcium silicate (C3S) has a nesosilicate structure with isolated

[SiO4] tetrahedra which are connected by Ca–O polyhedra. Because ofthe importance of alite, much research work has been done on thecrystal structure of C3S in the past 80 years. C3S exhibits a complexpolymorphism depending on temperature or impurities [1]. Due tothe complicated structure and the difficulty in preparing single crystalof C3S, it is difficult to obtain crystal structure information of individualpolymorphs. The analysis of the crystal structure of C3S has beenmainlybased on powder XRD, DTA and optical microscopy. Only threepolymorphic structures (R, M3 and T1) of C3S have been determinedby synthesizing single crystals [2–5]. In order to perform accurateRietveld quantitative phase analysis (QXRD), several studies in the lastdecade have focused on the crystal structure of alite [6–10]. Variousmodels are available for T1, T2, T3,M1, M3 and R polymorphs, however,there is still no structural model for the M2 polymorph. Since variousmodels are available for the polymorphs normally found in clinker,few studies have been done on the crystal structure of alite in the pastfive years.

The XRD pattern permits the determination of the dimensions andsymmetries of the various lattices, but little information can be derivedabout precise atomic positions in the structure. To resolve these prob-lems and to learn more details about the structure of C3S we recentlysynthesized single crystals of suitable size (4 mm) using the high tem-perature optical floating zone furnace [11]. The single crystal of pureC3S was tested using a CCD single crystal diffraction. 736 atomic param-eters were refined. The final crystallographic R factor is 3.58%, which ismuch smaller than that reported by Golovastikov (R = 9.7%) [12].This is by far the most exact determination for the structure of C3S.Space group: P-1(No.2), Lattice parameters: a = 13.719(2) Å, b =14.291(3) Å, c = 11.745(2) Å, α = 90.235(3)°, β = 94.395(3)°, γ =104.306(4)°, V = 2224.1(7) Å3, Z = 18. The structure is shown inFig. 1. It can be seen from Fig. 2 that the structure of C3S can be consid-ered as a three-dimensional assemblage of [O3Ca12] trimers. Three[OCa6] octahedra are connected to form a [O3Ca12] trimer, the trimerextends along the [112] direction and has a zigzag conformation. Itcan be seen in Fig. 3 that oxygen atoms are present in two differentcoordination environments. Among the 45 atomic sites, nine arepresent in special coordination environment such as O(5), O(6) andO(9). They are octahedrally coordinated by 6 calcium ions. It is believedthat these special oxygen atoms account for the high reactivity of C3S.The ordinary O in SiO4 is subject to sp3 hybridization, which is acovalent bond, whereas, the special O in [OCa6] is not connected withsmall radius Si, and the Ca–O–Ca is an ionic bond. The latter O wouldbe likely to have higher activity.

Based on the known structure, thedifferent polymorphs differ by thesilicate tetrahedral orientations. It is known that a higher symmetry of[SiO4] tetrahedron is attained as the structural symmetry of alite in-creases [13]. In any case, the accurate structure (atomic position) of dif-ferent polymorphs (especially for T2, T3, M1 and M2) is still not fullyunderstood.

2.1.2. Stabilization of high temperature polymorphs of alitePure C3S exhibits polymorphism when heated and can be triclinic

(T1, T2, or T3), monoclinic (M1, M2, or M3), or trigonal (R). While atroom temperature pure C3S only exists in the T1 (triclinic) form, thehigher temperature forms (T2, T3, M1, M2, M3 and R) are not stableeven if C3S is quenched [1]. A practical way to stabilize high temperaturepolymorphs is dopingwith foreign ions. Because thehydraulic reactivityof C3S polymorphs can be enhanced by doping with foreign ions, thissubject has gained great attention in recent studies.

2.1.2.1. Individual ions. The influence of individual ions on the structureof C3S has been extensively studied in the past decades [8]. It wasfound that the higher the crystal symmetry of C3S, the less stabilizingions can be incorporated. Based on the chemical structure of the ions,the basic patterns of substitution are summarized in Table 1. By defininga quantity called structure difference factorD [8], inwhich the structuralparameters, such as radius, electrovalence and electronegativity of thesubstituent ion were taken into account with respect to Ca2+, the rela-tionships between the chemical structure parameters of substituentions and their substitution patterns and abilities to stabilize thehigher-temperature forms of C3S have been established and quantified.As is shown in Table 1, the substitution position changes gradually fromCa to Si as the value of D increases, and at D ≥ 0.676 (D value for Ti4+)foreign ions start to replace silicon.

The presence of MgO has a stabilizing effect on C3S formation(decrease in temperature of formation, acceleration of the process,smaller crystals, M3 alite modification). Also an increase in C4AFcontent was observed in the presence of MgO [14–16]. A smallamount of Al2O3 was shown to be effective in stabilizing the M3-typepolymorph of alite [17]. Alite with a high P5+ concentration(P2O5 ≥ 0.5m.-%) can be stabilized as R-type alite [18]. The phase trans-formations of alite during reheating are mainly correlated with thepolymorphic form of alite and the kind and amount of foreign ions in-corporated. For example, for alite stabilized as M3 type and with highMg2+ concentration (2% MgO), the reversion to triclinic at 600 °C onthe reheating was blocked. The presence of Fe3+ has a reverse effect,i.e. the presence of Fe3+ promotes the transformation from M3 to Ttype [13]. In addition, it was also shown that the lattice parameters ofalite vary linearly with the amount of foreign ions up to their limits. A

Fig. 2. Schematic illustration of the structural elements for T1.

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discontinuity at particular concentration for phase transformationboundary or changes in the substitution pattern is observed that followsVegard's law [19,20].

The stabilization of C3S polymorphs by guest ions is shown schemat-ically in Fig. 4.

As shown in Fig. 4 the stabilization range of C3S polymorphisms isdependent on theD value suggesting that higher symmetry polymorphscan be stabilized with the increase in the difference between the sub-stituent ion and the parent ions.

Besides the nature of the foreign ions also the level of substitutionplays an important role in the stabilization of high temperature poly-morphs. Accordingly, it is known that higher temperature polymorphsare stabilized with increasing amounts of guest ions. Thus, the latticedistortion caused by ionic substitution could readily account for the sta-bilization of high temperature polymorphs.

Fig. 3. Schematic illustration of the coordination of O atoms.

2.1.2.2. Multiple ions. As constituents of the rawmaterials and fuels usedin the production of cement clinker, foreign ions such as Na2O, K2O,MgO, SO3, and P2O5 are unavoidably present during the formation ofalite. As a result, a limited amount of these oxides are introduced intothe chemical structure of alite. When several foreign ions are jointlypresent in alite, the structure and reactivity of alite are not additivelychanged but depend on the mutual interactions of these ions [13,18,21,22] which can be significantly different from the individual effectsof each ion. Due to the complexity of clinker, a vast number ofinteracting factors such as the amount of liquid formed, its appearance,temperature, viscosity, surface tension and other clinker minerals canbe involved in the effect of foreign ions on alite. Recently, the effect ofthe combined doping of the seven typical foreign ions (Na+, K+,Mg2+, Al3+, Fe3+, S6+ and P5+) in clinker has been studied systemati-cally by synthesizing pure alite phase. Compared with single doping,multiple foreign ions can more easily stabilize the higher temperatureforms of alite [13]. The combined doping with normal concentrationsof all the doped foreign ions could favor the formation of alite and pro-mote the M3-type alite stabilization. In this respect, Mg2+ and Al3+

seem to have the most significant effect on the stabilization of thehigher-temperature forms of alite. The absence of either Mg2+ or Al3+

resulted in the stabilization of T2-type alite.

2.1.2.3. Influence of minor or trace elements on clinker and alite formation.As stated before, the impact of foreign ions incorporated into the struc-ture of C3S influences the reactivity, but this effect depends strongly onthe incorporation level. Likewise the impact of foreign ions on the prop-erties of cement clinker is modified. It is known that minor or tracecomponents derived from rawmaterials or fuels affect both clinker for-mation and reactivity. During the past years, continuous efforts havebeen made to study the incorporation of various foreign ions (alone orin combinations) in the cement clinker and their effects on the clinkerformation and cement properties.

MgO and SO3 are the most common minor components of cementclinkers. Recently, the effects of MgO, SO3, temperatures and sinteringtimes (alone or in combination) on the clinker formation and alite poly-morphisms were studied by applying ex-situ XRD quantitative analysis[14,15]. It was found that the C3S and C2S contents were linearly relatedto the SO3/MgO ratio.MgO can enhance the formation of C3S both by ac-celerating the clinkerization reactions and by lowering the formation

Table 1[8] Chemical structure parameters of average ions incorporated in alite and their substitution patterns in C3S.

Ions Ionic radius (pm) Coordination number Electronegativity Substitution pattern D* value

O2− 140 – 3.44 – −2.021F− 136 – 3.98 F → O −1.114Li+ 60 6* 0.98 Li → Ca/interstitial or Ca/Si [6] −0.008Na+ 95 12 0.93 Na → Ca/interstitial −0.003Ca2+ 99 6,8 1 – 0Sr2+ 113 8* 0.95 Sr → Ca 0.014K+ 133 12 0.82 K → Ca 0.062Ba2+ 138 12 0.89 Ba → Ca 0.087Mn2+ 80 6 1.55 Mn → Ca 0.211Mg2+ 65 6 1.31 Mg → Ca 0.213Zn2+ 74 4 1.65 Zn → Ca 0.328Cu2+ 72* 4 1.9 Cu → Ca 0.491Ti4+ 68 6 1.54 Ti → Si 0.676Cr3+ 64 6 1.66 Cr → Ca/Si 0.7Al3+ 50 4,6 1.61 Al → Ca/Si/ hole 0.906Ga3+ 62 6 1.81 – 0.908Fe3+ 60 6 1.83 2Fe → Ca + Si 0.981Mn4+ 52* 4* 1.55 Mn → Si 1.044Si4+ 41 4 1.9 – 2.109P5+ 34 4 2.19 P → Si 3.907S6+ 29 4 2.58 S → Si 6.703

*Structure difference factor D (referred to Ca2+), D= Z*△x*(Rc − R)/Rc, where Z and R= the charge and radius of a ion respectively;△x= electronegativity difference between the el-ement and Ca; Rc = Ca2+ radius.

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temperature of C3S. Although SO3 hindered the formation of C3S, its neg-ative effects can be compensated by the addition ofMgO. The amount ofC4AF can be increasedwith the addition of bothMgO and SO3. Besides, itwas also found that MgO can favor the formation of M3 type alite,whereas SO3 promoted the stabilization of M1 type alite, just as it hasbeen described in the literature [23,24].

Another study on the use of fines and sludge generated out of theproduction of porphyry and dolomitic limestone for Portland clinkerproduction has shown that these alternative raw materials have someadditional advantages because of their higher MgO content. MgO com-bined in combination with alkali and SO3 has fluxing activities wherebythe alite formation is promoted which could improve the burnability ofthe alternative clinkers [25]. In high sulfate low alkali clinker, theamounts of the tricalcium aluminate (C3A) and alite as well as thealite/belite ratiowere decreased, which however leads to amodificationin the cement quality [26].

The distribution of P2O5 in the clinker minerals after the addition ofbone meal (BM) ash to the basic raw mixture was studied. It has beenfound that belite and alite accommodate nearly all the phosporoususin the clinker. P5+ enters the structure of calcium silicates with Al3+

and Fe3+ [27].

Fig. 4. [8] The relationship between D values* of substituent ions and their abilities to sta-bilize the higher-temperature forms of C3S.

2.2. Reactivity of alite

The fine changes in the chemical composition and structure inevita-bly lead to changes in the reactivity of alite with water. Because the re-activity of C3S and its polymorphs is too complex to be handled in thepresent review only principles on reactivity are discussed in thefollowing.

The hydration kinetics of alite with foreign ions may deviate signifi-cantly from that of pure C3S. Because many variables are involved, it isdifficult to compare the reactivities of different polymorphs. As a result,it is difficult or nearly impossible to handle these parameters indepen-dently. Back in the 1970's Fierens and Verhaegen [28,29] found thatthe influence of substituent ions on the reactivity of C3S was due tothe presence of defects, which could be studied by the irradiation in-duced thermoluminescence (TL). The complex relations between con-tent of substituent ions, polymorphism, defects and reactivity wererecently studied again [19,30,31]. As a result the findings of Fierensand Verhaegen [28,29] (relation between defect concentration and thehydration behavior of alite) remains open for discussion.

The impact of foreign ionswithin the structure of C3Swas investigat-ed by Stephan et al. [32] on Cr, Zn and Ni. It was found that Cr and Nilead to an accelerated C3S reaction whereas Zn shows retarding effects.But these effects were only evident at very high addition levels. In factBazzoni et al. [33] found an accelerating effect of Zn incorporated intothe structure of alite (0.98 and 1.16 wt.%). In another study it wasfound that the addition of Fe2O3 leads to a significant decrease in the hy-dration reactivity of alite [34]. The addition of phosphor containingma-terials leads to a decrease in viscosity of the melt and thus to theformation of larger alite crystals. This in turn may reduce the reactivityof the cement. The performance of cements obtained by addingphosporousous during clinker production is also varied. Early compres-sive strengths are slightly reduced due to incorporation of P (b1.0m.-%)and corresponding decreased amounts of alite and C3A. The 28d com-pressive strength is slightly increased (1.0 m.-% P2O5) and decreasesagain at increasing concentrations of phosphate (2.0 m.-% P2O5) on ref-erence values [35]. These results are in accordance to findings by others[36]. As a result of reduced C3A content the demand of calcium sulfate(as set regulator) decreases.

These findings are parts of the discussion on the causes of the induc-tion period during hydration of C3S or alite in general. This period occursafter first wetting of C3S and lasts for several hours before during the

Fig. 5. Recently discussed theories on the cause of the induction period.

Table 2Stabilization effect of foreign ions on β-C2S.

Ion addition Stabilization

P5+ Yes1,2

B3+ Yes1,2

As3+, As +5 No2, Yes1

V5+ Yes1,2

Al3+ No1,2

Cr2+, Cr3+, Cr+6 No2, No1,2, Yes1,2

Fe2+, Fe3+ No2, No1,2

Sr2+ Yes2

Mg2+ No1,2

Ba+2 No1, Yes2

Na+ No1, Yes2

K+ No1, Yes2

Mn2+, Mn3+, Mn4+, Mn6+ No1,2, No2, No2, No2

Data taken from [55,56].

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main hydration period the majority of C3S reacts to C-S-H phases andportlandite. Today's most discussed theories are the protective layertheory (first stated by Kantro et al. [37] and Stein and Stevels [38]),slow dissolution step hypothesis (Juilland et al. [39] and Nicoleau et al.[40] adapted from investigations on the dissolution rate of minerals[41,42]) and the aluminum induced formation of C-A-S-H phases [43].Fig. 5 gives an overview on the principles of the different theories.

According to the protective layer theory the surface of C3S is coveredby a product layer that is claimed tobe an intermediate phase in the pre-cipitation process of C-S-H phases [44–47]. The overall reaction processis believed to be controlled by that layer.

The slow dissolution step hypothesis on the other hand postulatesthat the slow reaction during the induction period is caused by a slowdissolution rate of C3S in conditions characterized by high ion concen-trations that are expected during the induction period [39,40,48]. Inthis case the formation of a layer is not necessary. Numerical modeling[49] of the hydration process that combines dissolution [50] and precip-itation reactions may contribute to understanding the mechanism dur-ing the early hydration stage of C3S and alite.

Besides these theories, the impact of aluminumon the early reactionof C3S has been investigated [43,48,51]. It is claimed that the formationof C-A-S-H phases lowers the number of C-S-H nuclei. Consequently, alonger period is needed to precipitate C-S-H phases that lead to theendof the induction period. The role of aluminum is of interest in partic-ular in cases when cement hydration is concerned including the use ofAFR. Further work is needed to clarify the role of aluminum in the hy-dration process.

3. Research on dicalciumsilicate (belite)

The need to reduce carbon dioxide emissions leads to a special focuson the reactivity of belite. By using alternative binders (e.g. calciumsulfoaluminate) the reactivity of belite is of central importance for theearly strength development of cements with low alite content. As a re-sult a number of works is published that aims at gaining new effortsin understanding the reactivity of belite.

3.1. Polymorphism, crystal structure and synthesis of belite

3.1.1. Polymorphism and crystal structure of beliteBelite is known to exist in six crystalline polymorphs. The poly-

morph present at very high temperatures is α-belite that transformsinto the αH′-polymorph at 1425 °C which is stable down to 1160 °Cbeing transformed to αL′-belite. The latter is replaced by the β-form atapprox 650 °C. Polymorphs from the α-family and the β-form have analmost identical crystal structure. When further cooled, β transformsinto γ that is stable at room temperature and the only polymorphwith a clearly distinct structure [1]. Recent results showed that twoother belite polymorphs exist. These two forms of belite are claimedto have a reactivity higher than alite. Themost reactive polymorph is in-deed an XRD-amorphous phase [52] with the same composition as thecrystalline polymorphs. The other very reactive polymorph is x-C2S[53]. These polymorphs have the identical chemical composition but amuch higher reactivity than polymorphs known from common cementmanufacture. The crystal structure of x-C2S is different to all other poly-morphs [54]. A particularity of x-C2S is the presence of small “tunnels”

Fig. 6. Phase diagram of the sub-system C2S - 7CaO-P2O3-2SiO2 [59].

Fig. 7. Production of highly reactive belite in a 2-step process.

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running parallel to the c-axis. These tunnels are possibly responsible forthe high reactivity.

3.1.2. Synthesis and stabilization of belite polymorphs

3.1.2.1. Stabilizing of high temperature polymorphs of belite.Despite beingunstable in the pure form at low temperatures, high temperature mod-ifications can be stabilized by rapid cooling, appropriate particle sizeand the incorporation of foreign ions. It was shown that Ba is beneficialforα, P forαH′, Sr forαL′whereas the β-polymorph can be stabilized bya broad range of elements. An overview of stabilization effects of foreignions on β-C2S polymorph is given in Table 2. It becomes evident that be-sides foreign element also the oxidation state influences the stabilizingeffect.

From the point of view of thermodynamics, the incorporation of for-eign ions leads to a slight reduction of the Gibbs free energy. Since thephase with the lowest Gibbs free energy is the stable one; the incorpo-ration of foreign ions could extend the zone of thermodynamic stability.This results in a changed phase diagramwith increasing content of for-eign ions, cf. Fig. 6.

Active belite cements produced by incorporation of foreign ions andfast cooling consist of β- and the various α-polymorphs [57]. However,sufficient cooling rates are difficult to realize and reactivity is also lowcompared to x- and amorphous-belite.

The addition of aluminum during burning of belite has been investi-gated by several researchers. It is found that silicate tetrahedra aresubstituted by aluminate which stabilizes β-C2S [36]. Furthermore it isargued that oxygen vacancies are generated which leads to an increasein water uptake and ionic conductivity. It is shown that even at hightemperatures protons or water remains in minor fractions in the crystalstructure [58].

Results gathered byNMR show that addition of sulfur during cementclinker production can cause also an increase in aluminum in silicate

phases [60,61]. Other researchers found that SO3 promotes C2S forma-tion at the expense of C3S [14,15]. It was documented that an SiO2 sub-stitution by SO3 of up to 4.4 wt.% stabilizes monoclinic β-C2S. By thismechanism the hydraulic activity increases up to 3 times with respectto the pure β-C2S [62]. This result is in contrast to observation that in-creasing sulfur additions decreases hydraulic reactivity of β-C2S [26,36].

Again by NMR spectroscopy [60,61] it was found that phos-porousous can enter the belite (β-C2S) structure which leads to its sta-bilization. The role of phosphates on clinker formation and reactivitywas studied in detail [35]. It was found that at low clinkering tempera-tures phosporousous leads to the formation of hydroxylapatite which isassociatedwith the consumption of calcium. At increased temperatures,decomposition of hydroxylapatite occurs. Thereby solid solutionsbetween C2S and C3P are formed. If the consumption of calcium is com-pensated in the rawmix then alite (with incorporations of P) is formed.If a higher amount of phosporousous is present in the mix then also adestabilization of C3A is observed because aluminum is increasingly in-corporated into C2S.

Similar to aluminum, also the addition of boron leads to a variationin the chemical structure of C2S polymorphs [60,61]. In this case silicatetetrahedra are substituted by borate tetrahedra. For charge compensa-tion also calcium is partly substituted by boron. In the same study, theimpact of Na/B was investigated showing a large variation in chemicalstructures of belite. For Rietveld refinement a new structure of boronactivated belitewas proposed. Itwas further stated that the stabilizationof belite is most effective in the presence of boron.

3.1.2.2. Synthesis of highly reactive belite polymorphs. The production ofhighly reactive C2S polymorphs was pioneered by Ishida [53] andGarbev et al. [63]. Supported by the presence of foreign ions it is basedon a two-step process that involves the synthesis of α-Ca2SiO4⋅H2O inan autoclave at 150–200 °C as a first step and its conversion into highlyreactive belite polymorphs by tempering at 400–800 °C in a second step,see Fig. 7. It was later shown by Link et al. [52] that a number of poly-morphs can be present in the same sample. The concentration of the

0

5

10

15

20

25

0 6 12 18 24 30 36

Hea

t re

leas

e ra

te [

J7(g

h)]

Time [h]

C3S

A420

B500

Fig. 8.Heat release rates for two samples with highly reactive C2S in comparison with C3Sanalyzed by isothermal calorimetry (modified from [52]).

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individual polymorphs depends on synthesis parameters of the twoprocess steps such as temperature, fineness, water vapor pressure andother details. It was also established that an amorphous belite materialcan be present in the samples after the dehydration step.

Alternative attempts to produce highly reactive belite with atwo-step autoclave process are also published. It is possible to producefly ash belite cements [64,65] or thermally activated hillebrandite(Ca2SiO4⋅H2O) [66–68]. These procedures result in the familiar β-α′-polymorphs and the reactivity is much lower compared to amorphous-and x-C2S.

3.2. Hydraulic reactivity of belite polymorphs

3.2.1. Reactivity of high temperature polymorphs of beliteBelite polymorphs in OPC clinker have a low reactivity which de-

pends on crystallographic modification, fineness, foreign ion substitu-tion and other parameters in the order α N α′ N β ≫ γ [69]. The β-C2Spolymorph commonly present in OPC reacts slowly whereas α-poly-morphs are higher in reactivity and γ-C2S is nearly inert.

Based on the chemical structure of C2S polymorphs thedifferences inreactivity are discussed [70]. According to the authors the difference incharge density of active O atoms in α′L- and β-C2S results in higherreactivity against water. In contrast γ-C2S exhibits no active O atomsand thus its reactivity is low. This finding can be the basis for under-standing the impact of foreign ions into the chemical structure of C2S.Nicoleau et al. [40] investigated the dissolution rate of C3S, C2S andCaO. They found that the dissolution rate decreases with an increaseof the ion concentration in the aqueous phase. Based on this, theyclaimed that dissolution theory is suitable to describe hydration kineticsunder paste conditions. Moreover, the decrease in dissolution rate isproposed to be caused by a superficially hydroxylated surface [71] ofthe silicates.

The role of foreign ions in the chemical structure of C2S was investi-gatedwith special focus on reactivity. It is known from earlier investiga-tions that the reactivity of C2S can be enhanced by mechanicaltreatment [72], fast cooling rate and incorporation of guest ions [73–76].

Along with the improvement of the reactivity of belite polymorphsdue to the presence of foreign ions during clinkerization, also mechani-cal activation of γ-C2S is of interest. This C2S polymorph is the maincompound of slags that originated from the production of stainlesssteel [77] which may be used as alternative raw material. It was foundthat high energy milling improves the reactivity of γ-C2S. However, bymeans of high energy milling only a slight improvement (9.1 MPaafter 90 days) of the γ-C2S reactivity can be achieved (sand:binderratio = 3, w/b = 0.75). Chemical activation by adding NaOHand Na2CO3 causes adequate effects. A combination between water

quenching and addition of mineralizer (NaF, Fe2O3) was shown to im-prove reactivity of belite [78].

3.2.2. Reactivity of highly reactive belite polymorphsIt was first demonstrated by Ishida et al. [67] that dehydration prod-

ucts from α-Ca2SiO4⋅H2O can react very fast with water. This was alsoconfirmed in the study by Link et al. [52]. In this study the process pa-rameters were investigated that have a direct impact on reactivity.Fig. 8 shows calorimetric data from two materials and conventionalC3S. Both C2S samples contain a high concentration of amorphous C2Sand x-C2S. Contrary to C3S, the two highly reactive C2S samples showan increased heat release rate during the induction period which indi-cates that the reaction to products occur. However other C3S samplescan show increased reactivity with respect to the C3S sample usedhere. According to the annealing conditions the maximum during themain stage of hydration is observed after approx. 12 h. This first maxi-mumof themain hydration period is related to the reaction of the amor-phous phase. The second maximum after approx. 24 h is associated tothe reaction of the x-polymorph. Thus, in relation to the C3S sampleused in this study amorphous C2S shows increased reactivity whereaswith respect to C3S the heat release rate of x-C2S is slower. It was alsoshown that the degree of hydration after 3 days is approximately 90%and thus higher than that of C3S after similar hydration times (37%). Itis expected that this high reactivity will result also in very high earlystrengths. Someof these samples containβ- orγ-C2S as a contaminationand investigations by X-ray diffraction confirm the low reactivity ofthese polymorphs.

Highly reactive belite cements have also other advantages com-pared Portland cements with alite as a major phase such as low spe-cific heat development required for mass concrete [1]. It is expectedthat highly reactive C2S will form a denser matrix with respect toOPC which results in good durability [79] (reduced capillary trans-port, higher frost resistance, etc.). Furthermore, the absence of AFmand AFt-phases is believed to be beneficial in terms of sulfate andfrost–thaw resistance. Based on unpublished work of the group atWeimar this binder is compatible with superplastisizers and stimulatesthe reactivity of latent hydraulic components. Finally, an intense grind-ing process is not required due to the high process related fineness ofthe material. Considering these properties, highly reactive belite ce-ments are a promising candidate to replace Portland cement clinkercontaining alite as main component.

4. Effect of AFR

A great amount of wastes from industrial, municipal, agriculturalsectors are used as AFR in the production process of cement clinkernowadays, for the reason of both lowering the consumption of naturalresources and reducing the emissions in cement industry andwaste dis-posal for other industrial sectors. These wastes are used for alternativefuels or alternative raw materials, depending on the characteristicssuch as compositions. Generally, wastes mainly consisting of combusti-ble organics contain a lot of energy and usually are used as alternativefuels for firing system, and wastes composed of CaO, SiO2, Al2O3, Fe2O3

and so on are usually used as alternative raw materials in the rawmeal preparation.

Coal is the primary fuel burned in cement kiln in most areas of theworld, but alternative fuels are commonly used in cement plants.Worldwide alternative fuels are used at a rate of below 10% (Fig. 9,[80]). Although in some European countries such as Germany, an aver-age of above 60% fuels used in cement kiln are alternative fuels [81]. Pe-troleum coke, used plastics and rubbers such as used tires, meat andbonemeal, biomass, solvent andwaste oil are traditional typical alterna-tive fuels used in cement industry. However, associated with large scalewaste disposal, sewage sludge and municipal wastes have become in-teresting alternative fuels in the cement industry.

Fig. 9. Solid Recovered Fuel (SRF)-substitution rate in cement industry (time period: 2000–2011, CPP = cement production plant) [80].

31H.-M. Ludwig, W. Zhang / Cement and Concrete Research 78 (2015) 24–37

4.1. Impact of alternative fuels on the production process and the clinkerquality

In contrast to primary fuels, significantly more ash is produced bythe combustion of most secondary fuels [82]. The amounts of ash are,particularly at high application rates of secondary fuels, of a scale thatmakes it necessary for them to be considered in the rawmeal. In situa-tions where the corresponding ashes contain less calcium andmore sil-icon, raw meals with higher lime saturation factor have to be used.Without this correction, this would lead to a significant decrease ofthe alite content in the clinker.

The resulting ashes can also have further implications, as they maycontain other property modifying oxides depending on the secondaryfuel. Through the use of meat and bone meal, as well as sewage sludgemore phosphorous may enter the clinker. The consequence of this is,as discussed in Section 2.1.2.1, the stabilization of belite a mixed crystalformation C2S and C3P and a decrease of the content of alite [35]. This isaccompanied by changes in the strength characteristics of correspond-ing cements. For example, the utilization of sewage sludge leads to

0 10 200

20

40

60 Alite Belite Aluminate phase Ferrite phase

phas

e co

nten

ts/%

(sewage sludge)/(raw meal) rate/%

Fig. 10. Phase content of clinker with the (sewage sludge)/(raw meal) ratio increasing[80].

variation in the content of clinker minerals as, for example, shown inFig. 10 [83], mainly due to phosphorous stabilizing belite phase andinhibiting the formation of alite frombelite andCaO in clinker formationprocess.

Fortunately, industrial trials suggest that the substitution of 10% ofthe heat demand by sewage sludge has an acceptable effect on the qual-ity of clinker [84]. It is believed that the addition of sewage sludge, ani-mal meal or bone meal has no negative impact on the development ofstrength when the phosphate content remains below 0.60 m.-% in theclinker [82].

Using different secondary fuels also leads to a size increasing of alitecrystals, which usually leads to a reduction in the early age compressivestrengths (12 h, 1d) and to an increase of the 28d compressive strength[85]. A large amount of sewage sludge used as AFR can lead to an in-crease of the alite size [86] (Fig. 11). Minor constituents, such as phos-phorous, sulfates, heavy metals, lead to changes in the properties ofliquid phase in clinkering process, the viscosity decreases and it be-comes easier for ions to diffuse in the liquid phase which leads tograin growth. Similar results are also reported by other authors [84,87].

Alitemay also grow large and quite perfectwith long and lazyflamesmost observed in the case of high levels of secondary fuels [80].

Minor amounts of alternative fuels in the range of amounts below20% usually do not alter much the properties of the clinker.

Alternative fuel derived frommunicipal wastes that has been buriedfor years was used in a cement manufacturing line in Beijing [88] to re-place about 10% of coal. The results show that strength of clinker, aswellas consistency, remains at the same level as without alternative fuel, be-sides setting time prolonged a little.

4.2. Heavy metals

The use of alternative raw and fuel material in clinker can also intro-duce various heavy metal ions, which could be dangerous to humanhealth and cause environmental problems. Significant attention hasbeen paid to the solidification and solidification mechanisms of heavymetals in cement clinker.

For waste with a high Zn content, the use of OPC as a binder for thesolidification/stabilization process can cause deleterious effect on ce-ment property. The validity of the alternative method to introduce Zninto the raw material before clinkerisation was assessed [89]. It has

Fig. 11. Alite phase in clinker with (sewage sludge)/(raw meal) ratio of 0 and 20% [80].

32 H.-M. Ludwig, W. Zhang / Cement and Concrete Research 78 (2015) 24–37

been shown that the immobilization during clinkerisation is a good al-ternative to treat waste with a high Zn content in Ordinary Portland Ce-ment (OPC).

Besides these components also the effect of heavy metals in traceson cement clinker production was investigated. Therefore thresholdlimits of Cu, Sn, Zn in a cement clinker (65% C3S, 18% C2S, 8% C3A, 8%C4AF) were defined as 0.35%, 0.7% and 1 wt.%, respectively [90] whichare quite high with respect to the current contents in clinker. It ap-peared that beyond the defined threshold limits, these elements haddifferent behaviors. Ni was associated with Mg as a magnesium nickeloxide (MgNiO2) and Sn reacted with lime to form a calcium stannate(Ca2SnO4). Cu changed the crystallization process and affected thereforethe formation of C3S. A high content of Cu in clinker led to the decompo-sition of C3S into C2S and of free lime. Zn, in turn, affected the formationof C3A. Ca6Zn3Al4O15 was formed while a tremendous reduction of C3Acontentwas identified. Cementsmadewith the clinkers at the thresholdlimits were at least as reactive as the reference cement. By others it wasshown that up to 6% Zn can be incorporated in C-S-H phases [91].

In leaching tests it was found that Cu, Cr andNi are trapped in clinkerphases andwere not liberated into the environment [92]. In general thestabilization of β-C2S increases (Cr3+ N Ni2+ N Zn2+), whereas copperhas a negative effect [93]. The incorporation of Sn in C3A causes changesin early hydration behavior. This effect depends on the level of Sn incor-poration. Therefore, additions of 0.5 and 1% Sn increase and 2% Sn de-crease C3A reactivity at early hydration period (up to 3 h). Sn alsostabilized the hydration products. Moreover, if C3A is mixed with gyp-sum the incorporation of Sn into the chemical structure of C3A leads toenhanced formation of AFt and AFmwithout changing the performanceof the mixture [94].

The solidification mechanisms for Cd and Ni during cement kiln co-processing of hazardous wastes were of three types [95]. One was theformation of a new solid. Substitution of the Ca in CaO to form a Ca–Cd–O solid was the main mechanism for Cd solidification in cementclinker. In the case of Ni, the metal tended to form a new inert com-pound, MgNiO2. A second process was the formation of an interstitialsolid solution; Cd and Ni could partly enter and be sited in the spacesof the C4AF lattice. The third typewas isomorphous replacement. There-by a small number of Cd2+ and Ni2+ ions substitute for Ca and Fe inC4AF, respectively, and are incorporated into the C3S crystal lattice byreplacement of Ca2+.

5. Alternative binder

5.1. SCMs

Due to the CO2 emissions in the traditional production of Portlandcement and the fact that forecasts project a doubling of cement con-sumption by 2050 [96], great efforts are being made to replace the tra-ditional Portland cement. Currently the most important option is the

use of so-called SCMs. Particularly through the use of the three compo-nents limestone, coal fly ash and blast furnace slag cement producerscould reduce CO2 emissions by about 18% with respect to the year1990 [97]. In some regions and for different applications Portlandcement has been largely replaced by cement with SCMs, whereasPortland cement clinker remains the basic material of these cements.Currently higher use rates of SCMs in the composite cements arehampered by problems in the early strength and durability [98].Furthermore, the availability of SCMs limits their use. Against this back-ground, researchers intensively investigate potential new SCMs cur-rently. Particularly promising SCMs are calcined clays [99–101] andmodified steelmaking slag [102–104].

5.2. Calcium Sulfoaluminate cements (C$A)

Besides the use of SCMs which today is the main instrument to re-duce material-related CO2 emissions, researchers are also investigatingcement production based on alternative clinkers. Further developmentof C$A cements and the production of novel cements based on α-C2SHare seen to be of special potential.

C$A cements have been known since the patent by Alexander Kleinin 1963 [105]. The classic C$A cements with ye'elimite as the mainphase have been produced and used in China since the 1970s [106].Fields of application are specific applications that exploit the specialproperties of the C$A cements such as rapid setting, high early strengthand shrinkage compensation. In addition, these cements were also usedsporadically for construction purposes [107,108]. Due to the high levelsof aluminum in the classical C$A cements (30–40%) and the requiredvery expensive bauxite as rawmaterial, these cements are not compet-itive as mass cements from an economic point of view. With regard tothe reduction of CO2 emissions C$A cements provide, however, a highpotential, since the formation of themain phase ye'elimite is associatedwith the release of significantly less CO2 compared to the clinker phasesof Portland cement:

Alite C3S ¼ 0:578 gCO2=gphaseBelite C2S ¼ 0:511 gCO2=gphaseAluminateferrite C2 A; Fð Þ ¼ 0:362 gCO2=gphaseCalciumsulfoaluminate C4A3$ ¼ 0:216 gCO2=gphase

The low burning temperatures (about 1250 °C) and the improvedgrindability of C$A also reduce the energy consumption and CO2

emissions. All in all, depending on the composition C$A cements emitabout 25 to 35% less CO2 during manufacturing compared to an OPC[109].

As already mentioned, with respect to total amounts these savingsare, however, currently not realizable in light of the high cost and avail-ability of raw materials. Therefore, recent research and developmentsfocus on so-called belite calcium sulfoaluminate cements (BC$A) with

Patents: 1. Klein US3251701 (1964) 2. Mehta US4036657 (1977) 3. Lafarge EP1781579 (2009) 4. Cemex EP0812811 (2003)5. Ost US3860433 (1975)6. JP 57-200252 (1982) 7. Italcementi EP1306356 A1 (2003)

Fig. 12. Patents in the system C4A3$-C2S-others [97].

33H.-M. Ludwig, W. Zhang / Cement and Concrete Research 78 (2015) 24–37

belite asmain phase and not ye'elimite. To lower the aluminum contentin the system, still mostly materials containing iron are added, so thatlarger amounts C4AF are formed and the Al2O3 content of cements canbe lowered to values between 14 and 17% [110].

Much of the ternary systemC4A3$-C2S-others has now been coveredby different patents (Fig. 12).

Amajor problemof the belite rich cements is the gap in reactivity be-tween the extremely fast reacting ye'elimite and the slow-reactingbelite. To solve this problem, a number of ideas have been developedand partially implemented in recent years.

An important starting point for closing the gap in reactivity is thestabilization of more reactive high-temperature modifications of belite,such asα-C2S andα′-C2S. For stabilization purposes B2O5 is particularlysuitable, but also Na2O and P2O5 [111]. Technically, the concept wasimplemented as part of a large-scale experiment in which 5500 t BC$A(AETHER™) with the composition 55% α′-C2S, 25% C4A3$ and 15%C4AFwere prepared [112].

A different approach is the technology of the so-called belitecalciumsulfoaluminate-ternesite (BCT), in which besides belite,ternesite (C5S2$) is a main phase [97,113]. Ternesite as sulfatespurrite has long been considered as not being hydraulically active.However, recent studies show that in the presence of reactive alumi-num ternesite is highly reactive. In the system BC$A ternesite istherefore able to close the gap between the reactivity of ye'elimiteand belite.

Another way to close the gap between the reactivity of ye'elimite-and belite is the introduction of alite-ye'elimite cements. The produc-tion of such cements is, however, associatedwith significant challenges.For the alite formation temperatures of 1450 °C are needed whileye'elimite decomposes above 1350 °C. To guarantee the coexistence ofboth phases it was previously proposed to either reduce the formationtemperature of the alite to a range between 1230 °C to 1300 °C by min-eralizers (fluoride, CuO, MgO) or to stabilize ye'elimite by suitable

Table 3Overview Celitement and Ca–Si–Bi.

Characteristics Binder

Celitement

Raw materials SiO2 and CaO containing materials (e.g. calciumhydroxide anFirst production step Autoclave (150–200 °C, 5 bar)Intermediate product Non-reactive α-C2SHSecond production step Activation grinding with a coarse SiO2-component (e.g. quarFinal reactive products Reactive calciumhydrosilicates (e.g. Ca2[SiO3OH](OH))Hydration products C-S-H-phases

measures to higher temperatures (e.g. addition of barium/strontium)[114,115]. Amore recent approach proposes a two-stage burningmeth-od in which firstly alite is formed at 1450 °C in a sulfate-rich raw mealand subsequently ye'elimite is obtained in a second burning step [116].

In addition to the change in the composition of the cements based oncalcium sulfoaluminate towards increased content of belite, the use ofwaste materials containing high levels of aluminum and sulfates cancontribute to cost reduction [117,118].

Owing to the considerable relevance today to reduce CO2 emissionand the potential offered by the group of C$A cements a variety of pub-lished studies on different aspects of these cements are available. Espe-cially the high temperature chemistry of clinker production [119,120]and the hydration of the BC$A and C$A cements [121–123] are thefocus of research. But also the possibility to include fly ash into C$A ce-ments in order to reduce CO2 footprint has been evaluated [124,125].

Decisive for the application of new cements in the field of concreteconstruction is the durability of concrete including also proper passiv-ation of reinforcement. Here, there have been few studies that partlycontradict each part. Because of their low porosity (very dense microstructure) concretes made with C$A cements possess a good resistanceagainst sulfate and chloride attack [126,127]. On the other hand resis-tance towards carbonation is estimated by most authors to be loweras compared to OPC concretes. One main issue seems to be the carbon-ation of ettringite [126,128].

Regarding the pH value of the aqueous phase in C$A concretes andtherefore potential for proper passivation of reinforcement, divergentstatements can be found in literature. One reason for the wide varietyof given pH valuedmight be of course also that the cements and respec-tive concretes differ largely in composition. The aqueous phase of C$Acements was investigated in detail by [129]. Here it was found thatthe pH value during the early hydration is low (approximately 10.5–11.0). Over the course of hydration an increase to pH of 12.8 was mea-sured. The maximum pH of 13.0 was found only by [130]. All other

Ca–Si–Bi

d quartz) SiO2 and CaO containing materials (e.g. calciumhydroxide and quartz)Autoclave (150–200 °C, 5 bar)Non-reactive α-C2SH

tz) Calcination (400–500 °C)Highly reactive belite polymorphs (e.g. x-C2S) and amorphous C2SC-S-H-phases and portlandite

34 H.-M. Ludwig, W. Zhang / Cement and Concrete Research 78 (2015) 24–37

determined pH values were less than 13 [131]. Concrete investigationsin the field did not detect significant corrosion of reinforcement [128].

Overall it must be noted that systematic investigations on concretedurability (especially with regard to composition of aqueous phaseand passivation of reinforcement) in dependence of systematic changesin C$A composition are still missing. Especially requested is durabilitydata also for promising belite containing C$A cements and concretesthereof.

Fig. 14. C-S-H formation in Ca–Si–Bi after 48 h.

5.3. Binder based on C-S-H

Another completely new technology of cement production is basedon the synthesis of initially nonreactive calcium silicate hydrate (for ex-ample α-C2SH). Referring to the composition of these intermediateproducts, the new binders are denoted as C-S-H binders. Currently,there are two fundamentally different products on this basis. While incase of “Celitement” theα-C2SH passes through a reactive grinding pro-cedure with quartz to form reactive calcium hydrosilicate, during thefabrication of the binder “Ca-Si-Bi” by calcination α-C2SH is convertedinto reactive polymorphs of belite. An overview of both binders isgiven in Table 3. Subsequently both binders will be discussed.

One of the first binders introduced of this type is the product calledCelitement (by Celitement GmbH, Germany) [132–134]. The produc-tion process of this binder is dived into several steps. First quartz andfree lime (or other CaO and SiO2 containing materials) are brought to-gether to a homogeneous rawmixture. The second step comprises reac-tion of this mixture at 150 °C–210 °C at 5 bar in an autoclave. Therebynon-hydraulic calcium-silicate-hydrates are formed. Preferential onlyα-C2SH is used for further Celitement production. To transfer thisnon-hydraulic material into a hydraulic binder a, so called, activationgrinding step is carried out. Thereby another SiO2 component (quartzsand or similar) is added. During this activation grinding process stabi-lizing hydrogen bonds of α-C2SH that are formed in the autoclave aredestroyed (“tribochemical” surface reaction). The newly formed hy-draulic binder consists of reactive calcium hydrosilicate, and is looselyattached to the added silicate component. The hydration starts after ad-dition of water at these interfaces (Fig. 13).

These phases are amorphous, reactive and water containing.Addition of mixing water induces a hydraulic reaction that furthertransforms these precursor C-S-H phases into C-S-H similar to thatformed during OPC hydration. The Ca/Si ratio of the final C-S-H phaseis lower than C-S-H formed during OPC hydration. According to thefirst studies on CO2 balance of the Celitement process up to 50% reduc-tion of CO2-compared to OPC production- can be achieved [132].

C-S-H-phases after 7 d hydration

Silicat material

Fig. 13.Microstructure of the hydrated Celitement [133].

Up to date the production of Celitement is limited to small scale pro-ductions. The upscaling of this pilot plant is planned for the next years.Alsomost of the performanceparameters of the newbinder are current-ly not available. Especially durability might be an upcoming issue sincecalcium hydroxide is not contained in the hydrated material. Thus in-vestigations on corrosion protection by passivation of reinforcementare urgently required. The question to be answered is if the low porosityof the binder ismaybe balancing the effect of amaterial inherent lowpHvalue that bears the main risk for reinforcement corrosion.

A further binder (Ca-Si-Bi) alreadymentioned in the Sections 3.1.2.2and 3.2.2 based on α-C2SH is introduced by studies [52]. The Ca-Si-Bitechnology is characterized by a calcination process at temperatures be-tween 400 and 500 °C after the autoclavewithout grinding. The productof this calcinations step is a highly reactive polymorph of belite (i.e. x-C2S) and amorphous C2S. During hydration of this binder C-S-H phasessimilar to those obtained during OPC hydration are formed. Contrary toCelitement, portlandite is additionally formed.

Interestingly, within the first 48 h hydration of the belite poly-morphs is nearly completed (i.e. C-S-H phases are formed, Fig. 14).

This is a significant difference to the hydration of belite contained inOPC, where after decades unhydrated belite is still found in concrete. Incontrast to the hydration of alite in OPC the reaction of the above de-scribed C2S binder produces more C-S-H phases and less calcium hy-droxide. Thus, a significant reduction in binder content can beexpected for concretes made with those newly developed C2S binders.Similar to Celitement the development of this binder is at an initialstate, i.e. only small scale production is possible. Thus investigationson durability of this binder and concretesmade thereof are also urgentlyrequired. Table 3 summarizes key points of Celitement and Ca-Si-Bibinders.

The two approaches discussed – C$A and cements based on α-C2SHprecursor – are characterized by the fact that reaction products similarto those of OPC hydration are formed. Completely different approachessuch as binders on the base of magnesium silicates [135], amorphouscalcium carbonates [136] orwollastonite [137] leave the area of hydrau-lic binders and are not discussed in the present review.

6. Conclusions and outlook

The present paper aims to summarize the development in the fieldof cement clinker chemistry in the last 4 years. Against the backgroundthat on the global scale CO2 emissions have to be reduced, themain goalof the cement industry is the reduction of the content of the Portland

35H.-M. Ludwig, W. Zhang / Cement and Concrete Research 78 (2015) 24–37

cement clinker in cements. But although promising approaches havebeen proposed in order to find alternatives, it is expected that Portlandcement clinker cannot be fully substituted in the near future. However,its content in cements can be reduced which results in the need of adeeper understanding of the impacts on cement clinker reactivity. Inparticular, a large amount of research is dedicated to understand the re-activity of belite since its production is associated with lower emissionsof CO2 as compared to C3S. Additionally, the use of AFR can impact theperformance of cements. We expect that the future cement will be agreen cement tailored more and more to the planed performance pro-duced with very different raw materials and fuels.

Acknowledgment

The authors would like to thank Thomas Sowoidnich and Dr. FrankBellmann for their support and helpful discussions.

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