inﬂuence of a long-term retarder on the hydration of ... · rickert and thielen on long-term...

Cement, Concrete, and Aggregates, Dec. 2004, Vol. 26, No. 2Paper ID CCA12315

Available online at: www.astm.org

Joerg Rickert1 and Gerd Thielen2

Influence of a Long-Term Retarderon the Hydration of Clinker and Cement

ABSTRACT: This paper summarizes the results of investigations on the effects of a long-term retarder (LTR) based on phosphonic acid (PBTC)on the hydration of several clinker phases (C3S, C3A, C4AF), Portland cement clinker, and Portland cement. It could be shown that long-termretardation is due to a thin sparingly soluble layer of calcium phosphonate on the particle surface. The formation of sparingly soluble calciumphosphonate requires 2.5 moles Ca2+ per mole PBTC. If enough Ca2+ is available, sparingly soluble calcium phosphonate precipitates and formsa layer around the cement particles, which retards further hydration immediately. In the contrary a lack of Ca2+ causes a short-term accelerationin hydration of the reactive clinker phase C3A. The investigations show that a controlled long-term retardation can only be achieved if the cementshows an optimal “natural” setting retardation; hence, when the sulphate ingredient is fully compatible with the reactivity of C3A.

KEYWORDS: C3A, C4AF, C3S, hydration, long-term retarder, phosphonate, Portland cement clinker, Portland cement, retardation

Introduction

Long-Term Retarders

Long-term retarders (LTR) are organic retarders, which on ac-count of their composition are capable of inhibiting the hydrationof cement very strongly, i.e., for several days. During their use asrecycling aids for wash water or residual concrete, they facilitatethe cleaning of truck mixers and concrete mixers, and enable therecycling of the wash water or the residual concrete at sites thathave no recycling water reservoir, or only a small one. Long-termretarders allowed in Germany as recycling aids consist of phospho-nates and mainly contain 2-phosphono-butane-1,2,4-tricarboxylicacid (PBTC) (Bayer AG 1998; Deutsche Bauchemie e.V. 1999;DIBT 2000). PBTC affects the hydration reactions of the cementwith the mixing water very strongly.

In addition to wash water and residual concrete recycling, long-term retarders are also used directly to control the hydration of thecement, e.g., in combination with accelerators for shotcrete in thewet-spray process for tunnel construction and for deep drilling op-erations, and in the production of bore piles and roller compactedconcrete (Persinski et al., 1976; Kinney, 1989; Bobrowski et al.,1990; Fischer, 1991; Bobrowski et al., 1991; Burge et al., 1992;MBT, 1993; Ramachandran, 1995; Bobrowski et al., 1996;Bodamer, 1997; Wenquan et al., 1997; Paolini et al., 1998; Okawaet al., 2000).

Contrary to the plasticizers, superplasticizers and air entrainingagents, where a wide range of knowledge has been gained concern-ing identification and mode of action in cement paste, mortar, andconcrete, knowledge concerning long-term retarding admixtures

Manuscript received; accepted for publication July 30, 2004; publishedDecember 2004.

1 Dr.-Ing., Research Engineer, German Cement Works Association (VDZ),Concrete Technology Department, Duesseldorf, Germany.

2 Prof. Dr.-Ing., Managing Director, German Cement Works Association(VDZ), Duesseldorf, Germany.

is still inadequate. Depending on the temperature and the time ofaddition, inverse reactions can occur with particular cements.For example, retarders may suddenly act as setting acceleratorsand/or severely impair the strength development of concretes(Goetz, 1967; Seiler, 1969; Meyer, 1979; Lewandowski, 1983;Richartz, 1983; Rickert, 2004a). In practice, this can lead to costlyconcrete removal and cleaning work, the loss of the mixing truckdrum, or even the destruction of entire bridge spans.

2-Phosphonobutane-1,2,4-tricarboxylic acid (PBTC)

PBTC is a hydroxycarboxylic acid with five acid sites, which candissociate in an alkaline medium. Figure 1 shows the structure ofPBTC in the deprotonated state. Above a pH value of approx. 12.7,five protons (H+) fully dissociated.

In this form, PBTC possesses five coordination sites, which aresuitable for the complexation with metal ions. Studies in Bier (1993)revealed that in the neutral pH region (pH 7), calcium, aluminium,cadmium, and magnesium ions are only complexed to a maximumof 3%, and other metal ions, e.g., chromium, copper, iron, man-ganese, lead, and zinc, to over 85%. In an alkaline medium, thecomplexing ability of PBTC with calcium and aluminum ions in-creases with increasing pH value (Bier 1993).

Influence of PBTC on the Hydration of Clinker and Cement

Skalny et al. (1980) reported that molecules with complex met-als can be converted from the extended molecular shape into sta-ble rings through the metal ion. Such chelating agents can retardthe hydration of cement particularly strongly. Here, the retarderaction generally increases with increasing number of free coordi-nation sites. According to Skalny et al. (1980), compounds witha β-hydroxycarboxyl group, which can form a stable 6-memberring with a metal ion, are particularly good chelating agents andvery effective retarders. PBTC can form similar ring structures bycomplexation with calcium ions. According to Kinney (1989),

Copyright C© 2004 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. 1

2 CEMENT, CONCRETE, AND AGGREGATES

FIG. 1—PBTC—deprotonated.

recycling aids based on PBTC retarded the hydration of C3S morestrongly than that of C3A. It is presumed that PBTC slows the for-mation of C-S-H and calcium hydroxide nuclei, and as a result finerand more compact hydrates can form.

In Bobrowski (1990, 1991, 1996), it is suggested that the retarderaction of phosphonic acids is due to the binding of calcium ionsin chelates. According to Paolini et al. (1998), Langenfeld et al.(1998), and Moeser et al. (1999), a PBTC-containing recycling aidretards the hydration of C3A and C3S very strongly. In Langenfeldet al. (1998) and Moeser et al. (1999), instead of the initial hydrationproducts, “spherical new formations” about 100 nm in size wereobserved on the surfaces of C3A and C3S after 20 min. Theseare thought to be chelate complexes. In experiments with C3S, theformation of these complexes resulted in higher hydration heat ratesin the initial minutes. In experiments with C3A, the initial evolutionof heat of hydration was decreased by PBTC.

In studies by Lipus et al. (2000) and Rickert (2004a), recyclingaids based on PBTC retarded all further hydration reactions ofPortland cement, except for the formation of small primary ettrin-gite crystals. The new formations (chelate complexes) observed inLangenfeld et al. (1998) were not observed on the surface of thecement particles.

In the literature, there are hardly any other references that ad-dress mechanism of action of PBTC on the hydration of clinker andcement. The literature yields only a few and to some extent contra-dictory findings concerning the influence of PBTC on the hydrationof clinker phases, clinker, and cement. In addition, the often verydifferent experimental conditions and procedures hinder the abilityto draw conclusions regarding the retarding mechanism of PBTC.

Aim and Extent of the Investigations

The aim of the current investigation is to extend the knowledgeconcerning the effects of the PBTC-based long-term retarder on thehydration of clinker phases, clinker, and cement.

Firstly, the effect of the long-term retarder on the hydration ofindividual clinker phases (C3A, C4AF, and C3S) and on the hydra-tion of mixtures of clinker phases, sulphate carriers, and/or calciumhydroxide, as stated in Table 1, was studied chemically and miner-alogically.

The retarding mechanism of long-term retarders associated withthe reaction of C3A or C4AF fractions with dissolved sulphate togive ettringite, was investigated by comparative studies betweentwo industrial clinkers and on cements produced from them bythe addition of sulphate. In this process, it was also necessary toexamine whether the formation of primary ettringite is a necessaryprecondition for the further planned retardation of cement pastes bylong-term retarders, or whether long-term retarders can take over

the regulation of the setting of clinker meal in place of sulphatecarrier optimization. In addition, the sorption behavior of the long-term retarder on clinker and cement was studied.

For a detailed description of the interactions between cement par-ticles and long-term retarders, the characterization of the chemicalcompounds responsible for the long-term retardation is necessary.Owing to the relatively small quantities of admixtures used and inparticular owing to their strong incorporation and sorption onto ce-ment hydration products in the hydrating multicomponent system,such compounds can be detected with difficulty. Consequently, thecompound that leads to the long-term retardation was synthesizedseparately, and chemically and mineralogically analyzed.

On the basis of the findings obtained, a model was derived withwhich the mechanisms of action of this admixture can be elucidated.

Test Procedure

An overview concerning the composition and designation ofthe samples and concerning the studies performed is given inTable 1.

Description of the Starting Materials

Clinker Phases, Industrial Clinkers, and Cements—The clinkerphases C3S, C3A, and C4AF in the form of granules were groundfiner than 90 µm in a swing mill with a tungsten carbide grind-ing vessel. Furthermore, calcium hydroxide (CH) p. a. and β-hemihydrate (CSH0.5) and anhydrite (CS) produced from gypsum(CSH2) p. a. were also used.

An unground Portland cement clinker was obtained from each oftwo cement works. By Bogue calculation and by X-ray diffractionwith Rietveld analysis, Clinker K I was C3A-free and containedabout 17.6 mass % C4AF and about 72.7 mass % C3S. Clinker K IIhad about 8.3 mass % C3A, 9.2 mass % C4AF, and about66.2 mass % C3S. Each clinker (K I and K II) was ground in aball mill to about Blaine 3200 to 3300 cm2/g. From one portionof each clinker meal, Cement Z I and Cement Z II, respectively,were produced by addition of sulphate carriers normally used inpractice. As sulphate carriers, β-hemihydrate (CSH0.5) and nat-ural anhydrite (CS) were used. For the production of the hemi-hydrate, gypsum (CSH2) was appropriately dehydrated. The sul-phate content as SO3 was adjusted to about 3.2 mass % of the ce-ment. This corresponded to sulphate dosages for these clinkers inpractice.

Long-Term Retarder

As long-term retarder (LTR) a commercially available long-term retarder (recycling aid for wash water), based on

RICKERT AND THIELEN ON LONG-TERM RETARDER 3

TABLE 1—Overview of experiments with clinker phases, clinkers, and cements.

CompositionDesignation (mass of solids) Water/Solid1 LTR in Mass %5 Studies

Clinker phasesC3A-1 C3A 1.0 0 ESEM, XRD, DSCC3A-2 2 SorptionC3A-37 5 Pore solution6

C3A-Ca(OH)2-1 C3A:Ca(OH)2 = 3.3 0C3A-Ca(OH)2-2 2

C3A-SO3-1 C3A:SO3 = 2.62 0C3A-SO3-2 2C3A-SO3-37 5

C3A-SO3-Ca(OH)2-1 C3A:SO3:Ca(OH)2 = 2.6:1:0.83 0C3A-SO3-Ca(OH)2-2 2C3A-SO3-Ca(OH)2-37) 5

C3S-1 C3S 0.5 0C3S-2 2

C4AF-17 C4AF 1.0 0C4AF-27 2

C4AF-SO3-17 C4AF:SO3 = 5.34 0C4AF-SO3-27 2

ClinkersK I-1 K I 0.50 0K I-2 2

K II-1 K II 0K II-2 2

CementsZ I-1 Z I 0Z I-2 2

Z II-1 Z II 0Z II-2 2

1 Admixture was included in the calculation of the water.2 C3A:CS:CSH0.5 = 3:1:1.3 C3A:CS:CSH0.5 : CH = 3:1:1:0.9.4 C4AF:CS:CSH0.5 = 6:1:1.5 Relative to solids.6 Only clinkers and cements.7 See (Rickert 2004b).

2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), was used.The active substance content (PBTC) was about 15 mass %. Ineach case, the quantities of LTR given below relate to the respec-tive mass of solids.

Experimental Procedures

For the studies, pastes were mixed in teflon dishes with a teflonspatula under argon using fully demineralized water. The admix-ture was diluted in the mixing water, with the amount of admix-ture water included in the calculation of the total water content.The total mixing time was about 2 min. Suitable portions of thepaste were filled into 50 ml polyethylene flasks, flushed with argonand stored tightly sealed at 20◦C over water in the desiccator. Ateach test time, a part of the sample was analyzed by Environmen-tal Scanning Electron Microscope (ESEM) and Energy-DispersiveX-Ray spectrometry (EDX). The hydration of the rest of the sam-ple was stopped with acetone and diethyl ether. The hydrate phaseswere also studied by Differential Scanning Calorimetry (DSC) andX-Ray Diffraction (XRD). The DSC studies were carried out over atemperature range of 25–630◦C with a heating rate of 30 K/min in anitrogen atmosphere. The XRD studies were carried out on powder

samples using a Bragg-Brentano diffractometer with CuKα radia-tion in the angular range from 5 to 53◦2θ and with a step width of0.04◦2θ.

The sorption behavior of the long-term retarder on clinker andcement was determined on suspensions (w/c = 3.75) over a pe-riod of 7 days by determination of the organic phosphorus contentin filtered pore water. The admixture content was 2 mass % and5 mass % relative to cement or clinker. The organic phosphoruscontent was calculated from difference of total phosphorus andinorganic phosphorus content.

The setting behavior was determined on the basis of EN 196-3(Vicat apparatus). Pore solutions were filtered off from fresh cementpaste with a vacuum pump, and pressed out in the case of hardenedcement paste.

By mixing of a solids-free, calcium hydroxide saturated solu-tion (pH 12.7) with a diluted long-term retarder solution (50%admixture, 288 mmoles PBTC/l) with constant stirring, the com-pound responsible for the long-term retardation was synthesized.During this, the content of long-term retarder was increased step-wise. The mixture was filtered with the aid of a vacuum pump.The filter residue was then dried to constant mass at 40◦C and thecomposition investigated by XRD, X-ray fluorescence analysis, IR


FIG. 2—C3S-1, w/C3S = 0.50, at age 28 days, normal hydration, de-velopment of C-S-H phases—paste hardened.

FIG. 3—C3S-2, w/C3S = 0.50, 2 mass % LTR, at age 180 days, no hydra-tion products—paste can still be processed.

spectroscopy, and ESEM/EDX. The carbon and hydrogen contentwas determined by elemental analysis.

Test Results

Influence of LTR on the Hydration of Synthetic Clinker Phases andon Mixtures Between Clinker Phases and Calcium Sulphate and/orCalcium Hydroxide

C3S Hydration—Figs. 2 and 3 show ESEM micrographs of thesamples C3S-1 and C3S-2, respectively, at ×10 000 magnification,at ages of 28 and 180 days respectively.

Comparison of the micrographs shows clearly that the LTR addi-tion of 2 mass % (Fig. 3) led to complete blocking of the hydrationof C3S. While in the case of the unretarded sample C3S-1 (Fig. 2) acompact structure of sharp needle-shaped C-S-H phases had formedas a result of hydration, no hydration products could be observedon the surfaces of the long-term retarded sample C3S-2 even after180 days. After this time, the paste displayed a soft consistency,similar to the freshly mixed paste. The light areas on the particlesin Fig. 3 display higher contents of oxygen, carbon, and phospho-rus compared to the remaining particle surface, which indicate aphosphorus-containing organic compound on the particle surface.In this region, the carbon/phosphorus ratio was between 2.7 and 2.9,and thus, approximately corresponded to the carbon/phosphorus ra-tio of the LTR (about 2.7). It must be presumed that this is a regionwhere the admixture has become concentrated. After a hydrationperiod of one year, the sample C3S-2 had hardened. At this age, themicrostructure, in addition to very compact structures due to the

FIG. 4—C3S-1, w/C3S = 0.50, DSC-curves.

FIG. 5—C3S-2, w/C3S = 0.50, 2 mass % LTR, DSC curves.

progression of the hydration, sporadically displayed relativelyporous regions. In these regions, poorly hydrated C3S particleswere present.

In addition to the ESEM micrographs, a comparison of the DSCmeasurement curves of both samples, shown in Figs. 4 and 5,also shows that the unretarded sample C3S-1 after one year had amarkedly higher calcium hydroxide content than the LTR-retardedsample C3S-2. This further confirms that the progress of the C3Shydration in this sample still remained lower.

C3A-, C4 AF Hydration—Figs. 6 and 7 show the course of hydra-tion of the Samples C3A-1 and C3A-2, respectively, up to 28 days onthe basis of DSC curves. Figures 8 and 9 show ESEM micrographsof the Samples C3A-1 (without LTR) and C3A-2 (with 2 mass %LTR), each at the ages of 3 min (top) and 28 days (bottom).

Whereas in Sample C3A-1 (Fig. 6 and Fig. 8) cubic C-A-H(C3AH6, hydrogarnets) could mainly be seen, in Sample C3A-2(Fig. 7 and Fig. 9) due to the addition of 2 mass % LTR in partic-ular, hexagonal C-A-H, such as for example CAH10, C2AH8, and


FIG. 6—C3A-1, w/C3A = 1.00, DSC curves.

FIG. 7—C3A-2, w/C3A = 1.00, 2 mass % LTR, DSC curves.

C4AH11-13, were formed. The formation of cubic C-A-H or theconversion of the hexagonal CAH into cubic CAH was stronglyimpeded by LTR. The same was observed with the hydration ofC4AF in the presence of LTR. X-ray diffraction studies at the ageof 28 days showed that LTR increased the C3A hydration (C3A-2).On the other hand, the hydration of C4AF was decreased by LTR(C4AF-2).

C3A-Ca(OH)2 Hydration—If C3A was hydrated in the presenceof Ca(OH)2 and LTR (Sample C3A-Ca(OH)2-2), the formation ofhexagonal C-A-H was increased. But compared to the system with-out Ca(OH)2, the conversion of C3A was decreased.

C3A-SO3, C4AF -SO3 Hydration—As can be seen from compar-ison of the DSC curves in Figs. 10 and 11 after a hydration time of3 min and 6 h, in sample C3A-SO3-2 (Fig. 11), the formation of pri-mary ettringite (E) was initially accelerated by the LTR for a shorttime. The further progress of the hydration of C3A was strongly

FIG. 8—C3A-1, w/C3A = 1.00, at age 3 min: formation of hexagonaland cubic C-A-H, and at age 28 days: conversion of hex. C-A-H to cubicC-A-H (C3AH6- hydrogarnet).

FIG. 9—C3A-2, w/C3A = 1.00, 2 mass % LTR, at age 3 min and 28days, hex. C-A-H predominantly, no conversion after 28 days.

retarded. The retardation commencing after the initially acceler-ated formation of ettringite affected all hydration reactions. Eventhe relatively fast reaction of hemihydrate (CSH0.5) with water togive gypsum (CSH2) was retarded. This was indicated by the pres-ence of hemihydrate in Sample C3A-SO3-2 after 28 days. Whereasin Sample C3A-SO3-1 after 28 days hexagonal C-A-H (C2AH8,


FIG. 10—C3A-SO3-1, w/solid = 1.00, DSC curves.

FIG. 11—C3A-SO3-2, w/solid = 1.00, 2 mass % LTR, DSC curves.

C4AH11-13) and cubic C-A-H (C3AH6), monosulphate (M), andettringite (E) were present. A similar situation was observed in theSample C4AF-SO3-2.

C3A-SO3-Ca(OH)2 Hydration—Comparison of the DSC curvesin Figs. 12 and 13 shows that in Sample C3A-SO3-Ca(OH)2-2 allhydration reactions, with the exception of the formation of ettrin-gite, were strongly retarded. Increased formation of ettringite dueto the LTR similar to the case of Sample C3A-SO3-2, was not ob-served. After 28 days, hemihydrate was still present in the retardedSample C3A-SO3-Ca(OH)2-2.

For all trials with calcium aluminates, no organic compoundsassociated with LTR as found on the surface of C3S could bedetected by EDX. This is due to the enlarged specific surface areacaused by the hexagonal C-A-H.

FIG. 12—C3A-SO3-Ca(OH)2-1, w/solid = 1.00, DSC curves.

FIG. 13—C3A-SO3-Ca(OH)2-2, w/solid = 1.00, 2 mass % LTR, DSCcurves.

Influence of LTR on the Sorption Behavior of Portland CementClinker and Portland Cement

Figure 14 shows the results of the sorption experiments on clink-ers (K I and K II) and cements (Z I and Z II) with varying additionamounts and addition times of LTR. It can be seen from the figurethat the long-term retarder was sorbed independently of the clinkertype, cement type, the amount added, and the time of addition.Shortly after the addition, in all cases the proportion of LTR de-tectable in the solution was already only about 7% relative to theoriginal amount added. As hydration progressed, the proportionsorbed increased further. After 7 days, the long-term retarder wascompletely sorbed on the cement and no longer detectable in thefiltered solution.


FIG. 14—Sorption of the long-term retarder onto clinkers (K I and K II)and cements (Z I and Z II) with varying addition amounts and times; 2s: 2mass % LTR, added with mixing water; 2n: 2 mass % LTR, added 2 minafter addition of mixing water; 5s: 5 mass % LTR, added with mixing water.

FIG. 15—K II-1, w/c = 0.50, DSC curves.

Influence of LTR on the Hydration of Portland Cement Clinkerand Portland Cements

Apart from a few details, the influence of LTR on the hydrationof the two clinkers (K I and K II) and the cements (Z I and Z II) pro-duced from them was almost identical. Therefore, only the resultsfor Clinker K II and Cement Z II are shown. Figure 15 containsthe DSC curves for the Sample K II-1 without LTR and Fig. 16 theDSC curves for the Sample K II-2 with 2 mass % LTR.

Comparison of the DSC measurement curves after 3 min showsthat with the Portland cement clinkers, LTR caused a short-termincrease in the formation of hexagonal calcium aluminate hydrates,such as for example C2AH8 and C4AH11-13. As a result, the stiff-ening or setting of the pastes and mortars was accelerated. On theother hand, the hydration of the calcium silicates was very stronglyretarded.

Pastes and mortars made of Portland cements showed a pro-longed workability by addition of LTR. Comparison of the DSCcurves for Sample Z II-1 (Fig. 17) with those for the Sample Z II-2(Fig. 18) clearly shows that the formation of primary ettringite wasnot affected by LTR. In contrast, the hydration of the silicate phaseswas particularly strongly retarded by LTR.

FIG. 16—K II-2, w/c = 0.50, 2 mass % LTR, DSC curves.

FIG. 17—Z II-1, w/c = 0.50, DSC curves.

FIG. 18—Z II-2, w/c = 0.50, 2 mass % LTR, DSC curves.

The comparative studies on Portland cement clinker meal (K Iand K II) and on Portland cements (Z I and Z II) produced fromthese clearly showed that the sulphate optimization of the cementsis necessary for a long-term retardation. The formation of primaryettringite initiated by calcium sulphate is essential for PBTC tofunction as a retarder.

During the retardation period, the pore solutions of the retardedclinker pastes or cement pastes always displayed higher contents ofcalcium or calcium and sulphate than the corresponding unretardedsample. This was attributable firstly to the briefly intensified hy-drolysis of the calcium aluminates and secondly to the stabilization


of submicroscopic crystal nuclei, e.g., of CaSO4 or ettringite, by theresidual LTR present in solution. After the retardation period, thecalcium and sulphate concentration in the pore solution decreasedas hydration recommenced (Rickert 2004b).

Whereas with pure C3S (Sample C3S-2, Fig. 5) the LTR re-sulted in complete blocking of the hydration up to 180 days, allpastes and mortars produced with clinker and cement showed after28 days approximately the same hydration progress as the corre-sponding unretarded reference sample (see Figs. 15 & 16 and 17& 18). This means that after the retardation a more rapid hydrationset in.

Synthesis of Calcium Phosphonate

The addition of LTR (containing 15 mass % PBTC) to a solids-free, saturated calcium hydroxide solution (pH 12.7) immediatelyproduced a milky turbidity. Up to an added amount of about6.8 mmoles PBTC /l calcium hydroxide solution (Ca/PBTC molarratio ≥ 2.5), a white precipitate was deposited. Analyses showedthat this was a sparingly water-soluble X-ray amorphous calciumphosphonate, which contained approximately 2.5 moles of calcium.In order to be able to bind this quantity of calcium, the active sub-stance PBTC must have been present in fully dissociated form (seeEq 1). The formation of calcium phosphonate presumably occursaccording to Eq 2.

C7H11O9P → [C7H6O9P]5− + 5H+ (1)(PBTC)

[C7H6O9P]5− + 5H+ + 2.5Ca2+ + 5OH−

←→ Ca2.5[C7H6O9P] · 3H2O + 2H2O (2)(Calcium phosphonate)

On further addition of LTR, the Ca/PBTC molar ratio fell below2.5 and soluble calcium phosphonate complexes of lower calciumcontent were formed. Hydrating cement makes available an almostinexhaustible supply of calcium, so that it can be assumed thaton addition of LTR to clinker, cement or calcium aluminates orcalcium silicates, sparingly soluble calcium phosphonate alwaysforms in accordance with Eq 2. This is most likely responsible forthe long-term retardation on account of its insolubility.

Mechanisms of Action of the PBTC-Based LTR

From the information gained in these studies, the main mecha-nisms of action of the LTR can be derived. By way of example,Figs. 19 and 20 respectively show the mechanisms of action ofthe LTR on Portland cement clinker (K II-2) and Portland cement(Z II-2) schematically.

In an alkaline medium, PBTC (C7H11O9P) releases 5 protonsand is present in a completely dissociated form ([C7H6O9P]5−). Forthe immediate formation of sparingly soluble calcium phosphonate(Ca2.5[C7H6O9P]), 2.5 moles of calcium are needed per mole ofPBTC. If LTR is added to Portland cement clinker without calciumsulphate carrier (Fig. 19), the calcium content in the pore solutionis not sufficient to form sparingly soluble calcium phosphonate.Because of the calcium demand of PBTC, there is generally ashort-term accelerated hydration of reactive clinker phases, such asfor example C3A. This causes an increased formation of hexagonalC-A-H and results in a marked stiffening or setting. The calcium

FIG. 19—Effect of LTR on the hydration of Portland cement clinkerwithout or with non-optimized supply of CaSO4, (K II-2).

FIG. 20—Effect of LTR on the hydration of Portland cement with opti-mized supply of CaSO4, (Z II-2).

ions released during this brief acceleration phase are bound byPBTC.

The sparingly soluble calcium phosphonate (Ca2.5[C7H6O9P])thus formed coats the surfaces of the individual clinker particlesdifferently. On the surfaces of relatively unreactive clinker phases,such as for example C3S and C2S, which are still largely freefrom hydration products when the sparingly soluble compound is


formed, essentially complete coating with calcium phosphonatecan take place. As a result, the further access of water (H) orfurther ion exchange are markedly hindered and the hydration isretarded. Already hydrated surfaces, such as the C3A, display arelatively large specific surface area, which cannot be completelycovered with sparingly soluble calcium phosphonate. The accessof water and ion exchange cannot be completely blocked. For thisreason, compared to the less reactive clinker phases, the hydrationof reactive calcium aluminates is only slightly retarded by calciumphosphonate formed from LTR.

Because of the sulphate carrier (CSHx) contained in the Portlandcement (Fig. 20), the concentration of calcium and sulphate inthe pore solution is much higher than in system containing onlyclinker. A thin layer of primary ettringite can form on the surfaceof the calcium aluminate. Moreover, sufficient calcium ions areavailable for the immediate formation of sparingly soluble calciumphosphonate. The calcium phosphonate coats both the surfaces ofthe clinker particle and also those of the still undissolved sulphatecarrier. As a result, further hydration is strongly blocked.

The formation of primary ettringite and the coating of the sur-faces with sparingly soluble calcium phosphonate constitute theprecondition for planned prolongation of the workability by thelong-term retarder. This means that sulphate carrier optimization isnot only the basis for the control of the setting of the cement, butalso a precondition for the desired mode of action of the admixture.

Conclusion

By systematic studies, the state of knowledge concerning theeffects of a long-term retarder (LTR) based on 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) on the hydration of clinker phases(C3S, C3A, and C4AF), clinker and cement has been substantiallyextended.

The following conclusions were drawn from the research results:

� In the hydration of pure C3A and C4AF, the retarder resultsin increased formation of hexagonal calcium aluminate hy-drate (C-A-H). Apart from this, the conversion of hexagonalC-A-H into cubic hydrates is strongly retarded. The conver-sion of C3A is increased, and that of C4AF decreased.

� The hydration of pure C3S was completely blocked for up to180 days.

� The retarder action is attributable to the formation of sparinglysoluble calcium phosphonate (Ca2.5[C7H6O9P] · xH2O). Theformation of sparingly soluble calcium phosphonate requires2.5 moles Ca2+ per mole of PBTC. If enough Ca2+ is availablein the pore solution, sparingly soluble calcium phosphonateprecipitates and forms a closed layer around the cement parti-cles, which retards further hydration immediately. Conversely,a lack of Ca2+ causes a short-term acceleration in hydrationof the reactive clinker phase C3A. Due to the formation ofC-A-H the specific surface area increases. Therefore, the for-mation of a closed layer of calcium phosphonate is impededand the hydration of the calcium aluminates progresses.

� With Portland cements, owing to the content of calcium sul-phate, sufficient quantities of dissolved calcium are presentfor the direct formation of calcium phosphonate. The forma-tion of primary ettringite and the coating of the surfaces withsparingly soluble calcium phosphonate are the preconditionfor planned prolongation of the workability by the long-termretarder. If calcium sulphate is absent or present in insufficientamount, for example in the case of Portland cement clinkers,

the calcium demand of the LTR active substance PBTC re-sults in a short-term acceleration of the hydration of reactiveclinker phases, e.g., C3A.

� The direct formation of an ettringite layer on the cement par-ticle and the simultaneous supply of calcium ions are not onlyimportant for controlling the setting of the cement but also forthe desired mode of action of the long-term retarder.

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