Cement and Concrete Research 39 (2009) 644–650

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Cement and Concrete Research

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Activation of blast furnace slag by a new method

F. Bellmann ⁎, J. Stark
Institute for Building Materials Science, Department of Civil Engineering, Bauhaus-University Weimar Coudraystraße 11, 99423 Weimar, Germany

a r t i c l e i n f o a b s t r a c t

Article history: Blast furnace slag is used as supplementary cementing material for the production of blended cement and
Received 2 May 2008 slag cement. Its latently hydraulic properties can be activated by several methods. Most applications employ
Accepted 28 May 2009 the use of high pH values in the pore solution (N 13.0) to accelerate the corrosion of the glass network of the
slag.
Keywords:
It is shown in this work that activation is also possible by lowering the pH to a range between 11.8 and 12.2
Acceleration
Hydration
by the addition of calcium hydroxide and soluble calcium salts. Among the salts investigated in this study are
Granulated blast furnace slag calcium chloride, calcium bromide, calcium nitrate, calcium formate, and calcium acetate. Other salts can be
Blended cement used alternatively as long as they are able to increase the calcium ion concentration and thus reduce the pH
in the pore solution via the calcium hydroxide equilibrium. Complex formation of organic anions with
calcium ions in the pore solution is a serious handicap when using organic calcium salts.
This concept was tested on a particular slag improving its early compressive strength. It was possible to
increase the strength of mortar bars produced from the pure slag from 3 MPa to 25 MPa after seven days by
adding calcium hydroxide, calcium carbonate and calcium acetate. The early strength of slag cement
containing 80% slag was increased from 6 to 16 MPa after two days by adding calcium chloride. The final
strength was increased from 36 to 53 MPa after 28 days (water/cement-ratio = 0.40, 20 °C).
Analytical data is included to demonstrate that application of the aforementioned concept is able to increase
heat liberation and degree of slag consumption.
© 2009 Elsevier Ltd. All rights reserved.

1. Introduction strength gain at later ages. The heat release during hydration at room
temperature is significantly lower than during hydration of Ordinary
Blast furnace slag is a by-product of the manufacture of pig iron Portland Cement (OPC). Due to its slow reaction, slag cement
from iron ore, limestone and coke. The liquid slag is rapidly cooled by produces a very dense microstructure and is highly resistant to
quenching to obtain an almost completely amorphous material. Its chemical attack. Blast furnace slag contains less lime than Portland
chemical composition mainly depends on that of the iron ore and cement clinker and calcium hydroxide is not formed during reaction
potentially contains 27–40% SiO2, 30–50% CaO, 5–15% Al2O3, and 1–10% of the slag particles. Instead of this, calcium silicate hydrate (C–S–H)
MgO. Recent reviews of the properties of blast furnace slag and its with a low calcium/silicon-ratio, hydrotalcite, and ettringite or AFm
utilisation for the production of blended cement were given by Taylor phases are formed. Sulfur contained in blast furnace slag is mainly
[1], Moranville-Regourd [2], Lang [3], and Glasser [4]. present in reduced form (S2−) accounting for the dark colour of the
Blast furnace slag has been used as a secondary cementing material concrete produced from cement with a high amount of slag before it is
for more than 100 years and there is a broad knowledge regarding its exposed to air. The hydration of slag proceeds via dissolution of slag
application. Most commonly it is used for the production of blended particles followed by precipitation of hydrate phases from the
cements and slag cements. Currently, there is high interest in the supersaturated pore solution. In this process, the dissolution of the
application of this alternative material because the production of unhydrous material is the time determining step. The higher the
Portland cement clinker contributes about 5% to the global man-made dissolution, the faster the reaction of the material to hydrated phases.
CO2 emissions. Despite the fact that the amount of available slag is Since the corrosion of the glass-network of slag can be accelerated by
limited, there is an increasing demand for slag cement to reduce the high pH values in the pore solution (N13.0), there is a tendency to use
CO2 emissions due to the production of cement and concrete [5]. Portland cement clinker with a high content of water soluble alkalis to
The hydration of blast furnace slag is slow when compared to produce blended cement containing slag.
Portland cement clinker, resulting in lower early strength and higher There are two other forms known to activate the latent hydraulic
properties of blast furnace slag, namely the addition of high amounts
of calcium sulphate and alkali activation. Blends of 10–20% calcium
⁎ Corresponding author. Tel.: +49 3643 584724; fax: +49 3643 584759. sulphate, mainly in the form of anhydrite (CaSO4) with very small
E-mail address: frank.bellmann@uni-weimar.de (F. Bellmann). amounts of OPC and 75–90% blast furnace slag are termed

0008-8846/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.cemconres.2009.05.012
 F. Bellmann, J. Stark / Cement and Concrete Research 39 (2009) 644–650 645

in the form of ettringite (C3A·3CaSO4·32H2O). Silicon released during

dissolution is bound in the form of C–S–H. To prevent the formation of
excessive amounts of ettringite and to avoid the presence of unreacted
calcium sulphate after hydration, a different sink for aluminium was
sought. Instead of calcium sulphate, a mixture of calcium hydroxide
(Ca(OH)2) and calcium carbonate (CaCO3) was added. By this
replacement, aluminium should be precipitated as monocarbonate
(C3A·CaCO3·11H2O) rather than ettringite. The co-precipitation of C–
S–H may proceed as in supersulphated cement. Binders made from
blast furnace slag and calcium hydroxide are termed lime-slag cements
[2]. A second concept has been used to accelerate the reaction of blast
furnace slag even further. The addition of calcium hydroxide as
reactant changes the composition of the pore solution. In the presence
of calcium hydroxide, the pH in the pore solution is 12.5 at room
Fig. 1. Strength development of four cements with different slag content. temperature and even higher in the presence of alkali hydroxide. This
is significantly higher than the pH in the pore solution of super-
supersulphated cement [2]. Well made cements of this type are able to sulphated cement (approximately 11.8–12.0). To reduce the pH in the
achieve final strengths in the same order as OPC. Their performance presence of calcium hydroxide, soluble calcium salts were added. They
depends critically on the pH value in the pore solution [6]. A serious maintain a very high calcium ion concentration in the pore solution.
drawback is that concretes made from supersulphated cement require Such high calcium ion concentrations are able to reduce the hydroxide
long curing periods, are sensitive to carbonation and frost attack. A ion concentration (expressed as pH) via the calcium hydroxide
special feature of these materials is that they are not compatible with equilibrium condition (Eq. (1)).
other hydraulic materials. Calcium sulphate is not completely
n o
depleted during hydration and can lead to sulphate attack at the
KSP = Ca
2+

fOH − g2 ð1Þ
interface between concrete from supersulphated cement with other
concretes.
Alkali-activated slag cements require the addition of up to 5% Na2O The concepts sketched above have been tested by measuring the
or K2O most commonly in the form alkali hydroxide, alkali carbonate or compressive strength development of mortar bars produced from
alkali silicate. The alkalis are added during mixing of the cement with laboratory-made binders with chemical admixtures. In connection with
water. They are partially bound in the hydration products during this, the composition of the pore solution during the hydration of
reaction. Concretes made from alkali-activated slag cements can have hardened cement paste specimen has been studied with a special focus
high strength, low permeability and high resistance to chemical attack. on the pH value. Additional details on the nature of the activation
On the other hand, problems have been reported with excessive mechanism were obtained by XRD, heat conduction calorimetry and
shrinkage and efflorescence. The most serious handicap is that the determination of degree of slag consumption of selected samples.
production of alkaline activator is very intensive with respect to energy
consumption and CO2 emissions. 2. Materials and methods
To the present time, supersulphated cement and alkali-activated
slag cement are rather niche-products when compared to other Different binders were produced in this study based on ground
cements such as OPC, blended cement or slag cement. This is due to the granulated blast furnace slag and commercially available CEM II/A-LL
disadvantages mentioned above. At present, most applications try to 32.5 R. The properties of these two materials are provided in Table 1.
activate blast furnace slag by maintaining a high pH in the pore The materials specified in Table 1 were blended with mineral
solution to corrode the slag particles by adding OPC or Portland cement additions of reagent grade quality to produce new binder types. These
clinker. However, this approach is only feasible with limited propor- additions comprised calcium hydroxide and calcium carbonate
tions of slag in the cement. If very high fractions of slag are used for the (2220 cm2/g). During hydration of these binders, the composition
production of cement, the early strength may be significantly reduced, of the pore solution was changed by adding admixtures such as
which is of major concern for application in the construction industry. calcium formate (Ca(COOH)2), calcium acetate (Ca(CH2COOH)2),
This is aggravated when a coarse slag is used. An example is displayed calcium chloride (CaCl2), sodium chloride (NaCl), calcium nitrate
in Fig. 1. The compressive strength of mortar bars made according to (Ca(NO3)2), and calcium bromide (CaBr2), all of reagent grade quality.
DIN EN 196-1:1995-05 is shown for a CEM III/B containing 72% of slag. The composition of all binders and the amounts of admixtures that
It is compared to the performance of supersulphated cement (SSC), were added during preparation of pastes and mortars are provided in
OPC, and CEM II/B-S containing 27% of slag.
Although the data presented in Fig. 1 is far from being systematic, it
Table 1
illustrates the fact that the early strength of slag cement with a high Chemical and physical properties of slag and cements used in this study (w.s. — water
amount of blast furnace slag can be very low when compared to other soluble).
cements. This prevents the use of this material for most construction
Slag CEM II/A-LL 32.5 R
purposes. After two days storage under water, the CEM III/B in the
SiO2 [M.-%] 35.2 17.0
aforementioned example achieves a strength of only 4 MPa whereas Al2O3 [M.-%] 11.9 5.0
the CEM II/B-S gets as high as 27 MPa. However, the performance of Fe2O3 [M.-%] 0.6 2.6
supersulphated cement with 12 MPa indicates that there is a much CaO [M.-%] 41.9 61.3
higher potential of hydraulic capacity that can be activated. It is the MgO [M.-%] 6.2 1.0
MnO [M.-%] 0.21 0.07
scope of the investigations reported here to identify a new way for
TiO2 [M.-%] 0.94 0.21
activation of blast furnace slag that can make use of the hydraulic K2O (w.s.) [M.-%] 0.44 (0.01) 0.47 (0.28)
potential of slag without applying such peculiar concepts as super- Na2O (w.s.) [M.-%] 0.42 (0.00) 0.11 (0.04)
sulphated cement or alkali-activation. SO3 [M.-%] 0.6 2.8
When supersulphated cement reacts with water, calcium sulphate L.O.I. at 1000 °C [M.-%] 0.6 8.6
Specific surface area (Blaine) [cm2/g] 6050 4550
serves as a reactant by precipitating aluminium dissolved from the slag
646 F. Bellmann, J. Stark / Cement and Concrete Research 39 (2009) 644–650

Table 2
Composition of binders, addition of admixtures, compressive strength and pH in the pore solution of samples investigated in this study.

Binder Admixture Compressive strength [MPa] pH in pore solution

2 days 7 days 28 days 2 days 7 days 28 days
100% slag – 0 2.6 17.0 11.8 12.2 12.4
85% slag + 10% Ca(OH)2 + 5% CaCO3 – 2.4 10.0 15.5 12.8 12.8 12.7
85% slag + 10% Ca(OH)2 + 5% CaCO3 5% CaCl2 12.2 33.1 49.9 11.8 11.8 11.8
100% slag 5% CaCl2 0.0 0.0 1.7 10.6 10.8 11.1
85% slag + 10% Ca(OH)2 + 5% CaCO3 5% NaCl 7.4 17.5 26.3 12.7 12.8 12.8
85% slag + 10% Ca(OH)2 + 5% CaCO3 5% CaBr2 8.6 22.8 36.7 12.0 12.3 12.3
85% slag + 10% Ca(OH)2 + 5% CaCO3 0.5% Ca(CH2COOH)2 6.9 22.3 34.5 12.5 12.5 12.5
85% slag + 10% Ca(OH)2 + 5% CaCO3 1.0% Ca(CH2COOH)2 5.0 25.0 40.0 12.4 12.4 12.4
85% slag + 10% Ca(OH)2 + 5% CaCO3 1.5% Ca(CH2COOH)2 3.6 23.6 40.6 12.1 12.2 12.3
85% slag + 10% Ca(OH)2 + 5% CaCO3 2.0% Ca(CH2COOH)2 2.7 19.3 34.8 12.1 12.2 12.3
85% slag + 10% Ca(OH)2 + 5% CaCO3 4.0% Ca(CH2COOH)2 0.7 11.0 33.3 12.0 12.0 12.1
85% slag + 10% Ca(OH)2 + 5% CaCO3 1% Ca(COOH)2 7.0 23.0 34.6 12.2 12.4 12.2
85% slag + 10% Ca(OH)2 + 5% CaCO3 2% Ca(COOH)2 6.3 19.7 38.7 12.0 12.0 12.2
80% slag + 15% CEM II/A-LL + 5% Ca(OH)2 – 5.6 24.6 36.5 12.8 12.9 13.0
80% slag + 15% CEM II/A-LL + 5% Ca(OH)2 3% CaCl2 15.6 36.8 52.9 12.0 12.2 12.3
80% slag + 15% CEM II/A-LL + 5% Ca(OH)2 3% CaBr2 15.2 37.8 48.8 12.2 12.5 12.6
80% slag + 15% CEM II/A-LL + 5% Ca(OH)2 1% Ca(COOH)2 6.1 31.9 47.4 12.4 12.6 12.8
80% slag + 15% CEM II/A-LL + 5% Ca(OH)2 3% Ca(NO3)2 11.2 36.7 52.5 12.2 12.5 12.6

Table 2. The amount of chemical admixtures is given in this table after heating the samples to 750 °C instead of 900 °C described in [8]. The
relative to the amount of binder. Details about ionic concentrations degree of hydration was calculated correcting for dissolution of
expected after mixing the salts with water are available in Table 3. unhydrated material and for the water content using the loss of ignition
Mortar bars were prepared, stored and tested according to DIN EN 196- results.
1:1995-05 (sand 0/4:cement=3.0, mortar bars 4 cm×4 cm×16 cm Isothermal heat conduction calorimetry has been used to measure
stored in water after demoulding) using binders and admixtures as the heat release in the first 100 h of hydration using a ToniCal Trio
detailed in Table 2. The only detail being different was the water/cement- 7339 instrument.
ratio that was fixed to 0.40. Compressive strength was tested in the age of
2, 7 and 28 days. In addition to strength development, the composition of 3. Results
the pore solution of hardened cement paste specimen was investigated at
the ages of 2, 7, and 28 days. For this purpose, cement paste was prepared The results of the compressive strength tests are provided in
at a water/cement-ratio of 0.50. All samples were stored in sealed plastic Table 2. Graphical presentation of this data is given below.
containers at 20 °C. The pore solution was obtained by the squeeze-out The mortar was prepared at water/cement-ratio of 0.40. Due to
method at a maximum pressure of 270 MPa at the age appropriate for this low water content, the samples showed low workability and
testing. The main focus of the investigations reported here was analysis of were hard to compact. Thus different air contents in the hardened
the pH value in the solution. It was carried out by employing a glass mortars bars may have affected the strength results to some extent.
electrode, calibrated on standard buffer solutions. The air content of mortar was not recorded. Beside compressive
XRD analysis was performed on selected samples after terminating strength, pH values measured in the pore solution are also available in
hydration via removal of liquid water at 38 °C. Before qualitative analysis, Table 2.
the hardened cement pastes were ground to pass a sieve with a mesh The blast furnace slag used in this study is of typical chemical
width of 63 µm. The instrument used was a Siemens/Bruker AXS D5000 composition for slags used for the production of blended cements in
equipped with a copper tube operating at 40 kV and 40 mA. An angular Germany. Its specific surface area measured according to Blaine
range from 4 to 60° 2θ was examined using a counting time of 2.5 s and a (6050 cm2/g) is higher than in most blended cements (3000–
step width of 0.05° 2θ. 5000 cm2/g).
The degree of slag hydration was measured by a selective dissolution The hydration of the pure slag is comparatively slow. Compressive
technique described by Lumley et al. [8]. Loss of ignition was determined strength was very low after 2 and 7 days (Fig. 2), whereas 17 MPa was

Table 3
Amount of salt added during mortar preparation and expected concentrations in the mixing water ignoring effects of ion association and interaction with solid phases contained in
the binder.

Amount of salt relative to weight of binder Salt concentration [mmol salt/100 g cement] Theoretical ionic concentration in solution after mixing [mmol/l]
3% CaCl2 27.0 [Ca2+] = 676 mmol/l, [Cl−] = 1352 mmol/l
5% CaCl2 45.0 [Ca2+] = 1226 mmol/l, [Cl−] = 2252 mmol/l
5% NaCl 85.6 [Na+] = 2139 mmol/l, [Cl−] = 2139 mmol/l
3% CaBr2 15.0 [Ca2+] = 375 mmol/l, [Br−] = 750 mmol/l
5% CaBr2 25.0 [Ca2+] = 625 mmol/l, [Br−] = 1250 mmol/l
3% Ca(NO3)2 18.3 [Ca2+] = 457 mmol/l, [NO−3 ] = 914 mmol/l
0.5% Ca(CH2COOH)2 3.2 [Ca2+] = 79 mmol/l, [CH2COOH −] = 158 mmol/l
1.0% Ca(CH2COOH)2 6.3 [Ca2+] = 158 mmol/l, [CH2COOH −] = 316 mmol/l
1.5% Ca(CH2COOH)2 9.5 [Ca2+] = 237 mmol/l, [CH2COOH −] = 474 mmol/l
2.0% Ca(CH2COOH)2 12.6 [Ca2+] = 316 mmol/l, [CH2COOH −] = 632 mmol/l
4.0% Ca(CH2COOH)2 25.3 [Ca2+] = 632 mmol/l, [CH2COOH −] = 1264 mmol/l
1% Ca(COOH)2 7.7 [Ca2+] = 192 mmol/l, [COOH −] = 384 mmol/l
2% Ca(COOH)2 15.4 [Ca2+] = 384 mmol/l, [COOH −] = 768 mmol/l
 F. Bellmann, J. Stark / Cement and Concrete Research 39 (2009) 644–650 647

The accelerating effect of calcium chloride may be attributed to

different aspects:
• Chloride ions may be consumed in an additional reaction forming a
hydration product containing chloride thus directly stimulating the
reaction of slag.
• A high calcium ion concentration in the pore solution is able to
lower the pH which may have an accelerating effect (see below).
Presence of calcium hydroxide is essential in this mechanism.
• The high calcium concentration is directly responsible for activation
of the slag. In this case, calcium hydroxide is not required.
To test the first hypothesis, samples of binder containing slag,
calcium hydroxide, and calcium carbonate were hydrated in the
presence of calcium chloride and sodium chloride, respectively. If
Fig. 2. Acceleration of blast furnace slag by the addition of calcium hydroxide, calcium
chloride ions have a positive effect on the reaction, the addition of
carbonate, and calcium chloride.
sodium chloride should be able to accelerate the reaction in the same
way calcium chloride does. The second and third hypotheses were
investigated by replacing the aforementioned binder by pure slag. If
reached after 28 days. Replacement of 15% of slag by a mixture of there is no influence of the addition of calcium hydroxide on the
calcium hydroxide and calcium carbonate is able to accelerate the performance, the calcium ion concentration is responsible for
reaction of slag. Even further acceleration is achieved by the addition acceleration instead of pH in solution. The results of these investiga-
of calcium chloride to the mixing water (Fig. 2). The compressive tions are presented in Fig. 4.
strength after 2 days is significantly higher than obtained for pure slag The data presented in Table 2 and Fig. 4 indicates that the addition
without accelerator. Despite the fact that it is lower than that of of calcium chloride to pure blast furnace slag is not able to improve the
Portland cement, it would warrant classification of this binder system strength development. The compressive strength after 2 and 7 days
in strength class 32.5 R or 42.5 N. However, the use of calcium chloride was not measurable because the samples were still soft. It can be
prohibits application in reinforced concrete, and therefore other inferred that calcium chloride does not act as a direct accelerator by
means of acceleration need to be selected. To identify other salts that reacting with the slag forming hydration products containing chloride.
could be used to accelerate the reaction of slag, information on the In contrast, a combined addition of calcium chloride, calcium
mode of acceleration and the mode of action are required. The hydroxide and calcium carbonate increases compressive strength
remaining part of this section is devoted to these investigations. and is thus able to activate blast furnace slag. This rules out the
Results of XRD analysis obtained on hardened cement paste aforementioned hypothesis that high calcium ion concentrations are
specimen stored under sealed conditions for 7 days are shown in directly able to accelerate the hydration of slag.
Fig. 3. There are no crystalline hydration products detected in the Replacement of calcium chloride by sodium chloride yields
sample consisting of pure slag and water. Also, in the presence of strength results that are better than those obtained for the pure
calcium chloride solution, no hydration product can be identified in binder consisting of slag, calcium hydroxide and calcium carbonate,
the pure slag. After mixing the slag with calcium hydroxide and but significantly lower than those obtained for calcium chloride. Thus,
calcium carbonate, aluminium from the slag can react with these sodium chloride is a much less effective activator than calcium
phases to monocarbonate or a similar AFm phase. This is observed by chloride. The hypothesis that acceleration is due to the presence of
XRD after 7 days hydration. Apart from AFm, some calcium hydroxide chloride ions can therefore be abandoned.
and calcium carbonate are still present in the sample. The results of The hydration of the aforementioned binders was investigated by
XRD analysis confirm that aluminium is precipitated as AFm phase isothermal heat conduction calorimetry (Fig. 5). There is only a
thus accelerating the reaction of blast furnace slag. The reaction is negligible output of heat during the reaction of the pure slag, even in
further accelerated by addition of calcium chloride as evident from the presence of calcium chloride. Activation with calcium hydroxide
Fig. 2. and calcium carbonate yields a substantial increase of heat release

Fig. 3. Results of XRD analysis after 7 day hydration (A = AFm, P = Portlandite, C = calcite).
648 F. Bellmann, J. Stark / Cement and Concrete Research 39 (2009) 644–650

Table 4
Degree of slag hydration in the age of 48 h for selected binders.

Binder Admixture Degree of slag hydration [%]

100% slag – 2
85% slag + 10% Ca(OH)2 + 5% CaCO3 – 8
85% slag + 10% Ca(OH)2 + 5% CaCO3 5% CaCl2 20
100% slag 5% CaCl2 1
85% slag + 10% Ca(OH)2 + 5% CaCO3 5% NaCl 15

calcium carbonate, the pH value is approximately 12.8, due to the

presence of calcium hydroxide and minor amounts of alkalis resulting
from slag dissolution. The use of 5% calcium chloride reduces the pH
value in the pore solution to 11.8 by establishing a high calcium ion
Fig. 4. Effect of calcium hydroxide, calcium carbonate, calcium chloride, and sodium
concentration in the presence of calcium hydroxide.
chloride on the strength development of slag.
It can be concluded from this data that the pH appears to play an
important role for the activation of blast furnace slag in the presence
rates. Further acceleration and higher rates are obtained when adding of calcium hydroxide. To confirm this conclusion, tests have been
inorganic salts that lower the pH in the pore solution such as sodium carried out in which calcium chloride was exchanged by calcium
chloride and calcium chloride. The effect is most pronounced with bromide (CaBr2). Such a replacement is also able to generate a high
calcium chloride. The amount of heat that is released during hydration calcium ion concentration and a low pH in the pore solution. Data
of a binder consisting of slag, Ca(OH)2 and CaCO3 in the presence of presented in Table 2 and Fig. 6 confirms that the addition of 5% CaBr2
CaCl2 is in the range of low heat cements. can accelerate the hydration of a binder consisting of slag, calcium
The degree of hydration of binder pastes at the age of 48 h is hydroxide, and calcium carbonate. However, the addition of an equal
presented in Table 4. Data obtained by selective dissolution confirms weight of calcium bromide appears to be less effective than calcium
that virtually no reaction is observed between the pure slag and water. chloride. This is due the higher molecular weight of calcium bromide
The degree of slag consumption (2%) is within the tolerance of the compared to calcium chloride (Table 3).
analytical method. The addition of calcium chloride to the mixing From the data presented here, it can be deduced that the reaction
water is not able to increase the degree of slag hydration (1%). Much of slag can be accelerated by maintaining a low pH value in the pore
higher degrees of hydration are observed when calcium hydroxide solution. Due to the addition of 5% calcium bromide, the pH is lowered
and calcium carbonate have been added to the slag. In agreement with from 12.8 to 12.0 after 2 days. It is even further reduced by the
results obtained by calorimetry, the extent of hydration depends on addition of calcium chloride (11.8), accounting for the better
the addition of inorganic salts that manipulate the composition of the performance of this admixture at equal weight.
pore solution (see below). The degree of slag hydration observed for The use of calcium chloride or calcium bromide allows for the
pure water (8%) can be increased by the addition of sodium chloride activation of blast furnace slag due to the maintenance of a high
(15%) or calcium chloride (20%). calcium ion concentration and reduction of pH in the pore solution.
In conclusion, the second hypothesis considering that a combined However, an application of such salts in the building industry is
addition of calcium chloride and calcium hydroxide is responsible for constrained by the fact that chloride and bromide ions promote pit-
acceleration is supported. However, it has to be tested by performing hole corrosion of steel. Consequently, the use of calcium chloride and
additional experiments. Calcium chloride can be replaced by another calcium bromide as an accelerator is prohibited for the production of
easily soluble calcium salt such as calcium bromide. In this case, the reinforced concrete. Other salts with similar properties need to be
pH in the pore solution will also be lowered by a high calcium ion identified. Inorganic calcium salts may potentially be used to
concentration and maintenance of the calcium hydroxide equilibrium accelerate the reaction of slag with calcium hydroxide and water,
(Eq. (1)). This equation indicates that calcium and hydroxide ion but the anions present in most of these salts may also affect the
concentration (an expression for pH in the solution) are connected in resistance of steel against corrosion. To avoid this situation, organic
the presence of calcium hydroxide. At high pH values, there is a very calcium salts were tested, namely calcium acetate (Ca(CH2COOH)2)
low calcium ion concentration in the pore solution. On the other hand, and calcium formate (Ca(COOH)2). Also the amount of admixture was
the pH can be shifted to values lower than the equilibrium value of varied. The results are displayed for calcium acetate in Fig. 7.
portlandite in pure water (12.5) by maintaining a very high calcium
ion concentration. This trend is confirmed by the data presented in
Table 2. In the binder system comprising slag, calcium hydroxide and

Fig. 6. Impact of additions of calcium chloride and calcium bromide on the strength
Fig. 5. Heat of hydration of selected binders. development of mortar bars made from slag, calcium hydroxide, and calcium carbonate.
 F. Bellmann, J. Stark / Cement and Concrete Research 39 (2009) 644–650 649

Fig. 7. Impact of the addition of calcium acetate (Ca(CH2COOH)2) on the strength Fig. 9. Modification of compressive strength development by partial replacement of slag
development of a binder composed of 85% blast furnace slag, 10% calcium hydroxide, by Portland limestone cement.
and 5% calcium carbonate.

hydroxide and soluble calcium salts. The early strength was increased
The addition of calcium acetate is able to increase the final but is still far from being able to compete with the performance of
compressive strength of a binder consisting of slag, calcium hydroxide, Portland cement or blended cement. In addition to this, a high amount
and calcium carbonate. Compressive strengths up to 40 MPa were of calcium hydroxide was added. The amount of this material has to be
obtained at the age of 28 days by adding 1.0% and 1.5% of admixture. The reduced in order to produce cements with a very low specific emission
final strength is not proportional to the amount of admixture added. of carbon dioxide. To improve the early strength and to reduce the
Quantities exceeding the aforementioned optimum of 1.0 to 1.5% amount of calcium hydroxide added during binder preparation, a part
showed adverse effects on the final strength. The same trend was of the slag was replaced by cement. A serious disadvantage is the high
observed for early strength results. After two days, the highest specific emission of carbon dioxide during Portland cement clinker
compressive strength was measured for samples containing 0.5% production. However, the benefits of this replacement are:
calcium acetate, higher amounts yielding a reduction of the early
• an improved early strength due to the reaction of the clinker,
strength. This behaviour could be attributed to complexation of calcium
• a lower requirement of the amount of calcium hydroxide due to the
ions by acetate ions. If the concentration of acetate surpasses a critical
liberation of calcium hydroxide during reaction of the clinker, and
level, the concentration of free, un-complexed calcium ions drops
finally
significantly. The lower the amount of acetate ions in solution, the less
• easier use of the product under existing standards.
calcium ions are bound in the form of complexes. Further discussion
requires knowledge of the complexation constant of calcium acetate [7]. The latter is due to the fact that the European standard for the
The pH in the solution did not reflect the optimum additions of production of concrete allows application of specific cement types
calcium acetate. It was found that the pH in solution decreases only in a defined range of environments. Such environment classes are
continuously with rising amounts of admixture (Table 2). The pH in not specified for a binder containing blast furnace slag and some
the pore solution falls from 12.5 (0.5% calcium acetate) to 12.0 (4.0% inorganic materials such as calcium hydroxide and calcium carbonate.
calcium acetate). This data does not support the discussion of the role They are, however, available for cement containing a very high
of pH and the influence of complexation of calcium ions by acetate. amount of slag. According to European regulation, such cements are
The accelerating effect of calcium formate was compared to the CEM III/B containing 66–80% blast furnace slag, 20–34% Portland
performance of samples containing calcium acetate (Fig. 8). Also, cement clinker and CEM III/C containing 81–95% blast furnace slag,
calcium formate is able to improve compressive strength develop- and 5–19% Portland cement clinker. Up to 5% of minor constituents
ment in the same way calcium acetate does. The final strength was such as calcium carbonate are allowed in both cements. For the
about 40 MPa for optimum additions of calcium formate. The early investigations reported below, a binder containing 80% blast furnace
strength is very low for both organic salts (b10 MPa after 2 days). slag, 15% cement and 5% calcium hydroxide was used.
Nevertheless, the aforementioned discussion demonstrates that The cement was a CEM II/A-LL 32.5 R with a very low content of
blast furnace slag can be activated by the addition of calcium alkalis (Table 1). A high alkali content would require a high amount of

Fig. 8. Comparison of compressive strength development when using calcium acetate

and calcium formate for accelerating a binder consisting of 85% slag, 10% calcium Fig. 10. Influence of chemical admixtures on mortar strength of a binder consisting of
hydroxide, and 5% calcium carbonate. 80% blast furnace slag, 15% CEM II/A-LL, and 5% calcium hydroxide.
650 F. Bellmann, J. Stark / Cement and Concrete Research 39 (2009) 644–650

The performance obtained here allows for the production of cement

with a high final strength containing a very high content of blast furnace
slag. Corrosion of steel reinforcement has to be avoided by proper
selection of chemical admixtures. Therefore, the use of calcium chloride
or calcium bromide has to be neglected. Instead, organic calcium salts
such as calcium acetate and calcium formate should be employed. In all
samples investigated in this study, the pH in pore solution was higher
than the threshold level for reinforcement corrosion. According to Taylor
[1] and Metha and Monteiro [9] corrosion will not take place at pH
values higher than 11.5. However, the pH values obtained in this study
are much lower than observed when blast furnace slag is activated by
the maintenance of a very high pH (13.0–14.0). This indicates a reduced
resistance to chloride induced pit hole corrosion.
Fig. 11. Compressive strength of mortar bars and pH in pore solution after 28 day Using a selective dissolution technique it was shown that a high
hydration of a blend of slag, calcium hydroxide and calcium carbonate with different degree of slag consumption correlates with a high compressive strength.
admixtures. The early strength development can be increased by adding more
clinker to the blend. Testing has to be repeated at a water/cement-
calcium salts to lower the pH value. Preference was given to CEM II/A- ratio of 0.50 but requirements of strength class 42.5 can be met at
LL instead of pure Portland cement CEM I. Limestone present in CEM higher amounts of clinker.
II/A-LL can replace calcium carbonate that was used for the
production of the binder described in the first part of this study. 5. Conclusions
Beside this, an increased amount of calcium carbonate may be
beneficial for reducing the induction period before the precipitation of Blast furnace slag is usually activated by mixing with Portland
C–S–H starts and may thus be able to improve the strength cement clinker. At very high pH values (N13.0) corrosion of the slag
development. Calcium hydroxide was also included in the mix (5%). particles is enhanced and the reaction of blast furnace slag is
The compressive strength of the aforementioned blend was accelerated. Alternatively, the use of low pH values (11.8–12.4) was
measured (Table 2) and compared to the performance of the binder investigated in this study. It has been shown that activation is possible
composed of slag, calcium hydroxide and calcium carbonate (Fig. 9). by the addition of calcium hydroxide and soluble calcium salts. The
The data displayed in Fig. 9 shows that the compressive strength of addition of calcium hydroxide has two benefits. First, it acts as
the binder is significantly enhanced by the addition of 15% cement. reactant precipitating aluminium in an AFm phase. Second, in the
The final strength is increased from 16 MPa to 36 MPa after 28 days. presence of calcium hydroxide, soluble calcium salts can decrease pH
Even without addition of admixture, the blend of 80% slag, 15 CEM II/ in the pore solution as long as the equilibrium condition of calcium
A-LL and 5% calcium hydroxide passes the requirements for strength hydroxide is maintained.
class 32.5. A further increase of compressive strength can be obtained It is demonstrated for selected binders that higher compressive
by adding admixtures. Again, easily soluble calcium salts were used, strength is linked to a higher degree of slag consumption and a higher
namely calcium chloride, calcium bromide, calcium formate and heat release during hydration.
calcium nitrate. The results obtained by these tests are provided in Inorganic salts such as calcium chloride and calcium bromide have the
Table 2 and Fig. 10. disadvantage that their anions stimulate corrosion of steel reinforcement.
It can be inferred from Fig. 10, that all soluble calcium salts used in Organic substances such calcium acetate and calcium formate are helpful
this investigation were able to accelerate the reaction of a binder to enhance late strengths but less effective than the inorganic calcium
consisting of slag, Portland limestone cement, and calcium hydroxide. salts.
Besides the slag activation, also the hydration of the clinker fraction is The aforementioned concept can be used to improve the
stimulated by calcium salts. Final strengths between 47 and 53 MPa performance of slag cements containing very high amounts of slag.
are obtained after 28 days. In the early stage, the strength depends
much on the anion of the calcium salt. Strengths below 10 MPa are References
obtained for organic anions. The addition of salts with inorganic ions
[1] H.F.W. Taylor, Cement Chemistry, Thomas-Telford, London, 1997.
such as calcium chloride, calcium bromide and calcium nitrate enables [2] M. Moranville-Regourd, Cements made from blast furnace slag, in: P.C. Hewlett
the production of mortar with compressive strength higher than (Ed.), Lea's Chemistry of Cement and Concrete, Arnold, London, 1998, pp. 633–674.
10 MPa after 2 days. [3] E. Lang, Blast furnace cements, in: J. Bensted, P. Barnes (Eds.), Structure and
Performance of Cements, Spon Press, London, 2002, pp. 310–325.
[4] F.P. Glasser, Chemical, mineralogical, and microstructural changes occurring in
4. Discussion hydrated slag-cement blends, in: J. Skalny, S. Mindess (Eds.), Materials Science of
Concrete II, American Ceramic Society, Westerville (Ohio), 1991, pp. 41–81.
[5] J. Lukasik, J.S. Damtoft, D. Herfort, D. Sorrentino, E.M. Gartner, Sustainable
The aforementioned results have shown that it is possible to development and climate change initiatives, in: Proceedings of the 12th Interna-
activate blast furnace slag by low pH values in the pore solution in the tional Congress on the Chemistry of Cement, 8.-13.7.2007, Montreal, MPL-1.
presence of calcium hydroxide. Pure slag blended with calcium [6] T. Matschei, F. Bellmann, J. Stark, Hydration behaviour of sulphate-activated slag
cements, Adv Cem Res 17 (4) (2005) 167–178.
hydroxide and calcium carbonate showed mortar strengths up to
[7] J.F. Young, A review of the mechanisms of set-retardation in Portland cement pastes
50 MPa after 28 days. The pH values in the pore solution after 28 days containing organic admixtures, Cem Concr Res 2 (4) (1972) 415–433.
are plotted against the compressive strength after 28 days in Fig. 11. [8] J.S. Lumley, R.S. Gollop, G.K. Moir, H.F.W. Taylor, Degrees of reaction of the slag in
The data shows that pH has a major impact on compressive strength some blends with Portland cement, Cem Concr Res 26 (1) (1996) 139–151.
[9] P.K. Mehta, P.J.M. Monteiro, Concrete, (2nd ed.), McGraw-Hill, New York, 1993.
under the conditions used in this study. The highest strength is
obtained at the lowest pH in the pore solution. However, some other
effects are at play and there is no straight line observed for the
interrelation of compressive strength-pH in solution.