Cement composition and method for the preparation thereof

The invention relates to a cement composition which can be hardened hydraulically and which consists of ground cement clinker material, granulated blast furnace slags and comminuted steel slags. The comminution of the steel slags is preferably carried out by grinding, in particular to a particle size of at most 0.2 mm and expediently to 0.09 mm. The ground steel slags have a Blaine value of at least 400 m.sup.2 /kg, expediently 473 m.sup.2 /kg, and a weight per unit volume of 2,800-4,000 kg/m.sup.3. The steel slags are chosen from air-cooled steel slags, foamed steel slags, granulated steel slags or a combination thereof. A method is also described for the preparation of a cement composition which can be hardened hydraulically, in which method cement clinker material and ground, granulated blast furnace slags are mixed with ground steel slags at normal or elevated temperature and pressure, the obtained mixture is cooled if desired, and comminuted to obtain a hydraulically hardenable cement composition. Use is made of a cement furnace if the present cement composition is prepared at an elevated temperature. Preferably a mixture of blast furnace slag cement and ground steel slags containing up to 30% of steel slags, or a mixture of Portland cement and ground steel slags containing up to 50% of steel slags is formed.

BACKGROUND OF THE INVENTION 
The invention relates to a cement composition based on ground cement 
clinker material and ground, granulated blast furnace slags and also 
comprising steel slags, and to a method for the preparation thereof by 
mixing the two first mentioned components with comminuted steel slags. 
PRIOR ART 
Cement compositions have already been known for a long time and are 
subdivided into various quality grades, based on the strength achieved 
under standardised conditions. Various types of cement exist, such as 
Portland cement and blast furnace slag cement. 
Portland cement is formed from the composite raw materials in a rotary 
furnace at a temperature of about 1450.degree. C. with the formation of 
the cement clinker as semi-finished product. The cement clinker is, 
however, much too coarse to be able to react with water at a reasonable 
rate. However, the surface area and with that the reactivity become 
sufficiently high on fine grinding in ball mills. In order also to achieve 
a favourable course of hardening when water is added to the cement 
composition, up to 5% gypsum is often added to the clinker during fine 
grinding. 
Blast furnace slag cement consists of a mixture of ground, granulated blast 
furnace slags and ground Portland cement clinker and usually also contains 
up to 5% gypsum. 
It is pointed out that it is generally known to convert iron products 
obtained from a blast furnace into steel in a converter, for example by 
blowing through oxygen. During this operation a steel slag is formed 
which, however, has disadvantages for practical use since it is difficult 
to process after solidification since it first has to be broken down in 
order to obtain particles of suitable size. For this reason, steel slags 
were virtually only processed as filler in a road construction material. 
Further, it is pointed out that an inorganic hydraulically-setting binder 
consisting of cement clinker material and steel slags is known from S. E. 
Aleksandrov, Stroitel'nye Materialy, 1979, (9), 25. It was found that such 
a binder had a shorter setting time when the amount of steel slags was 
raised, compared with a binder consisting of cement clinker material and 
blast furnace slags. 
Surprisingly, a cement composition has now been found which just has a 
longer setting time and can also give after hardening a product having a 
better strength. 
The cement composition according to the invention is based on ground cement 
clinker material and ground, granulated blast furnace slags, and is 
characterized in that the composition further partly consists of 
comminuted steel slags. 
The use of steel slags is further known from Chem. Abstr. vol. 104, nr. 26, 
June 30, 1986, abstract nr. 229557a. According to this disclosure, the 
steel slags are nevertheless firstly fused together with granite, 
thereafter cooled with water and pulverized to obtain pellets. These 
pellets are nevertheless especially designed for the iron processing 
industry. Such a product cannot be used as an inorganic hydraulic setting 
binder, such as the present cement composition, due to its low strength. 
NL-A-7111093 discloses a binder consisting of granulated blast furnace 
slags plus steel slags, in which the steel slags, due to their high 
content of combined CaO, can act as an initiator for the hydraulically 
acting blast furnace slag sand. 
In the cement composition according to the invention, on the contrary, the 
action of the granulated blast furnace slags is initiated by the 
(Portland) cement clinker material. 
A mixture of ground steel slags and blast furnace slags is also known from 
Chem. Abstr., vol. 112, nr. 2, Jan. 8, 1990, abstract nr. 11226k, but here 
again the cement clinker fraction is missing in the produced material. 
At last, it is known from FR 2.102,685 to use steel slags as a 
catalyst-accelerator in a mixture known as "grave laitier". "Grave 
laitier" consists of a mixture of coarse ground stone and granulated blast 
furnace slags. Here also the cement clinker material as used according to 
the invention, is missing. 
BRIEF SUMMARY OF THE INVENTION 
The cement composition according to the invention is based on ground cement 
clinker material and ground, granulated blast furnace slags, and is 
characterized in that the composition further partly consists of 
comminuted steel slags. The action of the granulated blast furnace slags 
is initiated by the (Portland) cement clinker material. 
The invention is not restricted to the use of air-cooled, comminuted 
steelslags. Any foamed steel slag or granulated steel slag can also be 
used advantageously. Such slags can moreover be finely ground requiring 
less grinding energy. 
Preferably, the steel slags are comminuted by grinding, in particular to a 
particle size of at most 0.2 mm, preferably at most 0.1 mm and expediently 
up to 0.09 mm. Such a particle size is advantageous since the commercially 
available cement powder usually has a particle size of up to .+-.140.mu., 
as a result of which an intimate mixture can be formed on mixing the two 
constituents. 
The ground steel slags used according to the invention in a cement 
composition are expediently essentially freed from iron. Iron is usually 
present in the form of ferrites. The removal of iron is effected in a 
simple manner by applying a magnetic field and provides a fraction which 
has a high ferrite content and a fraction which has a low ferrite content. 
The fraction with the high ferrite content can be re-used as such in the 
blast furnace for the preparation of pig iron. 
Preferably, the ground steel slags have a Blaine value of at least 400 
m.sup.2 /kg, preferably at least 450 m.sup.2 /kg. It is pointed out that 
the Blaine value for cement is about 400, depending on its quality. 
The weight per unit volume of the ground steel slags is expediently 2,800 
to 4,000 kg/m.sup.3 and in particular about 3,300 kg/m.sup.3. 
The invention also relates to a method for the preparation of a cement 
composition which can be hardened hydraulically and comprises at least 
cement clinker material and ground, granulated blast furnace slags, which 
is characterized in that cement clinker material and ground, granulated 
blast furnace slags are mixed with ground steel slags at normal or 
elevated temperature and pressure, the obtained mixture is cooled, if 
desired, and comminuted to obtain a hydraulically hardenable cement 
composition. 
The present cement composition is thus based on a combination of cement 
clinker material and ground, granulated blast furnace slags. Therefore, 
blast furnace cement or Portland cement are preferably used as starting 
materials. 
It has been found experimentally that optimum results can be obtained using 
a mixture of blast furnace slag cement and ground steel slags which 
contains up to about 30% steel slags, and using a mixture of Portland 
cement and ground steel slags which contains up to 50% steel slags. 
It is pointed out that the presence of the steel slags leads to a delay in 
the formation of cement setting. However, a delay in general leads to a 
higher strength and is therefore highly desirable. Since the ground steel 
slag itself already results in delayed hardening, no, or in any case less, 
gypsum has to be added to the cement composition. This constitutes an 
appreciable advantage in the long term. This is because the presence of 
gypsum (calcium sulphate) in cement can lead to the formation of 
ettringite. It is known that hardened concrete which contains ettringite 
can display crack formation if it comes into contact with 
sulphate-containing water. This crack formation occurs as a result of 
expansion of the ettringite. The absence of gypsum overcomes this problem. 
It is further pointed out that blast furnace cement consists of a mixture 
of (35-85%) granulated blast furnace slags and Portland cement clinker, a 
composition which is effective in practice for an end product of high 
strength, consisting of two parts of granulated blast furnace slags to one 
part of cement clinker. By now partially replacing the blast furnace slags 
and Portland clinker to be used for the production of the cement by finely 
ground steel slags, a cement is obtained which, it is true, hardens 
somewhat more slowly, but for which the strength obtained after hardening 
for 28 days is appreciably higher. 
Thus, according to the invention a cement clinker material can be formed 
using an intimate mixture of the raw materials customary for Portland 
cement clinker as well as ground steel slags as the starting material. On 
the other hand, it is also possible first to form a mixture of ground 
steel slags and ground, granulated blast furnace slags and to treat this 
mixture with conventional clinker raw materials in a cement furnace in 
order to obtain a cement according to the invention. The temperature of 
said furnace can then be as used for the preparation of clinker material, 
so up to about 1400.degree. C. 
However, it is also possible first to prepare a mixture of clinker raw 
materials and ground, granulated blast furnace slags and to treat this 
mixture at elevated temperature with the addition of ground steel slags, 
expediently also in a cement furnace, at the usual operating conditions. 
A preferred embodiment of the invention comprises forming a cement 
composition by treating an intimate mixture of essentially equal amounts 
of ground steel slags, ground, granulated blast furnace slags and cement 
clinker raw materials at elevated temperature and subjecting the resulting 
product to fine grinding after cooling. 
Mixing of the several components of the present cement composition can thus 
be effected at a temperature between room temperature and about 
1400.degree. C. 
Preferably, ground steel slags are used which have a weight per unit volume 
of 2,800 to 4,000 kg/m.sup.3, preferably about 3,300 kg/m.sup.3, and a 
Blaine value of at least 400 m.sup.2 /kg and preferably 473 m.sup.2 /kg. 
The particle size of the ground steel slags is at most 0.2 mm, preferably 
up to 0.1 mm and expediently up to 0.09 mm. 
It is pointed out that the invention is not restricted to the use of ground 
steel slags cooled in the air. Any foamed or granulated steel slag can 
advantageously be used requiring, moreover, less grinding energy for the 
fine grinding of such slags.

The invention is illustrated in more detail by reference to the following 
examples, provided solely by way of illustration. 
EXAMPLE 1 
Steel slags from a steel converter were cooled in air, and ground, in a 
manner known per se. A powder having the following analysis results was 
thus obtained. 
______________________________________ 
ANALYSIS RESULTS FOR STEEL SLAG POWDER 
Property Result 
______________________________________ 
Composition* (% m/m): 
CaO 43.7 
SiO.sub.2 18.3 
Fe.sub.2 O.sub.3 18.9 
Al.sub.2 O.sub.3 2.4 
MgO 3.1 
Na.sub.2 O 0.15 
K.sub.2 O 0.05 
Cl.sup.- 0.28 
Loss on ignition (952.degree. C.) 
3.9 
Insoluble matter (HCl) 
9.7 
Mass per unit volume (kg/m.sup.3) 
3314 
Blaine (m.sup.2 /kg)** 
473 
______________________________________ 
*total content expressed as oxides, digestion in acid 
**Blaine value of cement (depending on the quality) about 400 m.sup.2 /kg 
 
Although this steel slag powder contains a somewhat high amount of 
SiO.sub.2 and a somewhat low amount of Na.sub.2 O, this was found to be no 
drawback whatsoever when the powder was used in a cement composition, as 
can be seen from the results of a study on various mixtures of blast 
furnace slag cement and Portland cement using the steel slag powder shown 
above: 
__________________________________________________________________________ 
RESULTS OF STUDY ON MIXED CEMENTS 
Requirement 
in accord- 
Blast furnace 
90/10 
Portland 
80/20 
60/40 
40/60 
ance with 
Property 
Unit slag cement 
(a) cement 
(b) (c) (d) NEN 3550 
__________________________________________________________________________ 
WCF (water- 
% m/m 
27.8 26.6 
24.7 22.3 
21.2 
20.1 
-- 
cement factor) 
Standard 
mm 7 7 5 7 4 5 -- 
consistency 
Start of setting 
min 240 280 175 245 238 240 &gt;60 
End of setting 
min 285 330 200 275 265 280 -- 
Dimensional 
mm 0.5 1 1.5 1 3.5 2.7 &lt;10 
stability 
Tensile bending 
strength: 
1 day MPa 0.90 0.83 
2.00 1.59 
1.16 
0.36 
-- 
3 days MPa 3.7 4.0 3.2 3.9 3.1 1.7 -- 
7 days MPa 4.5 6.4 4.5 4.9 4.2 2.7 -- 
28 days MPa 8.4 8.8 6.8 6.5 5.8 3.9 -- 
Compression 
strength: 
1 day MPa 3.0 2.6 7.6 5.6 3.8 1.3 -- 
3 days MPa 15.0 16.6 
16.2 18.0 
13.2 
6.1 .gtoreq.11 
7 days MPa 30.0 31.4 
24.8 27.7 
19.3 
11.4 
-- 
28 days MPa 44.8 48.2 
39.8 34.2 
28.4 
17.3 
.gtoreq.35 and 
__________________________________________________________________________ 
.ltoreq.65 
(a): mixture of 90 parts of blast furnace slag cement + 10 parts of groun 
steel slags; 
(b): mixture of 80 parts of Portland cement + 20 parts of ground steel 
slags; 
(c): mixture of 60 parts of Portland cement + 40 parts of ground steel 
slags; 
(d): mixture of 40 parts of Portland cement + 60 parts of ground steel 
slags. 
It can be seen from the above results that the addition of ground steel 
slags to blast furnace slag cement delays setting by about 40 min., and in 
the case of addition to Portland cement delays setting by about 65 min. As 
yet, the delay in setting in the case of addition to Portland cement does 
not appear to be dependent on the amount added. 
It can be seen from the values found for the tensile bending strength and 
the compression strength that the use of ground steel slags provide a 
cement composition of outstanding quality. 
EXAMPLE 2 
The same steel slag powder as in Example 1, finely ground to a particle 
size of about 0.09 mm, was used as a starting material. 
The steel slag powder was mixed with the raw materials customary for 
Portland cement clinker, the amount of steel slags added being about 25% 
of the total amount of blast furnace slags present in blast furnace slag 
cement, and treated at a temperature of about 1400.degree. C. in a cement 
furnace. After cooling, the steel-slag cement clinker thus obtained was 
finely ground and mixed with ground, granulated blast furnace slags in 
order to obtain a cement composition. 
The properties of the material obtained after hardening of this cement 
composition show good comparison with those of blast furnace slag cement: 
the material according to the invention shows a somewhat delayed incipient 
hardening, but the compression strength obtained after hardening for 28 
days is higher. 
Repeating the above experiment but using an amount of steel slags of about 
50%, calculated with respect to the amount of granulated blast furnace 
slags, led to essentially the same result. 
EXAMPLE 3 
Example 2 was repeated, but ground, granulated blast furnace slags were 
also supplied to the cement furnace in an amount of 10%. The cement formed 
using the product obtained has an outstanding tensile bending strength and 
compression strength after hardening for 28 days. 
EXAMPLE 4 
An intimate mixture of equal amounts of ground, granulated blast furnace 
slags and ground steel slags was formed in a ball mill. The mixture thus 
obtained was baked at a temperature of about 1100.degree. C. and, after 
cooling, was finely ground and mixed with a finely ground cement clinker, 
in order to obtain a cement composition. 
The properties of the cement composition obtained are comparable with those 
of blast furnace slag cement.