1. Application of steel slag concrete in the foundation slab and basement wall of
the Labein-Tecnalia Kubik building
Idoia Arribas, José T. San-José, Iñigo Vegas, Jose A. Hurtado and Jose A. Chica
Abstract:
The study described in the present article is motivated by a desire for the
consolidation, on the market, of sustainability concepts in the construction hyper-
sector. Within the framework of the Kubik initiative, developed by Labein -Tecnalia
over the period 2008-2010, a concrete structure which incorporated black steel slag
was constructed, as the foundation for the Kubik building laboratory. The results set
out in this study cover the dosage phases of the steel slag aggregate concrete, with
volumes of over 75% black slag. It is a pioneering structural application involving
slightly over 140m3
of reinforced concrete (basement walls and foundation slab),
which was manufactured in a concrete factory (Hormigones y Minas SA) and poured
on site without interruption by means of a concrete pump.
Introduction
Basic cement materials are the most widely used materials in the world in terms of
investment and production volume. As an example, global cement production in 2007
was over 2,500 million tons. An estimated dosage of between 250 and 300 kg of
cement per cubic metre of concrete means that 8,000 to 10,000 million cubic metres
could be produced which is equivalent to 1.5 cubic metres of concrete per person. No
other construction material has been used in such quantities, and it appears unlikely
that another construction material could compete with concrete in terms of volume, in
the future. This is principally due to it being a relatively low-cost, easily manufactured
material, which has excellent qualities, both in terms of durability and mechanical
strength.
Over the last decade, the European construction sector has undergone periods of
stagnation with regard to its production levels. A downturn in Europe of -4.3% was
envisaged for 2009, in relation to the preceding year. There is widespread agreement
that the sector will be incapable of recovering earlier levels of growth before a
complete return to normality in the economy is confirmed. Accordingly, even if we
accept that in 2010 the economies of the Euro zone may start to free themselves

2. from sluggish growth levels, the construction sector will have to live through a further
full year of stagnation, before returning to the path of growth.
In the present situation, two tendencies may be observed among firms in the sector:
cost reduction maximization in response to falling demand or, to risk pursuing
innovation as a strategy that will result in greater strength when activity picks up
again. Some of the opportunities of the sector (civil building works or infrastructure)
lie in the development of systems (Glass et al 2008) with greater levels of
commitment towards global sustainability (better safety, low energy consumption,
reduced emissions, environmental compatibility, improved functionality, etc.).
In this first decade of the 20 c., greater environmental lobbying, on the part of public
authorities and social agents, has heightened interest in the assessment of steel slag
aimed at the production of steel slag aggregate. Ever since the 1990s, LABEIN-
Tecnalia, currently integrated in Tecnalia-construction, has been working in the field
of recycled industrial by-products (black slags, foundry sands, paper mill sludge,
etc.). Its technological focus has been on their incorporation in construction materials
(concrete aggregates, cements, bituminous asphalts, mixed asphalts, etc.),
generating a range of satisfactory products and solutions that perform to a required
standard (Vegas, 2009).
The industrial by-products examined in this study are none other than slags from the
first fusion of Electric Arc Furnaces (black slags), produced in steelworks. Towards
the end of the 1990s, a (nationwide) initiative was taken in the Autonomous
Community of the Basque Country [Comunidad Autónoma del País Vasco (CAPV)],
that led to the preparation of a White Paper on slags (Ihobe, 1999) applicable to the
Basque Country (CAPV). The regulatory standard became a reference in Spain that
made it possible to use black slags in resurfacing layers, road bases and sub-bases.
Given the high production of black slag in Spain (some 2 million tons/year), it is
considered necessary to diversify the commercial uses of steel slag aggregate, with
a view to ensuring greater penetration into potential markets. On this point, one of the
priority lines of research over the last five years has centred on the use of steel slag
aggregate to prepare hydraulic concretes in bulk, with low structural capacity.

3. Scientific progress has also led to the emergence of business interests around steel
slag aggregate. The next challenge associated with this line of research is to study
the performance of structural concretes made with steel slag aggregate: designing
new dosages, looking into technological improvements for on-site use, compatibility
between materials (use of cements with additions), widen knowledge on physico-
mechanical behaviour and aspects related to the durability of reinforced concrete,
development of structural/functional prototypes on a real scale and analysis of
different architectonic performances (acoustic, energetic or aesthetic efficiency).
Benefiting from the work of other Spanish research groups, located in the universities
of Catalonia (Vázquez-Ramonich et al. 2004 and Berridi, 2008), Burgos (Manso,
2001 and Rodríguez, 2008) and the Basque Country (Losáñez, 2005), the approach
of this research is at more of an industrial level, on a real scale. The purpose of the
work consists in validating, both technologically and industrially, the use of black slag
in basement elements which certainly have complex execution conditions,
summarized as: a need to use concrete pumps for on-site execution, a large
continuous concrete slab, thin elements (basement wall), high geometric quantities of
steel in its reinforcements and large distances between plant and work site.
The manufacture of concrete with black slag as a steel slag aggregate
The manufacture of steel in electric steelworks entails two very different processes at
present: load fusion and dephosphoration in the electric arc furnace (EAF) and
desulpheration and refining in the ladle furnace (LF).
The manufacturing process in an EAF begins with a pre-heated charge that is placed
in the furnace hearth. This charge is made up of pre-reduced steel scrap (main
component, in proportions of 50 -90%), and slag-forming materials (calcium, silicon,
magnesium, alumina) in suitable proportions so that they form slag and protect the
furnace lining. Subsequently, the electric arc generated between three graphite
electrodes is started up, in order to melt the load very rapidly. A pool or core of liquid
steel is left in the lower bowl, and the protective slag floats above it in the upper part,
also in a liquid state, given that the fusion point of the compounds so formed
(silicates and calcium and magnesium aluminates) is lower than that of the steel. The
presence of iron oxides and calcium silicates, together with other minority

4. compounds, form the EAF black slag, the black colour of which at atmospheric
temperature is due to the iron oxides.
The generation of black slag in the CAPV amounts to almost 50% of national
production in Spain (937,000 tons, according to data in 2007). The first studies in the
Basque country [CAPV] (San-José et al 2000), on the possibilities of making use of
EAF black slag, which date back almost 12 years, mainly covered physico-chemical
aspects for its use as granular material in the manufacture of road surfaces (Rubio,
1991). In the field of concrete, and in conjunction with other national initiatives
(Manso et al, 2005), important steps have been made over the past 10 years in the
incorporation of this by-product, which we shall call Steel Slag Aggregate (SSA), as a
material for possible future use in concrete: up until now in non-structural concrete,
and as shown in this present study, in structural concrete, with load bearing capacity.
The main concern for over a decade was to guarantee that steel slag aggregate
would be stable from the dimensional point of view (Frías et al 2004). The presence
of excessive free calcium and magnesia leads to the expansion of the aggregate
over time. The consequence of the hydration of free calcium that the slag might
contain is an increase in volume (it can even double in size) that generates internal
tensions, causing small “wedge effects”, which fracture the periphery of the calcium
nodules.
MgO that is not chemically bounded is known as free magnesia, which in its
crystalline form is called periclase, as well as magnesium wüstites the general
formula of which is (Fe2Mg)O with more than 70 % MgO in weight. Unlike free
calcium, the humidity reaction takes place in a considerably slower manner. The
negative influence due to free MgO has solely been observed to date in the case of
slags with over 4 % MgO in total weight.
At present, the producers of steel slag aggregate produced from EAF black slag
guarantee expansion values of almost 0%. Among other similar initiatives in the
Basque country, one example of a black slag processing plant is the Guipúzcoa
plant, which is managed by Corrugados Azpeitia, S.A and designed to produce
around 90,000 t-AS/year.

5. Design and execution of a foundation slab and basement walls
Following a laboratory study of dosages, as a preliminary step in the manufacture of
the definitive product, a study of the mechanical behaviour of the product was
undertaken at the Hormigones y Minas concrete plant in Mañaria – Durango
(Vizcaya), in order to evaluate difficulties associated with scaling up for the market.
The steel slag aggregate in use came from Corrugados Azpeitia (Grupo Alfonso
Gallardo) and its titration and storage for one month took place at the Arroa Bea-
Zestoa instalations of HORMOR, until the date of its delivery to the Mañaria plant of
Hormigones y Minas in September 2008.
A cement concrete mix of 375 kg/m3
was used, and the cement (MP-CEM II/B-M(V-
LS) 42,5R) originated from at the Rezola plant (Arrigorriaga-Vizcaya).
The evolution of the compressive strength values (28days: 50 - 58MPa), performed
at the concrete plant shows that the dosage is appropriate for placing on-site using a
concrete pump.
The following table sets out the adjustments to concrete dosages manufactured with
over 75% steel slag aggregate in the execution of the foundation slab and basement
walls of the KUBIK building.
Table 3. Dosage of the foundation slab and basement walls.
Dosage HA-30/F/20/IIa+Qa
Product
Foundation slab Basement walls
Cement: II/B-M(V-L-S)
42,5R*
375+/-15 kg 375+/-15 kg
Fine aggregate 46% 40%
Thick aggregate 54% 60%
Cement/aggregate 0.46+/-0.02 0.46+/-0.02
Additives 1+/-1.4spc 1+/-1.4spc
* FYM (Arrigorriaga factory)
(Source: Hormigones y Minas - FyM - Italcementi Group)

6. 0
10
20
30
40
50
60
70
80
90
100
0,010 0,100 1,000 10,000 100,000
F ULLE R OB TE NIDA
0.010 0.100 1.000 10.000 100.000
FULLER OBTAINED
0.010 0.100 1.000 10.000 100.000
FULLER OBTAINED
Figure 1 Dosage adjustment curve
(Source: Hormigones y Minas - FyM - Italcementi Group)
First of all, uninterrupted concreting of around 140 m3
(25 lorries) was carried out
using SSAC type HA-30/F/20/IIa+Qa.
Figure 2 Concreting and vibrating of SSAC in the foundation slab.
Subsequently, the concreting of the basement walls was executed in two parts, the
north half-wall and the south half-wall.
Unlike the foundation slab, these elements require greater vibration intensities and
more fluid consistencies to reach all of the crevices around the rebars. This structural
element has a thickness of 30cm and a height of 3m. Hence, concreting is carried out
in batches of approximately 60 cm. with continuous and sufficiently intense vibrating
(2 simultaneous vibrators) to avoid spalling. Nevertheless, some spalling appeared,
probably due to vibrating that was not sufficiently intense (and not always possible),

7. coupled with an excessively long delay of the two in-transit mixers, due to last minute
inaccuracies in the assembly of the shoring-reinforcement bars of the wall.
All these issues are not always unrelated to it being a large-scale execution (never
undertaken before in an international context, as mentioned beforehand), and
perfectly excusable in view of the complications associated with its uninterrupted
execution, in keeping with the construction schedule, and such thin elements.
Hormigones y Minas supplied a total of 26 m3
in 5 in-transit mixers from its plant at
Mañaria-Durango (Vizcaya) for the execution of the south half-wall. Likewise, 5 days
later, a further 5 in-transit mixers supplied a total of 28.5 m3
for the second concreting
phase of the north half-wall; excess production being returned to the plant for
recycling.
Figure 3 View of the foundation slab and basement walls constructed by means
of pumped SSAC.
Evolution of strength
Over the following months, Labein-Tecnalia took three concrete specimen test
samples of different sizes and volumes, with a view to implementing exhaustive
quality control and follow up of the properties of the SSAC.
Thus, breakage of 3 ø15 x 30cm specimens was performed at different curing ages:
3 days, 7 days, 28 days, 90 days and 180 days. Moreover, six ø15 x 30cm
specimens were used to characterize the modulus of longitudinal deformation,
Poisson’s ratio, the modulus of transverse deformation and the load-deformation

8. curve under compression. The remaining 9 specimens, at 180 days were exhibited at
the KUBIK experimental laboratory.
Prismatic specimens of different dimensions were also taken which were later
subjected to accelerated ageing tests. These specimens will be subjected to
systematic study in accordance with the testing-inspection-assessment cycle for
degradation evaluation, so as to draw conclusions on the envisaged behaviour of this
material in its lifecycle.
Strength control performed by Labein-Tecnalia on the concrete foundation slab gave
the figures that are shown below in Table 4.
Table 4. Compressive strength of concrete foundation slab.
Compressive strength (MPa)
1st
Sample
(20.9ºC Tª amb.
60%HR)
1st
Sample
(20.9ºC Tª amb.
60%HR)
1st
Sample
(20.9ºC Tª amb.
60%HR)
Age
Cone: 20 cm Cone: 17 cm Cone: 18 cm
28.1 23.6 23.6
26.3 24.4 24.4
3d
29.4 27.9 25.7 24.5 25.7 24.0
39.2 37.0 37.0
36.0 40.3 40.3
7
38.0 37.7 39.3 38.8 39.3 40.4
51.7 51.7 51.7
51.9 51.3 51.3
28d
51.1 51.5 51.4 51.5 51.4 57.1
55.7 54.6 54.6
53.0 57.7 57.7
90d
56.6 55.1 52.0 54.7 52.0 62.3
56.5 56.9 56.9
57.3 58.0 58.0
180d
58.3 57.4 57.1
57.3
57.1
64.3
Based on these values, the graphs were drawn up to show the evolution of the
compressive strength of S1, S2 or S3, which refer to the specimens taken during the
concreting of the slab.

9. Evolution of compressive strength
SLAB
0
10
20
30
40
50
60
70
0 28 56 84 112 140 168 196
AGE (days)
Compressivestrength(MPa)
SLAB 1 SLAB 2 SLAB 3 SLAB
Figure 4 Evolution of the compressive strength of the SSAC foundation slab
over 180 days.
This highlights the notable improvement in the strength of the SSAC that was applied
to the foundation slab at 180 days, which had increased by 10% with respect to its
value at 28 days.
The results of the strength control performed on the concrete applied to the
basement walls are presented in table 5.
Table 5. Compressive strength of concrete foundation slab.
Compressive strength (MPa)
South Wall
(20.9ºC Tª amb. 51%RH)
North Wall
(20.9ºC Tª amb. 60%RH)Age
Cone: 16 cm Cone: 17 cm
15.7 23.2
15.3 24.2
3d
16.4 15.8 22.3 23.2
33.5 36.2
34.5 36.1
7d
34.3 34.1 35.1 35.8
45.0 47.3
47.8 44.0
28d
48.5 47.1 49.1 46.8
51.7 54.6
50.4 53.7
90d
55.5 52.5 53.1 53.8
57.9 56.3
55.5 56.8
180d
57.3
56.9
57.8
57.0
On the basis of these values, the graphs showing the evolution of the compressive
strengths of the South Wall and the North Wall were drawn up:

10. Evolution of compressive strength
BASEMENT WALL
10
20
30
40
50
60
70
0 28 56 84 112 140 168 196
AGE (days)
Compressivestrength(MPa)
SOUTH WALL NORTH WALL WALL
Figure 5 Evolution of the compressive strength of the SSAC basement walls
over 180 days.
In figure 6, it may be seen that the SSAC applied to the basement walls at a curing
age of 180 days had undergone an increase of around 20% with respect to its
strength at 28 days.
Elasticity modulus
The moduluses of longitudinal and of transverse deformation were both calculated,
as well as the Poisson’s Ratio of the concrete foundation slab and basement walls,
as shown in table 6. In all three cases, the modulus of elasticity and Poisson’s ratio of
the concrete in both the foundation slab and the basement walls reaches very similar
average values of around 33 GPa and 0.26 respectively.
Future lines of research
Given that no two natural aggregates are exactly alike each other (morphology,
texture, granulometry, etc.), neither are there two exactly similar batches of black
slag. Some more immediate lines of work could be defined over coming years,
according to the following breakdown:
• Examine the technology of placing SSAC (pumping and spraying) in greater
depth.
• Explore other cementitious matrices.
• Define more practical and new methodological durability tests.
• Value the improvements/variations introduced by the new aggregate-paste
reactions in the new SSA environments, etc.

11. For all these reasons, the research team committed to this work believes it will be of
great interest in the future to look at questions such as pre-cast concrete structures,
hybrid structures, etc.
Conclusions
The construction of the foundation slab and basement wall has demonstrated the
viability of the structures in every sense. However, a new road is now open to explore
other improvements with regard to the technology of execution, variation of dosages
and components, the study of other aggressive environments, aspects of accelerated
durability, etc.
From a purely economic point of view, apart from the agreements reached between
different parties (producers/consumers), it should certainly be taken into account that,
according to the conclusions of a recent study by Labein, if the same volumes of both
black slag and natural aggregate had to be transported (same volume of works), then
the greater density of SSA would require a 21% increase in lorry transport (10-15%
denser aggregate), in comparison with the same volume of natural aggregate that
would have to be transported, which could, for example, be compensated by air
entrainment additives that would reduce its density. Furthermore, whilst maintaining a
global vision of the problem (lifecycle) at all times, this data should also be contrasted
with reductions in energy consumption and greenhouse gas emissions, due to:
1. Reductions in large amounts of energy and emissions needed for extraction
from the aggregate quarries.
2. Reductions in large amounts of energy and emissions required for crushing,
screening and cleaning the natural aggregate that is replaced by SSA.
3. Reductions in large amounts of energy and emissions needed for part of the
transport of the natural aggregate to concrete plants.
Thus, the solution to apply black slag from steelworks for mass and/or structural
concrete should undergo control processes identical to those for natural aggregate,
in addition to physico-chemical characterization of its inert properties.

12. Acknowledgements
The authors would like to express special thanks to Hormigones y Morteros Agote
S.L., represented by Modesto Etxeberria, for technical collaboration and the supply of
steel slag, and likewise for the funds made available through various research
programmes of Science and Innovation Ministry.
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