WO2017085667A2

WO2017085667A2 - Lightweight concrete with a high elastic modulus and use thereof

Info

Publication number: WO2017085667A2
Application number: PCT/IB2016/056932
Authority: WO
Inventors: Alessandro Morbi; Aronne Carminati; Alessandro ZAPPOLI
Original assignee: Italcementi S.P.A.
Priority date: 2015-11-18
Filing date: 2016-11-17
Publication date: 2017-05-26
Also published as: ITUB20155712A1; WO2017085667A3; EP3377459A2

Abstract

A structural lightweight concrete with a high elastic modulus is described, consisting of a mixture of cement, water, hollow spherical lightweight aggregates, inert aggregates, preferably calcareous or silico-calcareous inert aggregates, wherein the weight ratio cement/hollow spherical lightweight aggregates, in the dry mixture, ranges from 0.6 to 0.9, preferably from 0.80 to 0.85 and wherein the water/cement weight ratio is equal to 0.44 and its use as structural lightweight concrete for the production of structural elements, preferably pillars, beams, walls, etc.

Description

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LIGHTWEIGHT CONCRETE WITH A HIGH ELASTIC MODULUS AND USE THEREOF

The present invention relates to a lightweight concrete with a high elastic modulus and use thereof.

In particular, the present invention relates to a lightweight concrete with a high elastic modulus comprising light aggregates having the form of hollow spheres.

The demand for lightweight concrete building materials is continuously increasing: light structural materials characterized by a high mechanical strength and a high rigidity are of particular interest. The lighter the structural concrete is, at the same time satisfying the necessary mechanical specifications, the more possible it will be to produce structural elements which, with the same mechanical characteristics, will be lighter and more aerial, i.e. for example, thinner beams and narrower pillars can be used, etc..

In general, the greatest loads derive from the operative functioning of the structure (accidental loads), but it is also true that light structural elements reduce permanent loads allowing a reduction in the resistant sections.

Structural elements of this type also require a reduced quantity of concrete, with a consequent reduction in costs and energy consumption.

The main characteristics of lightweight concrete are associated with the density of the material which is lower than ordinary concrete. This results in:

- a reduction in its weight and therefore in the permanent loads applied to reinforced concrete structures;

a different relation between the two fundamental mechanical characteristics: compression strength and tensile modulus. In particular, in lightweight concretes, the ratio between mechanical strength (defined as the maximum mechanical stress of the material before breakage) and tensile modulus is higher than that of ordinary concretes. Eurocode 2 (i.e. European standards for the structural design of unreinforced, reinforced and pre-compressed concrete) provides two different relations for ordinary and for lightweight concretes. These relations are represented in Figure 1.

For a better understanding of the criticality of lightweight concretes, it is important to consider, for the purposes of structural calculation, the law that links the deformation of the material to the mechanical stress in the elastic-linear range, i.e. Hooke's law, expressed as follows:

σ = Εε (1)

wherein:

σ [MPa] : stress state→ force applied on the surface unit

ε [-] : relative deformation (strain) (ΔΙ/l) wherein "1" is the initial dimension and "Δ1" is the deformation measured (therefore non-dimensional)

E: elastic modulus -> expresses the tension theoretically required for a 100% compression of a test-sample.

Considering the example of a compressed pillar and assuming that the pillar is subjected to a load Q [N] and has a section A [mm ], the load/section ratio (stress state) is normally fixed at 1/3 of the mechanical strength of the concrete. Consequently, in the case of a concrete with Rck (characteristic resistance) equal to 30 MPa, the Q/A ratio will be 10 MPa.

The pillar is made of concrete characterized by an elastic modulus E [MPa] and the deformation of the pillar can be easily obtained from the relation (1) indicated above.

The stress state is given by the load applied divided by the section, and therefore by the relation:

by exerting the deformation ε:

is obtained. The deformation is therefore directly proportional to the load applied to the pillar and indirectly proportional to the section and elastic modulus. The product A * E is called rigidity of the pillar and corresponds to the capacity of opposing the deformations generated by a load and depends on the specific elastic modulus of the material.

In the case of an ordinary concrete (Rc=35MPa Q/A = 11.5; E 30,000 MPa):

8=11,5* 1/30,000 = 0.00038.

In the case of a lightweight concrete (Rc=35MPa Q/A = 11.5; E 20,000 MPa):

8=11.5* 1/20,000 = 0.00057.

The deformation of the pillar made of lightweight concrete is higher than that of the pillar made with ordinary concrete, even if the mechanical strength of both concretes is the same.

The only way of allowing the deformation of the lightweight concrete pillar to be comparable to the deformation of the ordinary concrete pillar is to increase the section (in this case almost 50%) of the pillar, reducing the stress state (Q/A).

The lightweight concretes according to the state of the art are generally produced using aggregates or porous inert materials, obtained with various methods. Said porous aggregates, which are certainly lighter than a classical aggregate, however, have problems of water absorption and mechanical strength, when used in the production of a light concrete.

The absorption of water leads to a heavier aggregate, which therefore loses its desired properties of lightness, and also an increase in the water consumption necessary for forming the lightweight concrete.

As an alternative to these porous aggregates, aggregates or hollow inert products made of a non-porous material have also been adopted, which, however, once used as aggregates in a lightweight concrete, have not allowed the production of a concrete characterized at the same time by a high mechanical strength, a high rigidity and with a high elastic modulus.

In order to overcome these problems, cellular concretes have also been used, i.e. concretes that incorporate a mixture of porous cement in which the cavities are produced from expandable polystyrene in the form of small particles, subjecting the material to sintering and thus causing the expansion of the polymeric material due to the heat during the polymerization (US 3021291).

The present invention proposes to find a lightweight concrete that does not have the disadvantages of the lightweight concretes according to the state of the art, which, as already mentioned, are not able to guarantee at the same time characteristics of lightness, mechanical strength, rigidity and elastic modulus, i.e. those characteristics that allow lightweight concrete to be optimally used as structural lightweight concrete.

The Applicant has in fact surprisingly found an improved lightweight concrete, that can be particularly effectively used as structural lightweight concrete, being provided at the same time with a high mechanical strength, a high rigidity and with a high elastic modulus, thus solving the technical problem addressed by the present invention.

An object of the present invention therefore relates to an improved structural lightweight concrete having a high elastic modulus, which comprises a mixture of cement, water, hollow lightweight spherical aggregates, inert aggregates, preferably calcareous or silico-calcareous inert aggregates, wherein the weight ratio of cement/hollow lightweight spherical aggregates in the dry mixture ranges from 0.6 to 0.9, preferably from 0.80 to 0.85 and wherein the water/cement ratio by weight is equal to 0.44.

A further object of the present invention relates to the use of a lightweight concrete having a high elastic modulus, which comprises a mixture of cement, water, hollow lightweight spherical aggregates, inert aggregates, preferably calcareous or silico-calcareous inert aggregates, wherein the weight ratio of cement/hollow lightweight spherical aggregates in the dry mixture ranges from 0.6 to 0.9, preferably from 0.80 to 0.85 and wherein the water/cement ratio by weight is equal to 0.44, as structural lightweight concrete.

An object of the present invention also relates to structural elements obtained with the lightweight concrete according to the present invention such as pillars, beams, walls, etc. Etc. A further object of the present invention relates to a hollow lightweight aggregate having the form of a hollow sphere, wherein said hollow sphere has a particle size ranging from 0.6 mm to 1.5 mm, preferably from 0.7 to 1.1 mm, with a wall thickness equal to about 1 mm and a smooth or rough outer surface, preferably rough, made of ceramic or cementitious material, and the use of said hollow lightweight aggregate in a structural lightweight concrete having a high elastic modulus.

A fundamental advantage of the lightweight concrete having a high elastic modulus according to the present invention is that it allows the production of lighter, more aerial structural elements, i.e. for example, it allows the production of thinner beams, narrower pillars, etc.. These structural elements, with the same mechanical characteristics, also allow a reduced quantity of concrete to be used, with a consequent reduction in costs and energy consumptions.

Furthermore, the particular lightweight aggregate having a hollow spherical form present in the lightweight concrete according to the present invention, also allows the problem of water absorption to be eliminated, a problem which is typical of porous lightweight aggregates according to the state of the art.

The term "cement" refers, according to the present invention, to a material in powder form which, when mixed with water, forms a paste which hardens by hydration, and which, after hardening, maintains its resistance and stability even under water. In particular, the cements according to the present invention comprise so-called Portland cement, slag cement, pozzolan cement, fly ash, calcined shale cement, limestone cement and so-called composite cements. Cements of type I, II, III, IV or V according to the standard EN197-1 , can be used, for example. Particularly preferred cements are CEM cement and CEM II cement. The particularly preferred class of cement is class 52.5 for CEM I cement.

The term "inert aggregate" according to the present invention refers to inert aggregates such as powders and sands, suitably selected from calcareous, silico-caleareous and siliceous aggregates in any form.

These aggregates can be distinguished in sands, having an average diameter ranging from 0.5 to 5 mm, and in powders or fillers, i.e. fine inert aggregates, having an average diameter ranging from 50 to 100 microns.

Calcareous or silico-calcareous aggregates are particularly preferred. The preferred aggregate consists of natural silico-calcareous or crushed sand and filler or fine calcareous filler.

The term "hollow lightweight spherical aggregate" refers, according to the present invention, to a lightweight aggregate having the form of a hollow sphere, characterized by a high mechanical strength and a high rigidity. The surface of the rigid shell that forms the hollow sphere is not porous, said hollow sphere being obtained from ceramic powders or sintered minerals. The hollow sphere has a thickness of about 1 mm, a density ranging from 1.5 to 1.7 (mg/m ) and a particle size ranging from 0.6 mm to 1.5 mm, preferably from 0.7 to 1.1 mm. The hollow lightweight spherical aggregates used in the lightweight concrete according to the present invention were produced using various possible matrixes for the rigid shell and more specifically

a material of the ceramic type;

a cement paste.

The cement paste can be hardened with accelerated aging in an autoclave with supersaturated steam (180°C and 13 atm) or it can be hardened with aging under "natural" conditions at a temperature of 22°C and a relative humidity close to 100%.

The aggregate made of cementitious material or ceramic material according to the present invention can be obtained by means of a process which comprises a first granulation step of a dry mixture of ceramic material or cement paste on spherical polystyrene nuclei sprayed with latex suspensions in a concrete mixer or granulator plate; and, in the case of an aggregate made of ceramic material, a subsequent ceramization step of the granulated spheres thus obtained, carried out in suitable electric ovens; or, in the case of an aggregate made of cementitious material, a subsequent hardening step of the granulated spheres thus obtained, with accelerating aging in an autoclave with supersaturated steam, at 180°C and 13 atm, or with aging under "natural" conditions at a temperature of 22°C and a relative humidity close to 100%.

The first step of the process for producing the lightweight aggregate in the form of a hollow sphere to be used in the lightweight concrete according to the present invention therefore comprises a granulation step of the dry mixture of ceramic material or cement paste on spherical nuclei of virgin polystyrene.

The ceramic-type material comprises three essential components: a clayey raw material, a melting component and a stabilizing component.

The clayey raw material can be selected from kaolite, bentonite, meta kaolin, etc., and is present in an amount ranging from 30 to 50% by weight with respect to the total weight of the ceramic material; the melting component can be selected from feldspars, limestone, dolomite, talc, and is present in an amount ranging from 20 to 40% by weight with respect to the total weight of the ceramic material; the stabilizing component can be selected from quartz sand and silica, etc., and is present in an amount ranging from 10 to 20% by weight with respect to the total weight of the ceramic material.

Clay is the basic raw material for preparing ceramic materials obtained by high-temperature baking, whereas the stabilizing component consists of minerals or rocks, used in their original form or thermally pre-treated, whose main requirement is dimensional stability or even a tendency to expand. When added to a ceramic paste, they form its backbone, thus countering the shrinkage to which the end-product is subjected, for reasons of a chemical nature (dehydration, thermal decomposition or the like) and/or of a physical nature (narrower structure of the particles forming the mixture). The melting component has a lower melting point than the base constituents of the ceramic end-products (silica and alumina), but has the property of further reducing the melting point, if present in the mixture with the same, and favours the sintering process of the clays.

In the case of ceramic materials, the following preparation is described, for illustrative purposes, starting from a crude mixture consisting of

- meta kaolin with the function of ceramizing base 65% by weight with respect

to the total weight of the crude mixture;

- quartz with the function of volumetric stabilizer, fraction 0 - 0.100 mm, 20%

by weight with respect to the total weight of the crude mixture;

- feldspar with the function of melting agent, fraction < 0.150 mm, 15% by

weight with respect to the total weight of the crude mixture.

The formation of the sphere takes place by means of a granulation process of the powder of dried/dry mixture wherein the nuclei from which the spheres grow are composed of virgin expanded polystyrene beads, preferably with two nominal particle-size distributions equal to 2-3 mm and 3-5 mm.

Latex suspensions are used for allowing the adhesion of the crude-mixture powder to the polystyrene beads (hydrophobic) and subsequently to continue the granulation, obtaining compact spheres.

As already specified, the thickness of the spheres is an important parameter: almost 200 g of dry powder are necessary for each gram of polystyrene in order to obtain a shell having a thickness of about 1 mm, regardless of the nominal particle size of the beads used, whereas the quantity of latex necessary is about 67.5 g, considering an average concentration of the dispersion of 30% by weight.

The thickness of 1 mm is the most adequate thickness for spheres having the desired particle size, which ranges from 0.6 mm to 1.5 mm, preferably from 0.7 to 1.1 mm, as a thickness lower than 1 mm would make them too delicate for sustaining the mechanical operations of the concrete paste and a larger thickness would negatively influence the nominal density parameter that considers closed cavities. Spheres with a substantially smooth surface and also spheres with a high roughness can be produced, as shown in figure 2 (2a and 2b, respectively).

The quantity of polystyrene beads corresponding to the quantity of hollow spheres to be produced is positioned in a suitable concrete mixer provided with a geared device which allows a continuous regulation of the inclination angle. The concrete mixer also has an adjustable rotation rate which varies linearly from 9.5 to 24 revolutions per minute. The granulation step is carried out at the maximum speed. The spheres can obviously also be produced with a granulator plate.

The dry powder is then fed, the quantity being defined in relation to the quantity of spheres to be produced, and the rotation of the concrete mixer is started.

The latex suspension is then sprayed, by means of a suitable nebulizer, onto the polystyrene beads, the latex wets the polystyrene beads which, rolling, end up in the mass of dry powder that slides on the drum wall, covering it.

The ceramization of the granulated spheres thus obtained is effected in suitable electric ovens, after drying the spheres in air and sieving them to remove the granulation waste and dust produced in every movement of the same spheres. This operation is necessary as, in the baking step, any possible non-removed waste, on baking, would stick to the spheres, gluing them together permanently.

The thermal baking cycle comprises the following three steps:

- in the first phase, the temperature is increased from room temperature to

300°C, with a gradient of 2°C/min, followed by an hour of maintenance. In this phase, there is the complete drying of the free water of the spere and, in the plateau phase, a first elimination of the hydration water. In this phase, there is the simultaneous degradation of the polystyrene of the nucleation bead and therefore the formation of cavities;

in the second phase, the temperature is increased from 300°C to 700°C, with a gradient of 2°C/min, followed by an hour of maintenance. In this phase, there is the complete elimination of the crystallization water and the first baking phase; in the third phase, the temperature is increased from 700°C to 1,300°C, with a gradient of 5°C/min, followed by three hours of maintenance. In this phase, there is the ceramization of the material which undergoes a consistent reduction in the volume that can be estimated as 20 to 30% with respect to the volume of the raw spheres.

As previously indicated, the lightweight aggregates in the form of a hollow sphere can also be obtained starting from cement pastes, wherein the cement paste can be hardened with accelerated aging in an autoclave with supersaturated vapour (180°C and 13 atm) or it can be hardened with aging under "natural" conditions at a temperature of 22°C and a relative humidity close to 100%.

The former case is referred to as a binder by aging in an autoclave and, for example, a binder is indicated, obtained from the following composition

· Scafa Clinker 70% by weight;

• Quartz silica 30% by weight

by co-grinding a Scafa clinker and a quartz silica to a high fineness, 6,000 Blaine (cm /g). The granulation step is carried out exactly as described for the hollow spheres of ceramic material with the only difference that the latex is only initially nebulized, i.e. until the polystyrene beads have been coated with a first "veil" of dry powder binder, the rest of the granulation is then continued, simply using water as liquid part.

For each gram of virgin polystyrene beads, there is a consumption of 200 g of binder, about 6.5 g of latex (ready to use) and 45 g of water.

The aging of the spheres produced with this binder takes place in two steps, the first at a low temperature at ordinary pressure and the second at a high temperature and pressure (180°C and 13 atm). The first aging step has a duration ranging from approximately 7 to 18 hours and has the aim of giving the spheres a minimum mechanical strength for sustaining the autoclaving step. The aging temperature is 60°C and the humidity is that established at this temperature in a closed environment with free water.

At the end of this first aging cycle in an oven, the spheres are introduced into the autoclave, set at 180°C and the equilibrium pressure, once the temperature has been reached, is about 13 atm. This step has a duration of about 24 hours.

In the case of a cement paste hardened with aging under "natural" conditions, this is referred to as a binder by natural aging and this binder is produced by the grinding of a Scafa clinker (100%) to a high fineness, 8,000 Blaine (cm²/g).

The granulation is carried out as described for the binder with aging in an autoclave and for each gram of virgin polystyrene beads, there is a consumption of 200 g of binder, about 6.5 g of latex (ready to use) and 57 g of water.

The aging phase of this type of sphere is carried out by means of a first step in which, at the end of the granulation and after a surface wetting, the spheres are put into a container, which is then closed and sealed. Within the first hour of aging, the hydration material develops a large amount of heat and the surface of the container reaches a temperature of about 80°C, producing a kind of steam aging, with autogenous heat. The rest of the aging, for a total of 28 days, takes place at a temperature of 22°C and a relative humidity close to 100%, corresponding to that established in the closed container with a small excess of water for wetting the spheres.

The hollow spheres made of ceramics and autoclaved cement paste, produced as described above, were characterized with reference to the physical characteristics as aggregates and more specifically, the density, water absorption and the crash test were measured, of which the results are indicated in Table 1 below. Table 1

The previous characterizations were carried out according to the following standards:

the volumetric mass or density was measured according to the method EN 1097-6:2013; the water absorption was measured according to the method EN 1097-6:2013;

the crash test was carried out according to the method EN 13055-1:2002.

The particle-size distribution of the hollow spheres, measured with a video-granulometer, is indicated in figure 3.

Table 2 below indicates the physical characterization for the hollow ceramic spheres shown in figure 2, smooth 2a) and rough 2b).

Table 2

The lightweight concrete with a high elastic modulus according to the present invention comprises a quantity of cement ranging from 300 to 550 kg/m , more preferably from 350 to

500 kg/m 3 , even more preferably from 420 to 480 kg/m 3. The preferred cement is CEM I class 52.5 and CEM II.

The lightweight concrete with a high elastic modulus according to the present invention comprises a quantity of water ranging from 132 to 242 kg/m , preferably from 160 to 230 kg/m³.

The water/cement ratio is equal to 0.44 (by weight).

The lightweight concrete with a high elastic modulus according to the present invention comprises a quantity of inert aggregate ranging from 450 to 800 kg/m . The inert aggregate according to the present invention more preferably consists of sand, preferably natural or crushed silico-calcareous sand, in a quantity ranging from 450 to 650 kg/m and fine aggregate, preferably calcareous filler, in a quantity ranging from 0 to 150 kg/m .

The lightweight concrete with a high elastic modulus according to the present invention comprises a quantity of hollow spherical lightweight aggregate ranging from 333 to 918 kg/m 3 , preferably from 500 to 612 kg/m 3.

The preferred aggregate consists of hollow spheres of ceramic material or cementitious material, preferably ceramic material.

The ratio between the quantity of cement and the quantity of lightweight aggregate is fundamental: the weight ratio, in the dry mixture, cement/hollow spherical lightweight aggregates ranges from 0.6 to 0.9, preferably from 0.80 to 0.85.

The lightweight concrete with a high elastic modulus according to the present invention can also comprise superplasticizing additives or other additives, in negligible quantities.

A lightweight concrete with a high elastic modulus according to the present invention comprises the following composition:

cement CEM I 52.5 in a quantity ranging from 350 kg/m 3 to 500 kg/m 3 ;

water in a quantity ranging from 160 kg/m 3 to 230 kg/m 3 ;

hollow spherical lightweight aggregates in ceramic material in a quantity ranging from 500 to 612 kg/m , wherein preferably 60% by volume of the aggregates has a rough surface and 40% by volume of the aggregates has a smooth surface, the volume percentages referring to the total volume of lightweight aggregates;

silico-calcareous sand in a quantity ranging from 450 kg/m 3 to 650 kg/m 3 ; calcareous filler in a quantity ranging from 0 to 100 kg/m ;

superplasticizer in a quantity ranging from 2 kg/m 3 to 5 kg/m 3 ; the water/cement ratio by weight being equal to 0.44 and

the ratio cement/hollow spherical lightweight aggregates, by weight, in the dry mixture, being equal to 0.82.

The solution according to the present invention has surprisingly identified a lightweight concrete with a high elastic modulus which contemporaneously has a compressive strength higher than 53 MPa (at 28 days), an elastic modulus higher than 25,000 MPa (at 28 days) and a volumetric mass or density in the hardened state ranging from 1,800 to 1,900 kg/m .

The lightweight concrete with a high elastic modulus according to the present invention has the advantage of being characterized, at the same time, by a high mechanical strength and a high elastic modulus which, contrary to expectations, allows the production of lighter, more aerial structural elements, such as, for example, thinner beams, narrower pillars, etc. These structural elements, with the same mechanical characteristics, also allow a reduced quantity of concrete to be used, with a consequent reduction in costs and energy consumptions.

Furthermore, the particular hollow spherical lightweight aggregate also allows the problem of water absorption to be eliminated, a problem which is typical of the porous lightweight aggregates according to the state of the art.

Further characteristics and advantages of the invention will appear evident from the following examples provided for illustrative and non-limiting purposes.

Example 1

A lightweight concrete with a high elastic modulus having the composition (C-l) indicated in Table 3 below, was prepared.

Table 3

The quantities indicated in the table are expressed in kg/m .

The cement is a Portland cement type I 52.5R in accordance with the standard UNI EN 197- 1, coming from the cement works Italcementi of Calusco d'Adda, having a fineness equal to

4,960 cm 2 /g (Blaine) and a density equal to 3.151 kg/dm 3.

The calcareous filler is a fine calcareous aggregate with a maximum diameter equal to about 100 Dm. Said aggregate has a density equal to 2.7 kg/dm .

The silico-calcareous sand is a washed alluvial sand, with a density equal to about 2.6 kg/dm³.

The ceramic lightweight aggregates having a hollow spherical form were obtained as indicated above, from the crude mixture composed of

meta kaolin with the function of ceramizing base 65% by weight

with respect to the total weight of the crude mixture;

- quartz with the function of volumetric stabilizer, fraction 0 - 0.100

mm, 20% by weight with respect to the total weight of the crude mixture;

feldspar with the function of melting agent, fraction < 0.150 mm,

15% by weight with respect to the total weight of the crude mixture

and they consist for 60% by volume of ceramic aggregates having a rough surface and 40% by volume of aggregates with a smooth surface, the volume percentages referring to the total volume of the lightweight aggregates.

The water/cement ratio is equal to 0.44, whereas the cement/lightweight aggregates ratio is equal to 0.82.

The superplasticizer, produced by SIKA Italia (Creactive 4K), is an acrylic additive with the function of water reducer.

For a complete homogenization, the cement, water, lightweight aggregate and other aggregates and additives are mixed in a concrete mixer or similar equipment, in the appropriate proportions, until a homogeneous lump-free mixture is obtained, which was suitably characterized with the measurement of the slump and density or volumetric mass. After hardening, the values relating to the volumetric mass, compressive strength, dynamic elastic modulus, static elastic modulus and thermal conductivity were measured, and are indicated in Table 5 below.

Example 2 (comparative)

For comparative purposes, a lightweight concrete was prepared using classical lightweight aggregates, i.e. expanded clay (C-2) and an ordinary concrete (without lightweight aggregates, C-3), having the compositions indicated in Table 4 below.

Table 4

The quantities indicated in the table are expressed in kg/m .

2 3

4,960 cm /g (Blaine) and a density equal to 3.151 kg/dm . The calcareous filler and silico-calcareous sand are the same as those used in Example 1. The water/cement ratio is equal to 0.44.

The superplasticizer is produced by SIKA Italia (Creactive 4K) and is an acrylic additive with the function of water reducer.

For a complete homogenization, the cement, water, the lightweight aggregate (if present) and the other aggregates and additives are mixed in a concrete mixer or similar equipment, in the appropriate proportions, until a homogeneous lump-free mixture is obtained, which was suitably characterized with the measurement of the slump and density or volumetric mass. After hardening, the values relating to the volumetric mass, compressive strength, dynamic elastic modulus, static elastic modulus and thermal conductivity were measured, and are indicated in Table 5 below.

The volumetric mass or density in the fresh state was measured according to the method UNI EN 12350-6.

The slump test was carried out according to the method UNI EN 12350-2.

The volumetric mass or density in the hardened state was measured according to the method UNI EN 12350-7.

The compressive strength was measured according to the method UNI EN 12390-3.

The thermal conductivity was measured according to the method UNI EN 12664 - Heat flow meter method.

The secant elastic modulus (MES) was measured according to the method UNI 6556.

The dynamic elastic modulus (MED) was measured according to the method ASTM C215. Table 5

Example 3

A lightweight concrete was prepared, with an identical composition to that indicated in Example 1 with the only difference that the ceramic lightweight aggregates having a spherical form are exclusively composed of ceramic aggregates with a smooth surface.

After hardening, the values relating to the volumetric mass, compressive strength and static elastic modulus were measured, and are indicated in Table 6 below.

Table 6

As can be seen from figure 4, the surface roughness does not substantially affect the elastic modulus, but it allows an improvement in the compressive strength.

From the previous comparisons, it is clear that the lightweight concrete with a high elastic modulus of the present invention has an optimum combination of physical properties that allow a lightweight concrete to be obtained, which, at the same time, is characterized by

a greatly improved compressive strength and static elastic modulus with respect to the lightweight concretes already known in the state of the art, physical characteristics which allow an optimal use of the lightweight concrete according to the present invention as a structural element;

- a reduced, but sufficient mechanical strength and a reduced, but sufficient elastic modulus with respect ordinary concretes (about 70% and 76% respectively of the values of the reference ordinary concrete of Example 2,) (Figure 5);

an elastic modulus about 10% higher with respect to what is estimated on the basis of the algorithms according to Eurocode2 (EC2) for a lightweight concrete (Figure 6) and consequently a greater rigidity; figure 6 indicates the Elastic Modulus values measured in the laboratory (red bars) for each concrete illustrated. These experimental values are compared with the estimation of the elastic modulus value (given the strength and density) according to the models of EC2 (blue bars). As can be observed, both in relative terms (between estimation and real - blue and red bar) and in absolute terms (final values of the red bars), the lightweight concrete according to the present invention has a higher elastic modulus (with the same density and design mix); these elastic modulus values allow, for example, a lightweight concrete pillar to be produced, which has a deformation very close to that of the same pillar produced with ordinary concrete, with the same mechanical strength, without having to increase the section;

- a reduced water absorption with respect to classical lightweight concretes.

Claims

1. A lightweight structural concrete with a high elastic modulus, comprising a mixture of cement, water, hollow spherical lightweight aggregates, inert aggregates, preferably calcareous or silico-calcareous inert aggregates, wherein the weight ratio cement/hollow spherical lightweight aggregates, in the dry mixture, ranges from 0.6 to 0.9, preferably from 0.80 to 0.85 and wherein the water/cement weight ratio is equal to 0.44.

2. The lightweight concrete according to claim 1, wherein the mixture comprises a

3 3 quantity of cement ranging from 300 to 550 kg/m , more preferably from 350 to 500 kg/m , even more preferably from 420 to 480 kg/m , said cement being preferably selected from CEM I class 52.5 and CEM II cements.

3. The lightweight concrete according to one or more of the previous claims, wherein the mixture comprises a quantity of water ranging from 132 to 242 kg/m , preferably from 160 to 230 kg/m³.

4. The lightweight concrete according to one or more of the previous claims, wherein the mixture comprises a quantity of hollow spherical lightweight aggregate ranging from 333 to

3 3

918 kg/m , preferably from 500 to 612 kg/m , said hollow spherical lightweight aggregate consisting of hollow spheres of ceramic material or cementitious material, preferably ceramic material.

5. The lightweight concrete according to one or more of the previous claims, wherein the mixture comprises a quantity of inert aggregate ranging from 450 to 800 kg/m , said inert aggregate being preferably sand, more preferably silico-calcareous natural or crushed sand, in a quantity ranging from 450 to 650 kg/m and powder or fine aggregate, preferably calcareous filler, in a quantity ranging from 0 to 150 kg/m .

6. The lightweight concrete according to one or more of the previous claims, wherein the mixture consists of

3 3

cement CEM I 52.5 in a quantity ranging from 350 kg/m to 500 kg/m ; water in a quantity ranging from 160 kg/m 3 to 230 kg/m 3 ;

hollow spherical lightweight aggregates made of ceramic material in a quantity ranging from 500 to 612 kg/m , wherein preferably 60% by volume of the aggregates has a rough surface and 40% by volume of the aggregates has a smooth surface, the volume percentages referring to the total volume of lightweight aggregates;

silico-calcareous sand in a quantity ranging from 450 kg/m 3 to 650 kg/m 3 ;

calcareous filler in a quantity ranging from 0 to 100 kg/m ;

superplasticizer in a quantity ranging from 2 kg/m 3 to 5 kg/m 3 ;

the water/cement ratio by weight being equal to 0.44 and

- the weight ratio cement/hollow spherical lightweight aggregates, in the dry mixture, being equal to 0.82.

7. The lightweight concrete according to one or more of the previous claims, wherein said concrete has a compressive strength higher than 53 MPa at 28 days, an elastic modulus higher than 25,000 MPa at 28 days and a volumetric mass or density in the hardened state ranging from 1800 to 1900 kg/m³.

8. Use of a lightweight concrete with a high elastic modulus, comprising a mixture of cement, water, hollow spherical lightweight aggregates, inert aggregates, preferably calcareous or silico-calcareous inert aggregates, wherein the weight ratio cement/hollow spherical lightweight aggregates, in the dry mixture, ranges from 0.6 to 0.9, preferably from 0.80 to 0.85 and wherein the water/cement weight ratio is equal to 0.44, as a structural lightweight concrete.

9. Use of a lightweight concrete according to one or more of claims 2 to 7, as a structural lightweight concrete.

10. Use of a lightweight concrete according to any of claims 8 or 9, for the construction of structural elements, preferably pillars, beams, walls.

11. A hollow lightweight aggregate having the form of a hollow sphere, wherein said hollow sphere has a particle size ranging from 0.6 mm to 1.5 mm, preferably from 0.7 to 1.1 mm, with a wall thickness equal to about 1 mm and a smooth or rough outer surface, preferably rough, said aggregate being made of ceramic or cementitious material.

12. The hollow lightweight aggregate according to claim 11, wherein the hollow sphere is made of ceramic material that can be obtained starting from a mixture comprising a clayey raw material, a melting component and a stabilizing component, wherein the clayey raw material is selected from kaolite, bentonite, meta kaolin, in a quantity ranging from 30 to 50% by weight with respect to the total weight of the ceramic material; the melting component is selected from feldspars, limestone, dolomite, talc, in a quantity ranging from 20 to 40% by weight with respect to the total weight of the ceramic material; the stabilizing component is selected from quartz sand and silica, in a quantity ranging from 10 to 20% by weight with respect to the total weight of the ceramic material.

13. The lightweight aggregate according to claim 1 1, wherein said aggregate made of cementitious or ceramic material can be obtained by means of a process comprising a first granulation step, wherein a dry mixture of ceramic or cementitious material is granulated on spherical polystyrene nuclei, sprayed with latex suspensions in a concrete mixer or granulator plate; and, in the case of an aggregate made of ceramic material, a subsequent ceramization step of the granulated spheres thus obtained, conducted in suitable electric ovens; or, in the case of an aggregate made of cementitious material, a subsequent hardening step of the granulated spheres thus obtained, with accelerated aging in an autoclave with supersaturated steam, at 180°C and 13 atm, or with aging under "natural" conditions at a temperature of 22°C and a relative humidity close to 100%.

14. Use of the hollow lightweight aggregate according to one or more of claims 11 to 13, as lightweight aggregate in a structural lightweight concrete with a high elastic modulus.