The present invention concerns a leightened cement-based extrudable composite material and a process for the manufacturing of leightened cement-based elements.

The invention relates to the field of building and more generally to the industry for the reduction of energy costs and for the improval of energy efficiency.

It is known that the production processes of lightened cement- based products in the construction industry are mostly constituted by vibratory presses which, by compressing and compacting (by the removal of voids) the material within steel formworks, disfigure the products in pieces of limited dimensions.

According to the prior art relating to cement-based lightened products, given the high pressures and vibrations, it is needed lightweight aggregates being natural (pumice or lava lapilli) or synthetic (EPS) or artificial (expanded clay, vermiculite, perlite) that are mixed to the cement base and which remain within the mass of the product by virtue of their affinity with the material of the base.

According to an alternative technology, a production process to make cement-based lightened products relates to products based on autoclaved aerated concrete. According to this technology, developed in 1924 by JA Eriksson, production has as its basic elements the quartz sand, lime and the cement. The quartz sand is finely ground and then mixed with lime and cement (binders), water and aluminum powder. The mixture thus obtained is poured, with a thickness of about one third of that of the end product, inside suitable molds, called formwork, generally of length equal to approximately 6m, to allow rising and hydration. After about 2 hours, the mixture levitates up to the achievement of the due thickness and of the suitable plastic consistency to make the cutting possible. Working with a material at a relatively soft stage is a simple practice, which requires a smaller amount of binder and minimizes the stand-by times. The sides and the profiles of each block are cut by means of a precision steel wire (cutter), first horizontally, in different planes according to the thickness that the blocks must have, then in height. The so cut blocks start the high-pressure curing process, which takes place in autoclaves for about 10-12 hours, at a temperature of 190°C and at a saturated vapor pressure of 12bar.

It is also known that it is particularly difficult to incorporate low density filler inside of cement-based construction products obtained by extrusion. In fact, the low density filler have difficulty to counteract the chemical and mechanical forces imparted by the extrusion process, commonly used for making slabs of cement reinforced with cellulose fibers. The fillers, as well as the air bubbles, can explode or break due to the pressure and the elevated temperature encountered during the extrusion process. For this, it is very difficult to control the distribution and the amount of low density filler in extruded cement materials.

WO2013082524A1 describes a cutting-edge technique in the field of industrialization of lightened extruded construction products, according to which it is proposed a low density cement product incorporating air bubbles in an amount controlled and distributed uniformly in the mass of the product. According to some embodiments described in WO2013082524, to create air bubbles in the product, thus decreasing the density and weight, while maintaining adequate strength, surface active agents or surfactants are used. Such agents can also be used directly in solution in the mixture of cement to create a foam, or are activated to generate a foam before being incorporated in the cement dough. The cementitious mix which incorporates the surface active agents or surfactants can be sent to an extrusion process, meeting high pressures, in which the air bubbles remain in the final cementitious panel.

In this context it is included the solution according to the present invention, which aims to overcome the limitations imposed by the known types of industrial processes: high production/extrusion pressures; high vibrations to process the material; expensive and cumbersome manufacturing and curing processes; need for prefabricated lightweight aggregates for the lightening of the mixtures, expensive raw materials and for the exclusive supply of the place of production, fibers and thickeners, although of biocompatible and non-polluting nature, preparatory to the production process.

These and other results are obtained according to the present invention by proposing a leightened cement-based extrudable composite material comprising starch and starch derivatives and a process for the manufacturing of leightened cement-based elements by extrusion with moderate values of the pressure thrust.

Purpose of the present invention is therefore to provide a leightened cement-based extrudable composite material and a process for the manufacturing of leightened cement-based elements that permit to overcome the limits according to the prior art and to obtain the previously described technical results.

A further object of the invention is that said composite material and said process can be realized with substantially contained costs, both as regards production costs and as regards the management costs.

Another object of the invention is to propose a leightened cement- based extrudable composite material and a process for the manufacturing of leightened cement-based elements that are simple, safe and reliable.

It is therefore a first specific object of the present invention a cement-based extrudable composite material obtained by trapping gas bubbles in a cement hybrid matrix in turn obtained by mixing cement and/or lime and water, together with a gel obtained from starch gelatinized with water and addition of acetic acid as catalyst and/or dispenser of carboxyl groups (COO ); together with calcium oxide or calcium hydroxide or hydrated lime as pH regulator and dispenser of calcium ions Ca ⁺ within said hybrid matrix, said gas being inert with respect to the components of said cement hybrid matrix.

In particular, according to the invention, the amount of cement and/or lime within said cement hybrid matrix, expressed as a function of the amount of gel, ranges from 1 to 5 times by weight, but preferably from 2 to 4 times by weight and more preferably is equal to 2,5 times the weight of the gel; and the amount of cement mixing water is comprised between 0,2 and 0,5 times the weight of the cement, and more in particular between 0,2 and 0,3 times the weight of the cement with a pobbible but non necessary use of plasticizers and/or super-plasticizers.

Moreover, always according to the invention, the amount of acetic acid within the gel as dispenser of carboxylic groups expressed as a function of the cement and/or lime within said cement hybrid matrix, is comprised between 0,010 and 0,25 times but preferably between 0,1 and 0,2 times the weight of the dry cement; whereas the amount of calcium oxide or calcium hydroxide or hydrated lime within the gel, expressed in relation to the amount of acetic acid, is comprised between 0,5 and 3 times the weight of the acetic acid and preferably between 2 and 2,5 times the weight of the acetic acid.

Preferably, according to the present invention, the composite material additionally comprises plasticisers, more preferably sorbitol or glycerol in addition to the gel and during the gelatinization step.

Preferably, according to the present invention, said starch is selected amongst rice starch, corn starch, wheat starch, potato starch, wheat starch.

Moreover it is a second specific object of the present invention a process for the manufacturing of the composite material, previously defined, comprising the following steps:

- preparing a first mixture comprising starch, acetic acid, water and calcium oxide or calcium hydroxide or hydrated lime and/or cement;

- heating the first mixture up to the starch phase transition temperature, preferably up to 90°C, maintaining constant the temperature until the completion of the phase transition,

- preparing a second mixture, comprising cement and/or lime and water,

- preparing a foam of bubbles of an inert gas with respect to the remaining components of said composite material, preferably air, in an aqueous solution, by insufflation of gas in the presence of a surfactant, - lowering the temperature of said first mixture down to 35-45X, preferably down to 40°C and mixing said first mixture together with said second mixture,

- progressively dispensing said foam in the mixture obtained.

Additionally, it is a third specific onject of the present invention a leightened cement-based element obtainable by extrusion of the composite material previously defined, wherein preferably said extrusion occurs with values of pressure thrust between 0,5 and 6 bar.

Moreover, in case said leightened cement-based element is obtained from a composite material comprising a plasticizer, said leightened cement-based element is coated with a plasticised coating layer.

Finally, said leightened cement-based element can be made of two or more extruded elements, as previously defined, superimposed and coupled.

The invention will be described below for illustrative, but not limitative, purposes with particular reference to some illustrative examples.

In particular, the present invention relates to the use of starch, especially one or more starches selected from rice starch, corn starch, wheat starch, potato starch, wheat starch, made of natural polymers (amylose, a linear polymer, and amylopectin, a branched polymer), in the form of microgranules, which are processed by heating, after the addition of a solvent, plasticizer and in the presence of a cross-linking agent, preferably but not necessarily under continuous stirring. The purpose is to achieve a phase shift of starch, from liquid to viscous-plastic gel, and subsequently its definitive polymerization. The continuous agitation can be useful in certain conditions to homogenize the mass of the gel which is forming, if there are temperature gradients between areas, in particular between areas close to sources of heat and the more distant areas. If instead the temperature of the thermostatic bath is uniformly distributed, including the surface exposed to the environment, the agitation is not required, since the gel that forms is free from not entirely gelatinized thickenings and then is sufficiently homogeneous and transparent without use of additional additives, by presenting a certain elasticity when not yet completely cooled: temp = 15 - 20 °C.

The amount by weight of the formulation of viscoplastic-gel in its simplest and most effective form are: 1 part of starch, 1 part of acetic acid, 5 parts of simple water 2 parts of calcium oxide or calcium hydroxide or hydrated lime

More generally, the weight ratio between the acetic acid and starches can vary between 50% and 150%, preferably between 60% and 120%. The acetic acid may have 99% or 80% degree of purity and preferably but not necessarily 99%.

The amount of cement by weight expressed as a function of the amount of gel ranges from 1 to 5 times by weight, but preferably from 2 to 4 times by weight and more preferably 2,5 times the weight of the gel. The cement mixing water is comprised between 0.4 and 0.5 with respect to the weight of cement. The fine sand as inert and for density more than 700 kg/m ³ goes from 0.5 to 1 with respect to the weight of cement. In this specific case the gel is calculated in the ratios referred herein above but according to the sum of dry cement and sand. The amount of foam is a function of the desired density.

This composition, during the intermediate step in the form of gels, mixed with the cement matrix, allows the input and the trapping of the inert gas preformed microbubbles in solution allowing the extrusion at low pressures and/or the molding of the lightened mixture thus formed .

Without wishing to be limited by theory, it is believed that the importance of the gel or xerogel obtained also thanks to hydrolysis of starch, and not from other polymers such as cellulose, through the thermal process, is in a marked syneresis effect evident even in a simple starch solution (in all ratios with water), with the loss of hydrolysis water during the step of curing of the lightened concrete, implying a further lightening of the finished product.

Among all the acids that are used in the literature in order to hydrolyze the starch and dispense functional groups in solution, the acetic acid was chosen for the fact of having an acceptable safety level and to be compatible with the binders of the hydration products and more in particular of cements. From a detailed analysis, based on the known literature, relating to chemical additives in addition to both cements that starches, it has been found that the acetic acid has a marked affinity for both mixtures and especially is the one that allows to incorporate a higher amount of foam produced by anionic surfactants, in order to lower the density of the final mixture, ensuring the extrudability. The choice of acetic acid is then due to the fact that it is the the most commonly used acid, readily available (also from renewable sources or from waste of other industries, such as vegetation waters, which are a waste of olive oil industry), it has a low cost (if not for food use), it is not particularly dangerous, it does not interfere with the curing processes, if not in the slowdown of the outlet of the cements, but only in case it is added as such. On the contrary, according to the invention, when added in its conjugated form, by reaction with lime as acetate salt, the effect is accelerating. Under the selected amount, the acetic acid maintains alkaline the pH of the resistant matrix, is extremely compatible with the proteic surfactant (preferably but not necessarily non-synthentic) and anionic derived from waste and specifically produced for cellular concrete.

From the literature it is known that starches are insoluble in water but if processed by heating (glass transition temperature 50 - 80 °C depending on the types of starch used) in an aqueous solution, the microgranules of which they are composed destructure passing from a turbid liquid phase to a viscoplastic-gel transparent phase (sol-gel phase also called starch solution). In this transition phase, the amylose polymers (soluble in water) and amylopectin (insoluble in water), contained in the microgranules, have long chains (linear, respectively in the case of amylose and branched amylopectin in the case), and, if not modified, have poor mechanical properties and strong tendency to retrograde to a crystalline state, after removing the excess water. To this it is also added that they do not allow the incorporation of foam by mixing with the cement.

To this limitation it has been remedied by a starch gel containing acetic acid diluted in water used as a solvent to gelatinize, the addition of lime to raise the pH of the solution and as a reagent of the acetic acid in order to synthesize lime acetate in solution with the simultaneous release of calcium ions and carboxylic ions, which will be introduced in the mixture with the cement by the starch gels.

In this regard, it is essential the release of functional groups COO ^" (carboxylic groups) and calcium ions Ca ⁺ . In fact, the acid in solution or the acid conjugated salt entered by reaction with lime and/or cement disperse in the hybrid mixture of gel/cement the carboxyl groups and the ions Ca ⁺ that allow the emulsification with the entered foam. Then, after the lowering of the temperature of the gel, and until the condition of equilibrium with the ambient temperature (8- 27 °C), the mixture allows the incorporation of foam microbubbles, without any coalescence and with a consistency such as to allow extrusion. Then, the formed emulsion is stable and homogeneous, with micropores incorporated perfectly and well distributed in the resistant matrix.

In addition, the acetic acid favors the deconstruction and modification of starch polymers, reacts with cement and/or lime paste, tends to emulsify compounds and dispense the functional groups in the gel-cement matrix, and allows entrapment of preformed microbubble by means of the anionic surfactant used for the formation of the foam.

For the part that concerns the resistant matrix, the cement hydrates as expected and curing occurs as in the prior art.

The material and the process according to the present invention assure the extrusion of lightened cement-based products with wet cure environment for complete hydration of the binder and without side effects or evident shrinkage.

In addition, the positive effects of the formation and breeding of starch polymers in the cement matrix can be seen especially on the surface of the extruded product, giving unusual properties which are unknown from the present art.

In addition to the components already said previously, it is possible to add synthetic polymers, in order to impart improvements to the pseudoplastic behavior of the compound at very low density.

However, it is not possible to replace the starch with viscosifying synthetic polymers, given their different nature of hydrocolloids and the different behavior of the obtainable gel, which would not allow the release of previously said functional groups and ions for the entrapment of the microbubbles and their stabilization inside the cement matrix or would have the effect of syneresis which is an advantage if the lightening of the mixture.

The size of the granules of starches or of the various types of starches do not have any effect on the production of extruded concrete according to the present invention. From the literature it appears that all starches have a glass transition temperature variable in the range 50 - 95 °C, depending on the carbohydrate extraction plant and from the present amylose and amylopectin content.

Furthermore, according to the present invention it is possible to add components to the mixtures in order to improve the mechanical and physical properties in general, while maintaining low production costs. In accordance with the present invention, the added compounds can be of various nature, such as: a blend of synthetic polymers, setting and hardening accelerators, viscosity modifiers, synthetic and/or natural fibers, synthetic and/or natural reinforcing networks, chain extenders for natural polymers, colorants, waterproofing, antibacterial additives and against the proliferation of molds, filler for polymers and/or mortars for reinforcing the matrix.

In the presence of specific plasticizers such as sorbitol and/or glycerol, the starch polymers undergo the "plasticization", ie the formation, simultaneously with the output phase of the product from the extruder, a protective layer or film of extruded products, essentially consisting of the plasticizers introduced into the matrix together with the binders. In fact, with the addition of these additives, when the gel loses the excess water it changes its viscous and miscible state with other substances and assumes a state of elastic solid. So, once the gel is hybridized with the hydrated cement and after curing, on the extrudate surface is formed a plasticised coating layer, which has a thin thickness and which depends on the surfaces of the extrusion matrix and on the extrusion pressure. The surface coating effect is also made possible by the coalescence of the micro bubbles, that crash in correspondence of the surface zones and due to rubbing with the forming matrix. The thickening of the surface layer, because of this phenomenon, is closely related to the amount of gel in relation to the other components.This means that larger amounts of gel in combination to a greater longitudinal extension of the extrusion matrix and thrust pressures at the upper limit, make it possible a greater thickening of the superficial layers of the product. The formation of the coating film, according to the present invention, can take place under controlled conditions and its thickness can be variable depending both on the physical characteristics conferred to the composite extruded element, both to the pressures at the output of the product. To this end it is possible to operate a narrowing of the section of the extrusion matrix, in correspondence with the exit of the products, in order to increase the pressure exerted on the extrudate surface. This technique allows the thickening, in a controlled manner and proportional to the thrust exerted for the extrusion, of the formed coating film.

In consequence of the formation of the elastic coating layer, the mechanical behavior of the extrudate is similar to a sandwich panel or structure, in which the skin is constituted by the coating layer and the inner core serves to increase the bending stiffness in the direction of the same while maintaining the cellular structure unchanged. In fact, the thickening of the superficial layers of the extruded creates a lack of homogeneity in the cross section. On the surface, not being present micro bubbles, since they are broken by rubbing with the contact surfaces of the forming matrix, the material is considerably more "dense" with respect to the extruded inner zones. On the contrary, the internal zones have the predetermined density and are homogeneous. This leads to higher mechanical resistance in the surface layers and greater thermal and noise inertia, due to lightening, in the inner areas of the extruded.

The extruded material, therefore, behaves like a sandwich, which is composed of a core of light and incompressible material (up to the compressive strength values expected, dependent on the characteristics of the cement matrix), covered by a laminate, ie the skin, that is inextensible. Subjecting this structure to bending, a side of the coating layer of the structure, as one of the skins of the sandwich, is stressed in traction, and the other side is biased in compression. Since the coating layer is made from a material with a Young's modulus greater than the internal structure, it opposes greater resistance to these stresses, behaving like materials inextensible and incompressible.

According to the present invention it is also possible the formation of elements consisting of a superposition of a plurality of extruded layers in accordance with the procedures previously described. It is in fact possible to couple together multiple layers one on the other using the own properties of the material just extruded to form a more resistant surface film. Immediately extruded, the surface of the material is adhesive and capable of binding to another superimposed surface. This means that, after curing, the layers are perfectly anchored with one another and that have better mechanical characteristics than the single layer of the same size.

According to the present invention it is therefore possible to obtain the formation of interlayers or skins of individual stacked sandwich, which result in adherence to the fresh phase and firmly united and connected when hardened. Always according to the invention, the thicknesses of the core and the skins of the various superimposed sandwich can be variable, as well as the sections of the extruded product.

Furthermore, it is possible to obtain the formation of a three- dimensional network of thermoplastic polymers formed in combination of the matrix essentially consisting of the binder. The formation of this network occurs during the step of mixing between the cement matrix, the viscoplastic-gel and the foam of micro-bubbles. The meshes of this network are formed by the hybridized matrix of gel and cement or binders in general, in which the bubbles homogeneously trapped form closed cells. Before the step of gripping of the binder and therefore of its subsequent hardening, the viscoplastic-gel has load-bearing function, while maintaining the weight of the entire hybrid matrix. The visco-plastic gel, given its characteristics in the fresh state, guarantees the shape imprinted by the extrusion matrix. After curing, the same three-dimensional network guarantees the mechanical properties of the cellular concrete, with in addition the mechanical properties coming from the polymers derived from the starch. The latter may be negligible, if considering the economy of the final product, but may be relevant and complementary if the polymers are added in a suitable manner in order to confer greater resistance and best properties.

In order to impart higher mechanical strength where there is a need and/or higher thermal and acoustic insulation characteristics, it is possible to use mixtures of air and hydraulic binders in various proportions.

Finally, it is possible the use of high strength pozzolanic cemente and portland cements, in combination with the other compounds described previously, in order to obtain high strength extruded products and at low thicknesses.

The components of the material according to the present invention ensure compostability and biodegradability of the residues of working the products made.

One of the embodiments of the present invention also provides for the introduction of inert waste from other processes but also and not only the use of raw materials derived from residues of other industrial sectors.

It is possible the use of carboxylic acids coming from suitably treated organic waste (acid fermentation).

The present invention overcomes the limitations of previous industrial processes, but does not exclude the use of raw materials used to date; rather, the choice of an additive mixtures is due to the only purpose of improving the mechanical and physical properties in general, while maintaining low production costs. Both the compositions of the material and the production methods of the present invention extend the fields of use of the products leaving, also, wider possibilities to realize more complex shapes of artifacts. One of the embodiments of the present invention provides high energy efficiency small-sized industrial installations to allow a wider diffusion of the insulating lightweight products and by significantly important performance for use in sustainable building.

The extrusion of the material according to the present invention can take place with thrust pressure values between 0.5 and 6 bar, depending on the density which is desired for the finished product.

For such values of thrust pressure it is possible to use extruders with a mechanical auger or a peristaltic pump or a screw pump, which allow the continuous extrusion of the lightened mixture, pushing it through the forming die. The products thus obtained can have an indefinite length and then be sectioned in the transverse direction of the desired length. The forming matrices may not necessarily be made of metal alloy but, thanks to the low extrusion pressures in play, it is preferred the use of plastic materials, even composite, low coefficient of friction.

The characteristics of the extrusion, according to the process of the present invention, allow the use of continuous or discontinuous mobile extruders with numerical control, in order to realize more complex shapes and also, but not necessarily more layers.

EXAMPLE 1.

To make a material according to the present invention, with fixed density equal to 700 kg/m ³ , calculated before curing, the production process can take place in a plurality of consecutive steps or in a single dosage and continuous mixing step.

The test mixture according to example 1 occupied a volume of 217 ml_.

In a first beaker 6.25g of 80% acetic acid were dosed together with 28.10g of water (solvent/main plasticizer).

Then, they were added, in succession, 12.5 g of hydrated lime and 6.25g of corn starch. Then, the components of the first beaker were mixed in a continuous manner and at the same time the temperature was gradually raised, up to the upper limit of 90 °C, temperature maintained for about 3-4 min, until the completion of the phase transition: swelling, gelatinization and visco-plasticization of the compound.

After cooling, 30 g of gel product were dosed and added to portland cement 42.5R powder, the whole was mixed by adding water in ratios of 0.2-0.3 with the cement. In particular, 75.00 g of cement CEM ll/A-LL 42.5R and 18.00 g of water were added.

Besides, by means of a commercial type generator with inert gas pressure control, set to a pressure of 3 bar, the generation of microbubbles of air in an aqueous solution was operated, by dosing of a surfactant, in particular the product Foamin C of the Italian MIBO Sri, in an amount equal to 3% by weight with respect to water.

Then, the mixture of components of the first beaker was mixed, 7g of air microbubbles were added in solution.

The obtained solution was mixed, with a percentage of 6% loss in weight of the mixture during the mixing phases.

A first portion of the mixture was extruded manually, by exerting a slight pressure thrust, by insertion of the material in an aluminium tubular section with a length of equal to 20cm, at a constant square section with each side of 2,5cm internal measures, and a thrust piston of deformable material adhering to the internal walls of the section.

The volume of the extrudate has remained equal to the volume of the inserted material and then the initial density was confirmed to be equal to the extrudate density. The extruded prototypes were left to cure in a laboratory environment, under not controlled conditions of temperature and humidity (and possible radiation from the light source).

A second portion of the mixture was extruded through mechanical extrusion with mild pressure thrust, using a crafted mini-extruder with a motor, with a screw conveyor with a variable section, made of steel, intubated and with an extruder or of an outlet matrix produced with Teflon and having rectangular section of dimensions 38mm x 28mm and a length of 100mm.

During the curing step, the extruded material has undergone a percentage of 15% loss in weight, without noticeable shrinkage.

It is evident the effectiveness of the leightened cement-based extrudable composite material and the process for the manufacturing of leightened cement-based elements of the present invention, that allow to realize extruded lightened products in a simple and economic manner, using as components raw materials of use common and widespread.

A suitable combination and mixing of the same components guarantees a highly efficient production process and a low environmental impact.

It is also evident the saving of energy resources compared to the present state of the art, in the process of the invention being possible to use renewable energy to polymerize the natural polymers, to power electrical machines that may be present and even being possible to use extracts of organic waste appropriately treated to obtain the carboxylic acids used in the process.

Another important advantage of the process according to the invention is that, during the step of extrusion and at the output of the products, it is possible to obtain the formation of surface layers of a different nature and thickness with respect to the inside of the extrudate. This phenomenon gives a mechanical behavior of the type of sandwich structure for the solid section extruded, but not only. In fact, depending on the pressure and speed of the extruder output, it is possible to obtain coating layers, of variable thickness, of the so-called "core", which identifies the inner part of the polymer-cement hybrid matrix. In dependence of this additional benefit it is also possible to extrude elements in multiple overlapping layers to form a laminated structure (or multi-layer) to improve the mechanical characteristics of flexural type of the finished product.

The present invention has been described for illustrative, but not limitative purposes, according to its preferred embodiments, but is to be understood that variations and/or modifications may be made by those skilled in the art without departing from the relative scope of protection, as defined by the appended claims.