Process for producing lightweight concrete

Abstract

A process for producing lightweight concrete, comprising; combining cement, aggregate, and water to form a mixture, aerating the mixture, casting the aerated mixture, irradiating the aerated mixture w

Description

Field of the Invention The present invention relates to processes for preparing lightweight concrete such as cellular lightweight concrete (also known as foamed concrete) or autoclaved aerated concrete (also known as aircrete). In particular, the present invention relates to an accelerated curing process. The invention also relates to concrete prepared by the processes. Background Concrete is a ubiquitous building material which hardens when the cement component reacts with mix water to form hydrated phases that impart strength and durability. Concrete can be high density or low density. High density concrete is usually made by mixing heavy aggregate, such as gravel, with cement and water. High density concrete is generally used as structural concrete, filling where mass is concerned and for strong and durable applications. Once the concrete has been mixed, it is left to harden. Hardening can be air curing which usually takes around 7-14 days but can take up to 28 days to reach full strength. Alternatively, the hardening can be steam curing performed in a steam atmosphere at low or zero pressure which takes at least three days to harden. Depending on their dry densities cellular lightweight concrete and autoclaved aerated concrete products can be used for non-load bearing and load bearing applications in the construction of buildings as walling, floors and roof slabs. The cellular structure of both types of products, typically with pore size diameters in the range 0.5mm to 2mm, provides excellent thermal insulation properties and ease of handling due to their lightness compared to dense concrete products. While the physical appearance of cellular lightweight concrete and autoclaved aerated concrete is similar their respective methods of production are different. Cellular lightweight concrete can be made by mixing cement, water and un-milled sand to form a slurry. A foam is then produced, for example by using a compressed air foam generator, water and either a protein based foaming agent or synthetic surfactant. The foam is mixed with the cement and sand slurry and the mixture cast into moulds either fitted with frames to form blocks after subsequent demoulding or reinforcement cages for panels. The products are then left in the moulds for 1 to 2 days to achieve sufficient ‘green’ strength before demoulding and relocation to a storage yard where they are left to achieve their required service compressive strength under ambient curing conditions for a period 14 to 28 days, periodically spraying with water to ensure cement hydration. This long period for curing can be a disadvantage as it can, for example, decrease production output. Autoclaved aerated concrete can be made, for example, by mixing cement, quicklime (calcium oxide), either milled sand or pulverised fuel ash, gypsum or anhydrite, water and a small percentage of aluminium powder. The mix is poured to part fill moulds, either empty for blocks or fitted with reinforcement cages in the case of panels, whereupon the aluminium powder reacts with the highly alkaline environment created by hydrating cement and the quicklime to produce hydrogen gas. The latter, being lighter than air, causes the mix to rise to the height within the mould for the required product density the gypsum or anhydrite, if used, controlling the rate of rise. The mould is transported to a waiting room area and after several minutes the hydrogen has formed a cellular structure within the mix and caused it to stiffen and retain its risen height. The hydrogen is then vented from the upper surface of the mould to be replaced by air. The mix is left in the waiting room area for typically 1 to 3 hours before being transported to a cutting line to form blocks or panels. The product is then loaded into autoclaves and cured over a period of, typically, 12 hours using steam at a temperature of above 180°C and a pressure of about 12 bar. After depressurisation the autoclave is emptied, and the product packaged and placed in a storage yard before despatch to the end user. Unfortunately, autoclave steam curing can be expensive, both in terms of energy usage and the ongoing costs associated with maintaining the steam system. Autoclave steam curing can therefore lead to a product with a large carbon footprint. Carbonation curing is an alternative to autoclave steam curing of dense concrete products. Carbonation curing involves saturating a cementitious mixture with CO2. An exothermic reaction then occurs between the CO2 and calcium-containing compounds in the cement resulting in the formation of calcium silicate hydrate and calcium carbonate. The concrete produced by carbonation curing has rapid strength gain, enhanced durability and enables permanent sequestration of CO2. However, carbonation curing is a moisture sensitive reaction and, typically, the process used to prepare lightweight concrete contains too much water for effective carbonation to occur -the carbonation can occur too slowly to be effective (for example from a cost or time perspective), or the carbonation can occur non-uniformly resulting in poor strength and durability. Therefore, carbonation curing has not previously been used in relation to curing of lightweight concrete products such as cellular lightweight concrete or autoclaved aerated concrete. There is therefore a need for a method for producing lightweight concrete which solves some of the aforementioned problems, or at least provides a viable alternative. Summary In a first aspect, there is provided a process for producing lightweight concrete, comprising combining cement, aggregate and water to form a mixture; aerating the mixture to provide an aerated mixture; casting the aerated mixture; irradiating the aerated mixture with microwaves or radiowaves to provide an irradiated mixture; and carbonating the irradiated mixture in the presence of CO2. In some embodiments, the aerated mixture has a water: solids ratio of at least about 0.6, more preferably at least about 0.7. In some embodiments, the irradiated mixture has a water: solids ratio of about 0.6. In some embodiments, the lightweight concrete has a density of up to about 1200Kg / m3, preferably up to about 1000Kg / M3, more preferably up to about 750Kg / M3. In some embodiments, the mixture is aerated using a foaming agent, or aluminum powder. Optionally, the foaming agent is a protein based foaming agent. In some embodiments, the microwaves have a frequency of about 900MHz to about 2450MHz, preferably about 2450MHz. In some embodiments, the radiowaves have a frequency of about 3MHz to about 30MHz, preferably about 14MHz. In some embodiments, the aerated mixture is irradiated for about 1 to 120 minutes, preferably about 2 to 60 minutes, more preferably about 3 to 30 minutes, even more preferably about 3 minutes. In some embodiments, the lightweight concrete has a compressive strength of up to about 4Mpa, preferably up to about 6MPa, more preferably up to about 8MPa, even more preferably up to about 10Mpa. In some embodiments, the step of irradiating the aerated mixture is conducted at atmospheric pressure. In some embodiments, the step of carbonating the irradiated mixture is conducted at atmospheric temperature. In some embodiments, the aggregate is fine aggregate, preferably selected from the group consisting of fine sand, fine crushed stone, Air Pollution Control residue (APCr), Pulverized Fuel Ash (fly ash), ora combination thereof. In some embodiments, when the aggregate is fine sand or fine crushed stone, the sand or stone is milled to a set fineness of 20 to 25% retained on a 53 micron sieve. In some embodiments, the cement is Portland cement. In some embodiments, the process further comprises a second cycle of irradiating and / or carbonating. In a second aspect, there is provided concrete prepared by the process described above. Detailed Description Concrete is typically prepared by combining cement, aggregate and water to form a mixture (sometimes referred herein as a “cementitious mixture”). The mixture is then allowed to harden. The present invention is concerned with low density, or lightweight, concrete. The terms “low density concrete” and “lightweight concrete” are used interchangeably throughout this disclosure. As outlined above, cellular lightweight concrete can be made by mixing cement, water and un-milled sand to form a slurry. A foam is then produced using, for example, a compressed air foam generator, water and either a protein based foaming agent or synthetic surfactant. The foam is mixed with the cement and sand slurry and the mixture cast into moulds either fitted with frames to form blocks after subsequent demoulding or reinforcement cages for panels. The products are then left in the moulds for 1 to 2 days to achieve sufficient ‘green’ strength before demoulding and relocation to the storage yard where they are left to achieve their required service compressive strength under ambient curing conditions for a period 14 to 28 days, periodically spraying with water to ensure cement hydration. This long period for curing can be a disadvantage as it can, for example, decrease production output. As also outlined above, autoclaved aerated concrete can be made by mixing cement, quicklime (calcium oxide), either milled sand or pulverised fuel ash, gypsum or anhydrite, water and a small percentage of aluminium powder. The mix is poured to part fill moulds, either empty for blocks or fitted with reinforcement cages in the case of panels, whereupon the aluminium powder reacts with the highly alkaline environment created by hydrating cement and the quicklime to produce hydrogen gas. The latter, being lighter than air, causes the mix to rise to the height within the mould for the required product density the gypsum or anhydrite, if used, controlling the rate of rise. The mould is transported to a waiting room area and after several minutes the hydrogen has formed a cellular structure within the mix and caused it to stiffen and retain its risen height. The hydrogen is then vented from the upper surface of the mould to be replaced by air. The mix is left in the waiting room area for typically 1 to 3 hours before being transported to a cutting line to form blocks or panels. The product is then loaded into autoclaves and cured over a period of typically 12 hours using steam at a temperature of above 180°C and a pressure of 12 bar. After depressurisation the autoclave is emptied, and the product packaged and placed in the storage yard before despatch to the end user. Unfortunately, autoclave steam curing can be expensive, both in terms of energy usage and the ongoing costs associated with maintaining the steam system. Autoclave steam curing can lead to a product with a large carbon footprint. Lightweight concrete typically has a dry density of not more than 1850Kg / m3. The concrete produced by the process disclosed herein may have a dry density of about 100-1200Kg / M3 In some embodiments, the concrete has a dry density of up to about 1200Kg / m3, preferably up to about 1000Kg / M3, more preferably up to about 750Kg / M3. In some embodiments, the concrete has a dry density of at least about 100Kg / M3, preferably at least about 460Kg / M3. For lightweight concrete, the aggregate is preferably a lightweight, or fine aggregate. The lightweight aggregate can be any lightweight, or fine aggregate used in the art. For example, the lightweight aggregate may be selected from the group consisting of fine sand, fine crushed stone, Air Pollution Control residue (APCr), Pulverized Fuel Ash (fly ash), or a combination thereof. In preferred embodiments, the fine aggregate is Pulverized Fuel Ash or fine sand. Where the fine aggregate is fine sand or fine crush stone, the sand or stone may have been milled to a set fineness of 20 to 25% retained on a 53 micron sieve. In the present invention, the cement may be any cement used in the art to produce low density, lightweight concrete. For example, the cement may be Portland cement. Portland cement may also be referred to as CEM1 together with a specified mortar test compressive strength at 28 days and whether the cement is normal or rapid hardening examples being CEM1 52.5N and CEM1 42.5R . Other cements containing blends of CEM1 with other materials can also be used in the invention, for example, blends with pulverised fuel ash (pfa) ground granulated blast furnace slag (GGBS) or limestone dust as designated in the international cement standard EN 197:2011. When making lightweight concrete in the present invention, the cementitious mixture is aerated to provide an aerated mixture. The aeration may be undertaking by any method known to the skilled person. For example, the mixture may be aerated using a foaming agent or aluminium powder. Where the mixture is aerated using a foaming agent, the foaming agent may be a protein based foaming agent. Examples of suitable foaming agents include, but are not limited to sodium or potassium salts of fatty acids, such as lauric or myristic acids, or synthetic surfactants such as sodium dodecyl sulphate. If a synthetic surfactant is used, glycerine might also be used to stabilise the foam. Where the mixture is aerated with aluminium powder, this may be flake powder grade having a median particle size in the range of 20 to 120 microns. Such aluminium powder is used in the art to produce autoclaved aerated concrete. Aerating the cementitious mixture provides an aerated mixture comprising a cellular structure of pores and channels. Such a mixture allows a low density porous concrete to be formed. The process for preparing low density concrete requires much more water than that for preparing high density concrete. This is because more water is required to effect both cement hydration and to achieve the required fluidity to form the cellular structure of low density concrete. The cementitious mixture for forming high density concrete typically has a water: solids ratio of around 0.13 (equivalent to a water: cement ratio of 0.4). The aerated mixture used to form low density concrete may have a water: solids ratio of at least about 0.5, for example, at least about 0.6, or at least about 0.7. In some embodiments, the aerated mixture used to form low density concrete may have a water: solids ratio of about 0.6. In some embodiments, the aerated mixture used to form low density concrete may have a water: solids ratio of about 0.7. The additional water that low density concrete requires, together with the cellular structure of pores and channels, results in the water occupying the pores and channels of the aerated mixture. Once the aerated mixture has been produced, it is cast and then left to harden. For example, the aerated mixture may be cast into moulds. Casting the aerated mixture can alter the water: solids content of the mixture. In some embodiments, casting may have the effect of increasing the watersolids content of the mixture, in other embodiments, casting may have the effect of decreasing the water: solids content of the mixture. The hardening can be air curing which usually takes around 7-14 days but can take up to 28 days to reach full strength. Alternatively, the hardening can be steam curing (autoclaved) which takes about 12 hours to harden. Carbonation curing is an alternative to autoclaving and air curing. Carbonation curing involves saturating the cementitious mixture with CO2. An exothermic reaction then occurs between the CO2 and calcium-containing compounds in the cement resulting in the formation of calcium silicate hydrate and calcium carbonate. The concrete produced by carbonation curing has rapid strength gain, enhanced durability and enables permanent sequestration of CO2. Due to the comparative speed of carbonation curing, the energy input and costs of producing concrete can be reduced. However, carbonation curing is a moisture sensitive reaction and generally requires a water: solids ratio which is lower than that used in preparing low density concrete. If attempts are made to carbonate a low density concrete aerated mixture at the typical water content, the CO2 cannot penetrate easily into the mixture due to the pores and channels being saturated with water. In extreme cases, the CO2 can only penetrate the solids by diffusion. This leads to very slow carbonation. Carbonation therefore takes a long time which negates the benefits of reduced energy input and costs. Furthermore, the carbonation that does occur is non-uniform and can result in a crust of calcite on the surface of the solids which prevents further carbonation. Non-uniform carbonation results in aerated concrete with low compressive strength. For successful carbonation of aerated concrete mixtures therefore, it is necessary to reduce the amount of water present in the pores and channels of the aerated mixture. Reducing the water in the pores and channels allows the CO2 to penetrate deeply and quickly into the solids. It also enables a uniform penetration. The carbonation therefore occurs quickly and produces a lightweight concrete with enhanced compressive strength. In the present invention, it has surprisingly been found that the amount of water present in the pores and channels of an aerated concrete mixture can be reduced by irradiating the aerated mixture with microwaves and / or radiowaves to form an irradiated mixture prior to carbonation. Irradiating the aerated mixture with microwaves and / or radiowaves has the effect of driving the water out of the channels and pores of the solids, which conditions the channels and pores of the solids more effectively for carbonation. The pores and channels which are unique to aerated concrete provide ready-made pathways for the water to leave the solids. Therefore, the time required to irradiate the aerated mixture is short. The carbonation occurs more quickly following irradiation because the CO2 can penetrate readily into the pores and channels. The carbonation also occurs more uniformly because the CO2 penetrates deeply throughout the solids via the already formed series of pores and channels. These combined effects results in carbonation which is fast, and therefore has a reduction in energy input and costs, and a product with increased strength due to uniform carbonation. This effect is unique to low density aerated concrete mixtures. High density concrete has a different pore structure which is much finer (capillary porosity). The finer pores are much more difficult for the microwaves and / or radiowaves to penetrate and the irradiation would take much longer than for aerated concrete mixtures. Furthermore high density concrete does not have pathways for the water to leave the solids as it does not have the cellular structure of pores and channels that aerated concrete mixtures have. Irradiating high density concrete would therefore need to heat up the water to evaporation point in order for the water to leave the solids. This requires a great deal of microwave and / or radiowave energy and is not energy or cost effective. This is demonstrated in the proceedings paper “Microwave-based Preconditioning Technique for Accelerated Carbonation Curing of Cementitious Materials”, presented at the 37th Cement and Concrete Science Conference. This paper discusses reducing the water / cement ratio in high density concrete by a preconditioning heating step involving heating the cementitious mixture to 70°C fora prolonged period of time. The heating can be by either water bath or microwaving. The paper then compares a preconditioning heating step with carbonation curing and without carbonation curing to demonstrate that carbonating increases the strength of the concrete. In the present invention, the aerated mixture is irradiated with microwaves and / or radiowaves. Microwaves are electromagnetic radiation with a frequency typically in the range of 100MHz (3M wavelength) to 300GHz (1mm wavelength). Radiowaves are electromagnetic radiation with a frequency typically from 300GHz (1mm wavelength) to 3kHz (100KM wavelength). In some embodiments, the aerated mixture is irradiated with microwaves having a frequency of about 900MHz (0.333M wavelength) to about 2450MHz (0.123M wavelength). Preferably, the aerated mixture is irradiated with microwaves having a frequency of about 2450MHz (0.123M wavelength). In some embodiments, the aerated mixture is irradiated with radiowaves having a frequency of about 3MHz (100M wavelength) to about 30MHz (10M wavelength). Preferably the aerated mixture is irradiated with radiowaves having a frequency of about 14MHz (20M wavelength). In some embodiments, the aerated mixture is irradiated simultaneously or sequentially with microwaves and / or radiowaves having a mixture of frequencies. As one example, the aerated mixture may be irradiated simultaneously with a combination of microwaves having a frequency of 2450MHz (0.123M wavelength) and 900MHz (0.333M wavelength). Using a combination of microwaves and / or radiowaves at different frequencies is advantageous as it effects deeper penetration into the aerated mixture. This can be particularly useful when preparing large concrete units such as a mold of concrete blocks. The irradiation may be for any period of time suitable for pre-conditioning the pores and channels in the solids such that the water: solids ratio is reduced. In some embodiments, the reduction in water content, or the water: solids ratio, after irradiation is about 5% w / w, for example, about 6% w / w or about 10% w / w. In some embodiments, the irradiated mixture has a water: solids ratio of about 0.6, or about 0.5, or about 0.4. In the present invention, the microwave and / or radiowaves only need to be applied for a short period of time. This is because, the microwave and / or radiowaves are not being applied to heat the aerated mixture and effect water evaporation. Instead, the microwave and / or radiowaves are applied to precondition the bridges between the pores in the aerated mixture by driving off the water. This enables the pores of the aerated mixture to receive CO2 gas more easily and therefore cure more quickly to a greater strength. The irradiation period required may be dependent on the volume of lightweight concrete being produced and the energy required to reduce water prior to carbonation. The ability to reduce water in the quickest time is generally in the order microwave 2450MHz frequency >microwave 900MHz frequency >radiofrequency 14MHz. However, the penetrative ability of the radiation type and time required for water reduction is the reverse. In addition, the time required for water reduction may also be dependent the orientation in which the lightweight concrete is presented to the radiation source; a single layer of blocks presented in a horizontal orientation will require less irradiation time than two layers of blocks stacked one upon the other vertically. In some embodiments, the aerated mixture is irradiated with microwaves and / or radiowaves for about 1 to about 120 minutes. In some embodiments, the aerated mixture is irradiated with microwaves and / or radiowaves for about 2 to about 60 minutes, for example, about 3 to 30 minutes. In may alternative be said that the aerated mixture is irradiated for up to 120 minutes, such as up to about 100, 80, 60, 40, 20, 10, 5 or 3 minutes. In preferred embodiments, the aerated mixture is irradiated with microwaves and / or radiowaves for about 3 minutes. The irradiation can be conducted at any suitable pressure and temperature. Preferably, the irradiation is conducted at atmospheric pressure. Preferably, the irradiation is conducted at atmospheric temperature however, the aerated mixture will heat up during the irradiation step and the internal temperature of the irradiated mixture will depend on the irradiation time and frequency of the microwave and / or radiowaves. Once the aerated mixture has been irradiated with microwaves and / or radiowaves, the irradiated mixture is carbonated in the presence of CO2. The carbonation can be performed using any suitable method known in the art. The carbonation step can be conducted at any suitable pressure and temperature. The carbonation may be conducted at atmospheric pressure. In some embodiments, the carbonation is conducted under increased pressure. In some embodiments, the carbonation is at a pressure of at least 3 bar, for example, at least 5 bar, or at least 10 bar, or at least 12 bar. Conducting the carbonation step under increased pressure may allow the accelerated curing process equipment to be retrofitting to replace steam curing in autoclaved aerated concrete production as the existing autoclaves can be used for carbonation under pressure after the product irradiation stage. Preferably, the carbonation is conducted at atmospheric temperature however, the irradiated mixture will heat up during the carbonation step due to the exothermic nature of the carbonation reaction. In some embodiments, the step of irradiating the aerated mixture is conducted at atmospheric pressure. In some embodiments, the step of irradiating carbonating the irradiated mixture is conducted at atmospheric temperature. The carbonation reaction is highly exothermic and produces water as a by-product. The pores and channels present in the aerated mixture assist with removal of this water. However, in some embodiments, the claimed process may include one or more further cycles of irradiation and carbonation to increase the efficiency of the carbonation. For example, the process may further comprise a second cycle of irradiation and carbonation. The lightweight concrete prepared by the present invention has a compressive strength that complies with appropriate national standards and end use requirements. In some embodiments, the lightweight concrete has a compressive strength of up to about 4Mpa, preferably up to about 6MPa, more preferably up to about 8MPa, even more preferably up to about 10Mpa. It may alternatively be said that the lightweight concrete has a compressive strength of at least about 2MPa, or at least about 4MPa, or at least about 6MPa, or at least about 8MPa. The present invention also has the advantage of sequestering carbon due to use of carbon dioxide in the carbonation step. It is envisaged that this carbon sequestration will offset any environmental impact of the carbon emissions associated in the manufacture and subsequent use of cement, and in the case of autoclaved aerated concrete manufacture and subsequent use of cement and quicklime, in lightweight concrete production and in so doing reduce the carbon footprint of such products during their respective life cycles. The claimed accelerated curing process can be retrofitted to a current production plant. This is advantageous because it avoids redundancy of existing machinery. The cellular lightweight concrete or autoclaved aerated concrete prepared by the process can be in the form of either blocks or reinforced panels and have a typical dry density ranging from 100kg / m3 to 1200kg / m3. It is estimated that the claimed accelerated curing process reduces the curing time for cellular lightweight concrete products by at least 80% and for autoclaved aerated concrete products by at least 50%. Examples The following example is a specific embodiment of the present invention but is not intended to limit the present invention. A slurry mixture containing CEM I cement (52.5N; Castle Cement Ltd.), pulverised fly ash (PFA), sharp sand (<4mm) and water (watensolids ratio 0.5) was prepared using a twin screw mixer. When uniformly mixed, a protein-based foam (comprising 4% foam and 96% water) was ‘folded’ into the slurry using a twin-screw mixer, on a low torque setting. The addition of foam produced an aerated mixture in the form of a cellular light weight slurry (water: solids ratio 0.5). The foamed slurry (aerated mixture) was cast in 100m cubes using pre-formed polystyrene moulds (water: solids ratio 0.5). The filled moulds were stored under ambient laboratory conditions for 24 hours. At one-day of age, the polystyrene moulds were gently removed from some of the cubes in preparation for curing under different conditions. The curing environments were: 1. Sealed curing in their moulds under ambient laboratory conditions 2. Exposure to 100 % CO2 for 24 hr. 3. Exposure to microwaving for 3 minutes using a 17 litre, 700 W domestic microwave producing microwaves having a frequency of 2450MHz. 4. Microwaved (as above) followed by exposure to 100% CO2 for 24 hr. Following curing, the cubes were placed in a sealed plastic container under ambient conditions for up to 28 days of age. Cubes were examined for key physical properties up to 28 days of age. The compressive strength (average from 4 cubes recorded at 28 days) follows: Curing Environment Strength (MPa) Difference from control (% @ 28 days) Ambient sealed cured 2.15 - CO2 (100%) cured (24 hr) 2.48 +15.3 Microwave cured (3 min) 1.8 -16.3 CO2 (100%) + Microwave cured (24 hr) 3.33 +54.9 These data show that preconditioning the aerated mixture by irradiation with microwaves and / or radiowaves, before carbonating the irradiated mixture, increases the strength of the concrete from 2.48 MPa to 3.33MPa (an increase of 34.3%). The application of microwaves alone, with air curing (without carbonation) actually decreases the strength of the concrete. However, adding in a carbonation step with the microwave preconditioning step saw an increase in the strength of the concrete of 85% (1.53MPa). It is proposed therefore that there is a synergistic effect of combining a microwave preconditioning step with accelerated carbonation curing. Without the microwave preconditioning step, carbonated curing increased the strength of the concrete by 0.33 MPa (15.3%). However, with microwave preconditioning, there was an increase of 1.18 MPa (54.9%). For the avoidance of any doubt, the terms “a”, “an” and “the” are intended, unless specifically indicated otherwise or the context requires otherwise, to include plural alternatives, e.g., at least one. "Optional" or "optionally" means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event or circumstance occurs and instances where it does not. Various other modifications to the present invention will be readily apparent to those skilled in the art.

Claims

1. A process for producing lightweight concrete, comprising:combining cement, aggregate and water to form a mixture;aerating the mixture to provide an aerated mixture;casting the aerated mixture;irradiating the aerated mixture with microwaves and / or radiowaves to provide an irradiated mixture; andcarbonating the irradiated mixture in the presence of CO2.

2. The process according to claim 1, wherein the aerated mixture has a water: solids ratio of at least about 0.6, more preferably at least about 0.7.

3. The process according to claim 1 or 2, wherein the irradiated mixture has a water: solids ratio of about 0.6.

4. The process according any one of claims 1 to 3, wherein the lightweight concrete has a density of up to about 1200Kg / m3, preferably up to about 1000Kg / M3, more preferably up to about 750Kg / M3.

5. The process according to any one of claims 1 to 4, wherein the mixture is aerated using a foaming agent, or aluminum powder.

6. The process according to claim 5, wherein the foaming agent is a protein based foaming agent.

7. The process according to any one of claims 1 to 6, wherein the microwaves have a frequency of about 900MHz to about 2450MHz, preferably about 2450MHz.

8. The process according to any one of claims 1 to 7, wherein the radiowaves have a frequency of about 3MHz to about 30MHz, preferably about 14MHz.

9. The process according to any one of claims 1 to 8, wherein the aerated mixture is irradiated for about 1 to 120 minutes, preferably about 2 to 60 minutes, more preferably about 3 to 30 minutes, even more preferably about 3 minutes.

10. The process according to any one of claims 1 to 9, wherein the lightweight concrete has a compressive strength of up to about 4Mpa, preferably up to about 6MPa, more preferably up to about 8MPa, even more preferably up to about 10Mpa.

11. The process according to any one of claims 1 to 10, wherein the step of irradiating the aerated mixture is conducted at atmospheric pressure.

12. The process according to any one of claims 1 to 11, wherein the step of carbonating the irradiated mixture is conducted at atmospheric temperature.

13. The process according to any one of claims 1 to 12, wherein the aggregate is fine aggregate, preferably selected from the group consisting of fine sand, fine crushed stone, Air Pollution Control residue (APCr), Pulverized Fuel Ash (fly ash), ora combination thereof.

14. The process according to any one of claims 1 to 13, wherein when the aggregate is fine sand or fine crush stone, the sand or stone is milled to a set fineness of 20 to 25% retained on a 53 micron sieve.

15. The process according to any one of claims 1 to 14, wherein the cement is Portland cement.

16. The process according to any one of claims 1 to 15, wherein the process further comprises a second cycle of irradiating and / or carbonating.

17. Concrete prepared by the process accordingly to any one of claims 1 to 16.17