Internal curing agents for concrete

Abstract

The present invention relates to a novel internal curing agent based on geopolymers from fly ash, which performs the same or better than the alternatives available on the international market, for use in concrete or mortar mixtures. The advantage of the internal curing agent is that it can be manufactured using local materials with a low large-scale production cost and it can be specifically designed for different applications of concrete construction.

Description

[0001] INTERNAL CONCRETE CURING AGENTS

[0002] FIELD OF INVENTION

[0003] The present invention relates to a new internal curing agent (ICA), with equal or better performance than alternatives available on the international market, for use in concrete or mortar mixtures. This new internal curing agent has the advantage of being manufactured with local materials using geopolymerization of fly ash, at a low large-scale production cost, and can be specifically designed for different concrete construction applications.

[0004] This new internal curing agent is added during concrete mixing and gives it the ability to cure autonomously, facilitating the hydration of concrete materials, reducing deformation in early stages, decreasing the potential for cracking, and reducing the harmful effects of drying such as shrinkage cracking.

[0005] BACKGROUND OF THE INVENTION:

[0006] Concrete curing is a fundamental requirement for obtaining high-quality concrete, ensuring the hydration of the concrete's constituent materials and achieving the strength and durability required. An adequate supply of moisture is required to ensure the hydration of the constituent materials, thereby reducing porosity and maximizing mechanical properties and, above all, durability.

[0007] The mixing water in conventional concretes is usually greater than the water required for cement hydration. However, water loss from the concrete to the environment can delay or even stop hydration, negatively affecting the development of concrete properties.

[0008] Conventional concretes, which traditionally have water-to-composite material (W / CM) ratios, contain sufficient water to hydrate the composition materials, but are often not properly cured, which promotes drying and compromises strength gains and adequate durability.

[0009] Additionally, curing improves the performance of low-permeability concretes with a low A / MC ratio, which require additional water to hydrate the underlying materials, thereby ensuring their hydration and achieving the required concrete properties. Effective concrete curing reduces variability in mechanical properties and decreases permeability.

[0010] With the increasing popularity of low W / MC concretes, many of the properties of concrete have been significantly improved. However, the need for curing has become more pressing than ever. Low W / MC concretes have a low water content and relatively high volumes of commodities, which increases the need for curing water. However, this type of concrete usually has low permeability, which makes it difficult for curing water to penetrate.

[0011] Given the importance of curing water and the difficulty of providing it from the outside in high-performance, low-permeability concretes, the idea of ​​providing water from within the concrete takes on special importance and interest.

[0012] This can be achieved through an internal curing process, which uses as its fundamental principle the incorporation of water into the mix, which is not available for hydration at the beginning (start of the internal curing process), but is released later when it is required to continue hydration.

[0013] Another important element to consider in the internal curing process of concrete is the internal curing agents used to supply water from within. The effectiveness of these agents depends on their size, internal porosity, and the ratio of absorbed to desorbed water (delivered to the environment). Currently, lightweight artificial aggregates (FLWAs, below 5 mm in size) have predominated in internal curing applications.

[0014] The use of natural (non-artificial) lightweight aggregates is a competitive option, but their availability can be limited regionally. In some cases, these aggregates may be available in areas near volcanoes. However, their properties are highly variable, requiring industrialization methods (production plants) that are not available in Chile, and their economic and environmental costs are very high. Another alternative is the use of artificial FLWA, available in international markets (mainly the USA and Europe), but not in Chile. These latter are not specifically manufactured for internal curing, so while they perform well, their internal pore structure does not allow for optimal water delivery and distribution. Furthermore, in some cases, they have poor mechanical properties that can create weak areas in the concrete.An additional disadvantage is that their availability is limited to certain countries and regions around the world where this type of lightweight aggregate is produced. Another alternative to the use of FLWA is superabsorbent polymers (SAPs). SAPs are hydrogels that absorb a considerable amount of water and release it to provide more moisture and act as a potential internal curing method for high-performance concrete. However, SAPs have the disadvantage that, by releasing water internally, they leave a high distribution of spaces in the material, affecting its mechanical properties.

[0015] Another alternative has been the use of cellulose fibers used in laboratory tests, but they have not yet reached commercial use in real-world projects and also present durability issues.

[0016] Thus, the currently available alternatives are expensive and not commercially available in Chile or Latin America, which explains why the commercial implementation of this technique, using agents imported from Europe and North America, has not been applied in the region.

[0017] The state of the art describes the use of geopolymeric materials, formed from raw materials derived from solid waste, such as municipal solid waste incineration ash, fly ash from combustion in thermal power plants, recycled rock dust, lithium slag, and other waste materials to reduce the environmental impact, carbon footprint, and associated costs of ultra-high-performance concrete.

[0018] The use of fly ash, rich in alumina and silicate, is known as a supplementary concrete material. Fly ash is used as a partial replacement for traditional ordinary portland cement (OPC). In concrete, fly ash can react directly with water (hydraulic reaction), generate a pozzolanic reaction, or both. The addition of fly ash to the mix has been found to improve concrete because it changes its composition, adding strength and durability. The resulting material is less porous than that using only OPC and is more resistant to deterioration. In addition to being more durable, concrete containing fly ash is more resistant to acid and fire, and has demonstrated higher compressive and tensile strengths at later ages.However, since fly ash is an industrial byproduct, its composition is highly variable, so its consistency must be verified before mixing it with conventional materials. The necessary controls must be performed to determine and verify that possible variations in its composition do not affect the concrete produced with it. This process is highly costly, and there are no processing plants for this purpose in Chile or Latin America. The present invention provides a method for manufacturing an internal curing agent based on alkaline-activated fly ash. The technique used is based on the geopolymerization of aluminosilicates in powder form, which, when reacted with an alkaline solution, form a synthetic solid material. This technique has been used for decades to produce more sustainable concretes that do not require or require the use of Portland cement in their composition.The difference with this technique is that geopolymerization is used to generate porous structures similar in size to sand, with controlled porosity that allows for internal curing in concrete. Among the advantages of this development is the ability to control the characteristics and efficiency of the lightweight aggregates produced, not only to produce internal curing but also to adjust it to different concrete construction applications.

[0019] STATE OF THE ART

[0020] The state of the art provides various disclosures related to geopolymer materials and their use. For example, document CN110776279A raises the difficulty of complex geopolymer preparation processes, their high cost, low stability, long solidification time, and difficulty maintaining good compressive and flexural strength. This document proposes as a solution to these problems the preparation of a geopolymer using three types of industrial waste as raw materials, including fly ash, furnace bottom slag, and water-rich calcium carbide slag, which are produced by combustion in a circulating fluidized bed. In addition, the geopolymer comprises an alkaline activator, common Portland cement, and a water-reducing agent.The alkaline activating agent is a 1-10 mol / L sodium hydroxide solution together with a combination of at least one 1-10 mol / L sodium silicate solution, 1-10 mol / L sodium sulfate solution, 1-10 mol / L sodium phosphate solution, 1-10 mol / L sodium carbonate solution. Although document CN1 10776279A discloses a composition comprising among its constituents fly ash, Portland cement and an alkaline activator that includes sodium hydroxide, it is not intended to produce lightweight aggregates, nor to apply it as an internal curing technology, nor the use of geopolymer for curing concrete, but rather a geopolymer gel is obtained that is mixed with cement for the formation of concrete, as traditional use.

[0021] Also within the state of the art is document US2019092688A1, which discloses fly ash-based geopolymer binders and activation solutions for their use in concrete formation, as well as methods for forming concrete. The geopolymer binders in this document are based on a combination of fly ash, silica fume, sodium hydroxide, and Portland cement, which form a paste that is mixed with aggregates to form concrete. This document does not disclose internal curing technology; however, it establishes that concretes formed from these binders can exhibit excellent compressive strength characteristics and are more cost-effective compared to other concretes.It is also indicated that the inclusion of Portland cement in the binders can allow a decrease in the sodium hydroxide content while maintaining desirable compressive strength characteristics. Since sodium hydroxide is a major contributor to the costs of geopolymer concrete, this can result in significant savings. Concrete produced with these binders can be cured under ambient conditions in a traditional manner, without the need for an external heat source to form a concrete product having desirable compressive strength characteristics. The technology of the present invention seeks objectives opposite to that of document US2019092688A1 .While said document seeks to maximize the mechanical properties of the solid produced, the present invention seeks to generate a porous structure, with a size in the range of fine aggregates, and with controlled internal porosity that allows for internal curing in different applications. In this way, instead of seeking to maximize strength, a low resistance of the agents produced is achieved, which helps generate internal curing in the concrete.

[0022] Documents US2013087078A and US2013087076A1 discuss the importance of fly ash as an additive that improves the properties of construction materials when added to cementitious mixtures with Portland cement, providing greater durability and reduced permeability of cementitious products. The use of fly ash as a supplementary material has been around for over 50 years internationally.

[0023] Both documents disclose inorganic polymer compositions that include 85% by weight or more of fly ash and no more than 10% by weight of Portland cement, anhydrous calcium sulfate (US2013087078A) or calcium aluminate (US2013087076A1), an activator that can be sodium hydroxide and / or citric acid, and optionally a retarder. The method for producing the inorganic polymer composition includes mixing for 2 seconds to 15 minutes the reactants comprising a reactive powder that includes fly ash and Portland cement, anhydrous calcium sulfate, the activator and, optionally, a retarder in the presence of water. This composition is mixed with the aggregates and the concrete is obtained.The above references, while considering the use of geopolymers in concrete production, are more specifically related to the production of a traditional geopolymer; that is, to produce a cement paste not based on Portland cement or one blended from Portland cement. Additionally, they do not provide a method for producing lightweight aggregates or for producing internal curing agents, such as the one included in the present invention.

[0024] Regarding internal curing technology, there is a variety of documentation that discloses this technology, where various elements of different origin are used. For example, document CN1 14057941 discloses an internal curing material for concrete based on sodium carboxymethyl starch; document CN1 13816696 refers to an ultra-high-performance concrete that includes as an internal curing agent a mixture of constituent vapors, including Portland cement, fly ash, and selected components of silica fume, glass beads, regenerated fine powder, regenerated fine aggregate, fine aggregate, polycarboxylate superplasticizer, and water.Document CN113321467, relating to an ultra-high performance concrete with lightweight aggregates with low shrinkage, is also manufactured using internal curing technology in which the concrete is composed of cement, fly ash and other constituents selected from silicon ash, river sand, modified ceramic sand, steel fiber, water reducing agent, antifoaming agent, blowing agent and water. Document CN1 13149501 also discloses an internal curing material for ultra-high performance concrete comprising acrylic acid, acrylamide, 2-acrylamide-2-methylpropanesulfonic acid, N,N-methylene-bis-acrylamide, ammonium persulfate, deionized water, modified porous quartz powder and liquid caustic soda. On the other hand, document CN112194756 refers to an internal curing agent for salt-tolerant concrete that includes a hydrophilic ionic monomer and a non-ionic monomer, physical and chemical crosslinking agents.Document CN1 11892315A relates to an internal curing sand composed of autoclaved crushed aerated lightweight concrete (ALC) waste particles, slag from power plant furnace bottoms, and foam cement board particles. Document CN1 10963737 relates to an internal curing agent comprising sepiolite, polyether macromonomers, carboxylic acid amide monomers, among others. Document CN110894266 proposes an internal curing material for concrete composed of agricultural waste, 2-ethylhexyl methacrylate, ethyl undecylenate, sodium carbonate, dibenzoyl peroxide, among other constituents. Document CN108751811 relates to an internal curing agent based on aluminosilicate industrial waste, specifically fly ash, coal dust, and polypropylene fiber.Document CN106866022 proposes an internal curing agent composed of fly ash, gypsum powder, magnesium oxide, aluminum dihydrogen phosphate, aluminum potassium sulfate, sodium polyacrylate, hydroxypropyl methylcellulose, sodium methylsiliconate and sodium gluconate.

[0025] Furthermore, document CN1 13735515 discloses a geopolymer curing material based on fly ash and red mud and its preparation method. The raw materials disclosed in this document comprise a solid raw material and a liquid raw material, wherein the solid raw material is preferably composed of 3% cement, 3% red mud, 4% fly ash and 90% polymetal-contaminated soil, and the liquid raw material is an aqueous solution containing an alkaline activator chosen from an anionic surfactant (preferably a polycarboxylic acid) and sodium silicate as a water-reducing agent, where the concentration of the alkaline activator is 5% and the concentration of the water-reducing agent is 0.75%. The solid raw materials and the liquid raw materials are mixed according to a water-cement ratio of 0.30-0.40, preferably 0.35.The objective of this product is to generate a solid material that captures harmful elements from contaminated soils, limiting or preventing their leaching. Although the term "curing" is used in its title, this refers to the preparation of the geopolymer and is not intended to provide any type of external or internal curing of the concrete.

[0026] Document CN11 1825378 discloses an internal curing material for concrete made from fly ash slag and its preparation method. The internal curing material disclosed in this document includes 5% to 7% water, 14% to 18% cement, and 5% to 15% in terms of mass ratio of fly ash slag, 15%-30% river sand and 45%-50% aggregates. The process disclosed in document CN11 1825378 provides concrete with this curing material comprising: selecting the fly ash particle size to a size less than 2.36 mm; pre-wetting the fly ash by immersing it in water for more than 24 hours to reach the saturated surface with a moisture content of 30-45%. Saturated surface (dry) fly ash is mixed with cement, sand and aggregates in a mixer, water is added while mixing for 2 minutes and poured into a mold.Although the internal curing agent comprises fly ash, and a particle size of less than 2.36 mm is sought, an internal curing agent comprising fly ash, cement, and an alkaline activator as particles of that size is not produced or provided. In this document, all materials are mixed without prior preparation of the curing agent. In summary, this document aims to take advantage of the water absorption capacity of fly ash in a powder size (similar to cement) to provide internal curing in hardened concrete. That is, lightweight aggregates are not produced from fly ash.

[0027] As can be seen, there are numerous references related to internal curing technology. However, the state of the art does not provide a technology like the one disclosed in the present invention that includes low-cost and less complex elements than the prior-art disclosures.

[0028] BRIEF DESCRIPTION OF THE FIGURES

[0029] Figure 1. Estimation of the role of internal porosity of lightweight aggregates in the internal curing process. The columns of the graph correspond to different types of lightweight aggregates and particle sizes. From left to right, the following are shown: (a) expanded glass with particle size between 0.1 and 0.3 mm; (b) 0.25-0.5 mm; (c) 0.5-10 mm; (d) 1.0-2.0 mm; (e) 2.0-4.0 mm; (f) 4.0-8.0 mm; (g) expanded clay with particle size between 0.15 and 3.0 mm; (h) 2.3-4.8 mm; (i) GP aggregate (internal curing agent of the present invention) with a particle size of 0.15-2.3 mm. The curing agent of the present invention exhibits a high solid volume to total porosity ratio, and a high useful porosity.

[0030] Figure 2. Compressive strength of mortars with water-cement ratios of 0.35, 0.40, 0.45, and 0.50 (tested according to ASTM C109). The reference mortars correspond to mixtures without internal curing agents. For each water-cement ratio, the results are shown for (a) the reference mortar and internally cured mortars with (b) expanded glass with particle size between 0.1 and 0.3 mm, (c) expanded glass with particle size between 1 and 2 mm, (d) expanded glass with particle size between 4 and 8 mm, (e) expanded clay with particle size between 0.15 and 0.3 mm, (f) expanded clay with particle size between 2.3 and 4.8 mm, and (g) GP aggregate (internal curing agent of the present invention) with a particle size of 0.15-2.3 mm.

[0031] Figure 3. Compressive strength of concretes with a water-cement ratio of 0.35, 0.45, and 0.55, with and without internal curing, measured 28 days after mixing. The reference concrete is compared to mixtures internally cured with the following internal curing agents: expanded glass with particle sizes between 1 and 2 mm, expanded clay with particle sizes between 2.36 and 4.75, and GP aggregate (the internal curing agent of the present invention) with particle sizes between 0.15 and 2.36 mm.

[0032] Figure 4. Tensile strength due to splitting of concretes with a water-cement ratio of 0.35, 0.45, and 0.55, with and without internal curing, measured 28 days after mixing. The reference concrete is compared to mixtures internally cured with the following internal curing agents: expanded glass with particle sizes between 1 and 2 mm, expanded clay with particle sizes between 2.36 and 4.75, and GP aggregate (the internal curing agent of the present invention) with particle sizes between 0.15 and 2.36 mm.

[0033] Figure 5. Evolution of the internal capillary pressure of reference mortars and those internally cured with GP aggregate (the internal curing agent of the present invention). For each mortar, mixtures with water-cement ratios of 0.35; 0.40; 0.45; and 0.50 were evaluated. After the mortar sets (end of the solidification process), the internal capillary pressure begins to rise. This pressure is responsible for early deformations and cracking of the material. The use of the internal curing agent can reduce the rate of pressure gain and shift the time it takes to reach maximum capillary pressure.

[0034] Figure 6. Evolution of the internal relative humidity of reference mortars (without the use of internal curing agents) and those cured internally with expanded glass and GP aggregate (the internal curing agent of this invention). Samples cured with the internal curing agent of this invention are able to maintain the mortar at higher relative humidities compared to a reference sample.

[0035] Figure 7. Evolution of autogenous shrinkage over 100 days or more, measured in pm / m (or pe), for internally cured mortar samples with: (a) expanded glass with particle size between 1 and 2 mm, (b) expanded clay with particle size between 2.36 and 4.75 (c) GP aggregate (internal curing agent of the present invention) with particle size between 0.15 and 2.36 mm, and (d) reference mortar (mixture without internal curing agents).

[0036] Figure 8. Evolution of the dynamic modulus of elasticity of concretes with a water-cement ratio of 0.35, with and without internal curing, measured over a period of 91 days from mixing. The reference concrete is compared with mixtures internally cured with the following internal curing agents: expanded glass with particle sizes between 1 and 2 mm, expanded clay with particle sizes between 2.36 and 4.75, and GP aggregate (internal curing agent of the present invention) with particle sizes between 0.15 and 2.36 mm.

[0037] Figure 9. Evolution of the dynamic modulus of elasticity of concretes with a water-cement ratio of 0.55, with and without internal curing, measured over a period of 91 days from mixing. The reference concrete is compared with mixtures internally cured with the following internal curing agents: expanded glass with particle sizes between 1 and 2 mm, expanded clay with particle sizes between 2.36 and 4.75, and GP aggregate (internal curing agent of the present invention) with particle sizes between 0.15 and 2.36 mm. Figure 10. Evolution of the total shrinkage of concretes with a water-cement ratio of 0.35, with and without internal curing, measured over a period of 91 days from mixing.The reference concrete is compared with internally cured mixtures with the following internal curing agents: expanded glass with particle size between 1 and 2 mm, expanded clay with particle size between 2.36 and 4.75, and GP aggregate (internal curing agent of the present invention) with particle size between 0.15 and 2.36 mm.

[0038] Figure 11. Evolution of the total shrinkage of concretes with a water-cement ratio of 0.55, with and without internal curing, measured over a period of 91 days from mixing. The reference concrete is compared with mixtures internally cured with the following internal curing agents: expanded glass with particle sizes between 1 and 2 mm, expanded clay with particle sizes between 2.36 and 4.75, and GP aggregate (internal curing agent of the present invention) with particle sizes between 0.15 and 2.36 mm.

[0039] DETAILED DESCRIPTION OF THE INVENTION

[0040] The present invention relates to an internal curing agent for concrete produced using the geopolymerization technique. The internal curing agent exhibits performance equal to or better than alternatives available on the international market when used in mortar or concrete mixtures. The invention has the advantage of being manufactured with local materials based on geopolymers, at a cost suitable for large-scale production, and for use in the production of self-curing concretes.

[0041] The internal curing agent of the present invention is a geopolymer obtained from fly ash from thermoelectric power plants. The ash is composed primarily of silica (SiO2), alumina (Al2O3), and free lime (CaO). In the geopolymerization process, the ash is mixed in a controlled manner with an alkaline activator, preferably a low-concentration sodium hydroxide (NaOH) solution. Additionally, a low amount of Portland cement is incorporated, which acts as a coactivator within the mixtures. The invention features a manufacturing process and a proportion of component materials such that the resulting lightweight aggregate has an internal porosity that allows it to function as an internal curing agent.

[0042] Example of realization

[0043] According to the present invention, the geopolymer-based internal curing agent requires the alkaline activation of a solid precursor. In this case, the precursor is fly ash. Fly ash is a by-product of power generation in thermoelectric power plants and can be used in its original size distribution or sieved to select specific particle diameters. The internal curing agent is produced from fly ash, a minor fraction of Portland cement that acts as a co-activator and an alkaline solution, and specifically comprises the components described in Table 1 . The present invention uses a NaOH solution. The molar concentration of the NaOH solution can vary between 0.5 M and 2.0 M, more specifically between 0.5 M and 1.0 M, and even more specifically the molar concentration is 0.75 M.The manufacturing process of the internal curing agent (ICA) involves forming a paste with an alkaline solution (activator) / cementing materials (fly ash + Portland cement (co-activator)) in a ratio of 0.5 by mass, considering the cementing agents as 95% ash and 5% pure Portland cement.

[0044] Table 1. Geopolymer dosage (example of implementation).

[0045] The ratio of NaOH solution to solid material (fly ash and Portland cement), by mass, varies between 0.4 and 0.6.

[0046] Preparation method:

[0047] A paste is produced by mixing the alkaline solution and solid materials in a ratio of 0.4 to 0.6 by mass. The solid material consists of 95% fly ash and 5% pure Portland cement.

[0048] The manufacturing process of the internal curing agent of the present invention is carried out according to the following steps:

[0049] (1 ) Dry the cementitious materials (fly ash and Portland cement) for 24-48 hours, until a zero moisture content is obtained.

[0050] (2) Homogenize the dry mixture in a mixer, for 2-10 minutes, preferably 2 minutes.

[0051] (3) Slowly add the NaOH solution and mix for 3-10 minutes, preferably 3 minutes, at medium speed. (4) Scrape the mixture from the sides of the mixer and check for lumps. Then, mix for 1-10 minutes, preferably 1 minute, until the paste has a homogeneous color and texture.

[0052] (5) Pour the mixture into 1 x 26 x 31 cm prismatic moulds, the surfaces of which have been treated with a release agent. The moulds are vibrated for 2-10 minutes, preferably 2 minutes, to eliminate bubbles inside the paste, and levelled to remove any excess.

[0053] (6) Leave the molds and the mixture in an oven at 70°C for 7-10 days, preferably 7 days. Then, the already hardened mixture is removed from the mold.

[0054] (7) Crush the hardened mixture until particles with sizes less than 2.36 mm are obtained.

[0055] (8) Sieve the mixture, leaving the portion passing through the ASTM No. 8 mesh (2.36 mm opening). In the case of particles retained above the No. 8 sieve, they are crushed again until the desired size is obtained.

[0056] (9) Wash the material on an ASTM No. 100 mesh screen (150 µm opening) to remove excess fines from the sample. The washed material is oven-baked at 105°C for 24 h to remove moisture. Once drying is complete, the aggregate is ready for use as an internal curing agent.

[0057] The development and optimization of the geopolymer manufacturing process of the present invention provides lightweight aggregates with specific characteristics to maximize internal curing performance. In this way, it is possible to obtain concrete capable of curing even if external curing is deficient or to complement external curing on site. Table 2 shows reference values ​​for the absorption and desorption of internal curing agents manufactured from geopolymerization. These physical properties are measured according to ASTM C1761. The absorption values ​​at 72 h (A72) and desorption at 73 h (WLWA) are satisfactory for the present invention.

[0058] Table 2. Absorption at 72 h (A72) and desorption at 73 h (WLWA) of the geopolymer.

[0059] The use of an internal curing agent, such as the one in this invention, reduces labor costs for the execution and inspection of reinforced concrete elements. Furthermore, the final product generated makes better use of the amount of cement incorporated in the mixture, which allows for increased strength (with the same cement dosage) or decreased cement dosage (with the same strength), increasing the sustainability and durability of reinforced concrete structures, and reducing cracking, among other benefits.

[0060] As indicated above, the most relevant specific characteristics for improving concrete performance through internal curing technology are the ability to absorb and release water at the appropriate times when the mixture most needs it. An optimal internal curing agent utilizes all of its porosity to store water and must also have the capacity to release it during the curing process, once the set or solidification of the paste has completed. Internal porosity that remains empty during water absorption and desorption does not contribute to the internal curing process and weakens the concrete structure. Figure 1 shows the role that the internal porosity of lightweight aggregate plays in the internal curing process. This estimation corresponds to a technique developed by the team that created this invention.It shows the relative volume of the aggregate corresponding to the solid (black region), the porosity that will not absorb water (gray region), the porosity that will absorb water but not release it (blue region), and the porosity that will release the absorbed water as part of internal curing, which is called "useful porosity" (light blue region). A lightweight aggregate that is effective as an internal curing agent will have a high solid-to-pore ratio and a high relative volume of useful porosity.

[0061] In both its physical characteristics and its performance in mortars and concretes, the product of the present invention (GP aggregate) compares with lightweight aggregates created from expanded glass and expanded clay. In each case, the numbers accompanying the name of the lightweight aggregate correspond to the upper and lower limits of the particle size, expressed in mm. The aggregate created by geopolymerization of the present invention compares favorably with other lightweight aggregates, as shown in Figure 1.

[0062] Small-scale development of lightweight aggregates based on geopolymers as an internal curing agent and results

[0063] The geopolymer-based lightweight aggregate of the present invention was manufactured according to the manufacturing method described above. The lightweight aggregate was subsequently saturated for 72 hours. The saturated aggregate was mixed as part of the component materials of concrete and mortar mixtures. The final concrete and mortar products were analyzed to determine the performance of the geopolymerized lightweight aggregate as an internal curing agent. Mortar is the term used for concrete that does not include coarse aggregates in its composition. Since the lightweight aggregate produced is in the size range of sand or fine aggregate, mortar samples were used to compare its role as an internal curing agent compared to existing lightweight aggregates.

[0064] Analysis of the efficiency of lightweight aggregates manufactured by geopolymerization as internal curing agents in concrete and mortar

[0065] To evaluate the performance of the internal curing agent of the present invention, different concrete and mortar mixtures were prepared with W / C (water / cement) ratios between 0.35 and 0.55. This, in order to have a wide range of concrete types. The dosage is based on a G35 (10) 20 / 8 concrete, with W / C ratio = 0.56, corresponding to a Pavement with Truss concrete. This base dosage was provided by Melón Hormigones. The reference mix was adjusted to produce mixtures of different water-cement ratios.

[0066] To compare the impact of the aggregate formed by geopolymerization of the present invention, two commercial aggregates were used: expanded clay (particle size between 2.36 and 4.75 mm) and expanded glass (particle size between 1 and 2 mm). The true dry density, absorption at 72 h (A72), and desorption at 72 h (WLWA) at 94% RH and 23 ° C according to ASTM C1761 are detailed in Table 3.

[0067] Table 2. Main characteristics of the LWAs analyzed.

[0068] The concrete and mortar mixtures were prepared according to ASTM C192. The concrete mixtures were mixed in a concrete mixer, while the mortar mixtures were mixed in an industrial mixer. All mixtures were compacted manually using a tamping rod. Curing conditions during the first 24 hours after placement depended on the requirements of each procedure. After 24 hours, all mixtures were air-cured in a Darwin Chambers® KB055 climate-controlled chamber with precise temperature control of 23 ± 0.5°C and relative humidity of 50 ± 0.5%.

[0069] The experimental procedure includes the following tests: a) Compressive and tensile strength

[0070] Compressive strength was evaluated on concrete and mortar samples. In each case, a reference mix was used, in which a fraction of the sand was replaced by lightweight aggregates that act as internal curing agents.

[0071] Figure 2 shows the compressive strengths of mortars made with water-cement ratios of 0.35; 0.40; 0.45 and 0.50. The test was performed according to ASTM C109. For each water-cement ratio, results are obtained for (a) the reference mortar and mortars internally cured with: (b) expanded glass with particle size between 0.1 and 0.3 mm, (c) expanded glass with particle size between 1 and 2 mm, (d) expanded glass with particle size between 4 and 8 mm, (e) expanded clay with particle size between 0.15 and 0.3 mm, (f) expanded clay with particle size between 2.3 and 4.8 mm, and (g) GP aggregate (internal curing agent of the present invention) with a particle size of 0.15-2.3 mm.

[0072] The addition of lightweight aggregates impacts compressive strength because they are more porous than traditional aggregates. By providing internal curing to mortars, GP aggregate (the internal curing agent of this invention) shows the least impact compared to other lightweight aggregates. This occurs because useful porosity is maximized, allowing internal curing without excessive porosity that significantly affects mechanical properties. This is repeated in all the water-cement ratios studied.

[0073] Figure 3 shows the 28-day compressive strengths of samples made from concrete at water-cement ratios of 0.35, 0.45, and 0.55. At each ratio, the reference concrete (concrete without lightweight aggregates, i.e., without the use of internal curing) is compared to concretes internally cured using expanded glass with particle sizes between 1 and 2 mm, expanded clay with particle sizes between 2.36 and 4.75 mm, and GP aggregate (the internal curing agent of the present invention) with particle sizes between 0.15 and 2.36 mm. As with the mortars, the GP aggregate (the internal curing agent of the present invention) shows the least impact on strength compared to the other lightweight aggregates used. This indicates that the GP aggregate is capable of providing internal curing without significantly impacting the compressive strength of the concrete.Figure 4 shows the tensile strength by splitting test, at 28 days, of samples made with concrete in water-cement ratios of 0.35, 0.45, and 0.55. This test was performed in accordance with ASTM C496. In each ratio, the reference concrete (concrete without lightweight aggregates; that is, without the use of internal curing) is compared with concretes internally cured using expanded glass with particle sizes between 1 and 2 mm, expanded clay with particle sizes between 2.36 and 4.75, and GP aggregate (internal curing agent of the present invention) with particle sizes between 0.15 and 2.36 mm. The impact of using GP aggregate as an internal curing agent is the least compared to the rest of the lightweight aggregates. The impact on tensile strength, which can be assessed in comparison to the strength of the reference mix, is zero or very low. b) Capillary Pressure.

[0074] The increase in the internal capillary pressure of a concrete material (cement paste, mortar, or concrete) is a naturally occurring phenomenon. This pressure increases significantly after the material sets; that is, after it has solidified and begins to gain strength. The generation of capillary pressure explains phenomena such as shrinkage or deformations that the material undergoes in its initial stages and, in uncontrolled cases, can cause cracking. Figure 5 shows the evolution of capillary pressure in reference mortars and in mortars that were internally cured with GP aggregate (the internal curing agent of the present invention). In each case, mortars were mixed at water-cement ratios of 0.35; 0.40; 0.45; and 0.50.When using GP aggregate, in all water-cement ratios, capillary pressure begins its accelerated increase process in later stages, the growth rate is lower, and the time at which maximum capillary pressure is generated occurs in later stages, in which the material has greater mechanical properties. This result indicates that internal curing is occurring effectively and that its impact on the material is positive for controlling deformation and cracking. c) Degree of Hydration and Evolution of Internal Relative Humidity.

[0075] One of the objectives of internal curing is the release of water during the self-drying of the concrete. As a result of this water release, concrete materials are expected to increase their degree of hydration; that is, the fraction of cement that hydrates. The degree of hydration of the mortars prepared for the compressive strength test was measured. Table 4. Degree of hydration of mortars at 28 days. The mortars are identified as reference (in the case of mortars that do not use internal curing) or with the name of the lightweight aggregate acting as the internal curing agent, along with the particle size range, in mm.

[0076] Mortar A / C = 0.35 A / C = 0.40 A / C = 0.45 A / C = 0.50

[0077] Reference 47.0 60.5 63.8 66.4

[0078] Expanded Glass 0.1 -0.3 mm 69.4 76.9 77.9 76.4

[0079] Expanded Glass 1 -2 mm 67.1 74.7 76.9 78.2

[0080] Expanded Glass 4-8 mm 76.6 82.8 85.3 69.4

[0081] Expanded Clay 0.15-0.3 mm 71.6 68.4 76.9 74.0

[0082] Expanded Clay 2, 3-4.8 mm 71.0 78.7 80.6 74.6

[0083] Aggregate GP 0.15-2.3 mm 66.5 68.3 72.0 75.2

[0084] Table 4 shows that, regardless of the water-cement ratio, there is a clear increase in the degree of hydration when using lightweight aggregates as internal curing agents compared to a reference mix. The higher degree of hydration indicates that internal curing is efficient in all cases, since the dosage seeks to provide the same amount of internal curing water. This also explains why no significant difference is observed between internally cured samples.

[0085] Another way to observe the internal curing phenomenon is through the measurement of the material's internal relative humidity. Samples of the reference mortar were exposed to controlled drying (temperature of 23 ± 0.5°C and ambient humidity of 50 ± 0.5%), while a replica was completely sealed, preventing any loss of moisture to the environment. Figure 6 shows the evolution of internal relative humidity over a period of 91 days. It is observed that, as expected, the sample exposed to drying decreases its internal relative humidity more rapidly. When using internal curing, both with expanded glass and with GP aggregate (the internal curing agent of the present invention), the rate of decrease in relative humidity is slowed. Internal relative humidity is directly related to cement hydration. Once humidity drops below -90%, the hydration rate decreases considerably. d) Autogenous Shrinkage

[0086] Autogenous shrinkage is the deformation of the material resulting from the cement hydration process. One of the objectives of internal curing is to control this type of deformation. Figure 7 shows the evolution of autogenous shrinkage of mortars over more than 100 days. Along with the reference mortar, the autogenous shrinkage of mortars with internal curing provided by expanded glass with particle sizes between 1 and 2 mm, expanded clay with particle sizes between 2.36 and 4.75, and GP aggregate (internal curing agent of the present invention) with particle sizes between 0.15 and 2.36 mm are shown. The results show that the use of internal curing is capable of significantly reducing autogenous shrinkage, bringing it to values ​​of low impact on the material. e) Dynamic Modulus of Elasticity

[0087] The dynamic modulus of elasticity is a non-destructive measurement of material stiffness. Therefore, it can be performed over time, using the same test samples. Figures 8 and 9 show the evolution of the dynamic modulus of elasticity of concretes with water-cement ratios of 0.35 and 0.55, respectively. In both cases, the reference concrete (without the use of internal curing) is compared with concretes internally cured by: expanded glass with particle sizes between 1 and 2 mm, expanded clay with particle sizes between 2.36 and 4.75, and GP aggregate (internal curing agent of the present invention) with particle sizes between 0.15 and 2.36 mm. The results show that the addition of lightweight aggregates, which are more porous than traditional aggregates, produces concretes with lower axial stiffness. However, this lower stiffness is related to the total porosity introduced into the system.Thus, GP aggregate, which has a lower porosity, shows the highest values ​​of the dynamic modulus of elasticity among the internally cured concretes. f) Hydraulic shrinkage.

[0088] Hydraulic shrinkage is the volumetric decrease in hardened concrete due to moisture loss into the environment. To estimate the effect of internal curing on this property, concrete specimens were exposed to controlled drying (temperature of 23 ± 0.5°C and ambient humidity of 50 ± 0.5%). Figures 10 and 11 show the evolution of total shrinkage (considering autogenous shrinkage and hydraulic shrinkage) over a period of 91 days for concretes with a water-cement ratio of 0.35 and 0.55, respectively.

[0089] Although internal curing is not designed to mitigate this shrinkage, the results show that internally cured concretes exhibit lower shrinkage than the reference mixture. In concretes with a higher water-cement ratio, the reduction in hydraulic shrinkage is greater compared to the reference concrete. This demonstrates that the use of GP aggregate (the internal curing agent of the present invention) is capable of reducing the hydraulic shrinkage of concretes when used as an internal curing agent.

[0090] Conclusions

[0091] The present invention considers the creation of lightweight aggregates, based on the geopolymerization technique, to act as internal curing agents for concrete. The manufacturing method produces internal porosity that maximizes the pores involved in internal curing, thereby reducing the impact on mechanical properties while controlling the initial deformations of the concrete.

[0092] The geopolymer produced in this invention has a particle size range corresponding to fine sand, which allows maximizing the area of ​​influence of internal curing water and covering a larger fraction of the concrete volume.

[0093] Additionally, the internal porosity size range is specifically designed to provide internal curing, which is not the case with commercial alternatives. Internal porosity captures water before its use in the material, which is delivered by increased capillary pressures in the concrete. For this reason, the internal pore size must be carefully controlled to allow for the delivery of water when it is most needed during the self-drying of the concrete. The present invention maximizes the presence of these pores, also reducing the pores that will not be used for internal curing.

[0094] As a result of the use of this invention, and as demonstrated in the tests included in this patent, the material achieves a higher degree of hydration, maximizes the fraction of pores that participate in the internal curing process, controls autogenous shrinkage, decreases the generation rate and delays internal capillary pressures of the material (which cause deformations and short-term cracking) and decreases total shrinkage (sum of autogenous and hydraulic shrinkage). Due to its greater efficiency with respect to the commercial alternatives compared in this patent (expanded glass and expanded clay), the use of the internal curing agent based on geopolymerization has a lower impact on mechanical properties.

[0095] Compared to other uses of the geopolymerization technique, such as replacing Portland cement to form alternative cement pastes, this invention uses a smaller amount of NaOH, resulting in lower costs and lower manufacturing complications. NaOH is a highly alkaline material, which can be corrosive in contact with skin. The NaOH solution considered in this patent is diluted and can be used with basic handling precautions. Thus, the present invention can be used in a wide range of concrete elements, controlling their deformation and cracking, thus obtaining more durable and higher-performing concrete construction.

Claims

CLAIMS 1. Lightweight aggregate for use as an internal curing agent based on the formation of a geopolymer, CHARACTERIZED in that it comprises: a NaOH solution; a solid material composed of fly ash and Portland cement; where the ratio of the NaOH solution to the solid material is 0.4 to 0.

6.

2. The lightweight aggregate according to claim 1, CHARACTERIZED in that the solid material corresponds to 95% fly ash and 5% Portland cement.

3. The lightweight aggregate according to claim 1 or 2, CHARACTERIZED in that the dosage of the geopolymer in weight / volume percentage is: 63.5% fly ash; 3.2% Portland cement; 33.3% NaOH solution.

4. The lightweight aggregate according to any of the preceding claims, CHARACTERIZED in that the NaOH concentration varies between 0.5 M and 2.0 M.

5. The lightweight aggregate according to any of the preceding claims, CHARACTERIZED in that the NaOH concentration varies between 0.5 M and 1.0 M.

6. The lightweight aggregate according to any of the preceding claims, CHARACTERIZED in that the NaOH concentration is 0.75 M.

7. The lightweight aggregate according to claim 1, CHARACTERIZED in that the fly ash comes from thermoelectric power plants.

8. A method for producing lightweight aggregate according to claim 1, CHARACTERIZED in that it comprises the steps of: a) Homogenizing a dry mixture of solid materials composed of fly ash and Portland cement, in a mixer, for 2-10 minutes. b) Slowly add a NaOH solution and mix for 3-10 minutes at medium speed until the mixture has a homogeneous color and texture. c) Pour the mixture into molds and vibrate them for 2-10 minutes to eliminate bubbles inside the mixture, and level them to remove any excess. d) Leave the molds with the mixture in an oven at 70°C for 7-10 days. e) Remove the hardened mixture from the mold. f) Crush the hardened mixture to obtain particles smaller than 2.36 mm. g) Sieve the mixture, leaving the portion passing through an ASTM No. 8 mesh screen (2.36 mm opening). h) Wash the material through an ASTM No. 100 mesh screen (150 µm opening) to remove excess fines from the sample. i) The washed material is baked at 105°C for 24 hours to remove moisture. j) Obtain the dry aggregate ready to be used as an internal curing agent.

9. The method according to claim 8, CHARACTERIZED in that it comprises the previous step of drying the solid materials corresponding to (fly ash and Portland cement) for 24-48 hours, until obtaining a moisture-free content.

10. The method according to any of claims 8 to 9, CHARACTERIZED in that during the mixing step in the mixer it includes removing the mixture from the edges of the mixer and checking that no lumps are observed, and mixing for 1-10 minutes, until the mixture has a homogeneous color and texture.

11. The method according to any of claims 8 to 10, CHARACTERIZED in that the molds are prismatic molds of 1 x 26 x 31 cm, the surface of which has been treated with a release agent.

12. The method according to any of claims 8 to 11, CHARACTERIZED in that in the sieving step the particles retained above sieve No. 8 are crushed again until the desired size is obtained.

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