WO2025043364A1 - Internal curing agents for concrete - Google Patents

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

INTERNAL CURING AGENTS FOR CONCRETE

FIELD OF INVENTION

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

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

BACKGROUND OF THE INVENTION:

Concrete curing is a fundamental requirement for obtaining good quality concrete, ensuring the hydration of the concrete materials and obtaining the required strength and durability of the concrete. An adequate supply of moisture is required to ensure the hydration of the concrete materials, in order to reduce porosity and maximize mechanical properties and, above all, durability.

Mixing water in conventional concretes is usually greater in quantity than the water required for cement hydration. However, water losses from the concrete to the environment can delay or even stop hydration, which negatively affects the development of concrete properties.

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

Additionally, curing improves the performance of low permeability and low A/MC ratio concretes, which require additional water to hydrate the underlying materials, thus ensuring the hydration of said materials and achieving the properties required in the concrete. Effective curing of concrete reduces variability in mechanical properties and decreases permeability.

With the increasing emergence of low A/MC ratio concretes, many of the properties of concrete have been significantly improved. However, the need for curing has become more necessary than before. Low A/MC concretes have a low water content and relatively high volumes of additives, 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.

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 inside the concrete takes on special importance and interest.

This can be achieved by an internal curing process, which uses as a 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 which is released later when it is required to continue hydration.

Another very important element to consider in the internal curing process of concrete is the internal curing agents used to supply water from the inside. The efficiency of the agents depends on their size, internal porosity and the ratio between absorbed and desorbed water (delivered to the environment). Currently, lightweight artificial aggregates, with a size under 5 mm (FLWA, for fine lightweight aggregates), have predominated in internal curing applications.

The use of natural (non-artificial) lightweight aggregates is a competitive option, but their availability may be limited regionally. In some cases, these aggregates may be available in areas close to volcanoes. However, their properties are highly variable, requiring industrialization methods (production plants) that are not available in Chile, and whose economic and environmental cost is very high. Another alternative is the use of artificial FLWA, available in international markets (mainly the USA and Europe), but not in Chile. The latter are not specifically manufactured for internal curing, so, although they perform well, their internal pore structure does not allow for optimal water delivery and distribution capacity. In addition, in some cases, they have low mechanical properties that can generate weak zones in the concrete. An additional disadvantage is that their availability is limited to certain countries and particular regions in the world where this type of lightweight aggregate is produced. Another alternative to the use of FLWA is superabsorbent polymers (SAP). SAPs are hydrogels that absorb a considerable amount of water and release it to provide more moisture and act as a possible 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.

Another additional alternative has been the use of cellulose fibers used in laboratory tests, but they have not yet reached commercial use in real works, in addition to presenting durability problems.

Thus, the currently existing 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.

The state of the art describes the use of geopolymeric materials, formed from raw materials derived from solid waste, such as ash from municipal solid waste incineration, 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.

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 benefit of adding fly ash to the mix has been found to be the ability to improve concrete, because it changes the composition of the concrete, adding strength and durability. The resulting material is less porous than using only OPC, and is more resistant to deterioration. In addition to being more durable, concrete with fly ash is more resistant to acid and fire, and has demonstrated greater compressive and tensile strength at later ages. However, since fly ash is an industrial by-product, it has highly variable compositions, so its regularity must be checked before being mixed with the supplementary materials, carrying out the necessary controls to determine and check that possible variations in its composition do not affect the concrete manufactured with it. This process is highly expensive and there are no processing plants for this purpose in Chile or Latin America. The present invention provides a method of 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 need or require the use of Portland cement in their composition. The difference presented by this technique is that geopolymerization is used to generate porous structures of a size similar to sand, with a controlled porosity that allows the implementation of internal curing in concretes. Among the advantages of this development is the possibility of controlling the characteristics and efficiency of the lightweight aggregates produced, not only to produce internal curing, but also to adjust it to different construction applications with concrete.

STATE OF THE ART

The state of the art provides various disclosures related to geopolymer materials and their use. For example, document CN110776279A addresses the difficulty of complex geopolymer preparation processes, their high cost, low stability, long solidification time and difficulty in maintaining good compressive and flexural strength. This document proposes as a solution to these problems the preparation of a geopolymer that uses as raw materials three types of industrial waste, including fly ash, furnace bottom slag and water-rich calcium carbide slag, which are produced by circulating fluidized bed combustion. 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 of 1 -10 mol/L sodium silicate solution, 1 -10 mol/L sodium sulphate 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 the geopolymer for curing concrete, but rather a geopolymer gel is obtained that is mixed with cement for the formation of concrete, as traditional use.

Within the state of the art is also the document US2019092688A1 which discloses geopolymer binders based on fly ash and activation solutions for use in the formation of concrete, as well as as the methods for forming concrete. The geopolymer binders in this document are based on a combination of fly ash, silica fume and sodium hydroxide and portland cement, which form a paste that is mixed with the aggregates to form the concrete. The internal curing technology is not disclosed in this document, however, it is stated that concretes formed from these binders can exhibit excellent compressive strength characteristics as well as being more cost effective compared to other concretes. It is also stated 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 one of the main factors contributing 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 those 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, of a size in the range of fine aggregates, and with a controlled internal porosity that allows the development of 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 to generate internal curing in the concrete.

Documents US2013087078A and US2013087076A1 discuss the importance of fly ash as additional additives that improve 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 supplementary additives is a use that has more than 50 years of history internationally.

Both documents disclose inorganic polymer compositions that include 85% by weight or more of fly ash and not more than 10% by weight of portland cement, anhydrous calcium sulfate (US2013087078A) or calcium aluminate (US2013087076A1), an activator that may 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 including 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, their application is rather related to the production of a traditional geopolymer; that is, to produce a cement paste not based on Portland cement or one mixed from Portland cement. Additionally, they do not provide a method for the production of lightweight aggregates or for the production of internal curing agents, such as that included in the present invention.

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 among which are mentioned Portland cement, fly ash and selected components of silica fume, glass beads, regenerated fine powder, regenerated fine aggregate, fine aggregate, polycarboxylate superplasticizer and water. CN113321467, relating to an ultra-high performance concrete with lightweight aggregates in low shrinkage, is also manufactured by 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, expansion agent and water. CN1 13149501 also discloses an internal curing material for ultra-high performance concrete comprising acrylic acid, acrylamide, 2-achlamide-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 nonionic monomer, physical and chemical cross-linking agents. Document CN1 11892315A refers to an internal curing sand composed of crushed autoclaved lightweight aerated concrete (ALC) waste particles, slag from power plant furnace bottoms, and cement foam board particles. Document CN1 10963737 refers 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 refers to internal curing agent based on industrial aluminosilicate waste, specifically fly ash, coal dust and polypropylene fiber. 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 methyl silicone and sodium gluconate.

On the other hand, 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% soil contaminated with polymetals 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 it refers to “curing” 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.

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 CN11 1825378 provides a concrete with this curing material comprising: selecting the particle size of fly ash 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%. Surface saturated fly ash (dry) is mixed with cement, sand and aggregates in a mixer, water is added while mixing for 2 minutes and poured into a mould. While the internal curing agent comprises fly ash and a particle size of less than 2.36 mm of the ash is sought, the internal curing agent comprising fly ash, cements and an alkaline activator is not produced or provided as particles having that size. In this document all materials are mixed without prior preparation of the curing agent. In summary, this document is aimed at taking advantage of the ability of water absorption of fly ash into a powder size (similar to cement), to provide internal curing in the hardened concrete. That is, no lightweight aggregates are produced from fly ash.

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

BRIEF DESCRIPTION OF THE FIGURES

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 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.

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.

Figure 3. Compressive strength of concretes with water-cement ratios of 0.35, 0.45 and 0.55, with and without internal curing, measured at 28 days after mixing. The reference concrete is compared with mixtures internally cured 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.

Figure 4. Tensile strength by splitting of concretes with water-cement ratios of 0.35, 0.45 and 0.55, with and without internal curing, measured at 28 days after 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.

Figure 5. Evolution of the internal capillary pressure of reference mortars and mortars cured internally with the GP aggregate (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 has set (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 is able to decrease the rate of pressure gain and shift the time at which the maximum capillary pressure is reached.

Figure 6. Evolution of the internal relative humidity of reference mortars (without the use of internal curing agents) and internally cured with expanded glass and with the GP aggregate (internal curing agent of the present invention). The samples cured with the internal curing agent of this invention are capable of maintaining the mortar at higher relative humidities, compared to a reference sample.

Figure 7. Evolution of autogenous shrinkage over 100 days or more, measured in pm/m (or pe), for mortar samples internally cured 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).

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.

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 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 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 11. Evolution of 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.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an internal curing agent for concrete produced using the geopolymerization technique. The internal curing agent has a performance equal to or better than the alternatives available in international markets when used in mortar or concrete mixtures. The invention has the advantage of being manufactured with local materials, based on geopolymers, with a convenient cost for large-scale production and for use in the production of concretes that can be cured autonomously.

The internal curing agent of the present invention is a geopolymer obtained from fly ash from thermal power plants. The ash is composed mainly of silica (SiOs), alumina (AI2O3) and free lime (CaO). In the geopolymerization process, the ash is mixed in a controlled manner with an alkaline activator, preferably a sodium hydroxide solution (NaOH) in a low concentration. Additionally, a low amount of Portland cement is incorporated, which acts as a coactivator within the mixtures. The invention presents a manufacturing process and a proportion of component materials, such that the lightweight aggregate obtained has an internal porosity that allows its role as an internal curing agent.

Example of implementation

According to the present invention, the internal curing agent based on geopolymers requires the alkaline activation of a solid precursor. In this case, the The precursor is fly ash. Fly ash is a by-product of power generation in thermal 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 acting 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 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 internal curing agent (ICA) comprises forming a paste with an alkaline solution (activator)/cementitious materials (fly ash + portland cement (coactivator)) in a ratio of 0.5 by mass, considering the cementitious materials as 95% ash and 5% pure portland cement.

Table 1. Geopolymer dosage (example of implementation).

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

Method of preparation:

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 is 95% fly ash and 5% pure Portland cement.

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

(1) Dry the cementitious materials (fly ash and Portland cement) for 24-48 hours, until obtaining zero moisture content.

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

(3) Slowly add the NaOH solution and mix for 3-10 minutes, preferably 3 minutes, at medium speed. (4) Remove the mixture from the edges of the mixer and check that no lumps are visible. Then, mix for 1-10 minutes, preferably 1 minute, until the paste has a homogeneous color and texture.

(5) Pour the mixture into 1 x 26 x 31 cm prismatic moulds, the surface of which has been treated with a release agent. The moulds are vibrated for 2-10 minutes, preferably 2 minutes, to eliminate bubbles inside the paste, and the excess is levelled off.

(6) Leave the moulds 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 mould.

(7) Crush the hardened mixture until obtaining particles with sizes less than 2.36 mm.

(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.

(9) Wash the material on an ASTM No. 100 mesh (150 µm opening) to remove excess fines in the sample. The washed material is placed in an oven at 105°C for 24 h to eliminate moisture. Once drying is complete, the aggregate is ready to be used as an internal curing agent.

The development and optimization of the manufacturing process of the geopolymer of the present invention provides lightweight aggregates with specific characteristics to maximize internal curing performance. In this way it is possible to obtain a concrete capable of curing even if the external curing is deficient or to complement the 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.

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

The use of an internal curing agent, such as that of the present invention, allows for a reduction in labor costs for execution and inspection of elements of reinforced concrete. Furthermore, thanks to the final product generated, the quantity of cement incorporated in the mixture is better used, which allows for increased resistance (with the same cement dose) or decreased cement dose (with the same resistance), increased sustainability and durability of reinforced concrete structures, reduced cracking, among other benefits.

As previously stated, the most relevant specific characteristics for improving concrete performance using internal curing technology is the ability to absorb and release water at the appropriate times when the mix needs it most. An optimal internal curing agent uses all of its porosity to store water and then must also have the ability to release it in the curing process, once the setting or solidification of the paste has finished. The internal porosity that remains empty during the absorption and desorption of water does not contribute to the internal curing process and weakens the structure of the concrete. Figure 1 shows the role played by the internal porosity of the lightweight aggregate 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 that corresponds to the solid (black region), the porosity that will not absorb water (gray region), the porosity that will absorb water but will not release it (blue region) and the porosity that will release the water absorbed as part of the internal curing and which is called "useful porosity" (light blue region). A lightweight aggregate that is efficient in acting as an internal curing agent will have a high solid-to-pore ratio and a high relative volume of useful porosity.

In both its physical characteristics and its performance in mortars and concretes, the product of the present invention (GP aggregate) is compared with lightweight aggregates created on the basis of expanded glass and expanded clay. In each case, the numbers accompanying the name of the lightweight aggregate correspond to the upper and lower limit of the particle size, expressed in mm. The aggregate created by means of geopolymerization of the present invention compares favorably with other lightweight aggregates, as reflected in Figure 1.

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

The manufacture of the lightweight aggregate based on geopolymers of the present invention was carried out according to the manufacturing method described above. Subsequently, the lightweight aggregate was 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 lightweight aggregate. based on geopolymerization as an internal curing agent. Mortar is the term used for a concrete that does not include coarse aggregates in its composition. Since the lightweight aggregate produced is in the size range of a sand or fine aggregate, the mortar samples were used to perform a comparison of the role as an internal curing agent, compared to the existing lightweight aggregates.

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

To evaluate the performance of the internal curing agent of the present invention, different concrete and mortar mixtures were prepared with W/C ratios (water/cement) 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 Truss Pavement concrete. This base dosage was provided by Melón Hormigones. The reference mixture was adjusted to produce mixtures with different water-cement ratios.

To compare the impact of the aggregate formed by geopolymerization of the present invention, 2 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.

Table 2. Main characteristics of the LWAs analyzed.

The concrete and mortar mixes were prepared according to ASTM C192. The concrete mixes were made in a concrete mixer, while the mortar mixes were made in an industrial mixer. Compaction of all mixes was manual, using a tamper rod. Curing conditions during the first 24 hours after placement depended on the requirements of each procedure. After 24 hours, all mixes are air-cured in a concrete mixer. Darwin Chambers® KB055 climate controlled chamber with precise temperature control of 23 ± 0.5°C and relative humidity of 50 ± 0.5%.

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

The 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.

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.

The addition of lightweight aggregates impacts the compressive strength, because they are more porous than traditional aggregates. When providing internal curing to the mortars, the GP aggregate (internal curing agent of the present invention) shows the least impact compared to the rest of the lightweight aggregates. This is because the useful porosity is maximized, which allows providing internal curing without having excessive porosity that significantly affects the mechanical properties. This is repeated in all the water-cement ratios studied.

Figure 3 shows the compressive strength, after 28 days, of samples made with concrete in 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 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. As in the case of mortars, GP aggregate (internal curing agent of the present invention) shows the least impact on strength compared to the other lightweight aggregates used. This indicates that 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, after 28 days, of samples made with concrete in water-cement ratios of 0.35, 0.45 and 0.55. This test was performed according to 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 internal curing agent is the least compared to the rest of the lightweight aggregates. The impact on tensile strength, which can be evaluated in comparison to the strength of the reference mixture, is null or very low. b) Capillary Pressure

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 has set; that is, after the material has solidified and begins to gain strength. The generation of capillary pressure explains phenomena such as shrinkage or deformations that the material undergoes in the 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 the GP aggregate (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, the capillary pressure begins its accelerated increase process in later stages, the growth rate is lower, and the time in which the maximum capillary pressure is generated occurs in later stages, in which the material has greater mechanical properties. This result indicates that internal curing is effectively occurring and that its impact on the material is positive for the control of deformations and cracking. c) Degree of Hydration and Evolution of Internal Relative Humidity

One of the objectives of internal curing is the release of water during self-drying of the concrete. As a result of this release of water, the concrete materials are expected to increase their degree of hydration; that is, the fraction of cement that is hydrated. The degree of hydration of the mortars prepared for the compressive strength test was measured. Table 4. Hydration level of mortars after 28 days. Mortars are identified as reference (in the case of mortar not using internal curing) or with the name of the lightweight aggregate acting as internal curing agent, together with the particle size range, in mm.

Mortar A/C = 0.35 A/C = 0.40 A/C = 0.45 A/C = 0.50

Reference 47.0 60.5 63.8 66.4

Expanded Glass 0.1 -0.3 mm 69.4 76.9 77.9 76.4

Expanded Glass 1-2 mm 67.1 74.7 76.9 78.2

Expanded Glass 4-8 mm 76.6 82.8 85.3 69.4

Expanded clay 0.15-0.3 mm 71.6 68.4 76.9 74.0

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

Aggregate GP 0.15-2.3 mm 66.5 68.3 72.0 75.2

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 shows 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 major difference is observed between internally cured samples.

Another way to observe the internal curing phenomenon is through the measurement of the internal relative humidity of the material. 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; that is, preventing any loss of humidity to the environment. Figure 6 shows the evolution of the internal relative humidity for a period of 91 days. It can be observed that, as expected, the sample exposed to drying decreases its internal relative humidity more quickly. By using internal curing, both with expanded glass and with the GP aggregate (internal curing agent of the present invention), the rate of decrease in relative humidity is reduced. The internal relative humidity is directly related to the hydration of the cement. Once the humidity drops below -90% the hydration rate decreases considerably. d) Autogenous shrinkage

Autogenous shrinkage is the deformation of the material as a result of 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 for more than 100 days. Together with the reference mortar, the autogenous shrinkage of mortars with internal curing provided by 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 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 Elasticity Modulus

The dynamic modulus of elasticity is a non-destructive measurement of the 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 ratio 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 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. The results show that the addition of lightweight aggregates, 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, the 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

Hydraulic shrinkage is the volumetric decrease in hardened concrete due to moisture loss to 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 1 1 show the evolution of total shrinkage (considering autogenous shrinkage and hydraulic shrinkage), over a period of 91 days, of concrete with water-cement ratios of 0.35 and 0.55, respectively.

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

Conclusions

The present invention involves the creation of lightweight aggregates, based on the geopolymerization technique, to act as internal curing agents for concrete. The manufacturing method produces an internal porosity that maximizes the pores that are part of the internal curing, thus reducing the impact on the mechanical properties, while controlling the initial deformations of the concrete.

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

Additionally, the internal porosity size range is specifically designed to provide internal curing, which is not the case with commercial alternatives. The internal porosity captures water prior to its use in the material, which is delivered by the increase in capillary pressures in the concrete. For this reason, the internal pore size must be carefully controlled to allow water delivery when it is most required during 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.

As a result of the use of this invention, and as set forth in the tests included in this patent, the material achieves a higher degree of hydration, maximizes the fraction of pores participating in the internal curing process, controls autogenous shrinkage, decreases the generation rate and delays internal capillary pressures of the material (which cause short-term deformations and 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 geopolymerization-based internal curing agent has a lower impact on the mechanical properties.

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, which results in lower costs and complications in its manufacture. NaOH is a highly alkaline material, which can be corrosive in contact with the skin. The NaOH solution considered in this patent is diluted and can be used with basic handling care. In this way, the present invention can be used in a wide range of concrete elements, controlling their deformations and cracking, in order to obtain a more durable and better performing concrete construction.

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 because 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 because the fly ash comes from thermoelectric plants. 8. A method of 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 off to remove the excess. d) Leave the molds with the mixture in an oven at 70°C for 7-10 days. e) Remove the already hardened mixture from the mold. f) Crush the hardened mixture to obtain particles with sizes smaller than 2.36 mm. g) Sieve the mixture leaving the portion passing through the ASTM No. 8 mesh (2.36 mm opening). h) Wash the material on an ASTM No. 100 mesh (150 µm opening) to remove excess fines in the sample. i) The washed material is placed in an oven at 105°C for 24 h to eliminate its 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 prior 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, whose surface has been treated with a release agent. 12. The method according to any of claims 8 to 11, CHARACTERIZED because in the sieving step the particles retained on sieve No. 8 are crushed again until the desired size is obtained.