Soft Means of Concrete Modification – Curing Conditions

The term “concrete curing” is not clearly defined in European standards. It appears many times both in PN-EN 206 [1] and, above all, in PN-EN 13670 [2], which are two basic documents regulating the principles of concrete technology and concrete works. However, it is not defined, but only described by giving methods, rules and requirements. Generally accepted definitions in concrete technology formulated e.g. in [3, 4], refer to the definition taken from the terminology dictionary of the American Concrete Institute (successive versions of ACI 116 [5] and currently subsequent versions of the ACI Concrete Terminology dictionary (current ACI CT-16 of 2016 [6]), which states: “curing - action taken to maintain moisture and temperature conditions in a freshly placed cementitious mixture to allow hydraulic cement hydration and (if applicable) pozzolanic reactions to occur so that the potential properties of the mixture may develop.” Other definitions often include the phrase “…actions taken from the moment of placing and compacting the concrete mix…” ([3, 4]), which actually limits the concept of maintenance to activities on the outer surface of the element after placing and compaction, which includes maintaining the appropriate temperature and humidity of concrete and its protection against atmospheric factors.

The classic definitions refer to the curing of cement concrete and do not take into account the different conditions for shaping the structure of concrete containing polymeric co-binder, which are the main subject of this study. In addition, for the purposes of this study, the concept of care has been extended to include issues related to activities undertaken at other stages of the technological process or related to other than surface impact on concrete, which are necessary and can have a significant impact on the formation of the concrete microstructure and the obtained properties of the composite. This made it possible to include issues of internal curing, issues of preparation of formwork related to curing (formwork inserts, anti-adhesive agents) or issues of active thermal curing during the initial hardening period. Proper curing, taking into account the specificity of the used binder, co-binders and modifiers, is a prerequisite for ensuring the durability of concrete, next to the correct selection of materials, construction design and technology of work.

Uncured or improperly cured cement concrete primarily shows weakening of the surface layer. This is due to physical and chemical phenomena related to two main factors: the flow of moisture and the flow of heat, while the scale of threats resulting from each of these factors is related to climatic conditions. At an ambient temperature above +10 ℃, the phenomena related to the lack of moisture are dominant, and the possible negative effects of errors are the more dangerous the higher the ambient temperature in which the concrete works are carried out and the higher the wind force. In low temperature conditions, ensuring proper humidity is equally important, but thermal curing is also becoming more important. In extreme cases, the effects of curing errors can be noticed in a short time, but often the defects are hidden in the concrete structure and reveal themselves during its use, causing deterioration of durability. One of the main defects of this type is the increase in the porosity of the concrete surface layer (i.e. the reinforcement cover), which deteriorates its protective properties and tightness [7‐12]. The proper scenario of PCC curing is also crucial for bond development to composites and tiles [13].

As a result, the mechanical properties of the near-surface layer of concrete in the structure deteriorate, as well as the durability characteristics, such as water tightness, frost resistance, chemical resistance, including resistance to chloride penetration and carbonation. This impact is significantly greater in the case of concretes containing mineral additives, used as a substitute for part of the cement. The level of the w/c ratio is also important: with medium and low w/c values, the role of curing is particularly important due to the optimal structure of the surface layer. At high w/c - when concrete is assumed to be weak (porous), the effect of curing decreases. The classification of cement concrete curing methods takes into account the method of its conduct (surface and internal), the type of surface (formed, unformed) and the method of impact (humidity, heat). Other forms of impact were also taken into account, including treatment in an environment with increased carbon dioxide concentration, which is the direction of sustainable development of concrete technology. In the above approach - regarding cement concretes as a base - it is difficult to take into account the specificity of polymer-modified concrete and polymer concrete curing. The so-called low-polymer cement-concretes, with a polymer content of less than 5% of the cement mass used as a modifying admixture, are subject to the curing methods and rules indicated above.

In case of cement-free polymer concretes (or PC) the curing conditions are different than in the case of ordinary concretes or PCC as there is no need to provide water to the internal structure needed for the hydration of cement. Moreover, it is not recommended or sometimes even not allowed to produce and cure the polymer concrete specimens in the atmosphere of increased humidity. In many cases the increased humidity may interfere with the binding process of the polymer binder (here specifically: resins), and in extreme cases, even prevent setting and hardening [14, 15]. Therefore, it is recommended to cure polymer concretes in a dry environment. What is more, in order to avoid moisture in the polymer concrete mix and the negative impact of the presence of water on setting, in many cases dried fillers are used for the production of polymer concrete. This approach is recommended and used in their research, among others by Sokołowska et al. [16‐18]. The ambient temperature during application and maintenance cannot be too low or too high, because it affects the dynamics of resin setting (delaying or accelerating setting, respectively), although the setting time can be adjusted by using appropriate amounts of chemical regulators dosed into the resin together with the curing system [18]. Many of the above mentioned methods do not apply to cement concretes with a higher polymer content (PCC), and those that do apply require a different approach related to different bonding conditions of both binders.

Concrete curing process effects in positive changes in concrete microstructure and properties, so in this context it could be treated as a soft mean of concrete modification. The positive effect of this modification is related to the optimal course of curing. The issue of the role of polymers in concrete curing processes can be considered in many aspects, including: the influence of the resin co-binder on the optimal course of concrete curing (Sect. 2), the role of superabsorbent polymers as an internal curing agent for cement concrete (Sect. 3), spray polymeric film-forming agents as a form of surface treatment of cement concrete (Sect. 4), plastic films as a material for the care of cement concrete, treatment of polymer-modified concretes in an environment with increased concentration of carbon dioxide (Sect. 5). Selected issues will be discussed in this article.

The curing of cement concrete with the use of superabsorbent polymers (SAP) is based on changes in water transport process in the pore network of hardening cement composites caused by this modification. Superabsorbent polymers absorb water as a result of high osmotic pressure present in the polymer structure - water absorption is electrochemical in nature. The ability to absorb significant amounts of water by superabsorbent polymers is related to high osmotic pressure caused by the accumulation of ions (e.g. sodium, potassium) in the polymer structure. The process of water absorption causes the polymer to swell, thereby moving the ions apart, which reduces the osmotic pressure. Assuming such a model of SAP operation, their ability to absorb is limited not only by reducing the osmotic pressure as a result of water absorption, but also by the influence of external pressures resulting from the change in polymer volume. This property of SAP determined their use in concrete technology, because in case of loss of equilibrium between the osmotic pressure of the water-saturated polymer and the internal stresses of concrete, SAP is able to reduce its volume, i.e. to release water. This is synonymous with the ability of these polymers to internally cure the composite. With the introduction of SAP to the concrete mix, the polymer grains absorb part of the mixing water. As a result of absorption, their physical properties change - from the form of dry granules in a water-unsaturated state, SAP goes into the form of a hydrogel, with a polymer structure stretched as far as it is allowed by chemical structure and the properties of the water absorption environment (e.g. alkali content in the environment).

From a chemical point of view, the group of superabsorbent polymers includes cross-linked polyelectrolytes (e.g. acrylic polyesters with acrylic acid) that swell when they come into contact with water. Many materials, both naturally occurring and synthetic, fit the definition of hydrogels. The classification of hydrogels is extensive and includes a number of factors. Hydrogels can be of natural origin (e.g. proteins such as collagen or gelatin) or synthetic, resulting from the polymerization process. Synthetic hydrogels are obtained by polymerizing one type of monomer (homopolymer hydrogels) or two or more (copolymer hydrogels). An example of copolymer hydrogels are IPN hydrogels (multipolymer interpenetrating polymeric hydrogel), made of two cross-linked polymer chains [44]. Currently, the most commonly used hydrogels are hydrogels of petrochemical origin, produced from acrylic monomers. Acrylic acid (AA) and its sodium or potassium salts and acrylamide (AD) are most often used in the production of hydrogels [45]. SAP is most commonly available as hard, dry, granular powders with particle sizes ranging from 100 to 1000 μm. They are produced by block polymerization (gel polymerization) or suspension polymerization [46, 47].

The SAP particles can be added into the concrete mix both in dry form and in the form of a hydrogel (i.e., SAP premixed with a portion of the mixing water). The effect of introducing SAP polymers in a dry form to the concrete mix is the absorption of part of the mixing water, which significantly exceeds the weight and volume of added polymer. In the case of a polymer previously mixed with mixing water (dosing in the form of a hydrogel), the mass ratio of saturated polymer to the mass of cement changes about a hundredfold. This is due to the different absorptivity of SAP in environments with different pH (at pH = 7 and pH = 13, the absorptivity of polyacrylate SAPs differs about 10 times). By introducing SAP into the cement composite, in a dry state or in the form of a hydrogel, after mixing with the other components of the composite, an additional phase of the material is created - in the form of quasi-pores filled with hydrogel, which over time, after fulfilling the function of ensuring continuity of hydration and after desorption of all water from the SAP structure, passes into the pore phase [44].

The effectiveness of internal curing with the use of SAP is based on its effect on the kinetics of water migration during the hydration of the binder in cement composites. Internal stresses in the cement matrix resulting from hydration and self-drying contribute to the release of previously absorbed water from the SAP structure. Taking into account a number of properties of cement composites that change as a result of the use of SAP - mechanical properties, degree of binder hydration, autogenous shrinkage, pore network distribution and others affecting the durability of the material - it is crucial to collect as much information as possible regarding both the course of absorption and desorption water from the SAP structure. The two mentioned methods have a different impact both on the issue of the homogeneity of the water absorption process and the characteristics of its course, and above all on the moment of the appearance of the stage of water desorption from the polymer structure over time. As a result of these differences, the impact of SAP introduced in different ways to the concrete mix is also different (Table 1).

The use of superabsorbent polymers is an interesting example of modification of cement composites by changing the rules governing the transport of water in the pore network of the material. By introducing an additional phase into the material which, depending on the circumstances, exhibits both solid and liquid characteristics, it is possible to exercise better control over the internal moisture of the pore network, and thus effectively influence a number of properties of the internally cured material. The introduction of SAP into the composite in the form of a hydrogel is the latest approach to the issue. The purpose of such a modification of the SAP dosing method is to eliminate the negative phenomena associated with the method of dosing SAP in a water-unsaturated state, including increasing the homogeneity of the distribution of SAP particles in the cement matrix, ensuring a positive impact of SAP on both material strength and durability-related properties. The authors, while emphasizing the advantages of internal curing, would like to strongly emphasize that internal curing cannot fully replace the activities that make up external curing - both forms of curing complement each other, using different mechanisms of influencing the properties and characteristics of the microstructure of the cement matrix and affecting different areas of concrete element.

Surface curing of cement concrete in the traditional approach is carried out “wet”, i.e. with the use of additional water, introduced to the surface of the treated concrete element. An alternative is the so-called coating curing, i.e. preventing the evaporation of the mixing water from the concrete by introducing a tight barrier material on the surface of the fresh concrete. Polymer materials are ideal for this role. For a long time, sheet materials have been used in this role, such as polyethylene foil, which can be spread on the surface several hours after concreting, so that it does not stick to fresh concrete. This delay in the start of surface protection is a disadvantage of the method, because curing in the first hours after concreting are the key to obtaining optimal effects. An earlier start of coating curing is possible with the use of film-forming polymer agents that can be applied to fresh concrete immediately after concreting, i.e. after a time sufficient to absorb the cement laitance from the surface. Film-forming agents are liquid substances used to cover the surface of fresh concrete in order to obtain a coating that protects against water evaporation. They can be divided into three groups: solutions of macromolecular substances in organic solvents, water emulsions that are dispersions of organic substances and low viscosity resins. Commercially available agents are in the form of resin solutions and in the form of emulsions. Currently, solutions of the following resins are most often used: acrylic, vinyl, styrene butadiene. These resins are diluted with highly volatile solvents. Film-forming agents in the form of emulsions are made with the use of wax or paraffin. In order to ensure uniform coverage of the concrete surface, a white dye is added to the film-forming preparations, which disappears in time. The requirements for these agents are currently not standardized within European standards. Old national guidelines [48] and American guidelines [49] are in use. Based on the research conducted by the authors, an estimated comparison of several care solutions is shown (Fig. 4). The effectiveness of each variant was assessed using a method based on the European standard draft CEN/TS 14754-1:2007 [50], which includes a unified method for assessing the effectiveness of film-forming preparations by assessing the reduction evaporation of water from the protected concrete surfaces. Water loss from the unprotected surface was assumed as 100%, for individual combinations of materials the loss was expressed as a % loss from the unprotected surface.

Nowadays we are struggling with the problem of excessive carbon dioxide emissions into the atmosphere. A tremendous contribution to CO₂ production comes from the construction industry which emits about 39% of CO₂ and the manufacture of construction materials which alone takes up as much as 11% of global carbon dioxide emissions [51]. A certain amount of carbon dioxide is captured during the service-life, the end-of-life and secondary usage stages of construction, but nevertheless there is crucial need to reduce these emissions and potential capture and avoidance of emitting CO₂.

Accelerated carbonation curing holds the key to capturing and storing CO₂ emissions from the cement industry for the production of value-added concrete products. Carbon sequestration is the process of separating and capturing carbon dioxide from exhaust gases in order to reduce its emissions to the atmosphere. Sequestration methods are categorized into direct, indirect and advanced.

Mineral carbonation is used in the construction industry. In favor of using it as a method of reducing carbon dioxide emissions into the atmosphere is supported primarily by the fact that it is a natural process, occurring in nature and the products resulting from this process are inert for the environment [52]. Mineral carbonation as a method of carbon dioxide sequestration was proposed by Seifritz (1990), and in 2005 was finally defined in the IPCC Special Report on Carbon Dioxide Capture and Storage, part 7. Mineral Carbonation and Industrial uses of Carbon Dioxide [53]. The idea of using CO₂ to cure building materials was proposed as early as the 1970s. However, the method was reluctantly considered because of the costly production of pure CO₂ and the possible negative effects of atmospheric carbonation. However, the need to reduce greenhouse gas emissions has led to renewed interest in the topic. Curing by CO₂ sequestration has a twofold effect on the engineering properties of products: in the early term, the rapid reaction of cement accelerates the development of concrete strength; in the longer term, durability is enhanced as a result of the change in the chemical composition and the microstructure caused by the precipitation of CaCO₃ in the cement paste. CaCO₃ precipitation impermeable microstructure which affects the reduction of total and capillary porosity, which ultimately modifies the transport properties of the paste, reducing the absorption and permeability of the concrete [54]. In the ECC (fiber reinforced concrete) with fly ash, after CO₂ curing, the early term tensile and compressive strength were accelerated by 57% and 41%, respectively. The matrix and fiber/matrix interface were found densified which increased the ultimate tensile strength by 22% compared to non-carbonated reference. Also dense fiber/matrix interface improved the chemical and frictional bonds leading to tighter crack width which reduced the ECC’s water permeability in loaded condition, despite a more permeable matrix due to a larger pore size associated with the lowered pozzolanic reaction [55]. The nature of the carbonation phenomena is similar in cement and polymer-cement concrete [42]. For PCC, besides the proper curing procedure, the duration of component exposition to CO₂, or the composite age, the important factors that affect the reaction with CO₂ are the polymer modifier type and its quantity. The lower carbonation was reported for polymer-modified cement because of the presence of the polymer in the pore which partially buffer the carbonation reaction despite the pore concentration abundance [56]. Referenced research establishes the viability of applying carbonation curing to polymer-modified concrete, with technical merits. Beyond the remarkable CO₂ sequestration capacity at the manufacturing stage, concrete after carbonation curing is anticipated to lower the lifecycle emissions as an example of beneficially utilizing CO₂ for durable precast construction products.

The scale of modification of concrete properties with the use of appropriately selected, effective curing methods can be significant, as shown in the research on polymer-cement composites presented in the article. This is particularly important in the context of durability, but also in the context of the cost of ensuring the expected utility of concrete. In this sense, research on the optimization of the concrete curing process is a part of the paradigm of sustainable development of concrete technology. This problem is of particular importance in relation to polymer-cement composites, in which the binder co-components need different conditions to optimally shape their structure - making mistakes in this case can be particularly expensive. The article also draws attention to other aspects of the use of polymers in the curing process of cement and polymer-cement composites. The possibility of curing PCC concretes with the use of carbon dioxide was also initially considered, pointing to this issue as an undiscovered research area.