Physicochemical Behavior of Concretes Admixed with Water-Based Polymers (PMC: Polymer-Modified Concrete)

Abstract

Portland cement concrete remains the most important of the materials used due to its mechanical properties combined with its low cost and ease of obtaining. However, conventional concrete structure suffers aging with gradual increase in porosity. This implies limitations and leads to the development of modifications to improve its general characteristics, like the so-called concrete modified by polymers that resulted in improvements in mechanical and microstructural properties, in relation to the original concrete. Concrete (cement)-polymer composites are the materials, which are made by replacing a part or all the cement hydrate binders of conventional mortar or concrete with polymers and by strengthening the cement hydrate binder with polymers. The composites are generally classified into three types by the principles of their process technology: 1. Polymer-modified mortar and concrete; 2. Polymer mortar and concrete; and 3. Polymer impregnated mortar and concrete. In this study, we evaluated the effect of the addition of the acrylic additive in the hydration reaction, comparing the result between the cement paste samples with and without added polymer. Beyond mechanical tests, such as compression resistance and traction, various thermal analysis techniques were used. The results showed great modifications on hydration process and mechanical behavior of PMC.

1. Introduction

In recent years, technical innovations in the construction industry have progressed considerably, and the research and development of high-performance and multifunctional construction materials have been actively pursued to cope with the innovations. Because of this worldwide interest in concrete (cement), polymer has become stronger.

Concrete (cement)-polymer composites are the materials that are made by replacing a part or all of the cement hydrate binder of conventional mortar or concrete with polymers and by strengthening the cement hydrate binder with polymers.

The concrete (cement)-polymer composites are generally classified into the following three types by the principles of their process technology:

1. Polymer-modified (or cement) mortar (PMM or PCM) and concrete (PMC or PCC)

2. Polymer mortar (PM) and concrete (PC)

3. Polymer-impregnated mortar (PIM) and concrete (PIC).

In this study, we considered only type 1—polymer-modified (or cement) mortar.

The Portland cement was modified by an acrylic polymer latex addition. The properties of the mortars and cements that were obtained by adding acrylic latex to Portland cement were an improvement in the mechanical properties and some changes in the hydration process. This addition alters the early-age hydration kinetics of polymer-modified cement. In other words, the physicochemical behavior of concretes admixed with water-based polymers (polymer-modified concrete).

Our experiments showed that the acrylic polymer retards the hydration process of the Portland cement. These changes were observed in different ways: mechanical properties and thermal analysis (TG/DTG—thermogravimetry and differential thermogravimetry; DSC—differential scanning calorimetry and semi-adiabatic calorimetry).

In the construction industry, many technical innovations have emerged in the latter years that have benefited the development of multifunctional and high-performance construction. Can be cited as examples the works buried as tunnels, buildings intelligent, and even lunar bases projects. In this way it is important, more and more, the development of new building materials that enable the care of these new high-performance requirements and include the environmentally friendly feature.

Portland cement concrete remains the most important of the materials used due to its mechanical properties combined with its low cost and ease of obtaining, which determine their advantages in relation, for example, to steel or other materials. However, conventional concrete has its limitations. Recent research shows that their structure suffers with aging, with a gradual increase in porosity as a whole [1]. This implies limitations and leads to the development of modifications to improve its general characteristics. One of these modifications led to the so-called concrete modified by polymers that, among other changes, resulted in improvements in mechanical and microstructural properties compared to the unlimited original concrete.

The concrete and mortar-modified mortars are increasingly used in applications such as tunnel coatings, reservoirs, roads, floors, repairs in old coatings, gluing for ceramic tiles, waterproofing, and chemical barriers.

In addition to meeting these new high-performance requirements already mentioned, the Portland cement concrete is exposed to a series of chemical species that can attack its microstructure. These chemical species can degrade concrete by dissolving its soluble constituents, such as calcium hydroxide. The dissolution of hydroxide calcium, in turn, leads to an increase in porosity by increasing the phenomena of percolation and diffusivity in this same microstructure.

Portland cement concrete can be modified by the addition of latices (aqueous polymeric dispersions), also known as latexes, changing their rheological and mechanical properties [2, 3]. Since 1950 in the USA, acrylic latex, defined as an anionic aqueous dispersion of an acrylic copolymer, has been studied as a modifier of mortar and Portland cement concrete with the objective of changing its original mechanical properties [3]. This modification with acrylics, which in general can be defined as a family of resins resulting from the acrylic acid polymerization, results in changes in the properties of healed concrete.

The rheological properties (fluidity), mechanical (compression resistance, traction, and abrasion), and permeability, which can be measured through resistance to ion penetration chloride [3]. In fresh concrete [3], for example, the addition of latex causes plasticizing rheological changes, besides reducing the WC factor (water/cement), which is a determining factor in the quality of the concrete.

The mechanism by which polymers interact with the main elements of Portland cement, such as silicates and aluminides, during the hydration reaction is of great importance in the study of its resistance to degradation. Various analysis techniques can be employed for the study of these interactions in the modified Portland cement microstructure. In addition to the traditional mechanical tests, the use of, among others, the scanning electron microscope [4], X-ray diffraction [4, 5], scattering of X-rays at low angles [6], spreading neutrons [7, 8], thermal analyzers [5, 9], and others can be mentioned.

In this work, we employ, beyond mechanical essays as resistance to compression and traction, the thermal analysis techniques, TG and DTG. In the present study, an effect evaluation of the addition of acrylic additive in the hydration reaction [10], comparing the result between the cement paste samples with and without added polymer. Beyond mechanical tests, such as compression resistance and traction, various thermal analysis techniques were used.

2. The Portland cement

Portland cement, as it is commonly known, is a mixture of compounds produced from the thermal decomposition of the limestone (CaCO3) and from clay at high temperatures ranging from 1400 to 1600°C with the addition of calcium sulfate after cooling. The production of Portland cement begins with the extraction of the limestone, which is then broken into small pieces by gigantic grinders. The ground limestone is then mixed with the clay, sand, and iron ore, apparently forming a homogeneous powder. However, this powder is microscopically heterogeneous [11, 12]. This resulting mixture is subjected to heat treatment at the temperatures mentioned above to obtain the clinker. By Portland cement clinker, the material is understood to be synthesized and pelletized, resulting from the calcination to approximately 1450°C of a proper limestone and clay mixture and eventually corrective components of silica, alumina, and even iron nature, employed to ensure the composition chemistry of the mixture within specific limits [11, 12]. This clinker, after cooling, is mixed with calcium sulfate for the correction of the catch time [11, 12], resulting in Portland cement itself.

3. Portland cement hydration: kinetic and mechanistic aspects

Portland cement contact with water triggers a series of reactive processes that lead to hydrated products resulting in a dense and stable microstructure. An important particularity of these complex reactions is that the initial reagents are in powder form.

The compounds present in Portland cement are anhydrous, but when put in contact with water, they react with it, forming hydrated products. Moisturizing cement Portland consists of the transformation of more soluble anhydrous compounds into compounds less soluble hydrated. During hydration, there is the formation of a gel layer in the grain of the original compounds, so that, in the transition zone (a zone intermediate between the primary crystal and the gel), the solution is supersaturated in relation to the hydrated compounds. The variations in concentration of solute and water generate a gradient of concentration, originating an osmotic pressure that will bring about the rupture of the gel, exposing new areas of the anhydrous compound to water action [12].

Two are the classic theories that seek to explain the hydration of Portland cement: Le Chatelier and Michaelis [12].

In Le Chatelier, crystallization hardening is explained by the shelter of crystals that are formed by the crystallization of a supersaturated solution of compounds that are hydrated but less soluble than the anhydrous. The binder, in definable terms, is a system of unstable anhydrous constituents that, in the presence of water, tend to give a system of stable hydrated constituents.

According to the colloidal theory of Michaelis, the hydration of the Portland cement gives rise to a supersaturated solution with the formation of crystals in needles and hexagonal reeds. There is formation of a hydrated monocalcic silicate, which gives rise to a gel colloidal of gellected mass, which imprints the crystals; the gel continues to drink water, the dough hardens and waterproof.

In this work, the percolation theory was used, where hydration is divided into three distinct phases: induction, nucleation, growth, and diffusion. More details of this theory will be presented below.

The study of hydration is important with the purpose of understanding, throughout its extension, as reactions occur throughout the process called cure. Studies of the hydration kinetics of each of the constituents of cement are found in the literature in isolation and usually serve as a reference [13, 14, 15]. Generally, the reactions that better represent the hydration process are mentioned below in a notation of the cement industry [16]:

1. 2C3S + 6H → C3S2H3 + 3CH

2. 2C2S + 4H → C3S2H3 + CH

3. 2C3A + 21H → C4AH13 + C2AH8

4. C4AH13 + C2AH8 → 2C3AH6 + 9H

5. C3A + 3CSH2 + 26H → C6AS3H32

6. 2C3A + C6AS3H32 + 4H → 3C4ASH12

7. C4AF + 3CSH2 + 29H → C6(A,F)S3H32 + (A,F)H3

8. C4AF + C6(A,F)S3H32 + 7H → 3C4(A,F)SH12 + (A,F)H3

It is interesting to note that in terms of reactivity with water, these different phases of minerals that make up Portland cement behave differently. The work performed by Jolicoeur and Simard [16] shows that the relative reactivity of the different mineral phases with water is usually given as C3A > C3S > C2S > C4AF (the absolute reactivities vary considerably depending on the degree of metal ion substitution in the phases and their crystal structure). Accordingly, the aluminate phases and their hydration products play an important role in the early hydration processes. In a general, simplifying sense, the early (O-l h) behavior of hydrating cements is governed by reactions of the aluminate phases, particularly C3A; the setting and early strength development behavior is mostly dependent on the hydration of silicates, particularly C3S. Because of the high reactivity of calcium aluminate and the undesirable properties of some of the products formed (e.g., hexagonal C-AH), the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminate sulfate products, namely ettringite (AFt) and monosulfoaluminate (AFm), i.e., reactions (5) and (6), respectively. Calcium sulfate added to the clinker can thus be viewed as a first ‘chemical admixture’ used to control the nature and properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration, and the influence of chemical admixtures on any process involving sulfates may be expected to be significant.

The overall process of cement hydration and setting results from a combination of solution processes, interfacial phenomena, and solid-state reactions. To help visualize the influence of admixtures on cement hydration, it is useful to recall the main events of the hydration process and the time scale over which they occur. A schematic representation of the evolution of a Portland cement hydration reaction with time is reproduced in Figure 1 (adapted from Jolicoeur and Simard-16). The latter identifies five distinct stages, the boundaries of which are determined by sharp variations in a reaction parameter, typically the heat flux measured as a function of time. These five stages correspond, respectively, to (times shown are approximate): I. Initial hydration processes (O–15 min); II. Induction period or lag phase (15 min–4 h); III. Acceleration and setting (4–8 h); IV. Deceleration and hardening (8–24 h); V. Curing (O–28 days).

Figure 1.

TG/DTG curves obtained with a heating rate of 10 K/min and a dynamic air atmosphere of 50 mm/min for an unmodified Portland cement sample with a hydration age of 28 days and a water/cement factor of 0.45.

Tricalcic silicate (C3S) is more reactive than dicalcic silicate (C2S), which, in turn, is more reactive than tricalcic aluminate (C3A), which has, approximately, the same reactivity as tetracalcic ironuminate (C4AF). According to these authors, complex reactions involving the mentioned species of chemicals cause the formation of a microstructure containing hydrated calcium silicates, represented by C-S-H; hydroxide in calcium, represented by CH; plaster; monosulfoalumines; and endothelium (a mixture of alumings and sulfates).

From the point of view of the reaction stoichiometry, according to the literature [16], for the Portland Type I (ASTM) cement, a water/cement ratio, known as the “water/cement factor (W/C), of 0.3 by weight is sufficient for the complete hydration of all phases of minerals that make up this same cement. In practice, however, this is not what occurs because the rheological characteristics such as viscosity should also be considered, which involve the processing of cementitious materials.

4. Portland cement samples evaluation

Figures 1 and 2 show the TG/DTG curves obtained for the cement samples unmodified and modified by the addition of latex, respectively. For the unmodified sample, two main stages of mass loss were observed. The first one can be attributed to dehydration that occurs from room temperature to approximately 600°C, with a total mass variation of 9.8%. At this stage, it was observed that between 25 and 200°C the mass loss is 5.5% and the DTG curve showed two distinct peaks at 45 and 87°C, indicating that in this material the water molecules are linked in different ways.

Figure 2.

TG/DTG curves obtained with a heating rate of 10 K/min and a dynamic air atmosphere of 50 mm/min of a modified Portland cement sample with the addition of 20% acrylic acid, a hydration age of 28 days, and a water/cement factor of 0.45.

A second mass loss, which occurs between 600 and 800^o C, corresponds to the release of 8.1% of CO₂ due to the thermal decomposition of 18.4% CaCO₃ present in the starting material. Figure 2 shows the presence of the polymer, whose pyrolysis peak occurs at 350.19°C. The temperature peak at 91.85°C indicates that after adding the polymer, the water combines differently because the peak temperature without the addition of polymer is 87.58°C.

The Portland cement hydration process is exothermic [17] and has a great influence on the final properties of mortars and concrete obtained. The temperature variations that occur inside large masses of concrete are the subject of study by many researchers.

One of the ways of studying the kinetics of this hydration and verifying the influence of factors such as the polymer concentration in relation to the weight of cement [18], among others, is by monitoring the heat generated over time by semi-adiabatic calorimetry. In this technique, the heat of reaction released due to cement hydration, modified or not, is recorded in the form of temperature under semi-adiabatic conditions. This technique allows the monitoring of temperature variations that occur in the hydration of Portland cement by the addition of latex. As can be seen, for example, in Figure 3.

Figure 3.

Hydration temperature as a function of time for unmodified Portland cement paste (0% latex addition).

The temperature versus time curves were obtained from 0 to 24 hours and plotted in graphs where the changes that occurred in the phase known as nucleation and growth of the hydration reaction were compared.

Due to the characteristics of the semi-adiabatic calorimeter used in the test, the nucleation and growth phases were isolated because they present the greatest manifestation of heat in relation to the dormant and diffusion phases in the hydration reaction. The linear regressions of each one of the hydration temperature curves of the Portland cement paste as a function of time show how the addition of the acrylic additive, in terms of concentration, alters the kinetics of the hydration reaction.

Figure 3 is important because, in a qualitative way, the following hydration phases of Portland cement can be observed:

1. An induction period that starts at time t = 0 hours and ends at time t = 2 hours, under the conditions of the experiment. Also known as the dormant period, this is the phase in which the cement constituents undergo irreversible dissolution in water.

2. Nucleation and growth that begin at the end of the induction period and last until time t = 11 hours. It is in this phase that most of the hydrates and calcium hydroxide are formed, and most of the heat of the hydration process is released.

3. Diffusion, which starts right after the end of the nucleation and growth phase, represented in the graph by the peak temperature of 70°C and is limited by the speed with which water can diffuse through the crystalline microstructure formed to find and react with the remaining silicates and aluminates.

The semi-adiabatic calorimetry, unlike the adiabatic one, allows the clear observation of the phase change between nucleation and growth and of diffusion. This is because, as there is a relative loss of heat from the system to the environment in the phase change due to the drop in the rate of heat release, there is a sharp drop in temperature as observed in Figure 3 (a peak of 70°C).

Once the hydration temperature of the unmodified Portland cement paste had been monitored under semi-adiabatic conditions, the same experiment was carried out for samples of Portland cement pastes modified by the addition of different levels of acrylic latex, as shown in Figure 4.

Figure 4.

Hydration temperature versus time as a function of modifications by the addition of acrylic latex to Portland cement.

From the observation of the graph in Figure 4, it can be seen how the addition of the lattice, in terms of a percentage increase, changes the generally accepted reaction mechanism for the phases or periods:

1. Induction. There is a small change in the final time of this period depending on the increase in the percentage of incorporated latex content.

2. Nucleation and growth. It is the most affected phase and is undergoing major changes, mainly in terms of heat generated;

3. Diffusion. Directly affected due to the greater difficulty for water to come into contact with the unreacted chemical species of cement after the nucleation and growth phases.

In general, unmodified cement obtains the highest peak temperature and therefore generates the most heat, which decays as the polymer concentration increases. This is explained because the acrylic latex acts as a retarder in the Portland cement hydration process.

On the other hand, its use in the modification of Portland cement leads to a decrease in the heat generated during hydration. This can be especially beneficial for situations where the addition, due to the complexity of the part or construction system, has a limiting factor in the quality of the concrete obtained in the release of heat.

The small increase in the induction period, the period in which the mortar and/or concrete are transported and cast, is also beneficial from a quality point of view, as it allows a longer time between manufacture and final molding.

Possible interactions between the polymeric latex and the cement particles occur, leading to this delay in the hydration process. Among these interactions, the following are presumed: reduction of the initial dissolution capacity of the chemical compounds of cement in water due to the presence of latex; formation of a polymeric film around the grains of the chemical compounds of cement, preventing the reaction with water; formation of complex compounds between polymeric particles and calcium ions in solution, with consequent reduction in the formation of calcium hydroxide crystals.

5. Conclusions

The modification of Portland cement by the addition of acrylic latex alters the behavior of its physical–chemical properties. When considering the hydration kinetics, the results obtained through the use of thermal analysis techniques allow us to conclude that the addition of polymers modifies the hydration kinetics of Portland cement at the considered initial ages. Probably due to the encapsulation effect caused by the involvement of Portland cement powder unreacted by the polymer, the contact and subsequent reaction with water becomes difficult.

Also, due to a decrease in porosity as the empty spaces tend to be filled by the polymeric chain, there is an increase in the difficulty of water diffusion in the resulting microstructure during the diffusion phase.

It can be noted by observing the TG and DTG curves that water tends to have two distinct behaviors: Without the addition of polymer, a peak is observed at 87°C; with the addition of 20% latex, the water peak shifts to 92.35°C.

This behavior indicates that water, when the polymer is added, tends to bind in the microstructure of hydrated Portland cement in a different way. The same behavior is verified for CaCO₃, whose peak for the unmodified Portland cement paste is observed at 771.65°C. When modified by the addition of latex, this peak shifts to a temperature of 760.34°C. Another peak at 716.68°C may indicate the presence of water bound to the C-S-H microstructure.