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Review

An Insight into the Chemistry of Cement—A Review

by
Luca Lavagna
1,* and
Roberto Nisticò
2,*
1
Department of Applied Science and Technology, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy
2
Department of Materials Science, University of Milano-Bicocca, INSTM, Via R. Cozzi 55, 20125 Milano, Italy
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2023, 13(1), 203; https://doi.org/10.3390/app13010203
Submission received: 23 November 2022 / Revised: 15 December 2022 / Accepted: 21 December 2022 / Published: 23 December 2022
(This article belongs to the Special Issue Concrete Structures: Latest Advances and Prospects for a Sustainable Future)
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Abstract

:
Even if cement is a well-consolidated material, the chemistry of cement (and the chemistry inside cement) remains very complex and still non-obvious. What is sure is that the hydration mechanism plays a pivotal role in the development of cements with specific final chemical compositions, mechanical properties, and porosities. This document provides a survey of the chemistry behind such inorganic material. The text has been organized into five parts describing: (i) the manufacture process of Portland cement, (ii) the chemical composition and hydration reactions involving a Portland cement, (iii) the mechanisms of setting, (iv) the classification of the different types of porosities available in a cement, with particular attention given to the role of water in driving the formation of pores, and (v) the recent findings on the use of recycled waste materials in cementitious matrices, with a particular focus on the sustainable development of cementitious formulations. From this study, the influence of water on the main relevant chemical transformations occurring in cement clearly emerged, with the formation of specific intermediates/products that might affect the final chemical composition of cements. Within the text, a clear distinction between setting and hardening has been provided. The physical/structural role of water in influencing the porosities in cements has been analyzed, making a correlation between types of bound water and porosities. Lastly, some considerations on the recent trends in the sustainable reuse of waste materials to form “green” cementitious composites has been discussed and future considerations proposed.

1. Introduction

Cement is a hydraulic binder; it consists of a finely ground inorganic material which forms a paste when mixed with water, is able to set and harden because of numerous exothermic hydration reactions (and processes), and is thus capable of binding fragments of solid matter to form a compact whole solid [1,2,3]. After hardening, cement retains its strength and stability, even under the effect of water. Cement forms a composite defined as mortar when mixed with water and fine aggregate (i.e., sand), whereas it forms concrete when mixed with water, sand and gravel (i.e., small stones) [4]. Cement-based materials, such as concrete, have been used for many centuries, mostly in the construction and civil engineering fields, thus becoming the most widely used material, and the second most consumed resource on Earth [5]. Among the different types of cement, ordinary Portland cement (OPC) is the most widely used one [6].
Even if the technology of cement seems quite well established, recent discoveries in nanotechnology and materials science (e.g., the discovery of graphene) opened the possibility of inducing novel smart functionalities in cement and concrete, allowing their use in advanced technological applications. Examples of such systems are self-healing systems [7,8], health-monitoring systems [9,10], conductive materials [11], water permeable materials [12], thermal energy storage materials [13], rubberized concrete [14,15,16,17], and sustainable composites containing bio-based fillers [18,19,20].
From the economic viewpoint, even if cement-based materials are low-cost products (with a maximum cost of 155 EUR ton−1), cements still represent raw materials of remarkable interest for the global market (i.e., cement accounted for 0.074% of total world trade in 2020). Countries leading in the worldwide exportation of cement in 2020 were Vietnam (USD 1.5 billion, covering almost 12% of the global market), Turkey (USD 1.3 billion, 10.3%), United Arab Emirates (USD 578 million, 4.7%), Thailand (USD 574 million, 4.6%), and Germany (USD 528 million, 4.3%) [21]. On the other hand, the largest global importers of cement-based materials are the US (USD 2.3 billion, 10.5%), China (USD 1.2 billion, 9.7%), Bangladesh (USD 513 million, 4.1%), Philippines (USD 464 million, 3.7%), and France (USD 452 million, 3.7%) [21].
This document aims to provide a useful survey of the chemistry behind cement and cement-based materials. Particular attention has been dedicated to the key role played by water in the hydration reactions occurring in OPC, and the different types of compounds generated during this step. In this context, the recent literature [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41] clearly highlights the pivotal interest still present in the development of new advanced cement and cementitious materials. Even if cement is a well-consolidated material, the chemistry of cement (and the chemistry inside cement) remains very complex and not obvious. Therefore, in this study, the different chemical compounds generated during hydration and setting are clearly investigated, keeping an eye on their mechanisms of formation. Furthermore, the physical and structural role of water in driving the entire process of pores formation has been considered. Lastly, a dedicated section regarding the latest developments in the field of sustainable recycling for the construction industry has been proposed, with a special focus on the use of recycled (waste) materials to produce a new generation of “green” concretes and mortars.

2. The Manufacture of OPC

OPC is a fine grey/white powder consisting of a mixture of calcium silicates, aluminates, and aluminoferrites. These raw materials can be classified into four distinct groups: calcareous, siliceous, argillaceous, and ferriferous. Such raw materials are thermally treated via pyrolysis and mechanically processed in order to obtain the desired product with specific compositions and defined mechanical properties. Cement manufacturing implies four different steps: quarrying, raw material preparation, clinkering, and cement preparation [42]. Figure 1 schematically represents the production line of an OPC [43]. The entire production process begins with the extraction of the previously described raw materials that are rock mixes mainly constituted by ca. 80% of limestone (i.e., primarily calcium carbonate mineral CaCO3) and 20% of either clays or shale (i.e., a source of silica SiO2, alumina Al2O3, and hematite Fe2O3) [44].
Once raw materials are extracted, they are pre-crushed inside a quarry. Subsequently, crushed raw material is prepared by applying a variety of blending and sizing operations to confer appropriate chemical and physical properties to the feed. These processing steps can be divided into two different approaches, namely, either dry or wet. Concerning the dry route, both limestone and clays are independently crushed, and then fed together inside a mill. The wet route, instead, requires clays to be mixed forming a paste inside a wash mill (i.e., a tank where clays are grinded in the presence of water), and crushed limestone is added only at the end. Finely ground materials are dried, thermally treated inside a kiln, and then cooled down. This series of steps is defined as clinkering and it confers the main relevant features of the OPC manufacturing. In fact, clinkering reshapes the starting raw materials mixture into clinkers, which are grey, spherically shaped nodules whose diameter is 5–25 mm in size [45]. The chemical reactions occurring inside the kiln, kept heated by the spontaneous ignition of the pulverized coal dusts introduced inside the system due to the high temperatures, is still not completely understood. These chemical reactions are influenced by several factors, such as the large variety in terms of starting chemical compositions, the operating parameters of processing, and the difficulties in performing an efficient in situ sampling under the processing conditions (i.e., high temperatures). However, the chemical reactions and physical processes involved in this step can be rationalized as follow:
  • Evaporation of the physically sorbed water molecules from the raw mix (20–100 °C).
  • Dehydration (100–430 °C) with the production of oxides, such as silica, alumina, and hematite.
  • Calcination (800–1100 °C) with the development of calcium oxide, according to the carbonate decomposition reaction:
    CaCO3 → CaO + CO2
  • Exothermic reactions (1100–1300 °C) with the formation of secondary silicate phases:
    2CaO + SiO2 → 2CaO · SiO2
  • Sintering and reactions occurring inside the melt (1300–1450 °C) with conversion of secondary silicate phases into both ternary silicates and tetracalcium aluminoferrites:
    2CaO · SiO2 + CaO → 3CaO · SiO2
    3CaO · Al2O3 + CaO + Fe2O3 → 4CaO · Al2O3 · Fe2O3
  • Cooling of the system, and the crystallization of the other mineral phases.
Finally, the resulting clinker is further united with gypsum and then ground. From the energetic viewpoint, the grinding process consumes ca. 60% of the total electrical energy involved in a standard cement plant (high cost). In a conventional cement making process, the electrical energy consumed is approx. 110 kWh/tons, with ca. 30% used for raw material preparation and ca. 40% for the grinding step. The resulting cement is pneumatically pumped toward specific silos for storage and is subsequently drawn either for packing inside paper bags or for storage inside bulk vessels [46].

3. Hydration Step Involving OPC

Hydration refers to chemical reactions between anhydrous compounds with water molecules, giving a plethora of hydrated compounds. The hydration phenomenon occurring in cement consists of the chemical reaction between non-hydrated cement and water that allows for important physicochemical transformations and mechanical changes. In this context, a distinction should be made between the partial hydration by means of the moisture contained in an air atmosphere, and the hydration achieved by the direct mixing of the cement with water [47]. In this last case (i.e., direct mixing), several chemical reactions take place, inducing the conversion from a workable paste with plastic features to a hard solid. Furthermore, since the hydration of OPC consists of a multitude of simultaneous single chemical reactions involving the different main components forming the cement powder with water, the resulting pathway is rather complex and non-obvious, and its degree of complexity is strongly influenced by the starting composition of the clinker that directly affects the transformation occurring within the cementitious matrices.

3.1. Typical Starting Composition of Cement and Mineral Phases

The starting composition of clinker is generally indicated by the oxide content (in wt.%). The main relevant oxides forming an OPC clinker are CaO, SiO2, Al2O3 and Fe2O3. These oxides form different mineral phases when forming the clinker. Since clinker primarily contains these four chemical species, it is possible to represent these phases with the Bogue formulae, thus indicating CaO with C, Al2O3 with A, Fe2O3 with F, SiO2 with S, and water with H [26]. Furthermore, there are also several further minor components deriving from natural clays, e.g., sodium (Na), potassium (K), magnesium (Mg), and other metal ions [48,49]. Table 1 summarizes the chemical composition of OPC, expressing the single component content in wt.%. Additionally, Table 2 describes the approximate composition of the cement clinker [50].

3.2. Chemical Reactions Occurring during the Hydration Process

A series of separate/independent (parallel and/or sequential) reactions involving water molecules and the principal mineral phases forming the cement led to the hydration of OPC. In general, the hydration process is strongly driven by the starting chemical composition, the dimensions and size (e.g., specific surface areas, roughness, and particle size distribution), the quantity of water added, the water-to-cement ratio (w/c), the curing temperature, and the presence of additives [51]. In order to simplify the comprehension of the process, the following hydration reactions are discussed separately by considering minerals individually. Figure 2 reports the principal hydrated products in OPC [52].

3.2.1. Reaction Involving C3S

The principal constituent of the OPC is the tricalcium silicate (C3S, or alite). Alite largely affects the cement mechanical properties (strength) and the hardening process. During the alite hydration, four stages may be distinguished. The first one is the pre-induction period (duration time: a few minutes, just after being exposed to water) where the rapid hydration of C3S occurs. When C3S is exposed to water, at the surface level, ions start to a rapidly dissolve, with release of hydroxyl and silicate ions in the liquid phase. As a result, the positive charge of Ca2+ is counterbalanced by the negative ones due to silicate and hydroxyl anions. Since the dissolution rate of C3S is faster than the diffusion rate of ions at the surface level, the liquid phase rapidly turns oversaturated considering the silicate hydrated phase. The direct consequence of this phenomenon is the precipitation of a first layer of C-S-H (i.e., tobermorite) nearby the surface. A high heat of hydration is produced in this first step of reaction [53]. Immediately after, the rate of hydration slows down and then increases, with a second heat release. Finally, the hydration process slows down again, entering the deceleration step [54]. The explanation of this specific behavior is the object of several theories, and some considerations regarding kinetic aspects are reported in the literature [55].
C-S-H is the main relevant binding phase, and is chemically defined as CaO · SiO2 · H2O. The stoichiometry of C-S-H in the cement paste is variable (and this is particularly true for the bound water). Additionally, a secondary product, calcium hydroxide or portlandite (i.e., Ca(OH)2, or CH), is generated in the case of a CaO/SiO2 molar ratio which is smaller than the alite one (namely, 3:1).

3.2.2. Reaction Involving C2S

In analogy with the previously described alite, the hydration of dicalcium silicate (C2S, or belite) also requires different reaction steps. In general, the induction time is significant, with a low hydration rate. Then, a second slight increment in the hydration rate occurs. Finally, the hydration rate reduces again in the third step. Belite evolves into both C-S-H and CH [56].

3.2.3. Reaction Involving C3A

Tricalcium aluminate (C3A, or celite), the most reactive component present in cement, is the third species involved in the hydration mechanism. Its role in the first step of hydration is of paramount relevance as it influences both the mechanical and rheological properties. Furthermore, gypsum can determine the chemistry of the products obtainable through the hydration of celite. Without gypsum, the formation of a gel-like material can be registered at the surface of cement as the first hydration product. Proceeding with the hydration, this material transforms itself into hexagonal crystals of C2AH8 and C4AH13 (or C4AH19). These products convert into cubic C3AH6. It is important to note that both C2AH8 and C4AH19 are metastable products, whereas the cubic C3AH6 is the calcium aluminate hydrate form stable at RT [57]. The fast precipitation of hexagonal platelets causes the phenomenon defined as “flash set”. A high temperature of hydration corresponds with a high rate of conversion towards the most stable hydration products. Additionally, at temperatures higher than 80 °C, C3AH6 directly forms from the hydration of C3A. Both C2AH8 and C4AH19 are considered as Afm, which indicates a subfamily of mono-hydrated calcium aluminate phases structurally related to hydrocalumite, with the representative formula of [Ca2(Al,Fe)(OH)6] · X · xH2O, where X corresponds to an exchangeable anion.
To prevent the fast setting of the paste, and elongate the period of workability, gypsum is usually added to the clinker. With gypsum, the principal hydration phase is ettringite (or C6AS3H32), which belongs to the Aft group, which indicates a subfamily of three-hydrated calcium aluminate phases with the representative formula [Ca3(Al,Fe)(OH)6 ] · X3 · xH2O, where X equals a doubly charged anion [58,59]. If sulfate ions are available in the system, the ettringite phase remains stable. When gypsum is fully consumed, ettringite chemically reacts with further C3A, forming Afm. The gradual consumption of ettringite, instead, favors the growth of hexagonal calcium aluminate hydrate, C4AH13 [60].

3.2.4. Reaction Involving C4AF

The last component that participates in the hydration reaction is calcium aluminoferrite (C4AF, or brownmillerite). This compound is the only variable phase present in cement with an unfixed composition. Brownmillerite constitutes a wide category of solid solution Ca2(AlyFe(2−y))O5, where the values of y are in the 0.00–1.33 range. Analogous to the previous component C3A, the hydration of C4AF is affected by the presence/absence of gypsum. The absence of gypsum favors the reaction of C4AF with water, forming metastable C(A,F)H hydrates (defined as hydroxy-Afm phase) onto the surface of the non-hydrated grains, and eventually the stable phase C3(A,F)H6. Experimentally, it has been demonstrated that the Al3+/Fe3+ ratio is usually lower in the products formed by the reaction with water with respect to the original C4AF [61]. The reduction in Al3+/Fe3+ in the hydrated products can be clarified by means of the production of a secondary by-product, namely, an amorphous iron-rich gel or iron hydroxide [62].
Conversely, in the presence of gypsum, C4AF produces hydration phases analogous to the case of C3A. However, the reaction rates of both components are quite different. In fact, in the presence of gypsum, C4AF reacts much slower than C3A, i.e., gypsum retards the hydration of C4AF more efficiently than in the case of C3A. However, the rate of hydration of C4AF is affected by the composition of the ferrite phase: the high amount of Fe corresponds to a low rate of hydration [63]. Therefore, gypsum favors the AFt phase as the main product of reaction, whereas in a successive reaction step, the AFt phase evolves into AFm by reaction with further C4AF.

3.3. Mechanism and Heat of Hydration of OPC

The kinetics of hydration defines both the cement microstructure and its final properties. The cement–water reaction causes both the dissolution of the anhydrous phases and the precipitation of the hydrated ones. Hence, its evolution with time is subject to the kinetics of dissolution [64], and the growth of the hydrated crystals with their rate of nucleation [65]. Figure 3 reports the kinetics of hydration for an OPC [1]. The hydration process of cement can be organized into four stages: pre-induction, induction (dormant), acceleration, and a post-acceleration period. Every step is discussed in the following paragraphs [66].

3.3.1. Pre-Induction Period

Once cement interacts with water, ionic species rapidly go in the solution allowing the formation of hydrates. From the dissolution of alkali sulphates, there is the release of free potassium, sodium, and sulphate ions. Gypsum dissolves until saturation, releasing Ca2+ and SO42−.
The literature evidences the creation of a coating growing over the surface of the cement grains immediately after contact with water [67,68]. This product is recognized as being C-S-H. The layer of C-S-H grows, covering most of the grain surface, until the exhaustion of reactants available at the surface. According to Thomas et al. [69], when the content in the C3S product is higher than the CaO/SiO2 ratio, during this first step of hydration, there is an increment in the concentration of Ca2+ and OH− in the liquid. Furthermore, adding water to the cement leads to the rapid dissolution of both gypsum and clinker minerals, producing Ca2+, OH−, SO42− ions, and ionic concentration differences through steps in the liquid phase, and causing the supersaturation phenomenon necessary for the formation of the ettringite phase.
Moreover, Jiahui et al. [70] pointed out that the formation of octahedral [Al(OH)6]3− is the time-determining step in controlling the AFt growth. Furthermore, it has been demonstrated that the ionic concentration affects the kinetic rate of the reaction. Among the different ions forming ettringite, [AlO2]− is the one with the lowest concentrated one, and is consequently the limiting species. There are only a small quantity of C2S hydrates within this first step. Furthermore, the belite (i.e., C2S) reaction rate is similar, but slower, in respect to that of C3S, and this is probably attributable to both belite’s high thermodynamic stability in respect to alite, and its different crystalline structure. Lastly, C2S presents high density, and the presence of holes/pores/channels in the structure of C3S facilitates the reaction with water [71].

3.3.2. Induction (Dormant) Period

After the fast hydration step of cement, the general hydration rate slows down for some hours. The explanation of this sharp slowdown is probably attributable to two main theories, considering both the apparent low reactivity of C3S and its high solubility calculated from the enthalpy of formation. The first theory is named as the theory of the protective membrane, and it considers the decrease in the hydration rate attributable to the formation of metastable C-S-H layers covering the reacting surface of cement particles, thus preventing the fast dissolution ability of C3S [72]. Several studies reported in the literature evidenced the tendency of C-S-H to rapidly form once it has interacted with water [73]. In principle, the formation of a low-permeable layer of hydrates, with enough density and coverage, should be able to slow down the ion diffusion rate from the anhydrous cement grains. However, experimental evidence confirms that C-S-H does not form a continuous layer around the grains. In this way, there is still contact with the pore solution, thus weakening this theory [74].
The second theory, the theory of dissolution, instead suggests that the slowdown rate is principally due to the decrease in the kinetic of dissolution of both C-S-H and alite when the system is evolving towards the equilibrium condition. In this context, it should be highlighted that the dissolution of C3S has been mostly simplistically considered in all models addressing the hydration kinetics of cementitious matrices. This is probably caused by the dissolution step, which in most cases has been studied independently from the other mechanisms that characterize the hydration step. As dissolution occurs together with the precipitation of hydrate phases at the surface, this might interfere with the dissolution process involving the cement components. Hence, this theory remains valid, but the role of the kinetics of dissolution should be highlighted [75].

3.3.3. Acceleration Period

During this step, hydration accelerates once again. The C3S hydration favors the formation of the “outer” C-S-H, which is different from the “inner” C-S-H. Tennis et al. [76] proposed a model describing the formation of calcium silicate hydrate, making a distinction between low-density (LD) C-S-H (“outer” product), and high-density (HD) C-S-H (“inner” product). Figure 4 shows a C3S grain after 96 h of hydration, where both C-S-H products are present [77]. In particular, the “outer” C-S-H forms during the early hydration step. It is easily recognizable as it remains away from the cement particle surface, and it is highly porous. On the other hand, the “inner” C-S-H forms during the late hydration step, and it has low porosity. Furthermore, the amount of “inner” C-S-H increases as the w/c ratio decreases [78].
During this step, a noticeable hydration of C2S is also registered. Moreover, Portlandite (i.e., CH) precipitates from the liquid, thus decreasing the amount of Ca2+ ions in the liquid. Gypsum, instead, completely dissolves. Quite surprisingly, the concentration of SO42− anions in the liquid decreases, probably because of the occurrence of two phenomena: (i) the formation of the AFt phase, and (ii) the adsorption of SO42− ions at the surface of the C-S-H phase.

3.3.4. Post-Acceleration Period

During this step, the hydration rate decelerates, and the “inner” C-S-H starts forming from hydration reactions involving both C3S and C2S. According to Bazzoni et al. [79], the acceleration period involves the nucleation (and growth) of C-S-H clusters onto the surfaces of the cement grains. The deceleration period, instead, involves a substantial decrease in the growth rate of C-S-H that, at this point of the process, covers much of the grains surface. In fact, this final period of hydration involves a low hydration rate, probably caused by the less available space. In particular, the formation of hydrates occurs only in space filled with water. Such a lack of available space is attributable to two phenomena: (i) the depletion of the water volume (necessary for hydrates’ precipitation), and (ii) the sequestration of the remaining water into pores whose critical pore sizes are smaller for precipitation [80].
Moreover, together with the formation of “inner” C-S-H, there is a continuous consumption of gypsum, until it is fully consumed. In this way, the concentration of the sulphate ions in the liquid decreases, and the AFt phase (produced during the earlier steps of hydration) begins to chemically react with further C3A and C2(A,F), thus forming the AFm phase [81].

3.3.5. Consideration over the Heat of Hydration

Figure 5 reports the heat of hydration of an OPC over the hydration time [1]. The curve profile evidences six different thermal phenomena. The first one is an initial sharp endothermic peak soon after mixing, probably due to the dissolution of potassium sulphate in water (i.e., this contribution is present only in cement containing potassium sulphate, as seen in Figure 5, step 1). The second one is an intense exothermic peak (with a maximum centered in the very first minutes) attributable to the initial hydration reactions involving C3S, C3A, and gypsum (Figure 5, step 2). The third one corresponds to the induction (dormant) period, and it is associated with a minimum of the heat of hydration (Figure 5, step 3). The fourth one is an intense exothermic peak attributable to the hydration of C3S and the consequent conversion into C-S-H and CH (Figure 5, step 4). The fifth one corresponds to a small descending branch of the principal peak, probably attributable to the AFt formation (Figure 5, step 5), whereas the sixth one corresponds to another small descending branch of the principal peak attributable to the Aft–AFm conversion (Figure 5, step 6) [1].

4. Setting of OPC

The setting time is the elapsed time from the addition of water to a cement mixture until it reaches a specified level of rigidity (measured by a specific technical procedure). Setting consists of the conversion of a plastic cementitious paste into a set material, which is no longer deformable [1]. This conversion is gradual and continuous, and the setting time is usually quantified by measuring the penetration resistance by means of a Vicat needle (i.e., ASTM C191-21) [82]. In general, setting is usually preceded by a stiffening of the paste (with an increment in the material viscosity, even if the mixture does not lose its plastic behavior). Furthermore, it is important to distinguish between the term “setting” and “hardening”. Hardening occurs after the setting step, and it refers to an increase in terms of mechanical properties (i.e., Young’s modulus, strength, and hardness) until the material reaches the final value of these ones.

4.1. Mechanism of Setting

The mechanism of setting passes through the contact between water and the OPC grains. Quite soon, formed particles of cement undertake flocculation during mixing, thus increasing the viscosity of the cement. After a few minutes of mixing, only coarse (approx. 10 µm) and fine (approx. 3 µm) particles are dispersed in the medium (i.e., water), with the formation of few aggregates of coarse particles, which entrap a fraction of the water. During the induction (dormant) period (i.e., hydration of C3S), flocculation is reversible, and aggregates are re-dispersed by remixing the paste. Once the acceleration period starts, C-S-H starts to precipitate from the AFt phase, and it is registered as an increment in the fraction of hydrated material and a consequent decrement in the volume of liquid. Thus, particles agglomerate with the fine ones. The principal hydrated compound is C-S-H, which is strongly 3D-connected, and agglomeration cannot be re-dispersed by simply mixing. By continuing with the hydration process, the quantity of products continues growing and the bonding between particles strengthens, thus providing a gradual increment in the strength of the cementitious set paste. Figure 6 reports the mechanism of the flocculation of the cement paste [83,84].

4.2. Flash Setting vs. False Setting

At this point, it is mandatory to clarify the difference existing between flash setting and false setting. “Flash setting” consists of the fast loss of plasticity of the pastes once mixed with water, thus reducing the effective time of workability of the cement. Such a rapid transformation is caused by the increment in the early reactions involving aluminate and ferrite chemical species, with the production of plates of AFm all over the material, and the release of a high heat of hydration [65]. The meshwork of AFm plates, responsible for minimizing the possibilities of remixing, is overcome by the more compact coatings of AFt phases formed in presence of gypsum, which is a set regulator. Gypsum dissolves in water, releasing Ca2+ and SO42− ions. As previously discussed, the quantity of hydrated C3A and C4AF in the pre-induction period is reduced, with the consequent formation of the AFt phase during hydration. This AFt phase precipitates at the surface of the cement grain forming a microcrystalline layer. In this way, the flowability and plasticity of the cement paste is maintained until the formation of the hydrated phases, such as C-S-H that induces the “normal setting”.
On the other hand, the term “false setting” refers to the formation of secondary gypsum (i.e., calcium sulphate dehydrated) crystals, in the presence of either a very low amount or an absence of C3A. In this case, this “false setting” is caused by the interlocking of gypsum crystals, and, simply, intensive mixing can restore the plasticity of the mixture. “False setting” is typical for cements containing high concentrations of either K2O (i.e., precipitation of K2Ca(SO4)2), or C3A (i.e., excessive formation of the AFt phase).

5. Pore Structure in OPC

5.1. Type of Porosities

In hardened cementitious materials, there are four types of pores, namely: gel pores, capillary pores, hallow-shell pores, and entrapped/entrained air voids [85].
Gel pores (size of approx. a few nm) form within the hydrated phases at the interface between the cement grains with the liquid. Due to their small size and the great affinity between gel surfaces with water molecules, the movement of the liquid water within gel pores does not contribute to the cement paste permeability.
Capillary pores (sized between a few nm to a few microns) form from the spaces in the fresh paste initially filled with water. During hardening, such spaces evolve into either interconnected channels or gel-pore interconnected cavities. Capillary pores are highly irregular, and their presence is due to the volume of water used during the hydration process. Since a high w/c ratio causes high porosity in the hardened paste, capillary pores are typically formed in formulation with w/c ratios larger than 0.42 [86].
Hollow-shell pores (size of several microns) are closed, ink-bottle pores that form within the void spaces at the borders of cement grains as they move back during hydration, whose shape is a relict of the cement particles.
Entrapped air voids form during mixing due to the high viscosity of the paste. These voids are irregular and typically isolated from each other. Entrained air voids, instead, are intentionally formed during the cement mixing. These voids are uniformly distributed, spherical in shape, and not interconnected with each other [86].

5.2. Type of Water

The introduction of water within the initial mixing of the cementitious paste allows for the formation of hardened cement through hydration. Moreover, there are three different categories of water: (i) chemically bound water, (ii) physically bound (gel) water, and (iii) free (evaporable) water. Chemically bound water forms a solid cementitious paste. Gel water, instead, is the water physically bound to C-S-H gel. The sum of chemically and physically bound water provides the minimum amount of water to fully hydrate a given quantity of cement, thus fixing the w/c ratio at 0.42 [86,87]. Lastly, free (evaporable) water is the water contained in the capillary network of pores.

5.3. Bleeding Phenomenon Involcing the Action of Water in Cementitious Paste

Bleeding is a particular type of sedimentation, where a definite volume of mixing water remains separated at the surface of the cementitious matrix. The implication of this is the reduction in the final volume of the hardened paste soon after the placement, resulting in interparticle distance reduction and variation in the effective w/c ratio, which results being smaller than the initial one. The properties of cement paste showing bleeding are defined by the sedimentation rate and the volume of bleeding water, calculated as the difference between the initial mixing water and the water effectively inside the cement. The bleeding-induced effect increases with the w/c ratio, thus for a high w/c ratio, it is mandatory to add anti-settling agents. Examples of anti-settling agents are bentonite, hydro-soluble polymers, and inorganic salts. Such anti-settling agents act in different ways, namely: adsorbing large quantities of water to preserve the homogeneity of the slurry (i.e., bentonite) [88], increasing the cohesiveness between cement hydrates and the viscosity of the medium (i.e., hydro-soluble polymers) [89], or entrapping water within weak-bounded hydroxide structures formed within the slurry volumes (i.e., inorganic salts) [90].

6. Future Perspective in the Use of Recycled (Waste) Materials in Cementitious Matrices

The use of recycled (waste) materials in the construction sector can achieve significant benefits in terms of both environmental preservation (i.e., saving natural resources, reducing the amount of greenhouse gas emissions and energy consumption) and processing costs. Quite recently, several studies report the use of waste materials as a substitute counterpart for the aggregate fraction in the preparations of concrete and mortar, trying to either maintain or (better) improve the final mechanical properties. The use of materials deriving from the demolition of structures (i.e., construction and demolition waste, CDW) is one of the major studied waste substrates that attract the interests of scientific literature. For example, Villoria Sàez et al. [91] proposed to examine and compare CDW generation in all EU member states in correlation with their respective national construction businesses, gross domestic product, and capital, together with an assessment of the policy framework and CDW recovery performance of each member state against the recovery target of the waste framework directive. The results show that Austria, Germany, the Netherlands, Belgium, and France are the highest CDW producers, whereas Croatia, Slovenia, Slovakia, Poland, Portugal, and Spain were the lowest ones. Liikanen et al. [92], instead, evaluated the role played by raw materials for wood-plastic composites (WPC) to achieve the CDW recovery target in Finland. Specifically, the objective of this analysis was to assess the environmental impacts of WPC production using specific CDW fractions (i.e., wood, plastic, gypsum board, and mineral wool) as raw materials, and to compare these impacts with the traditional situation in which these CDW fractions are treated by conventional methods. The results indicate that, compared with the traditional situation, the environmental impacts of CDW management can be reduced when CDW fractions are used in WPC production. Moreover, Coelho et al. [93] evaluated the economic implications of traditional demolition and selective demolition, analyzing a case study in Portugal. Several scenarios are considered, based on possible waste management options, some of which favor selective demolition over a conventional one. By considering the Italian situation, Borghi et al. [94] applied life-cycle assessment (LCA) methodology to assess the environmental performance of construction and demolition materials in the current context of the management of the Lombardy region, identifying critical aspects related to the management system of non-hazardous CDW and possible actions for improvement (e.g., increment and refinement of the quality of the recycled aggregate). Moving away from Europe, Contreras et al. [95] analyzed a case study in Brazil where CDW was used to produce new building materials, replacing the natural aggregate fraction to produce novel (sustainable) bricks showing superior average compression strength than standard bricks. This finding shows that it is possible to produce low-cost bricks with excellent physical properties using CDW as an aggregate and lime/cement as an additive. Ossa et al. [96] conducted a study regarding the use of recycled CDW aggregate to create asphalt mixtures for urban roads. Numerous tests were conducted to evaluate the susceptibility of the asphalt concrete samples to moisture damage and plastic deformation (typical for this specific application). The results indicate that it is possible to use CDW aggregates (up to 20%) to pave urban roads. In a recent study by Coelho et al. [97], the technological, economic, and environmental aspects related to operational CDW recycling facilities that produce medium- to high-quality recycled concrete aggregates have been reported (for details please refers to [97]). Interestingly, Marzouk et al. [98] evaluate the impact of two alternatives for CDW management, namely: either recycling or disposal. The results show that CDW recycling leads to significant reductions in emissions, energy consumption, global warming potential, and conserves landfill space compared to the disposal of waste in landfills. Gálvez-Martos et al. [99] summarize the key principles and best practices for CDW management across the entire construction value chain. Systematic implementation of these best practices could dramatically improve resource efficiency and reduce environmental impacts: reducing waste generation, minimizing transportation impacts and reuse/recycling, improving the quality of secondary materials, and optimizing the environmental performance of treatment methods. Lastly, Jesus et al. [100] studied the behavior of cementitious renderings incorporating very fine recycled aggregates from two types of CDW: recycled concrete aggregate (RCA, smaller one) and mixed recycled aggregate (MRA). Results pointed out that the modified mortars in most of the tests showed superior performance than the reference mortar (without CDW). An interesting and alternative use of recycled waste materials in concrete is reported by Ma et al. [101], who propose a novel route for utilizing recycled coarse aggregate in high-quality recycled manufactured sand, and the use of such high-quality sand to prepare recycled mortar with good mechanical strength and durability, thus further increasing the value of recycling concrete waste.
Furthermore, the use of alternative aggregates, besides those deriving from CDW, is another frequently discussed topic from the literature [102]. Among the possible recycled substrates, plastic waste that would be destined for landfills is attractive not only from the sustainability and economy viewpoint, but also for their very peculiar mechanical properties [103]. In this context, the literature proposes a multitude of different polymeric substrates to be exploited as aggregate fractions, fillers or fibers in the preparation of novel (green) functional concrete, such as polyethylene terephthalate (PET) [104,105,106,107,108,109,110,111,112], poly vinyl chloride (PVC) [113,114], high density polyethylene (HDPE) [115], shredded and recycled plastic waste [102,116,117], expanded polystyrene foams (EPS) [118,119], polycarbonate [120], or polyurethane foams [121,122]. Even if the use of polymeric materials in concrete often leads to a decrease in the mechanical properties due to several factors, such as the poor adherence between the plastic aggregates and the cementitious matrix, and the granular characteristics of the polymeric waste (which in most cases is not sufficient to achieve the optimum packing leading to the minimum of porosity) [117], the use of plastic waste can also be properly exploited for the introduction of some specific properties, unconventional for ordinary concrete. Currently, there is a growing interest in the use of recycled rubber (mostly derived from tire waste) in concrete as a partial replacement of the aggregate fraction [16,17,123,124]. In fact, the addition of rubber particles into concrete can introduce novel properties in cement, such as a limitation of the water absorption and a consequent improvement against corrosion, a reduction in noise propagation, and a further improvement in fire resistance [125]. Furthermore, in the literature, the use of rubber concrete in non-structural applications is also suggested [14,126]. In this context, an important point that should be considered is that the introduction of rubber fragments as aggregate fraction in cementitious matrices significantly affects the final mechanical properties of the cement/concrete in a proportional way in respect to the rubber content [16,17].

7. Conclusions

Due to their remarkable mechanical properties, cementitious materials are among the most largely exploited substrates used by the construction industry for building cities, and linking them with highways, roads, and bridges. However, even if cement is a well-consolidated material, its chemistry remains very complex and not obvious. In order to try to improve the comprehension of the chemistry of cement, the present document has been organized into five parts.
Part I (Paragraph 2) describes the manufacture process for obtaining ordinary Portland cement (OPC).
Part II (Paragraph 3) provides information on the chemical composition of OPC, which is a mixture of different inorganic oxides (i.e., mainly CaO, SiO2, Al2O3, Fe2O3), and deepens the understanding of the hydration mechanisms of every component forming the cement. Particular attention has been dedicated towards the evolution of the chemical composition in the cement system with time, after the reaction with water.
Part III (Paragraph 4) is focused on the mechanism of setting, fundamental for improving structural integrity in the final material.
Part IV (Paragraph 5) deals with the different types of porosities available in a cement. Additionally, a paragraph dedicated to the pivotal role of water in driving the pore formation has been provided.
Part V (Paragraph 6) reports the recent findings of the alternative use of recycled (waste) materials in cementitious matrices, thus highlighting the future trends for the sustainable development of cementitious formulations.
Based on the analysis of the literature, the following important statements can be delivered:
(1)
Water plays a fundamental role in the evolution of the cement matrix. Since the hydration process consists of a series of different chemical reactions involving the main components of the cement powder with water, the reaction products deriving from the hydration mechanism strongly affect the final chemical composition of the cement.
(2)
During the setting time, there is a conversion from a plastic paste towards a set material, whereas the mechanical properties of the formulation are defined after the hardening step.
(3)
It is possible to distinguish between three different types of water: (i) chemically bound water, (ii) physically bound (gel) water, and (iii) free (evaporable) water.
(4)
Every type of water influences the porosity of the cement, and consequently the mechanical performance.
(5)
It is possible to use recycled (waste) materials and reuse them in cementitious matrices to form sustainable (advanced) cementitious composites.
(6)
The recent interest in smart inorganic materials for advanced technological applications opens the possibility of using cement as a matrix for novel nanoscopic composites with improved properties (e.g., self-healing and health-monitoring materials).

Author Contributions

Conceptualization, L.L. and R.N.; writing—original draft preparation, L.L.; writing—review and editing, L.L. and R.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

Authors would like to thank M. Pavese, and E. Chiavazzo (Polytechnic of Torino, Italy) for fruitful discussions, and D. Burlon, and M.L. Aghemo for their precious help.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic representation of the process of cement manufacturing. Reprinted with permission from [43].
Figure 1. Schematic representation of the process of cement manufacturing. Reprinted with permission from [43].
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Figure 2. SEM micrograph of the fracture surface of hardened OPC paste after seven days of hydration. Reprinted with permission from [52].
Figure 2. SEM micrograph of the fracture surface of hardened OPC paste after seven days of hydration. Reprinted with permission from [52].
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Figure 3. Graph reporting the progress of hydration as a function of the hydration time for the principal components of an OPC: consumption of clinker phases ((a), top), and formation of hydrate phases ((b), bottom). Reprinted with permission from [1].
Figure 3. Graph reporting the progress of hydration as a function of the hydration time for the principal components of an OPC: consumption of clinker phases ((a), top), and formation of hydrate phases ((b), bottom). Reprinted with permission from [1].
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Figure 4. SEM micrograph showing the two types of C-S-H produced during hydration; namely, the “outer” product away from the cement particle surface, occupying the water-filled space and characterized by high porosity, and the “inner” C-S-H formed during later steps of hydration and characterized by low porosity. Reprinted with permission from [77].
Figure 4. SEM micrograph showing the two types of C-S-H produced during hydration; namely, the “outer” product away from the cement particle surface, occupying the water-filled space and characterized by high porosity, and the “inner” C-S-H formed during later steps of hydration and characterized by low porosity. Reprinted with permission from [77].
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Figure 5. Hydration heat evolution of an OPC vs. hydration time. Step 1: K2SO4 dissolution. Step 2: Early-stage period. Step 3: Dormant period. Step 4: Middle-stage period (C-S-H formation). Step 5: AFt formation. Step 6: Aft–AFm conversion. Reprinted with permission from [1].
Figure 5. Hydration heat evolution of an OPC vs. hydration time. Step 1: K2SO4 dissolution. Step 2: Early-stage period. Step 3: Dormant period. Step 4: Middle-stage period (C-S-H formation). Step 5: AFt formation. Step 6: Aft–AFm conversion. Reprinted with permission from [1].
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Figure 6. Image showing the mechanism of flocculation: (a) unhydrated section of polymineralic grain (scale of interstitial phase is slightly exaggerated), (b) after 10 min, (c) after 10 h, (d) after 18 h, (e) after 1–3 days, (f) after 14 days [83].
Figure 6. Image showing the mechanism of flocculation: (a) unhydrated section of polymineralic grain (scale of interstitial phase is slightly exaggerated), (b) after 10 min, (c) after 10 h, (d) after 18 h, (e) after 1–3 days, (f) after 14 days [83].
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Table
Table 1. Main components of an OPC.
Table 1. Main components of an OPC.
CompoundsChemical FormulaBogue FormulaAmount (wt.%)
Alite, or Tricalcium silicateCa3SiO5
[3CaO · SiO2]		C3S30–50
Belite, or Dicalcium silicateCa2SiO4
[2CaO · SiO2]		C2S20–45
Celite, or Tricalcium aluminateCa3Al2O6
[3CaO · Al2O3]		C3A8–12
Brownmillerite, or Tetracalcium aluminoferriteCa4Al2Fe2O10
[4CaO · Al2O3 · Fe2O3]		C4AF6–10
Gypsum, or Calcium sulphate dihydratedCaSO4 · 2H2O-4–8
Potassium oxideK2OK<2
Sodium oxideNa2ON<2
Table
Table 2. Chemical composition of OPC clinkers.
Table 2. Chemical composition of OPC clinkers.
ComponentAmount (wt.%)
CaO58.0–68.0
SiO216.0–26.0
Al2O34.0–8.0
Fe2O32.0–5.0
MgO1.0–4.0
SO30.1–2.5
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Lavagna, L.; Nisticò, R. An Insight into the Chemistry of Cement—A Review. Appl. Sci. 2023, 13, 203. https://doi.org/10.3390/app13010203

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Lavagna L, Nisticò R. An Insight into the Chemistry of Cement—A Review. Applied Sciences. 2023; 13(1):203. https://doi.org/10.3390/app13010203

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Lavagna, Luca, and Roberto Nisticò. 2023. "An Insight into the Chemistry of Cement—A Review" Applied Sciences 13, no. 1: 203. https://doi.org/10.3390/app13010203

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