Engineering Properties of New-age (Nano) Modified Emulsion (PDF)

applied sciences Article Engineering Properties of New-Age (Nano) Modified Emulsion (NME) Stabilised Naturally Available Granular Road Pavement Materials Explained Using Basic Chemistry Gerrit J. Jordaan 1,2,* and Wynand J. vdM. Steyn 3 ���������� ������� Citation: Jordaan, G.J.; Steyn, W.J.v. Engineering Properties of New-Age (Nano) Modified Emulsion (NME) Stabilised Naturally Available Granular Road Pavement Materials Explained Using Basic Chemistry. Appl. Sci. 2021, 11, 9699. https:// doi.org/10.3390/app11209699 Academic Editors: Luís Picado Santos and João Crucho Received: 24 July 2021 Accepted: 8 October 2021 Published: 18 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Civil Engineering, University of Pretoria, Pretoria 0002, South Africa 2 Jordaan Professional Services (Pty) Ltd., Pretoria 0062, South Africa 3 School of Engineering, Department of Civil Engineering, University of Pretoria, Pretoria 0002, South Africa; [email protected] * Correspondence: [email protected]; Tel.: +27-(0)-82-416-4945 Featured Application: The engineering properties obtainable during the construction of road pavement layers using reactive stabilising agents are traditionally determined using a trail-and- error approach based on material indicator tests, some developed more than a century ago. These tests inhibit the limitations associated with an empirically derived test or method, often lead- ing to costly failures. The introduction of proven and applicable nanotechnologies to treat or stabilise granular material for use in pavement layers could result in the rejection of these tech- nologies if not based on the understanding of the basic chemistry of both the stabilising agent and the mineralogy of the granular materials. This article gives a basic explanation of elemen- tary chemistry that will affect the physical engineering properties (stresses and strains) that can be expected using available, proven and cost-effective nanotechnologies to improve granular ma- terials for use in a pavement structure. Abstract: Nanoscale organofunctional silanes have been developed, tested and successfully applied to protect stone buildings in Europe against climatic effects since the 1860s. The same nanotechnologies can also be used in pavement engineering to create strong chemical bonds between a stabilising agent and granular material. The attachment of the organofunctional silane to a material also changes the surface of the material to become hydrophobic, thereby considerably reducing future chemical weathering. These properties allow naturally available materials to be used in any pavement layer at a low risk. In the built environment, scientists soon determined that the successful use of an organo-silane depends on the type and condition of the stone to be treated. The same principles apply to the implementation of applicable nanotechnologies in pavement engineering. Understanding the basic chemistry, determining the properties of the stabilising agent and the organofunctional modifying agent and the chemical interaction with the primary and secondary minerals of the material are essential for the successful application of these technologies in pavement engineering. This paper explains some basic chemistry, which fundamentally influences engineering outputs that can be achieved using New-age (Nano) Modified Emulsions (NME) stabilising agents with naturally available granular materials in all road pavement layers below the surfacing. Keywords: nanotechnology in pavement engineering; influence of emulsifying agents (surfactants); chemistry of surfactants in emulsions; stabilisations of naturally available granular materials; miner- alogy compatible nano-modified emulsions; organofunctional silanes; anionic emulsions; cationic emulsions; new-age (nano) modified emulsions (NME) 1. Introduction With the new millennium, the world also entered the fourth Industrial Revolution (4RI) [1] that will “broaden and deepen the connections between the biological, physical Appl. Sci. 2021, 11, 9699. https://doi.org/10.3390/app11209699 https://www.mdpi.com/journal/applsci  Appl. Sci. 2021, 11, 9699 2 of 25 and digital worlds in unprecedented ways” [2]. This phase of development will have an impact on all spheres of life, affecting all industries, bringing together multiple technologies in creating a new modern environment. Relatively new industries such as the Information Technology (IT) industry will experience the 4RI as a natural extension of ongoing develop- ments. However, many “traditional” industries will experience the 4RI as disruptive [3], requiring an extensive change in “set in stone practices,” together with a considerable change in the mindset of practitioners accustomed to traditionally operating procedures. The pavement engineering fraternity, with specific reference to the classification and use of naturally available granular materials, may well fit into the latter category. An under- standing of the basic scientific principles will greatly assist in the general acceptance and implementation of these technologies in traditional industries. Road pavement granular materials are traditionally classified using empirically de- rived indicator tests (some developed more than a hundred years ago). In contrast, pave- ment engineering design has evolved over the last few decades from empirically derived methods to a more fundamental scientific approach with the development and introduction into practice of Mechanistic–Empirical (ME) design methods. These methods use funda- mental failure theories based on the computer modulation of road pavement structures to analyse and predict expected pavement behaviour trends based on calculated basic physics, i.e., stresses and strains within the different pavement layers. Unfortunately, the testing and characterisation of granular materials for use in pavement structures have not shown the same advances. The successful introduction and general acceptance of cost-effective proven and avail- able nanotechnologies to improve granular materials for use in road pavement structures will not succeed using empirical tests and methods, relying on a trial-and-error approach. A scientific understanding of the mineralogy of the granular materials as well as the basic chemistry and chemical reaction caused by the use of proven/available technologies in a traditionally conservative industry is required in line with the expected developments during the 4RI—the use and combination of available information and technologies to understand the basic science behind technologies and predict, with improved confidence, results. Such an understanding obviously also applies to traditionally used reactive sta- bilising agents (e.g., cement, lime, modified bitumen emulsions, etc.), the general use of which often results in “unexplained” costly failures. These traditionally used stabilising agents are each not a single product but contain various combinations of different chemical components, which could react differently with different materials. This article is limited to the basic understanding of the chemical interaction of organofunctional nano-silane modified emulsions in combination with granular material for use in pavement layers below the surfacing. Nanotechnology products (nano-silanes) have been used in the built environment to improve, strengthen and protect stone buildings in Europe for almost 200 years [4]. The same technology could find immediate application in the field of pavement engineering to improve the cost-effective use of granular materials in road pavement structures. Scientists responsible for the development of the first nano-silane products in the built environment in the early 1800s [4–6] soon established that the type of stone (primary minerals) and the condition of the stone (presence and quantity of secondary minerals) considerably affected results achievable through any specific nano-silane treatment. These lessons learned from the built environment must form the basis for the successful introduction of these available, proven nanotechnologies in pavement engineering. It follows that engineers need to have at least a basic understanding of the chemistry involved in the use of these materials. The informed selection of applicable material com- patible nanotechnologies that are stable for practical application under often challenging practical field conditions will prove to be the difference between success and failure. It is not the intention to make pavement engineers chemists, but only to provide them with the necessary tools (basic knowledge) to make informed decisions in practice. The key is  Appl. Sci. 2021, 11, 9699 3 of 25 not to confuse with fundamental, detailed chemistry but to provide practical clarifications, explaining results and showing the consequences of insufficient specifications. Knowledge about these basic chemical concepts controlling material behaviour in the field will assist greatly in the informed application of materials and the use of material- compatible technologies by engineers specialising in road pavement engineering. The successful introduction of new-age nanotechnologies is disruptive to traditional pavement engineering practices, enabling the use of granular materials generally perceived as unsuit- able in quality. Hence, the understanding of the basic chemistry controlling nanotechnology characteristics in an emulsion, in combination with the mineralogy of naturally available materials, can facilitate the acceptance of the engineering benefits in road construction, leading to a substantial reduction in transportation infrastructure costs [7–9]. This article will also demonstrate the importance of more specific specifications with regard to the chemical properties of the emulsifying agent (surfactant) as a nano-particle used in the man- ufacturing of a specific stabilising agent (bitumen emulsion), which forms the foundation for the production of a material compatible New-age (Nano) Modified Emulsion (NME). The chemical characteristics of both nano-particles, i.e., the surfactant and the nano-silane modifier, are important factors in determining the engineering properties to be achieved when applied in practice for the treatment/stabilisation of available granular materials. 2. Background The use of nano-scale products for the stabilisation of granular materials in the con- struction of the pavement layers is nothing new. Per definition, products such as cement, lime as well as bitumen emulsion all incorporate nano-scale particles and can be considered as nanotechnology products, i.e., containing particles of which at least one dimension is between 1 and 100 nm in size [10]. The use of nanotechnology as a science only became of interest after the development of equipment enabling scientists (including chemists, physicians and engineers) to see and manipulate nano-scale particles at a molecular level in the 1980s/90s [11,12]. This ability to manipulate nano-scale products has had an impact on all industries, including the built environment where silicon-based nanotechnology products are being used to improve building materials across basically all spheres of ac- tivity. However, nano-silane products have been developed, tested and used in the built environment to protect stone buildings in Europe since the 1860′s [4]. The more than 150 years of “lessons learnt” from the built environment can assist pavement engineers to fast-track the implementation of these proven technologies to also protect and improve naturally available granular materials for use in the design and construction of roads. Ex- perience in southern Africa [13] has shown that considerable savings are a reality through the implementation of nano-silane technologies used for the treatment and stabilisation of granular materials for all layers below the surfacing. Materials used in road pavements are traditionally classified using empirically derived criteria based on material indicator tests dating back more than a century [14]. These mate- rial classification systems often classify naturally available granular materials in climatic regions of the world associated with a high potential for chemical decomposition (high temperatures in combination with seasonal rainfall [15]) as “non-standard,” “marginal” or even “sub-standard” [16]). Available and applicable nanotechnologies that could enable the use of these materials at a low-risk in pavement structures could substantially reduce the unit costs of road infrastructure, specifically in these regions. Bitumen emulsion technology dates back to the early 1900s [17] when a nano-scale particle was discovered that enables an oil substance (e.g., bitumen) to be mixed with an aqueous substance (i.e., a water-based substance). This nano-scale particle is commonly referred to as an emulsifying agent (known in chemistry as a surfactant and in engineering practice commonly referred to as a “soap”). Bitumen emulsion technology enables a rela- tively low bitumen content to be mixed with granular materials at ambient temperatures to construct road pavement layers. Emulsion technology incorporates several advantages, in- cluding the ability to accommodate considerably higher tensile strains in comparison to the  Appl. Sci. 2021, 11, 9699 4 of 25 unstabilised granular material as well as cement-bound materials [14,18]. However, simi- larly to the manufacturing of asphalt, it is still a requirement to use aggregate/stone/gravel granular materials of a relatively high quality together with the bitumen emulsion for the construction of a pavement layer meeting the required engineering specifications [19]. Similar to the manufacturing of asphalt for surfacings, the unmodified bitumen forms no chemical bonds with the granular aggregate/stone materials in the mix. Strength is only achieved through covalent bonds (relatively weak) and mechanical forces created through granular interlock and absorption of the bitumen into a porous surface of the aggregate [15]. For this reason, some aggregates containing a high silicon content, which usually result in relatively “clean breaks” during crushing, are notoriously difficult to use for the manufacturing of asphalt or bitumen emulsion stabilised layers meeting the engineering specifications. In asphalt and bitumen emulsion mixes, the use of materials conforming to specific grading envelopes are of major importance to create a firm granular matrix, which results in high interlocking mechanical forces being formed. The ability to create strong chemical bonds between the stabilising agent (e.g., bitumen or equivalent polymer) and the granular material to be stabilised (aggregate/stone/soil) can be achieved through the introduction and use of proven material compatible organofunc- tional nano-silanes. These nano-silane products attach to the granular materials, creating relatively strong ionic-chemical bonds. The organofunctional part of the nano-silane parti- cle is hydrophilic, rendering the surface of each granular particle of the material to become hydrophobic during consolidation, preventing water access to primary and secondarily minerals comprising each of the granular particles within the mix. The high chemical bond strengths and the enacted hydrophobicity enable materials classified as “non-standard,” “marginal” or “sub-standard” to be utilised successfully within any pavement layer be- low the surfacing, at low risk. The introduction of material-compatible organofunctional nano-silanes in the field of pavement engineering is a typical example of a disruptive technology [3], requiring traditional approaches to the use of materials in pavement engi- neering to become irrelevant. This combination of existing technologies in combination with an improved scientific understanding and knowledge forms the cornerstone of the 4RI in traditional industries, especially with regard to the cost-effective provision of macro infrastructure projects using “smart” materials in a cost-effective manner. The general acceptance of such disruptive technologies will require the necessary improved knowledge of the basic supportive science to become everyday practice. The key is not to overwhelm the practicing pavement engineer with complex fundamental chemistry but to simplify facts to be easily understandable in support of practical implementation. Many products have been introduced throughout the last few decades claiming to be able to provide the ability to improve granular material characteristics to enable the use thereof in road pavement structures. These so-called “snake-oils” have generally failed to meet expectations. In the absence of a scientifically-based approach to granular material investigations and tests indicative of engineering principles (e.g., stresses, strains and durability), the same can happen with the introduction of applicable/proven nano- silane technologies for the improvement/stabilisation of granular materials in pavement engineering [20]. The work performed by scientists in the built environment dating back almost 200 years established the basic requirements for the successful application of any spe- cific organofunctional silane technology to improve granular/stone material characteristics. These requirements are fundamentally based on the compatibility with the type of stone (primary minerals) and condition of the stone (presence and amount of secondary min- erals in the granular/stone that developed as a result of weathering due to chemical decomposition) [21,22]. It follows that material classification should, at least, include the scientific testing of the primary and secondary minerals present in the available granular materials [20]. Knowledge about the mineralogy of the materials and the environmental conditions favouring weathering through chemical decomposition [22] and the formation of secondary minerals in granular materials will enable engineers to select with confidence  Appl. Sci. 2021, 11, 9699 5 of 25 a material compatible technology to enhance/stabilise marginal granular materials for use in all of the pavement layers below the surfacing. The same principles apply to the characteristics of the nano-particles (surfactant) used to produce the emulsion stabilising agent (e.g., bitumen emulsion) and the effect thereof on the engineering properties when used in combination with an organofunctional silane modifying agent. In effect, the introduction of a material compatible nano-silane creates a modified emulsifying agent combining two different nano-particles. Each of these nano-particles could have a considerable influence on the engineering properties when used in the production of a modified emulsion stabilising agent and applied to specific granular material as a stabilising agent. 3. Traditional Use of Nanotechnology Products in Road Pavement Engineering The use of nano-scale material in the road industry (over and above the use of lime and cement) as stabilisation agents dates back more than a century, with the development of bitumen emulsions in the early 1900s [8]. As per definition [17,23–25], bitumen emulsion consists of bitumen, water and an emulsifying agent. The emulsifying agent is, in fact, a nano-scale particle commonly referred to as a “Janus” particle [26] (from the Greek mythology meaning “two-faced”) due to the dual nature of the emulsifying agent nano- particle, attracting oil on the one side and water on the other side. It is not the intention to discuss in detail the technology involved in the production of bitumen emulsions [17,23,24,26,27]. It is generally known that numerous production factors, bitumen rheology, etc., could influence the characteristics of bitumen emulsion, which are discussed in detail in numerous publications. The objective of this paper is to concentrate on the effects of the chemistry involved in the additives and modifications used in the manufacturing of bitumen emulsion as a stabilising agent. Understanding the role of the emulsifying agent nano-particle is comparable to that of organofunctional nano-silane products used in the built environment. Hence, the discussion in this paper is limited to the role of the emulsifying agent that enables water to be mixed with oil (organic substance, in this case, bitumen) substances, which, under normal conditions, do not mix. Not only does the emulsifying agent enable the oil (e.g., bitumen) to be mixed with water, but crucially, when modified with the addition of a material compatible nano-silane, the characteristics could also dramatically influence the engineering properties achievable when the nano-modified emulsion is used with a specific granular material. It follows that a basic understanding of the role of the nano-emulsifying agent also needs to be addressed. The mixing of oil (e.g., bitumen) and water is achieved through the addition of a chemical nano-particle (the emulsifying agent, soap or surfactant) at high shear (e.g., high mix revolutions), which forces the oil and water together with the emulsifying agent through small openings or between plates, which enables the bitumen (oil) particles to be separated and mixed and attached to the emulsifying agent already mixed and attached to the water molecules. The emulsifying agent typically has a hydrophilic (water-loving) head and a lipophilic (oil-loving) (hydrophobic) tail consisting of between 12 and 18 (or even more) carbon atoms [17]. The chemical composition of typical emulsifying agents is shown in Figure 1 (anionic) and Figure 2 (cationic). The hydrocarbon tail of the emulsifying agents is often replaced in chemical formulas by the letter “R”. The higher the number of carbon atoms in the hydrocarbon tail, the more firmly the emulsifying agent will attach to the organic stabilising agent (e.g., bitumen molecule). The hydrocarbon tail embeds itself into the bitumen molecule. In comparison, if the earth resembles the size of a bitumen particle, the hydrocarbon tail of a good emulsifying agent (high number of carbon atoms) will typically penetrate the crust of the earth to a depth of approximately 8 km (equivalent to approximately 5 nm in a bitumen molecule) and covers an area of approximately 10 km2 [25].  Appl. Sci. 2021, 11, 9699 6 of 25 Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 28 Figure 1. Typical composition of an anionic emulsifying agent. . Figure 2. Typical composition of a cationic emulsifying agent. Figure 1. Typical composition of an anionic emulsifying agent. Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 28 Figure 1. Typical composition of an anionic emulsifying agent. . Figure 2. Typical composition of a cationic emulsifying agent. Figure 2. Typical composition of a cationic emulsifying agent. The visual representation of the anionic emulsifying agent shown in Figure 2 is expressed in chemical formulation as follows [24,25]:  Appl. Sci. 2021, 11, 9699 7 of 25 CH3(CH2)nCOO− + Na+ (where n is normally a multiplier between 12 and 18 [17]: Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 28 The higher the number of carbon atoms in the hydrocarbon tail, the more firmly the emulsifying agent will attach to the organic stabilising agent (e.g., bitumen molecule). The hydrocarbon tail embeds itself into the bitumen molecule. In comparison, if the earth re- sembles the size of a bitumen particle, the hydrocarbon tail of a good emulsifying agent (high number of carbon atoms) will typically penetrate the crust of the earth to a depth of approximately 8 km (equivalent to approximately 5 nm in a bitumen molecule) and covers an area of approximately 10 km2 [25]. The visual representation of the anionic emulsifying agent shown in Figure 2 is ex- pressed in chemical formulation as follows [24,25]: CH3(CH2)nCOO− + Na+ (where n is normally a multiplier between 12 and 18 [17]: CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂ C ⫽ \ 0 O¯Na⁺ non-polar hydrocarbon group Ionic group (water-insoluble) (water-soluble) (1) Equation (1) is simplified to: CH₃(CH�)ₙ C ⫽ \ 0 O¯Na⁺ (2) with the value of “n” typically varying between 12 and 18 [17]. Formula (2) is further simplified to: O ⫽ "R" − C \ O¯Na⁺ (3) Similarly, a typical cationic emulsifying agent shown in Figure 3, is depicted as: "R" − H� | N� | \ H� − H� Cl (4) (1) Equation (1) is simplified to: Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 28 The higher the number of carbon atoms in the hydrocarbon tail, the more firmly the emulsifying agent will attach to the organic stabilising agent (e.g., bitumen molecule). The hydrocarbon tail embeds itself into the bitumen molecule. In comparison, if the earth re- sembles the size of a bitumen particle, the hydrocarbon tail of a good emulsifying agent (high number of carbon atoms) will typically penetrate the crust of the earth to a depth of approximately 8 km (equivalent to approximately 5 nm in a bitumen molecule) and covers an area of approximately 10 km2 [25]. The visual representation of the anionic emulsifying agent shown in Figure 2 is ex- pressed in chemical formulation as follows [24,25]: CH3(CH2)nCOO− + Na+ (where n is normally a multiplier between 12 and 18 [17]: CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂ C ⫽ \ 0 O¯Na⁺ non-polar hydrocarbon group Ionic group (water-insoluble) (water-soluble) (1) Equation (1) is simplified to: CH₃(CH�)ₙ C ⫽ \ 0 O¯Na⁺ (2) with the value of “n” typically varying between 12 and 18 [17]. Formula (2) is further simplified to: O ⫽ "R" − C \ O¯Na⁺ (3) Similarly, a typical cationic emulsifying agent shown in Figure 3, is depicted as: "R" − H� | N� | \ H� − H� Cl (4) (2) with the value of “n” typically varying between 12 and 18 [17]. Formula (2) is further simplified to: Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 28 The higher the number of carbon atoms in the hydrocarbon tail, the more firmly the emulsifying agent will attach to the organic stabilising agent (e.g., bitumen molecule). The hydrocarbon tail embeds itself into the bitumen molecule. In comparison, if the earth re- sembles the size of a bitumen particle, the hydrocarbon tail of a good emulsifying agent (high number of carbon atoms) will typically penetrate the crust of the earth to a depth of approximately 8 km (equivalent to approximately 5 nm in a bitumen molecule) and covers an area of approximately 10 km2 [25]. The visual representation of the anionic emulsifying agent shown in Figure 2 is ex- pressed in chemical formulation as follows [24,25]: CH3(CH2)nCOO− + Na+ (where n is normally a multiplier between 12 and 18 [17]: CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂ C ⫽ \ 0 O¯Na⁺ non-polar hydrocarbon group Ionic group (water-insoluble) (water-soluble) (1) Equation (1) is simplified to: CH₃(CH�)ₙ C ⫽ \ 0 O¯Na⁺ (2) with the value of “n” typically varying between 12 and 18 [17]. Formula (2) is further simplified to: O ⫽ "R" − C \ O¯Na⁺ (3) Similarly, a typical cationic emulsifying agent shown in Figure 3, is depicted as: "R" − H� | N� | \ H� − H� Cl (4) (3) Similarly, a typical cationic emulsifying agent shown in Figure 3, is depicted as: Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 28 The higher the number of carbon atoms in the hydrocarbon tail, the more firmly the emulsifying agent will attach to the organic stabilising agent (e.g., bitumen molecule). The hydrocarbon tail embeds itself into the bitumen molecule. In comparison, if the earth re- sembles the size of a bitumen particle, the hydrocarbon tail of a good emulsifying agent (high number of carbon atoms) will typically penetrate the crust of the earth to a depth of approximately 8 km (equivalent to approximately 5 nm in a bitumen molecule) and covers an area of approximately 10 km2 [25]. The visual representation of the anionic emulsifying agent shown in Figure 2 is ex- pressed in chemical formulation as follows [24,25]: CH3(CH2)nCOO− + Na+ (where n is normally a multiplier between 12 and 18 [17]: CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂ C ⫽ \ 0 O¯Na⁺ non-polar hydrocarbon group Ionic group (water-insoluble) (water-soluble) (1) Equation (1) is simplified to: CH₃(CH�)ₙ C ⫽ \ 0 O¯Na⁺ (2) with the value of “n” typically varying between 12 and 18 [17]. Formula (2) is further simplified to: O ⫽ "R" − C \ O¯Na⁺ (3) Similarly, a typical cationic emulsifying agent shown in Figure 3, is depicted as: "R" − H� | N� | \ H� − H� Cl (4) (4) The properties and stability of the emulsion is a function of numerous factors, includ- ing the chemical properties of the emulsifying agent (e.g., the length of the carbon-tail shown as “n”), the percentage of the emulsifying agent added during the emulsifying process, the manufacturing process and the properties of the bitumen. In terms of chemical stability, it is worth noting that the bond strengths between the various atoms in the emul- sifying agent differ substantially. These bond strengths could also play a major role in the stability of the emulsion, especially in combination with a second nano-particle and/or when a modification to the emulsification agent is introduced. The bond strengths between some of the major atoms comprising the emulsifying agent are summarised in Figure 3 (compiled from published information [28]). From Figure 3, it is seen that the bond strengths between the elements comprising an anionic emulsifying agent (pink arrow combinations) are considerably stronger than the bond strengths comprising the typical cationic emulsifying agent (green arrow combina- tions). This simplified chemistry explains the general trends found in the stability normally associated with anionic versus cationic bitumen emulsions in practice, assuming all manu- facturing processes are optimised in line with good practices. The implication in practice is that an anionic nano-modified emulsion will normally have a longer shelf life (due to higher stability) than a nano-modified cationic emulsion, an important practical factor, especially considering remote areas of implementation, uncertain climatic conditions and a construction industry often faced with unplanned delays due to political and managerial  Appl. Sci. 2021, 11, 9699 8 of 25 factors, not within the control of the contractor. The importance of the characteristics of the emulsifying agent nano-particle combined with a material compatible organofunctional nano-silane in the performance of the nano-modified emulsions as a stabilising agent for the treatment/stabilisation of granular materials is demonstrated in Section 7 of this article. The results show the variation in the engineering measured properties possible in practice with all input parameters carefully controlled. In this experiment, the only variable is the characteristics of the various emulsifying agents used by the various manufactures in the production of the nano-modified emulsions. Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 28 Figure 3. Comparison of the bond strengths between the various elements comprising the different emulsifying agents (anionic in pink and cationic in green) (compiled from published information [28]). The properties and stability of the emulsion is a function of numerous factors, includ- ing the chemical properties of the emulsifying agent (e.g., the length of the carbon-tail shown as “n”), the percentage of the emulsifying agent added during the emulsifying process, the manufacturing process and the properties of the bitumen. In terms of chemi- cal stability, it is worth noting that the bond strengths between the various atoms in the emulsifying agent differ substantially. These bond strengths could also play a major role in the stability of the emulsion, especially in combination with a second nano-particle and/or when a modification to the emulsification agent is introduced. The bond strengths between some of the major atoms comprising the emulsifying agent are summarised in Figure 3 (compiled from published information [28]). From Figure 3, it is seen that the bond strengths between the elements comprising an anionic emulsifying agent (pink arrow combinations) are considerably stronger than the bond strengths comprising the typical cationic emulsifying agent (green arrow combina- tions). This simplified chemistry explains the general trends found in the stability nor- mally associated with anionic versus cationic bitumen emulsions in practice, assuming all manufacturing processes are optimised in line with good practices. The implication in practice is that an anionic nano-modified emulsion will normally have a longer shelf life (due to higher stability) than a nano-modified cationic emulsion, an important practical factor, especially considering remote areas of implementation, uncertain climatic condi- tions and a construction industry often faced with unplanned delays due to political and managerial factors, not within the control of the contractor. The importance of the charac- teristics of the emulsifying agent nano-particle combined with a material compatible or- ganofunctional nano-silane in the performance of the nano-modified emulsions as a sta- bilising agent for the treatment/stabilisation of granular materials is demonstrated in Sec- tion 7 of this article. The results show the variation in the engineering measured properties possible in practice with all input parameters carefully controlled. In this experiment, the only variable is the characteristics of the various emulsifying agents used by the various manufactures in the production of the nano-modified emulsions. Figure 3. Comparison of the bond strengths between the various elements comprising the different emulsifying agents (anionic in pink and cationic in green) (compiled from published information [28]). 4. Basic Chemistry Applicable to the Understanding of the Successful Introduction of Material Compatible Nanotechnology Solutions for the Treatment, Improvement and/or Stabilisation of Granular Materials for Use in Road Pavement Layers below the Surfacing 4.1. Silicon Characteristics in the Material Sub-Strata and Applications within the Built Environment Silicon (Si) is the second most abundant element (after oxygen), comprising more than 26 per cent of the crust of the earth (by weight). In comparison, oxygen makes up more than 49 per cent by weight of the crust of the earth. The rest of the elements contained in the crust of the earth in combination makes approximately 25 per cent by weight of the crust of the earth. Silicon is found in nature as oxides (SiO2) or silicates (SiO4), forming the basis of most rock-forming minerals [15]. Commonly known naturally available materials such as granite, feldspar, hornblende, asbestos, clay and mica are a few examples of materials normally containing high percentages of silicon [22]. Silicon is also one of the most useful elements to mankind. In the form of sand and clay, it is commercially used as a cost-effective product to produce building components such as pottery, bricks, cement, glass, etc. Silicon also plays an important role in plant as well as animal life, forming part of cell structures and found in plant remains as well as skeleton structures. It is considerably versatile in application, inherent to most of the  Appl. Sci. 2021, 11, 9699 9 of 25 nanotechnology-based products currently in common use in the built environment, with “excellent mechanical, optical, thermal and electrical properties” [28]. The silicon present in nature as silicates can form 4 bonds with oxygen atoms, which may be orientated in various geometric structures, forming a three-dimensional infinite structure [29,30] to form minerals and the surface of the sub-strata or rock surface as commonly referred to. Figure 4 shows a very simplified illustration of the composition of naturally available materials (rocks), as numerous elements are found in nature that combines with the basic silicon lattice to form a large variety of minerals commonly found in all rock formations. Of importance is the illustrated attraction of exposed siliceous materials on the surface of granular materials to water molecules freely available in the atmosphere. Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 28 Figure 4. Simplified demonstration of the most common sub-strata or rock formations in nature. Rock surfacings contain “broken chains” of chemical bonds that form a layer of free energy [15] on the surface of granular materials (rock/gravel or soil). The surfaces of these Figure 4. Simplified demonstration of the most common sub-strata or rock formations in nature. Rock surfacings contain “broken chains” of chemical bonds that form a layer of free energy [15] on the surface of granular materials (rock/gravel or soil). The surfaces of  Appl. Sci. 2021, 11, 9699 10 of 25 these naturally available materials (with a few exceptions, e.g., talk) are hydrophilic (water- loving) and attract water molecules freely available in the atmosphere. Hence, freshly crushed stone is instantaneously covered by numerous layers of water molecules that (a water molecule is approximately 0.1 nm in size or 1 Angstrom (Å)). These layers of water molecules covering the area of the stone are invisible to the eye and other senses (touch/smell). Water molecules (H2O) are always accompanied by their natural derivatives of negatively charged oxygen (O−2) and positively charged hydrogen (H+1) in the atmo- sphere that surrounds the surface of granular materials (rock/gravel/soil) and is attached to the surface as shown in Figure 4. The freely available hydrogen (H+1) combines with the negatively broken oxygen chains of the Si elements to form (OH)−1 bonds on the surface. The siliceous surfacings containing O−2 and (OH)−1 bonds are hydrophilic in nature and hence, attract water. The simplified lattice shown in Figure 4 forms the basic understanding of the application of various silane-nanotechnologies developed using the properties of the silicon element, which have been used in the built environment for more than 150 years to protect buildings, monuments and statues [4]. These same basic technologies are also directly applicable to the field of pavement engineering to enable generally available natural resources (granular materials) to be used more cost-efficiently within road pavement structures. 4.2. Chemistry of Organofunctional Nano-Silanes for Application in Pavement Engineering Most of the nanotechnology products currently used in the built environment to protect, preserve and/or strengthen building materials are based on the silane (SiH4) derivative of silicon (the same derivative, which has found application as the basis for the development of stone consolidants for more than a 150 years [4]). Each hydrogen (H) atom in the silane (SiH4) can be replaced by any other element or group(s) to form a large variety of nano-products with numerous fields of application. The most common reactive groups are hydrogen (H), chloride (Cl), fluorine (F) and a RO group, where R is the general symbol for the alkyl group, also known as the organo-functional group in silane-chemistry (also referred to as the hydrocarbon group discussed and explained in the composition of the emulsifying agents demonstrated in Figures 2 and 3), which may, inter alia, include chemical compounds such as CH3 (methyl) or CH3CH2 (ethyl). The CH3-Si bond is a very stable (non-reactive) bond (known as organo-silane), with low surface energy with hydrophobic (water-repellent) or oleophilic (oil-loving) character- istics. The reactive group RO is referred to as an alkoxy (alkyl + oxygen) group [4]. Silicon (Si) forms the centre of the RO group together with the second functional group(s) (X) (e.g., methoxy, ethoxy, etc.) to form the chemical molecule RO-Si-X. These secondary functional (pendant) groups may be highly reactive (in cases where the element of carbon (C) is excluded from the formulation) and define the functionality of a specific nano-silane [4]. The functional group (R) will attach to organic material (such as bitumen or an alternative material compatible polymer with an oil basis), while the functional group(s) (X) will attach to inorganic material (granular material such as rock/gravel/soil). Hence, an organo- functional nano-silane combines the functionality of a reactive group with a non-reactive group in a single molecule. It should be noted that the organofunctional nano-silane is not replacing the stabilising agent in the stabilisation of granular road building materials. The molecule is too small to effectively bridge the gaps between the material fractions. However, in practice, the nano- silane can very effectively fulfil the role of a bonding agent (adhesive agent, a well-known concept in the asphalt industry), which is designed to permanently bind the stabilising agent (the bitumen or equivalent organic substitute with similar characteristics) to the granular materials (stone/aggregate/soil). Si–O chemical bonds are some of the strongest found in nature (refer to Figure 3). The relatively small size of the nano-silane particle (depending on the type of nano-silane and the quality of the product, the size of nano- silanes may vary from about 5 nm to less than 1 nm) results in a considerably high surface  Appl. Sci. 2021, 11, 9699 11 of 25 area per volume ratio [7]. In other words, a relatively small amount of the nano-silane is required to totally encapsulate all particles of the granular material (stone/gravel/soil) that is being treated/stabilised (for example, 1 litre of nano-silane could easily have the same coverage area of about a 1000 litres of bitumen, with a bitumen particle being in the order of 1000 to 5000 nm in size) [7,17]). As mentioned, the organofunctional nano-silane particle will, in effect, have the prop- erties of a “coupling agent” commonly referred to in the asphalt industry as an aggregate adhesive. Aggregate adhesives or aggregate promoters are well-known terminologies in pavement engineering, and such nanotechnologies have been used for at least three decades [31] as modifiers to bitumen emulsions. However, the use of silicon-based nan- otechnology products in pavement engineering is relatively new and has been developed mainly over the last 15 years. The functionality of the silane R-Si-X molecule in terms of an aggregate adhesive mainly refers to the ability of the (X) reactive group to be removed from the molecule when in contact with water (to effectively be chemically detached by water) and replaced by a hydroxyl (OH) group, during a process referred to as hydrolysis [4]. where: Hydrolysis: Si-X + H2O = Si-OH + XH (5) The one product of hydrolysis is the Si-OH binding referred to as silanol [4]. Silanols are now able to react with each other to form siloxane bonds in a condensation reaction when in contact with an inorganic material (e.g., granular material such as rock/gravel/soil) [4], covering the total area of each particle in the granular material. It should be noted that the by-product during condensation should preferably be water (H2O), as shown in the example (not all available nano-silane products produce water as a by-product, at worst a non-toxic alcohol should be allowed to form as a by-product during condensation—refer toxicology requirements [7]). where: Condensation: Si-OH + OH-Si = Si-O-Si + H2O (6) Of importance to the field of pavement engineering is that the organic (R) (organofunc- tional) group attached to the silicon by means of a direct Si-C bond is not affected by hydrolysis and will remain stable (non-reactive) also during condensation. It follows that in the case of CH3-Si-(OCH3)3 (methyl-trimethoxy-silane), the methyl group (CH3) directly attached to the silicon atom will remain stable throughout the process of hydrolysis and condensation, while the hydrogens in the methoxy ((OCH3)3) groups will react with water. These effects of hydrolysis and condensation are demonstrated as follows [4]: where: Hydrolysis: CH3-Si-(OCH3)3 + 3H2O = CH3-Si-(OH)3 + 3CH3-OH (7) Condensation: OH− OH− | | CH3 − Si − (OH)3 + (OH)3 − Si − CH3 = CH3− Si −O− Si −CH3 + H2O | | OH− OH− (8) In order to be effective as a consolidant, the silane-compounds must be able to form three-dimensional networks and hence, must have at least three reactive groups. The pavement engineering implications (macro-effect) of the above nano-silane formulations are demonstrated in Figure 5, which should be considered together with Figure 4 and the similarities with the emulsifying agent shown in Figures 1 and 2.  Appl. Sci. 2021, 11, 9699 12 of 25 Appl. Sci. 2021, 11, x FOR PEER REVIEW 13 of 28 Figure 5. Simplified demonstration of the chemical interaction between a stabilising agent, organofunctional nano-silane modification and the mineralogy of the rock/stone/soil granular material sub-strata. A 3-Dimentional matrix forms during consolidation of the nano-silane onto the material substrata to cover and surround all material particles fully and create a hydrophobic (water-repellent) surfacing with the organofunctional “R” part of the consolidated nano-silane facing outwards Figure 5. Simplified demonstration of the chemical interaction between a stabilising agent, organofunctional nano-silane modification and the mineralogy of the rock/stone/soil granular material sub-strata. The effective result shown in Figure 5 is that the surface of each particle comprising the granular material (rock/aggregate /soil) will chemically be changed from a hydrophilic (water-loving) to a hydrophobic (water-repellent) state that attracts an organic material (such as oil/bitumen or an equivalent material compatible polymer). Consequently, the  Appl. Sci. 2021, 11, 9699 13 of 25 water molecules that are naturally attracted to exposed surfaces of granular materials (stone/aggregate/soil) due to the presence of broken chemical bonds as discussed and demonstrated in Figure 4, will now actively be repelled together with the water that is the by-product formed during the condensation phase (as per the example shown in Figure 5). Figure 5 represents a most basic schematic explanation of the practical use of nano- silane science in pavement engineering. The changing and matching of reactive and non-reactive bonds to the silicon element can result in literary numerous different nano- products with different characteristics, which could match the numerous minerals available in naturally available materials such as stone, gravel or soils. At the same time, through creating the appropriate bonds, the surface of these ma- terials will become water repellent, negating the negative impact that water has on the durability of materials through the prevention of (or at least limiting) weathering due to chemical decomposition (a pre-requisite of the process of weathering of materials through chemical decomposition (chemical change) and the formation of resultant secondary min- erals in the presence of and access to water [15]). In pavement engineering design analyses, it is usually assumed that material mineral properties will stay unchanged during the design period. This basic assumption is incorrect, and even over a design period of 20 to 40 years, dramatic changes can occur (even in freshly crushed stone—given conducive environmental conditions [15]), which will influence the design assumptions considerably. This aspect alone shows the potential benefit of the use of organofunctional nano- silane products in pavement engineering as a protective agent for high-value, freshly crushed stone against chemical decomposition and deterioration over its design life (e.g., a 20-year normal design period). Rehabilitation investigations conducted on major freeways in the Gauteng province of South Africa have shown that high-quality newly constructed stone G1 (Figure 6) [32] base-course material can deteriorate over a period of 22 years to an equivalent G4 to G6 quality material. Similarly, secondary roads in the Gauteng region of South Africa have shown that a cement-treated base-course layer can deteriorate from a cement-treated layer (Unconfined Compressive Strength (UCS) between 750 to 1500 kPa) quality layer to an equivalent G7/8 quality material [32], over a period of...