Tribo Chemical Activation of Green Eco Cements

Konstantin Sobolev

Abstract Due to growing global demand for cement, the production of cement worldwide has significantly increased in the past 15 years, and this trend is the most significant factor affecting the technological and manufacturing advancements in the cement industry. While the increase in demand for cement reflects the growth of national economies, the production of cement clinker is ecologically harmful because it consumes considerable energy and natural resources, and it emits many pollutants into the atmosphere. Therefore, new ways to produce high volumes of cement clinker with less energy and less impact on the environment is greatly needed. One such approach is the production of tribo-chemically activated, high-volume mineral additive (HVMA) cement, which helps to improve the ecological compatibility of cements. This "green" technology is based on the intergrinding of portland cement clinker, gypsum, mineral additives, and a special complex admixture. Tribo-chemical activation increases the compressive strength of ordinary portland cements, improves the durability of cement-based materials, can be processed with a high volume of inexpensive indigenous mineral additives or industrial by-products, and which reduces energy consumption per unit of the cement produced. Additional ecological advantages for green HVMA cements include higher strength, better durability, less pollution at the clinker production stage, and fewer industrial by-products placed in landfills.

Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, EMS 939 3200 North Cramer Street, Milwaukee, WI 53211, USA e-mail: [email protected]

M. Nosonovsky and B. Bhushan (eds.), Green Tribology,

Green Energy and Technology, DOI: 10.1007/978-3-642-23681-5_15,

© Springer-Verlag Berlin Heidelberg 2012

15.1 Introduction

The most recent developments in cement and concrete technology can be summarized as follows [1-5]:

• Following a historical cycle of cement and concrete development, there is a strong need to increase strength. Continuous breakthroughs in knowledge allow the design and application of super high-strength concrete with a compressive strength of up to 250 MPa, high flexural strength, and remarkable ductility [2-9]. As a result, the design and application of effective structures built with high-strength concrete has been phenomenal in the past years, demonstrating strong growth potential.

• The use of industrial by-products and waste (IBPW) as mineral additives has become a valuable segment of cement and concrete technology. According to wide-scale investigations [2-6, 10-16], the performance of concrete with controlled volumes of IBPW can be significantly improved. Well-investigated mineral additives include granulated blast furnace slag, fly ash, and silica fume, which not only result in concrete with improved properties and economical effectiveness, but they also improve the eco- balance of cement and concrete.

• The use of chemical admixtures is essential in producing modern concrete. Added to the concrete mixture, relatively small amounts of chemical admixtures radically alter the behavior of fresh or hardened concrete. Modern admixtures can help realize almost any desired property of concrete [7-9, 17-19].

• Application of nanotechnology and nano materials is a relatively new direction focused on better understanding of the cement-based materials at the nanolevel and the application of nanomaterials (such as nano-SiO2 particles) in cement and concrete [20-23].

As a result of these developments, the concept of high-performance concrete (HPC) has been put forward and successfully applied. It is generally considered that concrete with improved properties (i.e., workability, strength, permeability, and durability) over the conventional levels can be classified as HPC. According to Forster [9], HPC is ''a concrete made with appropriate materials combined according to a selected mix design and properly mixed, transported, placed, consolidated, and cured so that the resulting concrete will give excellent performance in the structure in which it will be exposed, and with the loads to which it will be subjected for its design life.''

To realize the HPC concept, a variety of chemical admixtures and mineral additives are necessary, and modern concrete batching plant technology requires adequate equipment for precise control, dispatching, dosing, and batch processing [8, 9, 17-19]. The technology of high-performance cement was developed in order to simplify HPC production, thereby further extending the frames of its application [5, 6, 18, 24].

A novel approach for improving cement performance includes the application of chemical admixtures (modifiers) at the stage of cement grinding. Air-entraining,

Fig. 15.1 The mechanism of tribo-mechanically induced solid state reactions

Fig. 15.1 The mechanism of tribo-mechanically induced solid state reactions

High Temperature

hydrophobic, plasticized cements, and the family of cements manufactured with grinding aids are early examples of tribo-chemical and mechanochemical activation of cement surface layers resulting in the desired performance. The theory of tribo-chemical and mechano-chemical activation (TMCHA) has been applied to process nanopowders, pigments, fillers, binders, ceramic, and ferromagnetic materials, but has a limited use in cement modification.

TMCHA activation is used to describe the chemical conversions in solids induced by a mechanical process such as milling or grinding. The mechanical processing usually results in the formation of dislocations and other defects in the structure of the material. In the case of TMCHA, the mechanical impacts cause the development of elastic, plastic, and shear deformations leading to fracture, amorphization, and even chemical reactions in the solid state. Characteristic features of TMCHA are summarized in Fig. 15.1. Ball milling, and especially high-energy ball milling, breaks down the crystallinity of solid reactants and provides a transfer of mass required for chemical reactions. In addition, high pressure and shear stress facilitate both the phase transitions and the chemical transformations of solids. The energy in the form of various lattice defects, accumulated by the solid during the mechanical processing, can support/trigger various chemical transformations.

The application of advanced cements with high strength and superior durability is an attractive method of controlling concrete properties and the design of highperformance concrete. Such cements require the application of special clinkers, or they can be manufactured using blended cement technology (e.g., shrinkage compensating/expansive cements, high-early-strength cements, regulated-set cements). Advanced cements such as Ultimax cement, Pyrament, energetically modified cement, and silica fume cements are developed using this principle [6, 20, 24].

Tribo-chemistry can be used to improve the strength of conventional cement. For example, low water demand binder is produced by intergrinding cement and a dry modifier at a high energy. The application of a specially selected admixture-modifier at a relatively high dosage (about 4%) resulted in a cement with both reduced water demand and high strength [20, 24].

Fig. 15.2 Scanning electron microphotograph of TCA HP cement

Supersilica, a complex admixture for application in cement technology has been developed. It is based on a reactive silica-based sorbent, an effective surfactant, and some minor corrective components. Supersilica, a reactive silica-based complex admixture (RSA), was produced using this principle. It was proposed that, when added during the cement grinding process, RSA modifies the surface of cement particles and promotes the formation of highly reactive layers on the surface of cement grains. Scanning electron microscopy helps to reveal some details of interaction between RSA and cement, such as micro-or nano-indentations on the surface of cement particles (Fig. 15.2). Furthermore, the silica component of RSA also acts as a micro-filler and participates in a pozzolanic reaction. The tribo-chemical activation of cement with RSA results in a new product—high-performance cement.

High-performance cement can be defined as a material manufactured by the tribo-chemical activation of certain proportions of clinker, gypsum, a complex admixture (RSA) and, optionally, a mineral additive of industrial (IBPW) or natural origin. The application of HP cement imparts high strength and extreme durability to the concrete or mortar; and its high strength can be used for engineering cement with a high volume of mineral additives. The flowchart for the grinding unit for manufacturing HP/HVMA cement is presented in Fig. 15.3.

15.2 Tribo-Chemical Activation of Cement

The effect of tribo-chemical activation (TCA) on the properties of cement systems was investigated [20]. Three different modifiers were used in the experimental program: sulfonated naphthalene formaldehyde condensate (SNF, used in a dry form), sulfonated melamine formaldehyde condensate (SMF, used in a dry form), and polyacrilate/polycarboxylate superplasticizer (PAE, 31% concentration).

Melamine Formaldehyde Flowchart
Fig. 15.3 High-performance cement technology
Table 15.1 Composition and compressive strength of tribo-chemically activated cements

Cement type

Composition (%)

Flow (mm)

Compressive strength (MPa)

Silica fume

Supersilica

1 day

7 days

28 days

90 day

NPC

-

-

195

52.7

69.3

84.7

72.7

SFC

8

-

208

44.4

71.3

89.0

88.8

HPC-SNF

-

8

211

54.2

76.6

93.8

98.6

HPC-SMF

-

8

230

40.3

91.5

115.4

124.4

HPC-PAE

8

370

57.4

88.1

105.5

110.8

The modifiers were processed into complex admixture (Supersilica) by intermixing and granulating with silica fume (SF) at a ratio of 1:10, by weight [20]. Portland cement clinker conforming to ASTM Type I requirements and natural gypsum were used as the components of tribo-chemically activated (TCA) HP cements (Table 15.1). These cements were obtained by intergrinding clinker, gypsum, and complex admixture (or SF) in a standard ball mill for 3 h.

The monitoring of the crystalline structure (determined by X-ray diffraction) and particle size distribution (determined by laser diffraction method) was performed for the products obtained at different stages of intergrinding at the time intervals of 1, 2, and 3 h. Mortars were prepared at the sand-to-cement ratio (S/C) of 1 and water-to-cement ratio (W/C) of 0.3 (Table 15.1). The investigation of compressive strength of obtained cements was realized using 51 x 51 x 51 mm cubes following the ASTM C109 procedure. In addition to compressive strength, the flow of mortars was tested

25 30 35 40 45 50 55 20 Fig. 15.4 The effect of surface amorphization due to TCA [20]

using the flow table (according to ASTM C109); a 400 x 400 mm glass plate was used on the tabletop to accommodate the large flows.

The experiment demonstrates that the TCA method can be effectively used to modify cement properties and to design novel groups of cement-based materials such as HP cements, low water demand binders, and high-performance cements. Continuous grinding in a ball mill for 3 h results in significant amorphization of the surface of investigated cements (Fig. 15.4). Even after milling for 1 h, the main diffraction peaks of C3S were still sharp, with higher intensity levels obtained by the cement with SNF modifier; after milling for 2 and 3 h, the intensity of C3S peaks diminishes considerably (Fig. 15.4). These results correspond with the particle size reduction process (Table 15.2).

It can be observed that SNF was the most effective modifier to reduce the particle size and increase the surface area of investigated cements (Table 15.2). After milling for 3 h, the median particle size (D50) and specific surface area of SNF cement were 12.5 im and 415 m2/kg (vs. 13.6 im and 377 m2/kg for reference SF cement), respectively. Still, the efficiency of grinding in a ball mill was not very impressive—in spite of the particle size reduction by up to 40% (achieved with the additional 2 h milling when SNF/SMF modifiers were used), the surface area of investigated cements was not significantly affected (Table 15.2). The cement milled with PAE-based modifier was coarser than the reference cements.

The majority of reported data on TCA HP cements describes the behavior of such cements at a reduced W/C of less than 0.3, usually at the levels near to normal consistency [20]. It was demonstrated that the strength of wide-range compositions obtained at a different S/C and W/C follows the law of W/C (Table 15.1; Fig. 15.5).

Table 15.2 The effect of milling on particle size and surface area of investigated cements

Cement type

Duration of milling (min)

Superficial area (m2/kg)

Average diameter D50 (im)

Cement type

Duration of milling (min)

Superficial area (m2/kg)

NPC

60

227

22.1

120

302

16.5

180

340

12.8

SFC

60

256

21.6

120

311

16.9

180

377

13.6

HPC-SNF

60

285

20.8

120

388

13.5

180

415

12.5

HPC-SMF

60

285

20.8

120

388

13.5

180

415

12.5

HPC-PAE

60

247

24.6

120

274

22.2

180

300

18.8

Fig. 15.5 Strength of TCA Compressive Strength, MPa

HP cements as a function of W/C [20]

150 125

The necessity of using mortars with low S/C (<2, usually 1) for investigating superplasticized and high-strength compositions was already discussed [24]. This investigation was conducted at W/C = 0.3 in order to compare the effects of different modifiers on strength and workability (determined as a flow). It is assumed that mortars with higher flow will follow the W/C law (Fig. 15.5) and increase the strength at lower W/C when tested for constant flow (for example, as it is required by the GOST 310/ASTM C928). This adopted test procedure also accounts for the reduction of the superplasticizing efficiency of the modifier due to the change of oligomer's chain size and molecular weight distribution that occurs during TCA.

The flow of all investigated mortars was higher than that of reference portland-and SF-based cements (Table 15.1). While the SNF-based modifier almost lost its

Npc Cement
Fig. 15.6 The nanostructure of a NPC and b HP cement at early-stages of hydration [20]

plasticizing effect (vs. SF cement), the SMF and, especially, PAE modifiers preserved their supreme plasticizing efficiency.

A significant difference between NPC and TCA HP cements at the early stage of hydration (1 h) can be revealed with cryogenic SEM (Fig. 15.6). It can be observed that during this early period, NPC is hydrating rapidly and generating significant quantities of portlandite; in the same period, HP cement with SNF admixture still maintains the dormant period without visible formation of port-landite and C-S-H (Fig. 15.6). Such behavior can help to tailor the performance of cement systems to induce the controlled workability patterns and, at the same time, preserve the intensive rates of strength development.

The 28-day compressive strength of all investigated cements was very high, at the range of 93-115 MPa vs. the strength of reference cements of 72-89 MPa. The highest strength of 115.4 MPa was demonstrated by SMF-modified cement. It was reported that the strength of HP cements increases significantly from the very early ages of hardening (apparently, from 5 to 12 h) [20]; however, when tested at the same W/C, the 1-day strength of SNF and PAE modified cements was improved only by 5 and 12% vs. reference portland cement. On the other hand, the strength of these mortars was significantly higher than that of SF cement. The application of SMF additive reduced the 1-day compressive strength to 40.3 MPa. Meanwhile, the additives based on SMF and PAE were most effective in improving the 7-day strength, up to 91.5 and 88.1 MPa, respectively.

The 90-day strength of SMF-based mortar was quite remarkable, i.e., 124.4 MPa; this cement demonstrated steady growth during the investigated period throughout. Reduction of 90-day strength vs. 28-day level was observed for NPC; this is a common observation for finely ground cements, which can be related to the accelerated hydration and consequent stress accumulation.

In summary, tribo-chemical activation is a very effective method to manipulate the structure of portland cement-based systems at the nanolevel. The developed modifiers form the organo-mineral nanolayers (as mono/bi-molecular layers) or nano-grids (when the functional groups are attached only to the active centers) on the surface of cement particles. Using this method, the performance of the resulting cement can be tailored as desired—from high-strength materials to the products with controlled workability.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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