Macro-defect-free Research Articles

Processing of Optimized Cements and Concretes Via Particle Packing

It has been well-recognized for many years that the particle-size distributions of the cement and the grading of the aggregates play an important role in determining the properties and characteristics of cement and concrete products. DSP (densified with small particles) type cements and concretes, to a certain extent, MDF (macro-defect-free) cements, and optimized concretes are recently recognized outstanding examples of the application of this principle. The preset characteristics of the cementitious slurry are also strongly influenced by these factors. Both the workability of the fresh material, and the microstructure development are controlled to a considerable extent by these geometric parameters.Two seminal works in the areas of continuous particle size distributions and particle packing are those of Andreason and Furnas, respectively. Furnas deals mainly with discrete systems and Andreason with continuous distributions. As early as 1907, the concept of idealized particle packing was being used to optimize cements and concretes. Figure 1a shows an idealized cross section of a simple cubic packing of monodispersed spheres. This system has a maximum packing density of 0.65%. In an ideally packed system of discrete size ranges, the size of the next smallest particles would be such that they just fit in the gaps between the largest size particles, and so on for subsequent particle sizes; this system is represented schematically in Figure 1b. Not only the sizes but also the relative numbers of particles are important; Figures 1c and 1d show systems where some fraction of the smaller and larger particle sizes, respectively, are missing. Figure 1e shows a system where the size of the second largest particles is too large to fit into the gaps between the largest particles, resulting in a lower packing efficiency. Thus, both the particle size and fractions are important when considering packing efficiency.

Evaluation of interfacial mechanical properties in SiC fiber-reinforced macro-defect-free cement composites

The application of a micro-indentation technique for the measurement of interfacial mechanical properties in a fiber-reinforced cement composite has been examined. The composite was formed by placing aligned SiC fiber tows between macro-defect-free (MDF) cement sheets after which the compact was warm pressed at 6·89 MPa (1000 psi) at 80°C. The interfacial characteristics of the composite were varied by modifying the surfaces of the fibers prior to their incorporation into the matrix. These modifications included either an application of a stearic acid film or 1000°C gas phase treatments in air, oxygen, or nitrogen for one hour. These four fiber surface treatments served as the independent parameter for the mechanical testing analysis. The interfacial characteristics, including the shear, the residual axial fiber, and debond stresses, were evaluated with a mechanical properties microprobe by measuring the force/displacement curves generated during load-unload cycling. Estimates of these three stress values were obtained by matching the experimental force/displacement curves with data predicted from an existing model. For two of the four systems investigated, relatively high residual axial fiber compressive stresses were required adequately to describe the large fiber displacement recovery obtained after complete unloading. The stresses were believed to have resulted from the large differential shrinkage between the fibers and matrix during processing. The shear and debond stress values were highest for those fibers oxidized in air and lowest for fibers coated with stearic acid. The calculated interfacial shear stress value becomes significantly overestimated when the residual axial stress has a large magnitude and is not taken into account in the calculation.

Microstructural and Microchemical Characterization of a Calcium Aluminate—Polymer Composite (MDF Cement)

The microstructure of a macrodefect‐free (MDF) cement has been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and high‐resolution electron microscopy (HREM). The microchemistry of ultramicrotomed samples has been studied by energy dispersive spectrometry (EDS) and parallel electron energy loss spectrometry (PEELS). MDF cement consists of CaAl2O4 and CaAl4O7 grains randomly distributed in a polymer matrix. The ceramic/polymer interface contains an amorphous interphase inside of which are distributed very fine crystallites of the metastable hydration product Ca2Al2O5·8H2O. PEELS analyses of the interphase revealed the presence of carbon, indicating that Ca2Al2O5·8H2O was most likely stabilized by the intercalation of polymeric chains into its basal interlayers. The polymer phase preferably cross‐links with Al. In situ environmental cell electron microscopy showed that moisture uptake of MDF cements occurred by polymer swelling and interphase dissolution. The role of the interfacial interphase in dry and wet mechanical properties is discussed.

Environmental degradation of macrodefect-free cements

The effect of different environments on the flexural strength of a range of macrodefect-free (MDFs) cements was studied. The results showed significant differences in behaviour of the MDFs. These differences were found to be clearly related to the cement system the MDF was made from.

Cross-Linking Reactions in Macro-Defect Free Cement Composites

ABSTRACTThe key to the processing and properties of Macro-Defect Free (MDF) cement composites is the interaction between the cement and the polymer. In calcium aluminate (CA) / polyvinyl alcohol (PVA) MDF composites, it has been hypothesized that aluminate (Al(OH)4−) ions released from the cement cross-link PVA in a manner similar to the well known interaction of the borate (B(OH)4−) ions with PVA. In this investigation, stable aluminate/PVA solutions at different concentrations have been studied. Results indicate that the nature of interaction of aluminate ions is not similar to that of borate ions in dilute and semidilute polymer solutions.

Fracture toughness of macro-defect-free cement using small crack techniques

The fracture toughness of a macro-defect-free (MDF) cement was calculated from two measurement techniques: (1) indentation-strength method and (2) fracture surface analysis (FSA). It was found that the indentation-strength method, which showed good agreement with FSA, was applicable for estimating the fracture toughness of MDF cement. The ultimate toughness was found to be 1.25 MPa m1/2 for this MDF cement, which contained 3 wt. % polymer. An R-curve (crack-growth-resistance) bchavior was also observed. Scanning electron micrographs showed extensive microcracking on the fracture surface. Microstructural effects are discussed.

Dielectric properties of macro-defect-free (MDF) cements

Macro-defect-free cements, based mostly on Secar-71 and using polyvinyl acetate as a plasticizer, have been prepared. Dielectric constant and loss factor at 1 MHz and d.c. resistivity of these cements have been measured as a function of their composition, processing conditions and humidity. For some of the samples, flexural strengths have been measured in order to correlate with their electrical properties. Results of efforts to inhibit the effects of higher humidities on the electrical properties are also reported.

The Chemistry Of MDF Cements Produced From Polyacrylamide-Cement-Water Pastes

AbstractIt is becoming more generally accepted that the role of the polymers in these MDF (macro-defect-free) cement systems is more complex than simply acting as a processing aid or a filler. The polymer interacts in some way with the cations released from the cement to form a metal cross-linked polymer gel. Studies on complex formation in polymer gels have shown that there is often an unusual specificity in the ions, resulting in the highest degree of complexation. In addition, gel formation often occurs only within a narrow pH regime. Hence, the results shown here will attempt to examine the effects of altering the cement chemistry on strength development in calcium silicate-based systems.

Indentation Fracture of Macro-Defect-Free (MDF) Cements

AbstractThe fracture toughness of MDF (macro-defect-free) cement with different polymer contents has been investigated by the indentation-strength method and fracture surface analysis. It was found that the fracture toughness obtained by these two different techniques showed good agreement for the low (3 wt.%) polymer cement. The ultimate (apparent) toughnesses are 1.3 Mpam1/2 for low polymer content and 2.2 Mpam1/2 for high (8 wt.%) polymer content cement. Different microcracking and sponge-like microstructures have been observed and discussed.

A trimethylsilylation study of the silicate anion distribution of macro defect-free portland cement

The silicate anion distribution of two types of Macro Defect-free (MDF) Portland cement was determined using trimethylsilylation. The low hydration degree of these materials was confirmed, indicating essentially unhydrated cement grains bound by organic polymers and small quantities (<10%) of CSH gel. Spectroscopic and thermal analysis data are also presented.

Stability of macrodefect-free cement

The effect of water immersion on the strength and dimension stability of macrodefect-free (MDF) cement has been studied. The results show that the polymer phase in the MDF matrix is unstable in water. Prolonged immersion resulted in substantial expansion and leaching of the matrix. Microstructural changes were examined by scanning electron microscopy, X-ray diffraction, and infrared spectroscopy. The cause of weakening of the matrix is thought to be the modification of the cement/polymer interface through the hydration of anhydrous grains. Methods of improving the performance of MDF pastes in water have been considered.MST/692

Advanced materials from hydraulic cements

Hydraulic cements are energy-cheap relative to other common materials and this was the driving force that led to the development of macrodefect-free (MDF) cements in 1981. In this paper the scientific principles of such materials in terms of porosity, particle packing and rheology are demonstrated and the use of a range of different cement types will be reviewed. The role of the polymeric rheological aid is emphasized and distinguished from that of so-called super-plasticizers used with conventional cements. The distinction between MDF cements and polymer concretes of various types is highlighted and the similarity of MDF cements and chemically bonded ceramics (CBCS) when the polymer is removed is noted. Various properties of MDF cements are given with emphasis on their high degree of mechanical reliability. These properties place MDF cements in a unique position in the materials field, a position that makes the replacement of hitherto more advanced engineering materials, such as metals, polymers and ceramics, a viable proposition. Areas of use are discussed.

Macro Defect Free (MDF) Cements

SUMMARY Ordinary cement products based on calcium silicate (Portland cement) or calcium aluminate (refractory, rapid hardening cements) exhibit a number of interesting properties but are deficient in strength and toughness. The purpose of the ICI work on cements was to understand the cause of these deficiencies in order to design improved cement materials. Our early study of existing products showed that strong cements can be produced by high pressure moulding of cement powder, a phenomenon discovered about 50 years ago. Unfortunately this is only practical for small articles such as catalyst supports. A more recent empirical discovery was the improvement in properties of cements extruded using polymer extrusion aids. This process was patented in 1976. The problem was to explain these effects. Review of the literature also revealed a remarkable paradox in the theory of cement strength. Traditionally it had been believed that cement strength depended on the volume of pores in the material, which could be up to 30% porous. In contrast, it was also thought that cement behaved as a brittle material, in which case the presence of long cracks would be most critical. Our contribution was to propose and verify a theory which explained the separate effects of volume porosity and long crack-like pores. The study of cement microstructure and rheology demonstrated that large crack-like pores were the main problem in cements. Processes for removing these pores were developed and shown to give cement products an order of magnitude better than those on the market. The paper describes the progress of this development in detail.

Ultra-Low Dielectric Permitfivity Ceramics and Composites for Packaging Applications

AbstractTo accomplish the interconnect systems which will be required in the next generation of very high speed Ga:As digital ICs, it will be necessary to use strip line techniques for signal traces which must be deposited over very low permittivity dielectric substration. Materials with relative dielectric permittivities k 〈 3.0 and very low loss tangent up to microwave frequencies will be required. For ceramic systems such values are impossible to achieve in single phase monoliths, and composite approaches are required. Techniques for processing ceramic insulators which permit the introduction of controlled pore structures are discussed. The introduction of pores degrades some other desirable properties of the ceramic such as mechanical strength and thermal conductivity so that control of both scale and location of pores is desirable.Materials investigated include sol-gel processed silica films and monoliths, reactively sputtered silica, etched glass compositions and Macro-Defect-Free (MDF) cements.

Fibre toughening of MDF cement

Fibre reinforced cements are of considerable industrial importance, the production of asbestos cement amounting to about 5 m tonne per annum. The search for fibres other than asbestos has led to the investigation of glass, polypropylene, cellulose (and other fibrous materials) in a cement matrix. Whilst there has been much work on the fibrous component of cement-fibre composites little attention has been paid to the effect on the final composite properties following improvements in the cement matrix. The preparation of macro-defect-free (MDF) cements has recently been reported and the work presented here describes the preparation and the properties of fibre-reinforced MDF cement composites. The Young's modulus of elasticity and the strength of MDF cements are greatly improved and do not need fibrous reinforcement to show benefit. The main purpose of reinforcing MDF cement with fibres is to enhance toughness and impact performance. It is shown that small volume fractions of fibre (∼2%) in the MDF cement matrix produce composites with excellent toughness and impact properties.

The Properties and Potential Applications of Macro-Defect-Free Cement

AbstractThe properties of Macro-Defect-Free (MDF) cements are reviewed. Combinations of these properties place MDF cements in a unique position in the materials field, a position which makes direct replacement of current engineering materials such as metals plastics and ceramics a viable proposition. Perhaps of more importance is the invention of devices and components hitherto unavailable because of the property-restricted nature of available materials. Examples are given of unusual uses for cement. These include applications in the fields of cryogenics, ballistic protection, electromagnetic radiation screening and acoustics.