Macro-defect-free Research Articles

Ultra high performance concrete: recent applications and research

With advances in concrete technology, ultra high performance concrete (UHPC) has become a new focus for researchers and the concrete industry. UHPC is characterized by high compressive strength and excellent durability properties resulting in lighter structures and longer life. Different approaches have been adopted by researchers in achieving ultra high strength and associated other improved performances. As a result, several types of UHPC are available today such as Reactive Powder Concrete (RPC), Densifi ed with Small Particles (DSP) concrete, Special Industrial Concrete (BSI), Macro Defect Free (MDF) concrete, Self Compacting Concrete (SCC), Compact Reinforced Concrete (CRC) etc. A general description and their range of applications are reported here. In addition, Research at RMIT on Modifi ed Reactive Powder Concrete and very high strength (VHSC) and ultra high strength concrete (UHSC) is briefly outlined.

On the fracture toughness of calcium aluminate cement–phenol resin composites

Macro-defect-free (MDF) cement with high flexure strength has been an active research area over several decades. To study the tensile properties of these materials, it is essential to understand the mode I crack propagation. In this article, cleavage cracking in calcium aluminate cement (CAC)–phenol resin composites is analyzed based on an energy method. The crack-trapping effect of the cement particles is found to be significant. The fracture toughness rises with the particle size and is independent of the spacing between the particles. When the cement volume fraction is higher than a critical value the effective work of separation of the phenol resin decreases with the particle content with a coefficient of −1.88.

Synthesis, moisture resistance, thermal, chemical and SEM analysis of macro-defect-free (MDF) cements

The results, presented here discusses the Macro-Defect-Free (MDF) cements prepared from the blends of sulfoaluminate ferrite belite (SAFB) clinkers, ordinary Portland cement (OPC), Al2O3 and poly(butyl acrylate) (PBA), styrene/acrylonitrile co-polymer (SACP) or sodium polyphosphate (poly-P). Though MDF cements have several attractive properties, their utilization has been limited due to the insufficient moisture resistance. It is a very challenging task for scientists and technologist to improve the moisture resistance of MDF cements. Therefore, the new MDF cements were subjected to various moist atmospheres to investigate their moisture resistance. The most significance of this work is the improvement of moisture resistance of the studied MDF cements. The aim of this work was to understand the effects of polymers, Al2O3, OPC and SAFB clinkers in the raw mix and delayed drying on MDF cements and also on their subsequent moisture resistance and thermal stability as well as to discover the new properties of these materials. Their chemical, thermal and scanning electron microscopic (SEM) analysis was also carried out before and after exposure to moisture. PBA was found to be the most suitable polymer for MDF cement synthesis, since the samples containing PBA showed the highest resistance to moisture. There are three main temperature regions on TG curves of both series of MDF cement samples. The significant differences in SEM of MDF cements before and after moisture attack and with different polymers were observed.

Chemical reaction between polyvinyl alcohol and titanate coupling agent with x-ray photoelectron spectroscopy

The chemical reaction between polyvinyl alcohol (PVA) and tri (dioctylpyrophosphoryloxy) isopropyl titanate (NDZ-201) was studied using X-ray photoelectron spectroscopy (XPS). The results show that some C−OH functional groups of PVA react with the titanate coupling agent to form CPVA−O−Ti−O−CPVA bond. The cross-linking of the PVA chains occurs through the formation of CPVA−O−Ti−O−CPVA bonds and produces a three dimensional hydrophobic polymer network. Accordingly, the mechanism is proposed that the titanate coupling agent improves the noisture sensitivity of high alumina cement/polyvinyl alcohol (HAC/PVA) based macro defect free (MDF) composite material.

Macro-Defect-Free (MDF) Cements with High Moisture Resistance: Chemical, Thermal, SEM and Magnetometric Study

Macro-Defect-Free (MDF) Cements with High Moisture Resistance: Chemical, Thermal, SEM and Magnetometric Study

Macrodefect-Free Cements: The Effect of Chemical Composition on Microstructure

The mechanical properties and microstructures of three macrodefect-free (MDF) composites, prepared from Portland and high or medium alumina cements, were evaluated. Transmission electron microscopy...

Influence of organic coatings on the stability of macrodefect-free cements exposed to water

It is known that macrodefect-free (MDF) cements show instability in water, with swelling and reduction in strength. The weight and dimensional changes of three MDF cements, formulated from different aluminous cements, during immersion in quiescent tap water, were evaluated. The results show that the cement with the highest Al 2O 3 percentage had the lowest degradation after prolonged immersion. Hence, this material was selected for further analysis. As the time taken for water to diffuse into the body of MDF composites is considered a key factor in the collapse of their mechanical properties, the effect of organic coatings with different barrier properties on the composite behaviour, when exposed to water, was studied. The water permeability of seven surface coatings was evaluated by electrochemical impedance spectroscopy (EIS). Three of them were chosen for application to the selected MDF cement. The data presented show that the application of organic coatings, not necessarily characterised by extremely low water permeabilities, improves the stability of the composite when subjected to different conditions of exposure to water.

Changes of thermal events of macrodefect-free (MDF) cements due to the deterioration in the moist atmosphere

Sulfoaluminate ferrite belite (SAFB) clinker premixed with Portland cement in mass ratio 85:15 was combined with hydroxypropylmethyl cellulose (HPMC) or sodium poly-phosphate (poly-P) for the macrodefect-free (MDF) process and subsequent moisture treatment. Mass changes as the measure of the moisture resistance and thermoanalytical traces of MDF cements gave useful information. The mass change depends more on the humidity than on the composition (polymer and inorganic constituents) of the MDF cement or duration of the original MDF cement synthesis. The values of mass changes at 100% relative humidity (RH) and at ambient conditions are affected by the nature of the polymer, poly-P being highly advantageous. Attack by environmental water and carbon dioxide increases the contents of C 4( A, F) S̄H 12( AFm) and CaCO 3, represented by thermogravimetry (TG) effects at 250°C and differential thermal analysis (DTA) peaks at 600–700°, respectively. In contrast, the quantity of cross-linking, where the decomposition of Al(Fe)–O–C(P) bonds typically produces TG–DTA effects at 250–550°C, remains intact during the moisture attack.

Processing-Moisture Resistance and Thermal Analysis of Macro-Defect-Free Materials

The system of sulfoaluminate ferrite belite (SAFB) clinkers premixed with Portland cement (PC) in mass ratio 85:15 in combination with hydroxypropylmethyl cellulose (HPMC) or polyphosphates(poly-P) was used for the syntheses of Macro-Defect-Free (MDF) materials. The subsequent moisture treatment and thermal stability of these MDF materials were investigated. The effect of individual humidity upon the evolution of mass is more intensive than the effects of composition of MDF materials or duration of the original MDF material synthesis. Detailed values of mass changes at 100% relative humidity (RH) and under ambient conditions are strongly affected by the nature of polymer used. A significant improvement of moisture resistance of MDF materials is achieved when the materials are dried after 24 h of finishing the pressure application. In the inter-phase section of MDF material samples, the content ofC-(A,F)-S hydraulic phases, mainly tetracalcium aluminate ferrite monosulphatehydrate (AFm) decomposing by 250°C and CaCO3 decomposing at 600–700°C increase after the moisture attack, while cross-links in AFm-like section with typical thermoanalytical traces in temperature region 250–550°C remain intact.

建設材料 ホットプレス成形によるＭＤＦセメント硬化体の耐水性改善

In general, MDF (macrodefect-free) cements are produced by using portland cements and water-soluble polymers, and applying mechanochemical processing techniques at very low water-cement ratios, and have flexural strengths of 50 to 70MPa under dry conditions. However, MDF cements have a poor water resistance because of the nature of the water-soluble polymers used, therefore, the precast products made of MDF cements have not been commercialized in the world. MDF cements using a portland cement, a water-soluble polyacrylamide, an epoxy resin and a phenol resin are prepared with various mix proportions by applying a kneader mixing process, hot press molding process and heat cure, and tested for water resistance through a change in flexural strength and water absorption during water immersion. The effects of the mix proportions for the hot press molding process on the water resistance of MDF cements are examined to find out the effective processing techniques for improving their water resistance. As a result, the water resistance of MDF cements is markedly improved by applying the hot press molding process and heat cure to them with the optimum mix proportions, i.e., a polyacrylamide content of 6.0%, an epoxy resin content of 6.0%, a phenol resin content of 2.0% and a water-cement ratio of 8.0%. The application of the hot press molding process to MDF cements is considered to be effective for the simplification of their manufacturing process.

Applications of High-Performance Fiber-Reinforced Cement-Based Composites

Advanced composites and the fundamental understanding of their behaviour is a rapidly expanding branch within the field of civil engineering materials. In particular, fiber-reinforced cement-based materials have had a great evolution in these years, so that they are more and more utilized in the building sector. Besides, large efforts have been made to develop high-performance cements and concretes showing further performance improvements. High-performance fiber-reinforced cement composites include, for example, materials such as SIFCON (Slurry Infiltrated Fiber CONcrete), fiber-reinforced DSP (Densified with Small Particles), and fiber-reinforced MDF (Macro-Defect-Free) cements. Developments of these materials were possible due to: (a) the introduction of new reinforcement systems; (b) the development of high-performance cement-based matrices, which present greatly improved microstructural properties in terms of strength and durability; (c) the development of adequate processing techniques (including controlling chemical reactions) which allow us to obtain composite materials with surprising toughness properties. After a brief description of high-performance cement-based matrices and relative composites, two different examples among the above-cited products will be deeply presented.

Morphology of the poly(vinyl alcohol)–poly(vinyl acetate) copolymer in macrodefect-free composites: a 13C magic-angle-spinning nuclear magnetic resonance and 1H spin-diffusion study

A study was made of the macrodefect-free (MDF) composite based on aluminate cement and a poly(vinyl alcohol)–poly(vinyl acetate) (PVAc) copolymer by 13C cross-polarization magic-angle-spinning nuclear magnetic resonance. The spectra were run on both the copolymer and the MDF composite in order to observe the atomic environments of the carbon nuclei. An analysis of the intramolecular hydrogen bonds showed a stronger modification in the sample containing CaAl2O4 (CA) than in that containing Ca3Al2O6 (C3A). From the spectra, it was seen that deacetylation had occurred, and that there was interaction between acetate groups and the calcium and aluminium ions. Proton relaxation times both in the laboratory frame, T1(H), and in the rotating frame, T1ρ(H), were exploited for the study of the dimension of the polymer matrix in MDF composites. An extended interphase was identified in the composite containing CA. A comparison with the behaviour of the composite based on silicates Ca3SiO5–PVAc highlights the better mixing of the phases in the MDF composites containing CA.

Fibre-matrix adhesion in fibre reinforced CAC-MDF composites

Macro-defect-free (MDF) cements are cement-polymer mixtures characterised by high tensile and flexural strengths. The use of fibres in the MDF cementitious matrices can further improve their mechanical properties. This paper covers a study of the adhesion between steel, polyethylene (PE) and polyvinylalcohol (PVA) fibres and calcium aluminate cement (HAC) MDF pastes. Properly processed, single fibre MDF samples were subjected to pull-out tests in order to estimate the interfacial shear stress between the fibre and the matrix. The analysis of the resulting pull-out curves for the three fibre-cement matrix pairs has also been performed, yielding interface strength values for fibre-cement interface. Among the polyethylene, polyvinylalcohol and steel fibres used for this study, the first ones turned out to be most promising to improve toughness and impact performance of CAC-MDF matrices.

Evolution of mechano-chemistry and microstructure of a calcium aluminate-polymer composite: Part II. Mixing rate effects

Microstructure, microchemistry and mechanical properties of hardened macro-defect-free (MDF) composites processed at various rotor rates in a Banbury mixer were investigated. A quiescently formed calcium aluminate-polyvinyl alcohol composite served as a substitute for an unmixed system. Results from the Banbury studies in conjunction with microchemical analysis of the unmixed composite showed evidence that the polymer-particle interaction is a mechanically induced crosslinking reaction. The rate of the mechano-chemistry increases with mixing speeds. Scanning electron micrographs (SEM) and transmission electron micrographs (TEM) of hardened composites mixed for 15 min at 30, 50, 100, and 200 rpm indicate that much of the mechanical strength of MDF is due to the crosslinked interphase zones that blanket the cement grains. Stresses in the paste due to mixing can destroy the interphase layer, leading to a weaker hardened composite. Microchemical analysis revealed that the mechano-chemistry of the system did not vary with changes in the mixing conditions studied.

Effects of an Organotitanate Cross-linking Additive on the Processing and Properties of Macro-Defect-Free Cement

We have investigated the effects of an organotitanate cross-linking additive on the processing and moisture sensitivity of macro-defect-free (MDF) cements. The modified samples were formed by a two-part process: first, the as-received calcium aluminate cement powder was coated with the organotitanate compound, followed by the standard MDF cement fabrication procedure. It was estimated that 6 wt% (by weight of the cement powder) of the additive was adsorbed. Rheological measurements showed that cross-linking reactions between the organotitanate additive and the polymer (polyvinyl alcohol-acetate) used to form MDF cement occurred during the high shear mixing process. This led to a 65% reduction in the processing window for the modified pastes as compared to the standard MDF cement pastes. However, the organotitanate-modified MDF cement samples exhibited improved moisture resistance as compared to the standard samples when exposed to 100% relative humidity at 22°C. The most significant result was that the modified samples retained nearly 100% of their initial strength after 200 days of exposure to these conditions, whereas the strength of the standard MDF samples was decreased by approximately 45%.

Effects of an organotitanate cross-linking additive on the processing and properties of macro-defect-free cement

Abstract We have investigated the effects of an organotitanate cross-linking additive on the processing and moisture sensitivity of macro-defect-free (MDF) cements. The modified samples were formed by a two-part process: first, the as-received calcium aluminate cement powder was coated with the organotitanate compound, followed by the standard MDF cement fabrication procedure. It was estimated that 6 wt% (by weight of the cement powder) of the additive was adsorbed. Rheological measurements showed that cross-linking reactions between the organotitanate additive and the polymer (polyvinyl alcohol-acetate) used to form MDF cement occurred during the high shear mixing process. This led to a 65% reduction in the processing window for the modified pastes as compared to the standard MDF cement pastes. However, the organotitanate-modified MDF cement samples exhibited improved moisture resistance as compared to the standard samples when exposed to 100% relative humidity at 22°C. The most significant result was that the modified samples retained nearly 100% of their initial strength after 200 days of exposure to these conditions, whereas the strength of the standard MDF samples was decreased by approximately 45%.

Unreacted cement content in macro-defect-free composites: impact on processing-structure-property relations

The effect of unreacted cement content on the processing, structure, and properties of macro-defect-free (MDF) composites fabricated from calcium aluminate cement (CAC), α-alumina (Al2O3), and polyvinyl alcohol-acetate (PVAA) has been investigated. Three systems were formed with initial CAC: Al2O3 ratios of 50∶50, 35∶65, and 25∶75 by volume in their respective formulations. The amount of unreacted cement was reduced from 68.1 vol% which is present in standard (100% CAC) MDF cement, to 14.9 vol % for composites with an initial CAC: Al2O3 ratio of 25∶75, while the hydration product content was reduced from 18.1 vol % to 11.4 vol % for these respective systems. A hard core/soft shell continuum percolation model was used to determine that alumina substitution did not significantly affect the percolative nature of the interphase and bulk polymer regions. However, experiments showed that the reduction in unreacted cement content through Al2O3 substitution affected both the processing and microstructural features of these composites. The moisture absorption kinetics and flexural strength of composites exposed to 100% relative humidity were also evaluated, and it was found that their moisture sensitivity improved with decreasing unreacted cement content. A hypothesis is presented to explain the role of unreacted cement on the moisture sensitivity of these materials.

Viscoelastic behaviour of macro-defect-free cement

The viscoelastic behaviour of macro-defect-free (MDF) cement was studied by dynamic mechanical analysis. MDF specimens with various moisture contents in the range 1.70%–3.20% moisture, were measured at 1 Hz as a function of temperature from 34–96 °C and as a function of frequency from 0.05–5 Hz at 34°C. The viscoelasticity of MDF was found to be affected by moisture content through its plasticizing effect on the PVA binder. Time-moisture and temperature-moisture superposition of the shear moduli were found to be possible for MDF, and linear relationships between log time and linear moisture and between temperature and moisture were found. How the microstructure of MDF affects the viscoelastic response is also discussed through mechanical models. A comparison of the models with known experimental data suggests that the viscoelastic response arises from both direct connections between the inorganic particles and from connections through the polymer binder. Inorganic links are estimated to connect 45% of the inorganic phase.

Binder Distribution in Macro‐Defect‐Free Cements: Relation between Percolative Properties and Moisture Absorption Kinetics

Macro‐defect‐free (MDF) cement is fabricated from a calcium aluminate cement and a poly(vinyl alcohol‐acetate) (PVA) copolymer. For the composites studied, it was determined that the interphase regions comprised 63 voi % of the total binder content, while the bulk PVA regions comprised 37 vol% of this phase. Mercury intrusion porosimetry showed that a bimodal pore size distribution developed as binder was removed in increasing amounts from heat‐treated samples. Larger pores with a characteristic diameter above 30 nm resulted from the removal of bulk PVA, whereas smaller pores approximately 5 nm in size resulted from the removal of water and PVA from the interphase regions. Simulation results obtained from a hard‐core/soft‐shell continuum percolation model of the MDF microstructure indicate that both the bulk PVA and interphase regions form percolative pathways through the system. Dramatic changes in both moisture absorption kinetics and flexural strength were observed only when a percolative network of larger pores was present in these composites. Hence, the bulk polymer regions are the dominant transport pathway for moisture in MDF cement. Based on this knowledge, processing guidelines have been developed to improve the moisture resistance of these materials.

Microstructure-Property Relationships in Macro-Defect-Free Cement

The term “macro-defect-free” refers to the absence of relatively large voids (or defects) that are normally present in conventional cement pastes due to entrapped air or inadequate mixing. A decade ago, Birchall and co-workers developed a novel processing method that avoids the formation of these strength-limiting defects. This method, outlined schematically in Figure 1, consists of mixing hydraulic cement powder, a water-soluble polymer, and a minimal amount of water under high shear to produce a macro-defect-free (MDF) cement composite. Several cement/polymer systems can be processed by this flexible technique, although the calcium aluminate cement/polyvinyl alcohol-acetate (PVA) copolymer system is most common: MDF cements display unique properties relative to conventional cement pastes. For example, the flexural strength of MDF cement is more than 200 MPa as compared to values on the order of 10 MPa for conventional pastes. One can view MDF cements as a type of “inorganic plastic.” As is the case with plastic processing, fillers such as alumina, silicon carbide, or metal powders can be added to MDF cement to modify its performance properties (e.g., abrasion resistance, thermal or electrical conductivity, and hardness). The combined attractiveness of inexpensive raw materials and flexible, low-temperature processing has generated great interest in this new class of advanced cement-based materials.