Effective Properties of Textile Composites: Application of the Mori-Tanaka Method

An efficient approach to the evaluation of effective elastic properties of carbon-carbon plain weave textile composites using the Mori-Tanaka method is presented. The method proves its potential even if applied to real material systems with various types of imperfections including the non-uniform waviness of the fiber-tow paths, both along its longitudinal direction and through the laminate thickness. Influence of the remaining geometrical parameters is accounted for by optimal calibration of the shape of the equivalent ellipsoidal inclusion. An application of the method to a particular sample of the carbon-carbon composite laminate demonstrates not only its applicability but also its efficiency particularly when compared to finite element simulations.Comment: 18 pages, 6 figure

Effect of cyclic freezing and thawing on the microstructure of composite cements

Mixed performance of composite cements exposed to freeze-thaw has been reported. A detailed understanding of the degradation mechanism is also lacking. This study investigates the microstructure of composite slag cements with and without limestone subjected to cyclic freezing and thawing. Freeze-thaw was assessed on concrete samples in accordance with CEN/TR 15177 but with a modified temperature profile. Microstructure was characterized by SEM and thermogravimetric analysis. The results indicate decalcification through carbonation and then leaching as dominant degradation mechanisms. This has implications on the pore structure and hence the water suction capacity and progression of the ice-front in concrete

Effect of sulfate additions on hydration and performance of ternary slag-limestone composite cements

The global cement industry is striving to reduce its carbon footprint. Common approaches have included reduced clinker factors by blending cement clinker with supplementary cementitious materials (SCM). However supplies of SCMs are not sufficient to achieve replacement above about 30 %. Limestone ternary cements offer the opportunity to reduce the clinker factor of cements while maximizing the efficiency of SCMs. In these cements, calcite from limestone reacts with dissolved aluminates to form carboaluminate and in the process influence hydration of other constituents. However, sulfates which are conventionally added to regulate the early reactions in cement also compete for aluminates. Here we have used complementary techniques to investigate the effects of calcium sulfate additions on hydration, microstructure and performance of composite Portland clinker-slag-limestone cements. The results show that the presence of sulfate influenced the early-age reaction kinetics of the clinker phases and supplementary cementitious materials. However, even after sulfate depletion, the course of hydration and microstructures formed were significantly influenced. Increasing the sulfate level resulted in a gradual increase of the fraction of ettringite over AFm phases, coarser porosity and lower water content of the C-S-H. These microstructural changes impact the total porosity and hence cement strength in opposing ways, namely porosity is reduced with increasing ettringite fraction while the space filling capacity of the C-S-H is also reduced due to the lower water content of the C-S-H. These findings have important implications for optimizing the mechanical properties and durability of ternary blends

Relationship between cement composition and the freeze-thaw resistance of concretes

Concrete exposed to cyclic freezing and thawing may deteriorate by surface scaling, internally developed cracks or both in combination. The rate of deterioration tends to be accelerated in concretes containing higher levels of supplementary cementitious materials including slag and limestone. A fundamental insight into the relationship between cement composition and freeze–thaw resistance is therefore imperative for developing durable composite cement concretes. Concrete samples prepared from CEM I, binary slag cements and ternary limestone slag cement blends at 0·5 w/b ratio without air entrainment were investigated. The freeze–thaw test was based on the CIF method according to PD CEN/TR 15177. Additionally, phase assemblages in the concretes before and after freeze–thaw damage were evaluated. Before freeze–thaw testing, compressive strengths were similar but the composite cements were slightly more susceptible to carbonation. However, the scaling and internal damage resistance decreased in the order of CEM I, binary and limestone ternary blended cements. The composition of the scaled material differed from the bulk, revealing an absence of portlandite and a marked reduction in AFm and ettringite contents. A probable explanation for the reduced freeze–thaw resistance includes the porosity differences and the lower portlandite content compared to CEM I concrete

Freeze-thaw resistance of concrete: Insight from microstructural properties

Composite cements offer low carbon alternatives to conventional CEM I. These also generally tend to perform better than CEM I in aggressive chemical environments. However, the freeze-thaw resistance, evident through surface scaling and internal damage is usually impaired. Postulated theories on freeze-thaw induced damage do not fully explain the origin of this weakness in composite cement concretes. This contribution systematically presents the phase assemblage changes associated with the freeze-thaw of concrete specimen made from composite cements with and without limestone. The freeze-thaw test was performed on concrete according to CIF method based on CEN/TR 15177 and the corresponding cement pastes characterized by X-ray powder diffraction (XRD) and thermogravimetric analysis (TGA). In all investigated composite cements, portlandite was already depleted after the 7d capillary suction. The implications of this and other modified assemblages during the conditioning and the freeze-thaw test are consequently discussed

Impact of microstructure on the performance of composite cements: Why higher total porosity can result in higher strength

This paper describes the underlying principles behind the evolution in performance of ternary composite cements comprising Portland cement clinker, slag and limestone. By using the predicted phase assemblage as an input for the micromechanical model, the mechanisms underlying the evolution of mortar strength and Young's modulus were analyzed and quantified. This allowed the roles of hydrate assemblages and porosity distribution on the evolution of performance to be explained and quantified. Slag hydration results in the formation of a microstructure more efficient for development of compressive strength and elastic stiffness. Limestone further improves microstructure and enhances reactivity of the systems studied