Showing papers on "Cement published in 2013"

Modification of cement-based materials with nanoparticles

Abstract: This is a summary paper on the work being done at the Center for Advanced Cement-Based Materials at Northwestern University on the modification of cement-based materials with nanoparticles, specifically nanoclays, calcium carbonate nanoparticles, and nanosilica. The rheological properties of clay-modified cement-based materials are investigated to understand the influence of nanoclays on thixotropy. The influence of the method of dispersion of calcium carbonate nanoparticles on rate of hydration, setting, and compressive strength are evaluated. And an in-depth study on the mechanisms underlying the influence of nanosilica on the compressive strength gain of fly ash–cement systems is discussed. The motivation behind these studies is that with proper processing techniques and fundamental understanding of the mechanisms underlying the effect of the nanoparticles, they can be used to enhance the fresh-state and hardened properties of cement-based materials for various applications. Nanoclays can increase the green strength of self-consolidating concrete for reduced formwork pressure and slipform paving. Calcium carbonate nanoparticles and nanosilica can offset the negative effects of fly ash on early-age properties to facilitate the development of a more environmentally friendly, high-volume fly ash concrete.

Long-term mechanical and durability properties of recycled aggregate concrete prepared with the incorporation of fly ash

Abstract: This paper presents the findings of a long-term study on the mechanical and durability properties of concrete prepared with 0%, 50% and 100% recycled concrete aggregate that were cured in water or outdoor exposure conditions for 10 years. The recycled aggregate concrete (RAC) was prepared by using 25%, 35% and 55% class-F fly ash, as cement replacements. It was found that, after 10 years, the compressive strength and modulus of elasticity of the concrete prepared with 100% recycled concrete aggregate was still lower than that of the control concrete. Over this period, the highest gain in compressive strength and modulus of elasticity was recorded for the concrete mixture prepared with 55% fly ash. Fly ash improved the resistance to chloride ion penetration but it also increased the carbonation depth of the concrete.

Behaviour of cement concrete at high temperature

Abstract: The paper presents the impact of high temperature on cement concrete. The presented data have been selected both from the author's most recent research and the published literature in order to provide a brief outline of the subject. The effect of a high temperature on concrete covers changes taking place in cement paste, aggregates, as well as the interaction of these two constituents, that result in changes of mechanical and physical characteristics of concrete. This paper presents the effects of a high temperature on selected physical properties of concrete, including colour change, thermal strain, thermal strains under load, and transient thermal strains. In addition, changes to mechanical properties are discussed: stress-strain relationship, compressive strength, and modulus of elasticity. Moreover, the phenomenon of explosive spalling and the main factors that affect its extent are analysed in light of the most recent research. Interest in the behaviour of concrete at a high temperature mainly results from the many cases of fires taking place in buildings, high-rise buildings, tunnels, and drilling platform structures. During a fire, the temperature may reach up to 1100 ◦ C in buildings and even up to 1350 ◦ C in tunnels, lead- ing to severe damage in a concrete structure (1). However, in some special cases, even much lower temperature, may cause explosive destruction of concrete, thus endangering the bear- ing capacity of the concrete element. Nevertheless, concrete is considered a construction material that satisfactorily preserves its properties at high temperature. Owing to concrete's fairly low coefficient of thermal conductivity, the movement of heat through concrete is slow, and thus reinforced steel, which is sensitive to high temperature, is protected for a relatively long period of time. When concrete is heated under conditions of fire, the increase in temperature in the deeper layers of the ma- terial is progressive, but because this process is slow, signifi- cant temperature gradients are produced between the concrete member's surface and core inducing additional damage to the element. Fundamental issues related to the impact of high temperature on concrete involve identification of the complex changes that take place in concrete while heated. This con- cerns both the physical and chemical changes taking place in the cement matrix, as well as the phenomena involved in mass movement (gases and liquids). The analysis is complicated due to the fact that cement concrete is a composite consisting of two substantially different constituents: cement paste and aggregates. The effects of the various changes taking place in heated concrete are the alterations of its physical, thermal, and mechanical properties. Research has demonstrated (1, 2), that changes in the strength of concrete as a function of tem- perature are related to, inter alia, concrete composition the type of aggregate used, the water/cement ratio, the presence of pozzolana additives, etc. Important factors are also the rate of heating and the time of concrete exposure to high tempera- ture. The increase in temperature results in water evaporation, C-S-H gel dehydration, calcium hydroxide and calcium alu- minates decomposition, etc. Along with the increase in tem- perature, changes in the aggregate take place. Due to those changes, concrete strength and modulus of elasticity gradu- ally decreases, and when the temperature exceeds ca. 300 ◦ C, the decline in strength becomes more rapid. When the 500 ◦ C threshold is passed, the compressive strength of concrete usu- ally drops by 50% to 60%, and the concrete is considered fully damaged. The Eurocode method of calculating the load- bearing capacity of reinforced concrete members subjected to a fire is based on this assumption. In the 500 ◦ C isotherm method, sections of concrete surface where the temperature had exceeded 500 ◦ C are omitted from the calculations (3).

Effects of colloidal nanosilica on rheological and mechanical properties of fly ash–cement mortar

Abstract: The present study is aimed at investigating the combined effects of colloidal nanosilica (CNS) and fly ash on the properties of cement-based materials. The fresh and hardened properties of mixtures with CNS of 10 nm size and two Class F fly ashes were evaluated. Results revealed that CNS accelerates the setting of fly ash–cement systems by accelerating cement hydration, while fly ash can offset the reduction in fluidity caused by CNS. The early-age strength gain (before 7 d) of fly ash–cement systems was improved by CNS. However, the strength gain of mixtures with CNS diminished at later ages (after 28 d), where strength was eventually comparable to or exceeded by mixtures without CNS. Results showed that lack of Ca(OH)2, which results from the high pozzolanic reactivity of CNS at early ages, and the hydration hindrance effect of CNS on cement at later ages can be the critical reasons.

Investigation of the hydration and bioactivity of radiopacified tricalcium silicate cement, Biodentine and

Abstract: a b s t r a c t Objective. Novel root-end filling materials are composed of tricalcium silicate (TCS) and radiopacifier as opposed to the traditional mineral trioxide aggregate (MTA) which is made up of clinker derived from Portland cement and bismuth oxide. The aim of this research was to characterize and investigate the hydration of a tricalcium silicate-based proprietary brand cement (BiodentineTM) and a laboratory manufactured cement made with a mixture of tricalcium silicate and zirconium oxide (TCS-20-Z) and compare their properties to MTA AngelusTM. Methods. The materials investigated included a cement containing 80% of TCS and 20% zirconium oxide (TCS-20-Z), BiodentineTM and MTA AngelusTM. The specific surface area and the particle size distribution of the un-hydrated cements and zirconium oxide were investigated using a gas adsorption method and scanning electron microscopy. Un-hydrated cements and set materials were tested for mineralogy and microstructure, assessment of bioactivity and hydration. Scanning electron microscopy, X-ray energy dispersive analysis, X-ray fluorescence spectroscopy, X-ray diffraction, Rietveld refined X-ray diffraction and calorimetry were employed. The radiopacity of the materials was investigated using ISO 6876 methods. Results. The un-hydrated cements were composed of tricalcium silicate and a radiopacifier phase; zirconium oxide for both Biodentine TM and TCS-20-Z whereas bismuth oxide for MTA AngelusTM. In addition BiodentineTM contained calcium carbonate particles and

Hydration and strength development in ternary portland cement blends containing limestone and fly ash or metakaolin

Abstract: This paper reports the influence of limestone particle size and the type of (partial) cement replacement material on hydration and the mechanical properties of cement pastes. Limestone powders having median particle sizes of 0.7, 3, and 15 μm, at OPC replacement levels between 0% and 20% (volume basis), and two other replacement materials of differing reactivity (i.e., Class F fly ash or metakaolin) at replacement levels between 0% and 10% (volume basis), are used to proportion ternary binder formulations. Fine limestone accelerates early-age hydration, resulting in comparable or better 1-day compressive strengths, and increased calcium hydroxide (CH) contents as compared to pure cement pastes. The incorporation of metakaolin in conjunction with limestone powder alters the heat release (i.e., kinetic) response significantly. A ternary blend of this nature, with 20% total cement replacement demonstrates the highest 1-day strength and lowest CH content. Thermal analysis reveals distinct peaks corresponding to the formation of the carboaluminate phases after 28 days in the limestone–metakaolin modified pastes, whereas the incorporation of similar levels of fly ash does not change the response markedly. It is shown that the synergistic effects of limestone and metakaolin incorporation results in improved properties at early ages, while maintaining later age properties similar to that of traditional OPC systems.

Use of waste glass as sand in mortar: Part II – Alkali–silica reaction and mitigation methods

Abstract: Waste glass may be used in concrete provided that the potential deleterious expansion caused by alkali–silica reaction (ASR) could be mitigated. In this study, the influence of glass content, color and particle size on ASR expansion of mortar was determined by the accelerated mortar bar method. Two approaches to control ASR expansion were investigated for green, brown and clear glass sand mortar. They were: (1) by replacing cement with pozzolans, that is, 30% fly ash, 60% GGBS, 10% silica fume, or 20% glass powder; (2) by adding a suppressor, that is, plain steel fibers, and lithium chloride and lithium carbonate compounds. Test results showed that the ASR expansion increased with higher glass content in the case of clear glass sand mortar, but would decrease with increasing content for green and brown glass sand mortar. The ASR expansion also decreased with smaller glass particle size, regardless of glass color. Fly ash and GGBS were the most effective in mitigating ASR expansion, followed by silica fume, steel fibers and lithium compounds.