Lenka Scheinherrová

Monitoring of Kinetics of Pozzolanic Reaction

The pozzolanic additions are widely used as concrete component for numerous technical, economic and environmental reasons. Obviously the hydration process in a pozzolana containing system differs from hydration of Ordinary Portland Cement (OPC) what is indicated macroscopically by slower increase of strength and lower hydration heat. This paper aims to study pozzolanic reaction from perspective of chemical kinetics. From this point of view pozzolanic reaction and carbonation are two parallel reactions which are competing for portlandite (Ca (OH)2). The rate of each of these two reactions is characterized by rate constant and order of reaction. The system under study was 1:1 mixture lime – ceramic powder. The course of reaction was primarily studied by thermogravimetry which results were further subjected to kinetic analysis. MAS NMR spectroscopy was used for study of structural changes taking place in material in the course of pozzolanic reaction.

Moisture Resistance and Durability of the Ternary Gypsum-Based Binders

The moisture resistance of the ternary gypsum-based materials with metakaolin was investigated. Materials with metakaolin have lower porosity and higher strength than gypsum-lime binder without metakaolin. While material without metakaolin lost compressive strength in the wet environment, the strength of the ternary binders increased. Therefore it can be concluded that the gypsum – lime mixtures with the metakaolin can be used in the environment with higher humidity, e.g. in the exterior.

Microstructure and Pore Systems Development of Ternary Gypsum-Based Mortars

Paper deals with ternary gypsum-based mortars with higher resistance against water, prepared from gypsum, hydrated lime, several pozzolans (crushed ceramic, silica fume, granulated blast slag) and sand. The samples were stored in the water and the pore systems development in time was studied by mercury intrusion porosimetry. The change of pore structure in samples with pozzolans were observed, the size of pores shifted to the smaller pores during tested period. The elementary composition and microstructure of all materials were also studied by scanning electron microscopy (SEM).

Mechanical and Basic Physical Properties of High-Strength Concrete Exposed to Elevated Temperatures

In this paper, the effect of elevated temperatures on the mechanical and basic properties of two different newly-designed high-strength concretes is studied. The studied materials were prepared from Portland cement, steel fibers, reactive finely milled quartz powder and quartz sand, silica fume, plasticizer, and with a relatively low water/cement ratio of 0.24. The samples were stored in water environment for the first 28 days of hydration to achieve better mechanical properties. Then, after pre-drying at 105 °C to constant mass, the materials were exposed to elevated temperatures of 600 °C and 1000 °C where they were kept for 2 hours. The basic physical properties, such as matrix density, bulk density and open porosity were determined as a function of temperature. Mechanical properties (compressive and flexural strength) were also studied. The measured parameters exhibited a high dependence on temperature and the obtained results pointed to the structural changes of the studied materials. Spalling was not observed because of the pre-drying treatment.

Time evolution of pore system in lime - Pozzolana composites

The lime – pozzolana mortars and plasters are used in restoration works on building cultural heritage but these materials are also following the trend of energy – efficient solutions in civil engineering. Porosity and pore size distribution is one of crucial parameters influencing engineering properties of porous materials. The pore size distribution of lime based system is changing in time due to chemical processes occurring in the material. The present paper describes time evolution of pore system in lime – pozzolana composites; the obtained results are useful in prediction of performance of lime – pozzolana systems in building structures.The lime – pozzolana mortars and plasters are used in restoration works on building cultural heritage but these materials are also following the trend of energy – efficient solutions in civil engineering. Porosity and pore size distribution is one of crucial parameters influencing engineering properties of porous materials. The pore size distribution of lime based system is changing in time due to chemical processes occurring in the material. The present paper describes time evolution of pore system in lime – pozzolana composites; the obtained results are useful in prediction of performance of lime – pozzolana systems in building structures.

Effect of silica fume on hydration of air-cured blended cement pastes measured by DSC/TG analysis

In this paper, we study the influence of silica fume on hydration of cement pastes. Samples were prepared using Portland cement partially replaced by silica fume of 0, 4, and 12 mass%. Analyses were performed using TG/DSC in the temperature range from 25 °C to 1000 °C in an argon atmosphere. The amount of C-S-H gels decreases in time with an increasing amount of silica fume, which is in the contrary to the vaterite/calcite content. The portlandite content decreases with an increasing amount of silica fume which provides the pozzolanic activity.In this paper, we study the influence of silica fume on hydration of cement pastes. Samples were prepared using Portland cement partially replaced by silica fume of 0, 4, and 12 mass%. Analyses were performed using TG/DSC in the temperature range from 25 °C to 1000 °C in an argon atmosphere. The amount of C-S-H gels decreases in time with an increasing amount of silica fume, which is in the contrary to the vaterite/calcite content. The portlandite content decreases with an increasing amount of silica fume which provides the pozzolanic activity.

Thermal properties of SFR-HPC exposed to high temperatures

In this paper, a non-adiabatic method was used for the assessment of specific heat capacity of steel fibre reinforced high performance concrete in the temperature range 105–1000 °C. The tested SFR-HPC mix was produced from CEM II 42.5 R, ground granulated blast furnace slag, silica sand with maximum particle size of 2 mm, silica fume, brass-coated steel fibres, superplasticizer on polycarboxylate ether basis and batch water. For the studied material, properties after 2 hours thermal treatment at the temperatures of 105 °C, 200 °C, 400 °C, 600 °C, 800 °C, and 1000 °C respectively were tested. Among them, bulk density, matrix density, total open porosity and thermal parameters as thermal conductivity, thermal diffusivity and specific heat capacity were measured. The measured specific heat capacity exhibited high dependence on temperature and pointed to the structural changes that studied material underwent at high temperatures. Accordingly, the obtained residual parameters revealed the thermally induced dam...

The use of glass powder as a partial Portland cement replacement

Finely grinded waste glass powder can become material having suitable properties from the point of view of particle size and pozzolanic activity. Glass powder incorporation into cement paste and cement-based composites can bring improvement in porous structure resulting in increased mechanical strength and durability characteristics. On this account, two types of recycled glass powder are investigated in the presented paper as a possible partial Portland cement substitutes in cement blends. For raw glass powders, basic physical parameters and chemical composition are measured. The studied glass powders are applied as 5, 10 and 20 mass% of Portland cement replacement in cement paste mix composition, whereas water/binder ratio of 0.3 is used for all studied pastes. Fresh paste mixtures are characterized using initial and final setting time measurement. For hardened pastes cured 28 days in water, bulk density, matrix density, total open porosity and mechanical properties represented by flexural and compressive strength are accessed. Portlandite consumption by the pozzolanic reaction is monitored with TGA. The obtained results show effectiveness of a borosilicate glass powder that acts as a pozzolanic active admixture. This resulted in improvement of mechanical characteristics for cement substitution up to 10 mass%.Finely grinded waste glass powder can become material having suitable properties from the point of view of particle size and pozzolanic activity. Glass powder incorporation into cement paste and cement-based composites can bring improvement in porous structure resulting in increased mechanical strength and durability characteristics. On this account, two types of recycled glass powder are investigated in the presented paper as a possible partial Portland cement substitutes in cement blends. For raw glass powders, basic physical parameters and chemical composition are measured. The studied glass powders are applied as 5, 10 and 20 mass% of Portland cement replacement in cement paste mix composition, whereas water/binder ratio of 0.3 is used for all studied pastes. Fresh paste mixtures are characterized using initial and final setting time measurement. For hardened pastes cured 28 days in water, bulk density, matrix density, total open porosity and mechanical properties represented by flexural and compressi...

Simultaneous thermal analysis and thermodilatometry of hybrid fiber reinforced UHPC

Development of concrete technology and the availability of variety of materials such as silica fume, mineral microfillers and high-range water-reducing admixtures make possible to produce Ultra-High Performance Concrete (UHPC) with compressive strength higher than 160 MPa. However, UHPC is prone to spall under high temperatures what limits its use for special applications only, such as offshore and marine structures, industrial floors, security barriers etc. The spalling is caused by the thermal stresses due to the temperature gradient during heating, and by the splitting force owing to the release of water vapour. Hybrid fibre reinforcement based on combination of steel and polymer fibres is generally accepted by concrete community as a functional solution preventing spalling. In this way, Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) is produced possessing high mechanical strength, durability and resistance to water and salt ingress. Since UHPFRC find use in construction industry in tunnel l...

Hydration of calcium aluminate cement determined by thermal analysis

Calcium aluminate cements (CACs) are a very important type of non-Portland or special cements. Since they are considerably more expensive, they are not used as a simple substitute for Portland cement. Their structure allows them to achieve high compressive strength. They resist very well to high temperatures and temperature changes, or also to chemical attacks. The original motivation, why the CACs were developed, was the idea of finding new cement chemistries that would be more resistant to sulfate attack then Portland cements. Nowadays, the main usage of the CACs is in high temperatures applications. In this paper, we study the hydration of a CAC up to one year of age to control what happens in CACs structure during aging. The variety in the main products of hydration is studied using differential scanning calorimetry and thermogravimetry in the temperature range from 25 °C to 1000 °C with a heating rate of 5 °C/min in an argon atmosphere. The basic physical and mechanical properties are also determined.