Patrik Bayer

Thermal Properties of Alkali-activated Slag Subjected to High Temperatures

Alkali-activated slag belongs to prospective materials in the field of fire protection because it exhibits remarkable high-temperature resistance. In this article, specific heat capacity, thermal diffusivity, and thermal conductivity of an alkali-activated slag with sand aggregates are studied as functions of temperature up to 1200°C. The experiments are performed during the material exposure to high temperatures. The irregularities observed on the measured dependences of specific heat capacity and thermal diffusivity on temperature are discussed using the results of DTA and X-ray diffraction analyses. The crystallization of akermanite is identified as the most important factor affecting the properties of the studied material in high-temperature range.

Thermal and Hygric Parameters of Carbon-fiber-reinforced Cement Composites after Thermal and Mechanical Loading

The basic thermal and hygric parameters of two different types of carbon-fiber-reinforced cement composites are analyzed in this article. The thermal conductivity, specific heat capacity, moisture diffusivity, and water vapor diffusion resistance factor are determined as functions of thermal load and tensile load applied before the measurement as well as of the combination of both types of load. The tensile load up to failure is found to be not a very significant factor for all material properties analyzed except for the moisture diffusivity. On the other hand, the thermal load is observed to result already at 600 C in considerable changes in all investigated thermal and hygric properties except for the specific heat capacity. The combinations of thermal and tensile loads lead to similar results as the effect of the thermal load alone so that the domination of the thermal load is apparent. This is supposed to be due to the positive effect of randomly distributed carbon fibers that can reduce the damage of the pore structure by the tensile stress. The resistance of the materials studied to high temperatures expressed by the change of hygric and thermal properties after thermal load is found to be positively affected by the application of the high alumina cement and in the case of the Portland cement-based composite also by using the autoclaving procedure in the production process.

Influence of Guar Gum Derivatives on Hardened Properties of Aerial Lime-Based Mortars

This paper studies the possibility of usage of the guar gum and its derivatives (carboxymethylhydroxypropyl guar (CMHPG) and hydroxypropyl guar (HPG)) as admixtures for aerial lime-based mortars. The influence on the properties of mortars was studied on the aerial lime –based mortars prepared with quartz fine grained sand and doses of admixtures ranging between 0,5 and 10‰. The hardened bulk densities, flexural and compressive strength, porosity, water absorption coefficient due to capillarity action and carbonation rate were studied. The addition of the hydroxypropyl guar lowered the bulk density (due to an air intake), improved workability, slightly increased the strength, slowed carbonation rate, and nobbled the water transport in the mortar. The addition of carboxymethylhydroxypropyl guar does not impact the bulk density, the strengths were increased similarly to HPG: it does not impact carbonation rate significantly, so the long term strengths were comparative with the HPG. The water transport was slightly better in lower doses and slightly worse in larger doses in comparison with the reference mortar. The pure guaran was found not to be beneficial for the lime mortars for its only advantage is in the water transport, where in any dose the transport was better than any other mortar, but the benefit of this is questionable.

Modelling of interfacial transition zone effect on resistance to crack propagation in fine-grained cement-based composites

In this paper, the attention is paid to investigation of the importance of the interfacial transition zone (ITZ) in selected fine-grained cement-based composites for the global fracture behaviour. This is a region of cement paste around the aggregate particles which specific features could have significant impact on the final behaviour of cement composites with a crack tip nearby this interface under applied tension. The aim of this work is to show the basic interface microstructure by scanning electron microscopy (SEM) done by MIRA3 TESCAN and to analyse the behaviour of such composite by numerical modelling. Numerical studies assume two different ITZ thicknesses taken from SEM analysis. A simplified cracked geometry (consisting of three phases – matrix, ITZ, and aggregate) is modelled by means of the finite element method with a crack terminating at the matrix–ITZ interface. ITZ’s modulus of elasticity is taken from generalized self-consistent scheme. A few conclusions are discussed based on comparison of the average values of the opening stress ahead of the crack tip with their critical values. The analyses dealing with the effect of ITZ’s properties on the stress distribution should contribute to better description of toughening mechanisms in silicate-based composites.

The Effect of Aggregate Type on the Properties of Lime Mortars

Lime mortars represent indispensable building materials that have been used for centuries in civil engineering. Considering the necessity of numerous restoration work on historical buildings, a research of the applicability and suitability of various types of plasters for repairing the historical plasters has been developed. This work presents the applicability of limestone aggregate and limestone fines to aerial lime-based mortars. The role of aggregates on the properties of lime mortars is examined in this paper by comparing pure quartz sand and limestone aggregate and its quantity in the mortar. It was found that limestone aggregate produced the comparable or higher mortar strengths than quartz aggregate and slightly increased porosity, water absorptive capacity and carbonation rate of the mortars. Partially replacement of aggregate by limestone fines has caused dramatic growth in strength, especially in mortars with high content of binder. The applied limestone aggregate is convenient to lime-based mortars and the addition of limestone fines contributes to better mechanical properties of lime mortars.

Properties of Aerial Lime-Based Mortars with Chitosan Ethers

This paper studies the possibility of usage of chitosan derivatives (hydroxypropyl chitosan (HPCH) and carboxymethyl chitosan (CMCH)) as admixtures for aerial lime-based mortars. The physical-mechanical properties were studied on the specimens prepared with constant water/binder ratio and binder/aggregate ratio 1:3 by weight using siliceous sand with the grain size up to 4 mm and doses of admixture ranging between 0.5 and 10‰. The properties were studied on the specimens up to 365 days. The addition of admixtures improves workability of mortar, but in the early stages also decreases strengths of mortars, especially the compressive strength. The addition of HPCH significantly reduces the bulk densities of specimens, while CMCH decreases them just slightly. With the increasing amount of time, the difference in strengths between reference and tested mortars decreases, reaching the similar values after 90 days of curing time. The further ageing causes additional growth of strengths with a significantly higher increase in the case of CMCH.

Differences in Electrical Properties of Portland Cement and Alkali-Activated Slag Mortars

Alkali-activated slag is known as a building material for more than sixty years and is considered an alternative to Portland cement based binders. Compared to Portland cement it exhibits some superior properties such as higher resistance against chemical attack and exposure to elevated temperatures. Aluminosilicate binders are generally electrical insulators; however, electrical properties of building materials gain the importance in the new field of applications such as self-sensing or self-heating materials. This paper brings a comparison of the electrical properties, especially resistance and capacitance, between Portland cement and alkali-activated slag mortars. The measurements revealed that alkali-activated slag shows enhanced conducting properties due to the presence of mobile hydrated sodium ions and metallic iron microparticles.

Effect of Aggregate Type on Properties of Natural Hydraulic Lime-Based Mortars

In the current restoration practice, the mortars based on aerial or hydraulic lime with siliceous aggregate are used on repair / reconstruction of historical plasters. The resulting properties of lime mortars do not depend only on the type of aggregate, but also on its contamination, origin and a degree of hardening. For this reason, it is needed to know a detailed characteristic of the aggregate and the results obtained for the particular aggregate cannot be generally applied for the similar type of aggregate. This work presents the applicability of Czech local crushed limestone aggregate and limestone fines as a filler to natural hydraulic lime-based mortars. The role of aggregate and limestone fines on the properties of mortars is examined by comparing the mechanical strengths, porosity and frost resistance of the mortars with pure quartz sand and limestone aggregate and its quantity in the mortar. It was found that the limestone aggregate has produced lower strengths, higher total porosity of natural hydraulic lime-based mortars. It has resulted in low frost resistance of the mortars.

Sulfate Attack on Different Types of Concrete in Media Simulating Sewer System

The degradation of concrete due to ingress of sulfate ions from the environment plays an important role in the durability of concrete constructions. Microbiologically induced concrete corrosion (MICC) damages especially sewage collection systems. The most rapid cases of deterioration always occur in areas with elevated H2S concentrations, moisture, and oxygen in the atmosphere. During the MICC, the pH of the surface of concrete sewer pipes is reduced and it may lead to the steel depassivation and results in the corrosion of steel reinforcement. Damage due to a sulfate interaction can result in a cracking and softening, with a loss of strength of concrete. The formation of ettringite (AFt) from gypsum (forming by reaction of sulfate anion with calcium hydroxide) and C3A via monosulfate (AFm) is the main chemical reaction of sulfate attack on concrete. Ettringite and gypsum have considerably larger volume than initial compounds, which leads to increased pressure in concrete. This paper is focused on the sulfate attack on fine-grained concrete where the effect of 0.5% sulfuric acid, simulating MICC, and a solution simulating sewage water has been investigated on changes of the pH, content of sulfates and the porosity in various types of concrete. The aim of this study is to compare the changes in different types of concrete during the sulfate attack in two kinds of medium represented the bottom part of pipelines (waste water) and the sewer crown (0.5% H2SO4). It was found, that after 1 year in 0.5% H2SO4, a visible degradation of surface occurs in all investigated types of concrete. Samples over the year in waste water became dark. Concentration of sulfates in all studied types of concrete increased six times at least after one year sulfuric acid attack and also the reduction of the pH of their aqueous leaches was determined. The solution simulating sewage water did not cause such changes.

Influence of the Aggregate Type on the Properties of Dolomitic Lime-Based Mortars

Considering the requirement of numerous restoration work on historical buildings with dolomitic lime-based mortars, a research of the applicability and suitability of various types of plasters for repairing the historical plasters have been developed. This work presents the applicability of limestone aggregate and limestone fines to dolomitic lime-based mortars. The role of aggregates on the properties of dolomitic lime mortars was examined by comparing pure quartz sand and limestone aggregate and its quantity in the mortar. Limestone aggregate and limestone fines accelerated the carbonation reaction in the mortars, but the positive impact of limestone aggregates to strength properties of mortars was not observed. Mortars with limestone aggregate showed higher total porosity and water absorption than those with quartz aggregate. Similar results was obtained for mortars with limestone fines. It was found that limestone aggregate and limestone fines, used in this work, is not so suitable and applicable to the dolomitic lime-based mortar as to the aerial lime-based mortars.