Solid State Phenomena Vol. 325

Abstract: Recent experimental investigations on the nanoscale of hardened cement paste revealed that the tensile strengths of the microstructural phases present amount to several hundreds of MPa. Confrontation with macroscopic tensile strength testing, by e.g. Brazilian splitting, shows a decrease over two orders of magnitude. A computational model based on a hierarchical representation of hardened cement paste microstructure is presented in this paper, attempting to shed light on the factors affecting the scaling of strength from the nanoscopic scale up to the macroscopic scale. The model is validated on a case study featuring a Portland-limestone cement paste subjected to an external sulfate attack. Such conditions compromise the nanoscopic integrity of the C-S-H gel as a consequence of the progressive decalcification and affect the overall load-bearing capacity of the macroscopic cement paste specimen.

Abstract: Glass is an amorphous material, which could be a good pozzolanic material and can be beneficial in compressive strength gain. However, if waste of glass powder has undesirable contaminations (aluminium, clay impurities, sulphates, etc.) it can negatively affect hydration process. In the research were used two types of waste glass (with and without harmful impurities). Waste glass shards were obtained from a local waste recycling plant and its properties were investigated in the Portland cement hydration process. Properties of waste glass were analysed by SEM, XRD test methods, pozzolanicity of glass powder was investigated by Chappelle test method. The hydration process of Portland cement was researched by the semi-adiabatic test method and XRD analysis. Properties of hardened cement paste was analysed by: density, flexural and compressive strength test methods The main aim of this research is to analyse waste glass, which after primary cleaning is not suitable for secondary reuse and investigate its suitability in cement-based systems.

Abstract: The article describes the influence of chrysotile nanofibers dispersion introduction on the properties of the cement matrix. Comparison of the dispersion level of suspensions obtained using cavitation and ultrasonic processing methods is presented. The positive effect of chrysotile fibers application on the strength characteristics of the material has been confirmed. A 34% increase in the compressive strength of the samples was achieved on the 7th day of hardening, while on the 28th day it increased by 36% and with the steam treatment - by 38% compared to the reference sample. Laser particle size analysis confirmed the predominance of the nanosized component of chrysotile fibers in the suspension, which affected the structuring of the cement matrix. The results of the differential thermography, IR spectrometry, X-ray microanalysis and scanning electron microscopy of the samples are also presented. The analysis methods confirmed that introduction of chrysotile nanofibers suspension into the composition of a cement binder makes it possible to significantly vary the structure and morphology of new formations in fine-grained concrete. It also changes the quantitative and qualitative phase composition of the material with the formation of calcium silicate hydrates of lower basicity, leading to an increase in the strength of cement concrete.

Abstract: Portland-limestone cement materials are susceptible to sulfate attack at low temperature and high humidity, because such conditions facilitate the formation of thaumasite, detriment to the structural integrity of calcium silicate hydrates (C─S─H). In this work, the effect of the cation associated with sulfates, concentration of sulfate solution, and limestone content in cement, were thermodynamically simulated. MgSO₄ solution is of higher risk, degrading extensively the structural integrity of C─S─H. Although this phase is partially preserved under the effect of Na₂SO₄ and K₂SO₄ solutions, extensive expansion and thaumasite formation occur. The sulfate content of the corrosive solution and the limestone content in cement are the factors mostly intensifying the attack caused by MgSO₄ and Na₂SO₄/K₂SO₄ solutions, respectively.

Abstract: The current view on the determination of the abrasion resistance of a cement composite is mainly focused on the resistance of the composite to the effects of mechanical abrasion. However, many concrete structures are exposed to the abrasive effects of flowing liquids. One of the test procedures simulating this principle of abrasion is based on the creation of a very fast flow of liquids, often including abrasive media. Based on worldwide published research on the given topic, the use of the action of ultrasonic waves in a liquid, leading to the simulation of the cavitation stress of a composite, which is a very dangerous phenomenon, is considered a suitable method for creating the mentioned abrasion effects. The following article discusses new possibilities for simulating and evaluating the abrasive effects of cavitation on cement composites using the action of ultrasonic waves in a liquid. These effects will be monitored on cement pastes, which will be modified with several types of commonly used admixtures. Furthermore, the connection between the effects of the mentioned abrasive action and several physical-mechanical parameters will be monitored.

Abstract: Carbonatation represents one of the potential degradation processes whose can negatively affect the service life of constructions based on the inorganic binders. The carbonatation depth of the constructions when exposed to various environments is significantly dependent on the existing conditions. The most crucial parameters are the partial pressure of carbon dioxide and humidity. There were selected four environments for the deposition of samples made of the alkali-activated blast furnace slag mortars (exterior, interior, water and CO₂ chamber) in this study. These types of environments guarantee the variation of desired parameters influencing the carbonatation rate. The progress of carbonatation was evaluated with a selected technique in time intervals of 28; 56 and 84 days of the sample's exposition to the selected environments. The characterization was done using the destructive techniques (compressive and flexural strength, phenolphthalein method) as well as the non-destructive one like the Impact-Echo or the Ultrasound time passage measurement. The combination of these techniques allows to determine and evaluate the progress of carbonation without the destructive testing of the samples which is necessary for the real applications of these materials.

Abstract: When constructing deep wells for oil and gas production in difficult geological conditions, special lightweight oil-well cements are used. To reduce the density and water separation of the cement slurry as well as to increase the strength, corrosion resistance of cement stone and the quality of well cementing, opal-containing rocks, fly ash, microsphere and other lightening additives are introduced into the cement composition. The influence of sedimentary rocks, such as opoka, tripoli, and diatomite containing from 43 to 81% amorphous silica on the grindability, rheological and physical-mechanical properties of lightweight oil-well Portland cement has been studied. The twelve cement compositions with different content of additives (from 30 to 45%) that meet the requirements of the standard for density, spreadability, water separation, thickening time and flexural strength were selected. The introduction of 45% diatomite or tripoli significantly reduces the duration of cement grinding, provides the cement slurry with water-cement ratio of 0.9 with better density and flexural strength, respectively, 1480 kg/m³ and 1.1–1.5 MPa.

Abstract: The influence of the chemical composition of bauxite on the mineral formation and structure of sulfoaluminate-ferrite clinker (SAFC), and physical and mechanical properties of cement based on it is studied. Ferrous bauxite, marble crushing screenings and neutralized fluorine-anhydrite were used for the synthesis of SAFC. The compositions of raw mixes of low-and high-alkaline SAFC are calculated with modular characteristics: M_f (ferritic modulus) = 0.35 and 0.7, М_s (sulfate modulus) = 0.167 and 0.5, respectively. Clinker synthesis from a raw mix containing ferrous bauxite with a quality coefficient of 1.7 must be carried out at a temperature of 1250 °C, which is reduced to 1200 °C for burning a raw mix based on bauxite with a quality coefficient of 2.2. Intensive decomposition of anhydrite occurs with the release of sulfur dioxide into the atmosphere at a temperature of 1300 °C, so it is not recommended to burn SAFC at this temperature. The mineral composition of the obtained SAFC is represented by sulfoaluminate, calcium sulfoferrite and free anhydrite. There is no C₂S as a separate phase in clinkers, since in the process of their synthesis belite dissolves in 3С(A,F)·СŜ, mainly in the aluminoferrite phase. The use of bauxite with a quality coefficient of 2.2 in the SAFC raw mix increases the cement activity up to 54.2 MPa. Increasing its specific surface from 350 to 460 m²/kg significantly reduces the setting time of cement paste and increases the strength of sulfoaluminate-ferrite cement both in the early (up to 43.4 MPa) and late hardening periods (up to 67.3 MPa).

Abstract: Computational prediction damage in cementitious composites, as steel fibre reinforced ones, under mechanical, thermal, etc. loads, manifested as creation of micro-fractured zones, followed by potential initiation and evolution of macroscopic cracks, is a rather delicate matter, due to the necessity of bridging between micro-and macro-scales. This short paper presents a relatively simple approach, based on the nonlocal viscoelasticity model, coupled with cohesive crack analysis, using extended finite element techniques. Such model admits proper verification of its existence and convergence results, from the physical and mathematical formulation up to software implementation of relevant algorithms. Its practical applicability is documented on a sequence of representative computational examples.