The Structural Formation of Cement Stone Modified by a Solution of Superabsorbent Polymer

With the development of 3D technologies in construction, the development of formulations that are indifferent to the influence of the environment is in demand. Conditions of intense water loss from cement systems arise during the layer-by-layer printing process. This leads to a decrease in density, high shrinkage, and a decrease in the strength and durability of the composite. The use of superabsorbent polymer (SAP) solutions, in contrast to granules, will provide hardening Portland cement with a water supply for internal care of hydration processes. The aim of the work is to study the effect of SAP solution on the processes of structure formation of cement stone, hardening in unfavorable conditions. In this paper, the features of the structure formation of cement systems in the presence of SAP are established. It is shown that the use of polymer in an amount of no more than 1.5% by the weight of Portland cement provides the formation of a more perfect crystalline structure of the cement stone, which allows for an increase in the degree of cement hydration. When the amount of SAP is ≥ 1.5% by the weight of Portland cement, a decrease in the intensity of the maxima corresponding to hydration products is observed.

Review of studies on plugging compositions creation with controlled rheological properties and the ability to restore fluidity for completion, workover and abandonment of oil and gas wells

The main purpose of cementing oil and gas wells is zonal isolation of the formations exposed by the wellbore. During the entire life of the well, there should be no uncontrolled hydraulic communication between the developed formations and the surface, regardless of the composition and type of fluid (water, oil or gas). During the operation of the well, in addition to constant static ones, the casing and cement stone also experience various dynamic loads. The article presents an up-to-date review of experimental studies on the modification of grouting compositions and cement composites capable of autonomous selfhealing due to the introduction of various additives and nanomaterials. Such modification technologies significantly increase the tightness and resistance of cement to the effects of dynamic loads, the integrity of the cement stone. As a replacement for traditional cement materials, the authors propose the creation of grouting compositions with controlled physical and mechanical properties and the possibility of their re-liquefaction under the influence of temperature on the basis of bitumen or bitumen composites. Keywords: well plugging and abandoning; self-healing materials; autonomous self-healing; casing durability; impermeability of the cement stone; self-healing cement; bitumen and bitumen composites.

Determination of the safety factor of cement stone based on numerical modeling of the stress-strain state of the near-wellbore zone, taking into account the change in the elastic-strength properties of cement during its hardening and under the influence of an acid reagent

The results of laboratory studies of the elastic-strength properties of cement stone samples depending on the hardening time and the effect of an acid reagent, and approximated dependences of the change in the elastic modulus, Poisson's ratio and strength properties, depending on the time characteristics for two types of plugging materials are presented. A finite element scheme of the nearwellbore zone has been developed, taking into account the cement stone and the production casing. The results of numerical modeling of the stress-strain state of columns with a diameter of 146 and 178 mm, cement stone and reservoir rocks near the well based on an elastic model are presented. The analysis of the stress field for the occurrence of zones of destruction in the cement stone using the Coulomb-Mohr criterion is carried out. It is shown that, depending on the time of hardening and the effect of an acidic reagent, cement does not collapse and retains a sufficient safety factor. Keywords: cement stone; plugging material; elastic-strength properties; near-wellbore zone; numerical model; finite element method; stress-strain state; safety factor.

Influence of Graphite Powder on the Mechanical and Acoustic Emission Characteristics of Concrete

In this paper, uniaxial compressive strength (UCS) test and three-point bending (TPB) test, together with an acoustic emission (AE) system, were performed to investigate the mechanical properties and AE characteristic changes of concrete with different graphite powder (GP) content. The results show that: (1) Poor adhesion and low interlocking of graphite with cement stone increase the initial defects of concrete, reducing its elastic modulus and the cyclo-hoop effect, and thus weakening the compressive strength. (2) For concrete with a low graphite content, the second sharp rise in ringing counts or energy released during the compressive process can be regarded as a failure alarm. However, as GP content increases, the second sharp rise fades away, while the first sharp rise becomes more visible. At high GP content, the first sharp rise is better for predicting failure. (3) The initial defects caused by GP significantly lower the initial fracture toughness, but its bridging effect greatly increases the critical crack mouth opening displacement and thus significantly enhances the unstable fracture toughness of concrete, by up to 9.9% at 9% GP content. (4) In contrast to compressive process, the sharp increase in AE signals preceding failure during the fracture process cannot be used to predict failure because it occurs too close to the ultimate load. However, as GP can significantly increase the AE signals and damage value in the stable period, such failure precursor information can provide a safety warning for damage development.

STRUCTURE FORMATION OF DISPERSED-REINFORCED BUILDING COMPOSITES

The results of the study of the mechanism of structure formation of cement compositions reinforced with finely dispersed monofilament are presented in the article. The mechanism of microstructure organization of construction composites was studied on models of dispersed systems, with different qualitative and quantitative composition of linear and dispersed particles. At the same time, restrictions had been placed on particle size – fiber diameter and diameter of dispersed particles are proportional to each other. Study of cracking formation kinetics was carried out on disk-shaped samples made of water-clay and water-cement compositions. Physical and mechanical characteristics of dispersed-reinforced cement stone, including non-reinforced stone, have been defined on prisms-shaped samples of square section with size 40×40×160 mm. The analysis of physical models showed that cluster structures filling with particles of various nature and shape increases structural diversity of entire dispersed system. An inserting of linear particles changes nature of system structure formation. Depending on the characteristics, structural components of the system, substructures are formed, which differ in the periods of their formation and geometric parameters. It has been established that dispersed particles of different nature are structured in different ways into clusters with discrete fibers of different length. Linear particles were more active in the creation of structural aggregates (clusters) comparing to dispersed grains. The impact of highly dispersed fibers on the structure organization of the binder compositions was quantified by the damage coefficient determined on samples of different types. The presence of discrete fibers in the composition of the material leads to modify the qualitative characteristic of compositions cracking formation. Improvement of physical and mechanical properties of the dispersed-reinforced composite confirms the ability of the fiber to change a mechanism of material destruction due to a probable deposition of hydration products on monofilaments, to densify and strengthen the interfacial transitional zone.

Ultrafast transfer strength of reinforced concrete sleepers by Using complex additives

The influence of superplasticizers-polycarboxylates and their complex additives withaccelerants - electrolytes and dispersion of calcium hydroxylates on the early strength of concretefor concrete of concrete sleepers has been studied. It was found that the use of superplasticizerspolycarboxylates can improve the early strength of the concrete after heat treatment, but eachadditive must be checked for consistency with the cement used. Approx Complex additives withaccelerators in conditions of natural hardening ensure an increase in the early strength of concrete,but some of them, which accelerate natural hardening, can reduce the strength after WWTP and mustbe checked before use. Electron-microscopic examinations of the structure of cement stone withadditives were carried out and it was found that the additive of only policarboxylate does not changethe structure of hydration products. Complex additives lead to formation of additional number ofcrystalline hydrates of AFm- and/or AFt-phases. The scheme of development of the cement hardeningprocess and the initial stage of cement hardening without or with additives has been developed. Bymeans of which the highest accelerating effect of the complex addition of polycarboxylate andcalcium hydroxylate dispersions was established by experimental investigations, Dispersion particlesincrease surface area where crystallization (condensation) of cement hydration products takes placeand ensures faster filling of spaces between cement particles (mineral additives) with them, fillers)with establishment of lances with electro-heterogeneous contacts. As a result of potentiodynamic andmicroscopic investigations it was established that the tested additives in the dosages do not causecorrosion effect on the steel reinforcement bars. Transmission strength values of 32 MPa for sleepersafter 24 years of hardening under low-heat regime and after 2 days of natural hardening have beenachieved. The formula for economic efficiency of using additives to reduce the energy intensity ofproduction of concrete sleepers has been proposed.

Evaluation of photocatalytic activity of titanosilicate powders of synthetic origin

In this work, a comparative study of the photocatalytic activity of synthetic titanosilicate powders with differences in phase composition and specific surface area was carried out. Evaluation of the efficiency of decomposition of the dye methylene blue in the presence of titanosilicate additives in an aqueous medium under ultraviolet and visible light sources is given. The self-cleaning ability of titanosilicate-modified cement stone was studied by exposure to ultraviolet and visible light using ImageJ software. The strength of a cement stone modified with a titanosilicate additive has been determined, and its ability to self-cleaning under the influence of ultraviolet and visible light has been studied.

Development of alkaline concrete on the basis of active aggregates

The object of the research is the process of directed structure formation in the body of alkaline concrete, made using a reactive aggregate, in this case, basalt, and the process of deformation development in such concrete. The problem with using reactive aggregates is that they cause alkaline corrosion. It manifests itself in the form of cracks and layers of gel-like substances that form at the point of contact of the aggregate with the cement stone. During the research, methods of physical and chemical analysis were used (X-ray phase, differential thermal and thermogravimetric analyzes, electron microscopy, infrared spectroscopy, microprobe analysis). And also methods of mathematical planning of experiments have been used for the dependence of the physical and technical properties of cements and the directions of their structure formation. Also, the research has been carried out based on the analysis of world achievements in solving the problem of alkaline corrosion of concrete. The possibility of joint operation of the matrix of alkaline cements and active aggregates, represented by basalt, has been determined. The component composition of alkaline cement has been optimized and the need to increase the amount of the alkaline component in the system for the normal course of structure formation processes has been proved. The study of the influence of technical factors and conditions of hardening on the development of processes of structure formation of the investigated compositions has been carried out. The deformation properties of fine-grained concrete based on slag-alkaline cement and basalt aggregate have been investigated. It is shown that the expansion deformations of the samples, which accompany the process of alkaline corrosion of the aggregate in concrete, are directly related to the component composition and hardening conditions of the material. The obtained research results confirm the possibility of using active aggregates for the manufacture of building materials, in particular, based on alkaline cements. But for the safe course of the processes of structure formation, the component composition of the system has to be adjusted by introducing an active mineral additive and an additional alkaline component. The use of hydrophobizing additives makes it possible to increase the strength of the material even when operating under normal heat and humidity conditions.

X-ray diffraction of concrete composites of high structural strength and density

Аn analysis of the structure formation of concrete composites, compressive strength of which exceeds 120 MPa and a quantitative analysis of their qualitative composition and hydration products by X-ray diffraction, x-ray spectral analysis. The main factors affecting the physicomechanical parameters of the complex of various nanofillers and the formation of a denser cement stone structure, which mainly includes calcium hydrosilicates, calcium silicate hydroaluminates and hydroaluminates of various basicity, are studied.

Research of the mineral additives influence on aggregative stability of foam concrete mixture

An effective method for increasing the aggregate stability of non-autoclave heat-insulating foam concrete is proposed. This material is prepared using a two-stage technology on a turbulent-type installation. An increase in the stability of the foam in the mortar mixture by 9.5-23% has been established. An increase in the viscosity of the foam concrete mixture by 13.5% was revealed. Wollastonite and diopside are actively involved in the formation of a stable structure of foam concrete and are structurally modifying centers. The introduction of mineral additives contributes to the formation of a homogeneous stable structure of non-autoclave foam concrete. Thus, an increase in the stability of the cellular system in the technology of non-autoclave cement-ash foam concrete is possible due to the control of the processes of structure formation when using dispersed mineral additives of wollastonite and diopside. Due to the structural-modifying effect of additives as crystallization centers for neoplasms, a more complete hydration of the cement and a strong contact of the additives with the cement stone should be ensured