The benefits of using mineral additive as a partial replacement for cement in heavyweight concrete are discussed. This paper presents the strength development and temperature distribution of concrete using Class F of natural pozzolan (PU) sourced from Northern part of Vietnam. Based on the results of conducted studies, strengths of the natural pozzolan concrete at different ages were generally lower than those of control concrete. The 7-day compressive strengths of concrete with 20% PU decreases mostly by 30.1% and least by 12.3% at the age of 28 days in comparison with control concrete. However, natural pozzolan increases the workability of fresh concrete up to 16.67% in comparison with control concrete. By using the computer program Midas Civil, the maximum temperatures at the center of concrete block with 100% cement and of concrete block with 80% cement + 20% PU are 65.7600C and 52.4400C, respectively, after 48 hours from the beginning of pouring. In addition, temperature difference between the central point and the environmental temperature of the control concrete are higher than heavyweight concrete using 20% PU. Meaningfully, the risk of through thermal cracking of heavyweight concrete without pozzolan are higher than heavyweight concrete PU to replace 20% of mass cement.
Introduction. Currently, in all countries, Portland cement is used as a binder in the production of concrete, and its global production accounts for 10 % of the total carbon dioxide emitted into the atmosphere. Therefore, Portland cement can be partially or fully replaced by new cement-free binders, made of large-tonnage technogenic waste with a cementing effect, for example, by finely ground blast furnace slag, fly ash generated by thermal power plants and ash formed during the combustion of rice hulls. Aqueous alkaline solutions (NaOH and Na2SiO3 or KOH and K2SiO3) should be used as activators of setting and hardening of such binders, and calcium sulfate dihydrate can be used to adjust the setting time. Concrete containing new cement-free binders is called geopolymer concrete. Materials and methods. In order to reduce mixing water consumption and maintain the required workability of the fine-grained concrete mixture, a polycarboxylate superplasticizer was introduced into its composition. All raw materials, except for the superplasticizer, were of the Vietnamese origin. The following research methods were used: the composition of the geopolymer concrete mixture was analyzed using the absolute volume method, the workability of the concrete mixture was determined according to ASTM C1611-18 и TCVN 3106:2007, compressive and tensile strength of the concrete, subjected to bending, were tested pursuant to GOST 10180-2012, and the average density of concrete was tested according to GOST 12730.1-78. Results. The co-authors have developed the composition of the geopolymer concrete containing the alkaline cement-free binder. As a result of the heat treatment of the 28-days-old concrete for 6 hours at 100 °C, its compressive strength reaches about 60 MPa; hence, it can be used in the hot and humid climate of Vietnam. Conclusions. This cement-free concrete, in addition to its high strength, has good water resistance and low water absorption. This concrete has economic benefits, and its production will help to protect the environment due to the lower consumption of natural resources and the applicability of large-tonnage technogenic waste.
This paper considers the basic methods of physical-mathematical modeling that are used to describe the processes of non-stationary mass transfer of "free calcium hydroxide" in concrete structures placed in a liquid environment with a defined flow rate. The boundary value problem of "free calcium hydroxide" mass conductivity in dimensionless variables is obtained. To demonstrate the possibilities of the obtained solution, we will carry out a numerical experiment: in which the fluctuation in the field of dimensionless concentrations by the different values of the Fourier number, in accordance with the theory of similarity, will be considered as an indicator of the process time. The study indicated the results of calculating the concentration distributions "free calcium hydroxide" over the dimensionless thickness of the concrete structure at Fourier numbers 0.01; 0.1; 0.2; 0.5 and 1. The study also provides an example of determining the time of reaching the coastal structure surface critical concentration "of free calcium hydroxide" leading to the beginning of high-basic cement concrete. Analysis of the solution also makes it possible to determine the duration of the service life of a reinforced concrete structure, which is determined by the processes occurring at the interface: in concrete - mass conductivity (k), and in the liquid phase - mass transfer (β).
This paper presents the properties of high-strength concrete containing a high volume of N-class natural pozzolan sourced from the Northern part of Vietnam. Based on the conducted studies’ results showed that the mechanical properties of the natural pozzolan concrete were generally lower than those of control concrete. However, with effects combined of natural pozzolan and SR-5000F superplasticizer on the microstructure of concrete, mechanical properties of high-strength concrete increases with the amount of natural pozzolan from 10 to 20% by mass of cement, then descends. In addition, the maximum temperature of both conventional and High-strength concrete specimens significantly decreases with the increasing content of natural pozzolan. Furthermore, the relationship between compressive and flexural strength, modulus of elasticity and compressive strength of heavyweight concrete specimens are also determined in this investigation.
The purpose of this study was to evaluate the corrosion resistance of steel reinforcement in concrete samples containing modifying additives using an accelerated electrochemical corrosion method. It was found that the adhesion strength between the reinforcement and the cement stone of concrete largely depends on the corrosion resistance of the reinforcement in concrete. At the same time, the best result from the tested concretes was shown by the developed concrete of optimal composition, in which the adhesion strength between concrete and reinforcement is 1.7 times higher than that of control concrete without mineral additives. The obtained results can be explained by the fact that an increase in the density of concrete due to the modification of its structure with organo-mineral additives leads not only to an increase in its strength, but also to a better protection of steel reinforcement in concrete from corrosion. This will help to increase the durability of reinforced concrete structures of offshore hydraulic structures in Vietnam and increase the period of their maintenance-free operation.
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