Modified dry construction mixture for additive technologies

. The article briefly considers the process of construction methods development, naturally turning into modern additive technologies. The main problems of construction 3D printing are formulated. The results of laboratory studies of a dry building mix based on cement with the addition of sunflower ash are presented. The most efficient technology for mixing components has been proposed, the compositions of modified mixtures have been developed. Regularities of the effect of ash concentration on the rate of hardening and strength characteristics of the specimens have been established. The optimal content of the modifying additive from ash has been established, further ways of researching the material for additive building technologies have been proposed.


Introduction
The development of house building technology is one of the key branches of human civilization.From antiquity to the present day, man has constantly improved the methods and materials used in construction.From simple caves and huts to modern high-rise skyscrapers, the construction of houses has become a reflection of the progress of society and scientific achievements.
In ancient times, people used natural materials available to them, such as stone, clay, wood and straw.Stone structures such as the Egyptian pyramids and Greek temples were outstanding achievements of the time.Then came the time of urban development and the use of bricks.The Romans used brick mixed with cement to create stronger and more durable structures.Eastern civilizations developed techniques for building houses with wood, such as Japanese traditional houses using a frame building system.[1] With the development of industrialization in the 19th century, new materials and technologies appeared.The use of steel and reinforced concrete in structures made it possible to build taller and stronger buildings.Iconic structures such as the Eiffel Tower in Paris and skyscrapers in New York have become symbols of technological progress.[2] In the middle of the 20th century, the era of mass production of residential buildings began.Innovative construction methods, such as prefabricated houses and modular structures, have made it possible to quickly and cost-effectively build many residential buildings.This gave impetus to the development of new construction methods, including the use of prefabricated elements and large-block assembly.[3] With the development of computer technology and information models, the construction of houses has received a new round of development.Computer modeling and 3D design have changed the construction process.With the help of specialized software, engineers can create accurate virtual building models that allow potential problems to be taken into account before construction begins.This reduces construction time and costs, and improves the quality and safety of structures.[4] Modern construction methods also include the use of new materials.For example, environmentally friendly and energy-efficient materials have emerged, such as sorbent composite panels that can absorb air pollution, or materials based on straw or recycled plastic.These materials have a lower environmental footprint and contribute to sustainable and energy efficient buildings.[5] One of the most modern and innovative approaches to building houses is the use of 3D printing.3D printing makes it possible to create complex and precise building details right on the construction site or in the factory.This significantly reduces construction time and reduces waste.In addition, 3D printing can be used to create residential buildings from various materials, including concrete, plastic, and others.[6][7][8] However, with the advent of new technologies, special requirements arise for materials used in construction.When creating materials for formless 3D printing of construction objects, two problems need to be solved: -ensure plasticity and dimensional stability of mixtures; -to provide such a set of physical and mechanical properties of 3D-printed composites, which will allow us to abandon the traditional reinforcement of building structures.[9] Thus, the composition of the mixture should have a rapid set of strength, which could provide the necessary adhesion between layers that exert loads on the underlying ones, without deformation.To do this, the mixture must have an optimal yield strength and, at the same time, be quite liquid for extrusion.The basis of the optimal component composition should be fast-hardening concrete with the inclusion of various additives to improve its properties.[10,11]

Materials and methods
The presence in Russia of a developed industry for the production of binders, combined with rich natural reserves of mineral raw materials, is a powerful base for the domestic production of dry building mixes (DBM).If at the beginning of the development of this type of production, there were companies on the market that produced insignificant volumes of building materials, now there is a tendency towards a significant increase in production volumes.
In fact, dry building mixtures are a mixture of sand and cement, in which, depending on the purpose, various components are added, for example, inert fillers, special mineral additives, various polymers.It is due to the special properties of modern technologies and materials that dry mixes can be selected depending on such characteristics as the required strength, drying speed, humidity and temperature during operation.[12,13] A dry mixture is a powder product based on a binder: cement, gypsum or polymer, and a number of components, the content of which depends on the type of work for which this or that mixture will be intended.
The composition of the mixtures also includes fillers that give them an optimal volume (sand or marble chips, ground limestone, fibers of various origins).Changes in the properties of the mixture in the right direction are provided by modifying additives (plasticizers, a set of polymers, ash, clay, etc.).Among the parameters set in this way: color, hardening speed, which for the convenience of work can be both reduced and increased; viscosity (achieving optimal fluidity for a given amount of water); frost resistance (increased with the help of the so-called air-entraining additives that increase the porosity of the material); tear strength (adhesion), compressive strength, bending strength; moisture retention (reduction of moisture loss when laying tiles, which is necessary for uniform setting throughout the volume of the solution).[14][15][16][17] Based on the analysis of literature, hydraulic binder (cement), sand, mineral powder, and ash from burning sunflower husks were taken as the main components of a dry building mix for 3D printing.
To solve the problems, the following materials were used: 1. Portland cement CEM I 42.5N GOST 31108-2020.Cement plant "Pervomaisky", Novorossiysk   The use of fly ash in construction 3D printing can have several promising directions.First, fly ash can significantly reduce the amount of cement needed to create durable concrete, which will reduce construction costs and reduce the negative impact on the environment.
Secondly, ash can greatly increase the strength of a concrete structure and reduce its water absorption, making it more weather resistant and more durable.[18] In addition, ash can increase the plasticity and fluidity of the concrete mixture, which makes it easier to process and reduces the likelihood of cracks and deformations during the hardening process.This is especially important when creating complex geometric shapes and designs.
Finally, ash can serve as a catalyst for cement hydration and speed up the setting process.
The developed technological scheme for the production of dry construction mixtures allows the use of standard modern equipment used for the production of dry building mixtures.Requirements for the components of dry mortars may vary depending on the specific application and the standards to which the mixture must meet.Technological equipment must ensure dosage accuracy and uniform mixing of components.
Considering the technological chain for the production of dry building mixtures, the following stages can be distinguished: -preparation of aggregates and fillers; -sifting and separating large particles from binders; -dosage of components (additives are dosed separately); -mixing the components; packaging and storage.In addition, at all stages of production, it is required to protect the raw materials from moisture.Therefore, in laboratory studies, before mixing, all components were additionally dried in an oven at a temperature of 105 ± 5 °C for at least 2 hours until constant weight was reached.In industrial production, the use of drying drums is possible.
After drying, the sand was additionally sifted through a sieve with round cells (d = 2.5 mm) to separate large inclusions, crushed stone, impurities and debris.In production, this operation is performed on vibrating screens and is also necessary to speed up the cooling of the material after drying.
Then the stage of dosing and mixing of the components of the mixture by the automatic dosing system begins.Under laboratory conditions, the required proportions of materials were weighed manually on electronic scales with an accuracy of 0.1 g.For the preparation of mixtures, an automatic programmable mortar mixer was used.Matest KIT MIXMATIC.
When the mixer is operating, the rotation of the blade around its own axis and its planetary movement relative to the axis of the bowl must be carried out in opposite directions with a speed of rotation around its own axis (140 ± 5) rpm.with planetary movement relative to the axis of the bowl -(62 ± 5) rpm.
The process of mixing the components is the most important technological operation in the production of dry mortar [19].When mixing, the mixture should become homogeneous, the components should be evenly distributed throughout the volume.Homogeneity control was carried out visually.
It has been established that it is most effective to first mix fine fractions (cement, ash) for 1 minute, and then, after adding sand, continue mixing for another 2 minutes.With a reduction in time, foci of heterogeneity of the mixture were observed, as well as with the simultaneous introduction of all components.Thus, the total mixing time of dry components in laboratory conditions was 3 minutes with a total mass of the batch of 2-2.5 kg.
For subsequent testing of the material, mixing with water was carried out and the solution was prepared according to GOST 58277: -stirring for 120 s: stopping the mixer to remove the mixture stuck to the walls of the mixer bowl within 90 s; -stirring for 60 s.The total mixing time of the mixture from the moment of mixing with water is at least 3 minutes, excluding the time the mixer is stopped.
For testing, experimental samples of various compositions were made.The addition of ash was carried out by weight in excess of the mass of the main components.The table shows mass fractions of the total amount of material (Table 5).From the resulting mixtures, a solution was obtained by mixing with water (15% of the total mass of the mixture) and experimental beam samples 160x40x40mm were made.

Results
To assess the effect of ash waste on the final building material, laboratory tests of the prepared mixtures and sample beams were carried out.In an unhardened state: mobility of the mixture, setting time.In the solid state, the following were determined: material density, bending strength, compressive strength.
The results of studies of the main physical and mechanical characteristics of the manufactured experimental samples are presented in tables 7-9 and graphs (Figures 3-6) The data obtained indicate that with an increase in the ash content in the composition of the mixture, the setting time is reduced.The time to hardening was reduced by 48%, the hardening period by 63% (Composition No. 7).For the developed mixture for 3D printing, the following are optimal: the beginning of setting in 50-70 minutes after mixing with water, the hardening time is 45 ± 5 minutes.These requirements are met by compositions No. 4, No. 5, No. 6.
An increase in the average density of the material indicates a decrease in water absorption [20].This may indirectly indicate an increase in the frost resistance of the studied compositions.
The dependence of the tensile strength on the ash content is extreme.The maximum values correspond to the optimal ash content in 15% of the mixture (Composition No. 5).The increase is 15% during bending and 60% during compression.In addition, an acceleration in strength gain was observed, so on the 14th day, composition No. 5 gained 88% strength against 79% of the base one (composition No. 1).

Discussion
Construction 3D printing is one of the most promising areas in construction.Its application can lead to commercial benefits based on fewer employees needed and lower material costs.And the attractiveness of this niche contributes to the development of printing technologies, the introduction of more and more modern autonomous equipment and the minimization of construction waste.
Buildings in Russia, China, Europe, and America are printed using construction 3D printers, and the scale of 3DP construction in the near future will only grow due to growth of manufacturability and reduction of the cost of equipment.
In general, additive technologies in construction represent a potentially promising area that can change the way buildings are built.They have already proven their effectiveness in some areas of construction and are likely to be used more and more in the future.However, for now, it is necessary to continue research and development in order to make additive technologies in construction more accessible and efficient.

Conclusion
In the course of the work carried out, the existing components and modifiers for dry building mixtures were investigated, and the possibility of their use in building 3D printing was analyzed.
An experimental technology has been developed for the manufacture of a ready-made building mixture using sunflower ash.The main stages of preparation and mixing of materials are determined: the components are dried to a constant mass (no more than 4 hours), then finely dispersed components (cement, ash) are mixed in a mixer for 1 minute.After that, the remaining components are introduced, mixing lasts 2 minutes.It has been established that this technology, when using a mixer blade rotation speed of 140 rpm, is sufficient to obtain a dry mixture of a homogeneous composition.
Experimental samples of dry building mixtures for 3D printing have been made.Samples-beams with a size of 160x40x40mm with different content of sunflower ash were accepted for the study: 3%, 5%, 10%, 15%, 20%, 25% by weight of the remaining components.
The main physical and mechanical characteristics of the experimental samples were determined.Laboratory experiments showed positive changes in the main physical and mechanical characteristics of the manufactured compositions.The change in strength characteristics is extreme; the most effective is a composition with the addition of 15% ash.For this composition, the start and end times of setting were 61 and 103 minutes, respectively, the bending strength was 6.04 MPa, and the compressive strength was 25.52 MPa.An increase in the average density of the material indicates a decrease in water absorption and an increase in frost resistance.
Further research aimed at studying the influence of waste ash on the properties of building mixtures for 3D printing is relevant, since their use leads to a reduction in cost by reducing the number of imported modifiers in the composition of the mixtures, and the use of 3D printing in construction in the future may open up ample opportunities in the development of construction technologies that traditional production cannot compete with.

Table 1 .
Properties of cement

Table 2 .
Properties of sand

Table 3 .
Granulometric composition of sand

Table 4
Ash Properties

Table 5 .
Compositions accepted for the study

Table 6 .
Results of determining the setting time of the mixtures under study

Table 7 .
Bending test results for beam specimens

Table 8 .
Compressive test results for beam specimens