Portland cement with puzzolan additive with increased strength properties

. The following work refers to the issues of increasing the strength of portland cement with pozzolanic additive due to some multifunctional additive. Currently, it is the multifactorial additives for ready-mixed concrete that are in demand, which differ from others in their multicomponent composition. In this respect, lithoid pumice, which is a new type of aluminosilicate raw material for the cement industry, can make worthy competition.


Introduction
Nowadays, a completely new approach to concrete production technology is required, therefore, along with an increase in requirements for their main components, the level of requirements for mineral additives for concretes and mortars is growing [1][2][3]. It is impossible to obtain concretes that will meet all the characteristics complying with standards which are stated in the regulatory documents without the use of multifunctional additives that provide: -increasing the strength of concrete stone and mortar at the design age (10 ÷ 30%); -an increase in the plasticity (P) of the concrete mixture (from the index P1 ÷ P5), although the concrete mixture with the plasticity index P4 and P5 must be subjected to electric heating, protection of freshly laid concrete from negative temperatures by the thermomethod;reduction of water consumption by 15 ÷ 25%; -increased adhesion to reinforcement (more than 1.5 times); -increasing frost resistance (at least 50 cycles); -workability; -accelerated strength gain; -increased water resistance (more than W6); -increased salt resistance;increased connectivity (non-segregation); -improvement of the surface of products. It is almost impossible to accomplish such a complex task using one and two-component additives. Therefore, today it is precisely multifactorial additives for ready-mixed concrete that are in demand, which differ from other additives in their multicomponent composition. In this new quality, a multicomponent additive can compete with it: lithoid pumice, which is a new type of aluminosilicate raw material for the cement industry.

Methodology
The object of the research is Portland cements with pozzolanic additives with increased strength properties. Cements are known that include dried siliceous sedimentary rock -flasks, tripoli, diatomite. Mixing and grinding it with portland cement clinker and gypsum differ in the qualitative and quantitative composition of this mixture with the following ratio of components, wt. %: Portland cement clinker 45; gypsum 6; siliceous sedimentary rock 46, coal-bearing component 0.3 [4]. When receiving cement, the process of drying siliceous rock at a temperature of 150-190 ° C to a moisture content (W) of 0.3 ÷ 0.7% and the introduction of a carbon-containing component takes place, which leads to additional energy costs and increases the cost of cement production, although it also optimizes the grinding process Additionaly, this cement has a lower flexural strength 56.6% lower than the proposed substance. Thus, such a cement has a reduced strength at a high energy consumption. Also known is a cement containing portland cement clinker and a mineral additive, heat-treated at a temperature of 150-300 ° C until the loss of hydrated water 50-80%, consisting of a mixture of flask and phosphogypsum with the following ratio of components, wt. %: Portland cement clinker 50-90, additive 10-50 [5]. A considerable disadvantage of the known cement is its low compressive strength -13.4% lower compared to this portland cement, and the overrun of portland cement clinker -23% higher in contrast to the proposed substance. Also, the mineral additive is subjected to heat treatment, which is a sign of increased energy consumption. A cement is also known containing as a mineral additive cement dust-carryover of electrostatic precipitators of rotary kilns being characterized in the fact that it additionally contains a component with a specific surface area of at least 800 m2 / kg, one selected from the group: chalk, clay, limestone or a mixture of these components, when the ratio between the specified component and dust from 1: 1 to 1:10 and the content of alkaline oxides R2O in terms of Na2O in the additive is not more than 4%. A significant disadvantage of the known cement is a small amount of used mineral additive to cement -in the amount of 20% of its mass instead of 30% in the proposed substance. The high specific surface area of the components causes significant costs for their grinding, and fine grinding can disrupt the energy centers of crystallization of the material and reduce its strength. In the proposed substance, the strength is increased by 30% compared to the analogue due to finding the optimal composition, optimal grinding up to 290-300 m2 / kg, energy-intensive technology for producing high-quality cement [6,7]. Thus, the proposed pozzolanic portland cement is an optimal hardening product, consisting of three components, the strength activator of which is an alkaline component: lithoid pumice. The technical result of the proposed substance is to increase the strength properties of pozzolanic Portland cement and its manufacturability by increasing the plasticity of the mixture and the water-holding capacity of the finished product. The specified technical result is achieved by the fact that in pozzolanic portland cement, consisting of portland cement clinker, gypsum dihydrate and a mineral additive, lithoid pumice is used as a mineral additive with the following ratio of components, wt%: Portland cement clinker 62-67% Two-water gypsum 3% (based on the mass of Portland cement clinker) Mineral additive 30-35%

Results and discussion
In the proposed substance lithoid pumice quarry "Atik" from Charentsavan deposit is used as a natural pozzolana. Two-stage joint grinding of the initial components provides not only their fine grinding, but also the energy activation of particles and a change in the structure of their surface layers, which leads to an increase in the strength of the cement, and in some cases, its plasticity. Portland cement, obtained on the basis of this raw material, contains up to 1.4% R2O in the compositions indicated below, which meets the specifications of the regulatory documents. The lithoid pumice stone of the Atik quarry from Charentsavan deposit, which is part of the cement, is a natural alkaline aluminosilicate of the group of alkali feldspars of the KAlSi3O8 series, which are an important component of igneous rocks, such as granites, syenites, minerals of the nepheline and mariupolite group, of the following chemical composition, wt ... % (Table1). Small crystals in combination with minerals of Portland cement clinker form hydration products such as alkaline aluminosilicates, as well as silica gel-like shells, which promote the germination and bonding of potassium feldspar grains into a strong cementitious framework (a natural property of fine crystallinity of calsilite and potassium feldspar). The increase in strength can be due to the micro-reinforcement of the cement stone with mineral additives, as well as their effect on the process of cement hydration. In addition, the modulus of elasticity of the mineral additive is greater than that of cement stone, then, under the action of external loads, a greater level of stresses will fall on the additive material, which is more durable than cement stone. The mechanism is known of the processes of hydration and hardening of pozzolanic portland cements based on mineral additives, which are rocks of ultrabasic composition. Two silicate minerals (C3S and C2S) form, after interaction with water, calcium hydrosilicates (CSH) -the main carrier of the strength of portland cement and concrete, providing normal setting times. At the same time, the contribution of C2S to the strength of portland cement and concrete in the early stages (1-7 days) is small, the main role is played by alite -C3S. It should be noted here that the surface of a pozzolana in a highly dispersed state is characterized by a significant concentration of acid sites according to Bronsted, which was depicted in a number of works of the St. Petersburg school of researchers (founded by M.M.Sychev and continued by L.B.Svatovskaya, P.G. Komokhov and many others scientists), who postulated and then proved not only the presence of acidbase interactions, but also the significant importance of redox processes during hardening of normal Portland cement, as well as cement with additives. The given processes and pozzolanic additives get more importance given the fact, that during the hydration and hardening of cements from unburned and highly alkaline clinkers, impurities actively participating in them are added to normal objects . Pozzolanic additives in a highly dispersed state due to the above-mentioned active acid sites according to Bronsted and a significant increase in their presence of the action of additives -oxidizing agents can be considered as catalysts of redox processes occurring during the hydration and hardening of cements from both normal clinkers and from unburned and highly alkaline ones. The conclusion suggests itself: it is necessary to bind free lime during the hydration of cement with an active mineral additive introduced into the cement during grinding, and the harmful effect of underburnt clinker on the properties of cement and concrete can be significantly reduced [8,9].
Aluminosilicate rock is prepared as is decribed: rock with a grain size of less than 300 microns and a moisture content of no more than 7% is mechanically activated in the activation zone of the MPP-2 planetary mill (TTD). The feed rate of raw materials into the apparatus is 10 kg / h. As a result of rock processing on this apparatus, the dispersion of the raw material particles changes, that is, the crystallization centers of the material become more active. After the activation process, a crushed product with a particle size of less than 200 microns is obtained, which is the mineral additive with which Portland cement is mixed. Portland cement with a mineral additive is tested in accordance with GOST 30744 and compared with a control portland cement without any additives. The strength of portland cement is slightly reduced.
The research has revealed, that with a two-stage grinding in a rod vibration installation for 4 minutes, grinding a raw mixture consisting of 62-67% of portland cement clinker, 3% of gypsum dihydrate (based on the mass of portland cement clinker), 30-35% of a mineral additive -alkaline syenite, the strength increases in comparison with the known types of cements of this composition. The characteristics of the obtained values of pozzolanic portland cement are given in tables 1-3, in which the following designations of the main components are taken: K -Portland cement clinker, G -dihydrate gypsum, MD -mineral additive, TVO -heat and moisture treatment, Rсzh -ultimate compressive strength, Ri -ultimate strength in bending, Ssp -specific surface area, W / C -water-cement ratio, PPTs 500 D30 -pozzolanic portland cement grade 500. Analysis of the results of tables 2-3 shows that: -the proposed pozzolanic portland cement), which contains a mineral additive of 30%, has increased strength compared to ordinary portland cement grade; -the optimum is a composition containing 62-67% of portland cement clinker and 3% of gypsum dihydrate (based on the mass of portland cement clinker), 30-35% of a mineral additive; -with an increase in the consumption of the additive, starting from 40%, with a decrease in clinker to 52%, the strength of portland cement decreases; -all compositions of pozzolanic portland cement gain strength after 28 days of storage of samples in normal humidity conditions; -the strength of portland cement increases when the samples are stored for 28 days in normal humidity conditions (100% strength gain), which makes it possible to conclude about the strength gain at a later time of hardening; -all compositions of pozzolanic portland cement (PPTs 500 D30) gain strength after 8 hours of heat treatment, 2 days in water (TVO) and 1 day of storage of samples in laboratory conditions. When steaming samples for more than 8 hours of heat treatment, a significant increase in strength is not observed, however, when the samples are stored for more than 28 days in normal humidity conditions, an increase in strength can be seen, which gives reason to assume an increase in the strength of pozzolanic portland cement during a certain period of operation; cement, consisting only of Portland cement clinker and gypsum dihydrate, has a reduced strength 36.4% lower than the proposed portland cement, which contains a mineral additive -lithoid pumice stone, when all other conditions for hardening are equal ( Table 2, Table 3).

Conclusion
Thus, the cost of the product is reduced by reducing the grinding time of portland cement products, the average density of portland cement is reduced, which significantly affects the consumption of the product, a completed final product is obtained, when the samples are stored for more than 28 days, the strength of the product continues to increase, which is a particularly characteristic property for pozzolanic portland cements of this type.