STUDIES OF STRUCTURAL-PHASE CHANGES IN A MODIFIED CEMENT STONE METHOD BY DERIVATOGRAPHIC ANALYSIS The use of a complex nanodispersed additive consisting of a superplasticizer and nanoparticles (sol of nanosilica and carbon nanomaterial) have a significant effect on the hydration, hardening, and curing of building composites, predetermining their durability

The use of a complex nanodispersed additive consisting of a superplasticizer and nanoparticles (sol of nanosilica and carbon nanomaterial) have a significant effect on the hydration, hardening, and curing of building composites, predetermining their durability. The results of studies of phase transformations occurring in a cement stone modified with this additive are presented. The studies were carried out using the method of thermal analysis, which allows to obtain quantitative information about the change in the mass of the sample as a result of its heating and at the same time register this change. Thermal analysis has a number of advantages over other research methods, the flexibility of setting up an experiment, the rapid removal of information, the ability to automate data processing, and a small amount of substance. A comprehensive study of the properties of a material when it is heated allows a detailed and deeper study of the nature of the processes occurring in it. The study of physical and chemical processes was carried out on a combined thermogravimetric analysis device and differential scanning calorimetry METTLER TOLEDO, which is designed to measure the thermodynamic characteristics (heat and temperature of phase transitions and physicochemical reactions), as well as detect changes in the mass of materials in the temperature range from 25 to 1600 degrees C. The article discusses the intervals of weight loss of samples containing an additive of superplasticizer and a complex nanodispersed additive in age those 1 and 28 days. To find out what effect a superplasticizer has on a cement sample, an additional sample No. 1, consisting only of cement and water, was examined at 28 days of age.

a complex nanodispersed additive consisting of a superplasticizer and nanoparticles (sol of nanosilica and carbon nanomaterial) on the properties of the cement material.
Research Methods.

Prototypes.
Structural-phase changes in hardened cement stone were studied on samples of the following samples: No. 1 -control sample (cement-water), No. 2 -sample containing the addition of superplasticizer (SP), No. 5 sample containing a complex additive consisting of a superplasticizer and nanoparticles of nanoparticles (sol of nanosilica and carbon nanomaterial) [8][9][10].
The additive in samples No. 1,2 and 5 was introduced in an amount of 0.8% by weight of cement. The amount of mixing water for all samples was selected in such a way as to obtain a dough of normal density in all cases. Samples of beams with dimensions of 40x40x160 mm were made from cement paste of normal density. Tests were conducted at the age of 1 and 28 days.
Methodology for sample preparation. The preparation consists in removing water from the pores and micropores so that the cement hydration reactions do not continue further. It is impossible to simply dry the sample to remove pore water in an oven at 105 ° C, because together with the pore water, the sample will lose part of the hydrated water, which has already reacted with cement minerals and has become fixed in calcium silicate hydrates. After drying at 105 ° C we obtain, according to the data, an underestimated content of hydrated water and, accordingly, an underestimated content of calcium silicate hydrates. Therefore, drying was carried out in acetone at 50-60 ° C. Acetone is able to chemically bind water and evaporate at 50-60 ºС, taking away pore water with it, and at the same time crystallization hydrated water is not affected, which is fixed in hydrates of calcium silicates. Without acetone, it would be impossible to remove pore water without destroying calcium silicate hydrates.
The procedure was as follows: a) crushing the sample, b) abrasion to powder, c) adhering acetone to the powder, d) drying in a drying oven at 50-60 ºС in acetone for 30 minutes. If acetone completely evaporated earlier than after 30 minutes, then it was added additionally.
The equipment used.
The study of the physicochemical processes occurring during the high-temperature heating of cement stone samples was carried out using a combined thermogravimetric analysis and differential scanning calorimetry METTLER TOLEDO (Switzerland). This device is intended for measuring thermodynamic characteristics (heat and temperature of phase transitions and physicochemical reactions), as well as recording changes in the mass of solid and powder materials in the temperature range from 25 to 1600 degrees C.
Thermogravimetric analysis (TGA) allows you to obtain quantitative information about the change in mass of a sample as a result of heating, cooling, exposure at a constant temperature in accordance with a given temperature program and in a specific gas atmosphere.
Thanks to the unique design of the thermal sensors integrated in the measuring cell, the TGA / DSC1 also enables differential scanning calorimetry (DSC). Such a constructive solution makes it possible to carry out measurements of calorimetric quantities at different thermodynamic transitions accompanied by a change in the mass of the test sample at the same time in one experiment and on the same sample, and record this change. The device is made in the form of a single desktop measuring module, consisting of a temperature unit (furnace), a unit for measuring mass change (scales), a calorimetric sensor -a sample holder, placed in a furnace with adjustable temperature and heating speed, an electronic control and measurement unit.
The principle of measuring the amount of heat released or absorbed by the sample is based on the integration of a sensor signal over time that measures the difference in heat fluxes between two cups, one of which is filled with a sample, while being simultaneously controlled by the speed of heating in the furnace to a predetermined temperature controlled by the sensor.
The principle of operation of the scales is based on compensation for changes in the weight of the sample by electromagnetic force created by the automatic balancing system. The measuring unit of the balance is thermostatically controlled by an external circulation thermostat.
Main part. 1 interval. From 20-30ºС to 300ºС. The mass loss in this interval refers to hydrated water H2O, which was bound in the hydration reactions of alite and other clinker minerals, and transferred to the composition of calcium hydrosilicates and hydrates of calcium-aluminate and calcium-aluminoferrite. In this interval, there are minima on the DTG and DSC curves corresponding to the maximum rate of hydrated water removal. If we take the interval of 30-300 ºС, then sample No. 2 lost 7.6% of its mass in this interval, sample No. 5 is less -7.06%. The relative difference in this case is 7.6 / 7.06 = 1.076 = 7.6%. This difference is in favor of sample No. 2 without nanoparticles.
If the entire 1st and 2nd interval, as highlighted in the graphs, from 30 to 425 ° C are attributed to losses of hydrated water, then sample No. 2 lost 8.27% of the mass, sample No. 5 -7.72%, i.e. 7.1% less (8.27 / 7.72 = 1.071). It turns out that the proportion of hydrated water is again larger in sample No. 2.
2 interval. From 450 to 550ºС. In this range, weight loss is attributed to decomposition of Portlandite Ca (OH) 2 into CaO and H2O upon heating. Portlandite is formed as a result of hydration of alite and belite. On the graphs, the interval 425-491ºС is highlighted. In this interval, respectively, there are sharp minima on the DTG and DSC curves, confirming the decomposition of portlandite.
Sample No. 2 lost in this interval 425-491ºС 1.59% of the mass, sample No. 5 -1.48% of the mass. It turns out that in sample No. 2 the hydrated water associated with portlandite is relatively larger by 1.59 / 1.48 = 1.074 = 7.4%.
3 interval. This range corresponds to the decomposition of calcium carbonate CaCO3 into CaO and carbon dioxide CO2. The formation of calcium carbonate in cement stone is wholly attributed to the continuous carbonization of portlandite due to contact with carbon dioxide gas. Typically, calcium carbonate decomposes in the range of 780-820 ° C with corresponding sharp minima on the DTG and DSC curves. On the charts, these minima fall on the interval 540-800ºС. For sample No. 2 in this interval, the mass loss is 2.438%, for sample No. 5 -2.709%. If the losses are multiplied by the ratio of the molar masses of H2O and CO2 -18/44 = 0.409, then we get the equivalent loss of water by portlandite. In the range of 540-800ºС for sample No. 2 -0.9971%, for sample No. 5 -1.1079%. The relative difference is 1.1079 / 0.9971 = 1.111 = 11.1% in favor of sample No. 5. The total water associated with portlandite, by the sum for intervals 2 and 3 for sample No. 2, was 1.59 + 0.9971 = 2.5871%, for sample No. 5, 1.48 + 1.1079 = 2.5879%. The relative difference is 0.03%. Those. the amount of water associated with portlandite, and the amount of portlandite itself, formed during the hydration of alite and belite, are the same.
If we attribute the amount of water released in the 1st interval, when hydrates are decomposed, to the total amount of water corresponding to the 2nd and 3rd intervalsportlandite decomposition, then for sample No. 2 it is 7.6 / 2.5871 = 2.937, for sample No. 5 -7.06 / 2.5879 = 2.728, which corresponds to stoichiometry of the alite hydration reaction. The relative differences in interval losses at the level of tenths of a percent are negligible.
It turns out that hydration water, judging by the 1st interval, is larger in sample No. 2. The amount of portlandite in the samples is approximately the same.
The difference in hydrated water between samples No. 2 and No. 5 at the age of 1 day is not so significant as to make a confident conclusion about faster hydration in one of the samples. It is worth noting that the total mass loss in samples No. 2 and 5 in the range of 30-1000ºС differ even less -12.46% and 12.07%, i.e. only 12.46 / 12.07 = 1.032 = 3.2%.
Comparison at the age of 28 days was carried out between samples of samples No. 2 and No. 5. In order to find out what effect the superplasticizer has, sample No. 1 was additionally investigated. We examined 3 intervals of weight loss at the age of 28 days on the TG-curve (Fig.  3,4,5 The total water associated with portlandite, in the sum for intervals 2 and 3 for sample No. 1 -3.07 + 0.859 = 3.929%, for sample No. 2 -2.64 + 0.961 = 3.601%, for sample No. 5 -2.7 + 1.068 = 3.768%, i.e. water associated with portlandite, in sample No. 5 more than in No. 2 by 3.768 / 3.601 = 1.046 = 4.6%, but less by 3.929 / 3.768 = 1.043 = 4.3% than sample No. 1.

Conclusions:
1) In the method of thermogravimetry itself there is an inaccuracy associated with the choice of the boundaries of the intervals. For example, if the first interval is chosen within 30-300 C, then in sample 1, the mass loss is 13.29%. If the first interval is chosen within the range of 30-425 C, then the mass loss is 14.90%, i.e. relative change 14.9 / 13.29 = 1.12 = 12%.
2) Sample No. 1 has a mass loss greater than sample No. 2 in the 1st interval by 7.96%. In sample No. 2 modified with a superplasticizer, SP slowed down the hydration of cement.
3) The mass loss in sample No. 1 is greater than in No. 5. 4) Sample No. 5 at the age of 1 day showed a lower weight loss compared to sample No. 2, and at the age of 28 days a large weight loss (about 5%).