2.2 Creation and gypsum cement gypsum composite materials

The creation of composite materials formed by the Portland cement in mixtures with high amount of hemihydrate of calcium sulfate, requires the solution of the following tasks:

- to determine physico-chemical principles and develop technological ways of controlling the process of hydration hardening of the facility to optimize structural and mechanical properties of the formed stone;

to identify the factors contributing to the phase, and the resulting structural-mechanical transformations in cementing the stone in the later stages of curing and during its operation;

- to establish the mechanism of degradation of cement stone and to develop ways to control the mechanism to ensure its operational reliability and durability;

- to create and explore various types of rapid-hardening materials derived from Portland cement mixed with excess amounts of gypsum binder, to identify promising directions for their use in the national economy.

At the first stage of the study were subjected to two-, three - and four-part system for the following models:

Sulfo-lime-silica CaSО₄·0,5Н₂О -CaO-SiО₂ -Н₂О;

Sulfo-silicate CaSО₄·0,5Н₂О -ЗСаО * SiО₂-SiО₂-Н₂О;

Sulfo-aluminate CaSО₄·0,5Н₂О -ЗСаО*А1₂О₃-SiО₂ -Н₂О.

In the second stage were investigated four - and pathcomponent system, sulphonated, nitrated aluminosilicate models:

CaSО₄*0,5Н₂О-3СаО•SiO₂-3СаО•A1₂О₃-SiО₂-Н₂О.

In the third stage was investigated by models:

СаЅО₄•0,5Н₂О — clinker — H₂O;

СаЅО₄•0,5Н₂О — klinker — SiO₂-H₂O;

СаЅО₄•0,5Н₂О — klinker — modifiers — SiO₂—H₂O

In the fourth stage studied real gypsum cement gypsum compositions with different fillers. At all stages of investigations were carried out depending on the ratio of the components, duration and temperature and humidity conditions of hardening and destruction of the studied object, as well as other factors. At the first two stages were used chemically pure substances, and at the last stages used in industrial products, corresponding to requirements of acting normative documents.

2.3 Hardening of gypsum cement compositions

The core issue of physical-chemical mechanics, the development of which is associated with recent achievements in the theory of hardening of mineral binders, according to the views of the school of academician Rebinder is the structure hardening system.

According to Polak, in the most General form of a structure means the totality of emerging material properties contributing to its ability to resist external forces until the destruction. In a narrower sense, it differentialsa by three symptoms:

1) structure of matter, determines the mechanical and mineralogical composition and do not depend on geometric properties of the system;

2) system structure (pore space), characterized by a set of geometric properties;

3) the structure of relations arising from how the structure of substances and the structure of the pore space.

For hardening systems consisting of Portland cement mixed with Provodnik calcium sulfate, crucial management issues of formation of the structure of matter.

Control of the formation of structures is based on the mechanism and regularities of the kinetics of their development, as well as on the relationship between the mechanical and other properties at early stages of curing and the properties of the final material.

A systematic study of the formation of structures in the hardening of gypsum cement gypsum systems to date received insufficient attention, In this regard, the study of features and identification of regularities of processes of hardening of gypsum cement gypsum systems is due to both practical and theoretical necessity. The practical need is to justify the technological prie-MOUs impact on hardening the system with the aim of controlling the processes of formation of the optimal structure, hence the creation of the most high technical characteristics of the formed solids. The theoretical need to study the curing process is to identify the mechanism of Portland cement hardening in the presence of an excessive amount of hemihydrate of calcium sulfate.

To achieve these objectives, a prerequisite is to link the phenomena of hydration processes of structure formation and peculiarities of formation of the cementing properties of the stone in progressively more complex models of the object. This approach identifies the role of each major component of the gypsum cement gypsum system in the process of its hardening.

2.3.1 Sulfate lime siliceous system

When fully hydrated Portland cement clinker of normal composition at the expense of his transformations of silicate phases in the low-basic calcium hydrosilicates can theoretically produce up to 50% of the free calcium hydroxide (by weight digidratirovannogo clinker). The hemihydrate of calcium sulfate is the most reactive mineral binder within the composition gypsum cement gypsum systems. All this gives particular importance of the considered model system in the process of hydration hardening of gypsum cement gypsum compositions.

From figure 16 it follows that the hemihydrate of calcium sulfate makes fundamental changes in the processes of hydration izvestkovoglinistym system. These changes give rise to a significant reduction of the formation processes of crystallization and structure of multiple strength reduction occurring stone (figure 17).

It is possible to allocate three plots of structure formation depending on the composition of the original mixture system.

The first plot (l,0≥CSH0,5/T≥0,8) is characterized in that reduction of the sulfate component leads to a proportional reduction in the elasticity of the system is 1.3 times, and strength 5 times.

In the second section (0,8>CSH0,5/T>0.15) is observed two-step nature of the formation and a slight reduction in strength properties.

At the third site (0,15≥CSH0,5/T>0) a slight increase of the elastic properties and strength of stone.

These data confirm the presence of three distinctive areas of the content of gypsum in the stone of strength contacts in its structure. Fundamentally important is the fact that gypsum or izvestkovoglinistym binders, introduced each other in the amount of 15-20% of their total mass, reduce the strength of the contacts in the structure. This is accompanied by multiple reduction pragoprojekt characteristics of cement stone. For sulfosuccinates stone is characterized by processes of recrystallization, bringing to 14 days of hardening a number of contacts in the stone structure is much less than a day. In addition, in the range l<CSH0,5/T≤0,8 this recrystallization is accompanied by a significant reset of the margin.

Figure 16 – the Kinetics of the changes in pH, conductivity, degree of binding Ca(OH)2 (α) and its concentration in solution (C) at 20⁰C, a/T = 20 and C/S = 0.8 in the systems a – CaO-SiO₂-H₂O; b – CaSO₄•0,5H₂O-CaO-SiO₂-H₂O

1 – when CSH0,5/T = 1,0; 2 – 0,9; 3 – 0,8; 4 – 0,5; 5 – 0,4; 6 – 0,1; 7 – 0,0

Figure 17 – Kinetics of structure formation sulfosuccinates system

The nature of the decline in strength still does not answer the question about nature contacts. Replacement of semi-aquatic calcium sulphate dvuvodny ceteris paribus leads to a slight increase in induction period of structure formation. However, the formation curves between them are not fundamentally different, especially in the later stages of hardening systems. From this we can conclude that at the given concentration of calcium sulfate, the proportion of contacts continuous crystallization structure formed by hydration of gypsum semi-aquatic, very small. The bulk of contacts obra-zavyvaet the hydration products izvestkovoglinistym component of the system.

In this regard, of particular importance is the temperature factor of hydration hardening sulfosulfurone-samishii system. As can be seen from figure 18, at a temperature of +20°C and C/S>1 the hemihydrate of calcium sulfate prevents the interaction of calcium hydroxide with additives amorphous silica.

Figure 18 – Influence of calcium sulfate (1, 2) and temperature (3-5) on the rate of interaction of silica with calcium hydroxide, depending on their ratio in the starting mixture

The specified effect can be eliminated by ensuring that condition C/S<1 or increase in temperature. In particular, the increase in temperature from 20 to 80°C causes the reduction of induction period of structure formation system about 5 times, however, the strength of crystallization of the structure decreases (figure 19).

1 – at T = 20⁰C; 2 – T = 40⁰C; 3 – T = 60⁰C; 4 – T = 80⁰C;

Figure 19 – Kinetics of structure formation sulfosuccinates system

The established features of structure formation of the system are a consequence of changes in the conditions of education and vegetalization tumors. Thus, the crystals of the dihydrate of calcium sulphate, obtained in distilled water, pictures of electron microscopy have the form of prisms with a smooth surface. Under the influence of calcium hydroxide (C/S = =0,9), acting as moderator, formed the Bo-than large crystals of gypsum. Excess amount of calcium hydroxide form a solid, indistinguishable in the mass of aggregates on the surface of gypsum crystals and the space between them.

Introduction to the additives SiO₂ (C/S = 0.8) leads to the appearance of the globules of the calcium hydro-silicates of indeterminate configuration. These globules are agglomerated to form aggregates not only in space but also on the surface of gypsum crystals.

Regardless of the C/S original mixture (C/S=0,9-l,7) izvestkovoglinistym system at temperatures up to 80°C are formed of low-basic hydrosilicates of calcium, the basicity of which is from 0.88 to 1.24. An introduction to the system CSH0,5 leads to a decrease in the basicity of 8-10%, ceteris paribus.

This is due to the formation of calcium hydrosilicates, containing 8-10% of the introduced sulphate-anion, which prevents process of polymerization. This is what generates the rapid decline in the strength of the contacts in the structure of the stone during CSH0,5/T≤0,15.

Thus, all the components included in the composition sulfosuccinates system, play an active structural role, the intensity of which is identified by the hardening condition of the system.

2.3.2 Sulfate Silicate System

The significance of the system lies in the fact that included in its composition hemihydrate of calcium sulfate and tricalcium silicate in gypsum cement gypsum compositions dominated by content and are designed to ensure the formation of the main technical characteristics of cement stone.

Sulfanilamidna system is characterized by an almost classical type of integral curves of structure formation (figure 20).

1 – SiO₂ /C₃S = 0; 2 – SiO₂ /C₃S = 0,84; 3 – SiO₂ /C₃S = 1,68;

4 – when SiO₂ /C₃S = 2,52;

Figure 20 – Kinetics of structure formation system CaSO₄•0,5H₂O-C₃S-SiO₂ -H₂O

For values of parameters of structure formation this system is very close sulfosuccinates system when CSH0,5/T=0.9 and C/S=0,9.

As can be seen from figure 21, this similarity is due to the fact that the presence of hemihydrate of calcium sulfate significantly increases the content of CA(Oh)2 in the hydration products.

1 – at SiO₂ /C₃S = 0; 2 – at SiO₂ /C₃S = 0,42; 3 – the binding of hemihydrate calcium sulfate with hydration products

Figure 21 – the Impact of CaSO₄•0. 5H₂O on the content of CA(OH)₂ in the hydration products of tricalcium silicate

So, in the range of molar ratio of 0<CS/C₃S<0,36 content of CA(OH)₂ increased 2-fold; at 0,36<S/C3S<1 corresponds to the cleavage of C₃S from gidratirovana one of six calcium ions that calcium in the motive structure is in a separate hemisphere relative to the oxygen ion and is therefore particularly active.

Additive amorphous silica, which reduces the equilibrium concentration of CA(OH)₂ in the liquid phase of hardening systems intensifitsiruetsa the processes of structure formation in it.

From figure 22 it follows that the hemihydrate of calcium sulfate leads to the completion of the curing process sulfoxylates system during the day.

1, 2 - C₃S – H₂O; 3, 4 – CaSO₄•0,5H₂O – C₃S – H₂O

Figure 22 – Kinetics of increase of limit of compressive strength (1, 3) and the longitudinal wave velocity of ultrasound (2, 4) hardening systems

The number of contacts in the structure formed of the stone remains almost constant in time and is not depending on the presence and quantity of additives is SiO₂. Essentially, the presence of a sulfate component has led to the preservation of a cementing stone after curing during the day. The consequence of this is repeated reducing the target strength of the stone formed sulfoxylates mainly due to the decrease in the strength of the contacts in its structure, evaluated by the ratio C1/R.

The reduction of the final strength of the cement stone under the effect of calcium sulfate can be explained by the binding part gidratirovana C3S in sulfasalizine and complications in the formation of low-basic calcium hydrosilicates. In particular, according to table 2 the low-basic calcium hydrosilicates C-S-H(I) in the products of hydration are not detected if CS/C3S≥0,09. However, these phenomena cannot be explained by the fact of conservation (pause), which manifests itself in the presence of SiO₂ additives as these additives due to the interaction with CA(OH)₅ lead to the formation of additional quantities of C-S-H(I).

To clarify the reasons for the manifestation of such an unexpected process such as vosprepyatstvovat the impact of the sulfate component in the formation of C-S-H(I) and the conservation hardening, studied phase transformation in the hardening of the system for small values of CS/C₃S.

Table 2 – the Dependence of composition of products of hydration from the content of the silica and calcium sulphate in the original mixture sulfoxylates

Neoplasms	Attitude SiO2 /C3S	When CaSO4 / C3S to the mixture
		0	0,09	0,21	0,36	0,56	0,85	1,26	1,96	3,36	7,54
CaSO4·0,5H2O	0 0,42 2,52	- - -	+ + +	+ + +	+ + +	+ + +	+ + +	+ + +	+ + +	+ + +	+ + +
Са(ОН)2	0 0,42 2,52	+ + -	+ + -	+ + -	+ + -	+ + -	+ + -	+ + -	+ + -	+ + -	+ + -
С-S-Н(I)	0 0,42 2,52	+ + +	- + +	- + +	- + +	- + +	- + +	- + +	- + +	- + +	- + +

The absence of acidic anions in sulfanilamidnam stone is the cause of the loss of the ability to polymerization of the emerging calcium hydrosilicates. Such calcium hydrosilicates such monocrystal TSH(II), Plombieres, Teherani and some other varieties of natural hydrosilicates deprived of the ability to polymerization.

Thus, the negative structural and mechanical effects of calcium sulfate on the hydration process of hardening of C3S due to the very complex nature of the interaction CSH0,5 C₃S, leading to the complication of the formation of C—S—H(1), as well as the loss of ability to polymerization occurring hydrosilicates of calcium. Additives SiO₂ such an effect of calcium sulfate are significantly impaired, but unable to prevent it completely.

2.3.3 Sulphate aluminate system

Of all the model systems gypsum cement gypsum compositions sulfoaluminate system is the most thoroughly studied. So, for the first time in 1890 Candle first discovered the formation of hydrosulphuret calcium in hardened stone. Michaelis is similar to the double salt was synthesized and described by the formula 3CaO•Al2O3•3CaSO4•30H2O, calling it "cement Bacillus". Lerch, Ashton, and Bogue have established the existence of two forms of hydrosulphuret calcium — stable translvania form 3CaO•Al2O3•3CaSO4•31H2O and metastable monosulfate form 3CaO•Al2O3•3CaSO4•12H2O. The formation of these salts in the Quaternary system Cao-A12O3-СаЅО4-H2O and phase equilibrium in this system have extensively been studied by Jones and D ANS and Eyck.

These fundamental studies served as the theoretical basis of numerous studies performed in recent decades in this country, and abroad to address very complex issues about the role of gypsum in the hydration process of hardening of Portland cement. However, it should be noted that these studies used the Sul-fat calcium at least active — dihydrate form.

In this regard, with respect to gypsum cement gypsum compositions of the consideration of the processes of hardening sulfoaluminate in the system of the hemihydrate of calcium sulfate is a necessary and urgent task. This task is complicated by the fact that in sulfoaluminate system, as a rule, must be present and reactive silica.

In particular, the processes of hydration C2A occur with excessively high intensity, which eliminates the possibility of forming a structure of a solid. The self-destruction of structure hardening system is eliminated, if it is entered additives of SiO2.

In this case, after you shut of water (figure 23) immediately formed crystallization-coagulation structure, the elasticity which within 30-45 min rapidly increases in a linear relationship.

1 – at SiO2 / C3A =2; 2 – SiO2 / C3A =3; 3 – at SiO2 / C3A =4;

4 – when SiO2 / C3A =6

Figure 23 – Kinetics of structure formation of the system C3A-SiO2-H2O

After reaching the maximum of the index of elasticity almost immediately starting netsa reset. However, after 1.5-2 hours after mixing with water sets a conditional equilibrium, characterized by a very slow further increase the elasticity of the system. Such pulsating character has also the dependence of the change in time of the content of CA(Oh)2 in the hydration products of the system (figure 24).

a – at T=20⁰C; b – at T=80⁰C

Figure 24 – the Change in the content of CA(Oh)2 in the hydration products in the system C3A-SiO2-H2O (- - -) and CaSO4•0,5H2O - C3A-SiO2-H2O (----) for different values of SiO2 / C3A (indicated on curves)

The data indicate that mixing С3А water additives SiO2 interact with tumors arising in the process of hydration. The intensity of the effects of additives SiO2 depends not only basic parameters strukturoobrazovatelja system, but also the composition of the products of hydration. Last, depending on the SiO2 / C3A can be characterized by the presence not only highly basic, but low-basic calcium hydroaluminate (С2АН8 and САН10) and gidroperita — C2ASH8.

Hydration С3А in the presence of the hemihydrate of calcium sulfate due to the high reactivity of these components. The solubility of hemihydrate in water (20°C) is 60•10-3 to 7•10-3 mol/l for С3А. As a consequence, the interaction between sulfate and aluminate phases occurs directly at the interface gidratirovana С3А — rich sulfate-anions of the electrolyte, the reaction Products from the solution kristallizuetsya near the surface gidratirovana particles С3А and create, thus, gazorazdelitel film. This complicates the process of hydration С3А: he periodically pauses and resumes.

The structure formation processes of this system are determined by the conditions of necrotelicomnicon from a solution of ettringite together with gypsum, which, because of film formation involve in the stone structure non hydrated particles С3А. The latter again displayed the ability to interact with water and calcium sulfate. As a result of these processes formed the rock, which upon subsequent exposure to water is disrupted.

In the process of further hardening of the system as gypsum and SiO2 present in it individually, have a negative impact on the strength formation of cement stone. This impact is somewhat attenuated in the joint presence of additives of the hemihydrate of calcium sulfate and SiO2 in hardening the system.

However, this does not exclude phase transformation in stone, as indicated by the nature of the time variation of the relative strength of the contacts in its structure. This transformation is largely due to the kinetics of formation of ettringite, is fundamentally dependent on the SiO2 / C3A initial mixture.

Cementitious stone containing calcium sulphate, is characterized by a large number of large prismatic crystals of gypsum. In addition to the crystals of gypsum are highlighted in small prismatic crystals of ettringite length of 0.1-0.25 microns and a width of 0.03-0.1 MK. Druses of small crystals of ettringite are quite clearly distinguished on the surface of the individual Chris-for metal gypsum. Mainly small crystals of ettringite to form solid amorphous mass around the crystals of gypsum.

Additives SiO2 significantly change the mutual arrangement of crystalline phases. The main change should be considered a decrease in the proportion of amorphous ettringite and more uniform dispersion of its crystals in space.

2.3.4 Sulphate aluminate Silicate system

Sulfoaluminate system, having in its composition CaSO4•0. 5H2O, CaO, C3S, С3А and SiO2, the most complete models gypsum cement gypsum composition. In this regard, features of hardening of the system are important.

From figure 25 it follows that in the system C3S-С3А-H2O initial structural skeleton is formed mainly by the hydration products С3А, which is further strengthened by the products of hydration of C3S. With the introduction of the additives SiO2, the role of hydration products С3А in the formation of the initial frame is greatly weakened, resulting in almost completely disappear, differences in the nature of structure formation of systems C3S-С3А-H2O and C3S-H2O.

The special role of SiO2 in the processes of structure formation and inherited by the system CSH0,5-C3S-С3А-H2O. In this system, when SiO2/C3A=0 structure formation determines the hemihydrate of calcium sulfate, С3А and products of their interaction. The hydration products C3S do not have a practical effect on the processes of structure formation of the system.

1 – at SiO2/C3A=0; 2 – at SiO2/C3A=2; 3 – at SiO2/C3A=4;

4 – when SiO2/C3A=6; 5 – system C3S-H2O

Figure 25 – Kinetics of structure formation of the system C3S-С3А-SiO2-H2O

When SiO2/C3A=1 SiO2 additives interact with the hydration products С3А and kind of halfway remove them from participation in the education structure. The kinetics of formation and elastic properties of the structure in this case mainly determine the products of hydration of the hemihydrate of calcium sulfate and C3S.

If the additive SiO2 is introduced in the amount of SiO2/C3A≥2, then participate in the structure formation shows the hydration products not only С3А, but C3S. The nature of the structure of the system in this case is determined by the hardening sulfosuccinates system.

The system C3S-С3А-SiO2-H2O according to the amounts of elastic structures close to the system C3S-SiO2-H2O, however, in strength much superior to the latter. The change in the elastic properties and strength time spirit.

Of particular note are the extremely negative effects of additives SiO2 and semi-aquatic gypsum, and their mixtures on the ultimate strength of the structure of the system. All this testifies to the complex nature of the effects of these additives not only on the kinetics of hardening, but also on the final strength of cement stone. Specified complexity is largely due to the conditions of formation of ettringite during the curing altaloma-silicate system.

2.3.5 Features of the formation of a cementing stone

Each component part of a system that can interact with any other component or any components in their various combinations. This is the cause of the possible occurrence of a variety of reactions of interaction of components in the process of hydration of the system.

However, the General thrust of the interaction of components and, therefore, the result of hydration hardening the system as a whole, are subject to all reactions, but mainly one of the parallel running with the highest speed in these conditions. In relation to model systems gypsum cement gypsum compositions as the main factors determining the General orientation of the interaction of the components that stand out:

1) competition between the hemihydrate of calcium sulfate and silica in their interaction with the hydration products of tricalcium aluminate;

2) the competition between the hydration products of tricalcium aluminate and hydration products of tricalcium silicate in their interaction with silica.

Depending on which competitive ability has one or the other component relative to other components of the system, the overall thrust of their interaction can radically change.

Therefore, to identify the competing interaction of the named components are essentially causal factors contributing to the result of the hydration hardening of gypsum cement gypsum system.

The results of the study of model systems, we confirm that the competitive ability of a component system in relation to another or others can be adjusted by changing:

1) the conditions of their admission in the reaction medium (V/T, the temperature, etc.);

2) the ability of their interaction (temperature, conditions of film formation, etc.);

3) the sequence of their interaction (the order of introduction of components in the reaction mixture, a targeted film formation, etc.);

4) quantitative relationships between them.

In assessing common patterns of influence on the ratio of components on the character of physico-chemical processes of hydration hardening model systems it was found that the compositions of mineral binders Portland cement clinker and hemihydrate of calcium sulfate, can be characterized by a molar relations SO3/ЅО3 and C3S/C3A. The values of these ratios, ceteris paribus determine the phase composition of the formed cement stone.