Silica sol properties for construction

The article deals with a number of topical issues connected with transport construction and environmental pollution, such as heavy metal ion pollutions of soils, preservation of natural resources, technical properties of transport construction system, and building materials’ improvement. The main aim of the study is to present new problem solutions and technologies for transport construction concerning pure substance use for lithosphere. Silica sol solution was chosen as this substance. The choice was made because of silica sol nature. Having important properties, it is the purest substance for the environment. Experimental methods were carried out. The study introduces new technologies of silica sol solution use in transport construction systems: silica sol detoxication technology (SDT) for soils, silica sol absorption technology (SAT), soil strengthening technology (SST), silica sol blocking action technology (SBT). The article shows the examples of these technologies’ application and examines new main properties of construction systems.


Introduction
The paper presents new solutions of silica sol use in transport construction.3 topical issues of transport construction systems are especially important for today's overconsumption of natural resources.They are 2,5% per capita consumption of natural resources instead of 1%, lithosphere pollution by heavy metal ions and oil products during the construction of different structures, increase of building material durability to reduce the use of natural resources.It is known that more than 75% of lithosphere minerals are silicates and aluminosilicates, and the nature of these minerals makes them useful for lithosphere protection in transport construction.The main aim of the work is to show silica sol application in transport construction due to properties of the sol.
The papers [1][2][3][4][5] examine some properties of silica sol.The great positive argument is that silica sol is the purest substance for the environment.New opportunities of silica sol use technologies are of great interest now.The results of the work demonstrate the possibility of the study of silica sol useful properties to be applied in building systems and for environmental protection.The use of an artificial stone as a safe for absorbed pollutions from lithosphere and then their blocking by silica sol screen Thus, the  1).Thermodynamic calculation determines ΔG 0 298<0 for the reaction.Besides, another reaction can be performed for detoxication: The results of the reactions (1) and ( 2) are heavy metal ion detoxication in soils provided that solubility product of Me(OH)2 and MeO SiO2 2H2O is less than 10-15 and tolerable concentration (TC) of Pb (II) in a soil is 0,03 g/kg.The formation of substances with low solubility product is the base of SDT.
As an artificial stone has pores and capillaries it is possible to improve the surface of a stone when applying silica sol.The point of this technology is absorption of heavy metal ions by pores and capillaries of an artificial stone.That is why over such substances as SiO2 n H2O the level of a stone hardness and geosphere protection will be increased.It is the principle that SAT operates.
SST suggests the idea of silica gel's binding properties application.For soil strengthening Na2SiO3 is used and, as a result of Na2SiO3 reaction with water, silica gel and NaOH form: Na2SiO3+2H2O=2NaOH+SiO2•H2O.But the use of silica sol solution prevents a soil from NaOH pollution.Therefore, nowadays it is the purest substance for lithosphere.So, there is undoubted benefit from SST.
SBT uses the hard sol screen formed according to SAT.But before the screen formation an artificial stone is used to absorb oil products or solutions of oil with heavy metal ions.Then the polluted stone is covered by silica sol.In this case pollutions are blocked in the stone, and the material can be applied for useful purposes.That is the essence of SBT.

Methods
The experimental methods were carried out to examine these 4 technologies.For SDT about 30% silica sol solution was applied.The soil contained heavy metal ions to 1000 TC (tolerable concentration) was irrigated by silica sol solution for its watering and saturation.The size of sol particles was nearly 10 nm, pH of the solution -10.1-3 days later the aqueous extract from the soil samples were tested using selective electrodes for heavy metal ion presence.
For SAT 1,5-3% silica sol solution was used.After 28 days of hardening the foam concrete articles with density 400; 500; 600 kg/m3 were saturated with sol solution to improve the state of the surface and other technical properties.The test was done according to the Russian state standards.
For SST 30% silica sol solution was applied to saturate the soil.Because of sol→gel conversion the soil was strengthened by silica gel.The method suggests SiO2•nH2O use instead of Na2SiO3 for strengthening, but without Na (I) pollution, as mentioned above.
During SBT the articles saturated with oil products or oil with heavy metal ion solutions were covered by silica sol to stop pollution extraction from a stone.The aqueous extract from the samples was tested by means of selected electrodes and the oil product analyzer.It should be noted that after treatment by silica sol solution an artificial stone acts like a safe, therefore, the polluted stone can be named the "lithosafe".

Results and discussion
The solution of silica sol with the size of particles up to 10 nm and pH=10 was applied.For the model experiment of SDT the sandy soil polluted by heavy metal ions to 1000 TC was used.The first technique was the analysis of the aqueous extract from the polluted soil that was mixed with silica sol.After mixing one could see sediments of different colours.The sediments of the reactions were investigated applying derivatographic method after their drying in the room at a temperature ≈20-25ºC.The Table 2 indicates the results of the sediment examination.The analysis of Table 2 shows that: 1) water loss is different in various sediments; 2) light ion of Ca (II) (for comparison) increases the water loss.It makes possible to assume that the formation of calcium silicate hydrates takes place; 3) heavy metal ions decrease water loss.It makes possible to assume that the formation of heavy metal ions' silicates occurs.
The second technique was to mix the sandy soil with heavy metal ion pollution and after its saturation with silica sol for 3 days the aqueous extract of the sandy soil was tested for heavy metal ion presence.They were not found.Thus, thermodynamic and experimental values coincided.The practical experiment was done with the areas of different sizes ≈10m2 and 20 m2 in operation, the soil in the experiment had particles ≈3-5 nm in diameter.The sediments of the reactions between silica sol and heavy metal ions in the model aqueous extract of the polluted soil were obtained.The detoxication took place at the start of the interaction between the sandy soil and 30% silica sol solution.In 3 days the aqueous extract https://doi.org/10.1051/matecconf/201819303034ESCI 2018 of the soil was taken.Selected electrodes did not show the presence of Pb(II) in it.So, it is possible to detoxicate a soil with more than 1000 TC using silica sol solution.
The silica sol absorption technology (SAT) is new and quite effective.It is based on sol absorption through pores and capillaries.For SAT the foam concrete articles were used after process of hardening.After 28 days of hardening the foam concrete articles were saturated with silica sol during 1 day.The experiment has demonstrated the increase of the articles' properties.The Table 3 contains calculations of material consumption for the foam concrete articles during the experiment.With the surfaces of the foam concrete articles having been watered by 3% silica sol solution, the Table 4 illustrates the properties of foam concrete of different density after the treatment by silica sol.Moreover, it should be noted that when applying SAT the surface of the foam concrete articles did not have salt efflorescence, i.e. it was pure because of SiO2 H2O action.So, the problem of salt efflorescence on concrete surface can be solved, SAT being one of the solutions of this problem.
With reference to the Table 4 SAT can be named technology of preservation of natural substances in terms of cement technology.Taking into consideration the consumption of natural products, fuel and energy for 1m3 of concrete manufacture, the calculations demonstrate the decrease of natural product consumption.Therefore, the application of SAT is the best for motorways' construction.There are some problems of concrete surface in construction, especially in transport construction, e.g.motorways and railways in operation.The paper examines 2 surfaces of B20, B30 concrete used in motorways' construction.They are outside and inside surfaces.Being under loads, outside surface must have enough hardness, strength (compressive and bending), frost resistance and pollution resistance.Inside surface must have protection at least against water absorption.Road articles' treatment by silica sol was applied using 3% silica sol solution for saturation.The details of the technology, the class of concrete, its properties and examples of the use were examined.The research has shown new possibilities of the quality improvement using the new technology (SAT), particularly, pores and capillaries' healing by means of cement sol on the concrete surface that resulted in concrete hardness increase and water absorption decrease.Soil strengthening technology (SST) is the result of sol→gel conversion.The technology is nearly the same as SDT but, unlike this, during SST the process does not take place immediately because it takes more time for sol→gel conversion and for gel hardening.As the practical experiment has shown, it took a few days for strengthening because SiO2 nH2O as a gel hardened for a period of time.In the experiment there was the sandy soil with particles ≈0,5-1,5 nm, the concentration of silica sol solution having been not less than 30%.In that case the substance which is like sand was used but they were different in size.According to SST soil strengthening is 0,1-0,2 MPa, its value being the same as one when applying Na2SiO3.But silica sol is a pure substance for the environment.Thus, the properties of SiO2 nH2O are good for both strengthening process and the environment.

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Silica sol blocking action technology (SBT) is quite new one for transport construction and geoprotection.The result of the technology is formation of the special surface screen on polluted concrete.Oil pollutions were considered in the experiment that is why the concrete was saturated with oil products.Then the concrete having about 7-10% oil products absorbed was put into silica sol solution for saturation within a few hours.Then, in 3 days later, the system was saturated with water.In 1-3 days the aqueous extract was taken from the system for the oil product analyzer test.The oil products were not found in it.The research has shown that the screen formed on the surface of the concrete was 2-4 mm thick.It consisted of silica sol and some substances of interaction between SiO2 nH2O and the stone.
The Table 5 sums up the results of new technologies given above and their application in transport construction.According to the Table 5 it is possible to assume that when applying SDT silica sol can have a good strengthening effect because of both SST processes and the presence of Ca (II) [6][7][8][9][10][11][12][13].If Ca (II) is added in silica sol solution one can obtain calcium silicate that results in soil strengthening.Proceedings [14][15][16][17][18][19][20][21] investigate the mechanism of concrete's main properties increase over calcium silicate hydrates.In these papers some of them are described, namely: decrease of water absorption as well as salt efflorescence [18][19][20][21].As for SBT, it is quite good because of concrete surface screen.This technology is the best for environmental protection as it enables the useful properties of a stone, in particular: concrete can absorb oil products and then block them in the stone by silica sol screen which is 2-4 mm sick.
Apart from these technologies silica sol can be used for other purposes, e.g. a new method of identification of soil pollutions such as heavy metal ion pollution can be suggested.The method is qualitative one and it is based on the sediments' colour (see the Table 2).The point of the method is the reaction between heavy metal ions, Me 2+ or Me 3+ , in the aqueous extract of a polluted soil and silica sol solution.The result of the reaction is the formation of different MATEC Web of Conferences 193, 03034 (2018) https://doi.org/10.1051/matecconf/201819303034ESCI 2018 colours' sediments.For qualitative express method realization one must take 1 volume of a polluted soil and 2 volumes of water and mix them in order to get samples of the aqueous extract.Then these samples must be mixed with 30% silica sol solution.The Table 6 presents the results of the analysis of the soil polluted by heavy metal ions to 1000 TC.It is worth mentioning that one must know that if there is white color of Pb(II) or Cd(II) sediments it is important to perform other reactions, e.g. the reaction with I -ions which has yellow sediment for Pb(II) identification or the reaction with S 2-ions which results in the formation of the lemon-yellow sediment for Cd(II) identification.Thus, it is possible to use qualitative express method of free heave metal ion identification in soils.This method lets everybody easily determine geosphere pollution at once and detoxicate it by means of silica sol.

Table 1 .
Silica sol properties and technologies for transport construction.
Table 1 introduces 4 silica sol use technologies based on the properties of silica sol.Silica sol detoxication technology (SDT) is quite good for railway soil detoxication because railway soils are polluted by heavy metal ions.If there are heavy metal ions, Cu (II); Pb (II); Ni (II); Cd (II); Co (II), the reaction like (1) takes place (see the Table

Table 2 .
Water loss in the sediments.

Table 3 .
The material consumption for foam concrete.

Table 4 .
The improvement of foam concrete properties using sol absorption technology (SAT).

Table 5 .
Results of the study of new technologies for transport construction.

Table 6 .
Qualitative express method of heavy metal ion pollution identification.