Quality Improvement of Concrete Articles

In the paper it is shown that quality of concrete articles and structures may be significantly improved by silica sol solution absorption. Improvements include increase of compressive strength, resistance to low temperatures, coefficient of constructive quality, decrease of water sorption, contraction. The reason of improvement is discussed.


Introduction
It is well known that surface properties of construction materials, including acid-basic ones are very important [1][2][3][4][5].The main idea of the paper is the possibility of improvement of quality of concrete articles by means of saturation and modification of their surface with silica sol solution.Due to the presence of capillary pores concrete articles and structures possess ability to soak silica sol solution.After soaking process silica sol appears in cement based materials' pores and may participate in the reaction: Reaction ( 1) is characterized by negative value of Gibbs energy, therefore new calcium silicates are formed, which is good for improvement of the construction article quality [6][7][8][9][10][11][12].Due to presence of certain amount of capillary pores, their sizes and peculiarities of reaction (1), each cement based article can intake only certain limited quantity of silica sol.Thus we introduce new parameter to characterize concrete articles' ability to intake silica sol -absorption capacity C (kg/m3), mass of silica sol absorbed by unit volume of article.

Experiment, results and discussion
According to the preliminary experiment [8] it was chosen to perform experiments on quality improvement of construction articles using samplers of foam concrete with the following average density values D: 0.4; 0.5 and 0.6 Ton/m3.For the modification of samples we used 1.5% silica sol solution.Characteristics of solution: pH ~ 9, density of solution -1.02 g/cm3, particle diameter of silica sol dispersions -12 nm.
a Corresponding author : msychov@yahoo.comSamples were saturated by the silica sol solution on the third day of hardening process and then were examined.The process of saturating with silica was described using suggested above new parameter -absorption capacity, С, kg/m3, calculated according to equation (2).
where between m1 and m2 are masses of sample before and after the modification process, kg; V -volume of sample, m3.
After that modified sample of concrete article was tested according to common protocols such as water absorption, compressive strength, resistance to low temperatures, contraction and coefficient of constructive quality.Overall quality of cement articles was characterized by the value of specific compressive strength [10,13,14].
The main results are shown in the Table 1.One can see that quality of modified articles was improved and their properties are superior compering to ones of article without modification.Some two-fold increase of main characteristics of concrete articles was achieved by modification.Explanation of such kind of quality improvement is the healing of the pore structure and decrease of total volume of pores by means of silica sol saturation and occurrence of reaction (1).Parameter С reflects absorption of silica sol and formation of new phases in the pore space of articles according to the reaction (1).From the data of table 1 it is clear that value of parameter C increases with the rise of density of concrete article [15][16][17][18].In the Table 2 one can see results of the experiment on modification of concrete samples using different concentrations of silica sol.The level of the improvement of characteristics of modified heavy concrete samples is lower comparing to that of foam concrete, nevertheless 40…60% increase of strength was achieved [19,20].The main reason for the improvement of properties is the reaction (1) providing formation of additional amount of calcium silicates.That amount may be calculated taking into account reaction (1) and values of absorption capacity C. Examples of such kind of calculations are presented in the Table 3.

Conclusions
It was shown that such characteristics of foam concrete and heavy concrete articles as compressive strength, water absorption, resistance to low temperatures and contraction may the significantly improved by modification with silica sol solutions.Due to the presence of capillary pores intake of solution takes place resulting in formation of new silicate phases and healing of pores.

Table 1 .
Improvement of characteristics of foam concrete samples after modification.

Table 2 .
Increase of strength of concrete samples

Table 3 .
Hydrate calcium silicates in concrete and absorption capacity