Investigation of the bottom ash slags influence on the heavy concrete frost resistance

This paper addresses the issue of the influence of cement consumption, plasticizing additive, and compaction time on the strength and frost resistance of concrete manufactured with bottom ash slags from thermal power plants. The investigation was carried out using the threefactor experiment with variables varied at three levels. There were received response functions, which characterize the correlation between frost resistance and variables: cement consumption, plasticizer additives and compaction time. An analysis of the surface of the response function, where the frost resistance of the concrete was accepted as a response, revealed the optimal correlation between the components of the mixture and the compaction time of the seal. By using the STATISTICA 12 software complex, the values of the factors were specified, when the optimal components ratio with GLENIUM 51 superplasticizer is achieved for the production of concrete with high frost resistance that is greater than F300. The conclusions quantify the results of the investigation.


Introduction
During combustion of solid fuels for the production of heat and electricity at the thermal power plants waste is generated in the form of slag and ashes. Ash and slag are the largescale waste.
One of the ways to safely dispose of such waste is to use it in the construction industry for the manufacture of heavy concrete. The optimum content of ash in heavy concretes is determined by the selection of specific concrete mixtures under the condition that the necessary indicators of their quality are provided. This paper addresses the issue of the influence of cement consumption [1]. Concrete mixtures with ashes have a higher viscosity, better mobility, less water drainage and stratification, which makes such mixtures ideal for use in monolithic concrete. According to the sources [2], replacement of a part of cement by ash, as a rule, leads to lower frost resistance. This is explained, firstly, by an increase in the actual water-cement ratio and the formation of an additional volume of capillary pores. The reduction of frost resistance is also facilitated by the reduction of the entrained air content when the ash is introduced and the presence of ash in organic residues that swell in water and capable of forming compounds that destroy the binder.
Organic residues are usually contained in a coarse fraction of ash. Grinding of ash leads to the destruction of organic particles and their uniform distribution throughout the material, which positively affects the frost resistance of concrete.
A significant increase of the frost resistance of concrete with ash can be reached by the introduction of surface-active additives of hydrophobic-plasticizing type into concrete. The plasticizing component of such additives reduces the water consumption of the concrete mixture, and hence the volume of capillary pores. [3][4][5][6]. The hydrophobic component, in turn, affects the walls of pores and capillaries in concrete and thus prevents the water from entering them. Thereby, after 150 cycles of freezing and thawing of concrete with the replacement of 20% of cement by ground molded ash, combined with a complex hydrophobic-plasticizing additive, increased strength by 10% compared with concrete without ash. The use of a hydrophobic-plasticizer additive when replacing 25% of cement by fine ash allowed to obtain concrete that is not inferior to the frost-resistance of concrete without ash and capable of withstanding up to 200 freezing and thawing cycles [7]. However, the properties of such concrete are still remaining not investigated and the effect of ash slags on the frost resistance of concrete [8].

Materials for concrete with bottom ash slags
The main purpose of these investigations is to determine the effect of the composition of concrete mixtures with ash slags on the frost resistance of concrete, and the selection of the most optimal concrete with a strength class of C20/25 using bottom ash slags. The planning of the experiment and the results of research on concrete for strength using ash-slag are given in [9].
For this investigation, a mixture of the following materials was chosen: cement СEM I 42.5N which is characterized by high early strength, high content of С 3 S in clinker and low content of C 3 A [10].

Mathematical planning of experiments
The research was planned on the basis of using a three-factor experiment in three levels. As variable factors, there were selected: the consumption of cement, the compaction time of the concrete mixture, and the plasticizer additive consumption. In order to ensure the appropriate level of porosity and taking into account the features of the source materials structure, the water-cement ratio (W/C) of 0.4 was taken. In the research, control samples were made on quartz sand with strength of 31 N/mm 2 .

Regression analysis of the concrete frost resistance investigation
Frost resistance was determined using a "Concrete-Frost" device. After determining all the results were processed on the PC using the device programs, which allows determination of water absorption by volume of samples and obtain graphs of change in samples volume in time. In accordance with the composition of concrete for a series of samples, certain patterns are noticed. Namely, samples that were made with the maximum expands of plasticizer were distinguished by low frost resistance, as well as those containing the least amount of cement. Below is a detailed analysis of these phenomena. Using STATISTICA software, the coefficients of the regression equation (Table 1) are derived as well as the exact equation (1). x The diagram of the marginal average dependence of frost resistance on the variation factors was constructed.
The same dependence is noticed in the investigation of the dependence of frost resistance on the cement quantity ( Fig. 1): the greater the quantity of cement, the less frost resistance. For the quantity of cement at 350 kg/m 3 , sufficient compaction could not be provided.  The high consumption of plasticizer has an adverse effect on the frost resistance of samples as a chemically active filler (ash-slag) interacts with the organic components of the super-plasticizer additive, the result of which is the emergence of inclusions, which have a negative effect on the frost resistance of the samples.
Low consumption of plasticizer also has a adverse effect on frost resistance, since under such conditions the plasticization action of the supplement is not sufficiently manifested, it is not possible to provide an appropriate level of porosity, resulting in low frost resistance. As a result of the experiment, it was possible to determine the optimal sample compaction time, which is 3 minutes.
Interconnection of frost resistance and variation factors is obtained. For this purpose, the correlation coefficients have been calculated. From Table 2 it is getting obvious that plasticizer consumption has the greatest influence on the frost resistance. Table 2. Results of calculation of correlation coefficients.

Factor
Frost resistance, cycles The surfaces of influence are constructed for a clear reflection of the effect of variation factors on the frost resistance using STATISTICA software system. From Fig. 4 it is clear that the highest frost resistance is showed by samples for which the cement consumption is about 400 kg/m 3 , and the consumption of plasticiser -8 liters. Fig. 5 implies that the optimal compaction time is 3 minutes. Fig. 5 and 6 also suggest that the impact of the compaction time on frost resistance is much less than that of plasticizer and cement. Visual analysis of graphs and surfaces gives only approximate values of optimal component ratios. However, the software complex STATISTICA allows determining the degree of the factors at which the researched values reaches its maximum magnitude (Table 3). The predicted value of the response (frost resistance) for the above critical values of the input factors is of the order of 377 cycles.

Main conclusions
Utilizing the bottom ash slag as a fine filler allowed obtaining the concrete of the desired frost resistance class of F300. It should be noted that some samples manufactured using bottom ash-slag had a higher level of frost resistance than ones from the control group. The greatest frost resistance (the average value from the sample series is 317 cycles) was achieved for the cement consumption at 380 kg/m 3 . The frost resistance is negatively impacted by the increased (10 l) consumption of plasticizer while the average frost resistance of the samples was 222 cycles. Applying the bottom ash slag as a fine filler allows manufacturing the concrete with a strength of 36 N/mm 2 , that corresponds to C 20/25 class of concrete and F 300 frost resistance index. Using the bottom ash slag as a fine filler instead of sand in concrete mixture ensures not only the waste utilization but manufacturing the concrete for road construction as well.