Bending strength and phase composition of cement stone subjected to loading at an early stage

The article deals with the results of comparative tests of the phase composition of cement stone distinguished by strength under bending and compression. The comparison has been made between samples subjected to early loading and control samples. An increase in tensile strength in bending of samples subjected to early loading was recorded. The difference in compressive strengths does not exceed 5%. X-ray phase analysis indicates amorphous phase increase with the growth of strength properties, which is manifested in increased intensity of the amorphous phase arc in the X-ray diffraction pattern. The highest peak of the curve corresponds to the maximum flexural strength of the entire row of samples. Among the samples subjected to early loading, the peak intensity of the group of crystal tobermorites with interplanar distances 11.5-11.3Å and 12.5Å appeared to be higher than the intensity of the control samples. It was proposed that the amorphous structure affects the increase in tensile strength in bending of cement stone subjected to early loading.


1Introduction
Accelerated erection of buildings and structures from reinforced concrete has always been one of the highest priorities for construction industry. The paper of A.V. Satalkin [1] published in 1956 describes one of the first work experiences in the implementation of early loading in the course of erection of bridge span structures. Since then there have been many papers devoted to study of the properties of cement systems and composites subjected to early loading, (see [2][3][4][5][6][7][8][9]). Besides, concrete studies consider the properties of the cement at the later stages [10][11][12][13][14][15]. In the previous papers [16][17] we revealed an increase in flexural strength occurred in the samples subjected to early loading. The incremental growth reached 21% for Portland cement compounds. The change in compressive strength was less significant and did not exceed 10-15% The purpose of the paper is research of strength properties of cement stone from various batches subjected to early loading in regard with the phase composition. The study posed the following tasks: 1) determine the strength characteristics of samples loaded at early stage under bending and compression; 2) obtain X-ray diffraction patterns of the samples subjected to loading and control samples while holding thermal analysis; 3) overlay XRF curves within each batch and review phase analysis with regard to different strength characteristics; 4) make a general RFA with plotting for each sample; 5) compare derivatograms of loaded and control samples; 6) establish differences between structures and determine its relationship with the obtained strength data.

Methods
Binder to be applied -Portland cement "Verhnebakanskij" Cem I 42,5 Н as per GOST 31108. Chemical composition is given in the Table 1. Cement age for load application has been designated in accordance to Figure 1 [16] and is equal to 6 days. For this cement age increase of tensile strength in bending has been the highest, as it mentioned previously [16]. Duration of load application is equal to 7 days. There were two batches of samples being loaded (three prisms in each batch). Water-cement ratio was 0.3. Pressure load stood at 30% of cement stone prism strength in the age of loading, which was calculated as the arithmetic mean of 3 values of strength. The surface hasn't been isolated. Control samples were hardening under the same temperature and humidity conditions as the loaded ones (t=21-22 0 C, φ=55-60%).

Results and discussion
Arithmetic means of compression strength of samples with the fixed composition produced as per GOST 310.4, are given in the Table 2. The results were processed using statistical analysis techniques. X-ray patterns of the control samples are shown in Fig. 1, patterns for the samples subjected to loadingin Fig. 2. According to the data given in the Table 2 samples 1 and 2 have nearly the same compression strength. Flexural strength is higher for the cement stone of sample 2. By overlaying X-ray patterns we reveal displacement of amorphous phase arc of sample 2 within the angle range 2Θ = 5-13° toward a higher intensity of diffraction peaks. It may indicate that the structure is more amorphous [18]. Main phase -Tricalcium silicate C3Sis represented within the range 2Θ = 22,98°; 29,44°; 32,12°; 32,5°; 34,24°; 34,9°; 36,6°; 38,82°; 41,18°. The highest peaks of the amorphous crystalic hydrates CSH [19][20] in the X-ray diffraction patterns are seen in 2Θ = 7,52°; 7,86°; 8,6°; 9,6°; 10,54°; 11 Table 2) By reviewing Figure 1 and taking into account the difference in flexural strength (it was higher for control sample № 2see Table. 2), the following may be noted: for sample №2 with the highest value of Rtb the main peak of Portlandite at 2Θ = 17,94° [20] is more intensive. It may be due to higher degree of hydration of silicate minerals, as this value is one of the criteria of hydration process [21]; -ettringite peak at 2Θ = 9,04° for sample №2 is displaced higher, and its width is narrower in comparison with the peak of the same phase at these angles. It indicates the higher degree of crystallization of the structure [18]; -amorphous phase arc within the range 2Θ = 5 -11,78° in X-ray pattern of sample №2, which contains most of peaks related to CSH phase, is more intensive.
Reviewing the main differences of X-ray patterns of control samples with different flexural strength, it should be noted that Rизг, apparently is related to amorphous nature of the emerging structure and to morphology of generated hydrates, which has been already revealed in [22 -25]. With regard to these observed features, the X-ray diffraction patterns of samples subjected to early loading were compared with previously known strength characteristics (according to Table 2), shown in Fig. 2.  Table 2) For sample №2 of cement stone subjected to early loading, which had shown the higher flexural strength when tested, the main peak of Portlandite at 2Θ = 17,94° had the higher intensity, as well as in the Fig. 1. For ettrigite sharpness of the peak at 2Θ = 9,04° in according to review of sample №2 is more extreme, which could also signal more intensive crystallization in samples with higher flexural strength. There is also change in the amorphous phase arc which is 2Θ = 5 -11,78°. When comparing, it is evident that the arc is raising in line with increase of flexural strength, i.e. when the structure of the cement stone becomes more amorphous. Overlaying of four X-ray patterns representing samples subjected to early loading and control samples of the same composition is shown in Fig. 3.   Fig. 3 Overlaying of curved representing samples subjected to early loading and control samples (strength characteristics are given in Table 2) As the intensity of the diffraction peaks increases, the x-ray diffraction patterns are set up from the cement stone with the minimum Rtb = 4,25 mPa ("control sample" 1) to sample № 2 subjected to early loading with maximum flexural strength (Rtb = 5,34 mPf). There is a lift of amorphous phase arc of samples subjected to early loading (Fig. 3). Increase of amorphous properties observed in samples subjected to early loading due to hardening under compression, apparently, can be attributed to increase of absorbed bound water in the structure (as it is mentioned in [26]) as well as to formation of additional amorphous hydrates CSH (in particular, low-basic CSH-I with more amorphous structure, which are more difficult to detect by x-ray phase analysis [27]).
To identify differences between crystalline and amorphous-crystalline calcium hydrosilicates, X-ray phase analysis has been performed within a narrow range of angles 2Θ = 5 -16° (Fig. 4).

Fig. 4
Amorphous phase arc of control and subjected to early loading samples When X-ray diffraction patterns of samples with different tensile strengths at bending, i.e. flexural strength, (see Figure 4) are overlaid, the maximum arc lift occurs in a sample of loaded cement stone with the largest Rизг. Apparently, the degree of amorphousness acquired by the structure can influence the flexural strength.
The height of the peaks of ettringite at angles from 2Θ = 9,14° to 2Θ = 15,7° is increasing with the growth of flexural strength while width of the peaks becomes narrower (Fig. 4).
To clarify the surveillance data obtained in the course of X-ray phase analyze, thermal analyze has been performed. The results of thermal analyze are presented in Fig. 5 interpretation of the thermal effects on the thermograms is based on the reference data given in the works [19][20]28].
Shape of curves representing all the four samples is similar. Plot of the sample №1 of cement stone loaded at the early stage is located a bit lower. However, however, differentialscanning curves (DSC) reveal that it is close enough to sample 2 of cement stone loaded at the early stage. For ease of interpretation of the mass loss in the specific temperature intervals data is summarized in Table 3. The Table below contains temperature intervals in which occur endo-and exoeffects related to the phases of cement stone under consideration [28].  Table 2) Shape of curves representing all the four samples is similar. Plot of the sample №1 of cement stone loaded at the early stage is located a bit lower. However, however, differentialscanning curves (DSC) reveal that it is close enough to sample 2 of cement stone loaded at the early stage. For ease of interpretation of the mass loss in the specific temperature intervals data is summarized in Table 3.
The Table below contains temperature intervals in which occur endo-and exoeffects related to the phases of cement stone under consideration [28].
Despite the change in amorphousness, the average mass loss up to 105 0 С for samples of control and loaded at an early stage samples is 8.13% with maximum variation 2.76%. This indicates a similar amount of loosely bound water in all samples. In the temperature range from 130 to 400 0 С, when endoeffects related to dehydration of ettringite and hydrated calcium silicate are mainly observed (CSH) [28], the average value of the mass loss for earlystage loaded samples was higher -4.86 versus 4.58 for control ones.
This observation correlates well with the increase of intensity rate of diffraction maximum of ettringite and hydrated silicates of the tobermorite group (which are characterized by thermal effects within the range 100-180 0 С, 230 -240 0 С, 250 -300 0 С), as it is shown in Fig 2,3.