The ITZ microstructure, thickness, porosity and its relation with compressive and flexural strength of cement mortar; influence of cement fineness and water/cement ratio

Abstract

A new insight into the interfacial transition zone (ITZ) in cement mortar specimens (CMSs) that is influenced by cement fineness is reported. The importance of cement fineness in ITZ characterizations such as morphology and thickness is elucidated by backscattered electron images and by consequences to the compressive (F_c) and flexural strength (F_f), and porosity at various water/cement ratios. The findings indicate that by increasing the cement fineness the calcium silicate hydrate formation in the ITZ is favored and that this can refine the pore structures and create a denser and more homogeneous microstructure. By increasing cement fineness by about 25% of, the ITZ thickness of CMSs was reduced by about 30% and F_c was increased by 7%–52% and F_f by 19%–40%. These findings illustrate that the influence of ITZ features on the mechanical strength of CMSs is mostly related to the cement fineness and ITZ microstructure.

Graphical abstract

Keywords

cement fineness / interfacial transition zone / compressive and flexural strength

1 Introduction

1.1 Background and motivation

Microstructure characterizations of cement-based material (CBM) are important in understanding the mechanical behavior of a structural material [1–4]. Recent studies in this area have investigated the importance of appropriate mixing process and water/cement ratio (W/C) on the hydration process and on the resulting microstructure, and thereby also on mechanical properties of CBM [5,6]. Generally, the interfacial transition zone (ITZ) in a CBM microstructure is identified as a third phase and formed between two-phases of aggregate and cement paste. The ITZ plays a profound role in the compressive (F_c) and flexural strength (F_f) and cracking behavior of CBM [7–9]. Although the ITZ is formed of the same materials as the cement matrix, there are differences in microstructure, morphology, and other characterizations of ITZ compared with cement matrix [10–12]. The characterizations of this zone, namely thickness and porosity, are influenced by the size and amount of aggregate, W/C ratio, and particle size distribution of cement (i.e., cement fineness) [13–16]. Regarding the effect of aggregate, previous study has shown that decreasing the aggregate size appears to decrease the ITZ thickness [17]. In addition, the formation of the ITZ microstructure mainly depends on two well-known phenomena including the wall effect, which occurs near the larger aggregates, and the growth side effect, which relates to the distance from the aggregate surface [18]. The investigations indicated that W/C highly influences the quality of the cement paste and ITZ. For instance, by reducing W/C, the porosity of cement matrix and ITZ decreased, due to formation of dense microstructure and the filling of pores with hydration products [19,20]. Due to the high porous microstructure of ITZ, the compressive strength of CBM was reduced and the cracks propagated in this zone [21,22]. However, a stronger, denser, and more homogeneous microstructure of ITZ caused more ductility and greater elastic response [23]. Regarding the effect of cement, when cement is blended with aggregates, the normal particle size distribution of cement can be disrupted because of the wall effect; i.e., fine cement particles locate more easily than coarse ones in the vicinity of aggregate. Consequently, within the zone further away from the aggregate, cement particle content is higher than in the zones nearby [19]. This implies that the distribution of particle size of cement is the significant factor controlling the microstructure texture and ITZ characterization due to the effect of hydration process and the formation of hydration products. Indeed, cement fineness can be considered as the main factor because cement directly affects theW/C. Furthermore, both cement and aggregate affect the wall effect phenomenon. However, many studies concerning the influencing factors for the ITZ characterization and mechanical properties have been more focused on the size and amount of aggregate and W/C, while ignoring the undeniable role of cement fineness as a significant factor.

1.2 Research significance

The above-mentioned studies investigated the importance of appropriate mix design in term of W/C and size of aggregate on characteristics of ITZ for providing the best mechanical properties for cementitious materials. However, the role of cement in terms of different fineness is the first and foremost factor in determining adhesive action, followed by that of the W/C. How these factors affect the ITZ microstructure and its relation with mechanical properties of cement mortar are still unknown. This research investigated how these significant factors affect the formed microstructure, ITZ characteristics, and mechanical properties of CMSs with the aim of providing guidance that can be applied to evaluate the beneficial utilization of cement with suitable fineness in mix design.

2 Experimental procedure

2.1 Materials

Three cement strength classes (CSC) (BS EN 197-1) [24], with distribution shown in Fig. 1, were employed in this study. Table 1 tabulates the characteristics of the cements. Also, fine aggregates (sand) with maximum size, fineness modulus, and specific gravity of 4.75, 2.48 and 2.6 mm respectively, according to ASTM C778 [25], were applied.

The scanning electron microscope (SEM) micrographs of the three cement strength types, showing particles which are mainly irregular in shape with few having spherical shape, are illustrated in Fig. 2 and show the micro scale size of the particles. It was confirmed that finer particles of cement have greater capability to coat the aggregate surface than do coarser ones and finer particles could create closer contact within the resultant paste or mortar. Since the ITZ thickness is affected by the cement size [26], one would expect significant differences between the three cements employed in this study, and this will be examined in the next sections.

2.2 Mix plan and specimens preparation

The 18 mix designs of cement mortar specimens (CMSs) were prepared based on ASTM C305 [27]. The composition details of each series are given in Table 2. The homogeneous mixtures were prepared using a mixer and molds of 40 mm × 40 mm × 160 mm. After one day, CMSs were taken out of the molds and placed in water at (22 ± 2) °C until the age of 28 d.

2.3 Test methods

For the porosity test, the weight changes of CMSs from each mixture were measured. Firstly, the CMSs were thoroughly dried to reach a stable weight. Secondly, the weight of specimens was determined under water in two conditions of saturated and saturated surface dry. Finally, porosity was calculated for each CMS based on ASTM C830 [28]. F_f and F_c tests were done on CMSs with dimensions of 40 mm × 40 mm × 160 mm and 40 mm³ cubic from flexural CMSs, according to the ASTM C348 [29] and C349 [30]. Also, SEM (scanning electron microscopy) was done on CMSs (using Phenom ProX, Netherlands) with a voltage of 20 kV.

3 Image acquisition and analysis

To evaluate in-depth the property of the ITZ microstructure, the CMSs were investigated using a BSE detector in the environmental SEM with magnification of 1000 × and physical resolution of 0.18 µm. Images were randomly taken in regions around the aggregates.

3.1 Pre-processing

First, it is essential to draw the border of the aggregate. For this purpose, automatic computer settings were unable to fully differentiate regions. That is because the backscattered electron coefficients of hydrated cement products and aggregate are very similar, and the irregular shape of aggregates makes it hard to differentiate between them. Such an issue has also been pointed out by other researchers [31,32]. Accordingly, most of the previous researchers [13,17,32–34] have used image processing. In this research, image processing was performed by means of Digital Surf MountainsMap software [35]. For each image, aggregate boundary was specified and the area of delineated aggregate was separated by completely white pixels (i.e., grey-scale value of 255). After preprocessing, the obtained image was ready for further analysis.

3.2 Strip delineation

After preprocessing, the position of each area (aggregate, cement matrix, and ITZ) was determined. The distinction between these areas was made by the difference in their grey scale. The position of aggregate was specified by the absolute gray scale of 255. The position of surrounding cement matrix was indicated by gray scale values of less than 255. To differentiate ITZ from cement matrix based on previous research [13,32], the strips needed to be sketched. Different methods such as the conventional dilation–subtraction, manual delineation, and concentric expansion are generally proposed for drawing strips. Among these methods, the concentric expansion method is superior to other methods due to its ability to delineate narrow strips, decrease operation error, and fully cover convex boundaries; more details of concentric expansion method can be observed in [14]. An example of image processing is presented in Fig. 3. The successive narrow strips of 2.5 µm wide were delineated, as shown in Fig. 3(b).

3.3 Pore segmentation

To specify porous areas using the grey scale analysis, it is important to determine the bottom and top thresholds in grey scale values. Undoubtedly, the bottom value is set to zero. Regarding the top threshold, several methods have been proposed [36]. ‘Overflow criterion’ is a method in which the critical point in a threshold is considered as the overflow point. This method is more objective and reliable than others [14,36]. Hence, in this research, the overflow method was intended to specify the top threshold. Figures 3(c) and 3(d) show the overflow point and binary image. Within each strip, the percentage ratio of porous area to its total area was considered as the porosity of each strip. In the end, the corresponding porosity could be determined and the findings indicated how this varies with distance away from the aggregate.

4 Results and discussion

4.1 Porosity

The laboratory results of porosity for all the mixtures are presented in Fig. 4 and demonstrate that the porosity decreased noticeably when the CSC was increased from 32.5 to 52.5 MPa and W/C was decreased from 0.50 to 0.25. These changes show the porosity is influenced by CSC and W/C, so that porosity has high sensitivity to CSC.

4.2 ITZ morphology

Figure 5 illustrates that the ITZ is characterized by bright strips and irregular thickness. To better show the details, the images called II enlarge regions that are outlined in red in image I. In the CMSs with CSC 32.5 MPa (Fig. 5(a)) and CSC 42.5 MPa (Fig. 5(b)), ITZ was distinguishable; but for the CSC of 52.5 MPa, because of thinner thickness and light color of aggregate, its sharp identification was difficult (Fig. 5(c)).

The SEM image (Fig. 6) showed that the composition of ITZ changed with increase in the CSC, especially for CSC of 52.5 MPa, since finer particles bridge the gap between the aggregate and cement matrix. Therefore, the connection between cement matrix and aggregate improved and a denser ITZ was created. Also, the C-S-H formation in this zone was favored by increasing the CSC from 32.5 to 52.5 MPa, so that better hydration products of cement were formed, which could refine the pore structures and create a denser mix.

The same observation can be found in Fig. 7; although, with some differences, so that the microstructure of cement paste at W/C of 0.25 was more uniform than that of W/C of 0.5. It is implied from the SEM imagery that, with an increment from 0.25 to 0.5, the amount of C-S-H and CH decreased. Since the crystals of CH tend to fill the pore space inside the paste, the porosity increased and more pores were observed at higher W/C. As a result, the morphology of ITZ showed that increasing the CSC could improve the ITZ, reduce the porosity and size of pores and modify the area between the cement matrix and aggregates, which was especially apparent by decreasing the W/C.

4.3 ITZ thickness

Measuring ITZ thickness for three CSCs with minimum and maximum W/C values is illustrated in Figs. 8 and 9. For better exhibition of the analysis, comparison of these thicknesses for various W/C is presented in Fig. 10. Figure 10(a) illustrates that as CSC increased and W/C decreased, the ITZ thickness and F_c were clearly improved due to developing better distribution of size of cement particles similar to the distribution of particle size of aggregates [18,37]. This implies that the CMSs with higher CSC and lower W/C generated a better hydration process, along with thinner ITZ, as demonstrated in Fig. 10(b). In contrast, CMSs with higher W/C and lower CSC generated thicker ITZ (greater porosity and lower F_c) (Fig. 10(c)) [18,38–41]. In addition, increasing the CSC allowed higher W/C for the same ITZ thickness, which slightly increased the F_c.

4.4 Porosity distribution of ITZ

Generally, the ITZ porosity is related to the distance from aggregate [34,42]. Porosity profiles up to 100 µm from aggregate for CMSs with W/C of 0.25 and 0.5 are plotted in Fig. 11. The results show a gradual distribution profile in porosity so that it increased with decreasing distance from the aggregate surface and increasing W/C [19]; porosity value is more at 2.5 to 60 µm distance from the aggregate. Figure 11 reveals that the porosity of each strip decreased with the increase of CSC and a remarkable reduction was observed as the CSC increased to 52.5 MPa. It is obvious that changing W/C, curing age, and aggregate size affected the porosity of ITZ [19]. Moreover, it can be seen that, with a constant W/C, the porosity of ITZ still changes with the CSC (Fig. 11). In addition, higher CSCs could be used to improve ITZ porosity of CMS due to the influence of CSC induced by cement fineness [43,44], so that with a finer cement particle, a large amount of hydration was formed, the structure became more uniform, and the porosity decreased [45,46].

4.5 Mechanical strength

Figures 12 and 13 give the values of F_c and F_f of CMSs with different CSCs and W/C, respectively; it is clear that the highest F_c and F_f were obtained by CSC of 52.5 MPa and W/C of 0.25, with 63.63 and 12.03 MPa, which are similar to findings in previous research by the same authors [47–50]. Interestingly, with decrease of W/C and increase of CSC, as mentioned earlier, a decrease in the thickness and porosity of ITZ and an increase in the F_c and F_f of CMSs were observed. As the strength of ITZ was one of the significant parameters in F_c and mainly depended on the porosity [51], the relationship between F_c and porosity is drawn in Fig 14. It is obvious that the slope of the porosity reduction curve in the CMSs with CSC of 52.5 MPa was steep and about 0.35 to 3 times of those with CSC 32.5 and 42.5 MPa and those in Refs. [52–55], which showed the porosity was more sensitive to CSC than it was to other parameters such as W/C.

A closer look at the cracks caused by F_c in the CMSs containing CSC 32.5, 42.5, and 52.5 MPa with the W/C of 0.25 and 0.5 by electron microscope is presented in Fig. 15. Although there were no obvious cracks at the ITZ of CMS with CSC 52.5 MPa, the cracks were bordering the aggregates along the ITZ, which were deteriorated by decreasing the CSC. However, in the pumice concrete specimens, the cracks traversed lightweight aggregates [18]. Moreover, cracking along the ITZ became more recognizable by increasing the W/C in CMSs, due to its higher porosity and weakness [56]. The visible cracks along the ITZ indicated substantiation of crack and damage initiation in the ITZ, and suggested its significant impact on the damage of CMSs, confirming that this zone was the weakest part.

5 Conclusions

This study highlighted that the ITZ microstructure was sensitive to cement fineness. This was established by testing 18 mix designs of CMSs to evaluate the ITZ characterization and its impact on the F_c and F_f of CMSs using various cement finenesses with constant W/C, and also varying these parameters synchronously. The specific findings are as follows.

1) The results of microstructure analysis indicated that increasing the cement fineness (by about 25%) with a constant W/C can improve the morphology of ITZ, so that the CMSs containing finer cement (CSC 52.5 MPa) were characterized with more homogeneous and denser structures and narrower cracks than the coarser cement (CSC 32.5 MPa). This was especially apparent for decrease of the W/C.

2) The ITZ thickness of CMSs also decreased (by about 30%) and F_c increased (7%–52%) with the CSC increase from 32.5 to 52.5 MPa. At a lower W/C (0.25), this decrease was smaller and the similar ITZ thickness can be obtained by increasing CSC and W/C simultaneously.

3) The ITZ porosity had a gradual distribution profile and rose with decreasing distance from the aggregate surface and decreasing the CSC at constant W/C. The profile was sharper within 2.5 to 60 µm from the aggregate. Furthermore, using finer cement (CSC 52.5 MPa) can modify microstructure as well as F_f and F_c. However, CMSs with coarser cement and higher W/C had weaker and more porous ITZ. Porosity was significantly influenced by cement fineness. An increase of cement fineness led to a decrease of the porosity of CMSs, so that the slope reduction of this trend was more noticeable when higher cement fineness (CSC 52.5 MPa) was used.

4) Significantly, the results of this research will help to i) provide in-depth insights into the ITZ microstructure to identify its relationship to the strength properties of cement mortar and to investigate the mechanism governing the processes involved, and, ii) elucidate the effect of different cement fineness on ITZ characterization; hence the present results can serve as a basis for improving future cementitious materials mixes to obtain required properties, and, iii) facilitate future studies to find the cement with suitable fineness to benefit the mixture design.

Importantly, the conclusions drawn in this research relate to the effects of cement fineness and W/C as the foremost factors influencing ITZ microstructure and its relation with mechanical properties of cement mortar. In order to extend the current results in future research, it is essential to further evaluate the impact of additives to the mixture on the characteristics of ITZ.

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