The results of chemical shrinkage, compressive strength, density and UPV tests, data analysis and the correlations are reported in the following sections.
Chemical shrinkage
The results of chemical shrinkage of paste and mortar specimens during the first 24 hrs are presented in Figs. 5 and 6 respectively. The zero time was considered when water was added to the cement. At 24 hrs, the chemical shrinkage of paste with 0% LF reached a value of 0.026 ml/g. This value increases to a maximum value of 0.047 ml/g for pastes with 15% LF. As deduced from Fig. 6, the chemical shrinkage of mortars at 0% LF is 0.029 ml/g. This value goes up to 0.037 ml/g at 5% LF then subsequently drops with the incorporation of 10, 15 and 20% LF. This can be interpreted by the fact that the existence of LF causes an acceleration in the chemical shrinkage for the first few hours. This finding is consistent with that of Bouasker et al. (2008).
The results of chemical shrinkage of pastes and mortars for 90 days are shown in Figs. 7 and 8 respectively. Those figures show similar characteristics. For the first 3 days, the chemical shrinkage values are approximately similar for all mixes in paste and mortar specimens. At 90 days, the chemical shrinkage of pastes with 0% LF attains a value of 0.107 ml/g. This value increases to 0.133 ml/g at 5% LF then slightly drops with the addition of 10% LF (0.13 ml/g). After this slight decrease, the chemical shrinkage increases to the optimum value of 0.194 ml/g at 15% LF. For a replacement of 20%, the chemical shrinkage exhibits a sharp decline and reaches a value of 0.12 ml/g. During the 1st day, the inclusion of LF with the same percentages in mortars has mostly the same effect as pastes but with a smaller value. As shown in Fig. 8, the chemical shrinkage achieves a value of 0.121 ml/g at 0% LF. This value slightly decreases to about 0.117 ml/g for an amount of 5% LF then goes up to the maximum value of 0.132 ml/g at 10% LF. After that, the chemical shrinkage displays a sharp drop at 15 and 20% LF. The sharp decline occurs after the addition of 10 and 15% LF in mortar and paste samples respectively may be explained by the fact that with high amount of LF (˃ 10 and 15%), as the curing time goes up, the formation of calcium–carboaluminate hydrate could consumed CH. Therefore, the chemical effect of LF leads to the stabilization of the ettringite and could increase the solid volume of hydration products (external expansion) which decrease the total system volume (chemical shrinkage) (Wild et al. 1998; Menendez et al. 2003; Wang et al. 2010; Weerdt et al. 2011; Wang et al. 2018a, b). Besides, with the same w/b ratio, since LF has minimal cementitious or pozzolanic properties, the substitution of cement with different percentages of LF increases the free water to react with cement particles. That is known as dilution effect (Wang et al. 2018a).
The inclusion of different percentages of LF enhances the chemical shrinkage of pastes and mortars. In order to clarify this finding, three explanations can be made. First of all, as chemical shrinkage is a direct result of the hydration reactions, the acceleration of the cement hydration in the presence of LF which acts as a filler refines the pore structure, reduces the porosity of cement (Wang et al. 2018b) and makes an increase in chemical shrinkage. Additionally, LF can also interact slowly with the monosulfoaluminate (Afm) to create monocarboaluminate (Afmc), which has a higher density than the Afm phase (Weerdt et al. 2011). Last point, LF with fine particle size provides nucleation sites for hydration products to precipitate, accelerates the hydration process, and enhances the hydration degree of cement (Wang et al. 2018). It was mentioned that the advanced precipitation of C-S-H on the surface of LF was due to the propinquity between configuration of Ca and O atoms in calcite and CaO layers in C-S-H. With the increase of LF amount, additional nucleation sites would be available, and more hydration products could be absorbed (Wang et al. 2018a).
By comparing Figs. 7 and 8, it can be deducted that chemical shrinkage results for paste specimens are greater than those for mortars. For example, the addition of 5% LF in pastes exhibits a value of 0.133ml/g while shows a value of 0.117 ml/g in mortars. This is mainly due to the presence of sand which automatically reduce the amount of cement and LF and slow down the hydration process.
To further illustrate the influence of LF on the chemical shrinkage of paste and mortar specimens, Figs. 9 and 10 are displayed. The plots exhibit the change in volume against different percentages of LF at various curing ages (1, 7, 28, 45 & 90 days) for paste and mortar samples respectively. For pastes (Fig. 9), the extreme drop in volume occurs at 15% LF and then subsequently decreases. On the other side, the extreme drop in volume for mortars occurs at 10% LF then successively declines (Fig. 10). Proportionally, as curing time increases, the chemical shrinkage of pastes and mortars goes up as well. For example, the volume changes for 5% LF in mortar and paste samples increases about 8 and 9% from 1 to 90 days respectively. The chemical shrinkage becomes much steeper as curing time increases in pastes and mortars (e.g. at 7 days). The comparative proportions of phases present at specific curing time depends on LF content. For a specific LF content, as the curing time increases, CH is consumed (Wild et al. 1998; Wang et al. 2018a). Accordingly, the CH content of system drops and as a result, the CH/LF ratio will decline. Similarly, this last finding is also present when LF content increases (Wild et al. 1998; Wang et al. 2018a).
Compressive strength
The results of the compressive strength for pastes and mortars are presented in Figs. 11 and 12 respectively. Compressive strength measurements were carried out at 1, 7, 28 & 90 days. As shown in these plots, the compressive strength of paste and mortar specimens for 15% and 10% LF respectively displays the highest strength among all percentages. For example, at 7 days, the compressive strength of pastes is 31.5 MPa for the control mix. This value increases slightly with the addition of LF until achieving a maximum value of 35.8 MPa for 15% LF followed by a decrease in strength at 20%. This strength loss of mixes with higher amount of LF (˃15% LF) may be due to the significant decrease of the potential cementitious material content, which is known as dilution effect (Kovler et al. 2006; Menadi et al. 2009). Furthermore, the compressive strength of mortars at 7 days is 22.4 MPa at 0% LF. This value slightly increases to the maximum value of 23.4 MPa at 10% LF followed by a reduction in strength at 15 and 20% LF. This decrease is probably due to insufficient cement paste to coat all the LF and sand particles, which subsequently leads to a drop in compressive strength (Benabed et al. 2016). Those results are supported by previous studies (Soroka et al. 1976; Livesey et al. 1991; Voglis et al. 2005; Bentz et al. 2006; Bentz et al, 2009; Weerdt et al. 2010; Adel Mohammed et al. 2010; Güneyisi et al. 2011; Anusha et al. 2018). Same path is shown for the other curing durations. At 90 days, the maximum values of compressive strength are 70.4 and 31.6 MPa for pastes and mortars respectively. This can be explained by the fact that LF has fines particles which enable it to be more reactive. LF can fill the pores between cement particles, this is known as filling effect (Wang et al. 2018a). The pastes and mortars become more compacted, hence increase the compressive strength. Moreover, the presence of chemical reaction between LF & tricalcium aluminate (C3A) to form calcium –carboaluminate leads to decrease the porosity of pastes and mortars and consequently increase their compressive strength (Thongsanitgarn et al. 2012).
The results of compressive strength for pastes ad mortars are mainly consistent with those of density. The density increases up to 15% LF then drops for a higher amount of LF as shown in Table 4. For example, at 90 days, the density of pastes is 2.13 g/ cm3 at 0% LF. This value increases to the maximum value of 2.14 g/cm3 with the addition of 15% LF followed by a decline at 20%. This increase is almost negligible (only 0.5%). For mortars, the density at 0% LF is 2.43 g/cm3. This value goes up to the optimum value of 2.47 g/cm3 at 15% LF succeeded by a reduction at 20%. This is due to cement hydration resulting from the reaction between LF and cement. The reduction occurring in the external dimensions of cement particles generates voids, a perfect host for water. This phenomenon reduces durability as well as the density of paste. LF acts as a filler that fills the voids around cement particles up to the optimum. For higher filler content, voids are already totally filled. The extra amount of LF occupies the place of sand particles, thus decreases sand proportion and subsequently the density of mortars.
Besides, it is noted that the compressive strength values for pastes are greater than those for mortars. For example, the incorporation of 10% LF in pastes shows a value of 56.4 MPa at 28 days. However, it displays a value of 27.9 MPa in mortars. This is well expected because the amount of cement in paste samples is greater than that for mortars. Accordingly, the amount of cement and LF exists in small amount in mortar mixtures. Thus, the reaction between cement and LF as well as the hydration process will be reduced and negatively affects the compressive strength.
Table 4 Variation of density with different percentages of LF at various curing ages
|
Density (g/cm3)
|
1 day
|
7 days
|
28 days
|
90 days
|
Paste
|
P0
|
2.08
|
2.09
|
2.12
|
2.13
|
P5
|
2.06
|
2.08
|
2.09
|
2.1
|
P10
|
2.03
|
2.05
|
2.06
|
2.07
|
P15
|
2.09
|
2.11
|
2.13
|
2.14
|
P20
|
2.05
|
2.06
|
2.07
|
2.08
|
Mortar
|
M0
|
2.26
|
2.35
|
2.4
|
2.43
|
M5
|
2.28
|
2.36
|
2.41
|
2.44
|
M10
|
2.25
|
2.31
|
2.36
|
2.42
|
M15
|
2.3
|
2.39
|
2.42
|
2.47
|
M20
|
2.24
|
2.3
|
2.35
|
2.4
|
Ultrasonic pulse velocity
The UPV test is used to check concrete quality. Figs. 13 and 14 show the UPV for pastes and mortars versus curing ages for different percentages of LF. For paste samples, mix 4 with 15% LF exhibits a higher UPV value with a good quality of paste (3.88 km/s) among other mixes. Adding more than 15% LF reduces the quality of pastes. This means that whenever the percentage of LF increases beyond this limit, the LF may act as filler and does not contribute to an increase in UPV.
For mortar samples (Fig. 14), mix 3 with 10% LF displays higher UPV values than the other LF mixes. For example, at 90 days, mix 2, 4, and 5 with 5, 15, and 20% LF replacement levels display a value of 3.81, 3.7, and 3.73 respectively. However, mix 3 with 10% LF achieves a value of 3.91 km/s. It is well noticed that UPV results are consistent with those of compressive strength and chemical shrinkage. Adding 10 and 15% LF in mortar and paste samples respectively achieve maximum UPV, compressive strength and chemical shrinkage values. Besides, it is shown that UPV values for mortars are mainly greater than those of pastes. For example, the UPV value for a replacement of 10% LF in pastes shows a value of 3.78 km/s and a value of 3.91 km/s for mortar specimens. One possible explanation is that whenever the gel pores between cement and LF increase due to hydration, these voids are replaced by sand leading to a decrease in the time needed for the pulse to get throughout the mortar specimen.
Correlation between different properties
Figs. 15 and 16 show the correlation between compressive strength and UPV of pastes and mortars respectively for different percentages of LF at 1, 7, 28 and 90 days. This correlation appears to be linear for both pastes and mortars. For pastes, the compressive strength increases with the increase in UPV with a coefficient of determination R2 of 0.87, 0.87, 0.88, 0.83, 0.83 for 0, 5, 10, 15 and 20% LF respectively. Similarly, a positive correlation with a high coefficient of determination R2 above 0.94 for mortar mixes. This indicates that the presence of sand improves the linear correlation.
Additional correlations are shown between the compressive strength and chemical shrinkage at each curing age (Figs. 17 a-d). As chemical shrinkage increases, the compressive strength goes up. A positive correlation with a coefficient of determination 0.8 ˂ R2 < 0.99 and 0.75 ˂ R2 < 0.96 is shown for paste and mortar samples respectively. For pastes, the highest coefficient of determination is attained at 28 days. However, the highest R2 is realized at 90 days for mortars. It is also noted that the linear correlation for paste specimens is stronger than that of mortar specimens. One possible explanation is that the presence of sand in mortar samples (by delaying the hydration process) decreases the chemical shrinkage and accelerates the self – desiccation of samples. Therefore, compressive strength will be reduced. (Soroka et al. 1976; Voglis et al. 2005).
To illustrate the effect of LF on the chemical shrinkage of mortar and pastes, the slope of the regression line determined previously in Fig. 17 versus % LF is plotted in Fig. 18. It can be observed that the peaks are attained at 10 and 15% LF for mortars and pastes respectively. Those results are consistent with the chemical shrinkage results obtained previously. Therefore, the optimal percentage of LF as replacement of cement for pastes and mortars is between 10 and 15% respectively.