3.1 Representative molecular structures in simulation
The representative molecular structures for C–S–H gel and amine molecules / C–S–H gel are presented in Fig. 2, according to which systematic analysis is performed on the molecular structrual features of the as-generated models. Among these models, it should be noted that calcium atoms in intralayer region and interlayer region are labelled as Ca_intral and Ca_interl, respectively. There are two types of oxygen atoms linking with silicon atoms. One is bridging oxygen in silica chain, denoting as O_bri; the other is non-bridging in silica chain, denoting as O_nbri. The oxygen atoms in water molecules are described as O_water. By the way, organic molecules also contain atoms such as nytrogen (N), oxygen (O), carbon (C) and hydrogen (H). It is found that Ca_intral atom could form polyhedron with its surrounding O_nbri atoms as Ca–Si layer. Meanwhile, there are still additional water molecules in the intralayer region due to the diffusion of water molecules in the interlayer region through the defective silicate chains. The interlayer region exists in between two different Ca–Si layers, in which there are massive Ca_interl atoms, water molecules and amine molecules. It can be seen that amine molecule locates at the interlayer region of the corresponding model (refer to Fig. 2b-d). The effect of amine molecules intercalated into the interlayer region on the geometrical size and chemical environment will be detailed in the following sections.
3.2 Characterization of unsaturated intercalated amine molecules into C–S–H gel
3.2.1 Effect of amine molecules on geometrical size of nanocomposites
The existance of amine molecules in the interlayer region induces the change of model size. As can be seen from Fig. 3a that the space length varies from amine molecules. The baseline model is based on the as-generated C–S–H gel model with a dimension of 1.188 Å × 1.180 Å × 1.212 Å. Compared with C–S–H gel model, geometrical lengths for amine molecules / C–S–H gel models change in all x, y and z directions (Cartesian coordinate system refers to Fig. 2), different from a recent study that charactersitic length only elongated in z direction 16. However, the geometrical size of amine molecules / C–S–H gel model illustrates a molecular structure dependence. With the intercalation of linear TEPA molecule into pure C–S–H gel, the stable TEPA / C–S–H gel model demonstrates a dimension of 1.184 Å × 1.176 Å × 1.208 Å, a slight shrinkage in all x, y and z directions due to the interaction of polar amine functional groups with the opposite Ca–Si layer. Unlike TEPA / C–S–H gel model, the intercalation of branched PAM and TEA molecules into pure C–S–H gel leads to a volume expansion. For instance, the dimensions for PAM / C–S–H gel and TEA / C–S–H gel models increases to 1.259 Å × 1.251 Å × 1.285 Å and 1.197 Å × 1.188 Å × 1.221 Å, respectively. Compared to pure C–S–H gel model, the dimensions of PAM / C–S–H gel model in all x, y and z directions increases more than those of TEA / C–S–H gel model due to the greater steric hindrance of PAM molecule, which repells the Ca–Si layer more seriouly, thus forming a large interlayer region (refer to Fig. 2). As known, density of material is a function of volume for a given mass. The change of model volume in this study would bring a variation of density of nanopcomposites. As can be seen in Fig. 3b that the density for C–S–H gel, TEPA / C–S–H nanocomposite, PAM / C–S–H nanocomposite and TEA / C–S–H nanocomposite would be 2.26 g/cm3, 2.27 g/cm3, 1.92 g/cm3 and 2.23 g/cm3, respectively. As stated above, density is reciprocal to volume, indicating a same intrisinc mechanism. Therefore, amine molecular structures dominates the change of geometrical size and subsequent density of composite materials.
3.2.2 Effect of amine molecules onatom distribution of nanocomposites
As can be seen clearly in Fig. 2 that the unsaturated intercalated amine molecules into the layered Ca–Si structures interact with atoms in C–S–H gel, forming different bonds. Figure 4 gives the intensity distribution of Ca_intral, Ca_interl, Si, O_water and O atoms or N atoms in amine molecules along z direction. It can be seen from Fig. 4a that Ca_intral atoms and Si atoms distribute in a much more orderly manner due to the existance of sharp peaks, constructing the Ca-Si layer of C–S–H gel model. The region between two Ca_intral layer is defined as the interlayer region. Sharp peaks of Ca_interl atoms are observed in the interlayer region, implying Ca_interl atoms were mainly located at interlayer region. Besides, water molecules distribute in a relatively irregular way in the interlayer region of C–S–H gel model (refer to Fig. 4a), due to its diffusion capability through the defective region. However, the intercalation of amine molecules into C–S–H gel enables the appearance of minor sharp peaks of Ca_interl atoms, oxygen atoms and water molecules in the interlayer region (refer to Fig. 4b-d). This is due to the fact that polar functional groups in amine molecules break the equilibrium between atoms in the interlayer region, therein affecting atom distributions. In the meantime, it should be realized that atom distributions show amine molecule dependent characteristics towards calcium-silicate region (refer to Fig. 4b-d). In other words, the structure of amine molecule plays a crucial role in atom distributions. It also can be seen from Fig. 4b that the interaction between N atoms and Ca atoms promotes N atoms distributed close to the surface of Ca–Si layer in the TEPA / C–S–H gel model. In addition, no penetration of TEPA molecules into Ca–Si layer occurs due to rigidity of TEPA molecules. Similar to TEPA molecule in C–S–H gel, N atoms and O atoms in respective PAM and TEA molecules locate in the vicinity of Ca–Si layer as well, having better physical bonding strength. However, the N atoms of PAM molecule and Oxygen atoms of TEA molecule distributed closing to both Ca-Si layer in the interlayer region(refer to Fig. 4c and Fig. 4d). In comparison, the N atoms of TEPA moleculs were mainly gathering adjoining to one Ca-Si layer. All these characteristics affect greatly the final tensile strength of composites.
3.3 Interfacial bonding properties in the nanocomposites
3.3.1 Interaction between Ca atoms and O atoms
Radial distribution function (RDF) is widely accepted to represent spatial structure to describe the interaction between Ca atoms and O atoms in the nanocomposites. Figure 5 gives the RDF curves between Ca atoms and O atoms in C–S–H gel, TEPA / C–S–H gel, PAM / C–S–H gel and TEA / C–S–H gel, respectively. As shown in Fig. 5a, both Ca_intral atoms and Ca_interl atoms could form stable bonds with O_nbri atoms. The bond lengths for Ca_intral–O_nbri and Ca_interl–O_nbri are 2.35 Å and 2.45 Å, respectively. Compared to Ca_interl–O_nbri, bond length for Ca_intral–O_nbri is smaller due to more charges of Ca_intral atoms leading to greater interatomic forces, thus a shorter bond length. It is interesting to see that bond lengths for Ca_intral–O_nbri and Ca_interl–O_nbri remain unchanged even if there are intercalations of TEPA (refer to Fig. 5b), PAM (refer to Fig. 5c) and TEA (refer to Fig. 5d) molecules into C–S–H gel, implying a minor influence of amine molecules on bond length. However, the introduction of amine molecules into C–S–H gel can change the coordination number of Ca–O bonds. As listed in Table 1, the coordination numbers of interaction pair Ca_intral–O_nbri are 4.24, 4.38, 4.04 and 4.36 for C–S–H gel, TEPA / C–S–H gel, PAM / C–S–H gel and TEA / C–S–H gel, respectively. At the same time, the coordination numbers of interaction pair Ca_interl–O_nbri are 2.50, 2.68, 2.66 and 2.67 for the corresponding aforementioned nanocomposites. In addition, only a small amount of Ca_interl atoms could form bonds with O_bri atoms due to the large steric hindrance of O_bri atoms. As shown in Fig. 5a, there are no peaks in the Ca_intral–O_bri curves in the C–S–H gel model, indicating that Ca_intral atoms form no bonds with O_bri atoms. It is noteworthy that minor peaks appear as illustrated in Fig. 5b-d due to the intercalation of amine molecules into C–S–H gel, which disturbed the arrangment of Ca–Si layers thus giving more space for the formation of Ca_intral–O_bri bonds.
Table 1
Coordination number of interaction pairs in C–S–H gel, TEPA / C–S–H gel, PAM / C–S–H gel and TEA / C–S–H gel
Interaction pair
|
Coordination No.
|
C–S–H gel
|
TEPA / C–S–H gel
|
PAM / C–S–H gel
|
TEA / C–S–H gel
|
Ca_intral–O_nbri
|
4.24
|
4.38
|
4.04
|
4.36
|
Ca_interl–O_nbri
|
2.50
|
2.68
|
2.66
|
2.67
|
Ca_intral–O_water
|
1.70
|
1.47
|
1.37
|
1.65
|
Ca_interl–O_water
|
3.40
|
3.11
|
2.80
|
3.11
|
O_nbri–H_water
|
1.06
|
0.96
|
0.93
|
0.98
|
3.3.2 Interaction between water molecules and C–S–H gel
Apart from that, the intercalation of amine molecules into C–S–H gel affects the interaction between C–S–H gel and water molecules as well. In the models of C–S–H gel (refer to Fig. 6a), TEPA / C–S–H gel (refer to Fig. 6b) and TEA / C–S–H gel (refer to Fig. 6d), the bond lengths for Ca_intral–O_water, Ca_interl–O_water and O_nbri–H_water are respective 2.45 Å, 2.55 Å and 1.65 Å, implying a nearly constant bond length in spite of addition of TEPA and TEA molecules. In comparison, the bond lengths for Ca_intral–O_water, Ca_interl–O_water and O_nbri–H_water are respective 2.65 Å, 2.65 Å and 1.95 Å in the model of PAM / C–S–H gel (refer to Fig. 6c), demonstrating a weakened interaction between substrates and water molecules due to the addition of PAM molecules in C–S–H gel. Since PAM molecules contained double bonded oxygen atoms with strong polarities to enable water molecules move away from substrates, thus leaving a longer bonds. It can be seen from Fig. 6 (a, b, c, d) that there is no strong bond formation between O_bri atoms and water molecules as there is no obvious peak in RDF curve for all models.
As widely accepted, coordination number is an important parameter to characterize the influence of amine molecules on C–S–H gel chemical structures. It can be seen from Table 1 that in pure C–S–H gel, coordination numbers for interaction pairs Ca_intral–O_water, Ca_interl–O_water and O_nbri–H_water are 1.70, 3.40 and 1.06, respectively. It is interesting to observe that the intercalation of amine molecules into C–S–H gel leads to a decrease in coordination number (refer to Table 1). However, the minimum coordination numbers for all the interaction pairs in PAM / C–S–H gel (refer to Table 1) result from the large steric hindrance and strong polar oxygen atoms, which affect the interaction between C–S–H gel and water molecules more seriously, thus giving rise to a lower coordination number.
3.3.3 Interaction between amine molecules and C–S–H gel
As illustrated in Fig. 7, amine molecules not only influence the chemical structures of C–S–H gel, but also interact with C–S–H gel. The hydrogen bonds are calculated according to the rules in our previous publication4. It can be seen from Fig. 7a that Ca_interl atoms could form strong bonds with N atoms in TEPA with a bond length of 2.65 Å and coordination number of 0.08. However, there is no bond forming between Ca_intral atoms and N atoms in TEPA due to the reason that most of Ca_intral atoms get caught in the polyhedral of Ca–Si layer. Although no obvious peaks were observed in O_bri–H@N(TEPA) and O_nbri–H@N(TEPA), the active hydrogen atoms in TEPA molecules still could form hydrogen bonds with O_bri or O_nbri atoms, either. Averagely, each TEPA molecule could form 0.628 and 0.485 hydrogen bonds with O_nbri and O_bri atoms, respectively (Table 2).In contrast, both Ca_intral atoms and Ca_interl atoms could form strong bonds with O atoms in PAM molecules with the same bond length of 2.55 Å. Nevertheless, there is a difference between the coordination numbers, which are 0.01 for Ca_intral–O bonds and 0.04 for Ca_interl–O bonds. The higher coordination number indicates that the distribution of Ca_interl atoms locates at the interlayer region, thus offering more opportunity to encounter the O atoms. As shown in Fig. 7b, Ca atoms in intralayer regions could not form bonds with N atoms in PAM molecules, but Ca_interl atoms could form bonds with length of 2.95 Å, which is longer than the bonds between Ca_interl atoms and N atoms of TEPA molecules, because of the lower polarity and negaelectricity of the linkage between N atoms in PAM molecules and carbonyl groups. Besides, the active hydrogen atoms in PAM molecules could bond with O_nbri atoms and O_bri atoms through hydrogen bonding, and each PAM molecule could bond averagely 0.056 O_bri atoms and 1.082 O_nbri atoms (Table 2). Note that the amount of O_bri atoms involved hydrogen bonds was far lower than that between O_nbri involved hydrogen atoms due to its large steric hindrance.
Table 2
Coordination number of hydrogen bonds forming between amine molecules and C–S–H gel
Interaction pair
|
Coordination No.
|
TEPA
|
PAM
|
TEA
|
O_bri–H@N
|
0.628
|
0.00548
|
-
|
O_nbri–H@N
|
0.485
|
1.082
|
-
|
O_water–H@N
|
0.78
|
0.06
|
-
|
O_bri–H@O
|
-
|
-
|
0.0015
|
N–H_water
|
0.346
|
0.137
|
0
|
O–H_water
|
-
|
0.567
|
0.961
|
O_nbri–H@O
|
-
|
-
|
1.62
|
O_water–H@O
|
-
|
-
|
0.33
|
In TEA / C–S–H gel nanocomposites (refer to Fig. 7c), Ca_intral atoms could form minor bonds (bond length: 2.55 Å, coordination number: 0.01) with O atoms rather than N atoms in TEA molecules due to the lower polarity and higher steric hindrance of N atoms. Different from Ca_intral atoms, Ca_interl atoms could form bonds with both O atoms (bond length: 2.65 Å, coordination number: 0.09) and N atoms (bond length: 2.65 Å, coordination number: 0.04) in TEA molecules, whose bond lengths and coordination numbers are greater than those of Ca_intral involved bonds on account of more Ca_interl atoms than Ca_intral atoms providing additional opportunities to encounter and bond with TEA molecules. As stated above that coordination number for interaction pair Ca_interl–O@TEA is greater than that for Ca_interl–N@TEA for the reason that O atoms distributed at the ends of TEA molecule chain have smaller steric hindrance than N atoms entrapped in the core part of TEA molecules. Besides, the active hydrogen atoms of TEA molecules could form hydrogen bonds with O_bri atoms and O_nbri atoms. Averagely, each TEA molecule could form 0.002 hydrogen bonds with O_bri atoms, and 1.62 hydrogen bonds with O_nbri atoms.
By comparison, TEPA, PAM and TEA molecules prefer to bonding with C–S–H substrates than calcium atoms.
3.3.4 Interaction between amine molecules with water molecules
Besides formation of hydrogen bonds with the Ca–Si layer, the amine molecules could also form hydrogen bonds with surrounding water molecules. The number of hydrogen bonds between amine molecules and water molecules was calculated based on our previous publication4. Figure 8 describes the radial distribution function (RDF) between amine molecules and water molecules. It can be concluded that there is no sharp peaks of O_water–H(TEPA) and H_water–O(TEPA) curves within distance of 2.45 Å in the Fig. 8a, implying no regular arrangement of water molecules around NH functional groups of TEPA molecules. Averagely, each TEPA molecule could form 1.126 hydrogen bonds with water molecules (refer to Table 2). Figure 8b showed the RDF curves between PAM molecules and water molecules. Similarly, no obvious peaks are observed within 2.45 Å, indicating that no regular arrangement of water molecules was observed around N, O, active H atoms of PAM molecules. Averagely, each PAM molecule could form 0.764 hydrogen bonds with water molecules (refer to Table 2). However, the hydroxyl groups of TEA molecules could form strong interaction with water molecules through hydrogen bonding, but N atoms could not form hydrogen bonds with them due to large steric hindrance (refer to Fig. 8c). The oxygen atoms and active hydrogen atoms in TEA molecule could form strong interaction with water molecules, because hydroxyl groups are located on the tail of TEA molecules and have smaller steric hindrance to contact with surrounding water molecules. On average, each TEA molecule could form 1.291 hydrogen bonds with water molecules. By comparison, amine molecules with different structures could form various amount of hydrogen bonds with water molecules. It can be seen in Table 2 that the number of hydrogen bonds between TEA and water molecules is maximum, while that between PAM and water molecules is minimum. The influence of hydrogen bonds on the diffusion of water molecule in models will be discussed in Sect. 3.3.5.
3.3.5 Time correlated dynamic property
Characterization of the stability of bonds between amine molecules and C–S–H gel (refer to Fig. 9a) or water molecules (refer to Fig. 9b) can be described in terms of time correlated function (TCF) in the following Eq. 4, 39–40.
![](https://myfiles.space/user_files/83064_0857a92044b57365/83064_custom_files/img1628659085.png)
(1)
where δb(t) = b(t)-<b>, in which <b> is the ensemble average of all b(t) values, in which b(t) is a binary function at the moment t, b(t) = 1 if two atoms bond together, otherwise b(t) = 0. Note that TCF value ranges from 0 to 1. The greater TCF value indicates a higher bonding strength, thus a more stable bond.
Figure 9 illustrates the TCF values of bonds between amine molecules and C–S–H gel. It can be seen from Fig. 9a that N atoms in TEPA molecules, N atoms in PAM molecules, O atoms in PAM molecules and O atoms in TEA molecules could form stable bonds with Ca_interl atoms in such a sequence of stability that Ca_interl–N(TEPA) > Ca_interl–N(PAM) > Ca_interl–O(PAM) > Ca_interl–N(TEA) > Ca_interl–O(TEA).
As shown in Fig. 9b that no hydrogen bond forms between water molecules and TEPA molecule or PAM molecules. The lower TCF value of hydrogen bonds than that of Ca_interl–O(TEA) bonds or O_nbri–H@O(TEA) indicates a lower strength of hydrogen bonds than that of Ca_interl–O(TEA) bonds or O_nbri–H@O(TEA). Meanwhile, the slightly higher TCF value of O_water–H@O(TEA) than that of H_water–O@H(TEA) implies more stable bonds. Therefore, it can be concluded that bonds between Ca_interl and O(PAM) or O(TEA) make a key contribution on the interactions between C–S–H gel and PAM or TEA molecules corresponding to the observation of obvious peaks in Fig. 7b and Fig. 7c, respectively. However, the simultaneous formations of interaction pairs (Ca_interl–N(TEA), Ca_interl–O(TEA), O_nbri–H@O(TEA), O_water–H@O(TEA), H_water–O@H(TEA)) in the TEA molecules occur as illustrated in Fig. 9, probably influencing the final mechanical properties of nanocomposites. It also can be seen from Fig. 7 that the major interaction pairs for TEPA / C–S–H gel, PAM / C–S–H gel and TEA / C–S–H gel are Ca_interl–N(TEPA), Ca_interl–O(PAM) and Ca_interl–N(TEA), respectively. Though the stability of Ca_interl–N(TEPA) bonds is better than that of Ca_interl–O(PAM) bonds (refer to Fig. 9), the bond length for Ca_interl–O(PAM) (2.55 Å) is shorter than that of either Ca_interl–N(TEPA) (2.65 Å) or Ca_interl–N(TEA) (2.65 Å), indicating a stronger bonding strength. As demonstrated in Fig. 9a as well, the formation of hydrogen bonds between atcitive hydrogen atoms in TEA molecules and O_nbri atoms leads to a larger TCF value than Ca_interl–O(TEA) bonds. However, the gradual drop of TCF values for both O_nbri–H@O(TEA) and Ca_interl–O(TEA) bonds beyond a critical time indicates that the bonds become unstable with time elapsing (refer to Fig. 9a). Therefore, the intercalation of PAM molecules into C–S–H gel would offer a strong and stable bond with substrate, thus influencing the final mechanical propertie significantly.
It has been widely acknowledged that the mobility of water molecules in C–S–H gel is dependent on the impermeability of C–S–H gel, which can be further interpreted in terms of mean square displacement (MSD) as illustrated below 4, 16:
![](https://myfiles.space/user_files/83064_0857a92044b57365/83064_custom_files/img1628659139.png)
(2)
in which ri(0) is the initial position of atom i and ri(t) is the current position of atom i at time t. MSD(t) describes the deviation of atom i from its original position after elapsed time t. As shown in Fig. 10a that MSD values of water molecules in C–S–H gel, TEPA / C–S–H gel, PAM / C–S–H gel and TEA / C–S–H gel are 1.33 Å2, 3.12 Å2, 8.57 Å2 and 3.44 Å2, respectively. It is obvious to realize that MSD value of water molecules in amine / C–S–H gel is greater than that in C–S–H gel due to the fact that the introduction of amine molecules releases the confined water molecules by C–S–H gel, thus leading to an increased mobility of water molecules. The highest MSD value (refer to Fig. 10a) implies the best mobility of water molecules in PAM / C–S–H gel than that in either TEPA / C–S–H gel or TEA / C–S–H gel, because PAM molecules enlarge the interlayer region most, offering much more space for water molecule diffusion. It can be observed from Fig. 10a that MSD values for TEPA / C–S–H gel and TEA / C–S–H gel are almost the same with time elapsing, indicating a similar size in the interlayer region. Besides, the interaction between amine molecules and water molecules also affected greatly the diffusion of water molecules. Averagely, each TEPA, PAM and TEA molecules could form 1.126, 0.714 and 1.291 hydrogen bonds with water molecules. PAM confined the least water molecules, but TEPA and TEA molecules bonded almost the same water molecules, contributing final results. It is known from Fig. 10b that mobility of amine molecules can be ranked as follows: PAM > TEA > TEPA, as PAM / C–S–H gel possesses the largest interlayer region and therein subsequent maximum mobility of PAM molecules. Compared with TEPA molecules, the smaller molecule size of TEA molecules represents a greater mobility, thus a poor stability. This instability of PAM or TEA molecules (refer to MSD curves in Fig. 10b) arises from the change of molecule conformation and rotation of branched structures in the space.
3.4 Deformation and failure process of amine molecules / C–S–H gel under unixial loads
To enable the application of materials in engineering, characterization of their mechanical properties is a must. In this section, uniaxial tensile load was applied to C–S–H gel, TEPA / C–S–H gel, PAM / C–S–H gel and TEA / C–S–H gel, respectively along z direction (refer to Fig. 2). As stated in the review section, traditional investigations focused on mechanical properties of C–S–H gel with its interlayer region fully intercalated into organic molecules 16, rather idealistic than realistic. Therefore, exploration of the mechanical properties of C–S–H gel with an unsaturated intercalation of organic molecules into its interlayer is necessary, but remaining chanllenging.
3.4.1 Mechanical properties of amine molecules / C–S–H gel
Figure 11 shows the stress-strain curves of C–S–H gel, TEPA / C–S–H gel, PAM / C–S–H gel and TEA / C–S–H gel, from which it can be seen that the profiles of all the curves are similar with an initial increase until a peak followed by a rapid stress drop. Note that yield strength here is defined as the maximum stress in the stress-strain curve 41. Therefore, the yield strengths (refer to Fig. 11a) for C–S–H gel, TEPA / C–S–H gel, PAM / C–S–H gel and TEA / C–S–H gel are 2.76 GPa, 2.45 GPa, 1.67 GPa and 2.26 GPa, respectively. Though the nanocomposites seem to behave linearly at the beginning of deformation, Young’s modulus (refer to Fig. 11b) demonstrates a strain softening nature, indicating a visco-elastic behavior. Compared with pure C–S–H gel, intercalation of TEPA molecules into the interlayer region of C–S–H gel decrease both of the yield strength and initial Young’s modulus. This is due to the fact that TEPA molecules is rigid and could only bond one Ca-Si layer by forming hydrogen bonds with O_bri and O_nbri atoms, thus leaving a lower yield strength and initial Young’s modulus.
Moreover, it can be seen from Fig. 11a that there is a gradual stress drop for the post-yielding behavior, implying that strain softening dominates such post-yielding deformation. However, the different amine molecule structures might lead to the change of the strain softening post-yield behavior. For example, the post-yield behavior for pure C–S–H gel can reach to a strain as high as ~ 0.42 (εf, total failure / breakage of nanocomposites), the addition of amine molecules leads to a decrease of such value, implying an ease of bond breakage. The expansion of interlayer region is insignificant due to the linear and regular chain structure for TEPA molecule, which retains the structure of C–S–H gel, thus resulting in a minor decrease of stress-strain response. The post-yield behavior for TEPA can reach to a strain of ~ 0.32. However, the branched TEA and PAM molecules expand the interlayer region more seriously, giving rise to a lower mechanical response. As expected, the post-yield behavior for TEA / C–S–H gel can reach to a strain of ~ 0.22. The lower strain (εf) means an easy breakage of bonds in the enlarged interlayer region.
Unlike TEA / C–S–H gel, it is interesting to observe that there is an insignificant decrease of such strain (εf = ~ 0.36) for PAM / C–S–H gel compared with that for pure C–S–H gel. PAM molecules could bond simultaneously the both Ca-Si layer in the opposite directions by forming hydrogen bonds with O_bri and O_nbri atoms. The interaction between PAM molecule and calcium-silicate layer is so strong that leads to a local stress concentration in its neighbouring layer. Due to the unsaturatd intercalation of amine molecules into C–S–H gel, such adjacent layer is the pure C–S–H gel, which would domimate the post-yield behavior of PAM / C–S–H gel. In other words, micro-crack initiates in C–S–H substrates rather than the interlayer region where PAM molecule intercaltes into. Since the local concentrated stress is high enough to break the chemical bond, making an early yielding of PAM / C–S–H gel. As breakage occurs in the substrates, the C–S–H gel will govern the post-yield behavior of PAM / C–S–H gel. Thus a slight reduce of failure strain compared with pure C–S–H gel results from the lower yield strength.
3.4.2 Molecule interaction in amine molecules / C–S–H gel during uniaxial tension
Molecule interaction plays an important role during deformation process. Except for bonding, the nonbond interaction (Etotal) consists of van der waals interaction (Evan) and electrostatis interaction (Eelec). As Evan is much greater than Eelec, Evan contributes more during tensile process. It can be seen from Fig. 12 that Etotal initially decreases with elongation of nanocomposites followed by a balance of Etotal after strain exceeds ~ 0.4, indicating the failure of materials (refer to the corresponding stress-strain behavior in Fig. 11). However, minor oscillation of Etotal can be observed from Fig. 12 during uniaxial tensile process, implying the instable breakage of bonds due to the increased distance between atoms in the interlayer region. The initial decrease of Etotal mainly results from the desorption between amine molecules and C–S–H gel. As amine molecules contain a variety of atoms with distinguished polarity and property, the weak interaction tends to break earlier followed by the debonding of strong bonds. Therefore, the consumed Etotal would be the major part for energy absorption of nanocomposites.
Figure 13 gives the predicted deformation and failure process in C–S–H gel at different strain stages during the uniaxial tensile test. As can be seen from Fig. 13a that C–S–H gel deforms uniformly at the early loading stage. However, its initial elastic modulus is not constant (refer to Fig. 11b) due to the interaction between atoms, demonstrating a visco-elastic property. With the increase of the load to the peak stress (refer to Fig. 13b), the occurrence of breakage for both Ca–O bonds and hydrogen bonds gives rise to the initiation of micro-crack. Further loads during the uniaxial tension cause the fast propagation of micro-crack (refer to Fig. 13c and Fig. 13d), thus resulting in the total failure of C–S–H gel.
With the intercalation of TEPA molecule with linear structure into C–S–H gel (refer to Fig. 14), the applied uniaxial tensile load to the nanocomposite enables the elongation of TEPA / C–S–H gel. Similar to the deformation of C–S–H gel at the early loading stage (refer to Fig. 14a), TEPA / C–S–H gel deforms visco-elastically. With further loading, the failure of bonds between Ca atoms, O_bri atoms and O_nbri atoms and NH groups of TEPA molecule promotes the initiation of micro-cracks. As the NH groups locate in the chain, high steric hindrance leads to a low bonding energy between Ca atoms and N atoms (refer to Fig. 14b). However, bonding energy between Ca atoms and N atoms in NH2 groups of TEPA is much greater, remaining linked at this moment. The possible reason is that the NH2 groups present at the ends of TEPA molecules, therein a smaller steric hindrance and subsequent stronger linkage with Ca atoms. As shown in Fig. 14c, Fig. 14d and Fig. 14e, Ca–N bonds remain intact with the increase of deformation. Since the intercalated TEPA molecules could only bond with one Ca-Si layer, but no interaction with the other layer, resulting in the weak interaction between both Ca-Si layer. The intercalated interlayer regions are prone to break, implying a lower stress-strain response in TEPA / C–S–H gel.
Akin to TEPA / C–S–H gel, the intercalated TEA molecules into C–S–H gel demonstrates a similar deformation behavior as shown in Fig. 15. Nevertheless, different mechanism appears due to the branched molecule structures. As shown in Fig. 1c that N atoms locate in the center of the dendric TEA molecule structure with a high steric hindrance, resulting in the formation of no bonds between N atoms of TEA molecues and C–S–H substrates. However, the hydroxyl groups of TEA molecules could form bonds or hydrogen bonds with Ca atoms or Oxygen atoms of C–S–H gel. However, the intercalated TEA molecule into C–S–H gel could only bond with either of neighboring Ca–Si layers due to the small steric size. Therefore, the initiation of micro-cracks is probably due to the appearance of broken Ca–O_nbri bonds or hydrogen bonds among C–S–H gel in TEA / C–S–H gel subjected to a uniaxial tensile load (refer to Fig. 15b). As the ends of TEA molecules contain hydroxyl groups with strong polarity and low steric hindrance, forming stable bonds with C–S–H gel, which is much stronger than the interaction between TEA molecules. Consequently, the weak interaction between TEA molecules might be another possible reason for micro-crack propagation, thus dominating the failure of TEA / C–S–H gel.
Unlike any of the aforementioned nanocomposites, the intercalation of PAM molecules into C–S–H gel shows a more interesting deformation behavior (refer to Fig. 16). Since PAM molecule is large to stretch across the interlayer region to bond with both of its neighboring Ca-Si layers together, the adhesive strength between PAM molecules and C–S–H gel are much stronger than any others (refer to Fig. 7b). Besides, the PAM molecules affect the atom arrangement of substrates more seriously, leading to a poor mechanical property. As can be seen from Fig. 16a and Fig. 16b that hydrogen bonds are broken with the accumulation of tensile strain. Though the local stress is high enough to break the hydrogen bonds in the substrates, it is insufficient to break the bonds between polar NH2 groups in PAM molecule and Ca atoms or Oxygen atoms in the C–S–H gel substrate with the strain increase. Therefore, micro-crack initiates from the interlayer region without PAM molecules as shown in Fig. 16c, Fig. 16d and Fig. 16e, implying that mechanical property of C–S–H gel dominates the failure process of PAM / C–S–H gel. In a word, the unsaturated intercalation of amine molecules into C–S–H gel would significantly influence the mechanical property of the as-generated nanocomposites. Local stress concentration occurs in other interlayer region in spite of the formation of stronger bonds between PAM molecules and C–S–H gel, thus leading to a drop of mechanical properties.