1. Introduction
Due to the effective reinforcing effect,
versatility, compatibility with organic matrices, and low cost, glass fiber (GF)
is promising in developing sustainable cementitious composites (CCs), as
illustrated in Fig. 1(a). However,
early degradation due to chemical reaction between GF and cement matrix (CM)
has become a concern. The
applicability of silane coupling agents (SCAs) at the fiber/concrete interface has attracted growing interest due
to their ability to improve the adhesion between fiber and concrete, as
illustrated in Figs. 1(b) and 1(c). However, it is still largely unknown about the
interfacial bonding characteristics and interaction mechanisms of SCA-modified
GF (SCA-GF) bonded with CM. This study aims to investigating the
microstructure and interaction mechanisms between CC and SCA-GF with different
SCA concentrations using MD simulation.
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Fig.
1. Schematic diagram of silane coupling agent (SCA)-modified glass fiber-reinforced cementitious (GFRC) composite at multiple scales: (a) GFRC composite structure; (b) macroscale composite; (c) sub-macroscale composite; (d) mesoscale SCA-GF bonded to calcium silicate hydrate (CSH); and (e) nanoscale SCA-modified amorphous silica bonded to CSH substrate.
2. Model
Constructions
The CM layer is represented by calcium-silicate-hydrate (CSH). Amorphous SiO2 is used to represent the GF. The SCA used as an adhesion promoter is 3-aminopropyltriethoxysilane (APS). Pure CSH and nine cement models bonded to APS-modified SiO2 (APS-SiO2) with APS concentrations of 0.00%, 12.50%, 25.00%, 37.50%, 50.00%, 62.50%, 75.00%, 87.50%, and 100.00% are constructed, as illustrated on Fig. 2.
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Fig. 2. Atomistic modeling of: (a) CSH composite; (b) 3-aminopropyltriethoxysilane (APS); (c) grafted amorphous SiO2; and (d) CM/CM and CM/SCA-GF interfaces layers, where 0.00%, 12.50%, 25.00%, 37.50%, 50.00%, 62.50%, 75.00%, 87.50%, and 100.00% represent 0, 8, 16, 24, 32, 40, 48, 56, and 64 (fully-covered) APS molecules on amorphous SiO2 surface, respectively.
3. Results
(1) Stress-strain behaviors and failure modes
The interfacial stress-strain curves and corresponding microstructural changes of pure CSH and CSH/APS-SiO2 (0.00%–100.00%) obtained during the pulling simulation are shown in Figs. 3 and 4, respectively. It is noted that for all the interfacial systems, the stress increases to the maximum during Stage I. In Stage II, the stress reduces quickly, and in Stage III, the stress reduces to zero, as illustrated in Fig. 3(a). Pure CSH stress-strain curves behavior analysis demonstrate interfacial delamination, as illustrated in Fig. 4(a). With increasing APS from 0.00% to 37.50%, the failure mode transits from interfacial delamination to CSH cohesive failure, and with increasing APS from 37.50% to 100.00%, it transits to failure close to interface, as illustrated in Figs. 4(b) to 4(j).![]()
Fig. 3. Evolution of: (a) stress-strain curves and (b) normalized potential of mean force (PMF)-displacement curves.
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Fig. 4. Microstructural changes of: (a) pure CSH; (b-j) CSH/APS-SiO2 interfaces of cases: (b) 0.00%; (c) 12.50%; (d) 25.00%; (e) 37.50%; (f) 50.00%; (g) 62.50%; (h) 75.00%; (i) 87.50%; and (j) 100.00% interfaces during pulling simulation at a constant velocity of 1 m·s-1.
(2) Interfacial mechanical properties
The interfacial mechanical properties are measured to explore the effect of APS. The tensile strength is measured as 0.39 GPa for pure CSH. With increasing APS from 0.00% to 37.50%, the tensile strength increases from 0.48 to 0.58 GPa and with increasing APS from 37.50% to 100.00%, it decreases to 0.26 GPa. Meanwhile, the fracture strain value of pure CSH is measured as 35.12%. With increasing APS from 0.00% to 100.00%, the fracture strain displays a similar transition with the tensile strength variation. Furthermore, the interfacial adhesion energy derived by dividing the potential of mean force (PMF) by the surface area is measured, as shown in Fig. 3(b). The adhesion energy of pure CSH is measured as 0.56 J·m-2. With increasing APS from 0.00% to 37.50%, the adhesion energy increases from 0.71 to 0.90 J·m-2 and with increasing APS from 37.50% to 100.00%, it decreases to 0.23 J·m-2. It is noted that the 37.50% APS concentration leads to the highest enhanced interfacial system, which indicates that APS-SiO2 (37.50%) is the optimal concentration.4. Conclusions
In this work, MD simulation is used to evaluate the effect of APS concentrations on the interfacial bonding properties between CSH and APS-SiO2. The analysis of the stress-strain curve behavior and failure modes display distinct modes, with pure CSH demonstrating interfacial delamination. With increasing APS from 0.00% to 37.50%, the interfacial and mechanical properties continuously increase, and continuously decrease with increasing APS from 37.50% to 100.00%, which suggests that APS modifies the interfacial bonding and mechanical properties, with the 37.50% system as the optimal concentration. This study offers insights into optimizing the interfacial and mechanical properties of GFRC composites by systematically varying the APS concentrations, which promotes the development of sustainable CC with enhanced mechanical strength and durability during cementitious materials manufacturing.Paper Information:
Lik-ho Tam†, Askanderou Moundi†, Guoqing Jing, Jiaxing Ma, Bing Fu, Lu Ke, Huali Hao, Zechuan Yu* and Chao Wu*, “Molecular investigation on interfacial toughening between silane coupling agent treated glass fiber and cement”, Journal of Building Engineering, 2024, 95:110218. Link: https://doi.org/10.1016/j.jobe.2024.110218Contributed
by | Askanderou Moundi
Reviewed
by |
Lik-ho Tam
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