1. Introduction
As one of the most used materials in the construction industry, research on the durability of concrete, and its working and mechanical properties in the process of its production and application, has attracted more and more attention in recent years [
1,
2]. Some studies show that, in addition to the external environment and external load, damage to the concrete itself, including holes and microcracks, is the main factor causing a reduction of durability [
3,
4,
5]. Through these types of damage, harmful substances and ions can invade into the concrete with water as the carrier, resulting in neutralization, freeze–thaw loss, sulfate erosion, and other durability reductions [
6,
7,
8]. Therefore, controlling water intrusion is one of the main measures used to improve the durability of concrete [
9,
10].
On the basis of not changing the concrete base material, the current method to prevent moisture from invading the concrete interior is to carry out waterproofing treatment on the concrete surface [
11,
12,
13]. Among many waterproof materials, compared to traditional asphalt, such as epoxy resin, polyurethane, and acrylic resin, silane material can penetrate into the concrete interior and will not block the pores of the concrete, which has the effect of gas permeability and impermeability [
14,
15,
16]. Concrete with air permeability can make the water inside the concrete in the process of evaporation be smoothly discharged, reducing the risk of concrete cracking [
16]. In recent years, silane material has attracted the attention of researchers and engineers. Through a large number of experiments, it has been found that isobutyl triethoxysilane has good hydrophobicity [
17]. Silane emulsion can enhance silane stability and reduce volatilization, but can also reduce the effective silane ratio, thereby reducing the effect of silane material to some extent [
18]. At the same time, the effect of silane on the surface of early age concrete will affect the concrete early to a certain extent, as it is harmful to the development of concrete surface strength due to the hydration of concrete [
19].
Tetraethyl orthosilicate (TEOS) is prone to hydrolytic action to form polysilicic acid. Studies have shown that polysilicate has a certain volcanic ash effect and can react with the cement hydration product Ca(OH)
2 for two hydration reactions to generate hydrated silicate gel [
20,
21]. The Si–O bond after hydrolysis of TEOS can also be linked to –OH on the surface of cement-based material, which is helpful to improving the surface strength of concrete. Because TEOS is easy to hydrolyze, its long-term effect is not stable when it is used directly in cement-based materials, and it will cause a certain kind of environmental pollution.
Combined with the advantages and disadvantages of isobutyl triethoxysilane emulsion and TEOS, in this paper an attempt is made to prepare a new composite silane material that is waterproof and breathable and that can improve the surface strength of cement-based materials by sol–gel synthesis. TEOS and isobutyl silane emulsion were synthesized by the sol–gel synthesis method, and results show that the waterproof effect, gas permeability, and surface strength of concrete are enhanced by this method, all of which contribute to the further improvement of concrete durability.
3. Performance Characterization and Methods
3.1. Fourier-Transform Infrared Spectroscopy
Cement paste blocks coated with different waterproofing silanes were measured using Fourier-transform infrared (FTIR) spectroscopy (TENSOR 27, Bruker, UK; scanning range, 0–8300 cm−1; wavelength accuracy, 0.01 cm−1). The chemical groups of the cement paste blocks were analyzed and compared before and after the coating of the silanes. The quality of the bond between different silane waterproofing coatings and the cement-based materials was investigated, which revealed the underlying mechanism for the waterproofing coating effect.
3.2. Thermogravimetric Analysis and Differential Scanning Calorimetry
The cement-paste–powder samples with different coatings were measured using a comprehensive thermal analysis instrument (TA, SDTQ600, New Castle, DE, USA; determination range, 0.1 g to 200 mg; temperature range, room temperature to 1500 °C; heating rate, 5 °C/min), including thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal stability of the cement-paste–powder samples and the mass loss in different temperature ranges were analyzed and compared before and after coating. The reactions of Ca(OH)2 from cement with different waterproofing silanes were characterized.
3.3. Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy
The micro-structure of the cement-paste blocks coated with different waterproofing silanes was observed using scanning electron microscopy (SEM) (ZEISS, IGMA 300/VP, Jena, Germany). The Si content on the surface of the cement-paste test blocks was examined before and after coating using energy-dispersive X-ray spectroscopy (EDS) (EISS, IGMA 300/VP, Jena, Germany) (both spot and surface scanning).
3.4. Particle-Size Measurements and Distributions
Both the isobutyl-triethoxysilane emulsion and compound emulsion were measured to obtain the average particle size using a laser particle-size analyzer (Jinan Runzhi Technology, Rise-2008, Jinan, China). The distribution of the two emulsion types was observed with an inverted fluorescence microscope (Olympus, IX71-F22FL/PH, Tokyo, Japan).
3.5. Capillary Water-Absorption Test
A one-dimensional capillary water-absorption test was conducted to measure the weight of each block with absorption times of 0, 1, 2, 4, 6, 8, 12, 24, 36, and 48 h. According to the changes in every sample, a water-absorption coefficient was calculated using [
24]
where
A is the water absorption coefficient (g·m
−2·h
−0.5), Δ
W is the amount of water absorbed during the absorption time (g·m
−2), and t is the water-absorption time (h).
3.6. Gas Permeability Test
The gas permeability of concrete blocks (150 mm × 150 mm × 150 mm) coated with different waterproofing silanes was measured to determine the breathing properties of concrete before and after coating using the Auto-Clam Permeability System (Amphora Autoclam, Ireland, UK). According to the measured gas pressure reduction, the index for gas permeability of concrete was calculated.
3.7. Microhardness Test
The effect of coating composite emulsion on the surface hardness of cement-based materials was tested by a microhardness tester (Airma, FALCON 400, Shenzhen, China).
Twenty points were evenly selected on the coating surface of the paste test block, and the selection rules were as follows.
The diagonal length of indentation was L1 > L2 > 20 μm (to reduce the visual reading error), the distance between two longitudinal points d > 2L1 or 50 μm, the distance between two adjacent transverse points d, and the height difference h = 10 μm. The schematic is shown in
Figure 2.
The points with dense parts in the figure were selected, and the average value was calculated as the average Vickers hardness of the corresponding test block. The effect of composite emulsion on the surface hardness of cement-based materials was reflected by the Vickers hardness.