The chapter delves into the critical role of hydrophobic agents in protecting external thermal insulation composite systems from water damage. It examines various hydrophobic agents such as stearates, oleates, micronized wax, and silanes, detailing their chemical compositions and mechanisms of action. The study evaluates the impact of these agents on the physical and mechanical properties of cement screeds, including bulk density, flexural and compressive strength, water vapor permeability, and capillary absorption. Notably, the chapter highlights the significant long-term benefits of silane-based admixtures in enhancing water resistance without compromising mechanical strength. The comprehensive analysis, supported by experimental data, offers valuable insights into the optimal selection and application of hydrophobic agents in construction materials.
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Abstract
The paper discusses the possibilities of hydrophobization of cement screeds intended primarily for external thermal insulation composite systems. The hydrophobic character of the mixtures both with and without the addition of hydrophobic agents was investigated. The following hydrophobic agents were used: zinc and magnesium stearate, sodium oleate, two types of mixed product of stearates and oleates, micronized wax and a silane hydrophobic agent. The mixture containing 0.6% zinc stearate and 0.6% silane showed the lowest water absorption and high flexural and compressive strength. The microstructure of the selected mixtures was also monitored using a scanning electron microscope, where the pores covered with the layer of polymer admixture were observed.
1 Introduction
It is highly important to provide sufficient protection for the whole composition when designing the external thermal insulation composite system (ETICS), which is exposed to adverse weather conditions. It is particularly important to protect it from water influence. The hydrophobic concrete surface is characterized by a water contact angle of more than 90° [1]. It can be achieved by hydrophobic admixture, surface treatment [2], pore structure closure by sealant agents or impregnation [3]. The second method to increase water resistance is indirect hydrophobization by adhesive coating or hydrophobic composition adjustment of the internal structure [4]. To increase the hydrophobicity of the material, it is essential to use compounds with low surface free energy [5]. The surface wettability is mainly determined by the chemical composition and microscopic geometry of the material’s surface [6]. The chemical composition can be classified into two categories: inorganic and organic [2]. The long non-polar hydrocarbon chains of some compounds lead to increased hydrophobicity of such materials (for example stearates, oleates and micronized wax). Stearates are chemical compounds derived from stearic acid, a long-chain saturated fatty acid. The reaction of stearic acid with zinc oxide forms the zinc stearate. Magnesium stearate is produced by the reaction of magnesium oxide (or magnesium hydroxide) with stearic acid. The clusters of stearate molecules and long non-polar hydrocarbon chains predetermine the hydrophobicity. Sodium oleate is a sodium salt of oleic acid with sodium hydroxide. The structure of the oleate molecule, which has a long non-polar hydrocarbon chain and polar carboxylate group, leads to increased water resistance. The micronized wax has, due to its small particles (from 1 to 10 µm), special hydrophobic properties. Small particle size allows for a larger surface area, which can be exposed to liquid and be more effective in water repellence. The nature of micronized wax can lead to poor dispersion or compatibility with aqueous solutions. Highly efficient hydrophobic properties were also recorded with well-known silanes [7, 8], siloxanes, and their mixtures. They are composed of silicon and hydrogen atoms, with one or more organic group (alkyl or aryl) attached to the silicon atom. Silicon-hydrogen bonds in silanes can react with surfaces containing hydroxyl groups to form a covalent bond, which can further enhance the hydrophobic barrier by enlarging the contact angle and coarsening the pore surface [9]. The silane impregnation can provide a long-term hydrophobic effect even in applications where the long-term durability is demanded (over 20 years) [10]. Superhydrophobic coating treatment based on polydimethylsiloxane can reduce the water absorption of the foundry dust/Portland cement-based composites by more than 76% [11], and capillary water absorption of specimens impregnated with waterborne silane-based hydrophobic agents can suppress 5.4% of the value for the untreated mortars [12]. The process of molecular absorption can be adjusted by the structure of organic functional groups, number of hydrolytic groups, surface energy, and cross-linking degree of silanes [13].
2 Materials and Methods
The cement CEM I 42.5R was used as a binder. Grounded limestone (GL), with a high content of calcite and siliceous sand (SS), with particle size under 2 mm (Fig. 1) and content of SiO2 98.5%, Fe2O3 0.95%, Al2O3 0.18% and TiO2 0.21%, was used as the filler. Ethyl vinyl acetate, cellulose ether (as a stabilizer) and de-foaming agent were used as additives in the mixtures to increase the adhesion of material to the surface and durability.
Fig. 1.
Particle size of the fillers.
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The different polymers (zinc stearate (ZS), magnesium stearate (MS), sodium oleate (SO), mixed product of stearates and oleates (MPSO), micronized wax (MW), and silane hydro-phobic agent (SH), in amounts of 0.2, 0.4 and 0.6% for all) were used as internal hydrophobization (Table 1). The reference mixture (REF) did not contain any hydro-phobic agent.
Table 1.
Composition of the mortar’s mixtures
Hydrophobic agent
SS
LS
CEM
Additive
Water
w/c ratio
[% wt.]
[% wt.]
[% wt.]
[% wt.]
[% wt.]
[% wt.]
REF
0.00
16.60
40.40
38.00
5.00
24.00
0.63
ZS 0.2
0.20
16.63
40.48
38.08
5.01
24.00
0.63
ZS 0.4
0.40
16.67
40.56
38.15
5.02
26.70
0.70
ZS 0.6
0.60
16.70
40.64
38.23
5.03
26.70
0.70
MS 0.2
0.20
16.63
40.48
38.08
5.01
26.00
0.68
MS 0.4
0.40
16.67
40.56
38.15
5.02
26.00
0.68
MS 0.6
0.60
16.70
40.64
38.23
5.03
24.00
0.63
SO 0.2
0.20
16.63
40.48
38.08
5.01
26.70
0.70
SO 0.4
0.40
16.67
40.56
38.15
5.02
29.40
0.77
SO 0.6
0.60
16.70
40.64
38.23
5.03
31.30
0.82
MPSO I. 0.2
0.20
16.63
40.48
38.08
5.01
30.00
0.79
MPSO I. 0.4
0.40
16.67
40.56
38.15
5.02
30.00
0.79
MPSO I. 0.6
0.60
16.70
40.64
38.23
5.03
32.00
0.84
MPSO II. 0.2
0.20
16.63
40.48
38.08
5.01
26.00
0.68
MPSO II. 0.4
0.40
16.67
40.56
38.15
5.02
25.30
0.66
MPSO II. 0.6
0.60
16.70
40.64
38.23
5.03
26.00
0.68
MW 0.2
0.20
16.63
40.48
38.08
5.01
24.00
0.63
MW 0.4
0.40
16.67
40.56
38.15
5.02
24.00
0.63
MW 0.6
0.60
16.70
40.64
38.23
5.03
24.00
0.63
SH 0.2
0.20
16.63
40.48
38.08
5.01
24.00
0.63
SH 0.4
0.40
16.67
40.56
38.15
5.02
24.00
0.63
SH 0.6
0.60
16.70
40.64
38.23
5.03
23.00
0.60
The water permeability (according to EN 12086), coefficient of water absorption (accord. to EN 1015-18), water absorption coefficient due to capillary action (EN 12808-5), bulk density (EN 1015-10), three-point flexural and compressive strength (EN 13892-2), adhesive bond strength (ČSN 73 2577), shrinkage by the dilatometer, crack size of the reinforcing layer detected by the tensile test scanning electron microscopy, differential thermal analysis and mercury intrusion porosimeter were determined.
3 Results and Discussion
3.1 Bulk Density
The bulk density of samples was highly similar (approximately 1500 kg/m3), the exception being one of the mixtures with stearates and oleates (MPSO II) used as the hydrophobic agent (Fig. 2).
Fig. 2.
Results of bulk density.
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3.2 Flexural Strength
The results show that the screeds with the sodium oleate content led to decrease of flexural strength (Fig. 3). The presence of sodium in the structure of cement matrix can positively impact the workability of the cementitious material, but it can also lead to deterioration [14]. Sodium can with its alkali properties, contribute to the formation of the alkali–silica reaction, which leads to a weaker structure [15]. The degradation of the structure is more noticeable after 90 days, when the strength decreases are more obvious.
Fig. 3.
Results of the three-point flexural strength test.
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3.3 Compressive Strength
A similar trend to the flexural strength was observed with determination of compressive strength – see Fig. 4. Mixtures containing sodium oleate additives showed a significant decrease in compressive strength. This could be caused by improper presence of the sodium in the structure and weakening of the compactness of the microstructural bonds. The screeds with higher w/c ratio (MPSO) showed lower strength because a more porous structure was created, so the screeds were less resistant to mechanical stress.
Fig. 4.
Results of the compressive strength test.
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3.4 Water Vapour Permeability
The low permeability of concrete is the most important factor for protecting the material against sulphates, acids, carbonation, frost, alkali-aggregate reaction, efflorescence and other cement-based materials [16]. The lowest water vapour permeability was reached with the samples containing magnesium oleate – see Fig. 5.
Fig. 5.
Results of the water vapour permeability test.
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3.5 Coefficient of Capillary Absorption
When the cementitious material is more hydrophobic, it takes longer to reach the saturated water absorption stage [7]. In accordance with this finding were the lowest values of capillary absorption coefficient being observed in samples with micronized wax, silane and both magnesium-stearate-based hydrophobic agents – see Fig. 6. The sodium oleates did not behave as well as the other compounds.
Fig. 6.
Results of the coefficient of capillary absorption.
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3.6 Water Absorption
It is seemed from Fig. 7 that the hydrophobic admixtures rapidly decreased the water absorption of samples with hydrophobic agents based on both zinc and magnesium stearate, sodium oleate, micronized wax and a silane hydrophobic agent.
Fig. 7.
Results of the water absorption.
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Both types of mixed product of stearates and oleates increased the water absorption coefficient. The mixture of stearates and oleates can be used as an emulsifier in the presence of water or other polar solvents. The water absorption coefficient is closely related to the coefficient of capillary absorption: the higher the water absorption, the higher the capillary absorption.
3.7 Mineralogy and Microstructure
The XRD analysis and scanning electron microscope (SEM) identified the minerals calcite and portlandite and the C-S-H phases (Figs. 8, 9, 10 and 11). The pores of samples with hydrophobic agent were covered with the layer of polymer. This aligns with Liang’s conclusion that the hydrophobic components create the barrier for water intrusion in cement-based materials by covering the pores with a layer of low surface energy materials [4].
Fig. 8.
The SEM image of needle-like and reticular C-S-H phases formed inside the pore structure in the sample MPSO II and covered by the layer of hydrophobization agent: magnification 1 500×.
Fig. 9.
The SEM image of a well crystallized trigonal structure of portlandite in the sample SH: magnification 900×.
Fig. 10.
The SEM image of a partially filled pore of the cementitious matrix in the sample MPSO: magnification 900×.
Fig. 11.
The SEM image of calcite crystals in a fully filled pore in the sample MS: magnification 1 000×.
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According to Zhang et al. [17], the hydrophobic agent does not have an obvious effect on the hydration degree, amount and structure of hydration products. The current study results are consistent with these findings.
4 Conclusion
The addition of hydrophobic agents positively influenced the hydrophobic properties of the screeds. Hydrophobization based on sodium oleates had an adverse effect on the physical and mechanical properties and significantly affected the reduction of strength; however, this did improve in the long term. Highly favourable results regarding mechanical parameters and hydrophobic properties were achieved by the use of silane-based admixtures. These admixtures did not affect either the compressive or flexural strength, causing significant water absorption and capillary absorption, and in larger amounts increased water vapour permeability.
Acknowledgement
This paper was created with the financial support of the Czech Science Foundation (GACR), Standard project No. 22-08888S “Increasing the durability of cement composites using water-based hydrophobization”.
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