In infrastructure such as retaining walls, culverts, and small bridges that frequently suffer from repeated erosion by rainwater, snowmelt, and deicing salts, the water absorption problem of concrete has always been a key factor in durability decline. Although hydrophobic modification research exists for traditional ordinary Portland cement (OPC), research on hydrophobization of fast-setting, low-shrinkage, more eco-friendly calcium sulfoaluminate cement (CSA) remains very limited.
Based on relevant experimental results, this paper systematically summarizes the effects of polydimethylsiloxane (PDMS) and tetraethyl orthosilicate (TEOS) composite hydrophobic agents on the performance of CSA cement, providing clear insights for the development of high-durability cementitious materials.
1Research Background: Transforming Cement from "Water-Absorbing" to "Water-Repellent"
Water continuously intrudes into the cement interior through capillary pores, causing a series of diseases such as salt corrosion, freeze-thaw damage, and expansion cracking. Currently, the most stable and effective way to improve durability is to directly incorporate hydrophobic agents during mortar preparation, rather than only surface coating, avoiding problems such as coating peeling, blistering, and spalling.
Why Choose CSA Cement?
- Fast-setting and high early strength — suitable for emergency repair projects
- Low shrinkage — excellent volume stability
- Corrosion resistance — strong resistance to chemical corrosion
- Low calcination temperature — more environmentally friendly
This study uses PDMS-TEOS as the hydrophobic system with a fixed dosage of 0.5% by cement mass. By varying the ratio between PDMS and TEOS, we systematically investigate the mechanical properties, deformation, pore structure, water absorption, contact angle, and capillary pressure of hydrophobic CSA cement to determine the optimal ratio and mechanism.
2Core Conclusions: Hydrophobic Modification Significantly Improves Water Resistance
Through compressive strength, length change, mercury intrusion porosimetry (MIP), capillary water absorption, static contact angle, and other tests, the following key conclusions were drawn:
① Mechanical Properties and Deformation: Slight Strength Decrease, Significantly Improved Volume Stability
- As TEOS proportion increases, both early and later compressive strength slightly decrease, with maximum reduction of about 15%, mainly because ethanol produced from TEOS hydrolysis delays hydration
- Unmodified CSA cement shows obvious expansion due to ettringite formation; after hydrophobic liquid incorporation, except for the highest TEOS group, length changes in all other groups are significantly reduced, resulting in more stable volume
- Within the range maintaining good hydrophobicity, increased PDMS extends setting time, but the amplitude is very small and basically does not affect engineering applications
② Pore Structure: Finer Pore Size, Reduced Pore Connectivity
- The average pore diameter of all hydrophobic groups is smaller than the control group, with more dense pore structure
- Critical pore diameter significantly decreases, capillary pore connectivity reduces, fundamentally weakening water transmission channels
- Although total porosity slightly increases, water migration is significantly inhibited, indicating hydrophobic effect does not depend on low porosity but relies on interfacial "water-repellent" action
③ Hydrophobicity and Water Absorption: Achieving Transition from "Hydrophilic" to "Strongly Hydrophobic"
- After hydrophobic modification, mortar contact angle significantly increases, changing from hydrophilic to strongly hydrophobic
- Capillary pressure changes from positive to negative, meaning water is no longer "sucked in" but "repelled," making it difficult to intrude into the interior
- Capillary water absorption significantly decreases; optimal group water absorption is only about 1/5 of the control group, with greatly reduced internal water ingress area after immersion
④ Optimal Ratio: Balancing Strength, Hydrophobicity and Stability
Considering all indicators:
- PDMS:TEOS = 7:3～9:1 range performs best, corresponding to experimental groups T10, T20, T30
- These groups achieve comprehensive effects of strong hydrophobicity, low water absorption, volume stability, and minimal strength loss at only 0.5% ultra-low dosage
3Application Value: Suitable for High-Durability Structures with "Repeated Water Action"
The greatest significance of this study is: systematically demonstrating for the first time that PDMS-TEOS internal hydrophobization of CSA cement is completely feasible and highly efficient.
This hydrophobic CSA cement is particularly suitable for:
It is not just "surface waterproofing," but makes the cement matrix inherently water-repellent from the interior, providing more durable and reliable performance.
4Summary
By incorporating PDMS-TEOS composite hydrophobic agent, fast-setting calcium sulfoaluminate cement can achieve the transition from hydrophilic to strongly hydrophobic with only 0.5% dosage.
Capillary Suction Reversal
From "suction" to "repulsion"
Significantly Reduced Water Absorption
Optimal group only 1/5 of control
Improved Volume Stability
Expansion deformation significantly reduced
Controllable Strength Loss
Maximum decrease about 15%
Overall, hydrophobically modified CSA cement is a new type of cementitious material that combines fast-setting, low-shrinkage, green low-carbon and high durability. It is particularly suitable for long-term use in environments with repeated water infiltration and erosion, providing a simple, efficient, and engineerable new path for improving durability of underground engineering, municipal facilities, and lightweight concrete structures.