---
title: "Cement water proofers"
subject: "Civil Engineering Materials"
chapter: 5
key: "semester_3/civil_engineering_materials:cement-water-proofer"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/cement-water-proofer
site: BCT Engineering
---

# Cement water proofers

Civil Engineering Materials · Chapter 5: Cement

- Asked in: 2074 Asoj, 4 marks; 2075 Asoj, 1 marks (asked 2 times)
- Video: [Xypex - How to Waterproof Concrete Block Walls (CMU)](https://www.youtube.com/watch?v=w-BJ4Qb0s9U) — Xypex Chemical Corporation
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/cement-water-proofer
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-5

## Question

What is a cement water proofer? Explain.

## Short answer

A cement water proofer reduces permeability by pore-filling (fine fillers like fly ash, talc) or water repellency (soaps, stearates). Added at 2-3% by cement weight, it is used in basements, tanks, roofs; good compaction and low w/c ratio are also essential.

## Answer

A **cement water proofer** is a material added to cement mortar or concrete to make it impervious (water-tight) by reducing the water absorption and permeability of the hardened mass.

#### Types and action
1. **Pore fillers (pore-blocking):** very fine inert materials such as fly ash, talc, bentonite, hydrated lime, chalk or silica fume. They fill the voids between particles, so water cannot enter.
2. **Water repellents (hydrophobic):** soaps, stearates, fatty acids, oils and wax emulsions. They line the pores and make them repel water.
3. **Crystalline waterproofers:** react with water and free lime to grow crystals that block the pores.
4. **Integral liquid waterproofers:** ready-made liquid compounds mixed into the mixing water.

#### Uses
- Basements, water tanks, swimming pools, retaining walls.
- Roof slabs, terraces, bathrooms and toilets.
- Plaster for damp walls; damp-proof courses.

#### Points
- The dosage is usually 2-3% by weight of cement, as recommended by the manufacturer.
- Good compaction, a low water-cement ratio and proper curing are also needed; the water proofer alone is not enough.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does a cement water proofer do?
   A. Makes mortar or concrete impervious by reducing absorption and permeability
   B. Speeds up setting
   C. Increases heat of hydration
   D. Adds colour
   Answer: A. It reduces water absorption and permeability of the hardened mass. (Key point: Purpose)

2. How do pore fillers such as fly ash, talc or bentonite work?
   A. Repel water by lining pores
   B. Fill voids so water cannot enter
   C. Grow crystals in pores
   D. React to form gypsum
   Answer: B. Very fine inert materials block the voids between particles. (Key point: Pore fillers)

3. Which materials are hydrophobic water repellents?
   A. Fly ash, talc
   B. Calcium chloride
   C. Soaps, stearates, fatty acids, oils, wax emulsions
   D. Sugar
   Answer: C. They line the pores and repel water. (Key point: Water repellents)

4. What else is required with a water proofer?
   A. Nothing; the proofer alone is enough
   B. High water-cement ratio
   C. No curing
   D. Good compaction, low water-cement ratio and curing
   Answer: D. The proofer alone is not enough; dosage is usually 2-3% of cement weight. (Key point: Good practice)

## Related in other subjects

- [Effect of water cement ratio on strength](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Awc-ratio-effect-strength) — Concrete Technology and Masonry Structure, chapter 3, asked 4×
- [Gel/space ratio and strength (550 gm cement)](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Agel-space-ratio-550g) — Concrete Technology and Masonry Structure, chapter 4, asked 3×
- [Gel/space ratio and strength (600 gm cement)](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Agel-space-ratio-600g) — Concrete Technology and Masonry Structure, chapter 4, asked 3×
- [Strength of paste: 300 gm cement](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acement-strength-300g-150g) — Concrete Technology and Masonry Structure, chapter 4