CEE 395- Materials for Constructed Facilities

Civil Engineering Materials

Cement and It's Types

Week 4, Lecture 07

Portland Cement Background

Production of Cement

  • Two basic raw materials in Portland cement:

1. Calcareous material

Containing calcium oxide. Limestone (prevalent in WI), chalk, and oyster shells

2. Argillaceous material

Containing silica and alumina.

Examples include clay, shale, and blast furnace slag.

  • These materials are crushed, sorted, and stored.
  • Mixed by a wet or dry process.
  • Materials heated in kiln @1400-1650 C, melted in to a cement clinker, gypsum is added and
  • The mixture is ground into a fine powder (CEMENT !)
  • Cement is stored in bulk or sacks, dry. Standard sack is 94 pounds or 1 cubic foot of loose cement

Important Factors Affecting Properties:

Composition, Fineness, and Specific Gravity

1. Chemical Composition

  • The raw materials used are lime (C), silica (S), alumina (A), and iron oxide (F). Calcination occurs in kiln giving four compounds
  • C3S: hydrates and hardens rapidly Responsible for initial set and early strength
  • C2S: hydrates and hardens slowly Contribute to strength after 7 days
  • C3A: High heat release in early age Contributes slightly to early strength
  • C4AF: reduces clinkering temperature Hydrates rapidly, but little contribution to strength. Gives gray color
  • Other compounds, minor in amount, can have a high impact Alkalis (Na2O, K2O) can react with certain aggregates and adversely effect the properties.

2. Fineness

  • Fineness affects hydration rates.
  • The finer the particles => greater surface area => faster strength development => greater initial heat
  • Too much fineness may be detrimental (Too much heat !)
  • Size of PC particles: average is 0.01 mm, 95% < 0.045 mm
  • One kg of PC = 7 trillion particles
  • Surface area = 700-900 sq.m / kg ~ about 5 football fields of surface area in a 94 lb sack !
  • Fineness Tests: ASTM C204, ASTM C115, measures surface area of cement.

Also measured by percent passing the 0.045 mm sieve (ASTM C430)

3. Specific Gravity

  • The solid cement has a SG of 3.15, used for mixture proportioning. Found using ASTM C188.
  • Bulk density varies due to settling during transport or storage. Quantities are measured by weight, not volume.

Cement and It's Types (Con’t)

Week 4, Lecture 07

Hydration of Portland Cement (Table 6.2. in text)

The chemical reaction between the cement particles and water. It occur through two mechanisms:

I. Through-Solution

Formation of hydrates into solution

II. Topochemical

Solid state reaction occurring at the surface

Rate highly affected by A and S:

Setting Time
/ Concentration of aluminate (A) / Concentration of sulfate (S)
Normal / Low / Low
Quick Set / High / High
Flash Set / Low / High

Structure Development in Cement Paste (Figure 6.3 in text)

  • Begins immediately after water is added (a)
  • Water becomes alkaline in 10 minutes or less
  • Weak bonds form from hydrated C3A (b)
  • Further hydration stiffens mix (c)
  • Final set occurs when C-S-H phase has developed a rigid structure and all components of the paste lock into place
  • Continues as long as unhydrated cement particles and free water exist

Evaluation of Hydration Progress:

Can be done in several ways: heat of hydration, amount of CaOH in the paste, specific gravity of paste, amount of chemically combined water, amount of unhydrated cement paste.

>Most Popular: Strength of hydrated paste

Voids in Hydrated Cement

  • Important for strength, durability, and volume stability.

There are three types of voids:

1. Interlayer hydration space: 0.5 - 2.5 nm

2. Capillary voids: affected by W/C ratio, if > 50 nm, it

will reduce strength and durability

3. Air trapped during mixing: will reduce strength

4. Air entrainment: 0.01 to 1.0 mm, used for better

durability (Freeze-Thaw)

  • 1 and 2 are due to random growth of crystals and different types of crystal

Properties of Hydrated Cement

  • Setting, soundness, compressive strength

Setting

- Described by two levels: initial set and final set

- Initial set time must allow for handling and placing. Final

set is required to ensure sufficient hydration (strength development)

- Determined by Vicat or Gillmore penetration tests

- Factors that affect the set time

Fineness of the cement, the water-cement ratio, and the use of admixtures

Cement and It's Types:

Week 4, Lecture 07 (Con’t)

False Set

It occurs if the cement stiffens within a few minutes of being mixed, without the evolution of much heat

Soundness

Refers to the cement's ability to retain its volume after setting. Can be checked by the autoclave expansion test

Compressive Strength:

  • Is measured by preparing 50 mm cubes and subjecting them to compression according to ASTM C109
  • Affected by W/C ratio, type of cement, curing conditions.

Water-Cement Ratio

In 1918, Abrams found that the water-cement ratio influences all the desirable qualities of concrete

  • How much do we need?

Hydration requires 0.22 - 0.25 kg of water / 1 kg of cement.

  • Why do we need more water? For Workability.
  • Why excess water is not good? It results in more capillary voids, higher porosity and permeability, and less strength.

Types of Portland Cement

  • Why several types?

a - Rate of Strength gain, b - Volume Stability, and

c - Resistance to Sulfate Deterioration

  • Types vary by:

a - Ratio of Raw Materials (C3S, C2S, C3A, C4AF)

b - Rate of hydration also affected by fineness of cement

Standard Portland Cement Types (Table 6.3, 6.4, and 6.5 in text)

  • Type I (Normal)
  • Type II (Moderate Sulfate Resistance)
  • Type III (High Early Strength)
  • Type IV (Low Heat of Hydration)
  • Type V (High Sulfate Resistance)
  • Type IA, IIA, IIIA (Air-entrained)

Other types of Portland Cement:

  • White Portland Cement, Blended hydraulic cements, Masonry cements.

Expansive cements, Specialty cements