Concrete Exam 1: Aggregates, Cements

4 components of Cement

Lime/limestone (calcium)

Silica (clay/sand)

Iron (scrap metal/tires)

Alumina (often in clay)

Gypsum (sulfate)

Manufacturing phases of cement

Silicates - C2S and C3S

Aluminates - C3A and C4AF and Alkalis (Na & K)


cement clinker mfg in rotary kiln

Concrete components and vol%

PCC (7-15%) + Water (15-20%) + Sand (25-30%) + Coarse Agg(30-50%) + Air (3-5%)

Type I Cement

Ordinary PCC, normal proportions

Type II Cement

Moderate sulfate resistant & moderate low heat cement

Low C3A makes sulfate resistant and lower heat

Type III Cement

High early strength cement

Higher C3S and finer grind → higher early strength

Type IV cement

low heat cement

low C3S and C3A → less heat formation

Type V Cement

Sulfate resistant cement

very low C3A → best sulfate resistance

Types of Cement

Cement mfg process

Raw mix → Dehydration Zone → Calcination Zone → Clinkering Zone (melt forms with Alumina & Iron acting as flux for C3S formation) → Clinker out to grinding

Why add gypsum (sulfate)

helps offset undesirable effects of Al & Fe

keeps temp under control in hydration

AAR (Alkali-Aggregate Reactivity) - #1 concern for aggregate durability

ASR creates a gel that expands and fractures concrete and is similar to freeze-thaw damages due to expansion 

Alkali(Na & K) + Silica (minerals in aggregates) → ASR

ASR needs 1) reactive silica in aggregate 2) sufficient alkali in cement 3) sufficient moisture in concrete

Solutions for ASR

-identify and avoid problematic aggregate sources

-ASTMs for testing aggregate with an expansion threshold

-minimize water exposure

-minimize pore space/voids

Mixture proportioning considerations

economy of matls, labor, and equip

workability 

f’c (comp strength), w/c ratios, ASR resistant, etc


V(fine agg) = V(y=total vol) - (Vair+Vcement+Vwater+Vcoarse agg)

What can too little Gypsum cause?

too little gypsum leads to flash setting (setting in the truck nonreversible) b/c too little gypsum to slow down hydration reaction

*gypsums role is to slow down hydration reaction

What if gypsum gets too hot?

Leads to false setting (reversible) due to formation of gypsum crystals/dehydration