A Concrete Solution:

Admixtures Change PCC Performance

By Tom Kuennen, Contributing Editor

Portland cement concrete (PCC) is the most widely used structural material in transportation infrastructure, and it’s widely used for pavements as well. But the control of freshly mixed PCC properties is essential to ensure workability during placement, and compressive strength and long-term durability.

Not unlike modifiers in bituminous pavements (see Asphalt a la Carte: Modifiers Control Mix Performance, April 2012, pgs. 16-27), mineral and chemical concrete admixtures work both physically and chemically to improve the durability and quality of portland cement concrete, boost or retard set time, increase resistance to frost, sulfate attack and alkali-silica reactivity, and improve placement.

Adding chemical admixtures to PCC before or during mixing can manage its rate of early hydration, and regulate its fluid (rheological) properties. The most common chemical admixtures include air-entraining agents, water reducers, superplasticizers (high range water reducers), retarders, and accelerators. Concrete admixtures are classified as either mineral or chemical.

Cement and Concrete

Hydraulic portland cement is the product of the calcining, or pyroprocessing, of limestone (a calcium compound), aluminosilicates (clays and/or sand), and iron oxide at phenomenal temperatures, in the range of 2,700 and 3,000 deg. F. The raw materials are crushed, screened and deposited in an inclined, refractory tile-lined cement kiln, which rotates while the materials undergo calcination.

During calcining, the raw materials tumble in the rotary kiln, moving downward as they are exposed to heat. The chemically combined water and carbon dioxide from the raw materials is driven off, leaving behind new compounds such as tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite, according to the Portland Cement Association.

These compounds are contained in the resulting fused lumps – the size of a fist or smaller – called clinker. This clinker tumbles out the lower end, is cooled and is either shipped (exported) elsewhere for grinding into cement, or ground at the plant in a ball mill, which results in an hydraulic portland cement that is so fine it will pass through a sieve that will hold water.

Fresh concrete, of course, is the mix of aggregates (sand and gravel or crushed stone), water and portland cement. Cement makes up from 10 to 15 percent of the concrete mix, by volume, although in recent decades the cement industry has encouraged specification of equally performing cement containing up to 15 percent limestone fines.

The addition of water initiates hydration of the cement, which binds the sand and aggregates into a hardened product resembling stone. Compounds like calcium hydroxide and calcium silicate hydrate form within the paste. The strength of concrete is measured by its resistance to compressive forces after a month of curing (28-day compressive strength). But curing actually continues for decades, albeit at a much slower pace. Theoretically concrete never really stops curing.

How Concrete Cures

The curing or hardening of PCC is a physio-chemical process that begins at the molecular level. The scale at which this curing takes place is the realm of the atom and molecule, generally 10 times the diameter of a water molecule, one-billionth of a meter, called a nanometer.

Concrete is a porous material, ranging from air voids to nanometer-scale pores produced by the cement-water chemical reaction, say Ken P. Chong, National Science Foundation, and Edward J. Garboczi, National Institute of Standards and Technology, in their paper Smart and Designer Structural Material Systems (download at http://www.fire.nist.gov/bfrlpubs/build03/PDF/b03006.pdf).

“Since these nanoscale pores control the properties of the calcium-silicate-hydrate hydration product, which is the main ‘glue’ that holds concrete together,” they say, “concrete is in some ways a nanoscale material.”