With the use of supplementary cementing materials in concrete mix designs increasing substantially in recent years, Portland Cement Association has developed an educational program on the subject, which includes a training CD available for purchase at www.cement.org. As part of the effort to enlighten contractors and architects on the benefits of SCMs, PCA Education and Product Development Program Coordinator Michelle Wilson presented a paper entitled “Supplementary Cementing Materials for Use in Concrete” at the Construction Specifications Institute (CSI) Show, April 21-24 in Chicago. This article is adapted from her presentation.

Supplementary cementing materials (SCM) have an undeniable impact on the durability, workability, economy and sustainability of concrete. SCMs are materials that contribute to the properties of concrete when used in conjunction with portland cement or blended portland by reacting either hydraulically or pozzolanically. A hydraulic material reacts with water to form cementitious compounds. A pozzolonic material does not react alone with water, but reacts chemically with calcium hydroxide (or lime) in the presence of water to form compounds that have cementing properties.

The most commonly used SCMs are fly ash, slag and silica fume, all of which are byproducts from various industrial processes. Natural pozzolans, which originate from naturally occurring mineral deposits such as volcanic ash, calcined shale, calcined clays (such as metakaolin) and diatomaceous earth also are available. Processing of natural pozzolans often is carried out to increase their reactivity.

Traditionally, fly ash, slag, calcined clay, calcined shale and silica fume were used in concrete individually. Today, due to improved access to these materials, concrete producers can combine two or more of these materials to optimize mix properties. In 2000, SCMs were used in at least 60 percent of ready mixed concrete orders. The American Society of Testing Materials has three separate material specifications covering SCMs: ASTM C 618 (Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete); ASTM C 989 (Ground Granulated Blast-Furnace Slag for Use in Concrete and Mortars); and, ASTM C 1240 (Silica Fume for Use as a Mineral Admixture in Hydraulic-Cement Concrete, Mortar and Grout).

THE FRESH MIX

An SCM's effect on fresh concrete varies considerably in its degree of influence. The attributes of these materials when added separately to a concrete mixture also can be achieved in ternary or quaternary blended cements.

Water Requirements

Concrete mixtures containing fly ash generally require less water (about 1 to 10 percent less at normal dosages) for a given slump than concrete containing only portland cement. Higher dosages can result in greater water reduction. However, some fly ashes can increase water demand up to 5 percent.

The water demand of concrete containing silica fume increases with higher volumes of the material, unless a water reducer or plasticizer is used. Calcined clays and calcined shales generally have little effect on water demand at normal dosages; however, other natural pozzolans can significantly increase or decrease water demand.

Workability

Fly ash, slag and calcined clay and calcined shale generally improve the workability of concretes of equal slump. Silica fume may contribute to stickiness of a mixture; adjustments, including the use of high-range water reducers, may be required to maintain workability and permit proper compaction and finishing.

Bleeding and Segregation

Concretes using fly ash generally exhibit less bleeding and segregation than plain concretes. This effect makes the use of fly ash particularly valuable in mixtures made with aggregates that are deficient in fines. The reduction in bleed water is primarily due to the reduced water demand in fly ash concretes.

Concretes containing ground slags of comparable fineness to that of the cement tend to show an increased rate and amount of bleeding than plain concretes, but this appears to have no adverse effect on segregation. Slags ground finer than cement reduce bleeding.

Silica fume is very effective in reducing both bleeding and segregation. As a result, higher slumps may be used. Calcined clays, calcined shales and metakaolin have little effect on bleeding.

Air Content

The amount of air-entraining admixture required to obtain a specified air content is normally greater when flay ash is used. Class C ash requires less air-entraining admixture than Class F ash and tends to lose less air during mixing. Ground slags have variable effects on the required dosage rate of air-entraining admixtures. Silica fume has a marked influence on the air-entraining admixture requirement, which in most cases rapidly escalates with an increase in the amount of silica fume used in the concrete. The inclusion of both fly ash and silica fume in non-air-entrained concrete will generally reduce the amount of entrapped air.

The air-entraining dosage and air-retention characteristics of concretes containing ground slag or natural pozzolans are similar to mixtures made only with portland cement.

Heat of Hydration

Fly ash, natural pozzolans and ground slag have a lower heat of hydration than portland cement. Consequently, their use will reduce the amount of heat built up in a concrete structure. Calcined clay imparts a heat of hydration similar to moderate heat cement. Some pozzolans have a heat of hydration of only 40 percent that of Type I portland cement. This reduction in temperature rise is especially beneficial in concrete used for massive structures. Silica fume may or may not reduce the heat of hydration.

Setting Time

The use of fly ash and ground granulated blast-furnace slag will generally retard the setting time of concrete. The degree of set retardation depends on factors such as the amount of portland cement, water requirements, the type and reactivity of the slag or pozzolan dosage and the temperature of the concrete. Set retardation is an advantage during hot weather, allowing more time to place and finish the concrete. However, during cold weather, pronounced retardation can occur with some materials, significantly delaying finishing operations. Accelerating admixtures can be used to decrease the setting time. Calcined shale and clay have little effect on setting time.

Other effects SCMs can have on freshly mixed concrete include: a generally equal or improved finishability; improved pumpability; increased (for concrete with silica fume) to little effect on plastic shrinkage cracking; and little effect on curing time, although more time may be needed for certain materials with slow early-strength gain.

HARDENED CONCRETE

Strength

In contrast to using portland cement as the only binding material in a mix, SCMs contribute to a higher or lower strength gain of concrete. Because of the slow pozzolonic reaction of some SCMs, continuous wet curing and favorable curing temperatures may need to be provided for longer periods than normally required. However, concrete containing silica fume is less affected by curing and generally equals or exceeds the one-day strength of a cement-only control mixture. Silica fume contributes to strength development primarily between three and 28 days, during which time a silica fume concrete exceeds the strength of a cement-only control concrete.

The strength development of concrete with fly ash, ground slag, calcined clay or calcined shale is similar to normal concrete when cured around 73∞F. Concretes made with certain highly reactive fly ashes (especially high-calcium Class C) or ground slags can equal or exceed strength in one to 28 days. Some fly ashes and natural pozzolans require 28 to 90 days to exceed a 28-day control strength.

Freeze-Thaw Resistance

It is imperative for development of resistance to deterioration from cycles of freezing and thawing that a concrete have adequate strength and entrained air. For concrete containing SCMs to provide the same resistance to freezing and thawing cycles as a concrete made using only portland cement as a binder, four conditions must be met:

1. They must have the same compressive strength.

2. They must have an adequate entrained air content with proper air-void characteristics.

3. They must be properly cured.

4. They must be air-dried for one month prior to exposure to saturated freezing conditions.

SCM usage also can have an effect on hardened concrete's alkali-silica reactivity, sulfate resistance, drying shrinkage and creep, permeability and absorption, corrosion of embedded steel, carbonation and chemical resistance. SCM may slightly alter the color of hardened concrete as well. Color effects are related to the color and amount of material used. Some silica fumes may give concrete a slightly bluish or dark gray tint, and tan fly ash may impart a tan color to concrete. Ground slag and metakaolin can make concrete whiter. Ground slag can initially impart bluish or greenish undertones.