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Reducing Portland Cement in Concrete

Many of the environmental benefits of using slag cement in concrete are predicated on the reduction of portland cement in a concrete mixture.    

 

Portland cement is a critical ingredient in concrete, and comprises 10 to 20 percent of concrete’s weight. However, it contributes the largest portion of embodied energy and greenhouse gas in concrete.  The pie charts below (Prusinski, VanGeem and Marceau, 2004) show the proportion of weight, embodied energy and greenhouse gas emissions contributed by portland cement, aggregates, water, material transportation, and plant operations, for a typical 5,000 psi ready-mixed concrete mixture.  In this example, portland cement makes up 14 percent of the mass, but is responsible for 77 percent of the energy consumption and 92 percent of the greenhouse gas emissions.  Clearly, reducing the amount of portland cement in concrete with a lower energy/emissions product, like slag cement, will have a great impact on reducing embodied energy and emissions.


Matl Mfg and Tspt in cy conc.JPG 

 

Using slag cement in concrete can reduce the amount of portland cement required for a specific mixture.  Slag cement can achieve this in two ways: 1) Direct replacement for portland cement on a 1:1 basis and 2) reduction in total cementitious materials required in a mixture,

 

Slag cement, as well as other supplementary cementitious materials like fly ash and silica fume, can reduce the quantity of portland cement in a concrete mixture.  Slag cement, however, is a hydraulic cement and can offset higher quantities of portland cement in concrete than pozzolans (Note 1) Table 1 shows typical mixture percentages of slag cement, fly ash and silica fume in concrete (the table does not take into consideration potential reductions in total cementitious content).   In field practice, the proportion of slag cement in a concrete mixture is usually around twice the proportion of fly ash in a comparable mixture.  Silica fume proportions are usually lower than fly ash, but silica fume is normally used only in projects where certain high performance characteristics are being sought.  

 

Slag cement also can reduce the total amount of cementitious material required to achieve a specific performance objective in concrete.  Table 2 highlights two examples where using slag cement in a mixture can not only substitute for a portion of portland cement, but also can reduce total cementitious materials content (portland + slag).  In these examples, cementitious contents can be reduced because slag cement typically increases 28-day strength of concrete mixtures, the performance criterion in this illustration.

 

Case 1 in Table 2 shows two nominal 5,000 psi (@ 28 days) concrete mixtures, comparing a 100 percent portland mix to one with 47 percent slag, by weight.  The slag mix reduced the total amount of cementitious materials by 25 lb/cu yd, and yielded a 235 lb/cu yd, or 50 percent, reduction in portland cement used.  Case 2 illustrates two 6,000 psi (@ 28 day) mixtures, with 100 percent portland compared with a 35 percent slag mixture.  In this example, the 35 percent slag mixture resulted in a 50 percent reduction in portland cement content, because total cementitious was reduced by 100 lb/cu yd.  These two examples show that moderate to relatively large reductions in total cementitious materials content are possible with slag cement, when performance criteria are used to specify a concrete mixture design (Note 2). 

 

The benefit of higher levels of portland cement reduction is that larger amounts of a high-energy/emissions product (portland cement) are being replaced with a low energy/emissions recovered industrial material (slag cement).

 

 

 

Table 1: Typical Mixture Percentages for

Supplementary Cementitious Materials

 

Application

Slag Cement

Fly Ash

Silica Fume

General Concrete

25-50%

10-30%

0-10%

High-Performance

35-65%

15-30%

5-15%

Mass

50-80%

25-50%

0-10%

Precast/PS, Block

20-50%

10-30%

0-15%

Note:  The ranges shown are general estimates, and are meant to indicate typical ranges of SCMs in concrete in conventional mixtures.   The ranges shown can be exceeded, but special mix and construction considerations are often necessary (e.g. high level of chemical admixtures, longer curing times, increases in total amount of cementitious materials). 

 
 

Table 2:  Reducing Total Cementitious Materials Content with Slag Cement

 

Case

(28-day Target Strength)

%

Slag Cement

Cementitious Materials

 (lb/cu yd)

Actual 28-Day Strength (psi)

Reduction Due to Slag Cement Utilization

Portland Cement

Slag Cement

Total Cementitious

Δ Cementitious (lb/cu yd)

Δ Portland Cement    (lb/cu yd)

Δ % Portland Cement

Case 1:

5000 psi

(St. Marys, 2002)

0%

470

0

470

5470

25

235

50%

47%

235

210

445

6210

Case 2: 6000 psi

(ESSROC Cement Co., 2002)

0%

600

0

600

6167

100

275

46%

35%

325

175

500

6058

 

 

Notes:

 

Note 1: Hydraulic cements, like slag and portland cements, react chemically with water to produce calcium-silicate-hydrate (CSH), the “glue” that provides strength and durability to concrete. Pozzolans like fly ash and silica fume do not react chemically with water directly.  Instead, they react with a byproduct of the portland cement reaction, calcium hydroxide (CH), and water to form additional CSH.  CH is a soluble byproduct of the portland cement hydration process that does not contribute to strength or durability, so consuming CH and converting it to CSH is beneficial to concrete.  Slag cement, while principally hydraulic, also possesses pozzolanic characteristics and can consume excess CH.  Some fly ashes (e.g. Class C ashes containing higher levels of calcium) have some hydraulic characteristics, but are still mainly pozzolanic in nature.

 

Note 2: Cases 1 and 2 are laboratory mixtures, developed in different labs for different purposes.  They are cited to illustrate the concept of reducing total cementitious content.  Actual field use may be affected by other considerations, such as time of set and early strength development.  For instance, in Example 1, the 28 day strength of the 47 percent slag mix, 6210 psi, far exceeds the target of 5,000 psi.  However this may be necessary so that the early strength and time of set, in this relatively low-cementitious mixture, are reasonable for field placement/finishing/curing.  On the other hand, if early strength and time of set are not important considerations for the application (such as in some footings or insulated concrete forms), then total cementitious could potentially be further reduced.  Final choice of a mixture ultimately depends on both specification requirements, and constructability considerations (which are not necessarily specified). 

 

References:

 

Prusinski, J.R., Marceau, M.L, and VanGeem, M.G, “Life Cycle Inventory of Slag Cement Concrete,” Proceedings, Eighth CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete – Supplemental Papers, 2004, American Concrete Institute, Farmington Hills, MI.

St. Marys Cement Co., Unpublished test data investigating concrete compressive strength at 437, 470 and 564 lb/cu yd, with and without air entraining agent, 2002, Detroit, MI.

ESSROC Cement Corp., Unpublished test data investigating concrete with 400 to 600 lb/cy cementitious, portland cement, slag cement and Class C fly ash, 2002, Nazareth, PA.