In 2012, ASTM C595 and AASHTO M240 began to include Type IL cement, which contains 5 to 15% ground limestone. In the past few years Type IL cement has become ubiquitous in the United States—to the point where you can’t find Type I any more. Cement manufacturers maintain that it’s the same as Type I cement. But contractors are finding that it isn’t necessarily so. What’s the real story? And how should best practices change to accommodate it?
A brief history of limestone cements
Shortly after World War II, Europeans experienced severe shortages of cement as they rebuilt their cities. They began incorporating limestone into their cement to extend it. By now they have decades of experience using cements that sometimes contain high percentages of limestone.
Canada began incorporating small amounts (5% or less) of limestone in 1983. Keep in mind that if 5% is the maximum allowed, the cement will have 2 to 3% limestone in practice. In those quantities, the limestone doesn’t make much difference to the behavior of either fresh or hardened concrete. Finishers would sometimes observe that the concrete was a little easier to finish because of a slightly higher volume of fines. Limestone typically has a specific gravity of about 2.75, as compared to 3.15 for portland cement clinker. Because limestone is softer than clinker, the limestone grinds preferentially, resulting in particles finer than the clinker.
The compressive strengths were comparable. Several mechanisms offset the minor dilution of active ingredients:
- Particle packing. The fine limestone particles fill in the spaces between the clinker particles, making for more efficient packing.
- Nucleation. The limestone fines provide additional surface onto which the cement hydration products can precipitate out of solution. This effect slightly accelerates the initial hydration reactions and helps the hydration products distribute more uniformly.
- Carbo-aluminate reaction. The aluminate compounds in the cement react with the limestone, generating more solids.
For more than a decade, Canadian Type 10 and American Type I cements, which had historically been equivalent, could not be used interchangeably. Then in 2004, ASTM C150 began to allow up to 5% limestone. There was little fanfare and hardly anyone seemed to notice.
How are Type IL and Type I cements the same?
Cement manufacturers formulate Type IL cements to achieve comparable compressive strengths at 28 days. They maintain that Type IL is similar to Type I cement in many other ways as well:
- chemistry
- resistance to expansion due to alkali-silica reaction
- resistance to sulfate attack
- relationship between w/cm and strength
- setting times slightly faster
- admixture dosages
- handling and batching
- finishing
“Trust but verify” (доверяй, но проверяй)—Russian proverb quoted frequently by Ronald Reagan in the 1980s during nuclear arms reduction negotiations with the USSR.
However, as when you make any other changes to the materials you’re using on a project, qualify the concrete mixture. Test not only the compressive strength, but every aspect of the performance that matters to you. And once you’ve qualified the mixture in the lab, do mockups in the field with a batch size of at least 3 yd3. That way you can see how it will perform in the batch plant and the field.
And if you’re going to place the concrete in the summer, test it at summer-like temperatures. You’ll likely need to increase the dosages of water reducing and air entraining admixtures. To compensate for the shorter setting times, you may want to use a retarder or hydration control admixture.
How are they different?
Not surprisingly, there are differences between 5% and 15% limestone. With higher dosages of limestone, particle packing, nucleation, and carbo-aluminate reaction can’t fully compensate for the dilution of the clinker. So the clinker must be ground much finer, and the limestone particles will be finer still. Where a Type I cement might have a Blaine fineness of 350 to 400 m2/kg, a Type IL cement might have a fineness of 450 to 500 m2/kg or higher.
The dramatic increase in fineness will slow the rate of bleeding, so finishing will be delayed. There will be little to no bleed water, so the surface can easily dry out. That may result in a surface with unhydrated cement, higher porosity, weakness, or scaling. Flatwork is also more vulnerable to plastic- and drying shrinkage.
Something to be aware of in proportioning concrete is the difference in specific gravity. For a given mass, limestone has a greater volume than clinker. That may not have mattered with 2 to 3% limestone, but it’s significant with higher limestone contents. Batch weights should reflect the lower specific gravity of the cement.
Best practices for Type IL cement
When designed thoughtfully, PLC mixes remain compatible with supplementary cementitious materials … for an even greater reduction in embodied carbon. ..[Type IL cement concretes require] extra attention in the field—from batching to placement to curing—to deliver the quality and durability that owners expect.—Michigan Concrete Association
Tips from the Michigan Concrete Association:
- Add most or all of the water at the batch plant. Add water at the site no more than once; mix it in thoroughly before discharging the concrete.
- You may need to use more water reducer or a mid-range water reducer to get the workability you want.
- Don’t overwork the surface. Just consolidate, strike off, float, texture, and cure.
- In hot weather the concrete may set faster. If you can’t finish right away, use an evaporation retarder. A hydration control admixture will provide additional working time.
- In cold weather, the set will be slower.
- Finishing aids may help to prevent drying, crusting, and stickiness. Use colloidal silica finishing aids, not water.
- Keep finishing tools clean, as the mixtures are sticky and can build up on blades.
- Cure immediately after finishing.
- Check the evaporation rate and apply curing compound within 30 minutes of texturing. ACI sets a maximum evaporation rate of 0.20 lb/ft2/hr, but you may have trouble even at half that rate.
- Keep surfaces damp using wet burlap, plastic sheeting, or curing compound. If you use curing compound, apply it generously.
- Type IL cement concrete may need longer curing to achieve the desired performance.
- In hot weather, use fogging, evaporation retarder, or sunshades to keep the surface from drying.
- In cold weather, use insulating blankets or heated enclosures. Don’t remove the forms too early.
- Take special care with exposed edges; they can dry out easily.
Steve Waalkes explains how to work with Type IL cement.