On sulfate-bearing clay, straight lime grows ettringite and lifts the ground, sometimes years after the job closes. TxDOT research found the fix: 5 percent slag cement with 1 percent lime. We supply both materials, help size the blend to your soil, and can point your engineer to the TxDOT special provision that already exists for placing it.
Send the sulfate and PI numbers if you have them. Same-day response.
Lime stabilization works by putting calcium into the clay. Sulfate heave happens when that calcium meets gypsum and water inside the soil and grows a mineral called ettringite, which takes up roughly twice the space of what it replaced. The reaction runs until one ingredient is gone, and it can take years to show at the surface. Add more lime and you feed it. That is why TxDOT treats soils above about 7,000 to 8,000 ppm sulfate as unsuitable for lime alone.
All four together grow ettringite. Remove any one and the heave stops. You cannot remove the clay, the gypsum or the water, and you need some calcium to stabilize at all. So the answer is to use as little calcium as the chemistry allows and give it something better to react with.
Ground granulated blast furnace slag is a latent hydraulic material: it wants to become cement but sits inert until the pH passes about 10.5. One percent lime gets it there. The slag then consumes the free lime that would otherwise feed ettringite, and it keeps building strength for weeks.
Texas Transportation Institute, for TxDOT and FHWA, reports 0-4240-1 through -4. Everything below is read from the full reports. The recommended mix is 5 percent slag to 1 percent lime by dry weight of soil, and the reason is in the first table.
| Mix, College Station soil at 20,000 ppm sulfate | 3-D swell |
|---|---|
| 5% slag cement + 1% lime | 7% |
| 3% slag cement + 3% lime | 24% |
| 0% slag, 6% lime | 42% |
In the report's own words: it is important to only use enough lime to catalyze the reaction of the slag. Push the lime up and the swell comes back.
| Unconfined compressive strength | 6% lime alone | 5:1 slag-lime | Untreated |
|---|---|---|---|
| 7-day moist cure | 99 psi | 127 psi | 26 psi |
| 28-day moist cure | 82 psi, went down | 198 psi | n/a |
| After 4-hour soak, 20,000 ppm soil | 87 psi | 143 psi | fell apart |
Lime alone loses strength between 7 and 28 days. Slag-lime more than doubles, and it holds at 20,000 ppm sulfate. Values are read from report figures and are close rather than exact.
| Field test section | Sulfate | Result with slag-lime |
|---|---|---|
| SH 161, Dallas District | 21,000 to 27,000 ppm | Swell 4% below untreated. 6% lime on the same soil swelled 40% |
| SH 183, Dallas District | 20,000 to 30,000 ppm | Swell 6% below untreated. 6% lime swelled 35% |
| FM 3338 Sta. 107, Laredo District | 20,000 to 23,000 ppm | Swell 5% below untreated. Strength 99 psi vs 3 psi control |
| FM 3338 Sta. 103, Laredo District | 23,000 to 26,000 ppm | Swelled 2% more than untreated on a low-clay soil. Researchers said leave it alone |
The honest framing: against lime, slag-lime wins every time and it is not close. Against doing nothing, it wins on strength always but on swell only when the soil has enough clay to swell. Two sites 400 feet apart on FM 3338 got opposite answers. We will never quote a swell number without a test on your soil, and neither should anyone else.
Contractors buy a platform they can drive on, and that is a plasticity conversation. On a PI 41 soil, 6 percent lime took the plasticity index to 13. Five percent slag with 1 percent lime only reached 27. Going to 2 percent lime reached 17, a workable platform, still using a third of the lime of a straight lime job. Slag with no lime at all made the PI worse, 41 to 47. Slag is not a stabilizer on its own.
For high-PI soils the TxDOT researchers used a two-step addition: 1 percent lime first, six hours to mellow, then the remaining lime with all the slag. A 5:2 mix built that way performed like a 5:1 mix on swell. That is how you buy workability on fat clay without paying for it in heave.
From the special provision TxDOT wrote for the FM 3338 widening in Webb County. This is the construction sequence a crew will be handed, not theory.
TxDOT's own guidance says the most severe heaves from Dallas-Fort Worth to the border counties around Laredo sit on the Eagle Ford formation, which roughly parallels Interstate 35. The Geologic Atlas of Texas flagged six of the eight failures in the research before anyone set foot on site: look for selenitic or gypsiferous in the formation legend.
SH 161, SH 183, and US 67 west of Midlothian. The original failures and the reason the research began.
US 82 near Sherman. Surface read 4,160 ppm; 41 inches down the same location read 27,840. Gypsum washes down. Test at depth.
FM 3338 in Laredo, where an 8-mile job tested clean until a re-test found a 40,000 ppm section, and a test section near Eagle Pass above 32,000 ppm built with 4% slag and 1% lime.
US 287 on Blaine formation gypsum, SH 118 on the Aguja, SH 54 north of Van Horn. Sulfate is not only a Blackland problem.
Two field lessons that save jobs. Sulfate concentrates at depth, so a cut-and-fill job can bury a clean test and expose a bad one. Test again after the site is at grade. And one test across a project is not a test: sulfate runs in seams, so sample every 100 feet, read conductivity at time zero and again ten hours later because coarse gypsum dissolves slowly.
Grade 100 slag cement from a Houston producer on the TxDOT DMS-4620 list, delivered by pneumatic tanker in Houston or statewide. Concrete producers and contractors buying slag without the lime start there.
Materials load in Houston and deliver statewide. Blend design and sulfate testing support anywhere in Texas.
Calcium from the lime meets gypsum and water in the clay and grows ettringite, which takes up about twice the volume of what it replaced. The reaction runs for years. More lime feeds it. TxDOT treats soils above about 7,000 to 8,000 ppm as unsuitable for lime alone.
5 percent slag cement to 1 percent lime by dry weight of soil, per TxDOT project 0-4240. On fat clay that needs workability, 5:2 built in two steps. Never more lime than the plasticity requires.
No. Slag needs lime to raise pH above roughly 10.5 before it reacts. Without it, slag is inert filler and TxDOT testing showed it made plasticity worse. The lime is small but not optional.
The slag cement is a qualified material under DMS-4620 and TxDOT has written a special provision for placing slag-lime on FM 3338. Acceptance under a given stabilization spec item is a district materials engineer's call. We can point your engineer to the provision and the research.
Class F fly ash with lime reduced swell in the same research band as slag-lime, and TxDOT's own remedy on US 287 was lime and fly ash. It is a live second option, especially where the sulfate is gypsum rather than sodium sulfate, and it uses more lime. We supply fly ash too and will price both paths.
Hasten Energy Solutions, doing business as Hasten Chemical, is a Houston supplier of fly ash, lime and kiln dust blends for soil stabilization across Texas and Louisiana. Slag-lime is the system we recommend on sulfate soils, and we supply both halves of it.
Send the sulfate and PI numbers, the acreage and the depth. We will tell you whether slag-lime fits, and what it costs per treated ton.
Call 832-655-7763