In Texas, expansive clay soils present a serious challenge to pavement durability and foundation stability. With nearly 90% of the state’s population living in areas with these problematic soils, understanding how to properly treat them is essential. One question often arises: Can lime and Portland cement be used together in soil stabilization? The answer is technically yes—but with important caveats.
Lime and Portland cement are two of the most important and commonly used products in the construction industry. Here’s what you need to know about how these two materials function, when to use each, and why they aren’t interchangeable.
What Lime Soil Stabilization Does Best
Lime treatment is a proven method for stabilizing clay soils by permanently transforming their structure. Rather than excavating and hauling off expansive clays—a costly and labor-intensive process—lime allows engineers to treat the bad soil in place and turn it into a strong, uniform subgrade.
Key Benefits of Lime Stabilization
- Reduces excavation and disposal costs
- Creates a stable, uniform paving structure
- Minimizes expansion and swelling potential
- Improves foundation support and long-term strength
Lime excels in high plasticity clay by initiating pozzolanic reactions that kill swelling potential and create consistent strength across the treated layer. This makes it ideal for achieving subgrade moduli in the range of 30–50 ksi—strong enough for highways without being so stiff that cracking becomes an issue.
Can Lime and Portland Cement Be Used Together?
Yes, they can—but not as direct substitutes for one another. These materials have different chemical properties and purposes.
Lime
- Forms pozzolanic compounds using calcium from the lime, silica and alumina from the clay, and moisture
- Permanently alters the clay particles, creating long-term strength and stability
- Takes time to gain strength but provides deep, lasting improvement to high PI (plasticity index) soils
Portland Cement
- Reacts quickly (sets in about 3 hours), forming compounds with calcium, silica, and alumina already present in the Portland cement
- Encapsulates clay particles rather than transforming them
- Best suited for granular, low-PI materials—not expansive clay
Think of it this way: Lime is like an antibiotic—it treats the root of the problem by changing the soil structure. Portland cement is like a steroid—it boosts strength quickly but doesn’t address the core issue in clay soils.
Can Portland Cement Provide Additional Strength to Lime-Treated Soil?
In some projects, it makes sense to use lime to treat the clay soil first and then apply Portland cement if high early strength or additional compressive strength is needed for extremely heavy structures. This lime stabilized soil essentially acts like a granular material and can be further stabilized with Portland cement if needed. Once lime has reacted with the in-situ clay, the treated soil becomes a pozzolanic cement on its own, continuing to gain strength over time.
You need to fully stabilize the lime to get full strength from the soil (3-5x stronger) and allow it to build over time. Unstabilized clay soils typically have a sub-grade modulus value of 10 KSI or less. Lime stabilization of clay soils typically results in sub-grade modulus values of 30-50 KSI which is adequate for 99% of the applications where lime is used, such as soil stabilization for highway construction.
Important considerations include:
- High PI clays require lime to stabilize the expansive soil before any portland cement could be added
- Portland cement typically contains less than 2% free-calcium oxide, whereas lime is almost entirely composed of free-calcium oxide
- Stabilization success for clay soils depends on following the required design procedure which is TEX 121E Pt. 3 (also known as the Eades & Grimm procedure)
Why Lime and Portland Cement Aren’t Interchangeable
Portland cement is not a 1:1 substitute for lime in clay soil stabilization. Lime and Portland cement are both calcium-based stabilizers, however their chemical reactions with in-situ clay are very different.
Portland cement contains bonded calcium already tied to silica and alumina. It can add strength by encapsulating soil particles; however, it can’t transform the in-situ clay the way lime does, because it cannot trigger permanent structural change in expansive clay. Lime introduces free calcium to the soil, triggering permanent structural change in expansive clay. Lime-stabilized soils continue to gain strength with time creating a uniform, durable, and stable base.
A properly lime stabilized clay soil can be further enhanced by adding Portland cement to increase strength if needed. However, misusing or substituting Portland cement for lime or vice- versa can lead to costly failures and poor pavement performance. The bottom line is that Portland cement cannot do what lime does, and lime cannot do what Portland cement does.
