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(d) Carbonation. Atmospheric carbon dioxide can react with lime to form calcium
carbonate which can adversely affect lime-stabilization reactions. Proper and prompt mixing, storage,
compaction, and curing procedures that minimize the exposure of the lime-stabilized soil to atmospheric
carbon dioxide avoids the problem. Reported problems have been with highly weathered materials in
Africa that were poorly compacted and cured.
(e) Sulfate attack. Lime-stabilized materials are susceptible to sulfate attack if sulfates
are present in the soil or water in contact with the stabilized material or if they are present in materials
that are being stabilized. The sulfate attack reactions are expansive and highly disruptive. Technical
guidance on this problem is incomplete. If lime stabilization is contemplated where sulfates are present,
the HQUSACE (CEMP-ET), appropriate Air Force MAJCOM pavements engineer, or Naval Facilities
Engineering Service Center should be consulted for up-to-date guidance on this difficult issue.
(4) Suitable Soils. Clayey soils with a plasticity index of 12 or more are generally best suited for
lime stabilization. Organic soils and clays containing some iron compounds do not respond well to lime
stabilization, and some highly weathered soils may require a larger than expected dosage of lime
stabilizer to be effective.
k. Portland-Cement Stabilization. Type I portland cement and, more rarely, Types II, I/II, and III
meeting the requirements of ASTM C 150 may be mixed with soils or aggregates to provide a cohesive
cemented material often referred to as soil-cement, econocrete, lean concrete base, etc.
(1) Mechanisms. When mixed with water, portland cement develops cementing compounds
that bind the soil and aggregate particles together. Unlike lime, there is no necessary chemical reaction
with the soil particles themselves. Portland cement contains free lime as one of its constituents so the
same cation exchange and pozzolanic reactions with clayey soils will occur with portland cement, but
these are minor effects compared with the dominant formation of the conventional portland-cement
hydration compounds that serve to bind the particles together.
(2) Uses. Portland-cement stabilization can provide a material with compressive strengths
from a few MPa (few hundred) to well over ten MPa (several thousand psi), depending on amount of
stabilizer and soil properties. These higher-strength stabilized materials are often referred to as
econocrete, lean concrete, etc. with cement contents in the range of 134 to 223 kg/m3 (225 to 375
lb/yd3). Such high cement content and high-quality stabilized mixes are usually proportioned and placed
with the same techniques as conventional concrete. In general, cement stabilization of fine-grained soils
provides a lower strength than cement stabilization of coarse-grained soils. The reactions of portland
cement are faster than pozzolanic stabilizers such as lime. A major drawback for cement stabilization is
the formation of shrinkage cracks which can reflect up through surfacing layers. This is usually a severe
problem with cement-stabilized bases under asphaltic concrete surfaces, but it has also occurred with
concrete surfaces placed directly on high-strength cement-stabilized layers. To minimize problems with
reflective cracking, the Air Force limits the allowable content of portland cement in stabilized bases in
flexible pavements to a 4-percent maximum. A double application of curing compound is often sprayed
on cement-stabilized bases to reduce the chance of reflective cracking in overlying portland-cement
concrete surfaces in rigid pavements. Portland-cement stabilization is most often used for a relatively
high-strength layer that may provide a construction platform, an all-weather construction surface, or a
significant structural layer within the pavement. It is also probably the most expensive of the common
soil stabilizers. Materials stabilized with portland cement should be placed and compacted within
2 hours of the mix water coming into contact with the cement.