Lime Soil Reactions:

The concepts of lime – soil reactions can be classified into two parts:-- Concepts proposed before 1961.

- Concepts proposed after 1961.

2.7.3.3.7.1 Concepts Proposed Before 1961:

The published literature of lime – soil reactions up to and including 1960 was reviewed as follows [13]:

A- Ion Exchange and Flocculation:

The mechanism of flocculation or agglomeration was attributed to :-i- a base exchange reaction of Calcium (Ca⁺⁺) cations with other weaker cations such as (Na⁺ or K⁺) that might be held in the exchange positions at the clay surfaces.

ii- The crowding of more calcium cations (Ca⁺⁺) onto the clay surfaces. This mechanism was suggested by Hilt and Davidson [14]

after they had noticed that calcium saturated soils flocculated upon lime addition (i.e. mechanism (i) was not the single cause of clay flocculation) and that the rise in pH of the clay upon the addition of lime (when sufficient lime added to soils calcium dissolved in the soil water and created an alkaline environment) increased the cation exchange capacity of clay. This in turn led to extra unsatisfied exchange sites on the clay surfaces. To these sites extra calcium cations were believed to be attracted [6, 14].

The end result of extra calcium cations being attracted to the clay surfaces by one or both of the mechanisms (i) and (ii) identified above, was a suppression of the electrical charge distribution around the clay particles. This caused an attractive force between the particles and so flocculation occurred. The ability of lime to modify clay soils i.e. to improve plasticity, workability, swelling, shrinkage characteristics, was attributed to the flocculation mechanism.

B- Cementing or Pozzolanic Reactions:

It was suggested that the calcium cations in lime, react with the siliceous and aluminous minerals in the soil to form a highly

cementitious gel of calcium silicates and aluminates and these compounds created a bond between the soil particles. It was believed

that the cementing action requires several months for completion [10].

C- Carbonation:

This reaction involved the formation of calcium carbonate as a result of the reaction of carbon dioxide (CO₂) from atmosphere and these

carbonates form weak cements but also deter pozzolanic reaction and prevent the gain of normal strength [13].

2.7.3.3.7.2 Concepts Proposed After 1961:-

Many researchers had presented the results of their studies and added to the basic knowledge of lime – soil reactions the following concepts [8, 15]:

A- The Lime Retention Concept:

Previous work in lime – soil stabilization [14] had shown that addition of small amounts of lime considerably improves the workability of clay soils but contribute little improvement to strength. It was confirmed by [14] that certain quantity of lime added to clay soils must first satisfy an affinity of the soil for lime and this quantity was fixed in the soil and was not available for Pozzolanic reaction. They established that this certain quantity of lime required to attain the maximum increase in the plastic limit coincide with the minimum quantity required to start the strength gain reactions. They called this coincidence “The Lime Fixation Point “. The lime fixation points {Lm} of the montmorillonitic soils investigated [14] were found to be:

Lm= {of < 2 µmm clay + 1.25} % 35

The term was later altered in 1963 to “ Lime Retention Point “. [15].

The lime retention concept regarded the lime – soil interaction as two different types of reactions these are:

- The soil modification attributed to calcium cation being used in normal cation exchange and a pH dependent cation crowding (2.7.3.3.7.1.A) .

- The strength gain which should occur after the completion of cation crowding, attributed to cementation products by the pozzolanic reactions (2.7.3.3.7.1. B).

B- The physical Adsorption Concept:-

This concept was developed by Diamond and Kinter [8] . The concept was based on data obtained from experiments on lime rich suspensions and pastes of bentonite. Cation exchange, flocculation and carbnation were considered by these researchers to be inadequate to explain the soil – lime reaction for the following reasons:

i- Cation exchange :- It was found that many naturally occurring calcium – saturated soils required stabilization. It was also found that addition of lime in great amounts to dilute sodium clay suspensions did not cause complete cation exchange. Hence little exchange could not be expected to occur in normally compacted lime – soil mixtures.

ii- Flocculation:- Many naturally flocculated soils were found to be unstable and responded to lime stabilization .

iii- Carbonation:- It was reported that lime – soil mixtures sealed from contact with carbon dioxide developed the normal indication of stabilized soils. Hence carbonation was not an important stabilization reaction.

The lime retention concept (2.7.3.3.7.2. A) was considered by the two researchers to be unsatisfactory because the apparently unlimited crowding of positively charged cations onto the limited external surface of the montmorillonite would appear to be an unlikely phenomenon.

C- The Diffuse Cementation Concept:

This concept was developed [37] on the basis of extensive research on the physical, mechanical and mineralogical aspects of an Australian Calcium saturated montmorillonitic clay treated with 5% and 10% lime and compacted at its optimum moisture content.

It was confirmed that normal and pH – dependent cation exchange occurred, but these reactions consumed at most 0.5% lime (on oven dry soil weight basis). No initial lime adsorption over the whole of the clay surfaces (i.e. physical adsorption 2.7.3.3.7.2 B) was envisaged and all lime apart from that consumed in cation exchange was fully utilzed in the formation of cementation reaction products. The diffuse

cementation concept briefly described the net negative charge of the clay particles thus no cation crowding was envisaged. It was also found that the amount of lime adsorbed corresponded roughly to the lime retention point [15]. It was also believed that the clay particles

flocculated upon lime addition. This flocculation was attributed to the normal electrolytic effects of lime (i.e. not to the pH – dependent cation crowding ) and it did not stabilize the clay particles.

The above reaction was rapidly followed by limited cementation at points of contact between the edges and faces of the flocculated clay particles. This was thought to be responsible for the soil modifiction [37].