Lime Reactions

Silica which is soluble in about 2 to 3N hydrochloric acid can be taken as reactive silica. The acid-soluble silica can easily and accurately be determined by plasma emission spectrometry using a very small quantity of fly ash (Sivapullaiahet al. 1998).

3.3. Soil – Lime Reactions

The addition of lime to a soil initiates a two stage reaction. Short-term reactions show their effect right after the addition of lime, while long term reactions are accompanied by a period of time.

The short-term effect of the addition of lime to a clay soil is to cause flocculation and agglomeration of the clay particles, as explained in Section 3.1 on fly ash stabilization, for cation exchange takes place between the metallic ions of the clay particles and the

Because this lime is fixed in the soil and is not available for other reactions, the process has been referred to as lime fixation (Hilt and Davidson, 1960). The lime fixation point corresponds with the point where further addition of lime does not bring about further changes in the plastic limit. This therefore is the optimum addition of lime needed for maximum modification of the soil. Beyond this point excess lime can only produce cementitious compounds, which bind the flocculated particles and develop extra strength (Al-Rawas et al. 2002; Bell, 1996; Mathew and Rao, 1997). The long-term reactions are the pozzolanic reactions. The addition of lime to soil produces a highly alkaline environment, due to the OH- anions from the hydration of lime, which gives rise to a slow solution of silica and alumina from clay particles (Kinuthia et al.1999; Mathew and Rao, 1997). The cementation process develops from the reaction between calcium present in lime and dissolved silica and alumina from soil, forming calcium-silica-hydrates (CSH), calcium alumino-hydrates (CAH), and calcium-alumino-silica-hydrates (CASH) (Nalbantoğlu and Tuncer,

Carbonation is the reaction of lime with carbon dioxide from the air to form weak cementing agents, notably calcium carbonate (CaCO3). This is undesirable(Bell, 1993). Becoming unstable, calcium-silica, calcium-alimino and calciumalumino-silica hydrates may react with carbon dioxide to revert back to silica, alumina, and calcium carbonate if the pH of the lime-stabilized soil drops sufficiently low. Carbonation reactions are harmful to the long-term strength and durability of the lime-stabilized soil. Using sufficient amount of lime (to provide enough alkalinity), compaction of the soil to high density and prompt placement after mixing lime with soil (to minimize carbon dioxide penetration) can minimize potential carbonation problems. In a lime treated soil, if sulfates are present in the soil or water, then these sulfates may react with alumina released from clay and calcium from lime to form ettringite (Ca6[Al(OH)6]2.(SO4)3.26H2O) (Mohamed, 2000). Ettringite has the capability of imbibing large volumes of water and dramatically increases the swelling potential of the lime-stabilized soil. It would be prudent to test and understand whether lime, soil, and sulfate swell or not when mixed and exposed to moisture, if sulfates are present where lime stabilization is