Expansive clay
Expansive clay is a type of clay that is known as a lightweight aggregate with a rounded structure, with a porous inner, and a resistant and hard outer layer.
It is a clay or soil that is prone to large volume changes (swelling and shrinking) that are directly related to changes in water content. Soils with a high content of expansive minerals can form deep cracks in drier seasons or years; such soils are called vertisols. Soils with smectite clay minerals, including montmorillonite and bentonite, have the most dramatic shrink-swell capacity.
The mineral make-up of this type of soil is responsible for the moisture retaining capabilities. All clays consist of mineral sheets packaged into layers, and can be classified as either 1:1 or 2:1. These ratios refer to the proportion of tetrahedral sheets to octahedral sheets. Octahedral sheets are sandwiched between two tetrahedral sheets in 2:1 clays, while 1:1 clays have sheets in matched pairs. Expansive clays have an expanding crystal lattice in a 2:1 ratio; however, there are 2:1 non-expansive clays.
Mitigation of the effects of expansive clay on structures built in areas with expansive clays is a major challenge in geotechnical engineering. Some areas mitigate foundation cracking by watering around the foundation with a soaker hose during dry conditions. This process can be automated by a timer, or using a soil moisture sensor controller. Even though irrigation is expensive, the cost is small compared to repairing a cracked foundation. A laboratory test to measure the expansion potential of soil is ASTM D 4829.
Another important characteristic of the expansive clay is its vulnerability to physical changes, according to the amount of water. For example, in a wet season, the clay has the capacity of swelling, and on a dry season, it can shrink and form cracks. According to Biswas and Kriscna, “there are several types of clay minerals of which Montmorillonite has the maximum swelling potential”.
The expanded clay is obtained by heating of different types of clay, at a temperature of approximately 1200 °C, using a rotary kiln.
The principal characteristic of the expanded clay is to have a density three times smaller than normal aggregates. Usually, the density of this type of clay is 350 kg/m3.
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