Gypcrete or gypcrust is a hardened layer of

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

, consisting of around 95%

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywa ...

(

calcium sulfate Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris ...

). Gypcrust is an arid zone

duricrust Duricrust is a hard layer on or near the surface of soil. Duricrusts can range in thickness from a few millimeters or centimeters to several meters. It is a general term (not to be confused with duripan) for a zone of chemical precipitation and ...

. It can also occur in a semiarid climate in a basin with internal drainage, and is initially developed in a playa as an evaporate. Gypcrete is the arid climate's equivalent to

calcrete Caliche () is a sedimentary rock, a hardened natural cement of calcium carbonate that binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or semiarid regions ...

, which is a duricrust that is unable to generate in very arid climates.

Composition

Gypcrust horizons can be up to thick with a 75-97% gypsum (CaSO₄∙2H₂O) content. The majority of gypsum-rich layers occur where the average annual rainfall is less than 250 mm because gypsum is moderately soluble (c. 2.6 g⁻¹ at 25 °C) and is normally leached out under higher rainfall conditions. Gypsum cements are rarely, if ever, as strong as calcretes or silcretes.

Formation

Gypcrust forms in a manner similar to that of

caliche Caliche () is a sedimentary rock, a hardened natural cement of calcium carbonate that binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or semiarid regions ...

, which is composed of calcium carbonate. The development of gypcrust has 3 main stages. The first stage is primary crystallization of the surface brines or groundwater; the second stage is transportation and redeposition by wind or water; and the third stage is post-depositional alteration above or below the

capillary fringe The capillary fringe is the subsurface layer in which groundwater seeps up from a water table by capillary action to fill pores. Pores at the base of the capillary fringe are filled with water due to tension saturation. This saturated portion of ...

. Most gypcrust is formed either as a result of soil-forming processes or through the precipitation of cementing agents from groundwater.

Influence of groundwater on the formation of duricrusts

There are two models that are used to illustrate the influence groundwater has on the formations of duricrusts like gypcrust: ''per ascensum'' and ''per descensum''. The ''per ascensum'' model demonstrates a situation where the water table is relatively close to the surface, allowing solutions to be drawn upwards by evaporation and eventually cement near-surface sediments once they become concentrated enough to trigger precipitation. The ''per ascensum'' model is applicable to environments with high rates of surface evaporation like deserts. This type of system only produces thin duricrust layers since the process ultimately seals the surface horizons, which consequently decreases the potential for further evaporation. This model best depicts the formation of gypcrust. The ''per descensum'' model describes a system different from that of the formation of gypcrust in which precipitation of minerals occurs at a depth from downward-percolating solutions. This type of system explains the formation of thick duricrust horizons.

Conditions for formation

Gypcretes form in four distinct conditions: in well-drained soils, as buried evaporates, in hydromorphic soils, or by the exposure of subsurface horizons by erosion.

Profile

Gypcrete can be a loose and powdery deposit or a massive crystalline structure. The profile of a gypcrust outcrop can have three layers. The bottom layer is the sand rose horizon at the water table where gypsum develops as

aggregate Aggregate or aggregates may refer to: Computing and mathematics * collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the consistency of changes being made within the ...

s of crystals. The middle layer is composed of massive gypcrete cemented sand, which forms above the water table during evaporation from the capillary fringe; newly formed gypcrete will be hard, and will soften with age. The uppermost layer is usually rich in gypsified roots and has a banded or nodular structure.

Uses

Gypcrete has been used successfully for road construction in the Sahara. Well-cemented gypcrusts may also provide adequate bearing capacity for structures, however it must be ensured that the underlying uncemented material is not overloaded to avoid collapse.

References

{{Use dmy dates, date=April 2017 Soil