Minimization of Building Heave By Chemically Inhibiting Gypsum-induced Shale Expansion; A Preliminary Report

Floor heave is a continuing problem in structures built upon black shale as a result of gypsum growth within the shales. This problem might be minimized by periodic injections of powerful crystallization inhibitor chemicals into the vicinity of potential gypsum growth within the shales. In order to determine if this approach might be productive, the effect of sulfate scale inhibitory compounds upon the nucleation and growth of gypsum was investigated in laboratory systems simulating the acidic black shale heave environment. Of more than 25 different commercial and research inhibitors tested, nearly all reduced gypsum nucleation and growth under neutral pH conditions, but only three were even moderately effective at a pH of 2.5, similar to the acidity of black shale heave environments. At pH 2.5, however, many of the additives modified crystal growth morphology and size. Testing of expansion within rock cores in a consolidometer during induced gypsum growth within the cores showed that several additives modified the rate of expansion, apparently as a result of effects upon crystallization force pressures through habit modifications of growing gypsum crystals. These preliminary results suggest that periodic applications of crystallization inhibitor chemicals may be a viable approach to minimizing black shale heave.

Heaving of floor slabs and lightly loaded structures built on black pyritic shales is a construction engineering problem in many locations, including Norway, the United States, and Canada (Quigley, etal., 1973). The cause of the heave has been documented by many workers, including Grattan-Bellew and Eden (1975); Penner, Gillott, and Eden (1970); and Quigley and Voran (1970). These researchers agree that the growth of gypsum, CaS0₄ .2H₂0, is chiefly responsible. Although often there is little or no gypsum in the original shale, once a structure has been built on the pyritic shales, conditions often develop which lead to its growth. Sulfate ions are generated by the oxidation of pyrite or marcasite, FeS₂, and these ions react with available calcium ions from the dissolution of calcium-bearing minerals in the acid shales to form gypsum. The continued growth of gypsum eventually leads to shale expansion, in a manner similar to that in frost-induced soil expansion. As soon as overlying concrete slabs are cracked as a result of initial shale expansion, pressure of the shale in the heaved region is reduced and a pressure gradient may accelerate the migration of dissolved calcium and sulfate into the region of gypsum growth; this leads to an increased rate of shale heave.

Although there are potentially several different approaches in attempting to minimize or eliminate heave due to gypsum growth within black shales, one of the most economically attractive methods involves the inhibition, or prevention, of gypsum growth by chemical means. Conventional methods, such as sealing of foundation rocks to prevent pyrite oxidation, can be readily applied to new constructions, but heave alleviation beneath already existing structures is expensive. Other different approaches suggested to deal with this problem, such as floor bolting, backfill, or coating the underfloor with an impervious surface, all have limited applications.