Density and strength characteristics of foamed gypsum

Abstract

This paper deals with the effect of substances producing and assisting gas and foam formation on the properties of gypsum density and strength. The gas-based foaming method utilises aluminium sulphate, potassium alum or ammonium bicarbonate for the chemical production of gas bubbles in gypsum paste. The second method obtains foamed gypsum by means of air entrainment in wet gypsum paste. The foaming agents chosen were sodium lauryl sulphate and nonyl phenol ethoxylate that are widely employed in detergent production. Five different foaming techniques are obtained with these additives required for gas and foam production. Citric acid, as retarder, and carboxyl methyl cellulose (CMC), as viscosity increasing agent, were used for promoting foam and gas formation during the foaming experiments. Aluminium sulphate was discovered to be the one that achieves the most foaming in gypsum. The techniques utilising potassium alum, sodium lauryl sulphate and ammonium bicarbonate reduced the density of gypsum products by values varying between 30% and 35%. Foaming with nonyl phenol ethoxylate had an insignificant effect on density. The addition of citric acid and carboxyl methyl cellulose assisted with density reduction. Besides regulating the hardening time, citric acid supported density reduction by releasing gas through a reaction with marble powder. However, an addition of CMC above 0.1% or 0.2% causes a density increase by preventing bubbles to expand and merge with each other. The compressive strength of low-density gypsum products falls well below 10 Mpa, stipulated by TS 370 for building gypsum.

Introduction

Gypsum is a material with a relatively small weight per unit volume under normal conditions. To further reduce the density of gypsum, the water ratio is increased or lightweight aggregates such as vermiculite, perlite, expanded polystyrene are added. Beside these methods aeration of gypsum is also widely used. Two methods are commonly used for the production of porous gypsum. The first is based on the principles of forming gas bubbles in gypsum paste through chemical additives. The second method is entrapment of air in wet gypsum paste through whisking and tempering by using additives that reduce the internal surface tensions, resulting in aerated gypsum.

In the first method, it is important that the quality of gypsum is not impaired by the chemical additives. The fact that the calcium sulphate salt present in the material obtained at the end of the chemical reaction is insoluble in water prevents any degradation of gypsum. Sulphate salts of a weak alkali such as potassium aluminium sulphate (potassium alum), zinc sulphate, copper sulphate, aluminium sulphate additives for reducing the time for hardening or accelerating the reaction of gypsum cause the formation of gas in the mass of gypsum coupled with marble powder [1]. The investigation of the reaction of these materials with marble powder reveals that the final product is synthetic gypsum and can be defined as dehydrate which is the water-insoluble type of calcium sulphate salt. An adverse effect on the quality of gypsum does not occur. The production of porous gypsum is also confirmed to be possible through to the utilisation of synthetic gypsum that is resistant to acid action encountered in the production of hydrofluoric acid [2]. At present, large amounts of phosphogypsum and fluorogypsum are obtained during the production of hydrofluoric and phosphoric acid. A phosphate rock that belongs to the mineral of phosphorous rock salt dissolves in sulphuric acid and water, producing phosphogypsum, phosphoric acid and small amounts of hydrofluoric acid [2]. In the air-entraining gypsum production method, soaps [3], [4], oil acids [5], [6], [7], [8], [9], [10], [11], sulphates [12], [13], sulphonates [14], [15], [16], [17] and ethoxy groups [18] are added into the mixing water of gypsum. These additives that are called the surface active agents [19] provide for the stabilisation of bubbles and enable aeration [20].

The hardening time is observed to be the basic function of the formation of gas as well as foam in all applications of porous gypsum. Therefore determination is required of the appropriate times for hardening and consequently the type and ratio of the retarder or accelerator to be used. Citric acid, borax, tartaric acid, sodium citrate, lactose, keratin, maltose and saccharose are the most commonly used retarders for gypsum [21].When citric acid is used as a retarder in gypsum the observations made are the reduction in the number of open pores, the increase of the average pore diameter, the general reduction of small air bubbles in conjunction with the increase of pore diameters, and finally a reduction in the bending and compressive strengths and degree of hardness [2]. The entrapment of gas bubbles in the gypsum paste during the process of gypsum aeration is a factor that is as important as the determination of the setting time. The additives for increasing viscosity prevent the loss of gas and assist the aeration by enclosing the crystals of calcium sulphate in the wet gypsum paste so that the resistance to flow is increased.

The aeration methods described above were used in this research where reduction of weight per unit volume of gypsum was investigated. The effects of the percentages of additives for producing and assisting gas and foam formation and also the effects of the ratio of mixing water in the mixture on the physical and mechanical properties of gypsum are investigated.