Direct foaming and seeding of highly porous, lightweight gypsum
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This paper describes the direct-foaming of plaster of Paris (CaSO4½H2O) with up to 1.7 wt% of a nonionic surface-active agent to obtain highly porous, lightweight gypsum (CaSO42H2O) with 65–70% total porosity and a homogeneous, bimodal pore size distribution. Based on viscosity and temperature changes in the setting plaster of Paris slurry, the nonionic surfactant is seen to retard gypsum crystallization and thus extend the working time at higher water to solid ratios. The increase in viscosity during gypsum crystallization stabilizes the macropores formed during foaming. Gypsum foams of 32% density with a submicron matrix pore size, and relatively uniform macropore size of ;100 lm were obtained. Seeding the process with 0.5 wt% 100 lm diameter gypsum particles accelerates gypsum crystallization in the presence of the nonionic surfactant and results in a more uniform, finer (92 lm) macropore structure due to less time for bubble coalescence.
I. INTRODUCTION
Gypsum (CaSO42H2O) is widely used in the building industry because of its unique microstructure of high porosity (49% of theoretical density (qo 5 2.31 g/cm3)1,2 and characteristic submicrometer pores within a matrix of interlocking blade and needle shaped crystals.3,4 As a result gypsum has good strength and excellent thermal and sound insulation properties. The microstructure and morphology of the gypsum crystals depends on the gypsum crystallization conditions, and the presence and type of chemical additives. The scanning electron microscopy (SEM) image of the gypsum microstructure in Fig. 1 shows that the plaster of Paris (CaSO4½H2O) used in this study forms acicular and blade-shaped crystals of 10–20 lm in length by 1–2 lm in cross-section. It is well known that both the thermal and sound insulation properties of a material can be improved by increasing the amount of porosity. Thus, numerous strategies have been explored for producing porous materials. The foaming process appears to be most amenable for making lightweight gypsum because a critical process step during gypsum formation is the production of a slurry containing plaster of Paris and water. Surfactant molecules are typically used to produce stable foams because they reduce the air–water interfacial energy and thus make it easier to form bubbles. Surfactants also Contributing Editor: Paolo Colombo a) Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/. DOI: 10.1557/jmr.2016.246
serve as pore stabilizers by slowing foam collapse due to bubble coalescence.5–7 Gypsum forms by the hydration of calcium sulfate hemihydrate; commonly known as plaster of Paris (CaSO 4 ½H2 O). The reaction is exothermic and occurs as:4 1 3 CaSO4 H2 O þ H2 O ! CaSO4 2H2 O 2 2
:
ð1Þ
It is generally understood that plaster of Paris dissolves in H2O by an endothermic proce
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