THE ROLE OF GYPSUM IN CEMENT Gypsum Gypsum i s a soft, soft, tr ansparent or tr anslucent mi neral composed composed of cr ystall ystall ised ised calciu m sulphate (CaSO ), 4 f oun d natur all y pri mari ly i n sedimentary sedimentary depos deposits. its. I t i s use used in the manufactur e of of wall boards, boards, pape paper, r, pain ts, ts, plaste plasters rs and cement.
Gypsum is added to Portland cement to regulate the otherwise extreme setting reaction which occurs in the presence of water. The gypsum content content of cement cement is expressed in terms of its sulphate (SO 3 ) content. Sulphate is a readily determined measure of gypsum in cement. Portland cement usually contains around 5% gypsum, which is equal to appr oximately 2.1% SO 3. The maximum SO3 content allowed by AS 3972 Portland and Blended Cements is 3.5%. As well as influencing the setting time, gypsum also has an influence on other properties of cement. The more important of these properties are strength, strength development and volume stability. Gypsum also influences the grindability of the cement, which is the grinding energy required to produce cement.
THE EFFECT OF GYPSUM ON SETTING OF CEMENT
dormant period, ettringite crystals continue to grow and eventually are large enough to impede the mobility of cement particles. At this stage setting setting occurs.
The setting of cement involves the reaction of clinker minerals with water, to form a system of interlocking crystals, which lock the material together. The setting of cement is primarily due to the reaction of tricalcium aluminate (C 3A), which is the most reactive of the four clinker minerals present in cement. cement. The set regulating action of gypsum is due primarily to its reaction with C 3A.
F igure 1: 1: First stage - formation of a thin cover of
ettringite on the C 3 A surface In the absence of gypsum, the reaction of C 3A with water is very rapid. The C3A is converted to calcium aluminate hydrate (4CaO.Al 2O3.19H2O), which forms interlinking bridges of solid material between the cement particles. particles. These interlinking bridges reduce the mobility of the cement particles and produce a firm structure, which results in setting of the cement mixture. This rapid setting is called a flash set and is irreversible. The presence of gypsum in cement slows the reaction of the C3A. When mixed with water, water, gypsum dissolves dissolves and reacts with C3A to form ettringite (3Ca0.Al 203.3CaSO4. 32H20). This ettringite is initially formed as very very finegrained crystals, which form a coating on the surface of the C3A particles (Figure 1). These crystals are too small to bridge the gaps between the particles particles of cement. cement. The cement cement mix therefore therefore remains plastic. After the initial hydration reactions of cement, a dormant period occurs during which the rate of hydration of cement is greatly reduced. During this this
QCL Group Technical Notes The Role of Gypsum in Cement
F igure 2: 2: Second stage - a further amount of ettringite
is formed on the C 3 A surface A full explanation for this dormant period is still a matter of some some conjecture. conjecture. Different mechanisms mechanisms have been proposed by researchers. The classical explanation is that the ettringite coating which covers the cement particles reduces contact between the cement and water thereby retarding retarding hydration. hydration. However, this theory is being modified, as more data becomes becomes available.
THE EFFECT OF GYPSUM SOLUBILITY
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QCL GROUP
The rate of the reaction between C 3A and gypsum depends on the availability of dissolved gypsum in the mixing water. Gypsum (CaSO42H20) is a chemically stable naturally occurring mineral of moderate solubility. When gypsum is heated however, it dehydrates and is converted to plaster of paris (CaSO4.½H20) and at higher temperatures anhydrite (CaSO 4). Both these dehydration products are significantly more soluble than gypsum. The dehydration of gypsum occurs at relatively low temperatures: 100oC to 160 oC. Such temperatures can occur during the cement grinding process. Strict temperature controls are therefore exercised during the grinding process. Sustained high temperatures during the grinding of cement can result in the formation of significant quantities of plaster of paris and anhydrite. These minerals react readily with water to recrystallise as gypsum. This process is termed a false set, which, contrary to a flash set, is reversible. Extended or further mixing causes the stiffened cement paste to revert to a plastic state.
OPTIMUM GYPSUM CONTENT
: Variation of setting time with gypsum content F igure 3 (6.2% C 3 A)
: Variation of setting time with gypsum content F igure 4 (13.2% C 3 A)
The optimum gypsum content for a cement is dependent on a number of factors, chief among these are the C 3A content, the reactivity of this C 3A (which is a function of process, conditions, and trace elements present such as alkalis, strontium, etc), the solubility of the gypsum, and milling temperatures. The effect of gypsum on the setting time of a cement is demonstrated in Figures 3 and 4. The effect of C3A content is also shown.
STRENGTH AND VOLUME STABILITY The addition of gypsum to cement increases the strength obtained and reduces shrinkage. However, the addition of excessive amounts of gypsum causes expansion to occur, which is why cement standards place a limit on the sulphate content.
: Figure 5
Effect of gypsum content (as SO 3 ) on compressive strength
The level of gypsum at which expansion becomes a problem is dependent on the C 3A content and fineness of the cement. Figures 5 and 6 show the effects of gypsum on compressive strength and s hrinkage.
: Figure 6
Effect of gypsum content (as SO 3 ) on
shrinkage
QCL Group Technical Notes The Role of Gypsum in Cement
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