Portland cement setting control

CONTROL OF PORTLAND CEMENT SETTING BY THE USE OF CHEMICAL ADMIXTURES... 

The production of commercial building materials, e.g., portland cement and stucco, utilizes calcium sulfate in the form of the dihydrate or hemihydrate.f Gypsum is added in the production of portland cement to control set. Properties of cement can be adversely affected by the formation of hemihydrate during the grinding process. In the production of stucco, the hemihydrate is the preferred form. Consequently, investigators have adapted thermal analysis methods for estimating the quantities of each phase in these products. 

Regulated-set cement and jet cement are modified Portland cements in which the normal aluminate phase is replaced by CuA CaF, through the use of a raw mix containing CaF,. Uchikawa and Tsukiyama (U2I) gave chemical (Table 10.4) and phase compositions of two jet cements. Botli contained approximately 60% of alite, 20% of C, i A CaF,. 1% of belite and 5% of ferrite. Admixtures are required to control the rate of reaction of the C, 1 A CaFj and the nature of the products. One of the cements included a proprietary retarder based on citric acid, togetlier with 2"/o of CaC O, . The other contained 2.5% of hemihydrale. In each case, Na,S04 (T o) and anhydrite were also present. The specific surface areas were around 550 m kg . 

Up to 5.1 % of gypsum may be added to hydraulic limes to control the setting process, although it is not normally required when ordinary Portland cement is added. 

Use of gypsum in order to reduce the rate of calcium aluminate ration with water, as a set controlling agent is as old as Portland cement productioa It is commonly accepted that the CjA hydration is hampered by the ettringite layer covering the aluminate crystals and forming an impermeable barrier [30], This old hypothesi s has been proved recently [31-33]. 

The time for which the suspension preserves its flowabilily rrray vary between a few minutes and several hours, and may be controlled by the amoimt of added additives (Harma, 1977 Odler et al, 1978 Satava and Tyle, 1994). In general, the setting time is extended with an increasing amount of lignosirUbnate, whereas the amoimt of carbonate has little effect on this behavior. Unlike the behavior of ordinary Portland cement pastes, the transition from the plastic to the set state occurs very suddenly. 

The Portland clinker used should contain a high amount of tricaldum silicate, preferably more than 45%. This is necessary as the hydration of this phase produces the calcium hydroxide needed for a pozzolanic reaction of the ash. The hydration of the clinker minerals is mainly responsible for the setting and initial strength development of the cement, as the reaction rate of the fly ash is rather slow. The lydration of the ash contributes to strength only at longer hydration times, but also affects other properties of the hardened material. The calcium sulfate added in the form of gypsum or anhydrite serves to control the setting of the fresh paste in a similar way as in plain Portland cement. 

Subsequently, the high-almnina clinker is crashed and ground to a fine powder in a ball mill. The clinker is very hard to grind, and the process is associated with a high power consumption. Unlike Portland cement, no set controlling constituents, such as gypsum, need to be intergroimd with the burnt material. 

Introduction of phosphogypsum as an additive to cement accounts for about 4 million tonnes per annum. Portland cement usually contains about 5% gypsum, which is added to control (retard) the setting time. Phosphate, fluoride, and organic impurities in the phosphogypsum are undesirable and should be reduced to acceptable levels. Material from double crystallization phosphoric acid processes is satisfactorily employed in Belgium and Japan. 

Portland cement is made from limestone or other natural sources of calcium carbonate, clay (a source of silica), alumina, ferric oxide, and minor impurities. After grinding, the mixture is fired in a kiln at approximately 2500°F/1137°C. The final product is ground to a fineness of about 10 pm, and mixed with gypsum to control setting. When mixed with water, the portland cement forms a hydrated phase and hardens. As the cement hardens, chemical reactions take place. The two most important reactions are the generation of calcium hydroxide and of tricalcium silicate hydrate. The calcium hydroxide generated could theoretically... 

Salt derivatives for setting time control in Portland cements... 

In Portland cement, the hydration of the C3A phase is controlled by the addition of gypsum. The flash set is thus avoided. The C3A phase reacts with gypsiun in a few minutes to form ettringite as follows ... 

The exact mechanism by which gypsum addition to portland cement controls setting behavior is not known although much work has been reported in this field. It is clear, however, that interactions between C3A and gypsum are integral to the stiffening and hardening processes. 

The use of g3 sum to control setting in Portland cement accounts for considerable quantities of the use of this material.1 Control of the reaction rate of tricalcium aluminate (C3A),the constituent of cement that reacts most rapidly with water, is most commonly achieved through the addition of gypsum (sometimes hemihydrate is also used) to commercial Portland cement. This material is normally added to the cement clinker (in amounts of approximately 2 to 3%) before grinding. The cement manufacturers usually specify a sulfurtrioxide content of about 36%. Excess sulfate in the form of hemihydrate can cause flash set in portland cement. 

---