Chemical admixture-cement interaction

General Thoughts About Chemical Admixture-Cement Interactions... 

Jolicoeur C, Simard MA. Chemical admixture-cement interactions Phenomenology and physico-chemical concepts. Cement Concrete Comp 1998 20 87-101. 

Modem concretes often incorporate a mixture of chemical and mineral admixtures, each of which may interact with the various constituents of cements and influence cement hydration reactions. The admixture-cement interactions may in fact be viewed as the reaction between two complex chemical systems - the multicomponent, multiphasic inorganic materials in the cement and the organic compounds of multicomponent admixture systems. For example, lignosulfonate water-reducers are intrinsically complex mixtures of chemical compounds derived from the chemical degradation of lignin, while synthetic admixtures such as superplasticizers contain species with a broad distribution of molecular weights, reaction products, or other chemicals added for a specific purpose [125]. The performance of an admixture in concrete is highly dependent on many... 

In the following sections, physicochemical concepts which apply to admixture-cement interactions and the manner in which chemical admixtures can interfere in the cement hydration process and in particle- particle interactions are described. 

Reports of incompatibility between Portland cement and chemical admixtures have increased over the past 10 years. Rapid set, accelerated stiffening, increase in time of set, lack of water-reduction are some of the reported problems. Such effects produced by the interaction of the composition of the cement and that of the chemical admixtures has been often referred to as cement-admixture incompatibility . This is discussed below. 

Hewlett, P.C. and Young, J.F. (1983) Physico-chemical interactions between chemical admixtures and Portland cement. J. Mater. Educ., 9 (4), 389-435. 

The volume fraction of polymeric admixtures should be always adapted to the purpose and its results carefully checked from various viewpoints possible appearance of negative effects, material incompatibiUty, etc. Independently of the kind of admixture, some modifications in the cement hydration may be expected, because chemical and physical interactions occur. 

The particle size and SSA of a cement powder have a direct influence on hydration kinetics. This is illustrated in Figure 10.1, as the particle size of alite (main phase of cement) decreases, and consequently SSA increases, a significant acceleration of the hydration is observed. In addition, these parameters have a direct impact on the rheological and mechanical properties of concrete and on the interaction between the cement and chemical admixtures such as superplasticisers. Both parameters are routinely measured during cement production for quality control purposes. 

The admixture may remain in a free state as a solid, in solution, interact at the surface, or chemically combine with the hydrates. Physicochemical and mechanical properties of the concrete may be influenced by the type and extent of the interaction. Thus, the early hydration reactions of cement may be affected in diverse ways and it is possible that more than one effect occurs at the same time. These are sununarized below [125]. 

In principle, all the concepts of surface chemistry previously described are also expected to be vahd for cement particles. However, the highly dissolving and reactive nature of such particles introduces a dynamic and complex feature to the particle-water system. This makes it difficult to interpret the data obtained from cement-containing suspensions, which is probably the main reason for the paucity of publications available in the literature that focus on the interactions between additives and high-alumina cement particles. The concepts described herein are therefore based on sffidies performed in aqueous pastes of calcium silicate (Portland) cement, which have been more extensively researched in recent years due to their considerable importance for concrete technology (47-50). Although calcium aluminate and calcium silicate cements are known to display distinct reaction rates and form different hydration products, similar interactions between the cementious phase and the chemical additives (admixtures) are expected to occur in aqueous suspensions. 

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