Fresh Properties of Mortar made with Pozzolanic Cement and Water Reducers
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Fresh Properties of Mortar made with Pozzolanic Cement and Water Reducers María E. Sosa1, Yury A. Villagrán-Zaccardi1,2, Claudio J. Zega1,2 and Juan P.Peralta1 1 LEMIT, 52 entre 121 y 122, 1900 La Plata, Argentina 2 CONICET, Argentina ABSTRACT Natural pozzolans are supplementary cementitious materials (SCMs) that may be used to improve the properties of mortar and concrete, through the formation of additional hydration products by pozzolanic action. Water reducers (WR) play a main role in high performance concrete in terms of durability, strength and surface finishing. A first optimization of constituent proportions in paste and/or mortar is convenient to assess the compatibility between the WR and the cementitious materials. The compatibility between cement and WR may be affected by SCMs, as they can also interact with the molecules of the admixture. However, the practical implication may be variable. This paper deals with the influence of different types and dosages of WRs in mortars made with pozzolanic Portland cement. Both medium and high ranges WRs have been used. Mortar fluidity has been tested by the spread and the slump tests. Results show different fluidizing capacities of WRs, among which polycarboxylate-based WR was the most compatible with the pozzolanic cement.
INTRODUCTION The use of water reducers (WRs) is essential in high performance concrete production. This type of admixture not only modifies fresh properties of concrete, but also hardened properties when reducing mixing water and/or cement contents. Besides they allow contents of supplementary cementitious materials (SCMs) as they improve flowing. WRs act by being adsorbed on the surface of cement particles. Then, these particles are dispersed and the demand for mixing water is reduced for certain workability. According to their composition, WRs may be based on lignosulfonates, melamine formaldehyde, naphthalene formaldehyde or polycarboxylates. The dispersing action of the first three admixtures is by chemical effect, whereas polycarboxylates work by steric action [1]. The effectiveness of the WR is connected with its capacity for increasing flowing and maintaining its level over time. This depends on the characteristics of both the admixture and the cement, and the manner in which they are mixed (mixer capacity, manner of inclusion, mixing time), as well [2]. Pozzolanic Portland cement (PPC) contains up to a 50% of hydraulic natural pozzolan. This admixture additive does not necessarily modify mixing water demand for a certain flow level [3]. Several characteristics of the cement such as fineness, contents of aluminate, sulfate, alkalis, and SCM, and other related to the WR such as molecular weight and its distribution, chemical composition, structure and nature of the counter ion, have main impact on the performance of the system cement-WR [4-5]. Anhydrous and hydrated products of cement do not adsorb superplasticizer equally [6]. C3A phase has higher adsorption capacity of admixtures because it has more positive zeta potential than C3S and
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