ACI MATERIALS JOURNAL TECHNICAL PAPER Hydration and Carbonation of Pozzolanic Cements Some physical and chemical characteristics of hardened cement con- crete, which influence its strength and durability, are given quantita- tively, in terms of the chemical composition of the pozzolanic ce- ment. The chemical reactions of pozzolanic activity and hydration are presented and used to determine the final quantity of CSH, calcium hydroxide, etc., produced per unit weight of pozzolanic cement and the minimum water-cement ratio required for hydration. Expressions are given for the final value of porosity of pozzolanic cement paste, mortar, or concrete, as affected by hydration, pozzolanic action, and carbonation, in terms of the composition of cement and of concrete or mortar mix. The effect of the pozzolans on the pore-size distribu- tion of hardened paste is studied experimentally, and that on the de- gree of poresaturation is studied both experimentally and analyti- cally. The effective gas diffusivity of pozzolanic cement mortar or concrete is measured, and a semi-empirical expression, developed earlierfor ordinary port/and cement (Ope) concrete, is fitted to the measured values. The applicability of a fundamental, yet simple, predictive model for concrete carbonation is verified experimentally, and its parameters are specified for pozzolanic cement concrete. Conclusions are drawn on the favorable or adverse effects of OPC pozzolan or aggregate pozzolan replacement on CSH content, poros- ity, and rate of carbonation. Keywords: carbonation; cements; concrete durability; concretes; corrosion; diffusion; flyash; hydration; pore-size distribution; porosity; pozzolan ce- ments; pozzolans; reinforcingsteels. Blended (or pozzolanic) cements have attracted consid- erable interest, from both scientific and technological points of view, since the beginning of this century. 1-4 Interest in them increased after tne first oil crisis, as the addition of 10 tons of pozzolan to the clinker results in a savings of I ton of fuel. 1,2Pozzolans are composed of the same main oxides as the clinker andordinary port- land cement (OPC) but in different proportions and mineralogical compositions. The main difference is in the relative proportion of the oxides which combine to produce the CSH, i.e., the main strength component of hydrated cement and of hardened pozzolan. Pozzolans are rich in Si0 2 and poor in CaO, whereas the opposite holds for clinker and OPC. Therefore, calcium hy- droxide is produced from the excess CaO during the hydration of OPC constituents and serves as the main source of alkalinity for the pozzolanic activity, i.e., for the conversion of the Si0 2 of the pozzolan into CSH. ACI Materials Journal I March-April 1992 Steel bars in reinforced concrete are protected from corrosion by a thin oxide layer that forms on their sur- face due to the highly alkaline environment of the sur- rounding concrete. The alkalinity of concrete is due to Ca(OH)2 produced by the hydration of OPC, but is gradually destroyed due to its conversion into CaC0 3 by the atmospheric CO 2 , This process, called carbona- tion, ultimatelY.,leads todissolution of the protective oxide layer on the steel bars and to reinforcement cor- rosion. The rate at which carbonation proceeds de- pends on the concentration of Ca(OH)2 in the concrete mass, as well as on that of other compounds, such as CSH, which contain CaO. These compounds may compete with the Ca(OH)2 in reacting with the ingress- ing CO 2 , Is it clear, therefore, that the interplay of the reactions involving or producing Ca(OH)2 and CSH in a pozzolanic cement affect not only the porosity and strength of hardened concrete but also its durability. The literature is rich in papers dealing with blended cements and the mechanical and physical properties of blended cement concrete. What is lacking is a quanti- tative description of the composition of a hardened pozzolanic cement and of the effect that this composi- tion has on important physical and chemical character- istics of the pozzolanic cement concrete. This paper aims at filling exactly this gap: Starting with a clear presentation of the chemical reactions that lead to the hardening of a pozzolanic cement, it develops quanti- tative expressions for the quantity of the reaction prod- ucts, such as CSH and calcium hydroxide, and for the water-cement ratio required for complete hydration. Expressions are then given forthe porosity of a hard- ened pozzolanic cement paste, mortar, or concrete, as ACI Materials Journal, V. 89,No.2, March-April 1992. Received Jan.29, 1991, and reviewed under Institute publication policies. Copyright © 1992,American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright propri- etors. Pertinent discussion will be published in the January-February 1993 ACI Materials Journal if received by Oct. 1, 1992.
Hydration and Carbonation of Pozzolanic Cements
May 20, 2023
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