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Hardened cement paste is a heterogeneous system resulting from the grouping of particles, films, microcrystals and other solid structural elements bounded in a porous mass. The cement paste microstructure must be understood firstly due to its influence on concrete properties. The behaviour of concrete greatly depends on the conformation of localised special structures rather than on general structures found in the mass cement paste. The objective of this paper was to study the cement paste microstructure, as a function of the water–cement ratio, in order to interpret the variations of the steel–mortar bond strength and the developing of the corrosion process in steel–mortar specimens kept in tap water and 3 percent sodium chloride solutions for 1 year. A description of the steel–mortar interface was also provided.

Ordinary portland cement can be replaced, partially, by mineral admixtures which modify the properties of concrete and influence rebar corrosion. The most common mineral admixtures are puzzolans, granulated blast furnace slag, calcareous filler, fly ash with high and low lime content, condensed silica fumes, and rice hush ash. There is an increasing tendency to incorporate carbonate additions to mortars. The beneficial effect of calcareous filler addition has been long discussed although not many studies have been made on the effect of this addition on rebar corrosion. In this research, rebar corrosion in mortar containing calcareous filler was studied employing two water/cement (w/c) ratios: 0.50 and 0.65, respectively. It was found that rebar corrosion became important as the w/c increased. The presence of calcium carbonate altered the structure of the passive layer on rebars.

RESUMENEl aumento del consumo de las adiciones minerales activas en los materiales cementfceos contemporáneos ha determinado la revisión de algunos métodos de ensayo utilizados para determinar sus propiedades.En la evaluación de la durabilidad de los cementos compuestos, muchos ensayos de corta duración (de gran aplicación en cementos portland) dejan de tener validez, pues no permiten evaluar las mejoras que producen los materiales puzolánicos.El método propuesto por KOCH & STEINEGGER (1960) aparece como uno de los más apropiados para determinar el comportamiento de cementos con adiciones minerales activas frente al ataque de sulfatos.En este trabajo se presentan los resultados alcanzados con ente ensayo en la determinación del comportamiento de un cemento portland normal (CRN) y uno resistente a los sulfatos (CPARS), adicionados con ceniza volante de bajo contenido en óxido de calcio. La ceniza se incorpora con tres finuras (280,. Estos resultado indican que esta adición mejora notablemente la resistencia a los sulfatos del CPN y en menor medida la del cemento ARS. La finura influye en la evolución de la resistencia mecánica en agua y en la resistencia química en las primeras edades. SUf^MARYThe increase of active mineral admixtures consumption in contemporaneous cementiceous materials has stablished revision of some test methods.In the evaluation of blended cement durability, many accelerated tests of large application in portland cements become unvalid, because they don't allow to value the improvements produced by pozzolan materials.Koch-Steinegger Method appears as the most appropiate to evaluate sulfate resistance of cement with active mineral admixtures.In this paper are presented the results obtained with this test in the evaluation of an ordinary portland cement (CPN) and one resisting sulfates (CPARS), with low calcium fly ash addition. Fly ash is incorporated with three fineness (280, 420 and 480 m^/Kg Blaine). The results show that this addition improve sulfate resistance of CPN and in minor way of ARS cement. Fly ash influences evolution of mechanical strength in water and chemical resistance at first ages. PALABRAS CLAVE:Resistencia a los sulfatos, ceniza volante, ensayos acelerado, mortero, durabilidad.