Effect of supplementary cementitious materials on capillary sorption : relation with drying rate and testing time

The water sorption phenomenon in a cementitious matrix is responsible for the ingress of several deleterious agents, and it is directly related to pore connectivity and pore volume. One of the most common tests used to describe this mechanism is the capillary sorption test. Furthermore, the drying rate (DR) is a process that strongly depends on the transport properties and also provides valuable information related to porosity and durability. Supplementary cementitious materials (SCMs) are known to enhance durability-related properties, especially pore refinement with time due to the pozzolanic action. Therefore, changes in the pore structure could be assessed by means of the capillary sorption and drying rate. For this study, mortars with ground granulated blast-furnace slag, natural pozzolan and limestone powder at three different levels of replacement were made and the capillary sorption test was performed at 28 and 90 days. Weight loss was also assessed at 28 days and the DR was calculated. Calculation of the weight gain, weight loss, DR and capillary sorption rate (CSR) is made considering the stoppage of the test at different stages. A comparison between different approaches in the calculations is made. Also, the relation of DR and CSR is assessed. Results show the effect of SCMs with time, and also the influence of the calculations on the CSR and DR values

Recycled powder as filler admixture in cementitious systems : production and characterization

In concrete production, aggregate represents almost 75% of the materials used. The exploitation of natural sources for this purpose also causes an environmental impact, while deposition of wastes from construction industry pollutes soil and water. The feasibility of recycled coarse aggregate used as component of concrete has been amply proved. Fine recycled aggregate is a by-product derived from the processing of recycled coarse aggregate, but there are some technological difficulties for its use in concrete, because of the high water absorption and powder content. The aim of this study is to propose and analyse the alternative use of milled recycled fine aggregate as mineral admixture. For this purpose, dry recycled fine aggregate was processed in a laboratory ball mill for cement, with the aid of cylpebs. Grinding was carried out for 2:00, 2:45 and 3:30 hours, and the sampling of ground recycled fine aggregate (GRFA) was done after each period. For the three grindings periods, the characterization of GFRA was performed for assessing its suitability as mineral filler. Tests included determinations of contents of material < 45 μm and < 75 μm by wet sieving, density, particle size distribution by laser diffraction, chemical composition and Blaine specific surface. Additionally, water demand for standard consistency paste with ordinary Portland cement (OPC) was prepared as reference, and also determined for pastes with replacement of 15% and 30% of cement by GRFA for each grinding period. Also, setting times and strength were measured. Results showed limited filler effect from GRFA when incorporated in cementitious materials, with the consideration of the grinding period as an important factor. Thus, further feasibility studies are necessary in order to investigate different potential applications of this ground material

Lucas-Washburn vs Richards equation for the modelling of water absorption in cementitious materials

The Lucas-Washburn equation is still being applied by a significant number of researchers for the modelling of water absorption in cementitious materials. A modern approach considers the extended Darcy's law leading to the Richards equation instead. Three main assumptions are implied by the application of the Lucas-Washburn equation: the flow occurs in one direction only, the material is separated into one fully wet and one fully dry region, and pores are modelled as an assembly of parallel tubes of a particular radius. Its application to analyse experimental results allows defining these three assumptions as mere simplifications. Therefore, all the parameters comprised in the Lucas-Washburn model are apparent. Consequently, a very limited description of the transport properties of the material can be achieved. For many engineering purposes this would not be an issue, but for an intrinsic description of the material a more realistic model is required. This paper discusses the limitations of the Lucas-Washburn equation, and the advantages of the Richards equation regarding the modelling of water absorption in cementitious materials. The comparative analysis reveals the versatility of the Richards equation, with an approach that considers the material as a continuum and describes it through measurable parameters

Pore refinement action of GGBFS and fly ash on the primary and secondary capillary imbibition rates of concrete

Capillary imbibition is a transport phenomenon occurring in concrete structures exposed to weathering, frequently in direct connection with the resistance against different deterioration processes. This property depends on the volume and connectivity of pores. For ground granulated blast-furnace slag (GGBFS) and fly ash blended concrete mixes, the pore refining action of these supplementary cementitious materials plays a positive role in the disconnection of the capillary porosity and consequent reduction of the capillary imbibition rate. Moreover, for this particular transport process, primary and secondary transport rates can be defined in connection with different driving mechanisms. This allows a complementary description of the pore structure of concrete. In this paper, blended concrete mixes were prepared by substituting 20, 40 and 60 % of OPC by GGBFS, and 20, 30 and 40 % of OPC by fly ash. The pore structure of these concretes is assessed by water absorption under vacuum and mercury intrusion porosimetry after curing periods of 28 and 90 days. Long-term capillary imbibition tests were also performed and primary and secondary imbibition rates are computed by a novel approach that considers their linear evolution with the fourth root of time. Results show the refinement action of GGBFS and fly ash by a softening in the transition stage between the primary and secondary imbibition periods. A low water flow rate is consequently correlated with the increased tortuosity of samples

Trainable Weka Phase segmentation of SEM/BSE images of slag blended cement pastes

Scanning electron microscopy with backscattered electrons (SEM/BSE) is a powerful technique that allows the visualization of polished cross sections with good reproducibility and level of detail. It is widely used to study the microstructure of cement-based materials and identify different phases in the cement paste. However, in some cases it is difficult to distinguish between some phases due to a similar grey level, as in the case of slag and portlandite. Then, X-ray elements mapping is necessary to help in the differentiation according to composition, but it can be quite time consuming and tedious with standard detectors. A machine learning tool, trainable WEKA segmentation (TWS), can be used to train a classifier by means of pixel grey values and segment the different phases automatically without any assistance of compositional mapping, transforming the problem into a pixel classification issue. The trained models can be improved by adjusting each class. The application of the model to the images results in a segmented image that can be used for quantification. In this paper TWS is applied for segmenting SEM/BSE images without the need of elements mapping. Slag blended cement pastes at different ages are studied. Results are compared with image analysis through elements mapping and selective dissolution. From this comparison, some information regarding the image density of the portlandite is derived

Insight into the secondary imbibition rate of concrete and its relationship with curing time

Long‐term capillary imbibition in cementitious materials is relevant to describe their durable behaviour. After several weeks, when capillary rise is no longer progressing, a distinguished change in the slope (from the mass gain ‐ fourth root of time relation) is noticed. Primary and secondary imbibition rates (PIR and SIR, respectively) can be obtained from such behaviour. Contrary to the widely investigated PIR, usually defined as the capillary absorption rate, the SIR has been scarcely examined. To evaluate this unexplored process, we contrasted values of SIR and porosity. Moreover, using statistical inference we evaluated the evolution of the SIR of concrete mixes with different composition and curing age. This paper presents the effect of curing on the SIR and the relation between porosity changes and SIR. Despite the decrease in capillary and intrudable porosity, SIR increases with curing time having an opposite evolution than the PIR. Another interesting finding is this increase is irrespective of mix composition (including cement type, w/b ratio, aggregate amount, presence of SCMs). Results show that the PIR and SIR are influenced by different mechanisms with dissimilar correlations with porosity and curing time.Fil: Alderete, Natalia Mariel. University of Ghent; Bélgica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Área Tecnología del Hormigón; ArgentinaFil: Villagrán Zaccardi, Yury Andrés. University of Ghent; Bélgica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Área Tecnología del Hormigón; ArgentinaFil: de Belie, Nele. University of Ghent; Bélgic

Relationship between concrete resistivity and the indication of chloride penetration by ASTM C1202 in concrete made with OPC and admixed with slag and/or limestone powder

One of the most widespread methods to assess concrete ability to resist chloride penetration is the one described in ASTM C1202. This method consists in the determination of the amount of electrical charge that passes through a concrete sample in a 6 h period, whereas an electrical potential is applied. This highly empirical method has received some criticism due to its lack of representativeness of the actual process of chloride ingress into concrete. Moreover, the result of the test is a qualitative value that cannot be used for service life design based on performance. In this sense, even more practical methods can be considered with the same aim. This paper shows experimental results from the application of the method in ASTM C1202 and the measurement of resistivity in both conventional concrete and concrete admixed with slag and/or limestone powder, as these constituents significantly affect conductivity. A correlation between results from both methods is made, and some considerations are presented regarding the practicality of applying one method or the other in relation with the information they provide. The results reveal that the correlation between resistivity and results from ASTM C1202 is independent from the composition of the concrete

Influence of particle size distribution of slag, limestone and fly ash on early hydration of cement assessed by isothermal calorimetry

Supplementary cementitious materials (SCMs) enhance early hydration of cement through the provision of nucleation sites for the precipitation of hydration products. This effect contrasts with the dilution effect of the mineral addition to the cementitious system. In fact, the net effect on heat release during the first hours of hydration is the reduction of peak intensity. Then, only when the specific heat is evaluated in terms of cement weight the actual effect of the admixture on hydration may be assessed. In this sense, a clear difference may be expected in accordance with the replacement ratio and fineness of the SCM. Increased specific surface favors nucleation and it therefore enhances hydration. In this paper, cement pastes admixed with slag, limestone and fly ash in three different fineness levels and three replacing ratios were investigated through isothermal calorimetry. The filler effect of each SCM was in relation with the fineness and replacing ratio, with higher impact on the aluminate reaction than on the silicate reaction of cement

Neutron radiography based visualization and profiling of water uptake in (un)cracked and autonomously healed cementitious materials

Given their low tensile strength, cement-based materials are very susceptible to cracking. These cracks serve as preferential pathways for corrosion inducing substances. For large concrete infrastructure works, currently available time-consuming manual repair techniques are not always an option. Often, one simply cannot reach the damaged areas and when making those areas accessible anyway (e.g., by redirecting traffic), the economic impacts involved would be enormous. Under those circumstances, it might be useful to have concrete with an embedded autonomous healing mechanism. In this paper, the effectiveness of incorporating encapsulated high and low viscosity polyurethane-based healing agents to ensure (multiple) crack healing has been investigated by means of capillary absorption tests on mortar while monitoring the time-dependent water ingress with neutron radiography. Overall visual interpretation and water front/sample cross-section area ratios as well as water profiles representing the area around the crack and their integrals do not show a preference for the high or low viscosity healing agent. Another observation is that in presence of two cracks, only one is properly healed, especially when using the latter healing agent. Exposure to water immediately after release of the healing agent stimulates the foaming reaction of the polyurethane and ensures a better crack closure