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

Comparison of reaction degrees of slag and fly ash obtained by thermogravimetry and selective dissolution

The evolution of the reaction degree of supplementary cementitious materials (SCM) with time is an important aspect regarding the design and use of blended cements. However, this quantification is complex due to the multiple actions of SCM in the cementitious system, such as the filler effect, the consumption of CH and the formation of secondary calcium silicate hydrates. In this sense, different methods for the estimation of the reaction degree may be recommended for each type of SCM. Some of them are indirect methods that quantify hydration products and compute a reaction degree on that basis, e.g. thermogravimetric analysis, and others are direct methods that measure the amount of unreacted SCM, e.g. selective dissolution. In this paper, thermogravimetric analysis and selective dissolution are applied to estimate the reaction degree of cement pastes admixed with 40% slag or 30% fly ash. Curing periods ranged from 1 till 90 days. Results from both methods are compared and some comments on practicalities for their application are made. Although not enough evidence could be collected for any of the applied methods to be considered as a reference method, the simultaneous application of these two techniques offers increased reliability than when only one method is applied

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

On the quantitative thermogravimetric analysis of calcite content in hydrated cementitious systems

Calcite is a compound that is present in hydrated cementitious materials when carbonation of portlandite occurs or when limestone constituents are used. The quantification of its content in cementitious systems is then frequently necessary. Thermogravimetry (TG) measures the change in mass of a material (as a function of time) over a temperature range using a predetermined heating rate. It can be applied to estimate calcite content in the hydrated cement system, considering the temperature range at which it decomposes and releases carbon dioxide. However, the quantification is not easy because the onset of this decomposition is a function of many variables. The tangential method over the TG signal or the integration method over the derivative TG curve (DTG) are usually used to discount the background over the temperature range at which calcite decomposes. However, consistent underestimation of compounds is frequently described. The reasons for this are unclear and some hypothesis are discussed in this paper. Additionally, experimental quantitative TG of cement paste and aggregate containing calcite and diluted with low contents of analytical calcite are compared with the expected contents, as a calibration method regarding calcite content in the samples is given to improve the reliability of the results

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

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

Cuantificacion mineralogica de cementos mediante analisis rietveld de Drx. analisis cruzado de difractogramas experimentales y programas de refinamiento : Mineralogical quantification of cements determined by Rietveld analysis of XRD : Cross analysis of experimental pattern and software for refinement

Existen varias técnicas para conocer la composición mineralógica de los cementos, como las fórmulas de Bogue o el conteo con microscopía. En los últimos años la técnica que ha ganado mayor aceptación es la cuantificación mediante el método Rietveld para difracción de rayos X, por sus ventajas comparativas a los demás métodos (velocidad, precisión y la posibilidad de ser aplicado a cualquier tipo de cemento). Entre las variables que pueden afectar a la reproducibilidad de los resultados, se encuentra el conocimiento y la experiencia del operario que utiliza los programas de refinamiento. Para minimizar la incidencia del operario existen protocolos de refinamiento, a veces incluidos en el programa, o como en este caso elaborados manualmente. En este trabajo se analizaron muestras de cementos y adiciones minerales con dos programas de refinamiento y se compararon los resultados entre sí, y con ensayos de fluorescencia