Evaluación de la factibilidad de uso de arcillas provenientes de excavación de Bogotá, como material cementante suplementario (MCS) y su influencia en mezclas de concreto

Abstract

This research addresses the problem of fine excavation waste in the construction industry in Bogotá, capital of Colombia, proposing its use as a supplementary cementitious material through its transformation with a thermal treatment, in order to offer a sustainable and eco-friendly alternative in the sector. We start from the recognition that the inadequate management of these wastes generates significant environmental impacts, which makes it necessary to explore new strategies for their treatment and reuse. In experimental phase I, 115 samples of fine excavation soils were initially characterized by means of tests of liquid limit of soils INV E-125-13, plastic limit and plasticity index of soils INV E 126-13 and determination of particle sizes of soils INV E 123-13 and determination of natural moisture of soils INV E-122-13, taken from the Tunjuelo and Fiscala dump in Bogota, Colombia. A census of materials entering the dumps was carried out for one week and the samples were plotted with the test results on a geolocation map with QGIS with its attached image according to the soil classifications, Unified Soil Classification System (SUCS) and AASHTO (American Association of State Highway and Transportation Officials). In experimental phase II, 15 samples of fine excavation waste were taken taking into account the census carried out, these soil samples were characterized and the data were organized through the USC classification to achieve a better analysis of the results, the 15 samples were calcined at 850°C with a residence time of 3 hours, being characterized both calcined and uncalcined chemically, physically and mineralogically with 8 input parameters Granulometric curve Laser Below (45) μm (%), Fluidity of the mortar (mm), Specific gravity (Density) g/cm³, Loss on Fire % Burnt 950°C, Chemical analysis by RX fluorescence (SiO2 %, Al2O3 %, Fe2O3 %, CaO %, MgO %, SO3 %, Na2O %, K2O %) Amorphous Diffraction, Test to determine the fineness of cement by means of air permeability according to NTC 33 and two output variables (% compressive strength 1,3,7,28, days according to NTC 220 and pozzolanic activity index Zampieri method 1,3,7,28, days). The experimental design is 1 factor (temperature) with 2 levels in a randomized complete block design without replication, where the results showed significant effects with a confidence level of 95%. The free quartz content (%) was calculated through convolutional neural networks with a database of more than 750 clays in the area, where it was determined that only 2 samples range between 10% of free quartz and are of high interest, they could generate a material of high pozzolanic activity, 5 samples are between 10% and 30%, which are of interest for addition to concrete or as an additive for cement after a correct calcination and 8 samples that contain an undesirable amount of free quartz (fine sand), the option to use them as MCS would be to mix them before calcination with other materials that reduce this property, because it is an inert contaminant in terms of its functions within the cementitious matrix. In addition, the content of organic material was calculated where 5 samples ranged between 0% and 3% and 10 samples ranged between 4% and 24%, which defines that this type of waste has very high contents of organic material, this allows us to standardize this selection process in which Fine Excavation Waste of more than 10% of organic matter is necessary to mix them with other materials that decrease this % because it was observed that these materials when calcined release gases such as carbon dioxide (CO₂), in addition that it influences the kiln temperature, color, reactivity and efficiency in calcination processes. The pozzolanic activity was evaluated through the Zampieri method by measuring the ability of pozzolans to react with calcium hydroxide (Ca(OH)₂) in the presence of water, both calcined and uncalcined samples were tested, making 12 cubes per test failing at 1,3,7 and 28 days (3 replicates per day) in total were 360 cubes, mixed to 66. The fluidity was evaluated and the cubes were submerged at 60°c ±5° during the failure time. The results show a significant variability in the uncalcined samples and even most of them did not set on the 1st day, presenting values after 7 to 28 days, being moderate low results, determining that clays in their natural state have a limited capacity to react with calcium hydroxide and form cementitious compounds, in the calcined samples all show a significant increase in pozzolanicity with important values even on the first day of failure, its development in resistance is significant at 1,3,7 and 28 days, in most cases, determining that calcination at this temperature improves the pozzolanic reactivity of this fine excavation residue, taking into account that the samples with greater amount of SiO2 and free quartz are those that give lower pozzolanicity values. The % of pozzolanicity was determined with the compressive strength test according to NTC 220 with a proportion of 70% FID cement (fiber cement) and 30% addition (calcined and uncalcined samples) with 12 cubes per test, failing at 1, 3, 7 and 28 days (3 replicates per day of failure), a total of 720 cubes, taking into account that the pattern of the samples was carried out together with the addition sample. The pozzolanicity % was compared according to ASTM C618 as an artificial pozzolan where it must meet the SAI more than 75% where most of the calcined samples are above this value, only 1 sample was at 70% so 93.3% of the samples met the compressive strength and 46.6% of the samples are above 100% compared to the standard. Uncalcined materials are discarded as they significantly deteriorate the performance of the concrete. Regarding the mechanical activation by grinding method, a process that increases the pozzolanic activity, a blocking was carried out where the samples had to comply with a Laser Below (45) μm (97%) granulometric curve, where it was observed that the uncalcined samples present times that vary more significantly between 30 min and 45 min and in the calcined samples the grinding time is less, suggesting that the calcination process facilitates the grinding process because this treatment makes them harder, It significantly reduces plasticity and generates dehydroxylation in the materials, which is important due to the fact that at industrial level it is sought to optimize time and energy cost, in the same way it is concluded that the materials have a correct fineness and it is an option to increase the reactivity without modifying the thermal treatment but through the increase of the fineness of the material. Regarding the mineralogical composition variables, an exhaustive analysis of both XRD and XRF was carried out, crossing the information, it was determined that the results of amorphous phases vary from 11.79% to 23.79% of the samples analyzed, 26.6% are above 20% amorphous phase, in the correlation of the % of amorphous phase and % of pozzolanicity it was determined that . The % of clay compounds was analyzed, where it was determined that it varies from 6.99% to 24.4%, indicating that there is a significant difference in the amount of clay minerals present in the waste samples. it is possible to make a mining design in such a way that a good mix of materials can be guaranteed. In experimental phase III, the comparison of fine calcined excavation waste was carried out against 4 most relevant additions on the market in a concrete mixture at 10% substitution with respect to the weight of cement and maintaining the same design and materials for all the additions.Regarding the NTC 396 Initial settlement test, in comparison with the pattern, a loss of 2cm was obtained, however, in normalized workability it remains at 90%, in the NTC 673 compression resistance test at 1D (50%), 3D ( 100%), 7D (94%) and 28D (93%), as for the ASTM 237 air content test, the result was 3.8%, being the highest value. Regarding the density, the lowest of all the additions was observed with 2283 (kg/m3). This variable of % air and density influenced the compressive strength, the result of the water permeability test at the 56 days according to NTC 4483 was 29.8, a value very close to the standard which was 31.2, as for the ASTM 403 calorimetry test, most of the samples reach their peak temperature at 720 min (12 h), the Addition of RFE Calcined has the highest temperature peak, indicating greater heat release during hydration, influencing the development of early resistance and implying risks of cracking. Rapid Chloride Permeability Test (RCPT) The RFE gives 3549 Coulombs at 62 days and has the highest permeability to chlorides, with concrete being more susceptible to chloride penetration, which could reduce its durability in environments where reinforcement corrosion exists.