Relationship between Microstructure, Durability and Performance of CEM X Composite Cements

Cement production is a major contributor to anthropogenic CO2 emissions. Composite slag cement is one way of reducing the clinker factor, and hence emissions. Synergies between limestone and slag have been suggested to enhance the performance of such cement. The synergistic effect on mechanical performance has been ascribed to the carboalumination formation, which in turn prevents the conversion of ettringite to mono sulphoaluminate. Whilst the benefits of the synergy are documented, causation factors are not fully understood. In order to maximise the synergy, a clear understanding of the relationship between composition and the carboalumination-ettringite balance and how this impacts on performance is essential. Consequently, this study focused on the kinetics of reaction, microstructure and the implications on strength development and the freeze-thaw durability. The QXRD/PONKCS method proposed elsewhere has been extended to monitor the residual slag contents in ternary blended systems. The slag phase was modelled from the x-ray diffraction pattern of a 100 % slag specimen. The model was then calibrated on a 50:50 mixture of slag and corundum using the internal standard method. The measurement accuracy was found to be ± 2 % following implementation of the calibrated slag phase on simplified slag-corundum mixes of varying proportions. In hydrated cement systems, however, the accuracy established from the comparison between the results from QXRD/PONKCS and SEM/IA analysis was ± 6 %. The investigated hydration stopping methods except freeze-drying had minimal impact on the results as long as the mass attenuation coefficient (MAC) was accurately quantified. For implementing the QXRD/PONKCS method, freshly ground non-hydration stopped sample preparation is recommended. This method presented the least challenge in calculating the MAC and also preserve the other hydrates particularly the AFt/AFm phases. However, if hydration stopping cannot be avoided, a double solvent exchange regime using isopropanol and diethyl ether is recommended. The residual slag content is overestimated from freeze-dried samples. Multiple techniques including the QXRD/PONKCS method, SEM/IA, isothermal conduction calorimetry, chemical shrinkage and thermal analysis were subsequently used to investigate the kinetics of hydration. These revealed that limestone accelerates slag and clinker hydration. Alite hydration is accelerated by the supplementary cementitious materials including slag and quartz. However, while belite hydration is retarded in the binary slag mix, this is not the case in limestone ternary blends. Calcium carbonate also reacts with additionally dissolved aluminates instead of the latter converting ettringite to mono sulphoaluminate. Therefore, the synergy between limestone and slag is not only due to preserved ettringite but the additional hydrates and pore structure modification. The carboaluminate content is higher in the presence of limestone but the home to mono carboaluminate balance shifts with the limestone content. The aluminates available for incorporation into other hydrates and in the pore solution are lowered by carboaluminate formation. The pore structure and pore solution chemistry were analysed in order to further investigate the mechanisms for the accelerated dissolution of slag and the major clinker phases. The overall pore volumes as measured by MIP were similar in the slag containing mixes with or without limestone. Evaluation of the free water content at the end of experiment also indicated excess free water thus indicating that the pore structure and capillary pore water are not the limiting factors for slag hydration in the investigated mixes. The identified drivers behind the role of limestone in the synergy are the nucleation and dilution effects in the case of clinker, while for slag hydration pore solution effects are important. The incorporation of alumina into carboaluminates and C-S-H lowers the alumina in the pore solution and consequently promote slag dissolution. The freeze-thaw resistance of the ternary blends at the investigated 0.5 water/binder ratio is low compared to the CEM I 42.5 R concrete. Decalcification through carbonation during conditioning and leaching during freeze-thaw is identified to be the dominant microstructural difference between degraded and non-degraded samples. A chemo-mechanical degradation mechanism, which is similar to the glue-spall mechanism, is hypothesised for surface scaling during freeze-thaw. By this, freeze-thaw damage in concrete is caused by decalcification and spalling of successive layers under induced stresses of ice growth

Relationship between Microstructure, Durability and Performance of CEM X Composite Cements

Cement production is a major contributor to anthropogenic CO2 emissions. Composite slag cement is one way of reducing the clinker factor, and hence emissions. Synergies between limestone and slag have been suggested to enhance the performance of such cement. The synergistic effect on mechanical performance has been ascribed to the carboalumination formation, which in turn prevents the conversion of ettringite to mono sulphoaluminate. Whilst the benefits of the synergy are documented, causation factors are not fully understood. In order to maximise the synergy, a clear understanding of the relationship between composition and the carboalumination-ettringite balance and how this impacts on performance is essential. Consequently, this study focused on the kinetics of reaction, microstructure and the implications on strength development and the freeze-thaw durability. The QXRD/PONKCS method proposed elsewhere has been extended to monitor the residual slag contents in ternary blended systems. The slag phase was modelled from the x-ray diffraction pattern of a 100 % slag specimen. The model was then calibrated on a 50:50 mixture of slag and corundum using the internal standard method. The measurement accuracy was found to be ± 2 % following implementation of the calibrated slag phase on simplified slag-corundum mixes of varying proportions. In hydrated cement systems, however, the accuracy established from the comparison between the results from QXRD/PONKCS and SEM/IA analysis was ± 6 %. The investigated hydration stopping methods except freeze-drying had minimal impact on the results as long as the mass attenuation coefficient (MAC) was accurately quantified. For implementing the QXRD/PONKCS method, freshly ground non-hydration stopped sample preparation is recommended. This method presented the least challenge in calculating the MAC and also preserve the other hydrates particularly the AFt/AFm phases. However, if hydration stopping cannot be avoided, a double solvent exchange regime using isopropanol and diethyl ether is recommended. The residual slag content is overestimated from freeze-dried samples. Multiple techniques including the QXRD/PONKCS method, SEM/IA, isothermal conduction calorimetry, chemical shrinkage and thermal analysis were subsequently used to investigate the kinetics of hydration. These revealed that limestone accelerates slag and clinker hydration. Alite hydration is accelerated by the supplementary cementitious materials including slag and quartz. However, while belite hydration is retarded in the binary slag mix, this is not the case in limestone ternary blends. Calcium carbonate also reacts with additionally dissolved aluminates instead of the latter converting ettringite to mono sulphoaluminate. Therefore, the synergy between limestone and slag is not only due to preserved ettringite but the additional hydrates and pore structure modification. The carboaluminate content is higher in the presence of limestone but the home to mono carboaluminate balance shifts with the limestone content. The aluminates available for incorporation into other hydrates and in the pore solution are lowered by carboaluminate formation. The pore structure and pore solution chemistry were analysed in order to further investigate the mechanisms for the accelerated dissolution of slag and the major clinker phases. The overall pore volumes as measured by MIP were similar in the slag containing mixes with or without limestone. Evaluation of the free water content at the end of experiment also indicated excess free water thus indicating that the pore structure and capillary pore water are not the limiting factors for slag hydration in the investigated mixes. The identified drivers behind the role of limestone in the synergy are the nucleation and dilution effects in the case of clinker, while for slag hydration pore solution effects are important. The incorporation of alumina into carboaluminates and C-S-H lowers the alumina in the pore solution and consequently promote slag dissolution. The freeze-thaw resistance of the ternary blends at the investigated 0.5 water/binder ratio is low compared to the CEM I 42.5 R concrete. Decalcification through carbonation during conditioning and leaching during freeze-thaw is identified to be the dominant microstructural difference between degraded and non-degraded samples. A chemo-mechanical degradation mechanism, which is similar to the glue-spall mechanism, is hypothesised for surface scaling during freeze-thaw. By this, freeze-thaw damage in concrete is caused by decalcification and spalling of successive layers under induced stresses of ice growth

Effect of synthesis parameters on the performance of alkali-activated non-conformant EN 450 pulverised fuel ash

The fly ash reported in this paper is coarser than conventional pulverised fuel ash (PFA), with loss on ignition (LOI) exceeding 10.8%. Consequently, it is precluded from being used as a supplementary cementitious material (SCM) according to EN 450 and disposed in landfills. Alkali-activation of such PFAs is considered here. Three concentrations of sodium hydroxide (NaOH) were separately blended with water glass at different ratios to modify the silica modulus. Heat of reaction, setting time, compressive strength and drying shrinkage were investigated as a function of activator composition. Specimens were either cured at room temperature or hydro-thermally treated at 75 °C for five hours. The results show that by optimizing the activator composition, a binder with a 28 day compressive strength of 25 MPa can be synthesised from such PFAs even at room temperature. Among the activator parameters, the alkali content was observed to be most influential

SARS-CoV-2 infections among asymptomatic individuals contributed to COVID-19 cases: A cross-sectional study among prospective air travelers from Ghana

BackgroundThe spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) by asymptomatic individuals has been reported since the early stages of the coronavirus disease 2019 (COVID-19) outbreak in various parts of the world. However, there are limited data regarding SARS-CoV-2 among asymptomatic individuals in Ghana. The aim of the study was to use test data of prospective travelers from Ghana as a proxy to estimate the contribution of asymptomatic cases to the spread of COVID-19.MethodsThe study analyzed the SARS-CoV-2 PCR test data of clients whose purpose for testing was classified as “Travel” at the COVID-19 walk-in test center of the Noguchi Memorial Institute for Medical Research (NMIMR) from July 2020 to July 2021. These individuals requesting tests for travel generally had no clinical symptoms of COVID-19 at the time of testing. Data were processed and analyzed using Microsoft Excel office 16 and STATA version 16. Descriptive statistics were used to summarize data on test and demographic characteristics.ResultsOut of 42,997 samples tested at the center within that period, 28,384 (66.0%) were classified as “Travel” tests. Of these, 1,900 (6.7%) tested positive for SARS-CoV-2. The majority (64.8%) of the “Travel” tests were requested by men. The men recorded a SARS-CoV-2 positivity of 6.9% compared to the 6.4% observed among women. Test requests for SARS-CoV-2 were received from all regions of Ghana, with a majority (83.3%) received from the Greater Accra Region. Although the Eastern region recorded the highest SARS-CoV-2 positivity rate of 8.35%, the Greater Accra region contributed 81% to the total number of SARS-CoV-2 positive cases detected within the period of study.ConclusionOur study found substantial SARS-CoV-2 positivity among asymptomatic individuals who, without the requirement for a negative SARS-CoV-2 result for travel, would have no reason to test. These asymptomatic SARS-CoV-2-infected individuals could have traveled to other countries and unintentionally spread the virus. Our findings call for enhanced tracing and testing of asymptomatic contacts of individuals who tested positive for SARS-CoV-2

Influence of limestone on the hydration of ternary slag cement

The hydration kinetics, microstructure and pore solution composition of ternary slag-limestone cements have been investigated. Commercial CEM I 52.5 R was blended with slag and limestone; maintaining a clinker to SCM ratio of 50:50 with up to 20% slag replaced by limestone. The sulphate content was maintained at 3% in all composite systems. Hydration was followed by a combination of isothermal calorimetry, chemical shrinkage, scanning electron microscopy, and thermogravimetric analysis. The hydration of slag was followed by the implementation of QXRD/PONKCS method. The accuracy of the calibrated PONKCS phase was assessed on slag and corundum mixes of varying ratios, at different w/s ratios. Thus, the method was used to analyse hydrated cements without dehydrating the specimens. The results show that the presence of limestone enhanced both clinker and slag hydration. The pore volume and pore solution chemistry were further examined to clarify to the synergistic effects. The nucleation effects account for enhanced clinker hydration while the space available for hydrate growth plus lowering of the aluminium concentration in the pore solution led to the improved slag hydration

Impact of hydration stoppage on quantification of the GGBS content in ternary limestone cements using the PONKCS method

Most supplementary cementitious materials (SCMs) are predominantly poorly-crystalline or amorphous. Their reactivity is routinely evaluated through SEM image analysis (SEM/IA), which is a laborious and resource intensive technique. Quantitative X-ray diffraction (QXRD) provides an alternative, facilitating simultaneous evaluation of reaction kinetics and phase assemblages. However, QXRD requires relevant model structures, which are lacking for amorphous phases. In this study, we use the Phases of No Known Crystal Structures (PONKCS) method to model and calibrate ground granulated blast furnace slag (GGBS) and assess robustness in quantifying the GGBS content in synthetic and hydrated ternary blended CEM I-GGBS-limestone cements. Implications of sample preparation, in particular hydration stopping methods on the quantification was measured via the external standard method. Subsequently, the results are compared with SEM/IA calculations, based on backscattered images and magnesium maps. Robustness of the calibrated PONKCS phase is demonstrate with and without hydration stopping. However, X-ray absorption by the cements must be accounted in the attenuation co-efficient calculation. Freeze-drying destroyed water-rich phase assemblages and led to overestimation of the calibrated GGBS phase contents

Dataset for 'Influence of limestone addition on sodium sulphate activated blast furnace slag cements'

The data contained in this workbook has been used to create the following figures in the journal article: Marsh, A.T., Yue, Z., Dhandapani, Y., Button, K., Adu-Amankwah, S. and Bernal, S.A., 2022. Influence of limestone addition on sodium sulphate activated blast furnace slag cements. Construction and Building Materials, 360, p.129527. https://doi.org/10.1016/j.conbuildmat.2022.12952

Accelerating early age properties of ultra-low clinker cements for extrusion-based 3D printing

In this study, we investigated the influence of commercial sodium nitrate/thiocyanate accelerator compared to calcium sulfoaluminate cement addition on setting time, rheology and reaction kinetics of ultra-low clinker composite cement for extrusion-based 3D printing application. CEM I 52.5 N and a ternary composite cement with 70% clinker replaced by slag and limestone were evaluated. Results indicate that final setting time of 30 min and buildable yield stresses can be attained with less than 5% addition of calcium sulfoaluminate, with ettringite and C-(A)-S-H as main reaction products. This demonstrates the synergy between slag and calcium sulfoaluminate cements can be harnessed to control rheology and hardening. This is significance for evidencing suitability of ultra-low clinker composite cements for extrusion-based 3DCP, thus helping to fulfil its wider potential as a low-carbon concrete technology

Preparation of Ghanaian Dish 'Fufu' With Evaporated Milk

Abstract Fufu is a staple and well patronised traditional Ghanaian dish. It is mainly made from cassava or yam, mostly with added unripe plantain or cocoyam. The food is almost always eaten with soup. In the current study, evaporated milk has been used in preparation of the dish as well as the soup that mainly goes with the food. A ratio of 2:1 evaporated milk and water was used to prepare the food; and the soup was also prepared by using the same milk and ground-nut paste to obtain a creamy soup that had chicken as the accompaniment. Eighty percent (80%) of the five food evaluation experts who assessed the food indicated that they extremely liked the product of the new recipe; the remaining 20% also liked the food very much. The food was generally accepted by all the experts who recommended incorporation of milk into preparation of the meal from time to time. Preparation of the food with milk adds the rich nutrient components of the milk to the cassava which principally contains carbohydrate to make up for some nutrients that may be lacking in the food. Adding milk to the mainly carbohydrate-rich fufu is therefore highly recommended as this will make the food richer in nutrients required by the body. Furthermore, the study has shown that milk can be used in the preparation of main course meals apart from the normal beverages and light food in which the product is mainly used