Showing papers on "Cement published in 2010"

Measuring the eco-efficiency of cement use

Abstract: At present, the cement industry generates approximately 5% of the world’s anthropogenic CO2 emissions. This share is expected to increase since demand for cement based products is forecast to multiply by a factor of 2.5 within the next 40 years and the traditional strategies to mitigate emissions, focused on the production of cement, will not be capable of compensating such growth. Therefore, additional mitigation strategies are needed, including an increase in the efficiency of cement use. This paper proposes indicators for measuring cement use efficiency, presents a benchmark based on literature data and discusses potential gains in efficiency. The binder intensity (bi) index measures the amount of binder (kg m−3) necessary to deliver 1 MPa of mechanical strength, and consequently express the efficiency of using binder materials. The CO2 intensity index (ci) allows estimating the global warming potential of concrete formulations. Research benchmarks show that bi ∼5 kg m−3 MPa−1 are feasible and have already been achieved for concretes >50 MPa. However, concretes with lower compressive strengths have binder intensities varying between 10 and 20 kg m−3 MPa−1. These values can be a result of the minimum cement content established in many standards and reveal a significant potential for performance gains. In addition, combinations of low bi and ci are shown to be feasible.

Comparison of test methods to assess pozzolanic activity

Abstract: Assessment of the pozzolanic activity of cement replacement materials is increasingly important because of the need for more sustainable cementitious products. The pozzolanic activity of metakaolin, silica fume, coal fly ash, incinerated sewage sludge ash and sand have been compared using the Frattini test, the saturated lime test and the strength activity index test. There was significant correlation between the strength activity index test and the Frattini test results, but the results from these tests did not correlate with the saturated lime test results. The mass ratio of Ca(OH)2 to test pozzolan is an important parameter. In the Frattini test and strength activity index test the ratio is approximately 1:1, whereas in the saturated lime test the ratio is 0.15:1. This explains why the saturated lime test shows higher removal of Ca(OH)2 and why the results from this test do not correlate with the other test methods.

Use of natural zeolite as a supplementary cementitious material

Abstract: Natural zeolite, a type of frame-structured hydrated aluminosilicate mineral, is used abundantly as a type of natural pozzolanic material in some regions of the world. In this work, the effectiveness of a locally quarried zeolite in enhancing mechanical and durability properties of concrete is evaluated and is also compared with other pozzolanic admixtures. The experimental tests included three parts: In the first part, the pozzolanic reactivity of natural zeolite and silica fume were examined by a thermogravimetric method. In this case, the results indicated that natural zeolite was not as reactive as silica fume but it showed a good pozzolanic reactivity. In the second part, zeolite and silica fume were substituted for cement in different proportions in concrete mixtures, and several physical and durability tests of concrete were performed. These experimental tests included slump, compressive strength, water absorption, oxygen permeability, chloride diffusion, and electrical resistivity of concrete. Based on these results, the performance of concretes containing different contents of zeolite improved and even were comparable to or better than that of concretes prepared with silica fume replacements in some cases. Finally, a comparative study on effect of zeolite and fly ash on limiting ASR expansion of mortar was performed according to ASTM C 1260 and ASTM C 1567. Expansion tests on mortar prisms showed that zeolite is as effective as fly ash to prevent deleterious expansion due to ASR.

Compressive strength and microstructure of carbon nanotubes–fly ash cement composites

Abstract: In this work, carbon nanotubes of 0.5 and 1% by weight were added for the first time in a fly ash cement system to produce carbon nanotubes–fly ash composites in the form of pastes and mortars. Compressive strengths of the composites were then investigated. It was found that the use of carbon nanotubes resulted in higher strength of fly ash mortars. The highest strength obtained for 20% fly ash cement mortars was found at 1% carbon nanotubes where the compressive strength at 28 days was 51.8 MPa. This benefit can clearly be seen in fly ash cement with fly ash of 20% where the importance of the addition of carbon nanotubes means that the relative strength to that of Portland cement became almost 100% at 28 days. In addition, scanning electron micrographs also showed that good interaction between carbon nanotubes and the fly ash cement matrix is seen with carbon nanotubes acting as a filler resulting in a denser microstructure and higher strength when compared to the reference fly ash mix without CNTs.

Coupled effects of sulphate and temperature on the strength development of cemented tailings backfills: Portland cement-paste backfill

Abstract: Cemented paste backfill (CPB), which is a mix of tailings, water and cement, is subjected to the combined actions of temperature and sulphate during its service life. There is a need to acquire solid knowledge on the coupled effects of temperature and sulphate on the strength of CPBs for a safe, durable and cost-effective design of CPB structures. Hence, the main objective of this paper is to use an experimental approach to study the combined effect of temperature and sulphate on the strength development and microstructure (mineralogical composition of the hardened cement paste) of CPBs. About 200 CPB specimens with various initial sulphate contents (0, 5000, 15,000, and 25,000 ppm) and cured at different temperatures (0 °C, 25 °C, 20 °C, 35 °C, and 50 °C) are tested at different curing times (28, 90, and 150 days). The results show that the coupled effect of temperature and sulphate has a significant impact on the strength and mineralogical composition of the CPB. Depending on the curing time, temperature and initial sulphate content, the sulphate can have a positive or negative impact, i.e., leads to an increase or decrease of CPB strength. The obtained results show a strong indication that the absorption of sulphate by calcium–silicate–hydrate (C–S–H) could lead to the formation of lower quality C–S–H, thereby decreasing the strength of the CPB. This study has demonstrated that the coupled effect of sulphate and temperature on CPBs is an important factor for consideration in the designing of cost-effective, safe and durable CPB structures.

Microbial Concrete: A Way to Enhance Durability of Building Structures

Abstract: Natural processes, such as weathering, faults, land subsidence, earthquakes, and human activities create fractures and fissures in concrete structures which can reduce the service life of the structures. A novel strategy to restore or remediate such structures is biomineralization of calcium carbonate using microbes such as Bacillus species. In the present study, Bacillus sp. CT-5, isolated from cement, was used to study compressive strength and water absorption tests. The results showed 36% increase in compressive strength of cement mortar with the addition of bacterial cells. Calcite deposition on treated cubes absorbed nearly six times less water than the control cubes. The current work demonstrates that production of “microbial concrete” by Bacillus sp. on constructed facilities enhanced the durability of building materials.

Inclusion of carbonation during the life cycle of built and recycled concrete: influence on their carbon footprint

Abstract: When the service life (or primary life) of built concrete infrastructure has elapsed, a common practice is that the demolished concrete is crushed and recycled, then incorporated into new construction. LCA studies of CO2 emissions focus on the manufacturing and construction and occupancy/utilization phases, without consideration of the demolition and application of recycled concrete into a secondary construction application. Concrete has a documented ability to chemically react with airborne carbon dioxide (CO2); however, carbon capture (or carbonation) by concrete during the primary and secondary life, is not considered in LCA models. This paper incorporates CO2 capture during both primary and secondary life into an LCA model for built concrete. CO2 equivalent (CO2-e) emissions were estimated by calculation of the quantity of CO2-e emitted per unit of activity at the point of emission release (i.e., fuel use, energy use, manufacturing activity, construction activity, on-site demolition, etc.). Carbonation was estimated for built concrete during the primary life and also during the secondary life when the demolished concrete structure is crushed and recycled for a new application within a new built structure. Life cycle calculations for a built bridge structure are provided which contrasts the net effects of CO2 emission and capture. The study has analyzed a concrete bridge with primary life of 100 years. Following completion of the primary life, we have considered that the demolished concrete from the bridge will be crushed, recycled, and used in the construction of a replacement bridge. Due to damage caused by demolition and crushing, the quality of the recycled concrete is unlikely to be as high as quarried natural rock, and the recycled concrete is most likely to be used in a more temporary construction application with an assumed 30-year secondary life. Following the expiry of the secondary life, if recycling of recycled concrete aggregate (RCA) was to be undertaken, it is unlikely the quality of RCA will be suitable to enable a third construction application: therefore, our LCA includes the primary life of 100 years plus the secondary life of 30 years. Carbonation of the built concrete during the primary life is almost negligible compared with the emissions arising from manufacture of raw materials, concrete production, and construction. However, CO2 capture by recycled concrete during the secondary life is considerably greater: a factor that is not included in LCA estimates of the carbon footprint of built concrete. Crushed concrete has considerably greater exposed surface area, relative to volume, than a built concrete structure: therefore, a greater surface area of RCA, compared with a built structure, is exposed to CO2 and carbonates. This key factor leads to such high amounts of carbonation during the secondary life when compared with the built structure. While reducing the amount of solid landfill, recycled concrete provides significant capture of airborne carbon dioxide. The effects of carbon capture of recycled concrete aggregate within the secondary life is significant, imbibing up to 41% of the CO2 emitted during manufacture of a 100% Portland cement binder. Emission estimates can be overestimated by as much as 13–48%, depending on the type of cementitious binder in the built concrete and the application of recycled concrete during the secondary life. The effects of carbon capture of recycled concrete aggregate within the secondary life are significant and should be included in LCA calculations for reinforced concrete structures. Carbonation during the secondary life can be affected by the type of application and exposure of the RCA: gravel with fine particle size will carbonate more comprehensively than larger boulders (due to greater exposed surface area), while air-exposed RCA will carbonate more comprehensively than RCA located in a buried and moist environment. An approach is provided in this paper that incorporates carbon capture by concrete infrastructure into LCA carbon emission calculations. If carbonation is ignored, emission estimates can be overestimated by 13–48%, depending on the type of cement binder and the application of RCA during the secondary life. Due to the considerable number and types of built concrete structures, it is recommended that the LCA model be improved by incorporation of carbonation data obtained from recycled concretes that have been applied to a variety of construction applications and exposed to a range of exposure environments.