Preliminary study of strength of coal composites

Large scale floor convergence and sudden failure of pillars in room-and pillar underground mining have been reported in various countries over many years. Failure of these mines has been attributed, amongst other causes, to the stiffness criterion used by the mining operators and most importantly mine design practices, that tend to reduce the width-to-height ratio i.e. slender pillars in pursuit of greater coal recovery yields, in order to maximize coal recovery in the seam. It is recorded that the room-and-pillar method can leave behind about 40% of the total coal available for mining. The current study seeks to determine the appropriate width-height ratio for coal composite and stabilized coal pillars. Pure coal was mixed with granular soil and stabilized with cement to form cylindrical composite columns which were crushed to determine their unconfined compressive strength at different width-to-height ratios. The strength increased with the increase in W/H of the columns. Increase in coal percentage in a composite reduces the strength. The stabilized coal pillars mobilized less strength than the composite pillars. Based on limited data, relationships amongst column strength W/H and mix ratios were formulated and are proposed for the estimation of strength of mine support structures. More data is however required to formulate a general regression equation

Compressive strength and durability of fly ash stabilized dolomitic waste as a lightweight construction material

Due to the seriousness of environmental pollution and the production of huge energy consuming building and construction materials, the development of new eco-friendly and energy saving building materials to overcome these issues has been gaining increasing attention. South Africa has largest number of dolomite deposits in Southern Africa, if not in the entire Africa. Nonetheless most of the dolomite in South Africa has some impurities of which they are neglected in dumping site (millions of tons). This paper looked at finding alternative use for this dolomitic waste (DW) by stabilizing this waste with another waste called fly ash (FA), introduction of gypsum as an activator will trigger a pozzolanic reaction in presences of water. As a result of this, the product of this reaction will be a useful material in construction and as well solve the difficulties i.e. environmental, social and cost often associated with storing and handling of this waste. This work went as far as providing an alternative for low-cost construction materials that can help deal with South African housing problems. The waste material was analyzed from geomaterial and geochemistry view point, the chemical reaction between these materials: dolomitic waste and fly ash were illustrated. The XRF, SEM and EDS of the reacted product were looked at, the strength gained by the reaction after sample preparation for 28 days were also shown. However various geotechnical laboratory experiments were conducted ranging from proctor compaction test, compressive strength test and a mathematical model was generated to predict the compressive strength and these results were compared to compressive strength values obtained from the laboratory

Effect of wetting and air curing durations on strength of stabilized sands

Low cost houses in most developing countries are constructed from sandcrete blocks manufactured from sand-cement mortar. In South Africa, failure of many of these blocks has been experienced leading to collapse of the structures. Lack of sufficient compressive strength in the manufactured blocks was attributed as the main cause of the failures. The unconfined compressive strength (UCS) of sandcrete blocks can however be improved by curing the blocks under different conditions. Therefore the objective of this study was to evaluate the effect of different wetting and air drying durations would have on the strength and the stiffness of the cubes produced. Weakly cemented sand cubes, with different cement contents and cement-fly ash binder ratios compacted at the optimum moisture content, were prepared. The 28 days UCS was determined, modulus of elasticity and moisture contents of sample specimen was also determined. The results showed that curing cubes under water for 7 days and then air curing for 21 days yield maximum UCS and modulus of elasticity for sands cemented by plain Portland cement, and that 3 days under water curing produced maximum results for sands stabilized by cement-fly ash binder. It was however found that under water for 28 days results in minimum strength. Therefore the longer the cubes are cured under water minimum strengths were achieved

The Stabilization of Weathered Dolerite Aggregates with Cement, Lime, and Lime Fly Ash for Pavement Construction

An experimental program was performed on weathered dolerite specimens stabilized by adding varying percentages of cement (4, 8, 12, and 16) % and lime (6 and 12) % and a combination of lime and fly ash (6% lime + 12% Fly ash and 12% lime + 12% Fly ash) % by dry weight of soil. The strength was examined under three different curing methods, namely, membrane curing (MBC), alternate moist-air curing (MAC), and water curing (WAC), by conducting unconfined compressive strength (UCS) tests. Simple polynomial and linear functions (regression models) were used to define the relationships between the variables investigated. Membrane curing (MBC) gave results close enough to the water curing (WAC) to indicate that it can be confidently used on the field during pavement construction. From the results obtained, for class B (interurban collector and major rural roads) pavement construction, addition of 8% cement was recommended for road base construction with stabilized WDA. Also the addition of 12 + 12% Lime and Fly Ash was recommended for road subbase construction with stabilized WDA. Stabilized WDA against the prejudiced myths would perform satisfactorily for base and subbase construction in both heavily trafficked and low volume roads with economic quantities of cement, lime, and fly ash in South Africa