For mines with poor ore bodies and surrounding rocks,the general mining method does not allow the ore to be extracted from underground safely and efficiently.For these mines,the downward layered filling mining techniq...For mines with poor ore bodies and surrounding rocks,the general mining method does not allow the ore to be extracted from underground safely and efficiently.For these mines,the downward layered filling mining technique is undoubtedly the most suitable mining method.The downward filling mining technique may eliminate the troubles relating to poor ore deposit conditions,such as production safety,ore loss rate,and depletion rate.However,in this technique,the safety of the artificial roof of the next stratum is of paramount importance.Cementitious tailings backfilling(CTB)that is not sufficiently cemented and causes collapses could threaten ore production.This paper explores a diamond-shaped composite structure to mimic the stability of a glued false roof in an actual infill mine based on the recently emerged three-dimensional(3D)printing technology.Experimental means such as three-point bending and digital image correlation(DIC)techniques were used to explore the flexural characteristics of 3D construction specimens and CTB combinations with different cement/tailings weight ratios at diverse layer heights.The results show that the 3D structure with a 14-mm ply height and CTB has strong flexural characteristics,with a maximum deflection value of 30.1 mm,while the 3D-printed rhomboid polymer(3D-PRP)structure with a 26-mm ply height is slightly worse in terms of flexural strength characteristics,but it has a higher maximum flexural strength of 2.83 MPa.A combination of 3D structure and CTB has more unique mechanical properties than CTB itself.This research work offers practical knowledge on the artificial roof performance of the downward layered filling mining technique and builds a scientific knowledge base regarding the successful application of CTB material in mines.展开更多
Composite Portland cement （PC） played an important role in various kinds of construction engineering owing to low hydration heat,low-cost,and application of solid industrial waste,but its brittleness and low strengt...Composite Portland cement （PC） played an important role in various kinds of construction engineering owing to low hydration heat,low-cost,and application of solid industrial waste,but its brittleness and low strength limited its use in stress-bearing locations.The aim of this study is to improve the toughness and fracture resistance by incorporating CaCO3 whisker in cement matrix.Effect of different content of calcium carbonate whiskers on the mechanical properties of PC was investigated.The results showed that the flexural strength,impact strength and split tensile strength were increased by 39.7%,39.25% and 36.34% at maximum,respectively.Microstructure and elements of the whiskers in hardened cement were observed and analyzed by SEM/EDS.The mechanisms of the reinforcement of CaCO3 whisker on cement were also discussed,and the conclusion was that the improvement could be correlated to energy-dissipating processes owing to crack bridging,crack deflection,and whisker pull-out at the crack tips.展开更多
The effect of curing regime on the distribution ofAl3＋ coordination in hardened cement pastes within 28 d were investigated by 29Si and 27Al magic angle spinning （MAS） nuclear magnetic resonance（NMR） with deconvo...The effect of curing regime on the distribution ofAl3＋ coordination in hardened cement pastes within 28 d were investigated by 29Si and 27Al magic angle spinning （MAS） nuclear magnetic resonance（NMR） with deconvolution technique. The results indicate that the tetrahedral coordination Al3＋ incorporated in C-S-H structure mainly originate from the AP＋ incorporated in the alite and belite phases in the Portland cement. The curing regime of constant temperature of 20 ℃ is beneficial to the octahedral coordination Al3＋ transforming to tetrahedral coordination AP＋ incorporated in C-S-H structure. However, at curing regime of variable temperature, the temperature rising process is more advantageous to the transformation from ettringite to monosulphate, substitution of Al3＋ for Si4＋ in the C-S-H structure and the formation of the third aluminate hydrate （TAH） than that at constant temperature of 20 ℃. The high temperature of 60 ℃ in the holding temperature process promotes the decomposition of ettringite, and enhances the consumption of the Al3＋ incorporated in C-S-H phases and the Al3＋ in TAH for the monosulphate forming. The temperature decreasing promotes the transformation from monosulphate to ettringite, and increases the consumption of the Al3＋ incorporated in C-S-H phases, and then increases the quantity of the TAH.展开更多
The effect of curing regime on degree ofAl3＋ substituting for Si^4＋ （Al/Si ratio） in C-S-H gels of hardened Portland cement pastes was investigated by 29Si magic angel spinning （MAS） nuclear magnetic resonance ...The effect of curing regime on degree ofAl3＋ substituting for Si^4＋ （Al/Si ratio） in C-S-H gels of hardened Portland cement pastes was investigated by 29Si magic angel spinning （MAS） nuclear magnetic resonance （NMR） with deconvolution technique. The curing regimes included the constant temperature （20, 40, 60 and 80 ℃） and variable temperature （simulated internal temperature of mass concrete with 60 ℃ peak）. The results indicate that constant temperature of 20 ℃ is beneficial to substitution ofAl3＋ for Si4＋, and AI/Si ratio changes to be steady after 180 d. The increase of Al/Si ratio at 40 ℃is less than that at 20℃ for 28 d. The other three regimes of high temperature increase Al/Si ratio only before 3 d, on the contrary to that from 3 to 28 d. However, the 20 ℃ curing stage from 28 to 180 d at variable temperature regime, is beneficial to the increase of AI/Si ratio which is still lower than that at constant temperature regime of 20 ℃ for the same age. A nonlinear relation exists between the Al/Si ratio and temperature variation or mean chain length （MCL） of C-S-H gels, furthermore, the amount ofAl3＋ which can occupy the bridging tetrahedra sites in C-S-H structure is insufficient in hardened Portland cement pastes.展开更多
With OLYMPUS PMG3 metallograph, an abnormal three layer gradient structure ,i.e. coarse grain zone, binder enrichment zone and normal structure zone from the surface to the inner, was observed in Cr 3C 2 based cemente...With OLYMPUS PMG3 metallograph, an abnormal three layer gradient structure ,i.e. coarse grain zone, binder enrichment zone and normal structure zone from the surface to the inner, was observed in Cr 3C 2 based cemented carbide. In the binder enrichment zone, three different shapes of anomalous coarse carbides were observed. It is shown that the transverse rupture strength can be raised remarkably, up 20.7% from the alloy with abnormal gradient structure by removing the abnormal gradient structure. It is suggested that the abnormal gradient structure in the surface, especially the anomalous coarse carbides in the binder enrichment zone is the main reasons for the lower strength.展开更多
The chemical composition of cement has a crucial impact on the performance of concrete.Different brands of Portland cement are used for various construction works without prior knowledge of their performance.For econo...The chemical composition of cement has a crucial impact on the performance of concrete.Different brands of Portland cement are used for various construction works without prior knowledge of their performance.For economic and environmental reasons,the valorization of fly ash in concrete production has been increasingly studied.The aim of this study is to determine the mechanical performance of the cement in which hydrated cement has been injected,and to assess the environmental benefit of using the waste as a partial replacement.The experimental study consists of replacing cement,with high tricalcium aluminate(C3A)content,with cement with low C3A content.The obtained result shows that it is very feasible to valorize this material and to manufacture eco-environmental cement which has rheological and mechanical characteristics almost similar to or better than that of ordinary cement,where a resistance of 30 MPa has been obtained,after a substitution rate of 50%.The valorization by using cement with low C3A content makes it possible to strengthen the material at a young age and leads to obtain more economical and less polluting cement.展开更多
基金financially supported by the National Key Research and Development Program of China(No.2022YFC2905004)the National Natural Science Foundation of China(No.51804017)。
文摘For mines with poor ore bodies and surrounding rocks,the general mining method does not allow the ore to be extracted from underground safely and efficiently.For these mines,the downward layered filling mining technique is undoubtedly the most suitable mining method.The downward filling mining technique may eliminate the troubles relating to poor ore deposit conditions,such as production safety,ore loss rate,and depletion rate.However,in this technique,the safety of the artificial roof of the next stratum is of paramount importance.Cementitious tailings backfilling(CTB)that is not sufficiently cemented and causes collapses could threaten ore production.This paper explores a diamond-shaped composite structure to mimic the stability of a glued false roof in an actual infill mine based on the recently emerged three-dimensional(3D)printing technology.Experimental means such as three-point bending and digital image correlation(DIC)techniques were used to explore the flexural characteristics of 3D construction specimens and CTB combinations with different cement/tailings weight ratios at diverse layer heights.The results show that the 3D structure with a 14-mm ply height and CTB has strong flexural characteristics,with a maximum deflection value of 30.1 mm,while the 3D-printed rhomboid polymer(3D-PRP)structure with a 26-mm ply height is slightly worse in terms of flexural strength characteristics,but it has a higher maximum flexural strength of 2.83 MPa.A combination of 3D structure and CTB has more unique mechanical properties than CTB itself.This research work offers practical knowledge on the artificial roof performance of the downward layered filling mining technique and builds a scientific knowledge base regarding the successful application of CTB material in mines.
文摘Composite Portland cement （PC） played an important role in various kinds of construction engineering owing to low hydration heat,low-cost,and application of solid industrial waste,but its brittleness and low strength limited its use in stress-bearing locations.The aim of this study is to improve the toughness and fracture resistance by incorporating CaCO3 whisker in cement matrix.Effect of different content of calcium carbonate whiskers on the mechanical properties of PC was investigated.The results showed that the flexural strength,impact strength and split tensile strength were increased by 39.7%,39.25% and 36.34% at maximum,respectively.Microstructure and elements of the whiskers in hardened cement were observed and analyzed by SEM/EDS.The mechanisms of the reinforcement of CaCO3 whisker on cement were also discussed,and the conclusion was that the improvement could be correlated to energy-dissipating processes owing to crack bridging,crack deflection,and whisker pull-out at the crack tips.
基金Funded by the Major State Basic Research Development Program of China(973 Program)(No.2009CB623201)
文摘The effect of curing regime on the distribution ofAl3＋ coordination in hardened cement pastes within 28 d were investigated by 29Si and 27Al magic angle spinning （MAS） nuclear magnetic resonance（NMR） with deconvolution technique. The results indicate that the tetrahedral coordination Al3＋ incorporated in C-S-H structure mainly originate from the AP＋ incorporated in the alite and belite phases in the Portland cement. The curing regime of constant temperature of 20 ℃ is beneficial to the octahedral coordination Al3＋ transforming to tetrahedral coordination AP＋ incorporated in C-S-H structure. However, at curing regime of variable temperature, the temperature rising process is more advantageous to the transformation from ettringite to monosulphate, substitution of Al3＋ for Si4＋ in the C-S-H structure and the formation of the third aluminate hydrate （TAH） than that at constant temperature of 20 ℃. The high temperature of 60 ℃ in the holding temperature process promotes the decomposition of ettringite, and enhances the consumption of the Al3＋ incorporated in C-S-H phases and the Al3＋ in TAH for the monosulphate forming. The temperature decreasing promotes the transformation from monosulphate to ettringite, and increases the consumption of the Al3＋ incorporated in C-S-H phases, and then increases the quantity of the TAH.
基金Funded by the Major State Basic Research Development Program of China(973 Program)(No.2009CB623201)National Natural Science Foundation of China(No.51302070)
文摘The effect of curing regime on degree ofAl3＋ substituting for Si^4＋ （Al/Si ratio） in C-S-H gels of hardened Portland cement pastes was investigated by 29Si magic angel spinning （MAS） nuclear magnetic resonance （NMR） with deconvolution technique. The curing regimes included the constant temperature （20, 40, 60 and 80 ℃） and variable temperature （simulated internal temperature of mass concrete with 60 ℃ peak）. The results indicate that constant temperature of 20 ℃ is beneficial to substitution ofAl3＋ for Si4＋, and AI/Si ratio changes to be steady after 180 d. The increase of Al/Si ratio at 40 ℃is less than that at 20℃ for 28 d. The other three regimes of high temperature increase Al/Si ratio only before 3 d, on the contrary to that from 3 to 28 d. However, the 20 ℃ curing stage from 28 to 180 d at variable temperature regime, is beneficial to the increase of AI/Si ratio which is still lower than that at constant temperature regime of 20 ℃ for the same age. A nonlinear relation exists between the Al/Si ratio and temperature variation or mean chain length （MCL） of C-S-H gels, furthermore, the amount ofAl3＋ which can occupy the bridging tetrahedra sites in C-S-H structure is insufficient in hardened Portland cement pastes.
文摘With OLYMPUS PMG3 metallograph, an abnormal three layer gradient structure ,i.e. coarse grain zone, binder enrichment zone and normal structure zone from the surface to the inner, was observed in Cr 3C 2 based cemented carbide. In the binder enrichment zone, three different shapes of anomalous coarse carbides were observed. It is shown that the transverse rupture strength can be raised remarkably, up 20.7% from the alloy with abnormal gradient structure by removing the abnormal gradient structure. It is suggested that the abnormal gradient structure in the surface, especially the anomalous coarse carbides in the binder enrichment zone is the main reasons for the lower strength.
文摘The chemical composition of cement has a crucial impact on the performance of concrete.Different brands of Portland cement are used for various construction works without prior knowledge of their performance.For economic and environmental reasons,the valorization of fly ash in concrete production has been increasingly studied.The aim of this study is to determine the mechanical performance of the cement in which hydrated cement has been injected,and to assess the environmental benefit of using the waste as a partial replacement.The experimental study consists of replacing cement,with high tricalcium aluminate(C3A)content,with cement with low C3A content.The obtained result shows that it is very feasible to valorize this material and to manufacture eco-environmental cement which has rheological and mechanical characteristics almost similar to or better than that of ordinary cement,where a resistance of 30 MPa has been obtained,after a substitution rate of 50%.The valorization by using cement with low C3A content makes it possible to strengthen the material at a young age and leads to obtain more economical and less polluting cement.
