Effects of water content, water type and temperature on the rheological behaviour of slag-cement and fly ash-cement paste backfill

The pumping ability and placement performance of fresh cemented paste backfill(CPB) in underground mined cavities depend on its rheological properties. Hence, it is crucial to understand the rheology of fresh CPB slurry, which is related to CPB mixture design and the temperature underground. This paper presented an experimental study investigating the effects of binder type, content, water chemical properties and content, and temperature, on the rheological properties of CPB material prepared using the tailings of a copper mine in South Australia. Portland cement(PC), a newly released commercially manufactured cement called Minecem(MC) and fly ash(FA) were used as the binders added to the mine tailing materials. Various amounts of two different water types were added to the mixtures in the preparation of backfill material slurry. Six different temperatures ranging from 5 to 60 °C were to investigate the effect of temperature on CPB rheology. Overall, the increasing water content and decreasing temperature lead to lower yield stress. Based on the results obtained from the rheological properties of CPB slurry, it was found that at room temperature(25 °C), with regards to the unconfined compressive strength(UCS) performance, the replacement of 4% PC mixed CPB(28 days UCS 425 k Pa) to 3% MC mixed CPB(28 days UCS 519 k Pa), reduced the slurry yield stress from 210.7 to 178.5 Pa. The results also showed that the chemical composition of water affects the yield stress of CPB slurry and that MC mitigates the negative effect of mine-processed water(MW) and thus lead to improve the rheological properties of the slurry. However, the results suggested that the rheological properties of a mixture using MC is very sensitive to the water volume and temperature change. Therefore, using MC in backfill requires better quality control in slump mixing.

Influence of ultra-fine fly ash on hydration shrinkage of cement paste

Hydration shrinkage generated by cement hydration is the cause of autogenous shrinkage of high strength concrete. It may result in the volume change and even cracking of mortar and concrete. According to the data analysis in a series of experimental studies, the influence of ultra-fine fly ash on the hydration shrinkage of composite cementitious materials was investigated. It is found that ultra-fine fly ash can reduce the hydration shrinkage of cement paste effectively, and the more the ultra-fine fly ash, the less the hydration shrinkage. Compared with cement paste without the ultra-fine fly ash, the shrinkage ratio of cement paste reduces from 23.4% to 39.7% when the ultra-fine fly ash replaces cement from 20% to 50%. Moreover, the microscopic mechanism of the ultra-fine fly ash restraining the hydration shrinkage was also studied by scanning electron microscopy, X-ray diffraction and hydrated equations. The results show that the hydration shrinkage can be restrained to a certain degree because the ultra-fine fly ash does not participate in the hydration at the early stage and the secondary hydration products are different at the later stage.

Investigation of the Effect of Using Different Fly Ash on the Mechanical Properties in Cemented Paste Backfill

In the cemented paste backfill(CPB)method,which can also be used for fortification purposes in mines,different additive materials with pozzolanic properties can be employed as substitutes instead of cement that is the main binder.One of the most popular pozzolanic materials that can be employed instead of cement is fly ash,which is thermal power plant tailings.But the compositions of fly ash and tailings used in high amounts in the CPB method,as well as the chemical structures that these materials form by interacting with the cement binder,affect the mechanical properties of the material depending on time.In this study,fly ash with 4 different chemical compositions(TFA,SFA,YFA,and CFA)was used as a cement substitute in CPB.By substituting fly ash with different chemical compositions in different proportions,CPB samples were created and their strength was elucidated according to 28,56,and 90-day curing times.The results of the study revealed that TFA with the highest CaO/SiO_(2) and SO_(3) ratios remained stable at the strength values of 6 MPa(total 9% binder)and 10 MPa(total 11% binder)in the long term.However,CFA with the lowest CaO/SiO_(2),SO_(3),and the highest SiO_(2)+Al_(2)O_(3)+Fe_(2)O_(3) ratios resulted in the greatest strength increase at a 20%substitution rate(11% of the total binder).Nevertheless,it was found that the SFA,which is in Class F,increased its strength in the early period based on the CaO rate.

Adsorption and Desorption Characteristics of K^+ and Na^+ Ions in Fly Ash Blended Cement Pastes

Cement pastes containing 0%, 15%, 25% and 35% fly ash were prepared. After being cured for 90 days, all fly ash blended cement pastes were crushed and ground into powders with a particle size less than 80 μm and then the powders were immersed in alkali solutions. Adsorption characteristics of K^+ and Na^+ ions in the pastes were investigated. Meawhile, the desorption characteristics of the adsorbed alklai ions and the inherent K^+ and Na^+ ions in the pastes were also investigated. Results showed that the contents of K^+ and Na^+ ions adsorbed by the pastes increased with increasing the substitution levels of fly ash and/or the concentrations of alkali solutions. Each paste was characterized by having the same adsorption capacity for K^+ or Na^+ that was essentially independent of alkali concentration. Adsorption mechanism of K^+ and Na^+ ions by the pastes is believed to be an effect of charge compensation of the C-S-H gel. Adsorption-desorption of the adsorbed K^+ and Na^+ ions in the pastes is reversible. The inherent K^+ and Na^+ ions in the pastes entered rapidly into the de-ionized water during the first 120 minutes, and then they were released at a relatively slow rate. A steady-state alkali partition was reached at about 720 minutes. Some K^+ and Na^+ ions which were originally "bound" by the hydration products were considered to be released into de-ionized water. Leaching tests showed that there was no significant effect of fly ash on the retaining of available alkalis in the pastes. A part of the released alkali ions exists in the pore solutions and the other part may be physically adsorbed by the hydration products.

DURABILITY OF HARDENED FLY ASH PASTE

The mechanical properties and durability (mainly frost - resistance and carbonation resistance) of fly ash - CaO - CaSO//4 center dot 2H//2O hardened paste are studied. The relationship among durability of hardened fly ash paste, the quantity and distribution of hydrates and the initial paste texture of hardened fly ash paste is presented. (Author abstract) 3 Refs.

Deterioration Characteristics of Cement-Fly Ash Paste under Strong Ultraviolet Radiation and Low Temperature Conditions

The pastes containing different dosages of fly ash were taken into ultraviolet radiation and low temperature freeze condition simultaneously（URL）for 30 days and only ultraviolet radiation condition（UR）for 30 days after standard curing,so as to investigate the influences of the conditions on the deterioration characteristics of the pastes.Microscopic test methods,such as XRD,TG-DTA and SEM,were used to study the UR effect on the deterioration process of hardened paste.The results show that the deterioration tests,such as URL and UR,inhibit the common development of paste strength,especially after the standard curing age of 360days.With the increase of fly ash dosage,from 0 to 50%,the reference value decreases,especially at early age.While at the later age,i e,180 and 360 days,the paste strength cured for 30 days under URL and UR conditions all increase to different extent and the strength is slightly affected very low,especially for the paste containing25%fly ash.From XRD results,URL and UR dispositions do not influence the hydration product kinds but the amount,especially Ca（OH）2 and CaCO3.Deterioration experiments can decrease the diffraction peak of Ca（OH）2 sharply,and increase that of CaCO3 rapidly,especially under only ultraviolet radiation.From TG-DTA and SEM results,with the increase of curing age,the content of Ca（OH）2 decreases and that of CaCO3 increases.The Ca（OH）2 content of paste under continuous UR curing for 30 days is less than that under URL curing for 30days,which indicates that UR has more negative effects on the pastes than URL.

Utilization of Low-Alkalinity Cementitious Materials in Cemented Paste Backfill of Gold Mine Tailings

The purpose of this paper was to explore the possility of using low alkalinity cementitious materials as binders,in which ground blast furnace slag and fly ash acted as a partial replacement of ordinary Portland cement,and CaSO_(4),Na_(2)SO_(4),and CaO were used as a sulfate activator and alkali activated additives,to solidify gold mine tail-ings for preparation of a green,inexpensive cemented paste backill(CPB).For this target,the effects of cement/tailings ratio,superplasticizer dosage,solid content,tailings fineness on the mechanical properties of the CPB were inves tigated.Additionally,the hydration mechanism of the CPB was analyzed based on X-ray diffraction and scanning electron microscopy results.The results showed that the fuidity of the CPB slurry could be improved by adding polycarboxylic acid superplasticizer.The unconfined compressive strength(UCS)of the CPB specimens was increased with the increase of cement/tailings ratio and solid content.Under the same experi-mental conditions,the 28 d UCS of the CPB specimens was 3.8-4.9 times higher than that of ordinary Portland cement.The softening coefficient of the CPB specimens was increased with the increasing cement/tailings ratio,ranging from 0.83 to 0.92.The shrinkage rate of the CPB specimens was decreased from 0.70%to 0.54%with the increase of cement/tailings ratio from 1:12 to 1:4 The UCS of the full tailings CPB was the highest,followed by the fine tailings CPB specimens,and the UCS of the coarse tailings CPB specimens was the lowest.The low alka-linity binder was proved to be a promising material to improve the engineering performances of the CPB.The optimal mixing ratio is 1:6 cement/tailings ratio,0.15 wt% superplastizer dosage,and 70 wt%solid content.Pre-pared by this mixing ratio,the UCS values of the CPB after 3,7,and 28 d curing ages reached 1.85,5.87,and 9.16 MPa,respectively,which were suitable as CPB for the Zhaoyuan gold mine in terms of strength requirements.

Effect of Fly Ash on Rheological Properties of Cement Paste

This paper is aimed to study the factors influencing rheological properties of cement paste with fly ash,including the quantity of cement replaced by fly ash,types of the fly ash,degree of cement hydration and the microstructure of cement paste. By changing the water-cement ratio,mixing different amounts of fly ash,adding different types fly ash,the rheological parameters of cement paste are measured with a rheometer,and the needle penetration are tested by Vical needle. A ultrasound velocity test was aslo employed to monitor hydration of paste at early ages. Combinations of different factors on the rheological properties of cement paste are studied. The objective is to reveal the effect mechanism of material nature and proportion of each component on the rheological properties of cement paste with fly ash.