Investigation On Utilization Of Ti-extracted Residues And Red Gypsum For The Preparation Of Construction And Building Materials

Ti-extracted residues?TRs?and red gypsum?RG?are the two solid wastes from titanium dioxide industry.Both of them are simply landfilled,resulting in a series of severe environmental problems due to their difficulty for disposal.In view of these problems,three approaches in this work to prepare light-weight foamed concrete,low-clinker and or clinker-free binder with high performance were proposed as follows:?1?Light-weight foamed concrete was successfully prepared by using TRs and RG as main starting materials,and the technological parameters influencing the hydration of blending binder and properties of foamed concrete were investigated;?2?High-performance composite cementitious materials were synthesized via mixing finely ground TRs and RG,at the same time,the effects of key factors such as RG,waste anhydrite from RG,Portland cement on the hydration kinetics and properties of cementing binder were investigated;?3?The mixtures consisting of fine TRs and fly ash were activated by a small amount of sodium hydr oxide?Na OH?to prepare a cementless hydraulic material with high mechanical strength and the effects of fly ash on the evolutiuon of early hydration,mechanical properties and dimensional stability of binder were studied in this work.TRs and RG were utilized as raw materials to prepare the light-weight foamed concrete,and the influences of technological parameters on the properties of foamed concrete were investigated.The results revealed that the addition of quick lime and calcium sulfoaluminate cement evidently shortened the setting time of the complexing binder in foamed concrete,however,the excessive content of afore-mentioned two materials retarded the later hydration.It was also found that the effect of an increase of fineness for TRs on the setting time and early hydration is not visible.When Portland cement:TRs:RG and water to biner?W/B?were kept constant at 10:45:45 and 0.6,respectively,and the foam4.6%?by mass,the same below?,water reducer 0.2%,the optimum parameters for preparing foamed concrete was proposed as following:quick lime 2.0%,calcium sulfoaluminate cement 4.0%and Na₂SO₄ 0.4%.In such a condition,the maximum compressive strength and specific strength of foamed concrete with density of 437 kg/m³ reached up to 2.14 MPa and 4.90 k N·m/kg.Microstructure analysis indicated that the phases in wall section of foamed conctere mainly composed of floccular and petal-like hydrated gels,needle-like ettringite and unreactive bulk and plate-like gypsum.All the bulk density,compressive strength and thermal conductivity of A04,A08 and A10 grade specimens met the demand in a Chinese standard:JG/T 266-2011 named Foamed Concrete.One kind of high-strength cementitious materials was manufactured using TRs,RG and a small amount of Portland cement.The effects of RG's fraction,waste anhydrite and content of Portland cement on the early hydration,mechanical strength and softening coefficients at designated ages were investigated.A significant attentaion was payed to the influence of RG and waste anhydrite on the electrical resistivity of hydraulic binder.Results indicated that the onset of acceleration period of heat flow curves linearly correlated with the extreme point of resistivity curves well.Moreover,the isothermal calorimetry and electrodeless resistivity tests indicated that RG retarded the hydration within 24 hours but the addition of 20%RG replaced by anhydrite accelerated the hydration process within 72 hours and enhanced the early mechanical strength and water resistance.When 0-40%limestone was replaced by RG,it was found that the strengths and water resistance of binding system increased with an increase of RG content.The maximum compressive strength at three and 28 days and softening coefficient of mortars at 28 days can reach up to 18.8 MPa,47.9 MPa and 0.87,respectively,but the flexural strength at three and 28 days was slightly lower than the corresponding requirement in GB/T 175-2007 named Ordinary Portland Cement.X-ray diffraction analysis revealed that an increase of RG portion and the presence of waste anhydrite transformed Friedel's salt into ettringite.More attention should be payed to the enhancement of flexural strength at early ags and chloride binding behavior.In this paper,a type of high-strength clinker-free cement was achieved by the alkaline activation of mixtures consisting of ground TRs and fly ash by the addition of sodium hydroxide?Na OH?.The flexural and compressive strength of obtained cement were over 6.50 MPa and 52.5 MPa at 28 days.The experimental results indicated that the mechanical strength at three,seven and28 days decreased with an increase of fly ash.However,the mechanical property increased with the increasing portion of fly ash when the content of fly ash was less than 20%.A slight reduction of strength was also observed when fly ash's content reached to 30%.The evolution of drying shrinkage was significantly affected by the presence of fly ash at early ages and it was concluded that the change of dyring shringkage at later ages is not obvious.The Si-bearing and Al-bearing components in fly ash gradually dissolved and reacted with the components in TRs to form C-A-S-H,enhancing the later strength of composite cementitious materials.