Stripping of aggregate from mortar in waste concrete heated by microwave:Mechanisms of differential-temperature stress and vapor expansion pressure

Microwave heating,which is used for pre-treatment of concrete before it is comminuted,stands as a strong candidate for selective liberation of multiphase materials like concrete.This paper is concerned with the selective liberation of concrete's raw constituents(particularly aggregate)for recycling by considering the water content of concrete as a parameter of microwave heating for the first time.The deterioration law of the concrete's performance was characterized by the variation in the splitting tensile strength and relative dynamic modulus after heating by microwave at different water contents.Besides,tests were conducted to evaluate the performance of the interface transition zone(ITZ)between aggregate and mortar as well as to investigate the reasons for the stripping behavior of aggregate-mortar,which included the interface tensile strength test,temperature measurement,and porosity test.The deterioration law of splitting tensile strength and relative dynamic modulus revealed that the performance of concrete was subject to different degrees of damage depending on the water content.Furthermore,experimental results showed that interface bonding strength between aggregate and mortar was dramatically impaired,and a large temperature difference was generated between the aggregate and mortar during microwave heating.Meanwhile,the permeable pores increased considerably even when the specimens were dried.In the presence of water,the intactness of ITZ between aggregate and mortar was destroyed by microwave heating,and its performance was significantly lowered,which led to the occurrence of stripping behavior between aggregate and mortar.This was reaffirmed by the microstructure presented by scanning electron microscopy.Thus,the newly developed microwave pretreatment improved by providing appropriate water contents for concrete corresponding to different strength grades is a promising method for recycling aggregate from waste concrete.

Analysis of a Composite Admixture Based on Ready-Mixed Concrete Waste Residuals

Reasonable treatment and utilization of waste residuals discharged during the production of ready-mixed concrete is an important problem in the cement industry.In this study,a composite admixture was prepared by using ready-mixed concrete waste residuals,furnace slag,and water granulated slag.The grinding characteristics of such material were investigated.Moreover,the effect of such admixture on cement hydration and pore structure was analyzed by X-ray diffraction,thermogravimetric-differential scanning calorimetry,scanning electron microcopy and mercury intrusion porosimetry.As shown by the results:The grinding characteristics of the waste residuals can be improved significantly by mixing them with furnace slag and water granulated slag.Furthermore,the composite admixture does not change the composition of hydration products;rather it contributes to refine the pore structure of the matrix,thus improving the mechanical properties of these cement-based materials.

Utilization of Concrete Waste Aggregates Using Geopolymer Cement

Reuse of concrete waste, especially in large quantity, can save not only material but also cost for its disposal. This paper presents experiment results on the use of fine and coarse aggregates from concrete waste in geopolymer mortars and concretes. Geopolymeric cement is an inorganic compounds of aluminosilicates synthesized from precursors with high content of silica and alumina activated by alkali silicate solutions. Geopolymer in this experiment was synthesized from fly ash as the precursor and sodium silicate solution as the activator. Hardening of geopolymers was performed by heating the casted paste in an oven at -60~Cfor 3 to 36 hours. Compressive strength of geopolymer pastes and mortars using either fresh or waste fine aggregates were in the range of 19-26 MPa. Hardening time of 3 hours at 60~C followed by leaving the test pieces at room temperature for 7 day before testing results in similar strength to that of mortars cured for 36 hours at 60~C followed by leaving the samples at room temperature for 3 days. It suggests that optimum strength can be achieved by combination of heating time and rest period before testing, i.e the specimens age. Applying mix design with a target strength of 40 MPa, conventional Portland cement concretes using fresh aggregates reached 70% of its target strength at day-7. Compressive strength of geopolymer concretes with waste aggregates was -25 MPa at day-3 while geopolymer concretes with fresh aggregates achieved -39 MPa at day-3. It can be concluded that geopolymer concretes can achieve the target strength in only 3 days. However, the expected reinforcing effect of coarse aggregates in concrete was ineffective if waste coarse aggregates were used as the strength of the concretes did not increase significantly from that of the mortars. On the other hand, waste fine aggregates can be reused for making geopolymer mortars having the same strength as the geopolymer mortars using fresh aggregates.

Nano-treatment of Autoclaved Aerated Concrete Waste and Its Usage in Cleaner Building Materials

Autoclaved aerated concrete waste(AACW)was used as a raw material to prepare nucleation seed for acceleration of Portland cement.Nano AACW seed with median particle size of 324 nm was prepared by wet grinding method.Both the electrical conductivity and pH value of nano AACW suspension were obviously improved.Both the setting times and intensity of the main hydration heat peak were promoted by nano AACW,indicating the possibility of AACW suspension as nucleation seed.The early age compressive strength before 3 days was also clearly improved by nano AACW,with no negative effect on the late age strength.Furthermore,the reduced CH content with dosage of nano AACW indicates that nano AACW not only plays a role of nucleation seed in cement hydration,but also has a certain pozzolanic reaction.

Mechanical and Microstructural Analysis of Waste Ceramic Optimal Concrete Reinforced by Hybrid Fibers Materials: A Comprehensive Study

Combining different types of fibers inside a concrete mixture was revealed to improve the strength properties of cementitious matrices by monitoring crack initiation and propagation.The contribution of hybrid fibers needs to be thoroughly investigated,considering various parameters such as fibers type and content.The present study aims to carry out some mechanical and microstructural characteristics of Waste Ceramic Optimal Concrete(WOC)reinforced by hybrid fibers.Reinforcement materials consist of three dif­ferent fiber types:hook-ended steel fiber(HK),crimped steel fiber(CR)and polyvinyl alcohol(PVA)fibers and the effect of their addition on the waste ceramic composites’mechanical behaviour.Furthermore,a micro­structural analysis was carried out to understand the waste ceramic matrix composition and its bonding to hybrid fibers.Results showed that the ad­dition of hybrid fibers improved the strength characteristics of the ceramic waste composites.For instance,the existence of PVA-CR increased the tensile and flexural strength of the waste ceramic composite by 85.44%and 70.37%,respectively,with respect to the control sample(WOC).As well as hybrid fiber exhibits improved morphological properties as a result of in­creased pore filling with dense and compact structure,as well as increased C-H crystals and denser structure in pastes as a result of the incorporation of hybrid fibers into the concrete mix.The present experimental research shows the choice of using steel fiber with PVA as a reinforcement material.The idea of adding hybrid fiber is to prepare the economic,environmental,and technological concrete.Moreover,it offers a possibility for improving concrete’s durability,which is vital.Finally,it was concluded that steel fiber is more durable,and stiffer and provides adequate first crack strength and ultimate strength.In contrast,the PVA fiber is relatively flexible and improves the post-crack zone’s toughness and strain capacity.

Effect of Rubber Particle Modifi cation on Properties of Rubberized Concrete

To improve the combination of cement matrix and waste tire rubber particles in concrete, the rubber particles were treated with acrylic acid（ACA） and polyethylene glycol（PEG） for grafting hydrophilic groups on their surfaces. The X-Ray photoelectron spectroscopy（XPS） and surface contact angle were used to characterize the hydrophilicity and surface functional group of rubber particles. The effect of rubber particle modifi cation on fresh/hardened properties of rubberized concrete was studied. The experimental results show that the contact angle between rubber particle surface and water decreases when rubber particle is modifi ed. Compared with the unmodifi ed rubberized concrete（RC）, the unit weight of modifi ed rubberized concrete（MRC） changes slightly. However, the slump, air-entrainment, compressive strength, flexural strength, and impact performance of MRC are obviously improved. Under good condition of slump, the water-cement ratio of the MRC can be reduced from 0.4 to 0.38. And the compressive strength and fl exural strength of the MRC（10% rubber particle content） can be increased by 25.9% and 26.4%, respectively.

Reprocessing of Buildings＇ Demolition Waste and Utilization for the Manufacturing of New Products

Concrete is multicomponent composite material, consisting of coarse aggregate, fine aggregate, cement and water. Natural aggregates, as well as aggregates obtained after the reprocessing of buildings＇ demolition waste, can be used as coarse and fine aggregates. Characteristics of the hardened concrete depend on the raw materials, used for the preparation of concrete mixture, and their characteristics. The objective of the research is to analyse the sources of demolition waste, to describe the reprocessing technology of concrete waste, to investigate the production of the aggregate from the concrete waste, to analyse the main properties of these aggregates -- particles＇ density, bulk density, granulometric composition, hollowness and other properties, as well as to compare the obtained results with the requirements applicable to the aggregates based on natural materials.

Investigation of Water Stability of Concrete Wastes in Asphalt Treated Base

The use of concrete wastes in asphalt treated base (ATB) not only prevents environmental pollution but also protect stone resources. But the strength of the concrete wastes is often weaker than nature aggregates and the binding properties with asphalt is poor. So it difficult to meet the requirements of asphalt treated base. In this paper,The organosilicone waterproofing material is used to improve the performance of concrete wastes and the long term immersing frozen-thaw cycle test were used to investigate the anti-stripping performance of limestone asphalt mixture,concrete wastes asphalt mixture and concrete wastes asphalt mixture having treated by organosilicone waterproofing material. The experimental results show that organosilicone waterproofing material can improve the water stability of asphalt mixture. The use of concrete wastes in ATB is achievable.