Carbide Slag-based Composite Cementitious Material (CS-CCM) Performance Research And Environmental Impact Assessment

Carbide slag is the waste produced from PVC production by the calcium carbide hydrolysis method.The accumulation of carbide slag not only takes up a lot of land space but also causes certain pollution to the environment.To promote the recycling of carbide slag resources,the application of carbide slag to the cement industry is one of the current research directions.This article uses carbide slag waste as a cement admixture to prepare carbide slag-based composite cementitious materials（CS-CCM）.The strength evolution law and hydration mechanism of the hardened paste of CS-CCM were studied by strength test,XRD,SEM,FTIR,TG-DSC,and chemically combined water.Based on the LCA method,the environmental impact assessment of the preparation process of carbide slag admixture was carried out,and the correlation between the environmental impact and mechanical properties of CS-CCM was analyzed.The main conclusions obtained are as follows.（1）The carbide slag raw materials were pretreated with calcination temperatures of300°C,500°C,and 700°C.By increasing the calcination temperature of the carbide slag,CaO components with hydration activity gradually appear in the carbide slag phase.（2）The increase of water-cement ratio and carbide slag admixture can make the compressive strength and flexural strength of the hardened paste of CS-CCM gradually decrease.Increasing the calcination temperature of carbide slag,the compressive strength and flexural strength of the hardened paste of CS-CCM show an upward trend.The second-order polynomial regression model related to the compressive strength R_C and the flexural strength R_F of the hardened paste of the CS-CCM was established by using the response surface method（RSM）,and the predicted values from the prediction model were close to the actual test values.（3）HTP is the largest impact category in the total environmental impact of carbide slag blending,where HTP,GWP,and AP are the three main impact categories in the environmental impact of carbide slag blending,while several other impact types have a relatively small impact on the environmental impact of carbide slag blending.The power resources consumed by the calcination and sieving processes in the environmental impact category contribute most of the environmental impacts.The environmental impact results of carbide slag admixture show sensitivity to the calcination process and sieving process,but not to the transportation process and drying process;the comparative analysis of the results of several other evaluation methods verifies that the CML-IA baseline evaluation method chosen in this case has a certain degree of reliability.（4）With the gradual increase in the dosage of carbide slag,the proportions of HTP,AP,and ADP in the environmental impact of CS-CCM gradually increase,while the proportions of GWP,POCP,and EP gradually decrease,and HTP,GWP,and AP are the main impact categories in the environmental impact of CS-CCM.Under the premise of meeting the compressive strength requirements of CS-CCM,the CS-CCM with a small dosage of carbide slag are more conducive to reducing the environmental impact contributed by the unit compressive strength.（5）The hydration products of CS-CCM mainly include calcite,calcium hydroxide,ettringite（AFt）,tricalcium silicate,and dicalcium silicate.In the SEM image,irregular C-S-H gel,sheet-like Ca（OH）₂,and needle-like AFt can be observed.The increase of water-binder ratio,carbide slag dosage,and temperature of carbide slag all increase the dosage of Ca（OH）₂ in the weight loss stage from 430℃to 560℃.In the results of the FTIR spectrum,the C-O symmetry stretching vibration peak in the C-S-H gel around 874cm^-1 and the endothermic peak caused by the vibration of the O-H bond in the Ca（OH）₂around 3642 cm^-1 can be analyzed,while the peak at 1423 cm^-1 is the C-O characteristic peak.