摘要
对经CO2矿化后的再生骨料(RA)吸水率和表观密度的影响因素进行分析.结果表明:CO2矿化处理能有效改善再生骨料的物理性能,且随着固碳率的增加,RA的物理性能改善愈发显著;当RA含水率为1.3%时,在环境温度和相对湿度分别为50 ℃和98%,CO2体积分数、压力、碳化时间分别为99%、0.15 MPa、24 h条件下,RA的碳化强化效果最佳,与未经碳化处理的RA相比,其固碳率为1.77%,吸水率降低33.47%,表观密度增加2.06%.经CO2矿化处理后,RA中的Ca(OH)2和水化硅酸钙(C‑S‑H)凝胶与CO2发生碳化反应,在孔隙中生成方解石晶型的CaCO3,使得界面过渡区变得致密,孔隙率由22.07%降至12.78%,实现了RA对CO2的封存,改善了RA的物理性能.
混凝土是应用最为广泛的建筑材料,降低其碳排放,是建筑材料行业低碳转型的重点任
RA由于表面附着旧砂浆且因破碎形成了大量裂
鉴于此,本试验以废弃混凝土资源化利用得到的RA为研究对象,研究骨料含水率、环境温度和相对湿度、CO2体积分数、压力和碳化时间等不同碳化工艺参数对RA物理性能的改善效果和CO2固化能力的影响.并借助微观分析手段,从物相组成、孔结构和微观形貌等方面对RA碳化反应后的微观性能进行探究.
将建筑物拆除得到的废弃混凝土(经现场实测其强度等级为C30),破碎、清洗、筛分后获得粒径为5~20 mm的RA.测得RA的初始含水率(质量分数,文中涉及的含水率、吸水率等除特别注明外均为质量分数)为1.3%、吸水率为6.9%,表观密度为2 620 kg/m³.表面附着砂浆占RA总质量的25.39%,其中Ca(OH)2含量为1.78%,CaCO3含量为32%,无定形物(主要为水化硅酸钙(C‑S‑H)凝
将RA置于密闭的碳化反应釜中,研究RA含水率(w),环境温度(θ)、相对湿度(RH),CO2体积分数φ(CO2)、压力(p)和碳化时间(t)对RA碳化反应的影响.RA碳化试验参数如
Carbonation parameter | Value | Other condition |
---|---|---|
w/% | 0/1.3/3.2/6.8 | φ(CO2): 99%;θ: 25 ℃;p: 0.10 MPa; t: 24 h |
RH/% | 10/30/50/80/98 | |
φ(CO2)/% | 20/50/70/99 | w: 1.3%; RH: 98%; θ: 25 ℃; p: 0.10 MPa; t: 24 h |
θ/℃ | 25/50/70/90 | w: 1.3%; RH: 98%; φ(CO2): 99%; p: 0.10,0.15 MPa; t: 24 h |
p/MPa | 0.10/0.15 | w: 1.3%; RH: 98%; φ(CO2): 99%; θ: 25,50,70,90 ℃; t: 24 h |
t/h | 2/4/8/12/24 | w: 1.3%; RH: 98%; φ(CO2): 99%; θ: 25、50 ℃; p: 0.10 MPa |
(1)RA物理性能
依据GB/T 14685—2022《建设用卵石、碎石》,对碳化前后RA的吸水率和表观密度进行测试.
(2)RA固碳率(ζ,%)
ζ由碳化前后RA中的CaCO3质量差计算得到,计算式如式(
(1) |
(2) |
式中:m(CaCO3)为RA中CaCO3的质量,g;Δm(CO2)为热重(TG)分析结果中550~950 ℃内由于CaCO3分解释放CO2引起的样品质量损失,g;m(CaCO3)a为碳化后RA中CaCO3的质量,g;m(CaCO3)b为碳化前RA中CaCO3的质量,g;m(RA)为105 ℃干燥状态下RA的质量,g.
(3)RA物相组成
首先,将碳化前后的RA进行破碎,剔除原生骨料并保留附着砂浆;然后,将其置于研钵中研磨,得到粒径不大于75.00 μm的粉末;最后,将粉末样品在35 ℃的真空干燥箱中干燥72 h,取出冷却至室温,即可进行TG分析和X射线衍射(XRD)测试.
(4)RA孔结构
采用压汞法(MIP)对碳化前后RA的孔结构变化进行表征.测试前,选取粒径约为4.75 mm的RA,置于真空干燥箱中干燥72 h;之后采用AutoPore IV 9500型全自动压汞仪进行MIP测试,研究碳化前后RA的孔隙率(体积分数)变化.
(5)RA微观结构和形貌
采用JSM‑IT500型扫描电镜(SEM)中的背散射电子(BSE),分析碳化前后RA的界面过渡区(ITZ)和孔隙结构等微观结构的变
根据RA含水率与环境相对湿度的不同,将RA的碳化湿度划分为4个区:“内干外干区”(w<2.0%、RH<40%)、“内干外湿区”(w<2.0%、RH>70%)、“内湿外湿区”(w>4.0%、RH>70%)和“内湿外干区”(w>4.0%、RH<40%).RA含水率与环境相对湿度相互作用对RA碳化反应的影响如

图1 RA含水率与环境相对湿度相互作用对RA碳化反应的影响
Fig.1 Effect of interaction between water content of RA and environmental relative humidity on RA carbonation reaction
RA孔隙内的水分是其碳化反应的基础.当RA孔隙中的水分较少时,碳化反应所需水分不足,碳化效果较差;而当RA的孔隙被水填满时,CO2向内部反应区的扩散被阻碍,同样也会限制RA的碳化反
CO2体积分数对RA碳化反应的影响如

图2 CO2体积分数对RA碳化反应的影响
Fig.2 Effect of CO2 volume fraction on RA carbonation reaction
环境温度和压力对RA碳化反应的影响如

图3 环境温度和压力对RA碳化反应的影响
Fig.3 Effect of environmental temperature and pressure on RA carbonation reaction
由
升高环境温度可提高CO2的扩散速
由
碳化时间对RA碳化反应的影响如

图4 碳化时间对RA碳化反应的影响
Fig.4 Effect of carbonation time on RA carbonation reaction
RA中水化产物的碳化反应包括3个阶
当RA含水率为1.3%时,在环境温度和相对湿度分别为50 ℃和98%,CO2体积分数为99%、压力为0.15 MPa、碳化时间为24 h的条件下,RA的物理性能改善最为显著,固碳率最高.与未经处理的RA相比,其吸水率降低33.47%,表观密度增加2.06%,固碳率达1.77%.对未经处理的RA和碳化效果最佳的RA进行微观分析,以研究碳化处理对RA物相组成及微观结构的影响.
碳化前后RA的热重(TG)分析结果如

图5 碳化前后RA的热重分析结果
Fig.5 TG analysis result of RA before and after carbonation
由
由
为进一步确定碳化后RA的物相变化,进行了XRD分析,结果见

图6 碳化前后RA 的XRD分析结果
Fig.6 XRD analysis result of RA before and after carbonation
由上述TG与XRD分析可知:(1)RA中可碳化成分为Ca(OH)2、AFt、Mc和C‑S‑H凝胶.(2)RA碳化前,其中的Ca(OH)2和AFt含量较少;RA碳化后,Ca(OH)2和Mc几乎完全反
为区分碳化处理对不同类型孔隙的影响,统计了气孔(孔径d>1 000 nm),毛细孔(10 nm≤d≤1 000 nm),凝胶孔(d<10 nm

图7 碳化前后RA的孔隙分布
Fig.7 Pore distribution of RA before and after carbonation
采用原位观测手段,对碳化前后RA的微观结构进行图像分析,其BSE图像见

图8 碳化前后RA的BSE图像
Fig.8 BSE images of RA before and after carbonation
碳化前RA的微观结构松散多孔,为有害离子传输和水分进入提供了通道,导致RA吸水率和表观密度等物理性能较差.而碳化反应后RA中的Ca(OH)2生成了CaCO3,C‑S‑H凝胶生成了CaCO3和硅胶,反应生成物填充在孔隙和裂缝处,对RA的微观结构起到了良好的填充效应,使得RA的微观结构由疏松变得致密,进而改善了RA的物理性能.

图9 碳化前后RA的SEM照片
Fig.9 SEM images of RA before and after carbonation
(1)随着固碳率的增加,再生骨料(RA)的物理性能改善越显著.RA经CO2矿化处理后,附着砂浆中的Ca(OH)2和水化硅酸钙(C‑S‑H)凝胶等组分与CO2反应生成CaCO3,其沉淀并填充于微裂缝和孔隙处,使RA孔隙率降低,骨料与附着砂浆之间的界面过渡区更加密实,进而实现了RA对CO2的封存和自身物理性能的改善.
(2)CO2体积分数、压力和碳化时间的提高有利于RA固碳率的增加.RA含水率和环境相对湿度具有协同作用,当RA含水率较低且环境相对湿度较高或RA含水率较高且环境相对湿度较低时,均利于固碳率的增加.温度过低或过高均不利于RA固碳率的增加,即再生骨料存在最佳温度区间.
(3)当RA含水率为1.3%时,在环境温度和相对湿度分别为50 ℃和98%,CO2体积分数、压力、碳化时间分别为99%、0.15 MPa、24 h时,RA的物理性能最佳,固碳率最高.与未经碳化处理的RA相比,其吸水率降低33.47%,表观密度增加2.06%,固碳率为1.77%.
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