10.8
碳化收缩
大气中的 CO_2 渗入混凝土孔隙，在水分存在下形成碳酸，碳酸随后与水合水泥中的氢氧化钙发生反应，形成碳酸钙。这一过程会减少混凝土的体积，称为碳化收缩。
由于反应过程中产生的碳酸钙填充了混凝土的孔隙，混凝土的渗透性略有降低。此外，由于反应过程中释放的水促进了未反应水泥的水化，混凝土的强度略有提高。然…
在潮湿条件下，进入混凝土表面的大气二氧化碳会转化为碳酸，后者与水化水泥中的氢氧化钙发生化学反应，生成碳酸钙和水。
反应过程中生成的碳酸钙沉积在混凝土孔隙中，而释放出的水则促进未反应水泥的水化。
这会导致混凝土收缩，称为碳化收缩。
碳化从混凝土表面开始，并缓慢向内推进。
其进展取决于混凝土的渗透性、含水率、二氧化碳含量以及周围空气的相对湿度。
随着碳化的进行，水化水泥浆体的碱性逐渐中和。
因此，如果氧气和水分渗入混凝土，其中嵌入的钢筋很可能发生腐蚀。
通常，为了评估混凝土的碳化程度，会在新鲜断裂的表面喷洒酚酞溶液。
呈现粉红色的区域表明存在游离氢氧化钙，说明未发生碳化；而保持原有颜色的区域则表明已发生碳化。
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Q1: What chemical reaction causes carbonation shrinkage in concrete?
Atmospheric carbon dioxide enters concrete pores and forms carbonic acid in the presence of moisture. This acid reacts with calcium hydroxide in the hydrated cement, producing calcium carbonate and water. The calcium carbonate deposits in the pores, causing concrete volume to contract, while the released water aids hydration of unreacted cement.
Q2: How does carbonation progress through concrete?
Carbonation starts at the concrete surface and moves inward slowly. Its progression rate depends on the concrete's permeability, moisture level, atmospheric carbon dioxide content, and relative humidity. Water-filled pores slow carbon dioxide diffusion, so concrete shielded from rainfall but exposed to moist air experiences more significant carbonation than periodically rain-rinsed concrete.
Q3: What are the effects of carbonation on concrete properties?
Carbonation slightly reduces concrete permeability as calcium carbonate fills pores and slightly enhances strength through continued cement hydration. However, it neutralizes the alkaline nature of the cement paste. When carbonation reaches reinforcement steel, the loss of alkalinity allows moisture and oxygen to penetrate, significantly increasing the risk of steel corrosion.
Q4: Why does carbonation increase the risk of reinforcement corrosion?
The alkaline environment of hydrated cement normally protects embedded steel from corrosion. Carbonation neutralizes this alkalinity, removing the protective barrier. If oxygen and moisture then penetrate the concrete and reach the reinforcement steel, corrosion becomes likely, compromising the structural integrity of the concrete member.
Q5: How is carbonation depth measured in concrete?
A freshly broken concrete surface is sprayed with phenolphthalein indicator solution. Non-carbonated areas containing free calcium hydroxide turn pink, while carbonated areas retain their original color. This color contrast reveals the carbonation depth and helps assess how far the carbonation process has advanced into the concrete.
Q6: What factors influence the rate of carbonation in concrete?
Carbonation rate is controlled by concrete permeability, moisture content, atmospheric carbon dioxide levels, and ambient relative humidity. Lower permeability slows carbon dioxide diffusion. Higher moisture content in pores also reduces diffusion rates. Environmental conditions with moderate humidity and higher CO2 concentrations accelerate carbonation compared to very wet or very dry conditions.
Q7: How does carbonation differ from other forms of shrinkage in concrete?
Carbonation shrinkage results from a chemical reaction between atmospheric CO2 and cement paste components, causing volume reduction and potential reinforcement corrosion. Unlike drying shrinkage, which occurs from moisture loss, carbonation is driven by atmospheric carbon dioxide penetration and can occur even in moist environments, making it a distinct deterioration mechanism.