What are the varieties of retarders?
According to the chemical compositions, retarders can be divided into two categories, organic retarders and inorganic salt retarders. The organic retarder is a more widely used category of retarder. Common varieties are hydroxyl carboxylic acids and their salts (such as tartaric acid, sodium tartrate, potassium tartrate, citric acid, etc., of which natural tartaric acid has the best retarding effects), polyhydric alcohols and their derivatives, sugars (glucose calcium, gluconates, etc.) and other carbohydrates. Most of organic retarders typically have hydrophilic active groups, together with water-reducing effects, so they are also known as retarding water-reducing agents. Inorganic salt retarders include borax, zinc chloride, zinc carbonate and sulfates, phosphates and metaphosphates of iron, copper, zinc.
Why are retarders added into wet-mixed mortar?
Retarders can delay the hydration and hardening of cement, making fresh mortar maintain plasticity in a longer time and facilitating construction.
Wet-mixed mortar is transported to the assigned place by mixing truck after metering and mixing in the professional plant, then stored in special containers and taken when necessary. At present, wet-mixed mortar is mostly supplied by the concrete mixing station. Compared with concrete, mortar is much less in the amount. The mixing station usually concentrates in a certain period of time to prepare mortar, then transport to the site, so the transport volume once is often large. However, most of building mortar construction in China is currently manual, which is slower. The mortar transported to the site will not be used up soon and need be stored for a longer time, even for a day and night, which requires that the mortar will not be congealed in a longer period, thus facilitating construction operations and avoiding waste. Therefore, retarders are often added into wet-mixed mortar to adjust the setting time of mortar. The specific varieties and dosage can be determined by experiments according to the construction, weather, traffic, and etc.
Why special retarders are used in wet-mixed mortar?
The cement in the ordinary cement mortar or cement-lime mixed mortar is generally ordinary Portland cement. Its initial setting time is 2h and final setting time is 3-4h, so the setting time of mortar is 3-8h. Wet-mixed mortar is characterized by the high production capacity every time. However, most of construction on site is manual, which is slower, so the wet-mixed mortar at the construction site will not be used up soon, need be stored in closed containers, and will gradually be used up within the specified time. Therefore, special retarders are needed to extend the operable time of wet-mixed mortar. The setting time of wet-mixed mortar can be divided according to the requirements into 8h, 12h and 24h.
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