About curing you have to know 关于养护你需要知道的
In countries where the climate is favorable, green blocks are moist cured at normal temperature of 20°C to 37°C (as in the South of China). This type of curing which at 4 days would normally give 40% of its ultimate strength. Initially, green blocks should be placed in a shaded area or enclosed chambers for about 8-12 hours (depends on relative weather conditions i.e. temperature, humidity etc). After that, the blocks can be transported to an assembly yard for further curing for about 28 days to reach 99% of its maximum strength. For optimal final products, the fresh blocks need to be sprinkled daily, for the first 7 days (morning and evening) to maintain the moisture content for higher reactivity of cement with sand.
Low pressure Steam Curing
Steam curing at atmospheric pressure at temperature of 65ºC in a curing chamber accelerates the hardening process. The main benefit of steam curing is the rapid strength gain in the units, which allows them to be placed in inventory within hours after they are molded. 2-4 days after molding, the compressive strength of the blocks will be 90% or more of the final ultimate strength. Besides, steam curing produces units of lighter color than is usually obtained with natural curing.
Initial temperature of the concrete shall not be raised above 48ºC for a minimum of 2 hours after the units have been cast
The rate of increase after 2 hour period shall not exceed 15°C/hr and the maximum temperature shall not exceed 65ºC.
The maximum temperature shall be held for a period sufficient to develop the required strength (4-5 hours)
Rate of decrease in temperature shall not exceed 10ºC/hr.
Units shall be kept covered for a minimum of 24 hours after casting.
High pressure steam curing
This method uses saturate steam at pressures ranging from 125 to 150 psi and temperature of 178°C. This method usually requires additional equipment such as an autoclave (kiln). The strength of high pressure cured concrete masonry units at one day age are equivalent to the 28 days strengths of moist-cured blocks. This process produces dimensionally stable units that exhibit less volume change (up to 50% less). However, autoclave unit requires a much higher investment.
*Practical Suggestion for Curing
28-days curing to gain the full strength of masonry product is based on concrete which is slightly different when applying for the dry mix material for the block making. It is very commonly now cement is added with high quality Fly –Ash, and under the favorable conditions like the temperature and humidity, the compressive strength of the block/paver will gain up to 80% in less than 7 days curing. By using the #425 type cement and design the mix proportional at least 20% higher than the required compressive strength (Mpa), the block/paver will be qualified to be delivered to the clients.