Construction companies increasingly appreciate the significant benefits which accrue from using extended cements .
NEWS
CHRYSO admixtures key in extended cements
Construction companies increasingly appreciate the significant benefits which accrue from using extended cements .
Eddie Correia , general manager technical services from CHRYSO Southern Africa , says the move in South Africa towards the increased use of extenders , such as pulverised fuel ash ( PFA ) and slag in cement is in line with international trends . Blessed with an abundance of these extenders , South Africa has , in many cases , been at the forefront of this movement for a long time .
“ The need to conserve non-renewable construction material resources , as well as lower the rate of CO 2 emissions , is driving this move and the reference base of major South African projects where extended cements has been used successfully is steadily increasing ,” Correia says .
Both PFA and slag are by-products of industrial processes , which would normally be consigned to landfills . Both products are produced with a particle sizing either similar to or significantly smaller than pure cement . PFA is known to have a particularly spherical particle shape , which decreases concrete water demand and as a result increases concrete density .
Both products react with the by-products of pure cement hydration to form more hydration products in the cement paste pore structure . Under the right conditions , this process may continue for an extended period of time , extending from months to years in some cases . As a result , an ongoing densification of the concrete matrix is achieved , which in turn enhances impermeability and enhances durability . Slag is known for its chloride ion binding characteristics , further enhancing the protection of concrete from chloride ingress .
Correia says that understanding the characteristics of extended cements , and how they will perform , is critical to ensuring its successful use in construction and this places greater responsibility on admixture producers such as CHRYSO Southern Africa .
“ When replacing cement with PFA and slag the cost reduction is considerable , but you must know what you are doing ,” Correia says .
CHRYSO Southern Africa has been working with local contractors to facilitate the use of these extended cements without affecting the slump retention , workability and durability of the concrete . Correia says the use of the correct admixtures facilitates further extension of the concrete and CHRYSO has been involved in projects where up to 60 % PFA and even greater amounts of slag have been used .
Although extended cements do reach the required strengths , they react slower . This means that design mixes which include extenders must take factors such as slump retention , setting times and strength gains into account . “ The more pure cement that is replaced , the lower the
PFA is known to have a particularly spherical particle shape , which decreases concrete water demand and as a result increases concrete density . Image : Coralynne & Associates
early strength of the mix . This necessitates the introduction of activators to trigger the early strength characteristics of the concrete ,” Correia adds .
Typically slump retention ensures extended workability and is considered a critical characteristic of concrete on projects today , especially given some of the complex structures that are being constructed . Adding to this issue is the growing number of projects within the urban environment means that readymix trucks are subjected to traffic congestion while delivering .
In these applications , it is important to achieve acceptable slump retention , in conjunction with an acceptable early strength and for this admixtures with special polymers are key .
Correia says it is most important that contractors interact with CHRYSO from the start of a project as this enables the company to assess the complete cement and concrete requirements of the contract and make provision for the most appropriate admixture solution , based on the specific application needs .
“ By offering access to extensive applications knowledge and experience , coupled with our well proven range of products , it is a given that CHRYSO will provide a fit-forpurpose solution for extended cements ,” Correia says .
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