chapter  18
The EFFEC T of Limestonefillers on Sulphate Resistanceof High Alumina Cement compositers
Pages 16

A b s t r a c t T h i s paper deals with the e f f e c t of limestone f i l l e r s (powder and porous and dense aggregates) on sulphate re s i s t a n c e of HAC p a s t e s , mortars and c o n c r e t e s . A p p l i c a t i o n o f l i m e s t o n e f i l l e r s improves very s i g n i f i c a n t l y resistance of HAC a t normal temperature. I t i s mainly caused by t h e formation o f more st a b l e calcium cartoaluminate hydrate phase. That means t h a t conversion i s (considerably limited. However, higher temperature ( a b o u t 30°C o r 40°C) seems t o be a l s o d a n g e r o u s f o r u n f a v o u r a b l e p h a s e c h a n g e s . Keywords: Sulphate R e s i s t a n c e , Conversion, P a s t e , Mortar, Concrete, Limestone, Powder, Aggregate, Fhase Content, XRD, DTA

1 I n t r o d u c t i o n

I t i s g e n e r a l l y known t h a t HAC composites, i f only c o r r e c t l y made a r e of high s u l p h a t e r e s i s t a n c e . Due t o low r a t i o o f calcium oxide t o a c i d oxides i n HAC as w e l l as formation of AH3-gel f i l m s surrounding main aluminate hydrates o f lower b a s i c i t y CAHio and C2AHg, the attack of sulphate ions on HAC i s crjnsiderably limited (lea 1971 and Kurdowski e t a l . 1986). Also p h y s i c a l p r o p e r t i e s as p o r o s i t y pore s i z e d i s t r i b u t i o n and p e r m e a b i l i t y , being the r e s u l t s of HAC p a s t e s morphology, protect HAC composites from deep int e r n a l approach of sulphate ions and formation of e t t r i n g i t e . However, hexagonal calcium aluminate h y d r a t e s a r e metastable and t h e y change i n t h e conversion reaction i n stable cubic phase C 3 A H 5, AH3 and water. The conversion changes not only phase content but also physical features. The conversion products are of s u b s t a n t i a l l y higher density, than hexagonal products, and r e l e a s e of f r e e water reduce the volume of solids i n the cement paste fcy about 20% to 25% (Lea 1971, Kurdowski e t a l 1986 and George CM. 1980). I t i s equivalent to the same range i n c r e a s e i n p o r o s i t y of HAC p a s t e t h a t i n d i c a t e s t he g e n e r a l decrease i n s t r e n g t h and durability. Higher sulphate v u l n e r a b i l i t y of HAC catposites i s not only connected with increase i n porosity and permeability but a l s o w i t h lower r e s i s t a n c e of C 3 A H5 than t h a t of CAHIQ* Thus, converted HAC cannot be considered a s h i g h s u l p h a t e r e s i s t a n c e cement (Midgley, H.G. 1980), p a r t i c u l a r l y when the conversion had taken place very rapidly i n hardened composite, e.g. u n d e r h i g h e r t e m p e r a t u r e ( a b o u t 40°C).