described herein is the use of a combination of organic additives that is particularly advantageous for the preparation of cementitious compositions having a high conservation of the degree of white , which contain in mass a photocatalyst that is able to oxidize , in the presence of light , air and ambient humidity , pollutant substances present in the environment , the said photocatalyst being , in particular , titanium dioxide prevalently in the form of anatase . the above combination of additives comprises the following : a melamine resin ; a cellulose ether ; a polymer chosen from among an ethylene polymer , an acrylic polymer , and a terpolymer comprising as co - monomers at least one ester of acrylic acid with an alcohol and at least one ester of vinyl alcohol with a carboxylic acid , or alternatively , an acrylic , styrene or butadiene latex ; and a chemically modified starch .