SUMMARY 
 
 
 
 
 
Portland cement and gypsum are binder materials that in a separate way are widely used in construction. Once hardened, Portland cement has good strength and durability properties in wet environments; however it is expensive and has a slow setting process. On the other hand, the gypsum has the advantage of being cheaper and having a faster setting, however its use is limited due to its low mechanical strength and its sensibility to wet environments.    
 

The incompatibility of the combination of “cement/gypsum” in mixtures is widely known. In wet environment, they form “ettringite” and other compounds that cause a drastic decrease in the mechanical strength and excessive expansions that destroy them. However, some authors has proved that the addition of pozzolanic materials improve such incompatibility. 
 

In this research, the mechanical properties and durability of mixtures “cement/gypsum”, with the addition of pozzolans such as type F fly ash, are studied. To investigate the pozzolan action, the results are compared with mixes containing a non-pozzolanic material know as “limestone filler”. 
 

A complete laboratory experimental program was developed, where mortars binders based on different combinations of “cement/gypsum/pozzolan” were tested. The goal was to obtain a good mechanical strength in combination with a low level of volumetric expansion. 
 

The results show that the ternary systems “cement/gypsum/fly ash”, with low Portland cement content, presented remarkable mechanical characteristics and durability. It was noted in these systems the beneficial effect produced by the fly ash acting as a pozzolan. Based on the results, new and interesting applications arise considering the fact that an important percentage of cement can be replaced by fly ash, with an important improvement in the compatibility between cement and gypsum also. All this make the final product less expensive, widening the range of applications considering that in many regions, the fly ash and the gypsum are more accessible and cheaper. 
 

It was demonstrated that the addition of an adequate proportion of fly ash not only has a favorable effect in the compatibility between gypsum and the cement but also contribute to the increase in the final strength. Considering also the saving related to the replacement of the cement, the beneficial effect of the pozzolanic material in the durability is confirmed.     
 

The results of this research suggest a possible utilization of these mortars in construction applications, particularly in low income houses where a reduction in the amount of Portland cement due to the use of fly ash would produce a considerable reduction in the final cost.