<PAGE>   1
                                 EXHIBIT 10.20
<PAGE>   2
                AMENDMENT TO REVOLVING CREDIT FACILITY AGREEMENT

Amendment dated as of June 1, 1996 (the "Amendment") to that certain Revolving
Credit Facility Agreement dated as of September 1, 1994 (the "Credit
Agreement") between Lafarge Corporation, a Maryland corporation (the "Company")
and Wachovia Bank of Georgia, N.A.  (the "Bank").

WHEREAS, the Company and the Bank desire to amend certain provisions of the
Credit Agreement;

NOW, THEREFORE, the parties hereby agree as follows:

1.  Terms used herein shall have the same meaning as set forth in the Credit
Agreement.

2.  The definition of "Applicable Facility Fee Rate" in Section 1.01 is hereby
changed by deleting the percentages in Column B and substituting therefor the
following percentages:

<TABLE>
<CAPTION>
                                           Current Rate             Proposed Rate
                                           ------------             -------------
Column A                                                            Column B
--------                                                            --------
<S>                                        <C>                      <C>
Level I   Status                           .1250%                   .0750%
Level II  Status                           .1500%                   .1000%
Level III Status                           .1875%                   .1000%
Level IV  Status                           .2000%                   .1250%
</TABLE>

3.  The definition of "Applicable Margin" in Section 1.01 is hereby changed by
deleting the percentages in Column B and substituting therefor the following
percentages:

<TABLE>
<CAPTION>
                                           Current Rate             Proposed Rate
                                           ------------             -------------
Column A                                                            Column B
--------                                                            --------
<S>                                        <C>                      <C>
Level I   Status                           .2500%                   .1750%
Level II  Status                           .2500%                   .2000%
Level III Status                           .3000%                   .2500%
Level IV  Status                           .3750%                   .2750%
</TABLE>

4.  The definition of "Commitment" in Section 1.01 is hereby changed from
$15,000,000 to $30,000,000.

5.  Section 8.01 is hereby amended by deleting both references to the date
August 31, 1999 and substituting therefor the date June 1, 2001.

6.  In order to induce the Bank to enter into this Amendment, the Company
hereby (x) makes each of the representations and warranties contained in
Article V of the Credit Agreement on the
<PAGE>   3
Amendment Effective Date (as hereinafter defined), after giving effect to this
Amendment, and (y) represents and warrants that no Default or Event of Default
exists after giving effect to this Amendment.

7.  This Amendment shall become effective as of the date first above written
(the "Amendment Effective Date") when and if counterparts of this Amendment
shall have been executed and delivered by the Company and the Bank.

8.  On and after the Amendment Effective Date, each reference in the Credit
Agreement to "this Agreement", "hereunder", "hereof" or words of like import
referring to the Credit Agreement shall mean the Credit Agreement as amended
hereby. The Credit Agreement is, and shall continue to be, in full force and
effect and is hereby ratified and confirmed in all respects, as modified and
amended hereby.

9.  This Amendment may be executed in any number of separate counterparts, each
of which counterparts shall be an original and all of which taken together
shall constitute one and the same agreement. Delivery by facsimile of any
executed signature page hereof shall be effective as delivery of an executed
counterpart hereof.

10.  This Amendment shall be governed by, and construed in accordance with, the
laws of the State of New York.

IN WITNESS WHEREOF, the parties hereto have caused this Amendment to Revolving
Credit Facility Agreement to be duly executed by their respective authorized
officers as of the date first above written.


                              Wachovia Bank of Georgia, N.A.
                              
                              
                              By: /s/ K.A. SHREINER  
                                 ---------------------------------------
                              Title:  Senior Vice President
                                    ------------------------------------

                              
                              LAFARGE CORPORATION
                              
                              
                              By: /s/ KEVIN GRANT
                                 ---------------------------------------
                              Title:   Treasurer
<PAGE>   4
                AMENDMENT TO REVOLVING CREDIT FACILITY AGREEMENT

Amendment dated as of June 1, 1996 (the "Amendment") to that certain Revolving
Credit Facility Agreement dated as of September 1, 1994 (the "Credit
Agreement") between Lafarge Corporation, a Maryland corporation (the "Company")
and Royal Bank of Canada (the "Bank").

WHEREAS, the Company and the Bank desire to amend certain provisions of the
Credit Agreement;

NOW, THEREFORE, the parties hereby agree as follows:

1.  Terms used herein shall have the same meaning as set forth in the Credit
Agreement.

2.  The definition of "Applicable Facility Fee Rate" in Section 1.01 is hereby
changed by deleting the percentages in Column B and substituting therefor the
following percentages:

<TABLE>
<CAPTION>
Column A                                   Column B
--------                                   --------
<S>                                        <C>
Level I   Status                           .0750%
Level II  Status                           .1000%
Level III Status                           .1000%
Level IV  Status                           .1250%
</TABLE>

3.  The definition of "Applicable Margin" in Section 1.01 is hereby changed by
deleting the percentages in Column B and substituting therefor the following
percentages:

<TABLE>
<CAPTION>
Column A                                   Column B
--------                                   --------
<S>                                        <C>
Level I   Status                           .1750%
Level II  Status                           .2000%
Level III Status                           .2500%
Level IV  Status                           .2750%
</TABLE>

4.  The definition of "Commitment" in Section 1.01 is hereby changed from
$15,000,000 to $30,000,000.

5.  Section 8.01 is hereby amended by deleting both references to the date
August 31, 1999 and substituting therefor the date June 1, 2001.

6.  In order to induce the Bank to enter into this Amendment, the Company
hereby (x) makes each of the representations and warranties contained in
Article V of the Credit Agreement on the Amendment Effective Date (as
hereinafter defined), after giving effect to this Amendment, and
<PAGE>   5
(y) represents and warrants that no Default or Event of Default exists after
giving effect to this Amendment.

7.  This Amendment shall become effective as of the date first above written
(the "Amendment Effective Date") when and if counterparts of this Amendment
shall have been executed and delivered by the Company and the Bank and the Bank
shall have received new Promissory Notes reflecting its Commitment.

8.  On and after the Amendment Effective Date, each reference in the Credit
Agreement to "this Agreement", "hereunder", "hereof" or words of like import
referring to the Credit Agreement shall mean the Credit Agreement as amended
hereby. The Credit Agreement is, and shall continue to be, in full force and
effect and is hereby ratified and confirmed in all respects, as modified and
amended hereby.

9.  This Amendment may be executed in any number of separate counterparts, each
of which counterparts shall be an original and all of which taken together
shall constitute one and the same agreement. Delivery by facsimile of any
executed signature page hereof shall be effective as delivery of an executed
counterpart hereof.

10.  This Amendment shall be governed by, and construed in accordance with, the
laws of the State of New York.

IN WITNESS WHEREOF, the parties hereto have caused this Amendment to Revolving
Credit Facility Agreement to be duly executed by their respective authorized
officers as of the date first above written.


                              Royal Bank of Canada


                              By:  /s/ PETER D. STEFFEN
                                 ---------------------------------------
                              Title:   Senior Manager
                                    ------------------------------------

                              
                              LAFARGE CORPORATION
                              
                              
                              By:  /s/ KEVIN GRANT
                                 ---------------------------------------
                              Title:   Treasurer
<PAGE>   6


                AMENDMENT TO REVOLVING CREDIT FACILITY AGREEMENT

Amendment dated as of June 1, 1996 (the "Amendment") to that certain Revolving
Credit Facility Agreement dated as of September 1, 1994 (the "Credit
Agreement") between Lafarge Corporation, a Maryland corporation (the "Company")
and The First National Bank of Chicago. (the "Bank").

WHEREAS, the Company and the Bank desire to amend certain provisions of the
Credit Agreement;

NOW, THEREFORE, the parties hereby agree as follows:

1.  Terms used herein shall have the same meaning as set forth in the Credit
Agreement.

2.  The definition of "Applicable Facility Fee Rate" in Section 1.01 is hereby
changed by deleting the percentages in Column B and substituting therefor the
following percentages:

<TABLE>
<CAPTION>
Column A                                   Column B
--------                                   --------
<S>                                        <C>
Level I   Status                           .0750%
Level II  Status                           .1000%
Level III Status                           .1000%
Level IV  Status                           .1250%
</TABLE>

3.  The definition of "Applicable Margin" in Section 1.01 is hereby changed by
deleting the percentages in Column B and substituting therefor the following
percentages:

<TABLE>
<CAPTION>
Column A                                   Column B
--------                                   --------
<S>                                        <C>
Level I   Status                           .1750%
Level II  Status                           .2000%
Level III Status                           .2500%
Level IV  Status                           .2750%
</TABLE>

4.  The definition of "Commitment" in Section 1.01 is hereby changed from
$15,000,000 to $35,000,000.

5.  Section 8.01 is hereby amended by deleting both references to the date
August 31, 1999 and substituting therefor the date June 1, 2001.

6.  In order to induce the Bank to enter into this Amendment, the Company
hereby (x) makes each of the representations and warranties contained in
Article V of the Credit Agreement on the
<PAGE>   7
Amendment Effective Date (as hereinafter defined), after giving effect to this
Amendment, and (y) represents and warrants that no Default or Event of Default
exists after giving effect to this Amendment.

7.  This Amendment shall become effective as of the date first above written
(the "Amendment Effective Date") when and if counterparts of this Amendment
shall have been executed and delivered by the Company and the Bank and the Bank
shall have received new Promissory Notes reflecting its Commitment, Board
Resolutions covering this Amendment, and Incumbency Certificates for the
signers.

8.  On and after the Amendment Effective Date, each reference in the Credit
Agreement to "this Agreement", "hereunder", "hereof" or words of like import
referring to the Credit Agreement shall mean the Credit Agreement as amended
hereby. The Credit Agreement is, and shall continue to be, in full force and
effect and is hereby ratified and confirmed in all respects, as modified and
amended hereby.

9.  This Amendment may be executed in any number of separate counterparts, each
of which counterparts shall be an original and all of which taken together
shall constitute one and the same agreement. Delivery by facsimile of any
executed signature page hereof shall be effective as delivery of an executed
counterpart hereof.

10.  This Amendment shall be governed by, and construed in accordance with, the
laws of the State of New York.

IN WITNESS WHEREOF, the parties hereto have caused this Amendment to Revolving
Credit Facility Agreement to be duly executed by their respective authorized
officers as of the date first above written.


                              The First National Bank of Chicago


                              By: /s/ DIANE SELPH
                                 ---------------------------------------
                              Title:  Authorized Agent
                                    ------------------------------------

                              
                              LAFARGE CORPORATION
                              
                              
                              By:  /s/ KEVIN GRANT
                                 ---------------------------------------
                              Title:   Treasurer
<PAGE>   8
                AMENDMENT TO REVOLVING CREDIT FACILITY AGREEMENT

Amendment dated as of June 1, 1996 (the "Amendment") to that certain Revolving
Credit Facility Agreement dated as of September 1, 1994 (the "Credit
Agreement") between Lafarge Corporation, a Maryland corporation (the "Company")
and CIBC, Inc. (the "Bank").

WHEREAS, the Company and the Bank desire to amend certain provisions of the
Credit Agreement;

NOW, THEREFORE, the parties hereby agree as follows:

1.  Terms used herein shall have the same meaning as set forth in the Credit
Agreement.

2.  The definition of "Applicable Facility Fee Rate" in Section 1.01 is hereby
changed by deleting the percentages in Column B and substituting therefor the
following percentages:

<TABLE>
<CAPTION>
Column A                                   Column B
--------                                   --------
<S>                                        <C>
Level I   Status                           .0750%
Level II  Status                           .1000%
Level III Status                           .1000%
Level IV  Status                           .1250%
</TABLE>

3.  The definition of "Applicable Margin" in Section 1.01 is hereby changed by
deleting the percentages in Column B and substituting therefor the following
percentages:

<TABLE>
<CAPTION>
Column A                                   Column B
--------                                   --------
<S>                                        <C>
Level I   Status                           .1750%
Level II  Status                           .2000%
Level III Status                           .2500%
Level IV  Status                           .2750%
</TABLE>

4.  The definition of "Commitment" in Section 1.01 is hereby changed from
$20,000,000 to $35,000,000.

5.  Section 8.01 is hereby amended by deleting both references to the date
August 31, 1999 and substituting therefor the date June 1, 2001.

6.  In order to induce the Bank to enter into this Amendment, the Company
hereby (x) makes each of the representations and warranties contained in
Article V of the Credit Agreement on the Amendment Effective Date (as
hereinafter defined), after giving effect to this Amendment, and
<PAGE>   9
(y) represents and warrants that no Default or Event of Default exists after
giving effect to this Amendment.

7.  This Amendment shall become effective as of the date first above written
(the "Amendment Effective Date") when and if counterparts of this Amendment
shall have been executed and delivered by the Company and the Bank and the Bank
shall have received new Promissory Notes reflecting its Commitment.

8.  On and after the Amendment Effective Date, each reference in the Credit
Agreement to "this Agreement", "hereunder", "hereof" or words of like import
referring to the Credit Agreement shall mean the Credit Agreement as amended
hereby. The Credit Agreement is, and shall continue to be, in full force and
effect and is hereby ratified and confirmed in all respects, as modified and
amended hereby.

9.  This Amendment may be executed in any number of separate counterparts, each
of which counterparts shall be an original and all of which taken together
shall constitute one and the same agreement. Delivery by facsimile of any
executed signature page hereof shall be effective as delivery of an executed
counterpart hereof.

10.  This Amendment shall be governed by, and construed in accordance with, the
laws of the State of New York.

WITNESS WHEREOF, the parties hereto have caused this Amendment to Revolving
Credit Facility Agreement to be duly executed by their respective authorized
officers as of the date first above written.


                              CIBC, Inc.


                              By:  /s/ CHERYL ROOT  
                                 ---------------------------------------
                              Title: Authorized Signatory
                                    ------------------------------------

                              
                              LAFARGE CORPORATION
                              
                              
                              By:  /s/ KEVIN GRANT
                                 ---------------------------------------
                              Title:   Treasurer
<PAGE>   10

                AMENDMENT TO REVOLVING CREDIT FACILITY AGREEMENT


Amendment dated as of August 1, 1996 (this "Amendment") to that certain
Revolving Credit Facility Agreement dated as of September 1, 1994 as amended as
of June 1, 1996 (the "Credit Agreement") between Lafarge Corporation, a
Maryland corporation (the "Company") and The First National Bank of Chicago
(the "Bank").


WHEREAS, the Company and the Bank desire to amend certain provisions of the
Credit Agreement;

NOW, THEREFORE, the parties hereby agree as follows:

1.       Terms used herein shall have the same meaning as set forth in the
         Credit Agreement.

2.       The definition of "Commitment" in Section 1.01 is hereby changed from
         $35,000,000 to $30,000,000.

3.       The definition of "Debt" in Section 1.01 is hereby amended by adding:

         (ix)  all obligations of such Person arising under any Hedge
         Agreement.

4.       The following definition of "Hedge Agreement" is hereby added to
         Section 1.01 between the definitions of "Guarantee" and "Interest
         Period":

                 "Hedge Agreement" means any interest rate swap, cap, floor or
         collar agreement, interest rate future or option contract, currency
         swap agreement, currency future or option contract and any other
         similar agreement (including any option with respect to any of the
         foregoing) or any combination of the foregoing.

5.       The following definition of "Regulation G" is hereby added to Section
         1.01 between the definitions of "Regulation D" and "Regulation U":

                 "Regulation G" means Regulation G of the Board of Governors of
         the Federal Reserve System, as in effect from time to time.

6.       Section 3.03 (b) is hereby deleted and replaced in its entirety by the
         following:

                 (b)  If after May 1, 1996, the adoption or effectiveness of
         any applicable law, rule or regulation regarding capital adequacy, or
         any change therein, or any change in the interpretation or
         administration thereof by any governmental authority, central bank or
         comparable agency charged with the interpretation or administration
         thereof, or compliance by the Bank (or its Lending Office) or any
         corporation controlling the Bank with any request or directive
         regarding capital adequacy which is effective after May 1,
<PAGE>   11
         1996 (whether or not having the force of law) of any such authority,
         central bank or comparable agency, increases the capital required to
         be maintained by the Bank or such corporation as a consequence of its
         obligations hereunder, which increase results in a reduction of the
         rate of return on the Bank's or such corporation's capital to a level
         below that which the Bank or such corporation would have achieved but
         for such adoption, change or compliance (taking into consideration the
         Bank's or such corporation's policies with respect to capital
         adequacy) by any amount deemed by the Bank to be material, then from
         time to time, within 15 days after demand by the Bank, the Company
         shall pay to the Bank such additional amount or amounts as will
         compensate the Bank for such reduction.

7.       The following sections are hereby added  to Article V, Representations
         and Warranties:

                 SECTION 5.10.  Use of Proceeds.   The Company is not engaged
         in the business of extending credit for the purpose of purchasing or
         carrying any "margin stock" within the meaning of Regulation U, and
         will not use the proceeds of the Loans, directly or indirectly, for
         the purpose, whether immediate, incidental or ultimate, of purchasing
         or carrying any "margin stock".

                 SECTION 5.11.  Environmental Representation.  The operations
         and properties of the Company and each of its Subsidiaries are in
         compliance in all material respects with all applicable environmental
         laws, regulations and permits, all necessary environmental permits and
         approvals have been obtained and are in effect for the operations and
         properties of the Company and its Subsidiaries, and the Company and
         its Subsidiaries are in substantial compliance in all material
         respects with all such environmental permits and approvals, except to
         the extent that the failure to comply with any such environmental laws
         or regulations, or to comply with or to obtain any such environmental
         permits and approvals, would not have a material adverse effect on the
         business, financial position or results of operations of the Company
         and Lafarge Canada Inc. considered as a whole.

8.       The second sentence of Section 6.07 is hereby deleted and replaced in
         its entirety by the following:

                 The Company will cause each Material Subsidiary to maintain
         its corporate existence, and will not permit any Material Subsidiary
         to consolidate or merge with or into, or transfer all or any
         substantial part of its assets to, any Person other than the Company
         or a Wholly-Owned Consolidated Subsidiary; provided that the Company
         may permit a Material Subsidiary to merge with another Person if (A)
         such Material Subsidiary is the corporation surviving such merger, (B)
         immediately after giving effect to such merger, no Default shall have
         occurred and be continuing and (C) such surviving Material Subsidiary
         shall continue to be a Subsidiary.


                                      2
<PAGE>   12
9.       Section 6.08 is hereby deleted and replaced in its entirety by the
         following:

                 SECTION 6.08.  Use of Proceeds.  The proceeds of the Loans
         made under this Agreement will be used by the Company for the
         Company's general corporate purposes, provided that none of such
         proceeds will be used, directly or indirectly, for the purpose,
         whether immediate, incidental or ultimate, of purchasing or carrying
         any "margin stock" within the meaning of Regulation U or in violation
         of Regulation G or X.

10.      The following sections are hereby added to Article VI, Covenants:

                 SECTION 6.10.  Most Favored Nation.  The Company agrees that,
         in the event that (i) it agrees to enter into any amendment,
         supplement or other modification to the Other Bank Agreements (other
         than any such amendment, supplement or other modification which solely
         provides relief to the Company), it shall provide prompt notice
         thereof to the Bank and, if the Bank so agrees, enter into an
         amendment to this Agreement having substantially the same effect and
         (ii) after the occurrence of a Default, it voluntarily prepays any
         amount outstanding under any of the Other Bank Agreements or
         voluntarily reduces the amount of the commitments thereunder, it shall
         prepay the Loans or reduce the commitment, as the case may be, in such
         amount so that after giving effect to such prepayments or commitment
         reductions, as the case may be, under all the Other Bank Agreements,
         the amount so prepaid or reduced hereunder shall represent the Bank's
         pro rata share of the aggregate amount so prepaid or reduced under all
         the Other Bank Agreements.

                 SECTION 6.11.  Environmental.  The Company shall comply, and
         cause each of its Subsidiaries to comply, in all material respects,
         with all applicable environmental laws, regulations, permits and
         approvals, and obtain and renew and cause each of its Subsidiaries to
         obtain and renew all environmental permits and approvals necessary for
         its operations and properties, except to the extent that failure to
         comply with or obtain or renew the same would not have a material
         adverse effect on the business, financial position or results of
         operations of the Company and Lafarge Canada Inc. considered as a
         whole.

11.      Clause (c) (i) of  Section 8.08 is hereby deleted and replaced in its
         entirety by the following:

                 (i)  except in the case of such an assignment to an affiliate
         of the Bank, the Bank may not effect such an assignment without the
         prior written consent of the Company, which consent shall not be
         unreasonably withheld,

                                      3
<PAGE>   13
12.      The following is hereby added to Section 8.08:

                 (e)  Notwithstanding any other provision set forth in this
         Agreement, the Bank may at any time create a security interest in all
         or any portion of its rights under this Agreement (including, without
         limitation, the Loans and the Notes) in favor of any Federal Reserve
         Bank in accordance with Regulation A of the Board of Governors of the
         Federal Reserve System.

13.      In order to induce the Bank to enter into this Amendment, the Company
         hereby (x) makes each of the representations and warranties contained
         in Article V of the Credit Agreement on the Amendment Effective Date
         (as hereinafter defined), after giving effect to this Amendment, and
         (y) represents and warrants that no Default or Event of Default exists
         after giving effect to this Amendment.

14.      This Amendment shall become effective as of the date first above
         written (the "Amendment Effective Date") when and if counterparts of
         this Amendment shall have been executed and delivered by the Company
         and the Bank, and the Bank shall have received new Promissory Notes
         reflecting the amount of its Commitment.

15.      On and after the Amendment Effective Date, each reference in the
         Credit Agreement to "this Agreement", "hereunder", "hereof" or words
         of like import referring to the Credit Agreement shall mean the Credit
         Agreement as amended hereby.  The Credit Agreement is, and shall
         continue to be, in full force and effect and is hereby ratified and
         confirmed in all respects, as modified and amended hereby.

16.      This Amendment may be executed in any number of separate counterparts,
         each of which counterparts shall be an original and all of which taken
         together shall constitute one and the same agreement.  Delivery by
         facsimile of any executed signature page hereof shall be effective as
         delivery of an executed counterpart hereof.

17.      This Amendment shall be governed by, and construed in accordance with,
         the laws of the State of New York.


                                      4
<PAGE>   14
IN WITNESS WHEREOF, the parties hereto have caused this Amendment to Revolving
Credit Facility Agreement to be duly executed by their respective authorized
officers as of the date first above written.

                              The First National Bank of Chicago

                              By: /s/ KATHLEEN COMELLA 
                                 ---------------------------------------
                              Title:   VICE PRESIDENT
                                    ------------------------------------

                              
                              LAFARGE CORPORATION
                              
                              
                              By: /s/ KEVIN GRANT
                                 ---------------------------------------
                              Title:   Treasurer

                                      5
<PAGE>   15
                AMENDMENT TO REVOLVING CREDIT FACILITY AGREEMENT


Amendment dated as of August 1, 1996 (this "Amendment") to that certain
Revolving Credit Facility Agreement dated as of September 1, 1994 as amended as
of June 1, 1996 (the "Credit Agreement") between Lafarge Corporation, a
Maryland corporation (the "Company") and CIBC Inc.  (the "Bank").


WHEREAS, the Company and the Bank desire to amend certain provisions of the
Credit Agreement;

NOW, THEREFORE, the parties hereby agree as follows:

1.       Terms used herein shall have the same meaning as set forth in the
         Credit Agreement.

2.       The definition of "Commitment" in Section 1.01 is hereby changed from
         $35,000,000 to $30,000,000.

3.       The definition of "Debt" in Section 1.01 is hereby amended by adding:

         (ix)  all obligations of such Person arising under any Hedge
         Agreement.

4.       The following definition of "Hedge Agreement" is hereby added to
         Section 1.01 between the definitions of "Guarantee" and "Interest
         Period":

                 "Hedge Agreement" means any interest rate swap, cap, floor or
         collar agreement, interest rate future or option contract, currency
         swap agreement, currency future or option contract and any other
         similar agreement (including any option with respect to any of the
         foregoing) or any combination of the foregoing.

5.       The following definition of "Regulation G" is hereby added to Section
         1.01 between the definitions of "Regulation D" and "Regulation U":

                 "Regulation G" means Regulation G of the Board of Governors of
         the Federal Reserve System, as in effect from time to time.

6.       Section 3.03 (b) is hereby deleted and replaced in its entirety by the
         following:

                 (b)  If after May 1, 1996, the adoption or effectiveness of
         any applicable law, rule or regulation regarding capital adequacy, or
         any change therein, or any change in the interpretation or
         administration thereof by any governmental authority, central bank or
         comparable agency charged with the interpretation or administration
         thereof, or compliance by the Bank (or its Lending Office) or any
         corporation controlling the Bank with any request or directive
         regarding capital adequacy which is effective after May 1,
<PAGE>   16
         1996 (whether or not having the force of law) of any such authority,
         central bank or comparable agency, increases the capital required to
         be maintained by the Bank or such corporation as a consequence of its
         obligations hereunder, which increase results in a reduction of the
         rate of return on the Bank's or such corporation's capital to a level
         below that which the Bank or such corporation would have achieved but
         for such adoption, change or compliance (taking into consideration the
         Bank's or such corporation's policies with respect to capital
         adequacy) by any amount deemed by the Bank to be material, then from
         time to time, within 15 days after demand by the Bank, the Company
         shall pay to the Bank such additional amount or amounts as will
         compensate the Bank for such reduction.

7.       The following sections are hereby added  to Article V, Representations
         and Warranties:

                 SECTION 5.10.  Use of Proceeds.   The Company is not engaged
         in the business of extending credit for the purpose of purchasing or
         carrying any "margin stock" within the meaning of Regulation U, and
         will not use the proceeds of the Loans, directly or indirectly, for
         the purpose, whether immediate, incidental or ultimate, of purchasing
         or carrying any "margin stock".

                 SECTION 5.11.  Environmental Representation.  The operations
         and properties of the Company and each of its Subsidiaries are in
         compliance in all material respects with all applicable environmental
         laws, regulations and permits, all necessary environmental permits and
         approvals have been obtained and are in effect for the operations and
         properties of the Company and its Subsidiaries, and the Company and
         its Subsidiaries are in substantial compliance in all material
         respects with all such environmental permits and approvals, except to
         the extent that the failure to comply with any such environmental laws
         or regulations, or to comply with or to obtain any such environmental
         permits and approvals, would not have a material adverse effect on the
         business, financial position or results of operations of the Company
         and Lafarge Canada Inc. considered as a whole.

8.       The second sentence of Section 6.07 is hereby deleted and replaced in
         its entirety by the following:

                 The Company will cause each Material Subsidiary to maintain
         its corporate existence, and will not permit any Material Subsidiary
         to consolidate or merge with or into, or transfer all or any
         substantial part of its assets to, any Person other than the Company
         or a Wholly-Owned Consolidated Subsidiary; provided that the Company
         may permit a Material Subsidiary to merge with another Person if (A)
         such Material Subsidiary is the corporation surviving such merger, (B)
         immediately after giving effect to such merger, no Default shall have
         occurred and be continuing and (C) such surviving Material Subsidiary
         shall continue to be a Subsidiary.


                                      2
<PAGE>   17
9.       Section 6.08 is hereby deleted and replaced in its entirety by the
         following:

                 SECTION 6.08.  Use of Proceeds.  The proceeds of the Loans
         made under this Agreement will be used by the Company for the
         Company's general corporate purposes, provided that none of such
         proceeds will be used, directly or indirectly, for the purpose,
         whether immediate, incidental or ultimate, of purchasing or carrying
         any "margin stock" within the meaning of Regulation U or in violation
         of Regulation G or X.

10.      The following sections are hereby added to Article VI, Covenants:

                 SECTION 6.10.  Most Favored Nation.  The Company agrees that,
         in the event that (i) it agrees to enter into any amendment,
         supplement or other modification to the Other Bank Agreements (other
         than any such amendment, supplement or other modification which solely
         provides relief to the Company), it shall provide prompt notice
         thereof to the Bank and, if the Bank so agrees, enter into an
         amendment to this Agreement having substantially the same effect and
         (ii) after the occurrence of a Default, it voluntarily prepays any
         amount outstanding under any of the Other Bank Agreements or
         voluntarily reduces the amount of the commitments thereunder, it shall
         prepay the Loans or reduce the commitment, as the case may be, in such
         amount so that after giving effect to such prepayments or commitment
         reductions, as the case may be, under all the Other Bank Agreements,
         the amount so prepaid or reduced hereunder shall represent the Bank's
         pro rata share of the aggregate amount so prepaid or reduced under all
         the Other Bank Agreements.

                 SECTION 6.11.  Environmental.  The Company shall comply, and
         cause each of its Subsidiaries to comply, in all material respects,
         with all applicable environmental laws, regulations, permits and
         approvals, and obtain and renew and cause each of its Subsidiaries to
         obtain and renew all environmental permits and approvals necessary for
         its operations and properties, except to the extent that failure to
         comply with or obtain or renew the same would not have a material
         adverse effect on the business, financial position or results of
         operations of the Company and Lafarge Canada Inc. considered as a
         whole.

11.      Clause (c) (i) of  Section 8.08 is hereby deleted and replaced in its
         entirety by the following:

                 (i)  except in the case of such an assignment to an affiliate
         of the Bank, the Bank may not effect such an assignment without the
         prior written consent of the Company, which consent shall not be
         unreasonably withheld,

                                      3
<PAGE>   18
12.      The following is hereby added to Section 8.08:

                 (e)  Notwithstanding any other provision set forth in this
         Agreement, the Bank may at any time create a security interest in all
         or any portion of its rights under this Agreement (including, without
         limitation, the Loans and the Notes) in favor of any Federal Reserve
         Bank in accordance with Regulation A of the Board of Governors of the
         Federal Reserve System.

13.      In order to induce the Bank to enter into this Amendment, the Company
         hereby (x) makes each of the representations and warranties contained
         in Article V of the Credit Agreement on the Amendment Effective Date
         (as hereinafter defined), after giving effect to this Amendment, and
         (y) represents and warrants that no Default or Event of Default exists
         after giving effect to this Amendment.

14.      This Amendment shall become effective as of the date first above
         written (the "Amendment Effective Date") when and if counterparts of
         this Amendment shall have been executed and delivered by the Company
         and the Bank, and the Bank shall have received new Promissory Notes
         reflecting the amount of its Commitment.

15.      On and after the Amendment Effective Date, each reference in the
         Credit Agreement to "this Agreement", "hereunder", "hereof" or words
         of like import referring to the Credit Agreement shall mean the Credit
         Agreement as amended hereby.  The Credit Agreement is, and shall
         continue to be, in full force and effect and is hereby ratified and
         confirmed in all respects, as modified and amended hereby.

16.      This Amendment may be executed in any number of separate counterparts,
         each of which counterparts shall be an original and all of which taken
         together shall constitute one and the same agreement.  Delivery by
         facsimile of any executed signature page hereof shall be effective as
         delivery of an executed counterpart hereof.

17.      This Amendment shall be governed by, and construed in accordance with,
         the laws of the State of New York.


                                      4
<PAGE>   19
IN WITNESS WHEREOF, the parties hereto have caused this Amendment to Revolving
Credit Facility Agreement to be duly executed by their respective authorized
officers as of the date first above written.

                              CIBC Inc.

                              By:  /s/ CHERYL ROOT 
                                 ---------------------------------------
                              Title:   Authority Signature
                                    ------------------------------------

                              
                              LAFARGE CORPORATION
                              
                              
                              By:  /s/ KEVIN GRANT
                                 ---------------------------------------
                              Title:   Treasurer

                                      5
<PAGE>   20
                AMENDMENT TO REVOLVING CREDIT FACILITY AGREEMENT


Amendment dated as of August 1, 1996 (this "Amendment") to that certain
Revolving Credit Facility Agreement dated as of September 1, 1994 as amended as
of June 1, 1996 (the "Credit Agreement") between Lafarge Corporation, a
Maryland corporation (the "Company") and Wachovia Bank (the "Bank").


WHEREAS, the Company and the Bank desire to amend certain provisions of the
Credit Agreement;

NOW, THEREFORE, the parties hereby agree as follows:

1.       Terms used herein shall have the same meaning as set forth in the
         Credit Agreement.

2.       The definition of "Commitment" in Section 1.01 is hereby changed from
         $30,000,000 to $25,000,000.

3.       The definition of "Debt" in Section 1.01 is hereby amended by adding:

         (ix)  all obligations of such Person arising under any Hedge
         Agreement.

4.       The following definition of "Hedge Agreement" is hereby added to
         Section 1.01 between the definitions of "Guarantee" and "Interest
         Period":

                 "Hedge Agreement" means any interest rate swap, cap, floor or
         collar agreement, interest rate future or option contract, currency
         swap agreement, currency future or option contract and any other
         similar agreement (including any option with respect to any of the
         foregoing) or any combination of the foregoing.

5.       The following definition of "Regulation G" is hereby added to Section
         1.01 between the definitions of "Regulation D" and "Regulation U":

                 "Regulation G" means Regulation G of the Board of Governors of
         the Federal Reserve System, as in effect from time to time.

6.       Section 3.03 (b) is hereby deleted and replaced in its entirety by the
         following:

                 (b)  If after May 1, 1996, the adoption or effectiveness of
         any applicable law, rule or regulation regarding capital adequacy, or
         any change therein, or any change in the interpretation or
         administration thereof by any governmental authority, central bank or
         comparable agency charged with the interpretation or administration
         thereof, or compliance by the Bank (or its Lending Office) or any
         corporation controlling the Bank with any request or directive
         regarding capital adequacy which is effective after May 1,
<PAGE>   21
         1996 (whether or not having the force of law) of any such authority,
         central bank or comparable agency, increases the capital required to
         be maintained by the Bank or such corporation as a consequence of its
         obligations hereunder, which increase results in a reduction of the
         rate of return on the Bank's or such corporation's capital to a level
         below that which the Bank or such corporation would have achieved but
         for such adoption, change or compliance (taking into consideration the
         Bank's or such corporation's policies with respect to capital
         adequacy) by any amount deemed by the Bank to be material, then from
         time to time, within 15 days after demand by the Bank, the Company
         shall pay to the Bank such additional amount or amounts as will
         compensate the Bank for such reduction.

7.       The following sections are hereby added  to Article V, Representations
         and Warranties:

                 SECTION 5.10.  Use of Proceeds.   The Company is not engaged
         in the business of extending credit for the purpose of purchasing or
         carrying any "margin stock" within the meaning of Regulation U, and
         will not use the proceeds of the Loans, directly or indirectly, for
         the purpose, whether immediate, incidental or ultimate, of purchasing
         or carrying any "margin stock".

                 SECTION 5.11.  Environmental Representation.  The operations
         and properties of the Company and each of its Subsidiaries are in
         compliance in all material respects with all applicable environmental
         laws, regulations and permits, all necessary environmental permits and
         approvals have been obtained and are in effect for the operations and
         properties of the Company and its Subsidiaries, and the Company and
         its Subsidiaries are in substantial compliance in all material
         respects with all such environmental permits and approvals, except to
         the extent that the failure to comply with any such environmental laws
         or regulations, or to comply with or to obtain any such environmental
         permits and approvals, would not have a material adverse effect on the
         business, financial position or results of operations of the Company
         and Lafarge Canada Inc. considered as a whole.

8.       The second sentence of Section 6.07 is hereby deleted and replaced in
         its entirety by the following:

                 The Company will cause each Material Subsidiary to maintain
         its corporate existence, and will not permit any Material Subsidiary
         to consolidate or merge with or into, or transfer all or any
         substantial part of its assets to, any Person other than the Company
         or a Wholly-Owned Consolidated Subsidiary; provided that the Company
         may permit a Material Subsidiary to merge with another Person if (A)
         such Material Subsidiary is the corporation surviving such merger, (B)
         immediately after giving effect to such merger, no Default shall have
         occurred and be continuing and (C) such surviving Material Subsidiary
         shall continue to be a Subsidiary.


                                      2
<PAGE>   22

9.       Section 6.08 is hereby deleted and replaced in its entirety by the
         following:

                 SECTION 6.08.  Use of Proceeds.  The proceeds of the Loans
         made under this Agreement will be used by the Company for the
         Company's general corporate purposes, provided that none of such
         proceeds will be used, directly or indirectly, for the purpose,
         whether immediate, incidental or ultimate, of purchasing or carrying
         any "margin stock" within the meaning of Regulation U or in violation
         of Regulation G or X.

10.      The following sections are hereby added to Article VI, Covenants:

                 SECTION 6.10.  Most Favored Nation.  The Company agrees that,
         in the event that (i) it agrees to enter into any amendment,
         supplement or other modification to the Other Bank Agreements (other
         than any such amendment, supplement or other modification which solely
         provides relief to the Company), it shall provide prompt notice
         thereof to the Bank and, if the Bank so agrees, enter into an
         amendment to this Agreement having substantially the same effect and
         (ii) after the occurrence of a Default, it voluntarily prepays any
         amount outstanding under any of the Other Bank Agreements or
         voluntarily reduces the amount of the commitments thereunder, it shall
         prepay the Loans or reduce the commitment, as the case may be, in such
         amount so that after giving effect to such prepayments or commitment
         reductions, as the case may be, under all the Other Bank Agreements,
         the amount so prepaid or reduced hereunder shall represent the Bank's
         pro rata share of the aggregate amount so prepaid or reduced under all
         the Other Bank Agreements.

                 SECTION 6.11.  Environmental.  The Company shall comply, and
         cause each of its Subsidiaries to comply, in all material respects,
         with all applicable environmental laws, regulations, permits and
         approvals, and obtain and renew and cause each of its Subsidiaries to
         obtain and renew all environmental permits and approvals necessary for
         its operations and properties, except to the extent that failure to
         comply with or obtain or renew the same would not have a material
         adverse effect on the business, financial position or results of
         operations of the Company and Lafarge Canada Inc. considered as a
         whole.

11.      Clause (c) (i) of  Section 8.08 is hereby deleted and replaced in its
         entirety by the following:

                 (i)  except in the case of such an assignment to an affiliate
         of the Bank, the Bank may not effect such an assignment without the
         prior written consent of the Company, which consent shall not be
         unreasonably withheld,

                                      3
<PAGE>   23
12.      The following is hereby added to Section 8.08:

                 (e)  Notwithstanding any other provision set forth in this
         Agreement, the Bank may at any time create a security interest in all
         or any portion of its rights under this Agreement (including, without
         limitation, the Loans and the Notes) in favor of any Federal Reserve
         Bank in accordance with Regulation A of the Board of Governors of the
         Federal Reserve System.

13.      In order to induce the Bank to enter into this Amendment, the Company
         hereby (x) makes each of the representations and warranties contained
         in Article V of the Credit Agreement on the Amendment Effective Date
         (as hereinafter defined), after giving effect to this Amendment, and
         (y) represents and warrants that no Default or Event of Default exists
         after giving effect to this Amendment.

14.      This Amendment shall become effective as of the date first above
         written (the "Amendment Effective Date") when and if counterparts of
         this Amendment shall have been executed and delivered by the Company
         and the Bank, and the Bank shall have received new Promissory Notes
         reflecting the amount of its Commitment.

15.      On and after the Amendment Effective Date, each reference in the
         Credit Agreement to "this Agreement", "hereunder", "hereof" or words
         of like import referring to the Credit Agreement shall mean the Credit
         Agreement as amended hereby.  The Credit Agreement is, and shall
         continue to be, in full force and effect and is hereby ratified and
         confirmed in all respects, as modified and amended hereby.

16.      This Amendment may be executed in any number of separate counterparts,
         each of which counterparts shall be an original and all of which taken
         together shall constitute one and the same agreement.  Delivery by
         facsimile of any executed signature page hereof shall be effective as
         delivery of an executed counterpart hereof.

17.      This Amendment shall be governed by, and construed in accordance with,
         the laws of the State of New York.


                                      4
<PAGE>   24
IN WITNESS WHEREOF, the parties hereto have caused this Amendment to Revolving
Credit Facility Agreement to be duly executed by their respective authorized
officers as of the date first above written.

                              Wachovia Bank

                              By:  [SIG]
                                 ---------------------------------------
                              Title:  Vice President
                                    ------------------------------------

                              
                              LAFARGE CORPORATION
                              
                              
                              By:  /s/ KEVIN GRANT
                                 ---------------------------------------
                              Title:   Treasurer

                                      5
<PAGE>   25
                AMENDMENT TO REVOLVING CREDIT FACILITY AGREEMENT


Amendment dated as of August 1, 1996 (this "Amendment") to that certain
Revolving Credit Facility Agreement dated as of September 1, 1994 as amended as
of June 1, 1996 (the "Credit Agreement") between Lafarge Corporation, a
Maryland corporation (the "Company") and The Royal Bank of Canada (the "Bank").


WHEREAS, the Company and the Bank desire to amend certain provisions of the
Credit Agreement;

NOW, THEREFORE, the parties hereby agree as follows:

1.       Terms used herein shall have the same meaning as set forth in the
         Credit Agreement.

2.       The definition of "Commitment" in Section 1.01 is hereby changed from
         $30,000,000 to $25,000,000.

3.       The definition of "Debt" in Section 1.01 is hereby amended by adding:

         (ix)  all obligations of such Person arising under any Hedge
         Agreement.

4.       The following definition of "Hedge Agreement" is hereby added to
         Section 1.01 between the definitions of "Guarantee" and "Interest
         Period":

                 "Hedge Agreement" means any interest rate swap, cap, floor or
         collar agreement, interest rate future or option contract, currency
         swap agreement, currency future or option contract and any other
         similar agreement (including any option with respect to any of the
         foregoing) or any combination of the foregoing.

5.       The following definition of "Regulation G" is hereby added to Section
         1.01 between the definitions of "Regulation D" and "Regulation U":

                 "Regulation G" means Regulation G of the Board of Governors of
         the Federal Reserve System, as in effect from time to time.

6.       Section 3.03 (b) is hereby deleted and replaced in its entirety by the
         following:

                 (b)  If after May 1, 1996, the adoption or effectiveness of
         any applicable law, rule or regulation regarding capital adequacy, or
         any change therein, or any change in the interpretation or
         administration thereof by any governmental authority, central bank or
         comparable agency charged with the interpretation or administration
         thereof, or compliance by the Bank (or its Lending Office) or any
         corporation controlling the Bank with any request or directive
         regarding capital adequacy which is effective after May 1,
<PAGE>   26
         1996 (whether or not having the force of law) of any such authority,
         central bank or comparable agency, increases the capital required to
         be maintained by the Bank or such corporation as a consequence of its
         obligations hereunder, which increase results in a reduction of the
         rate of return on the Bank's or such corporation's capital to a level
         below that which the Bank or such corporation would have achieved but
         for such adoption, change or compliance (taking into consideration the
         Bank's or such corporation's policies with respect to capital
         adequacy) by any amount deemed by the Bank to be material, then from
         time to time, within 15 days after demand by the Bank, the Company
         shall pay to the Bank such additional amount or amounts as will
         compensate the Bank for such reduction.

7.       The following sections are hereby added  to Article V, Representations
         and Warranties:

                 SECTION 5.10.  Use of Proceeds.   The Company is not engaged
         in the business of extending credit for the purpose of purchasing or
         carrying any "margin stock" within the meaning of Regulation U, and
         will not use the proceeds of the Loans, directly or indirectly, for
         the purpose, whether immediate, incidental or ultimate, of purchasing
         or carrying any "margin stock".

                 SECTION 5.11.  Environmental Representation.  The operations
         and properties of the Company and each of its Subsidiaries are in
         compliance in all material respects with all applicable environmental
         laws, regulations and permits, all necessary environmental permits and
         approvals have been obtained and are in effect for the operations and
         properties of the Company and its Subsidiaries, and the Company and
         its Subsidiaries are in substantial compliance in all material
         respects with all such environmental permits and approvals, except to
         the extent that the failure to comply with any such environmental laws
         or regulations, or to comply with or to obtain any such environmental
         permits and approvals, would not have a material adverse effect on the
         business, financial position or results of operations of the Company
         and Lafarge Canada Inc. considered as a whole.

8.       The second sentence of Section 6.07 is hereby deleted and replaced in
         its entirety by the following:

                 The Company will cause each Material Subsidiary to maintain
         its corporate existence, and will not permit any Material Subsidiary
         to consolidate or merge with or into, or transfer all or any
         substantial part of its assets to, any Person other than the Company
         or a Wholly-Owned Consolidated Subsidiary; provided that the Company
         may permit a Material Subsidiary to merge with another Person if (A)
         such Material Subsidiary is the corporation surviving such merger, (B)
         immediately after giving effect to such merger, no Default shall have
         occurred and be continuing and (C) such surviving Material Subsidiary
         shall continue to be a Subsidiary.


                                      2
<PAGE>   27

9.       Section 6.08 is hereby deleted and replaced in its entirety by the
         following:

                 SECTION 6.08.  Use of Proceeds.  The proceeds of the Loans
         made under this Agreement will be used by the Company for the
         Company's general corporate purposes, provided that none of such
         proceeds will be used, directly or indirectly, for the purpose,
         whether immediate, incidental or ultimate, of purchasing or carrying
         any "margin stock" within the meaning of Regulation U or in violation
         of Regulation G or X.

10.      The following sections are hereby added to Article VI, Covenants:

                 SECTION 6.10.  Most Favored Nation.  The Company agrees that,
         in the event that (i) it agrees to enter into any amendment,
         supplement or other modification to the Other Bank Agreements (other
         than any such amendment, supplement or other modification which solely
         provides relief to the Company), it shall provide prompt notice
         thereof to the Bank and, if the Bank so agrees, enter into an
         amendment to this Agreement having substantially the same effect and
         (ii) after the occurrence of a Default, it voluntarily prepays any
         amount outstanding under any of the Other Bank Agreements or
         voluntarily reduces the amount of the commitments thereunder, it shall
         prepay the Loans or reduce the commitment, as the case may be, in such
         amount so that after giving effect to such prepayments or commitment
         reductions, as the case may be, under all the Other Bank Agreements,
         the amount so prepaid or reduced hereunder shall represent the Bank's
         pro rata share of the aggregate amount so prepaid or reduced under all
         the Other Bank Agreements.

                 SECTION 6.11.  Environmental.  The Company shall comply, and
         cause each of its Subsidiaries to comply, in all material respects,
         with all applicable environmental laws, regulations, permits and
         approvals, and obtain and renew and cause each of its Subsidiaries to
         obtain and renew all environmental permits and approvals necessary for
         its operations and properties, except to the extent that failure to
         comply with or obtain or renew the same would not have a material
         adverse effect on the business, financial position or results of
         operations of the Company and Lafarge Canada Inc. considered as a
         whole.

11.      Clause (c) (i) of  Section 8.08 is hereby deleted and replaced in its
         entirety by the following:

                 (i)  except in the case of such an assignment to an affiliate
         of the Bank, the Bank may not effect such an assignment without the
         prior written consent of the Company, which consent shall not be
         unreasonably withheld,

                                      3
<PAGE>   28
12.      The following is hereby added to Section 8.08:

                 (e)  Notwithstanding any other provision set forth in this
         Agreement, the Bank may at any time create a security interest in all
         or any portion of its rights under this Agreement (including, without
         limitation, the Loans and the Notes) in favor of any Federal Reserve
         Bank in accordance with Regulation A of the Board of Governors of the
         Federal Reserve System.

13.      In order to induce the Bank to enter into this Amendment, the Company
         hereby (x) makes each of the representations and warranties contained
         in Article V of the Credit Agreement on the Amendment Effective Date
         (as hereinafter defined), after giving effect to this Amendment, and
         (y) represents and warrants that no Default or Event of Default exists
         after giving effect to this Amendment.

14.      This Amendment shall become effective as of the date first above
         written (the "Amendment Effective Date") when and if counterparts of
         this Amendment shall have been executed and delivered by the Company
         and the Bank, and the Bank shall have received new Promissory Notes
         reflecting the amount of its Commitment.

15.      On and after the Amendment Effective Date, each reference in the
         Credit Agreement to "this Agreement", "hereunder", "hereof" or words
         of like import referring to the Credit Agreement shall mean the Credit
         Agreement as amended hereby.  The Credit Agreement is, and shall
         continue to be, in full force and effect and is hereby ratified and
         confirmed in all respects, as modified and amended hereby.

16.      This Amendment may be executed in any number of separate counterparts,
         each of which counterparts shall be an original and all of which taken
         together shall constitute one and the same agreement.  Delivery by
         facsimile of any executed signature page hereof shall be effective as
         delivery of an executed counterpart hereof.

17.      This Amendment shall be governed by, and construed in accordance with,
         the laws of the State of New York.


                                      4
<PAGE>   29
IN WITNESS WHEREOF, the parties hereto have caused this Amendment to Revolving
Credit Facility Agreement to be duly executed by their respective authorized
officers as of the date first above written.

                              The Royal Bank of Canada

                              By:  /s/ LORNA P. MENDELSON
                                 ---------------------------------------
                              Title:   ASSOCIATE
                                    ------------------------------------

                              
                              LAFARGE CORPORATION
                              
                              
                              By:  /s/ KEVIN GRANT
                                 ---------------------------------------
                              Title:   Treasurer

                                      5
<PAGE>   30
                AMENDMENT TO REVOLVING CREDIT FACILITY AGREEMENT


Amendment dated as of August 1, 1996 (this "Amendment") to that certain
Revolving Credit Facility Agreement dated as of June 1, 1996 (the "Credit
Agreement") between Lafarge Corporation, a Maryland corporation (the "Company")
and Bank of Montreal (the "Bank").


WHEREAS, the Company and the Bank desire to amend certain provisions of the
Credit Agreement;

NOW, THEREFORE, the parties hereby agree as follows:

1.       Terms used herein shall have the same meaning as set forth in the
         Credit Agreement.

2.       The definition of "Debt" in Section 1.01 is hereby amended by adding:

         (ix)  all obligations of such Person arising under any Hedge
         Agreement.

3.       The following definition of "Hedge Agreement" is hereby added to
         Section 1.01 between the definitions of "Guarantee" and "Interest
         Period":

                 "Hedge Agreement" means any interest rate swap, cap, floor or
         collar agreement, interest rate future or option contract, currency
         swap agreement, currency future or option contract and any other
         similar agreement (including any option with respect to any of the
         foregoing) or any combination of the foregoing.

4.       The following definition of "Regulation G" is hereby added to Section
         1.01 between the definitions of "Regulation D" and "Regulation U":

                 "Regulation G" means Regulation G of the Board of Governors of
         the Federal Reserve System, as in effect from time to time.

5.       Section 3.03 (b) is hereby deleted and replaced in its entirety by the
         following:

                 (b)  If after May 1, 1996, the adoption or effectiveness of
         any applicable law, rule or regulation regarding capital adequacy, or
         any change therein, or any change in the interpretation or
         administration thereof by any governmental authority, central bank or
         comparable agency charged with the interpretation or administration
         thereof, or compliance by the Bank (or its Lending Office) or any
         corporation controlling the Bank with any request or directive
         regarding capital adequacy which is effective after May 1, 1996
         (whether or not having the force of law) of any such authority,
         central bank or comparable agency, increases the capital required to
         be maintained by the Bank or such corporation as a consequence of its
         obligations hereunder, which increase results in a reduction of the
         rate of return on the Bank's or such corporation's capital to a level
         below that which the Bank or such corporation would have achieved but
         for such adoption,
<PAGE>   31
         change or compliance (taking into consideration the Bank's or such
         corporation's policies with respect to capital adequacy) by any amount
         deemed by the Bank to be material, then from time to time, within 15
         days after demand by the Bank, the Company shall pay to the Bank such
         additional amount or amounts as will compensate the Bank for such
         reduction.

6.       Section 3.03 (b) is hereby deleted and replaced in its entirety by the
         following:

                 (b)  If after May 1, 1996, the adoption or effectiveness of
         any applicable law, rule or regulation regarding capital adequacy, or
         any change therein, or any change in the interpretation or
         administration thereof by any governmental authority, central bank or
         comparable agency charged with the interpretation or administration
         thereof, or compliance by the Bank (or its Lending Office) or any
         corporation controlling the Bank with any request or directive
         regarding capital adequacy which is effective after May 1, 1996
         (whether or not having the force of law) of any such authority,
         central bank or comparable agency, increases the capital required to
         be maintained by the Bank or such corporation as a consequence of its
         obligations hereunder, which increase results in a reduction of the
         rate of return on the Bank's or such corporation's capital to a level
         below that which the Bank or such corporation would have achieved but
         for such adoption, change or compliance (taking into consideration the
         Bank's or such corporation's policies with respect to capital
         adequacy) by any amount deemed by the Bank to be material, then from
         time to time, within 15 days after demand by the Bank, the Company
         shall pay to the Bank such additional amount or amounts as will
         compensate the Bank for such reduction.

7.       The following sections are hereby added  to Article V, Representations
         and Warranties:

                 SECTION 5.10.  Use of Proceeds.   The Company is not engaged
         in the business of extending credit for the purpose of purchasing or
         carrying any "margin stock" within the meaning of Regulation U, and
         will not use the proceeds of the Loans, directly or indirectly, for
         the purpose, whether immediate, incidental or ultimate, of purchasing
         or carrying any "margin stock".

                 SECTION 5.11.  Environmental Representation.  The operations
         and properties of the Company and each of its Subsidiaries are in
         compliance in all material respects with all applicable environmental
         laws, regulations and permits, all necessary environmental permits and
         approvals have been obtained and are in effect for the operations and
         properties of the Company and its Subsidiaries, and the Company and
         its Subsidiaries are in substantial compliance in all material
         respects with all such environmental permits and approvals, except to
         the extent that the failure to comply with any such environmental laws
         or regulations, or to comply with or to obtain any such environmental
         permits and approvals, would not have a material adverse effect on the
         business, financial position or results of operations of the Company
         and Lafarge Canada Inc. considered as a whole.


                                      2
<PAGE>   32
8.       The second sentence of Section 6.07 is hereby deleted and replaced in
         its entirety by the following:

                 The Company will cause each Material Subsidiary to maintain
         its corporate existence, and will not permit any Material Subsidiary
         to consolidate or merge with or into, or transfer all or any
         substantial part of its assets to, any Person other than the Company
         or a Wholly-Owned Consolidated Subsidiary; provided that the Company
         may permit a Material Subsidiary to merge with another Person if (A)
         such Material Subsidiary is the corporation surviving such merger, (B)
         immediately after giving effect to such merger, no Default shall have
         occurred and be continuing and (C) such surviving Material Subsidiary
         shall continue to be a Subsidiary.

9.       Section 6.08 is hereby deleted and replaced in its entirety by the
         following:

                 SECTION 6.08.  Use of Proceeds.  The proceeds of the Loans
         made under this Agreement will be used by the Company for the
         Company's general corporate purposes, provided that none of such
         proceeds will be used, directly or indirectly, for the purpose,
         whether immediate, incidental or ultimate, of purchasing or carrying
         any "margin stock" within the meaning of Regulation U or in violation
         of Regulation G or X.

10.      The following sections are hereby added to Article VI, Covenants:

                 SECTION 6.10.  Most Favored Nation.  The Company agrees that,
         in the event that (i) it agrees to enter into any amendment,
         supplement or other modification to the Other Bank Agreements (other
         than any such amendment, supplement or other modification which solely
         provides relief to the Company), it shall provide prompt notice
         thereof to the Bank and, if the Bank so agrees, enter into an
         amendment to this Agreement having substantially the same effect and
         (ii) after the occurrence of a Default, it voluntarily prepays any
         amount outstanding under any of the Other Bank Agreements or
         voluntarily reduces the amount of the commitments thereunder, it shall
         prepay the Loans or reduce the commitment, as the case may be, in such
         amount so that after giving effect to such prepayments or commitment
         reductions, as the case may be, under all the Other Bank Agreements,
         the amount so prepaid or reduced hereunder shall represent the Bank's
         pro rata share of the aggregate amount so prepaid or reduced under all
         the Other Bank Agreements.

                 SECTION 6.11.  Environmental.  The Company shall comply, and
         cause each of its Subsidiaries to comply, in all material respects,
         with all applicable environmental laws, regulations, permits and
         approvals, and obtain and renew and cause each of its Subsidiaries to
         obtain and renew all environmental permits and approvals necessary for
         its operations and properties, except to the extent that failure to
         comply with or obtain or renew the same would not have a material
         adverse effect on the business, financial position or results of
         operations of the Company and Lafarge Canada Inc. considered as a
         whole.

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<PAGE>   33
11.      Clause (c) (i) of  Section 8.08 is hereby deleted and replaced in its
         entirety by the following:

                 (i)  except in the case of such an assignment to an affiliate
         of the Bank, the Bank may not effect such an assignment without the
         prior written consent of the Company, which consent shall not be
         unreasonably withheld,

12.      The following is hereby added to Section 8.08:

                 (e)  Notwithstanding any other provision set forth in this
         Agreement, the Bank may at any time create a security interest in all
         or any portion of its rights under this Agreement (including, without
         limitation, the Loans and the Notes) in favor of any Federal Reserve
         Bank in accordance with Regulation A of the Board of Governors of the
         Federal Reserve System.

13.      In order to induce the Bank to enter into this Amendment, the Company
         hereby (x) makes each of the representations and warranties contained
         in Article V of the Credit Agreement on the Amendment Effective Date
         (as hereinafter defined), after giving effect to this Amendment, and
         (y) represents and warrants that no Default or Event of Default exists
         after giving effect to this Amendment.

14.      This Amendment shall become effective as of the date first above
         written (the "Amendment Effective Date") when and if counterparts of
         this Amendment shall have been executed and delivered by the Company
         and the Bank, and the Bank shall have received new Promissory Notes
         reflecting the amount of its Commitment.

15.      On and after the Amendment Effective Date, each reference in the
         Credit Agreement to "this Agreement", "hereunder", "hereof" or words
         of like import referring to the Credit Agreement shall mean the Credit
         Agreement as amended hereby.  The Credit Agreement is, and shall
         continue to be, in full force and effect and is hereby ratified and
         confirmed in all respects, as modified and amended hereby.

16.      This Amendment may be executed in any number of separate counterparts,
         each of which counterparts shall be an original and all of which taken
         together shall constitute one and the same agreement.  Delivery by
         facsimile of any executed signature page hereof shall be effective as
         delivery of an executed counterpart hereof.

17.      This Amendment shall be governed by, and construed in accordance with,
         the laws of the State of New York.


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<PAGE>   34
IN WITNESS WHEREOF, the parties hereto have caused this Amendment to Revolving
Credit Facility Agreement to be duly executed by their respective authorized
officers as of the date first above written.

                              Bank of Montreal

                              By:  /s/ B.L. BANKE
                                 ---------------------------------------
                              Title:   DIRECTOR
                                    ------------------------------------

                              
                              LAFARGE CORPORATION
                              
                              
                              By:  /s/ KEVIN GRANT
                                 ---------------------------------------
                              Title:   Treasurer

                                      5