<PAGE>   1
                                                                    Exhibit 4(s)



                               AMENDMENT AGREEMENT

     THIS SECOND AMENDMENT AGREEMENT is entered into as of March 7, 2000 among
ZEMEX CORPORATION, a corporation established under the federal laws of Canada
(the "Company"),  ZEMEX U.S. CORPORATION, a corporation established under the
laws of Delaware (the "US Borrower") (the Company and the US Borrower called
the "Borrowers"), the several financial institutions from time to time parties
to the Credit Agreement (collectively, the "Banks"; individually a "Bank"),
Bank of America Canada as Agent for the Canadian Banks, Bank of America, N.A.
as agent for the US Banks, and Bank of America Canada as Arranger.

WHEREAS, the parties entered into a credit agreement dated as of May 21, 1999
(the "Credit Agreement") whereby the Banks provided certain credit facilities
to the Company and the US Borrower for working capital, short term liquidity
and general corporate purposes, including permitted Acquisitions;

AND WHEREAS, the parties have entered into a First Amendment to the Credit
Agreement dated as of September 24, 1999;

AND WHEREAS, the parties have agreed to make additional amendments to the
Credit Agreement, specifically, as required to incorporate a bridge loan
facility in favour of the US Borrower,  and the parties are entering into this
Second Amendment Agreement to amend the Credit Agreement accordingly;

NOW THEREFORE, in consideration of the mutual agreements, provisions and
covenants contained herein, the parties agree as follows:

1.   All capitalized terms used herein and not otherwise defined shall have
     the same meaning as those ascribed thereto in the Credit Agreement (as
     amended by the First Amendment


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     Agreement).

2.   The definition of "APPLICABLE MARGIN" is deleted and substituted with the
     following:


<TABLE>
<CAPTION>
                         LIBOR (EXCLUDING BRIDGE                   LIBOR LOANS
LEVERAGE                 LOANS),  B/A RATE        PRIME AND        FOR BRIDGE
RATIO                    LOANS AND L/CS           BASE RATE LOANS  LOANS ONLY
--------                 -----------------------  ---------------  -----------
<S>                      <C>                      <C>              <C>
UNDER 2.75                   1.25%                     .25%           1.625%

FROM AND INCLUDING 2.75
TO LESS THAN 3.00            1.375%                    .375%          1.75%

3.00 OR MORE                 1.50%                     .50%           1.875%
</TABLE>


3.   The following definitions are added to the Credit Agreement:

     "BRIDGE LOAN COMMITMENT" HAS THE MEANING SPECIFIED IN SECTION 2.01(D);

     "BRIDGE LOAN FACILITY" MEANS THE TERM CREDIT FACILITY TO BE MADE
     AVAILABLE TO THE US BORROWER BY THE BRIDGE LOAN LENDER UNDER SECTION 2.20
     HEREOF;

     "BRIDGE LOAN LENDER" MEANS BOFA;

     "BRIDGE LOAN" MEANS A US BORROWING MADE PURSUANT TO SECTION 2.20 HEREOF;

     "BRIDGE LOAN REPAYMENT DATE" HAS THE MEANING SPECIFIED IN SECTION 2.20
     HEREOF.

     "NET PROCEEDS" MEANS, AS TO ANY DISPOSITION, PROCEEDS IN CASH,
     CHEQUES, OR OTHER CASH EQUIVALENT FINANCIAL INSTRUMENTS AS AND WHEN
     RECEIVED BY EITHER BORROWER OR A SUBSIDIARY, NET OF:



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     (A)  THE DIRECT COSTS RELATING TO SUCH DISPOSITION EXCLUDING
          AMOUNTS PAYABLE TO A BORROWER OR ANY AFFILIATE OF SUCH BORROWER,

     (B)  ANY INCOME, SALE, USE OR OTHER TRANSACTION, TAXES PAID OR
          PAYABLE BY THE BORROWER OR SUBSIDIARY AS A DIRECT RESULT THEREOF.

4.   The definition of "Commitment" is deleted and substituted with the
     following:

          "COMMITMENT", AS TO EACH BANK, IS THE AMOUNT SET FORTH
          IN SCHEDULE 2.01 OPPOSITE EACH BANK'S NAME AND SHALL
          INCLUDE THE BRIDGE LOAN COMMITMENT.

5.   The definition of "Credit" is deleted and substituted with the
     following:

          "CREDIT" MEANS THE REVOLVING CREDIT FACILITY OF UP TO
          $20,000,000 (OR THE CANADIAN DOLLAR EQUIVALENT) ESTABLISHED
          BY THE BANKS IN FAVOUR OF THE BORROWERS, AND SPECIFICALLY
          EXCLUDES THE BRIDGE LOAN FACILITY.

6.   The definition of "Majority Banks" is deleted and substituted with the
     following:

          "MAJORITY BANKS" MEANS AT ANY TIME AT LEAST TWO BANKS THEN
          HAVING AT LEAST 100% OF THE COMMITMENTS, OR IN THE EVENT
          ONLY ONE BANK HAS COMMITMENTS, SUCH BANK."

7.   The definition of "Loan" is amended by the addition of ", BRIDGE LOAN"
     following the phrase "Swingline Loan" in lines 2 - 3 thereof.

8.   The definition of "Pro Rata Share" is amended by the addition of the
     following sentence at the end of such definition:



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          "IN THE CASE OF THE BRIDGE LOAN FACILITY, THE BRIDGE LOAN LENDER'S PRO
          RATA SHARE OF THE BRIDGE LOAN COMMITMENT IS 100%."

9.   Section 2.01(a) is amended by the addition of ", PREPAY UNDER SECTION
     2.21 " following the phrase "prepay under Section 2.07" in the last
     sentence thereof.

10.  Section 2.01(b) is amended by the addition of ", PREPAY UNDER SECTION
     2.21 " following the phrase "prepay under Section 2.07 " in the last
     sentence thereof.

11.  Section 2.01(c) is amended by the addition of the following sentence at
     the end thereof:

          "ONLY THE BRIDGE LOAN LENDER SHALL MAKE BRIDGE LOANS TO THE
          US BORROWER."

12.  Section 2.01 is further amended by addition of the following:

     "(D) THE BRIDGE LOAN LENDER AGREES, ON THE TERMS AND CONDITIONS SET
          FORTH HEREIN, TO MAKE A SINGLE LOAN TO THE US BORROWER (THE "BRIDGE
          LOAN") IN AN AMOUNT NOT TO EXCEED $50,000,000 (THE "BRIDGE LOAN
          COMMITMENT"). AMOUNTS BORROWED AS A BRIDGE LOAN MAY NOT BE REBORROWED.

13.  Section 2.05(a) is amended by the addition of "OR A BRIDGE LOAN"
     following the phrase "a Swingline Loan" in line 1 thereof.

14.  Section 2.06 is amended by the addition of the following:

     "(G) THE PROVISIONS OF THIS SECTION 2.06 PERMITTING CONVERSION OF US LOANS


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          SHALL NOT APPLY IN THE CASE OF THE BRIDGE LOAN."

15.  Section 2.08 is amended by the addition of "(EXCLUDING THE BRIDGE LOAN)"
     following the phrase "amount of US Loans outstanding" in line 3 thereof.

16.  Section 2.10(b) is amended by the deletion of the third sentence thereof
     and substitution of the following:

          "INTEREST SHALL ALSO BE PAID ON THE DATE OF ANY PREPAYMENT
          OF LOANS, INCLUDING ANY PAYMENT UNDER SECTION 2.07 OR 2.21,
          FOR THE PORTION OF THE LOANS SO PREPAID AND UPON PAYMENT
          (INCLUDING PREPAYMENT) IN FULL THEREOF AND, DURING THE
          EXISTENCE OF ANY EVENT OF DEFAULT, INTEREST SHALL BE PAID ON
          DEMAND OF THE APPLICABLE AGENT AT THE REQUEST OR WITH THE
          CONSENT OF THE MAJORITY BANKS."

17.  Section 2.11(b) is amended by the addition of "(EXCLUDING THE BRIDGE LOAN
     COMMITMENT)" following the phrase "of the Total Commitments" in the first
     sentence thereof.

18.  Section 2.17(a) is amended by the addition of the following:

     "(VII) NO REALLOCATION REQUEST MAY BE MADE IN RESPECT OF THE BRIDGE
            LOAN."

19.  The following section is added to the Agreement as a new section 2.20.

"2.20 BRIDGE LOAN FACILITY

     (A)  THE BRIDGE LOAN LENDER SHALL MAKE A ONE TIME BRIDGE LOAN TO
          THE US BORROWER BY WAY OF LIBOR LOANS IN AN AMOUNT NOT TO EXCEED
          $50,000,000.  THE PROCEEDS FROM THE BRIDGE LOAN SHALL BE APPLIED BY
          THE US BORROWER TO THE REPAYMENT OF


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          THE SENIOR SECURED NOTES.  THE BRIDGE LOAN SHALL BE MADE UPON THE
          US BORROWER'S IRREVOCABLE WRITTEN NOTICE DELIVERED TO THE US AGENT
          SPECIFYING: (I) THE AMOUNT OF THE BRIDGE LOAN; (II) THE REQUESTED
          BORROWING DATE, WHICH SHALL BE A BUSINESS DAY; AND (III) THE
          DURATION OF THE INITIAL LIBOR PERIOD.

     (B)  SUBJECT TO (D) BELOW, THE US BORROWER SHALL REPAY IN FULL THE
          BRIDGE LOAN AND ALL ACCRUED INTEREST ON OCTOBER 31, 2000 (THE
          "BRIDGE LOAN REPAYMENT DATE").

     (C)  REPAYMENT OR PREPAYMENT (INCLUDING PURSUANT TO SECTION 2.21)
          OF ALL OR ANY PART OF THE BRIDGE LOAN SHALL PERMANENTLY REDUCE THE
          BRIDGE LOAN FACILITY.

     (D)  IF AT ANY TIME THE OUTSTANDING AMOUNT OF THE LOANS IS LESS
          THAN $10,000,000, THE US BORROWER SHALL IMMEDIATELY REPAY THE FULL
          AMOUNT OUTSTANDING OF THE BRIDGE LOAN. SUCH REPAYMENT MAY BE MADE
          BY WAY OF CANADIAN LOANS OR US LOANS."

20.       The following Section is added to the Agreement as a new Section 2.21:

    "2.21 MANDATORY PREPAYMENT

          (A)  IF THE COMPANY OR THE US BORROWER OR ANY SUBSIDIARY SHALL AT ANY
               TIME OR FROM TIME TO TIME MAKE A DISPOSITION, THEN (I) THE
               COMPANY OR THE US BORROWER, AS THE CASE MAY BE, SHALL PROMPTLY
               NOTIFY THE CANADIAN AGENT OF SUCH PROPOSED DISPOSITION (INCLUDING
               THE AMOUNT OF ESTIMATED NET PROCEEDS TO BE RECEIVED BY THE
               COMPANY, US BORROWER OR SUBSIDIARY IN RESPECT THEREOF) AND (II)
               PROMPTLY UPON RECEIPT BY THE COMPANY, US BORROWER OR SUBSIDIARY
               OF THE NET PROCEEDS OF SUCH DISPOSITION, THE COMPANY OR THE US
               BORROWER AS THE CASE MAY BE, SHALL REPAY LOANS (EXCLUDING L/C
               LOANS) IN AN AGGREGATE AMOUNT EQUAL TO 100% OF THE AMOUNT OF SUCH
               NET PROCEEDS. IN NO EVENT SHALL THE DATE FOR PAYMENT OF THE NET
               PROCEEDS BY THE COMPANY OR THE US BORROWER TO THE APPLICABLE


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               AGENT (THE "NET PROCEEDS PAYMENT DATE") BE LATER THAN TWO
               BUSINESS DAYS FOLLOWING RECEIPT BY THE COMPANY, US BORROWER
               OR SUBSIDIARY, AS THE CASE MAY BE, OF THE NET PROCEEDS OF
               SUCH DISPOSITION.  AT LEAST TWO BUSINESS DAYS PRIOR TO THE
               NET PROCEEDS PAYMENT DATE, THE COMPANY OR THE US BORROWER, AS
               THE CASE MAY BE, SHALL PROVIDE THE APPLICABLE AGENT WITH AN
               IRREVOCABLE NOTICE OF PREPAYMENT, IDENTIFYING THE NET
               PROCEEDS PAYMENT DATE AND A DETAILED CALCULATION OF NET
               PROCEEDS FROM THE SUBJECT DISPOSITION. WHEN SUCH NOTICE IS
               GIVEN BY THE COMPANY OR THE US BORROWER, THE COMPANY OR THE
               US BORROWER, AS THE CASE MAY BE, SHALL MAKE SUCH PREPAYMENT
               ON THE NET PROCEEDS PAYMENT DATE, TOGETHER WITH ACCRUED
               INTEREST TO SUCH DATE ON THE AMOUNT PREPAID.

          (B)  THE COMPANY OR THE US BORROWER, AS THE CASE MAY BE, SHALL ALSO
               PAY TO BANKS ANY AMOUNTS PAYABLE PURSUANT TO SECTION 4.04 AS A
               RESULT OF A PREPAYMENT PURSUANT TO THIS SECTION 2.21.

          (C)  UPON RECEIPT BY AN AGENT OR AGENTS OF A NOTICE OF
               PREPAYMENT PURSUANT TO SECTION 2.21(A) ABOVE, THE AGENTS SHALL
               CALCULATE THE TOTAL OUTSTANDING AMOUNTS OF EACH CANADIAN LOAN
               (EXCLUDING L/C LOANS), US LOANS (EXCLUDING L/C LOANS AND BRIDGE
               LOANS) AND BRIDGE LOANS. THE TOTAL AMOUNT OF SUCH CANADIAN LOANS
               AND US LOANS ARE CALLED THE "REVOLVING LOANS". THE AGENT(S) SHALL
               APPLY THE NET PROCEEDS FROM A DISPOSITION RATABLY AGAINST THE
               REVOLVING LOANS AND THE BRIDGE LOAN. SPECIFICALLY, APPLICATION OF
               THE NET PROCEEDS SHALL BE DETERMINED OR CALCULATED IN PROPORTION
               TO WHICH THE REVOLVING LOANS AND THE BRIDGE LOAN EACH BEAR TO THE
               TOTAL AMOUNT OF ALL LOANS OUTSTANDING (EXCLUDING L/C LOANS).
               DISBURSEMENT OF PAYMENTS BY THE APPLICABLE AGENT TO EACH
               APPLICABLE BANK SHALL BE MADE ON THE NET PROCEEDS PAYMENT DATE
               (OR OTHERWISE IN ACCORDANCE WITH SECTION 2.13) IN AMOUNTS EQUAL
               TO EACH BANK'S PRO RATA


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               SHARE, BUT APPLIED IN THE FOLLOWING PRIORITIES:

               (I)  IN THE CASE OF REVOLVING LOANS FIRST
                    TO PRIME RATE LOANS AND BASE RATE LOANS, AND SECOND TO
                    B/A EQUIVALENT LOANS AND LIBOR LOANS;

               (II) IN THE CASE OF THE BRIDGE LOAN, FIRST
                    TO THE LIBOR LOANS."

21.  Section 4.04 (d) is amended by the addition of "OR SECTION 2.21"
     following the phrase "Section 2.07" in line 1 thereof.

22.  The following Section is added to the Agreement as a new Section 5.03:


     5.03 CONDITIONS TO BRIDGE LOAN BORROWINGS.

          IN ADDITION TO THE PROVISIONS OF SECTION 5.01 AND 5.02, THE OBLIGATION
          OF THE BRIDGE LOAN LENDER TO MAKE THE BRIDGE LOAN IS SUBJECT TO THE
          SATISFACTION OF THE FOLLOWING CONDITIONS PRECEDENT ON OR BEFORE THE
          EFFECTIVE DATE OF THE INITIAL BRIDGE LOAN:


          (A)  PREPAYMENT AMOUNT.  EVIDENCE OF THE ARRANGEMENTS FOR
               PREPAYMENT OF THE SENIOR SECURED NOTES ON TERMS SATISFACTORY TO
               THE AGENTS.

          (B)  CONFIRMATION OF EXISTING GUARANTEES.  EACH GUARANTOR SHALL
               CONFIRM IN WRITING TO THE BANKS, THE AGENTS AND THE BRIDGE LOAN
               LENDER THAT THE GUARANTEE GRANTED BY EACH RESPECTIVE GUARANTOR
               DATED MAY 21, 1999 REMAINS IN FULL FORCE AND EFFECT AND APPLY TO
               THE CREDIT AGREEMENT AS AMENDED;



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          (C)  SECURITY CONFIRMATION.  WRITTEN CONFIRMATION FROM THE
               BORROWERS AND EACH GUARANTOR THAT THE EXISTING SECURITY DOCUMENTS
               GRANTED PURSUANT TO THE CREDIT AGREEMENT REMAIN IN FULL FORCE AND
               EFFECT AND APPLY TO THE CREDIT AGREEMENT, AS AMENDED.

          (D)  LEGAL OPINIONS.  (I) AN OPINION OF STIKEMAN, ELLIOTT,
               COUNSEL TO THE COMPANY AND ADDRESSED TO THE AGENTS AND THE BANKS,
               IN FORM SATISFACTORY TO THE AGENTS, THE BANKS AND THEIR COUNSEL;
               AND (II) AN OPINION OF HOGAN & HARTSON, COUNSEL TO THE US
               BORROWER AND ADDRESSED TO THE AGENTS AND THE BANKS, IN FORM
               SATISFACTORY TO THE AGENTS, THE BANKS AND THEIR COUNSEL.

23.  Section 8.24 is amended by the deletion of the first sentence thereof and
     substitution therefor with the following:

          "NEITHER BORROWER SHALL USE OR PERMIT TO BE USED ANY
          PART OF THE CREDIT, THE BRIDGE LOAN OR ANY LOAN
          PROCEEDS FOR THE DIRECT OR INDIRECT ACQUISITION OF AN
          EXCLUDED SUBSIDIARY."

24.  Section 9.01 is amended by the addition of the following paragraph:

          (N) DISPOSITIONS.  THE COMPANY OR THE US BORROWER
          FAILS TO SUBMIT THE NET PROCEEDS FROM ANY DISPOSITION
          TO THE BANKS IN REPAYMENT OF THE LOANS.

25.  Section 11.08 is amended by additions of the following:

     (F)    IN EACH INSTANCE WHERE A DECISION IS REQUIRED TO BE MADE BY THE
MAJORITY BANKS UNDER THIS AGREEMENT, EACH BANK SHALL ACT IN GOOD FAITH AND USE
ITS BEST EFFORTS TO REACH


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AGREEMENT WITH THE OTHER BANKS WITHIN FIVE BUSINESS DAYS, OR, IN THE CASE OF
ACTION OR DECISION REQUIRED PURSUANT TO SECTION 9.02, WITHIN 48 HOURS. IN THE
EVENT THAT THE BANKS ARE UNABLE TO REACH AGREEMENT WITHIN SUCH TIME PERIODS,
THE CHASE MANHATTAN BANK ("CHASE") SHALL BE DEEMED TO HAVE ASSIGNED ITS
COMMITMENTS AND ALL OTHER RIGHTS AND OBLIGATIONS OF CHASE HEREUNDER TO BofA,
AND BofA SHALL BE DEEMED TO HAVE ACCEPTED AND ACQUIRED ALL SUCH COMMITMENTS,
RIGHTS AND OBLIGATIONS FROM CHASE. THE COMPANY AND THE AGENTS WILL BE DEEMED
TO HAVE CONSENTED TO SUCH ASSIGNMENT. THE PARTIES SHALL EFFECT SUCH
ASSIGNMENT IN COMPLIANCE WITH THE PROVISIONS OF THIS SECTION 11, INCLUDING
DELIVERY OF AN ASSIGNMENT AND ACCEPTANCE.

26.  Schedule 2.01 is replaced with the following:


                                  SCHEDULE 2.01

                      BANK COMMITMENTS AND PRO RATA SHARES


<TABLE>
<CAPTION>
CANADIAN BANKS            COMMITMENT   PRO RATA SHARE
------------------------  -----------  --------------
<S>                       <C>          <C>
BANK OF AMERICA CANADA    $2,000,000         10%

US BANKS
------------------------
BANK OF AMERICA, NA       $9,000,000          45%
THE CHASE MANHATTAN BANK   9,000,000          45%
                          -----------
                 TOTAL    $20,000,000

BRIDGE LOAN LENDER
------------------------
BANK OF AMERICA, NA       $50,000,000         100%
</TABLE>


27.  For greater clarification and certainty, the parties agree that any
     obligation to make the Bridge Loan is only that of the Bridge Loan Lender
     and the other Banks are under no obligation to participate directly or by
     way of risk participation in any Bridge Loan.




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28.  The parties confirm the terms and conditions of the Credit Agreement as
     amended by the terms of the First Amendment Agreement and as amended by
     this Second Amendment Agreement.

29.  This Second Amendment Agreement may be referred to as being dated March
     7, 2000, notwithstanding the actual date of execution.

30.  This Second Amendment Agreement may be executed in any number of separate
     counterparts, each of which, when so executed shall be deemed an original
     and all said counterparts taken together shall be deemed to constitute one
     and the same instrument.

31.  The representations and warranties in Article VI of the Credit Agreement
     remain true and correct with the same effect as if made on and as of the
     date of this Second Amendment Agreement.


[The remainder of this page is intentionally left blank.]




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IN WITNESS WHEREOF the parties have executed this Agreement on March 7, 2000.

                                          ZEMEX CORPORATION, AS COMPANY

                                          By:___________________________________

                                          Title:________________________________

                                          ZEMEX U.S. CORPORATION,
                                          AS US BORROWER

                                          By:___________________________________

                                          Title:________________________________





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                                          BANK OF AMERICA CANADA,
                                          AS CANADIAN AGENT AND AS A BANK

                                          By:___________________________________

                                          Title:________________________________






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                                         BANK OF AMERICA N.A.
                                         AS US AGENT

                                         By:____________________________________

                                         Title:_________________________________





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                                          BANK OF AMERICA N.A.
                                          AS A BANK

                                          By:___________________________________

                                          Title:________________________________






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                                     - 16 -

                                          BANK OF AMERICA N.A.
                                          AS BRIDGE LOAN LENDER

                                          By:___________________________________

                                          Title:________________________________





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                                     - 17 -

                                          THE CHASE MANHATTAN BANK
                                          AS A BANK


                                          By:___________________________________

                                          Title:________________________________